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DISTRIBUTION PLANNING GUIDE

A GUIDE TO ASSIST PLANNING OF 11000 AND 400 VOLT OVERHEAR AND UNDERGROUND DISTRIBUTION SYSTEM

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BANGLADESH POWER DEVELOPMENT BOARD SEPTEMBER 1985

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FOREWORED

There was a long-standing need of the Bangladesh Power Development Board to have its own “Distribution Planning Guide” for development of reliable & efficient distribution system. So long we have been following unwritten guides based on heterogeneous concepts, which gave rise to sub-standard system reticulations resulting in poor voltage and high loss.

In this background this “Distribution Planning Guide” will fill up the vacuum and fulfill the requirement for proper planning and execution of distribution networks.

All the concerned engineers of PDB are hereby directed to follow the “Distribution Planning Guide” strictly for implementation of 11kV and 400 volts distribution networks.

I hope that this document will be kept up to date with time-to-time revision as and when necessary.

(Brig. A. Muhith Chowdhury) (Retd.)Chairman

Bangladesh Power Development Board

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INDEX PAGE

1. PROLOGUE 01

1.1 General 01

1.2 Firm Capacity of Substation 02

1.3 Definition of Planning 02

1.4 System Configuraiton-11kV 03

1.4.1 Urban Area 03

1.4.2 Rural Area 04

1.5 System Configuration - L.V. 05

1.6 Earthing Practice 06

1.7 Approach to Planning 06

1.8 Design Parameters 08

2. CONSTINUOUS CURRENT RATING 09

3. LINE VOLTAGE REGULATION 09

4. FAULT LEVELS 10

4.1 Maximum Value 10

4.2 Minimum Value 11

4.3 Section Points 11

4.4 Induction Motor 11

4.5 Generator and Synchronous Motor 11

4.6 Calculations 11

5. MOTORS 11

5.1 Continuous Rating 11

5.2 Starting Current 12

5.3 Starting Method 12

5.4 Regulation 13

5.5 Rating Limits 13

5.6 Calculations 14

6. PROTECTION 14

6.1 General 14

6.2 Distribution Substations 15

7. CONSUMER SUPPLY VOLTAGE 16

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INDEX PAGE

8. LOAD ESTIMATING 17

8.1 Power Factor 17

8.2 Residential & Commercial 17

8.3 Industrial and Large Commercial 17

8.4 Load Diversity 18

8.5 Future Load 18

9. CONSUMERS SERVICE 19

9.1 Current Rating 19

9.1.1 Insulated Aerial Conductors 19

9.1.2 Down Load to Meter 19

9.1.3 Non-Insulated Service Conductors 19

9.1.4 Underground Cables 20

9.2 Service Limits-Insulated Aerial Conductors 20

9.3 Service Equipment 20

9.4 Calculation Examples 21

9.4.1 Example – 1 21

9.4.2 Example - 2 21

10. DRAWINGS 22

10.1 General 22

10.2 Geographic and Single Line 22

10.3 Key Diagrams 22

10.4 Topo Survey 23

10.5 Electrical Symbols 23

APPENDICES

APPENDIX - I Line Voltage Regulation Limits

APPENDIX - II Voltage Drop Calculations

APPENDIX - III Examples in Determining Service Requirements

APPENDIX - IV Fault Calculations: Symmetrical Three Phase

APPENDIX - V Fault Calculation: Earth Fault

APPENDIX - VI Determination of Motor Starting Current

APPENDIX - VII General Notes on Selection and Setting Protection Devices

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TABLES

TABLE - A Overhead Line Constants

TABLE - B Medium Voltage Insulated Conductors and Cable Constants

TABLE - C 11kV Cable Constants.

TABLE - D Volt Drop Factors – 230 volt Single Phase

TABLE - E Volt Drop Factors – 400 volt Three Phase

TABLE - F Volt Drop Factors – 11,000 volt Three Phase

TABLE - G Overhead Lines – Percentage Resistance are Reactance per mile to a 10 MVA

Base

TABLE - H Cables – Percentage Resistance and Reactance per mile to a 10 MVA Base

TABLE - I Transformers – Percentage Reactance to a 10 MVA Base.

TABLE - J Equivalent 11kV Source Impedance Referred to 400 volt.

TABLE - K Equivalent Impedance of 11,000 volt Overhead Lines Referred to 400 volt.

TABLE - L Equivalent Impedance of 11,000 volt Cables Referred to 400 volt.

TABLE - M Equivalent Impedance Distribution Transformers Referred to 400 volt.

TABLE - N Load Assessment – Residential

TABLE - O Load Assessment – Commercial

TABLE - P Aerial services – Maximum Span Lengths and Erection Tensions.

TABLE - Q Service Equipment – Single Phase

TABLE - R Service Equipment – Three Phase

TABLE - S Application of H.T. Dropout Fuse and Molded Case Circuit Breaker.

TABLE - T 11kV Overhead Line Constant (Zero Sequence)

TABLE - U 11kV Cable Constant (Zero Sequence)

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FIGURES

Fig.1 Time/Current Curve – 100 amp M.C.C.B. (Type XA Fuse)

Fig.2 Time/Current Curve – 200 amp M.C.C.B. (Type XA Fuse)

Fig.3 Time/Current Curve – 350 amp M.C.C.B. (Type XA Fuse)

Fig.1a Time/Current Curve – 100 amp M.C.C.B. (Type K Fuse)

Fig.2a Time/Current Curve – 200 amp M.C.C.B. (Type K Fuse)

Fig.3a Time/Current Curve – 350 amp M.C.C.B. (Type K Fuse)

Fig.4 Electrical Symbols

Fig.5 Typical Key Diagram

Fig.6 Characteristic – Time Lag Fuses

Fig.7 Operating Characteristic and Outline Template for a Standard 3 Second I.D.M.T.

Relay

Fig.8 Setting Envelope for a Standard 3 Second Over Current Relay

Fig.9 Example-1

Fig.10 Example-2

Fig.11 Example-3

DRAWING

Drawing No.-100

Maps

Map - 1 : 1200 Scale Sample Survey Map

Map - 2 : 5000 Scale Sample Survey Map

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1. PROLOGUE

1.1 GENERAL

This planning guide is for Engineers who have received specific training on distribution planning already.

The objective of this guide is to serve as a means of standardizing the planning of 11kV and L.V. distribution systems throughout Bangladesh. The provisions of this guide must therefore be followed most strictly and deviations will only be allowed when authorized by the Chief Engineer (P&D), PDB.

It is realized that there are differences in conditions between urban areas and rural areas, particularly caused by the higher density of load in urban areas. This will necessitate different methods of planning in the two areas as is detailed in these instructions.

The task of a distribution planning engineer in Bangladesh is to plan 33kV, 11kV and 400V distribution system. His task is not concerned with the 132kV transmission system. His task is to take electricity from “bulk supply points” in the form of 132/33kV substations and distribute it at 33kV, 11kV and 400V to the consumers.

Responsibility of distribution planning will generally lie with Superintending Engineer (P&D) under each Zonal Chief Engineer. Planning of 11kV and 400 volt distribution is to be done by the development divisions while 33kV by the Superintending Engineer (P&D).

The steps involved are (a) load survey (b) conceptual layout on appropriate key maps (c) voltage drop studies (d) sectionalizing studies (e) preparation of detail layouts with detail mapping.

Field planning and development offices under the Chief Engineers should be in direct communication with the Chief Engineer (P&D)’s offices in the headquarters for technical assistance as required from time to time.

Limit of Development Division

The divisional planning engineer will rarely be concerned with 33/11kV substation. He may be required to liaise with the Superintending Engineer (P&D) to ensure that 33/11kV substations are designed to suit the requirements of the distribution of his area.

The divisional planning engineer may be involved in extensions to the main 11kV switchgear board in 33/11kV substations, if his planning shows that such extension is required.

His other and most important involvement with 33/11kV substations will be to ensure that they do not go “out-of-firm”. An “out-of-firm” condition is said to exist when the maximum load on a 33/11kv substation is greater than “the total 33/11kV transformer capacity minus the rating of the largest transformer”. The reason for not letting these substations go out-of-firm is to allow a substation to supply its maximum load demand while one transformer is out of commission through a fault. (The repair replacement of such a faulty transformer would take a considerable time and the alternative requirement of load-shedding for such a time would be unpalatable).

1.2 FIRM CAPACITY OF SUBSTATION

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At the time of writing this guide it is noted that many 33/11kV substations are out-of-firm. This situation has arisen because of the lack of adequate planning activity, systematic approach and because of financial constraints vis-a-vis extremely rapid rate of load growth being experienced. This is a temporary situation, which will be rectified as soon as possible. Thereafter out-of-firm situations will only be allowed to arise if such are authorized in writing by the respective Chief Engineer. Such authorizations will only be given when the out-of-firm situation is clearly seen to be rectifiable within a short time by creation of new bulk sources.

The standard 33/11kV substation now being installed contains 210/13.3 MVA transformers and thus has a firm capacity of 13.3 MVA. All 33/11kV substation smaller than this will be up-rated as and when required, circumstances permitting. There are also some 33/11kV substations with 310/13.3 MVA transformers and thus the firm capacity is 26 MVA.

1.3 DEFINITION OF PLANNING

The term “planning” has a different meaning with respect to electrical distribution than with other engineering fields so it is worth defining the term here. The output of planning of distribution systems will identify the following things:

1) The route of all overhead lines and cables;2) The required size or capacity of all overhead lines and cables;3) The location and capacity of all transformers;4) The location of sectionalizing points and specifying the type of all switch and fuse

gear;5) The settings or rating of all protective devices (in liaison with the protection

engineer).

Thus the term should not be confused with another word “Design”. “Design” with respect to electrical distribution systems consists of specifying the detail of the line materials and equipment of a distribution system, such as the height of poles, pole top details, conductors, hardware etc. and quantifying the same.

1.4 SYSTEM CONFIGURATION – 11KV

1.4.1 Urban Area

The most basic concept of 11kV system planning must be to give two sources of supply to as many substations as possible in order to reduce the outage time during inevitable faults to a minimum. The only exceptions to this concept which should be allowed are small loads (below 1 MVA) or where the provision of a second source of supply would be in-ordinately expensive difficult. However, supplies to persons or areas designated by management as very important shall always be given 2 sources of supply irrespective of the size of the load involved.

The simplest and most preferred way of allowing the facility of duplicate supplies is to arrange the 11kV distribution system in the form of a ring which will be operated with one point of the ring normally open. This configuration is shown as type ‘A’ on the attached drawing No. 100.

However, to allow for other geographical conditions and to more easily allow the incorporation of existing networks into new planning, the configurations shows as ‘B’ and ‘F’ may also be allowed. Both of these give 2 sources of supply.

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In some circumstances, in particular where there is a heavy concentration of load situated a large distance from a 33/11 kV substation, it cab be considered advisable to establish a small bulk distribution point in the form of a switching station as shown in the same drawing. This switching station shall be equipped with circuit breaker panels, one of which must be a bus-section switch or C.B. It shall be fed by 2, 11kV feeders, preferably from the same 33/11kV substation or else from 2 different substations. From these switching stations simple open rings shall be arranged, as shown.

Substation interconnections (type ‘G’) may be planned if the circumstances permit, one only per substation.

Small loads may be catered for with short redial feeders of tees off overhead lines as shown in configuration types ‘C’ and ‘D’.

Every effect must be made to avoid over-complicating the 11kV system since this will make the work of operating and maintaining it un-necessarily difficult. Thus interconnections between rings, such as those shown as type ‘H’ shall be avoided. These also cause the necessity for additional switchgear, which is very expensive.

Circuit breakers shall only be installed in 33/11kV substations and the type of switching station shown in drawing No.:100. The switchgear in ground mounted distribution substations (11kV/400V) shall consist of outdoor type ring-main units (RMUS). These shall consist of isolating switches controlling the incoming and outgoing cables and fuse switches controlling the transformer.

Otherwise, ground mounted substations, if feed from and overhead line may consist of a consumer-supplied 11kV C.B. or a PDB supplied fuse switch controlling the transformer. The substation shall be teed off the overhead line with a pole mounted 11kV switch controlling the cable.

Pole-mounted substations shall consist of transformers teed off the overhead line, the transformers being controlled by fuse-isolators.

Overhead lines shall have switches installed in the line every 2 KM or at Tee-off points to assist with fault location and sectionalization. As far as is practical and possible each 11kV ring shall be either overhead or underground and not a mixture of the two. Overhead lines shall be sited so as to give at least 1 M clearance from buildings.

Tee-joints on underground 11kV cables will not be allowed.

For the supply to very important consumers or areas underground systems shall, as far as possible, be installed in order to ensure a high reliability of supply. In all other areas, in order to reduce the cost of the installation, overhead systems shall be installed unless the actual site conditions make the construction of overhead lines impossible or impractical.

All overhead 11kV distribution lines shall be planned to run roughly parallel to roads although not necessarily following every curve of a winding road. This is intended to make operation and maintenance easier.

Main lines shall be kept as short as possible. It is much preferred to feed the various loads by tees off the main line rather than to deviate the main line in a zigzag manner from load to load.

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1.4.2 Rural Areas.

The situation regarding the rural areas of Bangladesh will be found to the different form the urban areas; in particular the distances involved will be greater, the load density will be less and the importance of the loads will be less. Thus the configuration of rural distribution systems will differ from those in urban areas. In general, REB standard of planning the distribution will be followed in rural areas.

Rural 11kV distribution systems will be almost entirely overhead. The great majority of 11kV feeders will be the radial type ‘D’.

Long main lines, especially if they have many tees-off shall have one or two auto-reclosers installed in the line so as to divide the line into equal sections. Long tees-off may also control at or near the tee-off point by an auto-recloser. Reclosers shall be sited in locations easily accessible from roads.

It will generally be found necessary to build long main lines with Merlin conductor, other main lines and long tees with Dog conductor and all other tees with Rabbit conductor.

Line routes shall be such as to avoid repeated crossing and re-crossing of roads.

1.5 SYSTEM CONFIGURATION – L.V.

400 volt circuits will normally be supplied from one point only but, where practical, facilities for connecting to an adjacent 400 volt circuit may be provided.

Distribution transformers should be sited as close as possible to the load centre of the system they are supplying.

The source point for all L.V. networks is the 11kV/400V distribution transformers mentioned in paragraph 4. The transformers will be protected from faults on the L.V. networks by M.C.C.B’s (molded-case circuit-breakers) installed adjacent to the transformers themselves. The M.C.C.B. rating shall be as mentioned in paragraph 6.2 of the main planning guide. No additional fuse or M.C.C.B protection shall be installed on L.V. lines at points remote from the transformer.

Preference must be given to overhead lines rather than underground cable systems for economic reasons. Very important consumers or very important areas may be given an underground L.V. system however.

From consumer substations the L.V. network will consist of cable from the transformer to the consumers L.V. switchgear. From all other substations the L.V. network shall consist of a number of networks lines (the minimum number being given in paragraph 6.2 of the main guide).

The lines will be sited to run along the side of roads to give close access to house and other buildings. As many as necessary tees-off from these radial feeders may be installed. Closed rings of L.V. feeders shall not be used. The phase conductors of adjacent L.V. networks shall not be connected together to parallel the supplies from the 2 transformers. However, to ensure low earthing

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resistances, if is necessary to connect together the neutrals of adjacent networks. Thus the ends of adjacent networks should be terminated on the same pole.

Overhead lines shall be sited so as to give 1 meter clearance from all buildings and other obstructions or else the conductors shall be insulated.

1.6 EARTHING PRACTICE

The electrical system at 400V, 11kV, 33kV and transmission voltage are solidly earthed, that is, the star points of 3 phase system is connected to earth to allow full short circuit current to pass in the event of a fault to earth, limited only by the resistance of the path to earth between the point of supply and point of fault (the entire system of BPDB however may not be effectively earthed particularly away from the S/S). In general BPDB system earthing is provided at supply S/S. Resistance of S/S earth is limited to maximum 1 (ohm), maximum earth circuit resistance, including the fault resistance should not exceed 40 (ohms) at 11kV/6.33kV for effective protection. No practical limit of earth circuit resistance can be specified for 400V system so as to actuate molded case circuit breakers without earth leakage feature, in case of earth fault.

Therefore maximum 40 (ohms) may be considered for calculation of minimum earth fault current in the 11kV system.

1.7 APPROACH TO PLANNING

The initial planning step is to estimate the magnitude of the load to be supplied, including anticipated load development within the period under review, within the specified geographic area.

a) Identifying the Area.

Identify the area to be supplied. The whole project area should be divided into zones, one supply zone per 33/11kV S/S or into sub-zones: one sub-zone per 11kV switching station.

b) Mapping

Collect maps and/or mains records of the area to be supplied. 1:5000 and 1:1200 scale maps shall be used for all planning works. For small planning jobs where no proper maps are available, sketch maps (not to scale) may be used. For larger jobs the Surveyor General shall be requested to survey and map the project area. This procedure can take months or years depending on the size of the area involved so that suitable notice must be given. If time does not permit for long delay, this may be done through professional private survey companies after taking necessary approval.

c) Recording existing system

Collect details of the existing system in the subject area. This should be assembled on the geographic main records maps of the area and on the relevant single line diagram.

Due to the poor state of main record keeping in the past it should not be assumed that record drawings are up to date and correct. Records should always be checked physically on site and amend as necessary.

d) Survey existing Load

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Collect as much detail as possible of the existing loading conditions in the subject area. This should comprise the maximum loads on all 11kV feeders and all distribution transformers. 11kV feeder loading are always available from substation records. In time a programmed of checking and recording the maximum load at peak time of the year on all distribution transformers on the system will be started. In the meantime, the data routinely available from the O&M Circles may be utilized. Alternatively, loads may be estimated by counting the number and type of services with their connected load and applying the diversity factors as mentioned in the planning guide.

e) Forecasting the Demand

Estimates must be made for at least 5 years ahead of the load to be catered for a maximum of 7 years forecast must be made for sizing a 33/11 kV substation. This must be done by assuming an appropriate annual growth rate over the existing loads. Past records may be a guide line in this respect. In addition, all known and possible future large loads (200 kW and above) must be taken into consideration. This will take the form known of possible future developments such as office buildings, factories, residential areas etc. Location of all of those loads must be identified in order to plan the distribution feeders.

f) Survey of Existing Facilities

The condition of all exiting installations must be assessed in order to decide whether any of them are in such poor condition as to require replacement. This may be done by studying the records kept in O&M files and by close physical inspection by the planning engineer himself. Alternatively, this information may be requested from O&M Circles although the planning engineer must himself verify that there is a valid justification for scraping such equipment.

g) Planning the System

The position and size of all transformers shall be decided. However, there must also be taken into consideration the routes of cables and lines to be decided in ‘h’ hereafter. The two are interdependent. The location of all new and rehabilitated L.V. lines must then be decided. They should all run alongside roads close to the buildings they are to supply.

h) Feeder Loads

i) From ‘e’ it should be possible to work out the maximum load to be fed from the subject 33/11kV substation divide this into sub-areas and draw up a system plan composed of the various configurations shown in figure 100. The maximum use of existing installation, in satisfactory condition, shall be made and new cables or overhead lines planned to complete the system. Each leg to a single ring, a 3-legged ring and interconnected distributor shall be planned to carry a maximum load of 2.5 MVA in normal circumstances. In case, one leg is out of the near end of substation and sectionalizing switch will be closed, it would carry a maximum of 5 MVA. This is based on a 3c185mm2 (Al) cable or Dog-conductored overhead line within both carry a maximum load of 5 MVA, approximately. The sub-system on a switching station can carry a maximum load of 7 MVA if the express feeders are 3c185mm2 Copper U.G.C. or Merlin O.H.L. and 5 MVA if the express feeders are 3c185mm2 (Al) to carrying a maximum of 50% of the current carrying capacity. Subject to voltage conditions they may carry up to 100% of their ratings.

ii) Feeder Size

In deciding the size of necessary conductors and cable voltage drop calculations must be done. However, in urban areas it will generally be found that because of the higher load density, voltage drop will rarely be a serious factor in deciding cable or line sizes. In rural areas voltage drop calculations are essential and it will frequently be found that a line loaded to its maximum current carrying capacity will result in excessive voltage drop. Power loss will be additionally opted. In deciding the size of lines and cables, certain problematic loads must also be taken into consideration, namely motors, arc furnaces, welding machines and

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equipment generation harmonics. These shall be handled in the manner described in the planning guide.

iii) Sectionalizing Switches

For overhead line systems the position if all 11kV switches shall be fixed as mentioned in section 5.

iv) System Configuration

All equipment and the planned 11kV add L.V. networks must then be drawn up on the drawings mentioned in (b) and (c) and shown in sample map 1 and 2, 1:5000 scale shall be used for zonal 11kV system planning while 1:1200 scale shall be used for detail planning of 11kV and 400 volt system.

1.8 DESIGN PARAMETERS

All data given herein is based on the following parameters:

Maximum ambient air temperature : 45˚CGroup temperature at one meter : 30˚CGroup thermal resistivity : 1.2˚C/m/W

Maximum Conductor temperature:

PIL and PVC insulated cables : 70˚CERP and XLPE insulated cables : 80˚COverhead line conductors : 75˚CMinimum Wind Speed : 1.6 km/hrSolar intensity : 0.12W/sq.cm

Maximum three phase r.m.s. Symmetrical fault levels:

33,000 volt system : 1,000 MVA (17.5 kA)11,000 volt system : 250 MVA (13.1 kA)400 volt system : 29 MVA (41.9 kA)

Overhead line conductor spacing:

11,000 volt lines, equilateral formation : 930 mm

400 volt lines, vertical formation : 300 mm

Temperature coefficient of resistance at 200C ():

Copper (annealed) : 0.00393 per ˚CCopper (hard drawn) : 0.00381 per ˚CAluminum : 0.00403 per ˚C

All planning shall be carried out in accordance with the latest issue of the Electricity Act, 1910 (Bangladesh), and any other Regulation made in pursuance thereof.

2.CONTINUOUS CURRENT RATING

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The maximum continuous current ratings for overhead lines and underground cables are listed in Tables A-C.

These ratings have been calculated on the basis of the parameters given in Section-1.3.

For other equipment, transformers, switchgear etc., the manufacturer’s quoted current rating will be used after applying any appropriate de-rating factor. For example, the information given on the rating plate may refer to its operation at a different ambient temperature than that in which it is installed.

The load current should not exceed the de-rated current rating under normal operating conditions.

The continuous maximum current ratings for consumer’s services are considered in section-9.

3. LINE VOLTAGE REGULATION

For planning and design purposes, and as shown in Appendix-I, the following line regulation at the points farthest from the supply point, shall not be exceeded under normal operating conditions: -

a) 11,000 volt system : 3%

b) 400/230 volt system : 5%

c) Services : 1%

In calculating the line regulation the following power factors shall be assumed:

a) 11,000 volt systems : 0.80 lagb) 400/230 volt system & services:

i) Predominantly heating and lighting loads : 0.95 lagii) All other combinations of load : 0.85 lag

For certain loads where a low power factor is inherent to the equipment the consumer is required to install power factor correction equipment to the extent that the power factor is improved to a value between unity and 0.85 lags.

For individual circuit analysis for motors the manufacturer’s stated power factor and efficiency should be used. Where these are no available the following may be assumed: -

Motor RatingSingle Phase Three Phase Motor

Efficiency Power factor Efficiency Power factor

Up to 3 HP 75% 0.8 lag 80% 0.8 lag

3 to 10 HP 80% 0.83lag 87% 0.87lag

Over 10 HP 85% 0.87lag 90% 0.9lag

Conductor KW-Yard and KW-Mile voltage factors are given in Tables D, E & F and examples of volt drop calculations are given at Appendix-II.

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4. FAULT LEVELS

4.1 Maximum Value

The maximum prospective three phase r.m.s. symmetrical fault levels must not exceed the following values:-

a) 33,000 volt system : 1,000 MVA (17.5 kA) r.m.s. symmetrical

b) 11,000 volt system : 250 MVA (13.1 kA) r.m.s. symmetrical

c) 400/230 volt system : 29 MVA (41.9 kA) r.m.s. symmetrical

These are the maximum values. The equipment must be checked and the manufacturers rated value used if lower.

Calculations shall be carried out to determine the maximum fault level at points in the system where switchgear and fuse gear is installed, to show that the equipment rated fault capacity is adequate and that protective devices will operate satisfactorily. In the latter case it is also necessary to calculate the minimum fault level.

The source impedance from the next higher voltage network shall initially be assumed to be the value corresponding to the prospective three phase r.m.s. symmetrical fault rating. If the calculated fault level does not then exceed 80% of the equipment rated capacity the system will be satisfactory.

4.2 Minimum Value

When calculating the minimum fault level, the source impedance shall be assumed to be 25% of the prospective three phase r.m.s. symmetrical fault rating.

4.3 Section Points

When making calculations for an interconnected system the points normally open must be clearly indicated. Open points, which if closed could cause the equipment rated capacity to be exceeded must be clearly indicated and noted to this effect.

4.4 Induction Motors

Fault contribution from motors shall be neglected unless:-

a) 400 volt system : The circuit aggregate motor capacity exceeds 250 kW.b) 11,000 volt system : The circuit aggregate motor capacity at 11,000 volt

and 400 volt exceeds 2,000 kW.

4.5 Generators and Synchronous Motors

Fault contributions from generators and synchronous motors, which can operate in parallel with the distribution systems, shall be determined by detailed analysis.

4.6 Calculations

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Reactance values for calculation purposes shall be those given in Tables A, B and C.

Calculations and examples are given at Appendix-IV.

5. MOTORS

5.1 Continuous Rating

Motors may be rated in Kilowatts or Horse Power. The line current may be determined as follows:

a) Single phase Motor

I =Horse Power 746

Amp230 Efficiency Power factor

OrKilowatt 1000

Amp230 Efficiency Power factor

b) Three phase Motor

I=Horse Power 746

Amp1.732 400 Efficiency Power factor

OrKilowatt 1000

Amp1.732 400 Efficiency Power factor

The manufacturer’s values for efficiency and power factor should be used. Where these are not available the values given in Section 3 should be adopted.

5.2 Starting Current

When starting a motor the initial stating current is several times greater than the continuous rated current and this can cause excessive voltage dips on the system and be a nuisance to other consumers due to visible flicker in Tungsten lamps.

It is therefore necessary to consider together with the size of the motor the following factors to determine if the motor can be connected as proposed:

a) The method of motor starting,b) The method of connection to the supply system,c) The frequency of motor starts andd) The voltage dips on starting.

5.3 Starting Method

Apart from direct on-line starting various methods are used to reduce the initial starting current to a more acceptable level. Depending on the type of motor one of the following methods of starting may be used:

a) Star-Delta (Approx. 60% of direct-on-line starting current)

b) Auto Transformer (Approx. 40-70% of direct-on-line starting current)

c) Rotor Resistance

d) Stator Resistance

e) Capacitor

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5.4 Regulation

Starting regulation depends on the number of starts with respect to time.

For motors which subject to infrequent starting, i.e. start intervals of two hours or more, the initial. Starting current should not normally cause a volt dip exceeding 3% in the phase to neutral voltage at the point of common coupling with other loads connected to the supply authority distribution system.

For motors subject to frequent starting, i.e. start intervals of less than two hours, the corresponding value is 1%.

5.5 Rating limits

It is convenient for both the supply authority and the medium voltage consumer if range 400/230 volt motors can be accepted for direct-on-line starting without the need for special study.

The permissible rating of such a motor is dependant on the frequency of motor starts; the distribution transformer rating and its location within the high voltage system; and the point of common coupling, within the 400/230 volt distribution system, with other consumer’s loads.

The point of common coupling is where the specific consumer service is connected to the

400/230 volt distribution main, and may be at any location between the terminals of the distribution substation and extremity of the distribution system.

Based on the general conditions above, motors may normally be accepted for direct-on-line starting up to the following ratings :-

a) With Common Coupling Occurring on the 400/230 Volt Distribution Main

Motors subject to infrequent start up to the following ratings may be accepted for connection to the 400/230 volt distribution system without qualification as to the method of starting :-

Single Phase, 230 volts - 1.0 HP (0.75 kW)

Three Phase, 400 volts - 3.0 HP (2.25 kW)

Motors subject to frequent start (including lift and hoist motors and other motors subject to similar duties) up to the following ratings may be accepted for connection to the 400/230 volt distribution system without qualification as to the method of stating.

Single Phase, 230 volts - 0.33 HP (0.25 kW)

Three Phase, 400 volts - 1.0 HP (0.75 kW)

b) With Common Coupling Occurring at 400/230 volt Terminal of the Distribution Transformer

Motors up to the following ratings may be accepted for connection to the 400/230 volt terminals of distribution transformer without qualification as to the method of starting :-

Three Phase Transformer Rating 11/0.4kV

MOTOR RATING – H.P.INFREQUENT START FREQUENT START

KVA Single phase Three phase Single phase Three phase25 2 4 0.75 1.550 3 6 1 2

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100 6 12 2 4200 10 20 3.5 7315 15 30 5 10500 20 40 7.5 15800 30 60 10 201000 35 70 12 24

When the point of common coupling occurs on the 11,000 volt distribution system (i.e. a high voltage consumer) it must form the subject of a special study as illustrated at Appendix-V.

5.6 Calculations

Examples showing the method of determining the maximum direct-on-line start motor are given at Appendix-V.

6.Protection

6.1 General

Protection devices, i.e. relays, fuses, molded case circuit breakers (MCCB), miniature circuit breakers (MCB) are designed for two purposed:-

a) To protect equipment from prolonged or excessive current which may otherwise cause damage?

b) To quickly and automatically isolate from the supply system any equipment that has been damage.

In order to provide protection against prolonged overloads the fuse or circuit breaker or current transformer operating into a circuit breaker relay must be rated as closely as possible to the circuit full load current. It is seldom practical to select a device with a rating equal to the circuit full load current. It is usually necessary to install a device with the next higher standard current rating.

Current transformers to operate over current relays on 11kV circuit breakers may require more than one ratio. For example if a 400 amp circuit breaker is initially to control a load of 140 amp circuit breaker is initially to control a load of 140 amp it would be in order to use a 200/5 amp current transformer. In such a case it would be reasonable to install a 400/200/5 amp current transformer allow the circuit breaker to be utilized up to its rated capacity at some later date.

Protection devices, characteristics and recommended setting values can be obtained from a separate report ‘Electrical Protection for the Greater Dacca Transmission and Distribution Systems'.

The protective devices should be discriminative so that only the device nearest to the

equipment affected operates thus isolating a minimum of the system from the supply.

Where protective devices are installed in series it is essential that they be properly time/current graded to ensure their correct operation.

For example, consider a radial 11kV overhead line from an 11kV switching station supplying a number of distribution transformers each with its own medium voltage distribution system. The protective devices installed may include a relay operated circuit breaker at the 11kV switching station, 11kV fuses at each of the distribution transformers, an MCCB on each medium voltage circuit from the transformers and a fuse or MCB at each consumer’s metering point.

When correctly graded the protective devices should operate so that only the minimum of the system is isolated from the supply as follows :-

a) Overload or damage within the consumer’s premises – service, fuse or MCB only.b) Overload or damage on a distribution transformer – the MCCB controlling the medium -

voltage circuit only.

c) Damage to the distribution substation - the 11kV fuses only.

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d) Overload or damage on the 11kV overhead line or cable-the circuit breaker at the 11kV switching station only.

Where small motors, such as unit air conditioners, are not automatically disconnected from the supply in event of power failure, allowance must be made for this heavy, short time, current on restoration of supply. The selection of the MCCB should permit this short time excess current.

6.2 Distribution Substations

The following arrangements for the protection of distribution substations will normally be adopted in order to give discriminative operation of protective devices:-

a) Pole Mounted Transformers

TRANSFORMER RATING

11KV TYPE XA FUSE

M.C.C.B.NO. OF

CIRCUITSRATING

MAGNETIC SETTING NO.

KVA AMPS NO. AMPS NO.50 15 1 100 5

100, or 30 1 200 3100 30 2 100 3

200, or 50 1 350 3200 50 2 200 2

b) Ground Mounted Transformers

TRANSFORMER RATING

11kV H.R.C FUSE

M.C.C.B.NO. OF

CIRCUITSRATING

MAGNETIC SETTING NO.

KVA AMPS No. AMPS No.300 40 3 350 5500 50 4 350 5800 80 5 350 41000 90 6 350 4

The Time/current characteristic curves for the above M.C.C.B.’s and 11kV fuses are shown at Figures 1, 2 and 3.

The magnetic current setting 1 refers to ‘HI’ and 5 to ‘LO’ on the M.C.C.B. (These settings refer to one manufacturer’s M.C.C.B.’s. Other manufacturer’s M.C.C.B.’s may require different settings).

7.CONSUMER SUPPLY VOLTAGE

The voltage of supply and number of phases, to consumers shall normally be as follows:

a) Up to 10 kVA : 230 volt, single phase, two-wire from the distribution system.

b) 10 to 30 kVA : 400 volt, three phase, four-wire from the distribution system.

c) 30 to 175kVA : 400 volt, three phase, four-wire direct from the terminals of an 11/0.4kV distribution system.

d) Over 175 kVA : 11,000 volt, three phase.

Note: A 400 volt, three phase supply may be necessary because of the nature of the load although the maximum demand may be less than 10 kVA.

See also Table – s for NEMA standard 11kV Fuse.With commercial/industrial loads the effect of motor starting currents may necessitate the

installation of a transformer, or a high voltage supply, solely for a single consumer.

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The above is considered a guide for use in all Electric Supplies and should be read in conjunction with B.P.D.B. tariff conditions ruling at the time.

8. LOAD ESTIMATING

8.1 Power Factor

The values of power factor specified in Section 3 shall be applied as necessary.

8.2 Residential and Commercial

The after diversity maximum demand (A.D.M.D.) of residential and commercial consumers may be assessed using the Tables N and O respectively.

The following notes apply to these tables :-

a) All loads to be converted to watts using the appropriate power factor and efficiency.

b) All indoor lighting points, including table and standard lamps supplied from socket outlets or adaptors, to be rated at 100 watts.

c) All outdoor security lighting points, floodlighting, to be rated at 100 watts.

d) All floodlight points to be rated in accordance with the equipment installed.

e) All other equipment to be rated in accordance with manufacturer's rating plate.

f) For commercial consumer’s stand-by plant need not be taken into account provided it cannot be used at the same time as the main plant.

8.3 Industrial and Large Commercial

The A.D.H.D. of all plant installed to be assessed on information obtained from the consumer (mode of operation), the manufacturer (plant ratings) and site inspection.

Stand-by plant need not be taken into account provided it cannot be used at the same time as the main plant.

8.4 Load Diversity

On any feeder, because the maximum demand of each individual consumer may not occur at the same time, a diversity factor may be applied to the total sum of each individual consumer’s maximum demand to the access the feeder maximum demand.

No fixed diversity factor to be applied to a feeder can be given as any particular feeder may supply a variety of combinations of different types of consumers with their peak loads occurring at different times. It is also difficult to find out diversity factor.

For example, a single distribution feeder may supply a number of commercial, industrial and residential consumers. One industrial consumer may work a single shift with peak load occurring at 11.00 to 13.00 hours, another may work twenty four hours with the peak load occurring at 01.00 to 06.00 hours, commercial consumers may have peak load occurring 08.00 to 15.00 hours, and

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residential peak loads occurring 18.00 to 20.00 hours. In addition the peak load of any consumer may vary depending on the season of the year.

In general each feeder must be examined individually to assess the diversity factor applicable to that particular feeder.

With medium voltage feeders supplying predominantly residential consumers the situation is not so complicated as the majority of consumers are in the same category with their individual peak loads occurring at the same time and the same season of the year. This also applies to small commercial consumers (shops) within a residential area.

The following diversity factors may be applied to the sum of the individual residential/shop consumer after diversity maximum demands, the consumers all being supplied from the same medium voltage distribution feeder :-

Up to 5 Consumers - 1.0 to total A.D.H.D.6 to 10 consumers - 0.9 to total A.D.H.D.Over 10 Consumers - 0.85 to total A.D.H.D.

Any other consumer supplied from the same feeder, but not within the above category, must be assessed separately and the feeder maximum demand adjusted accordingly.

It may be necessary, as experience is gained, to revise the above diversity factors.

8.5 Future Load

When extending the distribution system to establish power supplies in a new area, the area should be investigated and a load study, to determine the probable development in the following 5 to 7 years should be carried out and the distribution system planned in accordance with the results or this study. It: should be understood that only those works with immediate economic benefit need be implemented in the short term, the balance of the works can be deferred until the development proceeds. The institution of medium term (5 year) planning will enable deficiencies in existing systems to be identified in adequate time to allow corrective steps to be taken.

9. CONSUMERS SERVICES

9.1 Current Ratings

9.1.1 Insulated Aerial Conductors

The current ratings for duplex and quadruplex cables for the various installation conditions are listed in Table B. It should be noted that for overhead services where the down leads to the meter are installed in conduit, this is the limiting conductor loading condition - See 9.1.2.

9.1.2 Down Leads to Meter

Where practical the service down leads to the meter shall aerial service conductors continued unbroken from the service, via the wall attachment and conduit, to the meter.

In certain circumstances in order to utilize the full current rating of the aerial service, whether it be duplex, quadruplex or bare conductor, it may be advantageous to use conductors of larger cross sectional area in the conduit.

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The current ratings of PVC insulated and sheathed cables installed in conduit for this purpose are:-

6 mm2 Cu. PVC/PVC - 20 amperes16 mm2 Cu. PVC/PVC - 40 amperes25 mm2 Cu. PVC/PVC - 55 amperes

For example a 16 mm2 cable in conduit (40 amps) will enable the full rating of a 10 mm 2

aerial service (40 amps) to be utilized.

9.1.3 Non-Insulated Service Conductors

Occasionally it may be necessary to install bare (or PVC insulated) Gnat or Ant conductors as a service.

Generally it is best to consider such lines as part of the distribution system rather than as services.

The current ratings for these conductors are given in Table - A.

9.1.4 Underground Cables

Copper cables, PVC insulated, wire armoured and PVC sheathed shall be used for underground services.

The current ratings given by Eastern Cables are for ground installation at 30˚C ambient and for air installation at 35˚C ambient. The following de-rating factors should be applied to these current ratings:-

For installation in air at 45˚C ambient - 0.85

For installation in ground at 30˚C ambient - 1.00

For installation in ground at 1 meter air at 45˚C ambient - 0.95

For installation in single way ducts - 0.83

The de-rated current ratings for underground cables are given in Table-B.

9.2 Service Limits - Insulated Aerial Conductors

The maximum service span lengths for duplex and quadruplex conductors are given in Table - P.

These values have been adopted to simplify the installation of the aerial service and to ensure that the maximum sag at a conductor a temperature of 70˚C does not exceed 5 feet (1.52 mete). The erection tensions selected are suitable for all air temperatures in the range 15-45˚C.

Note: Owing to its low tensile strength 24 mm2 duplex shall not be                      used at road crossings.

9.3 Service Equipment

The current ratings of standard metering equipment, the matching cable sizes for various service load requirements and the service length for 1% service volt drop are given in Tables Q and R.

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The following notes apply to these tables:-

a) Duplex and quadruplex cable sizes are selected on the current ratings for installation in conduit. Power Factor not less then 0.85 lag.

b) Underground cable sizes for services are selected on the current ratings for installation in air and exposed to solar radiation. Power factor not less than 0.85 lag.

c) For three phase services a power factor of 0.85 lag and a load out-of-balance of 20% have been assumed.

d) The volt drop factors of Tables D and E should be used to determine the maximum service lengths for 1% volt drop.

e) The maximum service lengths are based on the kW load given in column 1 of the cable.

The maximum service length for an intermediate load not tabulated may be calculated as follows:-

= maximum service length (ft) for required load.

The MCB and meter ratings will remain as for the higher load.

f) The effect of motor stating current has not been taken into account and where this is necessary larger cables than those selected in the tables may be required, or a separate substation for the consumer. See Appendices III and V.

9.4 Calculation Examples

9.4.1 Example - I

A consumer requires supply for a heating and lighting load of 5kW. The total service length from the overhead line to the meter position is 115 feet.

As a single phase service is suitable for this load use Table Q. Against the required load (5 kW) in column1, read off :-

a) Under Column 2 the required MCB - 30 amp.

b) Under Column 3 the required meter - 10-40 amp.

c) Under Column 6 select the next highest service length (126 ft) and against this read off the duplex conductor size in Column 4 - 16 mm2 (or if underground service is required use Columns 7 and 5).

9.4.2 Example - 2

Determine the service equipment and maximum service length using a 50 mm2 underground cable for a three phase load of 35 kW.

As it is a 3-phase load, Table-R refers.

The next highest load is 40KW in column l. Read off against this :-

a) The required MCCB in column 2 - 100 mpb) The required meter in column 3 - 80 ampc) The service length in column 7 against the cable size (50 mm2) in - 212 feet

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column 5.

The maximum service length for a 33kW load is = 242 feet

10. DRAWINGS

10.1 General

In order to plan distribution systems, or extensions to them, and to carry out necessary design calculations, accurate Geographic Plans, Single Line and Key Diagrams must be prepared.

10.2 Geographic and Single Line

Separate drawings are normally prepared for each voltage and should contain the following information:-

GEOGRAPHIC SINGLE LINEPole locations YES (Note i) -Line routes YES -Cable routes YES -Cable and Conductor sizes YES YESCable and Conductor types YES YESCircuit route lengths - YESAll normally open points YES YESIsolators YES YESFeeder Pillars YES YESRing Main Units YES YESAuto re-closers YES YESSwitching Stations YES YESSubstations YES YESTransformers YES YESTransformer kVA rating YES YESDrawing No. of Key Diagrams - YESCross reference of Drawing Numbers (Geographic/Single Line)

YES YES

Note: i) Required on all 1:1200 scale geographic maps, otherwise only if          

practical. ii) All symbols to be used are to be in accordance with Fig. 4.

10.3 Key Diagrams

A Key Diagram is a single line drawing of installed equipment, e.g. a switching station, a 33/11kV substation.

The drawing is to be fully detailed and to include, in addition to the switchgear and transformers, the following :-

For Power Transformers For SwitchgearVector Group Current rating of circuit busbarsVoltage ratio Current rating of circuit breakerskVA rating Fault rating in MVA% Impedance Current transformers and ratiosEarthing Voltage transformers

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Current transformers and ratios InstrumentationMetersRelays

A typical Key Diagram showing the layout and information required, and the standard symbols to be used, is shown at Fig. 5.

10.4 Topo Survey

The negatives of this survey at a scale of 1:1200 will be considered the Master Negative. The only information to be inserted, and kept up to date, on the Master Negative are all Electric Supplies Poles and the pole numbers, plus revisions and extensions to the survey as area development progresses.

Other negatives will be made from the Master Negative for permanent records as required, e.g.:-

a) 400 volt lines and cablesb) 11000 volt lines and cables

In addition a reduced scale version, approximately 1:5000, are to be prepared primarily for the planning of the high voltage system but also to record the overall picture of the ‘as installed’ high voltage system for each 33/llkV supply area.

10.5 Electrical Symbols

The electrical symbols in most frequent use on Geographic, Single Line and Key Diagrams are shown at Fig. 4.

For symbols not given in this drawing reference should be made to the Chief Engineer (P&D).

APPENDIX – l

LINE VOLTAGE REGULATION LIMITS

1. Regulations require that at medium voltage the voltage: at the consumer’s terminals be kept within the limits of 5% of the declared voltage of supply of 400/230 volts, i.e. 420/380 volts three phase and 24l.5/218.5 volts single phase.

For planning purposes the following are the normal maximum voltage regulation limits for 11,000 volt and 400/230 volt distribution systems, after allowing for load development, and these limits should not normally be exceeded :-

100%11.22kV 10.88 kV 409.4 Volt

3% 4% 393 volt

2.75% 1%

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11kV BUSBARS 10.725/0.415 389 volt at 33/11 kV

AT TAP CONSUMERS

Substation TERMINAL

a) A primary 11kV bus bar voltage at a 33/11kV substation or 11.22 kV.

b) An 11kV distribution line drop of 3%, i.e. 10.88kV at the distribution transformer high voltage terminals.

c) With the distribution transformer tap change set at

(10.725/0.415kV) and allowing 2.75% transformer internal regulation towards full load, an output voltage at the transformer medium voltage terminals of 409.5/235.5 volts.

d) A medium voltage distribution line drop of 4% (this is equivalent to a uniformly distributed load of 70 kW along 350 yards of Wasp conductor) giving an end of mains voltage of 393/226 volts.

e) A consumer's service volt drop of 1% giving a voltage at the consumers terminals of 389/224 volts

By planning to these limits the voltage at the consumer's supply terminals will be within statutory limits.

2. In a new system, or a reinforced system, where the initial load may be small compared to that for which it is designed, it may be necessary to set the tap change on the distribution transformer to its nominal ratio or possibly 11kV plus 2.5% tapping to compensate for the lower line voltage regulation and to reduce the voltage of supply to those consumers near to the transformer.

Not withstanding the general case described, variation in the consumer voltage beyond the statutory limits may be experienced following a change to the system. Where permanent change is made to the network the voltage can be adjusted by use of the manual tap changing switch of the 11/0.415kV distribution transformers. Four tap positions are provided to give variations of 2.5% and 5% on the high voltage winding.

3. The 11kV primary bus bar at the 33/11kV substation will normally be maintained at 11.22 kV by means of automatic tap changing equipment.

APPENDIX – II

LINE VOLTAGE REGULATION LIMITS

VOLT DROP CALCULATIONS

1. General

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1.1 The above vector diagram applies to a three phase system where ES and ER are the phase to

neutral voltages sending and receiving respectively, I is the line current of a three phase load, R and X are the line resistance and reactance and Cos is the load power factor.

The values for R (at the temperature corresponding to the maximum current rating of the conductor) and X, in ohms per mile are given in Tables A, B and C.

The voltage drop is given by the equation

ES - ER = RI Cos + XI Sin = I (Cos + X Sin ) ….... (1)

1.2.1 For a medium voltage system with nominal voltage of 400 volts phase to phase:

The line current for a balanced three-phase load is obtained from the equation:

Where E is the Phase to phase voltage (400 volts)

Substituting for I in (1) :-

ES - ER volts/mile ….... (2)

For 1% volt drop ES - ER = 2.309 volts

Substituting for ES - ER in (2) :-

2.309

Assuming a unit length of one mile of line

KW-Mile for 1% volt drop

Or,

KW-Yard for 1% volt drop

For medium voltage at a power factor of 0.95 the volt drop factor for 1% volt drop is:-

2675 KW-YARDS (Three phase)

0.95R + 0.3122 X

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And for a power factor of 0.85:-

2675 KW-YARDS (Three phase)

0.85R + 0.5268 X

1.2.2Similarly for a high voltage system with nominal voltage of 11,000 volts phase to phase, and a

power factor of 0.8 the volt drop factor for 1% volt drop is:-

968 KW-MILE (Three phase)

0.8R + 0.6 X

1.2.3Considering a medium voltage single phase system, of nominal voltage of 230 volts phase to

neutral, assuming phase and neutral conductors are identical, for 1% volt drop:-

I =KW 1000

=KW

230 Cos 0.23 Cos

Substituting for I in (3) and assuming a unit route length of one mile :

2.3 =KW 2 (R Cos + X Sin) x 1 MILE

0.23 Cos

Or, KW-MILE =2.3 0.23 Cos

2 (R Cos + X Sin)

=0.529 Cos

2 (R Cos + X Sin)

Or, KW-YARD =0.931 Cos

for 1% volt drop2 (R Cos + X Sin)

For medium voltage at a power factor of 0.95 the volt drop factor for 1% volt drop is :-

884KW – YARDS (Single phase)

1.9 R + 0.6245X and for a power factor of 0.85

791KW – YARDS (Single phase)

1.7 R + 1.0536X

1.3Single and three-phase volt drop factors for various sizes of conductors and cables are

listed in Tables D, E and F. The single-phase factors assume identical phase and neutral conductors.

In using the tables the following power factors should be assumed :-

a) 11,000 volt loads : 0.80 lag

ES – ER = 2.30 = 2 (RI Cos + XI Sin)= 2I (R Cos + X Sin) ...... (3)

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b) 400 and 230 loads, predominantly : 0.95 lag (heating and lighting) c) All other 400 and 230 volt loads : 0.85 lag

1.4 As shown in the examples that follow, calculations are made to determine the KW-Yard (KW-Mile) loadings of each circuit section between load tapping points. This value is divided by the appropriate factor to give the percentage volt drop at the end of the section.

2. EXAMPLES

2.1 Examples 1

RABBIT

3 MILES 500 kVA BUSBAR 3-ph

11.22kV

For the system shown determine the voltage at the high voltage terminals when the transformer is on full load.

a) High voltage power factor (see Para 1.3) = 0.80 lagb) Load = KVA 0.8 = 500 0.8 = 400 kWc) Line Loading = 400 3 miles = 1,200 kW-Miled) From Table F the volt drop factor for

Rabbit conductor at 11,000 volts is 790

e) Volt drop = = 1.52%

790f) The voltage at the transformer H.V.

terminals is = 11.05kV

2.2 Example 2

For the above example what is the maximum through load that can be transferred with a volt drop of 3%.

a) Maximum line loading = 3 (%) Volt Drop Factor= 3 790 = 2370 kW-Mile

b) Maximum line loading = =

Or =

2.3 Example 3 50 kVA F

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A 0.95 MILE GNAT

DOG B DOG C DOG D 0.75 MILE 0.45 MILE 0.65 MILE 0.55 MILE

BUSBAR ANT 11.22kV E 500 kVA 300 kVA 750 kVA

For the system shown determine the percentage volt drop to the high voltage terminals of each transformer assuming each transformer at full rating with a power factor of 0.8 lag. All distances are in miles.

a) Converting all loads to KW the loading diagram is:- F 40 A 40kW 0.95 MILE

1280 kW B 880 kW C 640kW D 0.75 MILE 0.45 MILE 0.65 MILE

400 kW 240 kW 600kW 0.55 MILE E 600

b) By calculating the KW-Miles for each section and dividing by the appropriate volt drop factor from the tables, the percentage volt drops are obtained for each section.

Volt drop factors:- Dog - 1180 (Table F)

Ant - 810 (Table F)Gnat - 470 (Table F)

Volt drop at B =1280 0.75

= 0.81%.1180

Volt drop at C =880 x 0.45

+ 0.81 = 1.15%1180

Volt drop at D =640 x 0.65

+1.15 = 1. 5 0%1180

Volt drop at E =600 x 0.55

+ 1. 50 = 1.91%810

Volt drop at F =40 x 0.95

+ 1. 50 = 1.58%470

2.4 Example 4

As example 3 but with an additional load of 1,000kVA for connection at point D. What line reinforcement over section A - D, in order that the maximum volt drop at E, does not exceed 2.5% ?

a) Volt drop over section D - E is = 0.41%

b) Maximum permissible volt drop over section

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A - D will be 2.5 - 0.41 = 2.09%

c) Additional load = 1000 kVA = 1000 0.8 = 800 kW

d) Existing line loading section A-D is(1280 O.75) + (880 O.45) + (640 O.65) = 1772 kW-Miles

e) Additional line loading section A-D inrespect of the new 800kW load is 800 (0.75 + 0.45 + 0.65) Miles = 1480kW

f) Total new line loading section A-D is1772 + 1480 = 3252 kW-Miles

g) To restrict the volt drop over, section conductor with a volt drop factor not less than

= 1556 will be required.

h) From Table F the conductor with the next larger volt drop factor, i.e. 1600 for 0.15 in2

copper, will be required.

i) Section A-D should be re-conductor to 0.15 in2 copper (or aluminium equivalent) giving

a volt drop at D of = 2.03%.

j) New volt drop at E (maxm) in 2.03 + 0.41 = 2.44%

APPENDIX – III

EXAMPLES IN DETERMINING SERVICE REQUIREMENTS

Example - 1

A single phase overhead line service is required to supply a heating and lighting load of 3.5KW. The service length, including the down lead to the meter in conduit, is 45 feet.

Using Table - Q

a) Select next highest kilowatt load in column 1 (4kW) b) The corresponding figure in Column 2 is the MCB rating - 30 amps.c) The corresponding figure in column 3 is the required meter rating – 10(40) amp. d) In column 6, corresponding to 4kW in column 1, select the next higher service length to 45 feet, i.e. 97 feet. Corresponding to this value read of the service size in column 4, i.e. 10 mm2 Duplex.

The service requirements are:-

M.C.B. rating - 30 ampsMeter rating - 10(40) amp Duplex service - 10 mm2

The actual service volt drop is :-

Actual load (KW) Actual Service length (ft.)Maximum load (KW Maximum service length (ft)

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Alternatively using Table - D

a) Kilowatt - Yard loading

b) Using Table - D select the size of duplex and it volt drop factor corresponding to the next higher kilowatt rating, i.e. 10 mm2 duplex with a volt drop factor of 130 KW-Yds and a rating of 6 KW.

c) The load current corresponding to a load of, 3.5 KW at 0.95 power factor is :-

The next highest standard M.C.B. rating is 30 ampsThe standard meter corresponding to this current is 10-40 ampsThe duplex service size is 10 mm 2 The service volt drop is

Example – 2

An existing consumer has a maximum demand of 3.5kW. An additional load of 3.5kW for a small cooker is to be connected.

Details of the existing service are :4 mm2 duplexTotal length of service is 75 ft.15 amps M.C.B.10(40) amp. single phase meter.

a) Total estimated maximum demand is 3.5 + 3.5 = 7kW. As this is only just within the limit of a single phase service (7.5KW) it should be confirmed with the consumer that no additional load will be added.

b) Using Table - Q as in example 1,Corresponding to 7.5KW in Column 1:-

M.C.B. rating required is - 60 ampMeter rating required is - 10-40 amp

Next higher service length is 84 feet Corresponding to a duplex size of - 16 mm2

c) Actual service volt drop is

d) The following service alterations are required: Change M.C.B. to 60 ampere. Replace entire existing service with 16 mm2 duplex.No change to meter.

Example 3

A residential consumer with an existing load of 13KW requires an additional 10KW for air conditioning.

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The existing service is 16 mm2 quadruplex of length 115 ft. with a 30 amp MCB and a 25 amp meter.

a) Total new load is 13 + 10 = 23 KW

b) Using Table - R the next highest load in column 1 is 25 KW with a corresponding MCCB rating of 60 amps and a meter rating of 50 amps.

c) Corresponding to 25 KW in column 1 the next longer service length in column 7 is 122 ft. in respect of a 16 mm2` underground cable in column 5.

d) The complete service should therefore be changed to :-60 amp M.C.C.B. - Three Phase 50 amp Meter - Three Phase· 16 mm2 underground cable.

Example 4

A small inqustria1 consumer requests s supply of 45 kVA at 400 volts and 0.85 power factor by underground cable from an overhead line pole. The motor with the highest starting current is 7.5 KW at 0.8 power factor and 0.9 efficiency. Motor starting will be infrequent, and only one motor will be started, at a time. The length of service cable from the nearest pole is 25 yards and to nearest substation 100 yards. A ducted road crossing is involved.

a) Rating of motor is which is

equivalent to 15.04 amps at 0.8 power factor or (0.8 15.04) + j (0.6 15.04) = 12.032 + j 9.024 amp

b) With reference to section 5.5 the motor cannot be supplied from the overhead line but may be supplied direct from the substation provided the distribution transformer is of 100 kVA or higher rating.

c) Total load = 45 kVA or 64.95 amp at 0.85 power factor = (64.95 0.85) + j (64.95 0.5268) = 55.208 + j 34.216

d) Load current less motor = (c) - (a) = (55.208 + j 34.216) – (12.032 + j 9.024) = 43.176 + j 25.192 amps

e) Assuming a starting current of five times full load current at a power factor of 0.3 lag,

starting current = 15.04 5 = 75.2 amp at 0.3 power factor = (75.2 0.3) + j (75.2 0.9539) = 22.56 + j 71.733

f) Total current at motor start = (d) + (e) = (43.176 + j 25.192) + (22.56 + j 71.733) = 65.736 + j 96.925 = 117.11 amp at 0.56 power factor.

g) The cable selected should have a current rating of 117 amps under worst conditions of installation (air, ground or duct). From Table - B suitable cables are 4 95 mm2

aluminium and 4 50 mm2 copper. h) Total running load is 45 kVA at 0.85 power factor = 38.25 KW

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i) KW cable length = 38.25 100 = 3825 KW-Yard.

j) From Table-E, volt drop factors are:-

4 95 mm2 aluminium cable - 4030 KW-Yard 4 50 mm2 copper cable - 3400 KW-Yard

And the service volt drop will be:-

aluminium cable

Copper cable

In this case, as the connection is made direct from the distribution transformer terminals, the service volt drop limit of 1% does not apply, but the voltage at the consumer’s terminals must be kept within 400/230 5% at all times.

The service involving the least capital expenditure may therefore be installed.

(Note:- For cables in ducts, the ducted rating should be adopted (a) where any duct length exceeds 10 meters, or (b) the total ducted length, considering only individual lengths in excess of 2 meters, is greater than 50% of the total route. It should be appreciated that where the in-duct rating is neglected, the heat conduction along a cable is negligible at distance of approximately 2 meters from the point under consideration. A cable in a duct of say 4 meters or more in length may have its life reduced due to operation at excessive temperatures. This is significant with PVC insulation.

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APPENDIX – IV

FAULT CALCULATIONS: SYMMETRICAL THREE PHASES

1. General

Then consideration new or projected extensions to supply systems, it is usually necessary to consider the prospective symmetrical three phase fault values at particular points in the system. This appendix shows how symmetrical fault levels may be estimated. In these approximate methods often the reactance only are taken into account. Discounting the resistances results in the calculated fault level being higher then the actual value. Resistances should be taken into account when matching to the rating of equipment and when calculating the minimum fault level.

2. Method of Calculation

The usual methods of approach to this problem are baaed on :- a) Percentage valuesb) Ohmic values

Both methods give the same result. Choice of method normally depends upon the form in which the circuit data is given.

3. Formula

Definitions

Rating MVA - The rating of the equipment in MVA.Base MVA - The normal MVA on the basis of which calculations

are made.Fault current - The abnormal magnitude of current created due to

fault on the system expressed in absolute MVA or in per unit.

Fault MVA - The abnormal apparent power created due to fault expressed in absolute MVA or in per unit.

E - The phase-to-phase voltage expressed in kilovolt.

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EB - The normal voltage on the basis of which calculations are made.

ZP - The impedance expressed us a percentage at s rated MVA.

Z - The impedance expressed in ohms at a stated voltage.

Z1 - The positive sequence impedance expressed in ohm at a stated voltage.

Z2 - The negative sequence expressed in ohm at a stated voltage.

Z0 - The zero sequence impedance expressed in ohm at a stated voltage.

Formula for percentage method for 3 phase symmetrical

Fault MVA = ... (1)

ZP = ... (2)

ZP = at Base MVA = ... (3)

Formula for Ohmic Method

Fault value in MVA= = ... (4)

Z = ... (5)

Z at Base KV = ... (6)

Relation between percentage and ohmic value

ZP =

=

= Z ... (7)

Z = ... (8)

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4. Application of Formulae

4.1 Percentage Reactance Method

Normally the reactance of transformers, generators etc. is expressed as a percentage reactance at the nominal volt-ampere rating of the equipment.

Before any calculations can be made the various percentages reactance must all be converted to the same MVA base using formula (3). The percentage source reactance is obtained by using formula (2).

The reactance of overhead lines and cables is usually stated in ohms and these must be converted to percentage values at the selected MVA base by using formula (7).

When all reactance have been converted to percentage values at a common MVA base formula (9) and (10) must be applied to determine the total reactance from the supply point to the point at which the fault value is required. The reactance of the supply unit (source reactance) must be included.

Formula (1) is now applied to determine the fault value in MVA.

4.2 Ohmic Reactance Method

All the reactance must be expressed in ohms by being converted at the selected voltage by using formula (8). Reactance’s expressed in ohms at different voltages must all be converted to the equivalent reactances at the selected voltage by using formula (6).

The total reactance in now obtained by using formulae (9) and (10) and the fault value in MVA determined by applying formula (4).

5. Resistance and Reactance

5.1 Ohmic Values of Resistance and Reactance

Ohmic values of resistance and reactance are listed 1n Tables A, B and C.

5.2 Percentage Values of Resistance and Reactance to a 10 MVA Base

Percentage values of resistance and reactance to a 10 MVA Base are listed in Tables - G and H.

Table - I lists typical reactance and percentage reactance to a 10 MVA base for transformers. Wherever possible the manufacturer’s value should be used.

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5.3 Calculation of Percentage Resistance and Reactance to a 10 MVA Base

For calculations the following formulae have been used:-

5.3.1 Resistance - Copper

R (at t˚ C) = R (at 20°C) {1 + (t - 20)}

Where, = 0.00393 per ˚C at 20˚C for cables, and 0.00381 per ˚C at 20˚C for overhead lines.

For cables (P.I.L and P.V.C) : R (at 70°C) = 1.1965R (at 20°C)Overhead lines : R (at 75°C) = 1.20955R (at 20°C)Cables (EPR & XLPE) : R (at 80°C) = 1.2358R (at 20°C)

5.3.2 Resistance - Aluminium

R (at t˚ C) = R (at 20°C) {1 + (t - 20)}

Where, = 0.00403 per ˚C at 20˚C

For Overhead lines : R (at 75°C) = 1.22165R (at 20°C)Cables (EPR & XLPE) : R (at 80°C) = 1.2418R (at 20°C)Cables (PIL & PVC) : R (at 70°C) = 1.2015R (at 20°C)

5.3.3 Equivalent Conductor Spacing of Overhead Lines (G.M.D.)

The equivalent conductor spacing is defined as the cube root of the product of the conductor spacing Red to Yellow, Yellow to Blue, and Blue to Red.

For 400 volt are arranged lines the conductor spacing is equal at 300mm and they are arrange vertically or horizontally.

G.M.D. = (300 300 600) = 378mm

Or 1.24 feet

For 11,000 volt lines the conductor spacing is equilateral at 930 mm spacing.

G.M.D. = (930 930 930) = 930 mm

Or 3.05 feet

5.3.4 Resistance and Reactance

The formulae of 5.3.1 and 5.3.2 have been applied resistance values published by the manufacturers, or by the Electrical Research Association, to obtain the values at maximum working temperature.

The reactance of cables has been obtained from manufacturer's publications.

The reactance of overhead lines has been obtained from the Electrical Research Association publications.

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5.3.5 Percentage Resistance and Reactance to a 10 MVA Base

At 400 volt using formula (7) :

ZP = Z = = 6.250 Z

At 11,000 volt using formula (7) :

ZP = Z = = 8.264 Z

Transformers using formula (3) :

ZP = =

6. Calculation of Fault Values

The following examples illustrate the use of the above formula calculating prospective symmetrical three phase fault values.

6.1 Example 1

11kV 11kV

1.5 mile

3 185 mm2

25 MVA P.S. Al. cable S/S 15%

For the system shown determine the 11kV fault values (F.V.) at the power station (PS) and substation (S/S) bus bars.

FORMULA (F)OR TABLE (T)

(a) Fault Value at P.S. =

= = 166.67 MVA F - 1

(b) Using Ohmic Reactance Method

Reactance of F.V. at P.S. at 11kV

Z = F - 5

=

= 0.726 ohms

Reactance of 185 mm2 AL. 11kV cable T - C is 0.147 ohms per mile. Reactance of cable

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from P.S to S/S = 1.5 0.147 = 0.2205 ohm.Total reactance generator to S/S

= (0.726 + 0.2205) = 0.9465 ohms

11kV F.V. at S/S = = = 127.84 MVA F - 4

Or alternatively

(c) Using Percentage Reactance Method

% Reactance of the generator to a 10 MVA base:

ZP = F - 3

=

= 6%

% reactance of 185 mm2 AL 11kV cable to a T - H10 MVA base = 1.215% per mile,% reactance of cable from P.S to S/S = 1.5 1.215 = 1.8225%

Total % reactance generator to S/S = (6% + 1.8225%) = 7.8225%

11kV F.V. at S/S = F - 1

=

= 127.84 MVA

(d) Taking Circuit Resistances into Consideration

Reactance of F.V. at P.S. at 11kV

Z = F - 8

=

= 0.726 ohms = 0 + j 0. 726

Impedance of 11kV Cables T - C = 1.5 (0.326 + j0.147) = 0.489 + j0.2205

Total Impedance = (0.489 + j0.9465) = 1.0654 ohms.

11kV F.V. at Substation = = = 113.57 MVA F - 4

This is less than the value (127.84) obtained above.

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6.2 Example 2

B C DOG RABBIT

0.5 1.5 11kV 1.3 95mm2 AL Cable 250 MVA DOG

A 3.2 D

For the 11kV system shown determine the fault values at substations B, C and D. The system is operated as a closed ring. All distances are given in miles.

Solution:

The percentage source reactance at A to a 10 MVA base is

ZP = ... F - 2

=

= 4%

The circuit % reactances to a 10 MVA base are :-

Line A to B = 4.686 1.5 = 7.029 T – GLine B to C = 4.983 0.5 = 2.429 T - GLine C to D = 1.331 1.3 = 1.730 T - HLine D to A = 4.686 3.2 = 14.995 T - G

F. V. at Sub-station B

% reactance from A to B direct = 7.029% reactance from A to B via D and C = (14.995 + 1.730 + 2.492) = 19.217

These two reactance are in parallel, hence % reactance A to B is

= = 5.147

Total % reactance to B is 5.147 plus the % source reactance 4%= 5.147 + 4.0= 9.147

F.V. at B = F - 1

=

= 109.325 MVA

F. V. at Sub-station C

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% reactance from A to C through B = (7.029 + 2.492) = 9.521% reactance from A to C through D = (14.995 + 1.730) = 16.725Hence % reactance A to C is 9.521 and 16.725 is parallel, i.e.

= = 6.067

Total % reactance to C is 6.067 plus the % source reactance 4%

= 6.067 + 4.0= 10.067

F.V. at C = F - 1

=

= 99.334 MVA

F. V. at Sub-station D

% reactance from A to D through B and C = (7.029 + 2.492 + 1. 730) = 11.251

% reactance from A to D direct = 14.995 Hence % reactance A to D is 11.251 and 14.995 in parallel, i.e.

= = 6.428

Total % reactance to D is 6.428 plus the % source reactance 4%= 6.428 + 4.0= 10.428

F.V. at D = F - 1

=

= 95.90 MVA

6.3 Example 3

33kV 11kV 15MVA WASP 1.5 400V 12.5% ANT 2.75 D 15 MVA 185 Al C 200 kVA 12.5% Cable 4.5% B

A

For the system shown determines the 11kV fault values at A, B and C, and the 400 volt fault value at D. Distance are in miles.

Solution:

% source reactance to a 10 MVA base is

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ZP = ... F - 2

=

= 1%% reactance of each transformer (33/11 kV) to a 10 MVA base is

ZP = F - 3

=

= 8.33

% reactance of the two transformers in parallel

= = 4.165

Total % reactance to A is (1.0 + 4.165) = 5.165

F.V. at A is F – 1

=

= 193.611 MVA

% circuit reactances to a 10 MVA base are :Line A to B = 4.338 1.5 = 6.582Cable A to B = 1.215 1.5 = 1.823

% reactance of line and cable in parallel = = 1.427

Total % reactance A to B is 1.427 plus total % reactance to A is 5.17= (1.427 + 5.17) = 6.597

F.V. at B is F – 1

=

= 151.584 MVA

% circuit reactance for line B to C is (4.686 2.75) = 12.886 T – G

Total % reactance to C is 12.886 plus total % reactance to B is 6.597 = (12.886 + 6.597) = 19.483

F.V. at C is F – 1

=

= 51.327 MVA

% reactance of 200 kVA transformer to a 10 MVA base is

ZP = F – 3 or T – I

=

= 225%Total % reactance to D is 225 plus total % reactance to C is 19.483

= (225 + 19.483) = 244.483

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F.V. at D is F – 1

=

= 4.09 MVA

Appendix – V

Fault Calculation: Earth Fault

1. General

(To be circulated Later)

APPENDIX – VI

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DETERMINATION OF MOTOR STARTING CURRENT

1. General

The starting Current taken by an electric motor depends upon the size and type of motor and the method of starting. When the starting Current is drawn from the distribution system there is a dip in voltage which must be kept within certain limits in order to avoid annoyance to consumers. Whether or not there is annoyance depends on the magnitude of the voltage variation and the frequency of its occurrence.

The method recommended to avoid annoyance is to specify levels of voltage dip at the point of common coupling with other consumers' loads that must not be exceeded when the motor under consideration is started. Two levels voltage dip are considered according to the frequency of the motor starts. These are:-

Infrequent motor starting - 3% phase to neutralFrequent motor starting - 1% phase to neutral

2. System Data.

Table - J lists the equivalent impedance between the supply source and the lower voltage bus bars of a primacy substation.

Referring to Appendix-IV and substituting formula 5 in formula 6 the equivalent impedance is determined as

XO = , where E = 0.4kV

Tables - K and L list the equivalent impedance per phase in a 400 volt system corresponding to one mile of 11,000 volt overhead line or cable. These values have been determined using formula 6 of Appendix IV and the ohmic resistance and reactance values of Tables - A and C.

Table - M lists the equivalent impedance per phase in a 400 volt system corresponding to a distribution transformer of stated rating. The values have been determined using formula 8 of Appendix IV and the manufacturers' values of percentage resistance and percentage reactance.

The values listed in Table - M in respect of the single phase transformers and the 25, 75, 315 and 750 kVA transformers are typical only as no manufacturer’s values were available.

3. Examples

The following examples illustrate the use of Tables - J, K, L and M in determining the magnitude of motor starting currents.

Having determined the maximum permissible motor starting current, it can be ascertained from the manufacturer’s data whether a particular motor can be accepted, and whether it is necessary to stipulate a particular method of starting to retain the starting current within the desired limit.

3.1 Example 1 - Three Phase 400 Volt Motor

a) Fault level at primary 11kV bus bars (33/11 kV substation) 100 MVA

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b) From the primary substation to the distribution substation there are 2 miles of 185 mm2 EPR Al. cable, 1.5 miles of 100 mm2 Al and one mile of 50 mm2 Al of 11,000 volt lines/cables.

c) The distribution substation is a 50 kVA, 11/0.4 kV transformer

d) From the distribution transformer to the point of common coupling (i.e. Point of attachment of the consumer's service) there is 300 yards of 50 mm2 Al 400 volt overhead line.

Table Section of SystemEquivalent Resistance

Equivalent Resistance

Ohms Ohms

JSource to 11 kV bus bars at the primary substation

0 0.00160

L 185 mm2 11kV cable 0.000862 0.000388 K 100 mm2 11kV line 0.001058 0.001053 K 50 mm2 11kV line 0.001416 0.000750 M 50 KVA transformer 0.0755 0.1165 A 50 mm2 400 volt line (300 yds) 0.182557 0.08131

TOTALS =>0.261393 0.201601≈ 0.2614 ≈ 0.2016

Assuming a motor starting current power factor of 0.3 lag the volt drop per ampere of starting current will be

R Cos + X Sin = (0.2614 0.3) + (0.2016 0.9539) = 0.2707 volt per ampere of starting current

Assuming the direct on-line starting current to be five times the full load current:-

Case 1 - Frequent Start

Permissible voltage dip is 1% of 230 volt = 2.3 volt

Maximum starting current is = 8.49649 ≈ 8.496 amp

Assuming a motor efficiency of 80% and power factor of 0.8 lag, the maximum rating of a direct on-line motor is:

Volts Efficiency Power Factor (kW)

= 1.732 0.4 0.8 0.8 kW

= 0.753 kW

Or, = 1.009383 ≈ 1.01 H.P.

Case 2 - Infrequent Start

Permissible voltage dip is 3% of 230 volt = 6.9 volt

Maximum starting current is = 25.48947 ≈ 25.489 amp

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Assuming a motor efficiency of 80% and power factor of 0.8 lag, the maximum rating of a direct on-line motor is:

Volts Efficiency Power Factor (kW)

= 1.732 0.4 0.8 0.8 kW

= 2.26 kW

Or, = 3.02949 ≈ 3.03 H.P.

3.2 Example 2 - Three Phase 400 volt Motor

a) Fault level at primary 11 kV busbars - 75 MVA b) From the primary substation to the distribution substation there are 3 miles of 100 mm2 and one mile of 50 mm2 Al overhead lines. c) At the distribution substation a 50 kVA transformer d) From the distribution transformer to the point of common coupling there is 400 yards of 50 mm2 Al 400 volt overhead line.

Table Section of SystemEquivalent Resistance

Equivalent Resistance

Ohms Ohms

JSource to 11 kV busbars at the primary substation

0 0.00320

K 100 mm2 11kV line (3 miles) 0.002115 0.002106 K 50 mm2 11kV line (1 miles) 0.001416 0.00075 M 50 KVA transformer 0.0755 0.1165 A 50 mm2 400 volt line (400 yds) 0.243409 0.108409

TOTALS =>0.32244 0.230965≈ 0.3224 ≈ 0.2310

Assuming a starting current of five times full load current at a power factor of 0.3 lag, The volt drop per ampere of starting current is

R Cos + X Sin = (0.3224 0.3) + (0.2310 0.9539) = 0.317 volt

Case 1 - Frequent Start

Permissible voltage dip is 1% of 230 volt = 2.3 volt

Maximum starting current is = 7.255 amp

Assuming a motor efficiency of 80% and power factor of 0.8 lag, the maximum rating of direct on-line motor is:

Volts Efficiency Power Factor (kW)

= 1.732 0.4 0.8 0.8 kW

= 0.643 kW

Or, = 0.86193 ≈ 0.86 H.P.

Case 2 - Infrequent Start

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Permissible voltage dip is 3% of 230 volt = 6.9 volt

Maximum starting current is = 21.7666 ≈ 21.766 amp

Assuming a motor efficiency of 80% and power factor of 0.8 lag, the maximum rating of a direct on-line motor is:

Volts Efficiency Power Factor (kW)

= 1.732 0.4 0.8 0.8 kW

= 1.93 kW

Or, = 2.58713 ≈ 2.59 H.P.

3.3 Example 3:- Single Phase 230 Volt Motor (from three phase supply)

(a) Fault level at primary 11kV basbars (33/11kV substation) -100 MVA. (b) From the primary substation to the distribution substation there are 3 miles of 100 mm2 and one mile of 50 mm2 Al overhead line. (c) At the distribution substation a 50 kVA three phase transformer. (d) From the distribution transformer to the point of common coupling there is 400 yards of 50 mm2 Al. distribution line. In this example it is necessary to use voltage factor to convert the equivalent impedance per phase in a 400 volt system to equivalent impedance per phase in a 230 volt system.

The voltage factor is = 0.33

The return path of the single phase load current must also be taken into account.

SECTIONOF

SYSTEM

BASIC EQUIV. RESISTANCE

BASIC EQUIV. REACTANCE VOLTAGE

FACTOR

EQUIVALENT RESISTANCE

EQUIVALENT REACTANCE

Ohms Ohms Ohms OhmsSource to

11 kV BusbarTable - J

0 0.00160 0.33 0.000528

11kV O/HLine

Table – K

0.002115 0.002106 0.33 0.00071 0.0006950.002115 0.002106 0.33 0.00071 0.0006950.001416 0.00075 0.33 0.000467 0.0002480.001416 0.00075 0.33 0.000467 0.000248

TRANSFORMERTable – M

0.0755 0.1165 0.33 0.024915 0.0384450.0755 0.1165 0.33 0.024915 0.038445

230 VoltLine Table - A

0.24341 0.108410.24341 0.10841

TOTAL0.539004 0.296124≈ 0.5390 ≈ 0.2961

Assuming a starting current of 5 times full load current at a power factor of 0.3 lag, The volt drop per ampere of starting current is

R Cos + X Sin = (0.5390 0.3) + (0.2961 0.9539) = 0.4441 volt

Case 1 - Frequent Start

Permissible voltage dip is 1% of 230 volt = 2.3 volt

Maximum starting current is = 5.1802 amp

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Assuming a motor efficiency of 75% and power factor of 0.8 lag, the maximum size of direct on-line motor is:

Volts Efficiency Power Factor (kW)

= 0.23 0.75 0.8 kW

= 0.143 kW

Or, = 0.191689 ≈ 0.192 H.P.

Case 2 - Infrequent Start

Permissible voltage dip is 3% of 230 volt = 6.9 volt

Maximum starting current is = 15.5405 amp

Assuming a motor efficiency of 75% and power factor of 0.8 lag, the maximum size of direct on-line motor is:

Volts Efficiency Power Factor (kW)

= 0.23 0.75 0.8 kW

= 0.4289 ≈ 0.429 kW

Or, = 0.575067 ≈ 0.575 H.P.

3.4 Example 4: Single phase motor (from Single Phase transformer)

As example 3 but with a 15 kVA single phase distribution transformer,

The equivalent resistances and reactance remain the example 3 except for the transformer. From Table – M the transformer resistance is 0.11 and the reactance is 0.13. Note that these values are not multiplied by two in this case.

Total equivalent resistance is now 0.5992 Total equivalent reactance is now 0.3492

The volt drop per ampere of starting current is R Cos + X Sin = (0.5992 0.3) + (0.3492 0.9539) = 0.5129 ≈ 0.513 volt

Case 1 - Frequent Start

Permissible voltage dip is 1% of 230 volt = 2.3 volt

Maximum starting current is = 4.483 amp

Assuming a motor efficiency of 75% and power factor of 0.8 lag, the maximum size of direct on-line motor is:

Volts Efficiency Power Factor (kW)

= 0.23 0.75 0.8 kW

= 0.124 kW

51/77

Or, = 0.16622 ≈ 0.166 H.P.

Case 2 - Infrequent Start

Permissible voltage dip is 3% of 230 volt = 6.9 volt

Maximum starting current is = 13.4503 amp

Assuming a motor efficiency of 75% and power factor of 0.8 lag, the maximum size of direct on-line motor is:

Volts Efficiency Power Factor (kW)

= 0.23 0.75 0.8 kW

= 0.3712 ≈ 0.371 kW

Or, = 0.4975871 ≈ 0.498 H.P.

3.5 Example 5: Three Phase 400 volt Motor direct from transformer terminals

a) Fault level at primary 11kV busbars - 100 MVA

b) From the primary substation to the distribution transformer there are 3 miles of 100 mm2 and one mile of 50 mm2 Al overhead line.

c) At the distribution substation there is a 50 kVA 3-Phase transformer.

TABLE SECTION OF SYSTEMEQUIVALENT RESISTANCE

Ohms

EQUIVALENT REACTANCE

OhmsJ Source to 11 kV Busbars 0 0.0016K 100 mm2 Al. 11kV line (3 miles) 0.002115 0.002106K 50 mm2 Al. 11kV line (1 mile) 0.001416 0.00075M 50 KVA Transformer 0.0755 0.1165

TOTAL0.079031 0.120956≈ 0.0790 ≈ 0.1210

Assuming a starting current of 5 times full load current at a power factor of 0.3 lag, The volt drop per ampere of starting current is

R Cos + X Sin = (0.079 0.3) + (0.121 0.9539) = 0.1391 volt

Case 1 - Frequent Start

Permissible voltage dip is 1% of 230 volt = 2.3 volt

Maximum starting current is = 16.5468 ≈ 16.547 amp

Assuming a motor efficiency of 80% and power factor of 0.8 lag, the maximum size of direct on-line motor is:

Volts Efficiency Power Factor (kW)

= 1.732 0.4 0.8 0.8 kW

= 1.467 kW

Or, = 1.966488 ≈ 1.97 H.P.

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Case 2 - Infrequent Start

Permissible voltage dip is 3% of 230 volt = 6.9 volt

Maximum starting current is = 49.6403 ≈ 49.64 amp

Assuming a motor efficiency of 87% and power factor of 0.87 lag, the maximum rating of a direct on-line motor is:

Volts Efficiency Power Factor (kW)

= 1.732 0.4 0.87 0.87 kW

= 5.206 kW

Or, = 6.978552 ≈ 6.98 H.P.

3.6 Example 6: Single Phase 230 volt Motor direct from 3-phase transformer terminals

a) Fault level at primary 11kV busbars - 100 MVA.

b) From the primary substation to the distribution substation there are 3 miles of 100 mm2 and one mile of 50 mm2 Al overhead 11kV line.

c) At the distribution substation there is a 25 kVA, 3-Phase transformer.

SECTIONOF

SYSTEM

BASIC EQUIV. RESISTANCE

BASIC EQUIV. REACTANCE VOLTAGE

FACTOR

EQUIVALENT RESISTANCE

EQUIVALENT REACTANCE

Ohms Ohms Ohms OhmsSource to

11 kV BusbarTable - J

0 0.00160 0.33 0 0.000528

11kV O/HLine

Table – K

0.002115 0.002106 0.33 0.00071 0.000695 0.002115 0.002106 0.33 0.00071 0.000695 0.001416 0.00075 0.33 0.000467 0.000248 0.001416 0.00075 0.33 0.000467 0.000248

TRANSFORMERTable – M

0.17 0.21 0.33 0.0561 0.0693 0.17 0.21 0.33 0.0561 0.0693

TOTAL 0.114554 0.141014≈ 0.1146 ≈ 0.1410

Assuming a starting current of 5 times full load current at a power factor of 0.3 lag, The volt drop per ampere of starting current is

R Cos + X Sin = (0.1146 0.3) + (0.1410 0.9539) = 0.1689 volt

Case 1 - Frequent Start

Permissible voltage dip is 1% of 230 volt = 2.3 volt

Maximum starting current is = 13.61753 ≈ 13.62 amp

Assuming a motor efficiency of 75% and power factor of 0.8 lag, the maximum size of direct on-line motor is:

Volts Efficiency Power Factor (kW)

= 0.23 0.75 0.8 kW

= 0.38 kW

53/77

Or, = 0.50938 ≈ 0.50 H.P.

Case 2 - Infrequent Start

Permissible voltage dip is 3% of 230 volt = 6.9 volt

Maximum starting current is = 40.85258 ≈ 40.853 amp

Assuming a motor efficiency of 75% and power factor of 0.8 lag, the maximum size of direct on-line motor is:

Volts Efficiency Power Factor (kW)

= 0.23 0.75 0.8 kW

= 1.128 ≈ 1.13 kW

Or, = 1.51475 ≈ 1.51 H.P.

3.7 Example 7: High Voltage Consumer (11000 volts) 3-Phase Motor

a) Fault level at primary 11kV busbars (at 33/11kV substation) is 100 MVA

b) From the primary substation to the point of common coupling on the 11000 volts distribution system there are 800 yards of 185 mm2 Aluminium cable and 2400 yards of WAPS three phase line.

TABLE SECTION OF SYSTEMEQUIVALENT RESISTANCE

EQUIVALENT REACTANCE

OHMS OHMS

JSource to 11 kV Bus bars at the primary substation

0 0.001600

L 185 mm2 11kV cable (800 yards) 0.000196 0.000088

K 100 mm2 11kV line (2400 yards) 0.000961 0.000957

TOTAL 0.001157 0.002645

Assuming a motor starting current power factor of 0.3 lag, The volt drop per ampere of starting current will be:

R Cos + X Sin = (0.001157 0.3) + (0.002645 0.9539) = 0.00287 volt per ampere of starting current

Assuming the direct-on-line starting current to be 5 times the full load current

Case 1 - Frequent Start

Permissible voltage dip is 1% of 6350 volt = 0.0635 kV

Maximum starting current is = 22.1254 ≈ 22.13 amp (at 11kV)

Assuming a motor efficiency of 90% and power factor of 0.9 lag, the maximum rating of a direct on-line motor is:

Volts Efficiency Power Factor (kW)

= 1.732 11 0.9 0.9 kW

= 68.303 kW

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Or, = 91.558981 ≈ 91.56 H.P.

APPENDIX VII

GENERAL NOTES ON SELECTION AND SETTING PROTECTION DEVICES

1. INTRODUCTION

The notes in this Appendix are intended to provide general guide to the selection and setting of protection devices in order to provide discriminate operation under fault conditions. The notes are not intended to provide a definitive guide, for which reference should be made to technical guides and text published by manufacturers.

2. PROTECTION DEVICE:

2.1 General

There are two types of device common in distribution networks. Firstly the fuse, which is breaks to interrupt the current. Secondly the circuit breaker which is controlled by an external device, e.g. a relay designed to automatically open the circuit breaker if the current exceeds a present value (both devices can provide protection against overload and faults).

2.2 Fuses

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Fuses are available in a wide range of voltages, ratings and operating characteristics. Depending upon the duty, i.e., speed of operation, prospective peak current etc. selection should be made in accordance with the manufacturers recommendations.

Within the distribution networks 11KV fuses are used to protect 11/0.4 KV transformers, for which rating are shown in Section 6 of the Guide. Operating characteristics for these fuses are shown in Figs.1, 2 and 3, which also show the characteristics for some 400 V MCCB’S.

2.3 Molded Case Circuit Breakers (400V)

MCCB's are used in the networks to control 400V supplies out of 11/0.4 KV transformers. These have two control systems, a thermal element to operate during prolonged overloads and an instantaneous element to operate during a fault. The elements are sometimes adjustable.

Application Consideration: Rating must be compatible with expected load. Interrupting capacity should be commensurate with duty to be concountered. For 200 KVA transformers no less than 14 KA interrupting rating should be selected to be on safe side. Molded case circuit breakers may be erratic from continued exposure to current beyond its rating. High ambient, particularly in enclosure would de-rate it. Sources of heat are loose terminal connections, poor contact, undersized wires, corroded terminals and eroded contacts. A molded case circuit breaker should not be loaded beyond 80% of its continuous rating when applied in a panel board where such load is to continue for 3 hours or more. Breakers which are calibrated at 25°C should be further de-rated by about 20% when applied at shed temperature of about 40°C. A de-rating @ 20% is expected with MCCB’s and accordingly, application should be planned not be exceed 80% rating for continuous duty.

2.4 Oil Circuit Breakers (11000V )

OCB’s are usually controlled through on Inverse Definite Minimum Time (IDMT) relay or in order switchgear through a Time Lag Fuse. Operating characteristics for the fuses is given in Fig.6.

2.5 IDMT ( Inverse Definite Minimum Time ) Relays

The operating characteristic for a Standard 3 second IDMT relays shown is in Fig 7.

The following method can be used to locate the characteristic on a Figure. First plot the operating curve onto a page of log/log graph paper, using points determined from the manufacturers data (3 inch decade log/log scale is used through the Figures in the Guide). Extend the operating curve to a practical limit, as shown in Fig.7. Stick the template to a piece of cardboard or preferably plastic and cut this out to provide a working template.

By definition, the Standard relay operates in 3 seconds at 10 times setting. The setting is calculated by multiplying the primary turns of the CT supplying the relay by the percent current plug on the relay, which is generally arranged in an over current relay in seven equal steps from 50% to 200%. The relay incorporates also a time multiplier graded from 10% to 100%. At 10setting the multiplier can shorten the operating time from 3 second (100% multiplier) to 0.3 seconds (10% multiplier).

Fig.8 shows an example of the IDMT over current relay setting envelope, obtained when the relay is supplied from a 100A primary turns CT (the CT secondary turns must be equal to the relay rated current, normally 5A). The Figure also shows the simple calculations required to enable the relay curve to be correctly marked onto the Figure, with respect to time and current scales. When using vertical and horizontal units shown in the Figure, it can be seen that changing the relay plug on the primary turns on the CT, causes the relay template to move

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horizontally across the page. Changing the time multiplier causes the relay template to move vertically. This method of graphical illustration will be used in subsequent examples to estimate settings for IDMT relays.

(Note, depending upon the manufacturer, the plugs may be marked in percentage values or alternatively in Amperes, i.e. 0.25A to 10A. The time multiplier may be marked in per-unit values from 1.0 to 0.l). Due to scalar markings it is usually necessary to set a time multiplier to a value rounded off to the nearest 5%.

3. DISCRIMINATION

3.1 General

Discrimination is the ability of the protection devices to operate in a timed sequence, such that the device nearest to the fault is first to operate and the devices towards the source operate progressively slower until the device at the source is last to operate. Experience shows that devices are having an extreme inverse operating curve, e.g. a fuse which will have an operating curve tending to be a vertical line in the Figures, should be located at the load end of a feeder. IDMT relays should be used towards the source end. If this pattern is not maintained device operating times are unlikely to discriminate.

3.2 Fuse

Fault current discrimination times between fuses will depend upon make and type as well as current rating. It is important to note that if a marker's range incorporates fuses rated from 10 A to 100A in 10A steps, this dies not imply a 40A fuse will discriminate under fault conditions with a 50A fuse. In practice it could require 80A fuse or even higher rating to discriminate with the 40A fuse. Fuse selection should be made in accordance with the maker’s recommendations.

3.3 Consumer’s Devices

At 400V, the discrimination between consumer's fuse of MCB and the MCCB at the 11/0.4 KV transformer, is unlikely to create any practical problems, which can be checked as shown in the later examples.

3.4 400V MCCB's and 11 kV Fuses

Discrimination between 400V MCCB's and 11KV fuses on the 11/0.4KV substation is determined largely by experience. The recommendations in this Guide will provide satisfactory operation (Table S) and Fig. 1a, 2a, 3a.

3.5 11 KV Fuses and IDMT Relays

Discrimination can be varied according to the fault current value at relatively high current, say more then 20 times the fuse rated current. The IDMT relay should operate in not less then 0.15 seconds. At lower fault current discrimination for most practical cases will be obtained by setting the relay operating characteristic close to, but not touching or cutting the fuse character1etic.

3.6 IDMT Relays

Discrimination between relays will vary according to the age and type of switchgear and relay. In Bangladesh where there is variety in both type and age, a time interval of 0.5 seconds at the maximum fault current is recommended. This interval will allow for CT and relay error, relay overshoot and breaker opening time.

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4. EXAMPLES

4.1 General

The object in the examples which follow will be to set devices to levels which provide the minimum discrimination intervals as given in the previous Section 3. Establishing longer intervals serves no practical purpose and enforces a slower fault clearance action throughout the networks.

Fault currents in the examples have been chosen for illustration purposes. In practical cases these need to be calculated.

4.2 Example No. I

Calculate the setting for the 11kV IDMTL over current relay for the circuit shown in Fig. 9. Assume the fault current is 8000A maximum and the minimum current is unspecified. As the first step, mark scales on the current and time axes. Now examine the load-end protection and select the fuse with the longest operating time. In this example the three fuses are of the same type and rating. Draw the fuse operating curve onto the Figure.

Calculate the circuit maximum load at 3 1000 kVA or 158A at 11kV. Plugging the relay to 75% of 200A gives 150A, which is marginally too low for the load current.

The minimum relay plug must be 100%, 200A. Since the relay is to discriminate with a fuse, assume the time multiplier (TMS) at 0.1 will be satisfactory.

Place template on the page such that the 10 setting line corresponds with 200A 10 = 2000A. Move the template vertically until the time marker at 10 setting on the template is at 0.3 seconds. This curve is shown dotted in the Figure.

Inspect the relationship between the fuse and relay curves. Note the relay cuts the fuse curve at 450A, below which value the relay will operate before the fuse. Since the minimum fault current is not known this may or may not be acceptable.

To obtain complete discrimination move the template to the right hand side until the relay curves does not cut the fuse curve and continue until a standard plug point is achieved, at 150%.

Check the relay operating time at maximum fault current, which is 0.2 seconds. This is satisfactory in that is exceeds the minimum recommended time of 0.15 seconds.

The setting for the IDMT relay to provide completed discrimination with the 90A fuse is a plug at 150%, and a TMS of 0.1.

4.3 Example 2

Extending the previous example, assume the 11kV bus bar is now supplied through an 11kV feeder from a second 11kV bus and the fault current remains unchanged at 8000A as shown in Fig. 10. Calculate the setting for the two IDMT relays on the 11kV feeder to maintain discrimination.

Draw the curve to show operation of relay ‘A'.

On the vertical line equal to the maximum fault current, 8000A, mark the time equal to that in which relay 'A' operates plus 0.5 second, equal to 0.7 second. Place the template on the graph with the 10 setting point to coincide with this mark. This is clearly beyond the maximum current setting of the CT at 300 200% 10A. Move the template to the left, keeping the curve

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touching the 0.7 second marker until the 10 setting marker coincides with 6000A. (The template will be forced to move up the page to achieve this). A setting of 200% will not provide overload protection in the 300A feeder. Assume this is required, move the template further left until the 10 setting lies on the 3000A line again moving the template up the page until a practical time multiplier is reached, at 0.35, equal to 1.05 seconds.

In order to set relay 'C' add 0.5 second to relay 'B' operating time and follow the same procedure used for relay 'B', except relay 'C' does not need provide overload protection and a plug at 150% has been selected with a time multiplier at 0.5. The relay could be set at 200% plug with a time multiplier at 0.45 to achieve a similar result.

4.4 Comment

Example 2 has illustrated the possibility of varying relay plug and time multipliers to achieve the same end result, allowing for different network operating needs which influence the current setting. If relays C and B are in unattended substations and if the maximum demand on the feeder is expected to approach 300A, there is justification in setting one relay to protect against overloading. If overload protection is, not critical, relay 'B' could equally be set at plug 150% with TMS 0.3 and relay 'A' at plug 200% and TMS 0.45 without significant difference to the end result. The specific setting of a relay, providing the discriminatory time interval is obtained, is somewhat arbitrary and can be varied according to the network operating needs, subject to one qualification. The IDMT relay will operate accurately between the 2 and 10 setting points. At current values above 10 times the accuracy is not guaranteed. Therefore, the closer the 10 times limit can be placed to the actual fault current, the more reliable the practical result.

It should be noted that the template operating characteristic continues in a uniform operating mode for currents in excess of 10 setting. This may not be true in practice, where saturation and disc inertia may cause the operating time to reach a constant minimum time. Where timing is considered to be critical, data on the relay and CT performance should be obtained from the manufacturer.

4.5 Example 3

The earlier examples considered a series of devices operating at a comment fault current. The example shown in Fig. 11 considers a 33/11kV substation having two transformers operating normally in parallel, when a phase fault occurs on an outgoing 11kV feeder.

In this example the maximum fault current with two transformers in service is assumed to be 8000A, equal to 4000A through each transformer. With one transformer in service the current is assumed to be 5000A.

Plot the curve for the feeder relay, assumed as that shown in Fig. 10 relay C. This relay must discriminate with the relays on the 11kV side of the incoming transformers at conditions:-

(a) 4000A through relay B and 8000A through relay A.

(b) 5000A through relay B and relay A.

Place a mark on the Figure on the 5000A line 0.5 second above relay A operating time. Draw a curve for relay B to cut this mark, establish the resultant setting for relay B at 100% plug, 0.5 TMS.

Check the interval between relays A and B for condition (a) above. Relay A operates in 1.2 sec approximately at 8000A and relay B in 2.4 seconds at 4000A, providing adequate discrimination.

Proceed to set relay C to operate 0.5 second slower than relay B at 5000A, remembering to refer to the 33kV CT primary turns to 11kV, multiplying the turns by the voltage ratio, 3.

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This example illustrates the important necessity of understanding how the network can be operated before undertaking any protection grading, as well as the need to identify the value of the fault current which can flow in each branch.

TABLE – A

OVERHEAD LINE CONSTANTS

CONDUCTORTYPE AND SIZE

OHMS PER MILE MAXIMUM

CONTINUOUS

CURRENT

AMPS

A.C.

RESISTANCE

400 VOLT

REACTANCE

X1 = X2

11KV

REACTANCE

X1 = X2

A.C.S.R. Bare or PVC

GOPHER (25 mm2) 2.161 - 0.625 110

RABBIT (50 mm2) 1.092 - 0.603 165

DOG (100 mm2) 0.601 - 0.567 240

Aluminium Bare or PVC

GNAT (25 mm2) 2.139 0.504 0.594 110

ANT (50 mm2) 1.071 0.477 0.567 165

FLY (60 mm2) 0.888 0.466 0.557 180

EARWIG (75 mm2) 0.718 0.450 0.547 195

WASP (100 mm2) 0.533 0.442 0.531 240

Copper:-

No. 8 s.w.g. 2.659 0.554 0.645 80

6 s.w.g. 1. 845 0.535 0.627 120

4 s.w.g. 1.263 0.516 0.609 150

3 s.w.g. 1. 070 0.508 0.599 165

2 s.w.g. 0.891 0.501 0.590 185

1/0 s.w.g. 0.646 0.482 0.572 225

2/0 s.w.g. 0.560 0.477 0.566 240

3/0 s.w.g. 0.490 0.470 0.558 255

7/10 s.w.g. 0.599 0.480 0.572 220

7/8 s.w.g. 0.384 0.458 0.549 270

7/0.136 (0.1 sq. in) 0.531 0.466 0.556 240

7/0.166 (0.15 sq. in) 0.356 0.445 0.538 300

Note: 400V reactance is based on equivalent spacing (1.24 feet) as normally used for 400V system.

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11KV reactance is based on equivalent spacing (3.05 feet) as normally used for 11KV system.

TABLE - B

MEDIUM VOLTAG INSULATED CONDUCTOR AND CABLE CONSTANTS I

CABLETYPE AND SIZE

OHMS PER MILEMAXIMUM CONTINUOUS

CURRENT AMPSA.C.

RESISTANCE400 VOLT

REACTANCEAIR GROUND DUCT

Aluminium PVC/SWA

2 16 mm2 3.655 0.138 50 70 60

4 16 mm2 3.655 0.138 45 60 50

4 35 mm2 1. 679 0.132 72 95 80

4 50 mm2 1.241 0.131 85 115 95

4 95 mm2 0.620 0.127 135 160 140

4 185 mm2 0.319 0.124 205 240 200

Copper PVC/SWA

2 6 mm2 5.932 0.150 40 50 45

2 10 mm2 3.524 0.149 55 70 60

2 16 mm2 2.216 0.142 70 90 75

2 25 mm2 1.400 0.140 95 120 100

2 35 mm2 1.011 0.131 115 150 120

4 16 mm2 2.216 0.142 60 80 65

4 25 mm2 1.400 0.140 80 105 85

4 35 mm2 1.011 0.131 95 125 100

4 50 mm2 0.747 0.130 125 150 120

4 95 mm2 0.373 0.126 180 210 175

4 185 mm2 0.195 0.122 270 310 260

Copper Dup1ex/Quadp1ex AIRATTACHED

TO WALLCONDUIT

4 mm2 8.877 0.159 20 20 15

10 mm2 3.524 0.149 40 40 30

16 mm2 2.216 0.142 55 50 40

The above current ratings are for a single cable circuit only. If more than one circuit is installed additional de-rating for cable and duct arrangements will be necessary.

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TABLE - C

11 kV CABLE CONSTANTS

CABLETYPE AND SIZE

OHMS PER MILEMAXIMUM CONTINUOUS

CURRENT AMPSA.C.

RESISTANCE11000 VOLT

REACTANCEAIR GROUND DUCT

Aluminium EPR/SWA

3 95 mm2 0.629 0.161 150 185 170

3 185 mm2 0.326 0.147 225 280 240

Aluminium XLPE/SWA

33/0 awg (83 mm2) 0.676 0.164 130 160 145

3350 MCM (174 mm2) 0.324 0.148 210 260 220

Copper PVC/SWA

30.5 sq. in. 0.103 0.123. 380 420 370

Copper PIL/SWA

30.04 sq.in. 1.300 0.172 85 100 85

30.10 sq.in. 0.510 0.148 150 145 150

30.15 sq.in. 0.348 0.140 185 200 185

30.20 sq.in. 0.262 0.137 225 240 225

30.25 sq. in. 0.209 0.134 260 275 260

30.30 sq.in. 0.170 0.130 295 305 295

30.40 sq. in. 0.127 0.126 350 350 350

30.50 sq.in. 0.103 0.122 395 385 395

The above current-ratings are for a single cable circuit only. If more than one circuit is installed additional de-rating for cable and duct arrangements will be necessary:

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TABLE D

VOLT DROP FACTORS : 230 VOLT SINGLE PHASE

CONDUCTOR/CABLE SIZE/TYPE

FACTORS FOR 1% VOLT DROP IN KW-YARDS

COS = 0.85 COS = 0.95

FACTOR KW FACTOR KW

Aluminium Conductor

GNAT (25 mm2) 190 20 200 25

ANT (50 mm2) 340 30 380 35

WASP (100 mm2) 580 45 680 50

Aluminium Cable PVC/SWA

16 mm2 125 10 125 15

Copper Conductor

8 s.w.g. 155 15 165 15

6 s.w.g. 215 20 230 25

4 s.w.g. 295 30 325 30

2 s.w.g. 390 35 440 40

1/0 s.w.g. 490 45 580 50

7/0.064 (14 mm2) 290 15 320 20

3/0.104 (16 mm2) 330 20 360 25

Copper Cable PVC/SWA

6 mm2 70 8 80 10

10 mm2 130 12 130 13

16 mm2 200 15 210 20

25 mm2 320 20 330 25

35 mm2 440 30 450 30

Copper Duplex

6 mm2 50 3 50 3

10 mm2 130 6 130 6

16 mm2 210 7.5 210 7.5

KW ratings are based on air installation for overhead lines, ground installation for cables and conduit installation for Duplex cables.

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TABLE - E

VOLT DROP FACTORS - 400 VOLT THREE PHASE

CONDUCTOR CABLESIZE/ TYPE

FACTORS FOR 1% VOLT DROP IN KW-YARDS

COS = 0.85 COS = 0.95

FACTOR KW FACTOR KW

Aluminium ConductorGNAT 25 mm2 1150 65 1220 70

ANT 50 mm2 2060 95 2290 105

FLY 60 mm2 2390 105 2700 115

EARWIG 75 mm2 2820 115 3250 125

WASP 100 mm2 3490 140 4150 155

Copper Conductor8 s.w.g. 940 45 990 506 s.w.g. 1290 70 1390 804 s.w.g. 1780 85 1970 1003 s.w.g. 2030 95 2280 1102 s.w.g. 2340 105 2670 1201/0 s.w.g. 2980 130 3500 1452/0 s.w.g. 3290 140 3930 1553/0 s.w.g. 3600 150 4370 1657/10 s.w.g. 3140 130 3720 1457/8 s.w.g. 4220 160 5270 1757/0.136 (65 mm2) 3430 140 4120 155

7/0.166 (100 mm2) 4460 175 5610 195

Aluminium Cable PVC/SWA16 mm2 750 35 760 40

35 mm2 1600 55 1630 60

50 mm2 2130 70 2190 75

95 mm2 4030 95 4260 105

185 mm2 7110 140 7830 155

Copper Cable PVC/SWA16 mm2 1220 45 1240 50

35 mm2 2580 75 2670 80

50 mm2 3400 90 3570 100

95 mm2 6240 125 6790 140

185 mm2 10400 180 11980 205

Copper Quadruplex

4 mm2 310 8.5 320 9.5

10 mm2 780 17 790 19

16 mm2 1220 23 1240 26

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KW ratings are based on air installation for overhead lines, ground installation for cables, and conduit installation for Quaduplex cables.

TABLE - F

VOLT DROP FACTORS - 11000 VOLT THREE PHASE

CONDUCTOR/CABLESIZE/TYPE

FACTOR FOR 1% VOLT DROP CONTINU0USKW - MILES KW

A.C.S.R. ConductorGOPHER (25 mm2) 460 I680RABBIT (50 mm2) 790 2510DOG (100 mm2) 1180 3660Aluminium ConductorGNAT (25 mm2) 470 1680ANT (50 mm2) 810 2510FLY (60 mm2) 930 2740EARWIG (75 mm2) 1070 2970WASP (100 mm2) 1300 3660Copper Conductor

6 s.w.g. 520 18304 s.w.g. 710 22903 s.w.g. 800 25102 s.w.g. 910 28201/0 s.w.g. 1130 34302/0 s.w.g. 1230 36603/0 s.w.g. 1330 38907/10 s.w.g. 1180 33507/8 s.w.g. 1520 41207/0.136 (0.1 sq. in) 1280 36607/0.166 (0.15 sq. in) 1600 4570

Aluminium Cable EPR/SWA95 mm2 1620 2820185 mm2 2780 42703/0 AWG 1520 2440350 MCM 2780 3960

Copper Cable PVC/SWA0.5 sq.in. 6200 6,400

Copper Cable PIL/SWA0.04 850 15200.10 1950 25100.15 2670 30500.20 3320 36600.25 3910 41900.30 4530 46500.40 5460 53300.50 6220 5870

Ratings are based on air installation for overhead lines and ground installation for cables.

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TABLE – G

OVERHEAD LINES

PERCENTAGE RESISTANCE AND REACTANCE PER MILE TO A 10 MVA BASE

CONDUCTORTYPE AND SIZE

400V – 10 MVA BASE 11,000 – 10 MVA BASE% R/MILE % X/MILE % R/MILE % X/MILE

A.C.S.R

GOPHER (25 mm2) - - 17.860 5.165

RABBIT (50 mm2) - - 9.024 4.983

DOG (100 mm2) - - 4.967 4.686

For DOG %X/KM = 2.911

Aluminium

GNAT (25 mm2) 13370 3150 17.677 4.909

ANT (50 mm2) 6690 2980 8.851 4.686

FLY (60 mm2) 5550 2910 7.338 4.603

EARWIG (75 mm2) 4460 2810 5.892 4.520

WASP (100 mm2) 3330 2760 4.405 4.388

Copper

No. 8 s.w.g. 16620 3460 21. 974 5.330

6 s.w.g. 11530 3340 15.274 5.182

4 s.w.g. 7890 3230 10.437 5.033

3 s.w.g. 6690 3180 8.842 4.950

2 s.w.g. 5570 3130 7.363 4.876

1/0 s.w.g. 4040 3010 5.339 4.727

2/0 s.w.g. 3500 2980 4.628 4.677

3/0 s.w.g. 3060 2940 4.049 4.611

7/10 s.w.g. 3740 3000 4.950 4.727

7/8 s.w.g. 2400 2860 3.173 4.537

7/0.136 (0.1 sq. in) 3320 2910 4.388 4.595

7/0.166 (0.15 sq. in) 2230 2780 2.942 4.446

1 Mile = 1.609344 Km

1 Km = 0.621371192 Mile

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TABLE – H

CABLES

PERCENTAGE RESISTANCE AND REACTANCE PER MILE TO A 10 MVA BASE

CABLETYPE AND SIZE

400V – 10 MVA BASE 11000V – 10 MVA BASE

% R/MILE % X/MILE % R/MILE % X/MILE

Aluminium – EPR or XLPE/SWA

95 mm2 - - 5.198 1.331

185 mm2 - - 2.694 1.215

3/0 AWG - - 5.586 1.355

350 MCM - - 2.678 1.223

Aluminium – PVC/SWA

16 mm2 22840 863 - -

35 mm2 10490 825 - -

50 mm2 7760 819 - -

95 mm2 3880 788 - -

185 mm2 1990 775 - -

Copper – PIL/SWA

0.04 sq.in - - 10.743 1.421

0.10 sq.in - - 4.215 1.223

0.15 sq.in - - 2.876 1. 157

0.20 sq.in - - 2.165 1.132

0.25 sq.in - - 1.727 1.107

0.30 sq.in - - 1.405 1.074

0.40 sq.in - - 1.050 1.041

0.50 sq.in - - 0.851 1.008

Copper PVC/SWA

16 mm2 13850 888 - -

25 mm2 8750 875 - -

35 mm2 6320 819 - -

50 mm2 4670 813 - -

95 mm2 2330 788 - -

185 mm2 1220 763 - -

0.50 sq.in 0.851 1.016

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TABLE – I

TRANSFORMERS

PERCENTAGE REACTANCE TO A 10MVA BASE

VOLTAGE RATIOkV

RATINGMVA

REACTANCE%

% REACTANCE TO10 MVA BASE

11/0.415 0.050 4.5 900

0.100 4.5 450

0.200 4.5 225

0.300 4.5 150

0.315 4.5 143

0.500 4.5 90

0.750 4.75 63.3

1.000 5.0 50

33/0.415 0.100 4.5 450

0.200 5.0 250

0.500 5.0 100

33/11 5.0 10.0 20

15.0 12.5 8.33

The transformer reactance’s given above are typical values. Wherever possible the manufacturer’s value should be used.

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TABLE – J

EOUIVALENT 11KV SOURCE IMPEDANCE REFERRED TO 400 VOLT.

FAULT LEVEL AT 11 kV BUSBARS OF PRIMARY

SUBSTATION

EQUIVALENT IMPEDANCE PER PHASE IN 400 VOLT SYSTEM CORRESPONDING TO STATED FAULT LEVEL

AT 11 kV BUSBARS

MVA R-Ohms X-Ohms

250 0 0.00064

225 0 0.00071

200 0 0.00080

175 0 0.00091

150 0 0.00107

125 0 0.00128

100 0 0.00160

75 0 0.00213

5O 0 0.00320

25 0 0.00640

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TABLE – K

EOUIVALENT IMPEDANCE OF 11000 VOLT OVERHEAD LINES REFERRED

TO 400 VOLT

CONDUCTORTYPE AND SIZE

EQUIVALENT IMPEDANCE PER PHASE IN 400 VOLT SYSTEM CORRESPONDING TO ONE MILE OF 11000 VOLT LINE

R-Ohms R-Ohms

A.C.S.R

GOPHER (25 mm2) 0.002858 0.000826

RABBIT (50 mm2) 0.001444 0.000797

DOG (100 mm2) 0.000795 0.000750

Aluminium

GNAT (25 mm2) 0.002828 0.000785

ANT (50 mm2) 0.001416 0.000750

FLY (60 mm2) 0.001174 0.000737

EARWIG (75 mm2) 0.000944 0.000723

WASP (100 mm2) 0.000705 0.000702

Copper

8 s.w.g. 0.003516 0.000853

6 s.w.g. 0.002440 0.000829

4 s.w.g. 0.001670 0.000805

3 s.w.g. 0.001415 0.000792

2 s.w.g. 0.001178 0.000780

1/0 s.w.g. 0.000854 0.000756

2/0 s.w.g. 0.00740 0.000748

3/0 s.w.g. 0.00648 0.000738

7/10 s.w.g. 0.00792 0.000756

7/8 s.w.g. 0.000508 0.000726

7/0.136 (0.1 sq. in) 0.000702 0.000735

7/0.166 (0.15 sq. in) 0.000471 0.000711

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TABLE – L

EQUIVALENT IMPEDANCE OF 11000 VOLT CABLES REFERRED

TO 400 VOLT

CABLE

TYPE AND SIZE

EQUIVALENT IMPEDANCE PER PHASE IN 400

VOLT SYSTEM CORRESPONDING TO ONE MILE OF

11000 VOLT LINE

R-Ohms X-Ohms

Aluminium – EPR or XLPE/SWA

95 mm2 0.000832 0.000213

185 mm2 0.000431 0.000194

3/0 AWG 0.000894 0.000217

350MCM 0.000428 0.000196

Aluminium – PVC/SWA

0.04 sq.in 0.001719 0.000227

0.10 sq.in 0.000614 0.000196

0.15 sq.in 0.000460 0.000185

0.20 sq.in 0.000346 0.000181

0.25 sq.in O. 000276 0.000177

0.30 sq.in 0.000225 0.000172

0.40 sq.in 0.000168 0.000167

0.50 sq. in 0.000136 0.000161

Copper – PIL/SWA

0.04 sq.in 0.000136 0.000163

71/77

TABLE - M

EQUIVALENT IMPEDANCE OF DISTRIBUTION TRANSFORMERS

REFERRED TO 400 VOLT

TRANSFORMER EQUIVALENT IMPEDANCE PER PHASE IN 400 VOLT

SYSTEM CORRESPONDING TO A DISTRIBUTION

TRANSFORMER OF STATED RATINGTYPEMVA

RATINGR-Ohms X-Ohms

SINGLE PHASE 0.005 0.40 0.32

230 VOLT 0.010 0.18 0.18

0.015 0.11 0.13

THREE PHASE 0.025 0.1700 0.2100

400 VOLT 0.050 0.0755 0.1165

0.075 0.0500 0.0800

0.100 0.0320 0.0659

0.200 0.0135 0.0334

0.300 0.0085 0.0260

0.315 0.0079 0.0250

0.500 0.0045 0.0148

0.750 0.0030 0.0100

0.800 0.0027 0.0097

1.000 0.0020 0.0072

72/77

TABLE – N

LOAD ASSESSMENT - RESIDENTIAL

(NOTE: READ IN CONJUNCTION WITH SECTION 8.2)

EQUPMENT

1 2 3 4 = 1 2 3

NO.UNITS

WATTSEACH

DIVERSITYFACTOR

MAXIMUM DEMAND WATTS

INDOOR LIGHTS First 10 Units 100 0.8

Additional Units 100 0.25

OUTSIDE LIGHTS Normal 100 0.8

Floodlights 1.0

FANS First Two Units 1.0

Additional Units 0.5

WATER HEATERS* Largest Unit 1.0

Additional Units 0.5

COOKERS* Less than 3.5 kW 1.0

Over 3.5 kW 0.8

HOT PLATE 1.0

DRIER* 1.0

AIR CONDITIONERS* First Two Units 1.0

Additional Units 0.6

REFRIGERATORS 1.0

DEEP FREEZERS 1.0

WASHING MACHINE* 1.0

WATER PUMP 1.0

PORTABLE

APPLIANCES

Kettle 1.0

Iron 1.0

SUNDRIES (TV, Radio, cleaner,

etc)

1.0

OTHERS 1.0

TOTAL = A.D.M.D IF LESS THAN 5 MAJOR APPLIANCES =

* A.D.M.D. IF MORE THAN 5 MAJOR APPLIANCES = TOTAL 0.85 =

TABLE – O

73/77

LOAD ASSESSMENT – COMMERCIAL

(NOTE: READ IN CONJUNCTION WITH SECTION 8.2)

EQUIPMENT1 2 3 4 = 1 2 3

NO. UNITS

WATTSEACH

DIVERSITYFACTOR

MAXIMUM DEMAND WATTS

INDOOR LIGHTS All Units 100 0.8OUTDOOR LIGHTS All Units 100 0.8FLOODLIGHTS All Units 1.0FANS All Units 1.0WATER HEATERS Largest Unit 1.0

Additional Units 0.8COOKERS Less than 3.5 kW 1.0

Over 3.5 kW 0.8HOT PLATE First Two Units 1.0

Additiona1 Units 0.8DRIVERS First Two Units 1.0

Additiona1 Units 0.8AIR CONDITIONERS First Four Units 1.0

Additiona1 Units 0.8AIR CONDITIONERS Central Plant 1.0REFRIGERATORS First Two Units 1.0

Additiona1 Units 0.8FREEZERS/COOLERS All Units 1.0COLD STORAGE ROOM All Units 1.0WASHING MACHINE First Two Units 1.0

Additiona1 Units 0.8WATER PUMP All Units 1.0GRINDERSMILLERS All Units 1.0PORTABLE

APPLIANCES

Kettles 1.0Irons 1.0Cleaners 1.0

SUNDRIES TV, Radio, etc. 1.0OTHERS 1.0TOTAL = A.D.M.D. IF LESS THAN 8 MAJOR ITEMS =* A.D.M.D. IF MORE THAN 5 MAJOR ITEMS = TOTAL 0.8 =

TABLE – P

74/77

AERIAL SERVICES

MAXIMUM SPAN LENGTHS AND ERECTION TENSIONS

(NOTE: READ IN CONJUNCTION WITH SECTION 9.2)

SERVICE CONDUCTOR

DUPLEX/QUADRUPLEX

MAXIMUM SPAN ERECTION TENSION U.T.S.

FEET METRES Lbf. Kgf. Lbf.

2 4 mm2 65 20 11 5 370

2 10 mm2 80 25 40 18 940

2 16 mm2 100 30 77 35 1480

4 4 mm2 65 20 18 8 370

4 10 mm2 80 25 65 30 940

4 16 mm2 100 30 118 53 1480

NOTE: Owing to its low tensile strength 2 4 mm2

Duplex may not be used at road crossings.

75/77

TABLE – Q

SERVICE EQUIPMENT – SINGLE PHASE

(NOTE: READ IN CONJUNCTION WITH SECTION 9.3)

1 2 3 4 5 6 7

MAX. LOAD

M.C.B. RATING

METER RATING

COPPER SERVICE CABLEMAXIMUM SERVICE

LENGTH FOR 1% VOLT DROP -FEET

kW Amp. Amp.

AERIAL DUPLEX

mm2

UNDERGROUNDPVC/SWA/PVC

mm2

AERIALDUPLEX

UNDERGROUND

CABLE

1 5 0-10 4 - 150 -- 6 - 240

10 10 390 390

2 15 0-10 4 - 75 -- 6 - 120

10 10 195 195

3 15 10-40 4 - 50 -- 6 - 80

10 10 130 13016 16 210 210

4 30 10-40 - 6 - 6010 10 97 9716 16 157 157- 25 - 247

5 30 10-40 10 10 78 7816 16 126 126- 25 - 198

6 30 10-40 10 10 65 6516 16 105 105- 25 - 165

7.5 60 10-40 - 10 - 5216 16 84 84- 25 - 132- 35 - I80

TABLE – R

76/77

SERVICE EQUIPMENT – SINGLE PHASE

(NOTE: READ IN CONJUNCTION WITH SECTION 9.3)

1 2 3 4 5 6 7

MAX. LOAD

M.C.B. RATING

METER RATING

COPPER SERVICE CABLEMAXIMUM SERVICE LENGTH

FOR 1% VOLT DROP - FEET

kW Amp. Amp.AERIAL

QUDUPLEX mm2

UNDERGROUND PVC/SWA/PVC

mm2

AERIALDUPLEX

UNDERGROUND CABLE

7.5 15 25 4 - 103 -

10 - 260 -

16 16 406 406

10 30 25 10 - 195 -

16 16 305 305

15 30 50 10 - 130 -

16 16 203 203

- 35 - 430

20 60 50 16 16 152 152

- 35 - 322

- 50 - 425

25 60 50 - 16 - 122

- 35 - 258

- 50 - 340

30 60 80 - 35 - 215

- 50 - 283

- 95 - 520

40 100 80 - 35 - 161

- 50 - 212

- 90 - 390

50 100 100 - 50 - 170

- 90 - 312

TABLE – S

APPLICATION OF

77/77

H.T. DROP CUT FUSE AND MULDED

CASE CIRCUIT BREAKER

FOR PROTECTION OF DISTRIBUTION TRANSFORMERS

(11KV/400V)

Transformer 3-phase MCCB (400 volt)Magnetic

Adjustment11kV Fuse

RatingCapacityCurrent rating

Frame Size Thermal Trip rating

3-Phase

KVA. Fig. Amp. Amp.Amp.

1-Fdr. 2-Fdr.250 1 350 400 - a) 350 75-100% - 65 K200 2 280 400 a) 350 b) 200 75-100% 65 K100 3 140 225-250 b) 200 c) 100 75-100% 40 K50 4 70 225-250 c) 100 - 75-100% 15 K

1-Phase25 5 315 400 d) 350 d) 200 75-100% 65 K50 6 210 400 e) 250 e) 140 75-100% 65 K25 7 105 225-250 f) 140 f) 75 75-100% 40 K15 8 65 225-250 g) 75 - No adjustment 15 K

Thermal trip calibrated for 40˚C ambientK stands for NEMA size

3-Phase

250 KVA 200 KVA 100 KVA 60 KVA

a a a b b b c c c

Fig-1 Fig-2 Fig-3 Fig-4

1-Phase

75 KVA 50 KVA 25 KVA 60 KVA

e f f g h h h i i i

Fig-5 Fig-6 Fig-7 Fig-8