DESIGN OF A 3MVA DISTRIBUTION NETWORK

AMR SALEM AHMED BA HAKIM

COLLEGE OF ENGINEERING

UNIVERSITI TENAGA NASIONAL

2013

DESIGN OF A 3MVA DISTRIBUTION NETWORK

By

AMR SALEM AHMED BA HAKIM

Project Supervisor:

DR. MARAYATI BTE MARSADEK

THESIS SUBMITTED IN PARTIAL FULFILMENT OF

THE REQUIREMENTS FOR THE DEGREE OF

BACHELOR OF ELECTRICAL POWER ENGINEERING

COLLEGE OF ENGINEERING

UNIVERSITI TENAGA NASIONAL

2013

i

DECLARATION

I hereby declare that this thesis, submitted to University Tenaga Nasional as a partial

fulfillment of the requirements for the degree of Bachelor of Electrical Power

Engineering, has not been submitted as an exercise for a degree at any other university. I

also certify that the work described here is entirely my own, except for quotations and

summaries whose sources have been appropriately cited in the references.

This thesis may be made available within the university library and may be

photocopied or loaned to other libraries for the purpose of consultation.

29 JANUARY 2013 AMR SALEM AHMED BA HAKIM

EP083687

ii

DEDICATION

This thesis is dedicated to all people around me especially for those who involved and

contributed a lot of expenses in completing this thesis. First to my beloved father

SALEM AHMED BAHAKIM, who has always motivated me and played a great role in

bringing the accomplishment of this thesis. Second, I dedicate this thesis to all my friends

who gave me their real friendship and advice and motivated me to accomplish this thesis.

iii

ACKNOWLEDGEMENT

First of all, praise to Allah, the Lord of the universe for bestowing me strength,

opportunity, knowledge, ideas, physical health and support needed during completing this

project.

I would like to express my gratitude to my supervisor Dr. Marayati Bte

Marsadek for helping, continuously advising, generous comment, supporting and

professional guiding me for the completion of this project as well for all the knowledge

she had pass to me in all aspects of Electrical Engineering.

I would like to thank my beloved parents whose support and blessings have truly

lightened my road. I hope to repay their kindness in the future.

My due respect and high appreciation is expressed to all lecturers and staff of

Universiti Tenaga Nasional for their support and sincerity in immeasurable ways.

Finally, I would love to thank my friends Magd A. Qaher, Mohammed Adnan,

Hitham Taresh and Radhwan Alnofaish for their help and cooperation.

iv

ABSTRACT

Electrical power distribution is required to be designed and planned with anticipation of

distribution network. This thesis demonstrates the modeling and simulation of a

distribution system for a new township. A power distribution system of a ten Load points

with a distribution network of voltage 11KV was modeled and analyzed using ETAP

software. Power distribution design criteria were reviewed in details. Technical aspects of

each required component in the system were studied and chosen in terms of rating and

sizes. The complete electrical power supply design was achieved by software simulation.

ETAP software was used to examine the system performance and verify the design

criteria in the aspect of power flow and voltage drop. Some issues related to the design

namely power protection, and reliability of the supply have been considered with all the

specifications and given the solutions as well. Results indicated that the network has the

ability of meeting the current and future demand. Moreover, the installation of the

protection devices has extensively improved the reliability of the network. This network

design may be used in rural areas, which are expected to have a larger electricity demand

in the coming years.

v

CONTENTS

DECLARATION................................................................................................................. i

DEDICATION.................................................................................................................... ii

ACKNOWLEDGEMENT ............................................................................................... iii

ABSTRACT ....................................................................................................................... iv

LIST OF TABLES ......................................................................................................... viii

LIST OF FIGURES .......................................................................................................... ix

LIST OF SYMBOLES ...................................................................................................... xi

CHAPTER I INTRODUCTION

1.1 Overview .................................................................................................................... 1

1.2 Project Overview ........................................................................................................ 1

1.3 Project Objectives ...................................................................................................... 2

1.4 Scope of Thesis .......................................................................................................... 2

1.5 Scope of Project ......................................................................................................... 2

CHAPTER II LITERATURE REVIEW

2.1 Introduction ................................................................................................................ 3

2.1.1 Power System Network ....................................................................................... 3

2.1.2 Power Generation ................................................................................................ 4

2.1.3 Transmission ........................................................................................................ 5

2.1.4 Distribution .......................................................................................................... 6

2.2 Distribution Network Components ............................................................................ 8

2.2.1 Transformer ......................................................................................................... 9

2.2.2 Cables ................................................................................................................ 16

2.2.3 Switchgear ......................................................................................................... 20

2.3 Power Flow ............................................................................................................. 23

2.3.1 Power Flow Solution ......................................................................................... 23

2.3.2 Power Flow Analysis ......................................................................................... 25

vi

2.3.3 ETAP Software .................................................................................................. 25

2.4 Fault Analysis ........................................................................................................... 26

2.4.1 Types of Fault .................................................................................................... 26

2.4.2 Causes of faults in the electrical system ............................................................ 28

2.4.3 Methods of Fault Calculation ............................................................................ 29

2.5 Protection of Sub-Station and Distribution System ................................................. 29

2.5.1 Function of Protection ....................................................................................... 30

2.5.2 Characteristic required of the protective system ............................................... 33

2.5.3 Types of Protection ............................................................................................ 34

2.5.4 Transformer Protection ...................................................................................... 34

2.5.5 Feeder Protection (cables) ................................................................................. 36

2.5.6 Busbar Protection .............................................................................................. 39

2.6 Reliability and Quality of Supply ............................................................................ 40

2.6.1 Power Quality -TNB’S Limits on Quality of Supply ..................................... 41

2.6.2 Effects of Harmonics in Distribution Components ........................................... 42

CHAPTER III METHODOLOGY

3.1 Introduction .............................................................................................................. 43

3.2 Design Criteria ......................................................................................................... 46

3.3 Design Procedure ..................................................................................................... 46

3.3.1 Demand Estimation ........................................................................................... 46

3.3.2 Selection of Supply Voltage (Income-Intake) ................................................... 48

3.3.3 Distribution Network Type Selection ................................................................ 50

3.3.4 Components Selection (rating) .......................................................................... 52

CHAPTER IV RESULTS AND DISCUSSIONS

4.1 Introduction .............................................................................................................. 59

4.2 Maximum Demand Based on the Area’s Population ............................................... 59

4.3 Power Flow .............................................................................................................. 61

vii

4.4 Fault Analysis ........................................................................................................... 70

4.5 Reliability Assessment. ............................................................................................ 73

CHAPTER V CONCLUSION AND RECOMMENDATION

5.1 Conclusion ............................................................................................................. 78

5.2 Recommendation ...................................................................................................... 79

REFERENCES ................................................................................................................. 80

APPENDICES .................................................................................................................. 83

APPENDIX A ................................................................................................................... 84

APPENDIX B ................................................................................................................... 85

APPENDIX C ................................................................................................................... 86

viii

LIST OF TABLES

Table No. Page

2.1 Overhead lines Vs. Underground cables 6

2.2 Effect of frequency in core loss 16

2.3 Functions of each layer in the cable Construction 18

2.4 Types of protection scheme used in TNB distribution system` 37

3.1 Range of maximum demand (M.D) for domestic consumer 47

sub-classes

3.2 Coincident factor for different consumer group 48

3.3 Level of Security in TNB 51

3.4 Number of substation required 53

4.1 Circuit breaker input data 72

4.2 Fuses input data 72

4.3 Reliability input data 75

ix

LIST OF FIGURES

Figure No. Page

2.1 Power System Network 4

2.2 Types of Networks 7

2.3 Characteristics of Different Network Configuration 7

2.4 Transformer schematic 9

2.5 The principle of the transformer 10

2.6 The main transformer components 12

2.7 Transformer’s Connections 13

2.8 Parallel operation of two transformers 15

2.9 Single core XLPE cable 17

2.10 Three cores XLPE cable 17

2.11 XLPE cable constructions 17

2.12 Wound Primary CT 22

2.13 Ring CT 22

2.14 Voltage transformer 22

2.15 Short-circuited-phase faults 27

2.16 Time/Current Characteristics of Fuse Links: (a) 200K; 32

(b) 200T Fuse Link

2.17 Differential Protections 35

2.18 Restricted Earth Fault Protections 36

2.19 Unit Protections 39

x

3.1 Flow Chart of the Project 45

3.2 ESAH-MD Supply Schemes 49

3.3 key-in utility’s parameters into the software 50

3.4 Transformer’s input data 55

3.5 Cable input data 56

3.6 One line diagram of the network 58

4.1 Electricity consumption per capita for Malaysia 60

4.2 Power flow results 62

4.3 Transformer loading 64

4.4 Cable loading 65

4.5 Voltage drop across the cables 66

4.6 Voltage drop across the transformers 67

4.7 Power losses across cables 68

4.8 Power losses across transformers 69

4.9 Fault levels of MV buses 71

4.10 Average interruption rates 74

4.11 Annual outage duration 75

4.12 Comparison of SAIDI 76

4.13 Reliability assessment test 77

xi

LIST OF SYMBOLES

f Frequency

Ns Synchronous speed

P Power

T Torque or temperature

W Speed

R Resister

I Current

H Hysteresis losses

E Eddy losses

kh Constant Hysteresis losses

ke constant Eddy losses

V Voltage (U)

S Apparel power

t Time

C 3-phase rating of the capacitor bank (kVA)

X line reactance per km

d distance from substation

N Number of turns

Φ flux Ci No of interrupted customers

Di Restoration time of each interruption event (in min)

n No of interruption events

N Total No of customers

xii

SAIDI System Average Interruption Duration Index

CAIDI Customer Average Interruption Duration Index

SAIFI System Average Interruption Frequency Index

CHAPTER I

INTRODUCTION

1.1 Overview

The main task of the power distribution system is to supply the loads connected to the

system with real and reactive power. In order to supply electricity to industrial users

as well as general public, the power needs to be delivered from power plants by

distribution systems. Electrical Distribution system is built in such way that “satisfies

customers at an economic overall cost in terms of electricity supply” [1]. Provide secure,

reliable, quality and safe supply. Moreover, it is important to have a proper network design

with appropriate equipment rating in order to overcome all contingency conditions.

1.2 Project Overview

This project is going to emphasize on how to design a small distribution

network. This study will include basic review about the key network components and how

each component functions. Designing any small distribution networks is always based on

the maximum demand of 3MVA. It requires constructing 10 load points (costumer

entrance) with 11kV network and 3MVA load capacity. ETAP software will be used to

check the reliability of the network.

2

1.3 Project Objectives

This project is mainly aimed to:

i. Study key network components used in distribution.

ii. Perform the analysis on distribution network (load flow).

iii. Perform fault network analysis.

iv. Perform reliability analysis for the network in order to check the quality of the

supply.

v. Analyze the results.

1.4 Scope of Thesis

The report objective is to design a low Voltage distribution system for a new township.

The content of this report is mainly proposed of all equipment ratings for the distribution

system such as transformers, cables, switchgears and protection settings. The proposed

distribution system network design is to provide secure supply whilst fully meeting customer

demand and at the most economic overall cost consistent with the nature of the load. Perform

power system analysis (load flow) using ETAP software. Protection setting shall be carried

out base on fault analysis.

1.5 Scope of Project

The scope of this project is targeted to electrical power engineering undergraduate

students. The project will study the requirements needed to build small distribution

network with 3MVA load capacity. Justify the selection of network components and rating

analysis. Perform power system analysis as well as short circuit analysis and reliability

assessment for the network.

CHAPTER II

LITERATURE REVIEW

2.1 Introduction

This chapter contains a general overview of the power system network and the role of

distribution network in power system. Moreover, the key network components used in

distribution, types of fault and protection requirements are going to be highlighted.

2.1.1 Power System Network

The electrical power system consists of three major parts, which are power generation,

transmission and distribution as shown in Figure 2.1. Each one has its own philosophy

and responsibilities. The aim of electrical power system is to generate, transmit and

distribute the electrical energy without risk to the consumer and environment with

minimum cost.

4

FIGURE 2.1 Power System Network

From the generators side, the voltage is stepped up from about 15kV to 400kV, which

makes it ready for transmission over a long distance.

2.1.2 Power Generation

Power generation is the process of converting non-electrical energy to electricity to be

transmitted and distributed to customers. Electricity is generated in power plants by

electromechanical generators, which are fed by different sources of energy. Usually the

power plant consists of source of energy, boiler and turbines. The type of the power plant

is related with the type of turbine used. The types of turbines can be steam, gas or hydro

turbines.

Electricity generated in Malaysia is mostly by burning natural gas, hydro

and a small amount is being generated by diesel. The main generators are gas turbine or

steam turbine. Electricity starts all the way from the generation side. In Malaysia there

are many power stations such as Cameron Highlands hydroelectric Power Station,

and Manjung Power Station and more. There are mainly three types of power

stations in TNB, which are the Gas Power Plant, Steam Power Plant and

Hydro Power Plants. Although gas and steam can be used to produce electricity, the

steam turbines produce more power than the gas turbine. The gas turbines produce 60MW

5

to 150MW of power while the steam turbines can go up to 600MW of power. The speed

of the all generators used in Malaysia is 3000 cycle per minute, which gives 50 Hz as

shown in the following equation

f =

(2.1)

If the power needs to be increased with the same synchronous speed (Ns), the torque

needs to be increased as shown in the following equation

P = T.W (2.2)

2.1.3 Transmission

Power Transmission is the process of transmitting power from the generation to the

distribution side. The power is transmitted using different voltage levels; Malaysia

use 375, 220 and 132kV as transmission voltage level. The usage of high voltages

during power transmission will give us the benefit of reducing power losses since low

currents will be used during the transmission according to the following formula.

Ploss = I ² x R (2.3)

The power is transmitted in two ways, either using overhead lines or underground cables.

Each way has its own advantages and disadvantages. Table 2.1 shows a comparison

between overhead lines and underground cables.

6

TABLE 2.1 Overhead lines Vs. Underground cables

Category Overhead lines Underground cables

Cost

Economical for long distances, while it’s

not for short distances

Economical for short distances, while it’s

not for long distances

Maintenance Easy to maintain Difficult to maintain

Problems Having more problems than UC Having less problems than OH

View of the city Doesn’t look good for modern cities It’s good for the view of the city

2.1.4 Distribution

Power distribution is the last stage of power system network, where the power

generated by the power plants is being delivered to the consumers. The distribution

network usually contains medium and low voltage levels. The voltage levels used in

distribution are 33kV, 22kV, and 11kV. The distribution network delivers power

from the transmission source to the consumer. There are various types of networks

used in distribution system and each network has its own characteristics.

2.1.4.1 Types of Distribution Network

During the early days utility’s company used to design a radial network. In radial

networks, the costumers are being connected to the system via a tree like shape, as shown

in Figure 2.2 (c). A radial network is cheap and easy to construct but consumers will be

largely affected if any failure was to occur at any substation, such breakdown will affect

the entire radial network (low reliability).

Second loop network (ring) connecting all the substations in a ring shape, with

normal open point (NOP) in the middle of the loop, were later discovered as shown in

Figure 2.2 (b). Distribution network is generally radial even the loop network behaves

like a radial with the NOP open. Ring networks have a good reliability, in case of a

breakdown happens from one side then the network still can continue to supply power

form the other side and close NOP. This way of connection shortens the duration of

7

black out in the area (an N-1 criteria). Each types of network have different

characteristics and positive and negative score as shown in Figure 2.3.

FIGURE 2.2 Types of Networks

FIGURE 2.3 Characteristics of Different Network Configuration

8

2.1.4.2 Types of Substations

The supply authority will determine the type and size of the substation requirements base

on the place and the load:

Pole-mounted

Unenclosed, rating up to 500KVA.

Pad-mounted

Metal enclosed kiosk-type mounted on ground, rating up to 1000KVA.

Outdoor (fenced)

Enclosure for ratings up to 3000KVA.

Outdoor (building)

Enclosure Dedicated small building, rating up to 5000KVA.

Indoor

Basement or ground-floor substation as above, but located indoors.

2.2 Distribution Network Components

Types of electrical equipment used in the distribution system are transformers, cables,

switchgears and protection devices. In order to understand the design of a distribution

network, it is imperative that the functions and applications of each component are

thoroughly understood.

9

2.2.1 Transformer

Transformer is a device for changing alternating current with an increase or decrease of

voltage [2]. Transformer's simple structure consist of two sets of winding (coil) which are

connected together through a core made of iron, each winding set is basically an inductor

as shown in figure 2.4. The side of winding where AC Voltage is being applied is called

Primary winding while the other winding is called the secondary winding.

FIGURE 2.4 Transformer schematic

2.2.1.1 Basic Theory of the Transformer

Magnetic flux is being generated around the conductor, when an alternative current runs

through it. If a different conductor were placed in the field induced by the first conductor

in a way that the flux lines connect the second conductor, then a voltage is being applied

into the second conductor. The transformer’s theory and applications are being built

based on the principle of using the magnetic field from the first coil to induce a voltage

into a second coil, as shown in figure 2.5.

10

FIGURE 2.5 The principle of the transformer

U = N .dΦ / dt (2.4)

Φ = N . I / Rm (2.5)

2.2.1.2 The Main Components of the Transformer

i. Core

The main duty of the core is to provide a magnetic path to transfer the flux. Inside the

core there are some thin strips of high grade steel called laminations, which are

electrically alienated by a thin coating of insulating material, as shown in figure 2.6 (A)

[3].

ii. Windings

Current-carrying conductors wrapped around the core are basically the windings, they

have to be perfectly insulated, cooled and supported in order to handle operations and test

conditions [3].

11

iii. Oil

The insulating oil provides an insulation medium as well as a heat transferring medium

that carries away heat produced in the windings and iron core. Since the electric strength

and the life of a Transformer depend chiefly upon the quality of the insulating oil, it is

very important to use a high quality insulating oil [3].

iv. Bushings

Bushings are the main connection between the transformer windings and external

source and load. In its simplest form a bushing is a cylinder of insulating material,

porcelain, glass resin, etc. with the required radial clearance and axial clearance to suit the

electric strength [3]. Normally these have the conducting material passing through the

center and the external insulator material surface at ground potential as shown in Figure

2.6 (B).

v. Tap changer

Voltage regulation as well as the phase shifting is being accomplished by tap changer,

shown in Figure 2.6 (C). This is done by varying the transformer’s ratio. There are two

types of the tap-changer, On Load Tap-Changer and Off Load Tap-Changer [3].

vi. Cooling equipment

Small transformers are natural air cooled, but as the size and voltage increase, fans

and oil pumps are required to be fitted to assist cooling. As the transformers get larger,

radiators are fitted to increase the surface area of the tank to farther aid cooling as they

get larger fans are installed in the radiators [3]. Life span of power transformers

depends on integrity of insulation which depends on the temperature of transformer.

Transformer can be overloaded if and only if a proper cooling system were installed (i.e.

transformer rated at 15MVA can be loaded at 20MVA with an improved cooling system

without effecting transformer’s life time).

12

Figure 2.6 Main transformer Components

2.2.1.3 Type of Power Transformer

i. Step - Up transformers

Also called generator transformers, are used in power plants to transform the electric

energy from the voltage of generator up to the level of the power grid, for example from

18KV up to 400KV. The typical power ratings of these transformers are between 10 and

500MVA.

ii. Transmission or power transformer

They are used to transport large amounts of electric power between high voltage

networks. Their nominal power ratings vary between 100MVA and 700MVA. The

voltage ratios of these transformers can for example be: 400 / 220KV or 400 / 132KV.

iii. Supply transformers

They are used to large industrial users or distribution substation. The rated power varies

between 4 and 40MVA and the primary and secondary voltage goes up to respectively

132 to 33KV or 11KV.

13

iv. Distribution transformers

They are used to distribute electric power to (small) industries and domestic consumers.

They transform the voltage from values between 3.3 and 25KV to 400 or 240V. The

typical power and voltage levels vary with the type of equipment that has to be fed [12].

2.2.1.4 Connection of the Transformer

i. Star Connection:

Low voltage (LV) normally star-connected to avoid triple harmonics, more economical

for high current low voltage windings as shown in Figure 2.7 (A).

ii. Delta Connection:

HV always delta-connected and most common connection for step down transformer is

DYN11 as shown in Figure 2.7 (B).

FIGURE 2.7 Transformer’s Connections

14

2.2.1.5 Transformer Loading

Loading of transformers in operation is important since it determines losses and

temperature of windings which will result deterioration of insulation and reducing

lifetime of transformer. The transformer load varies with the temperature rise due to flow

of current in winding, which can cause deterioration of paper insulation and oil. When

design a distribution network the optimal percentage of transformer loading is 50% - 60%

of transformer load because of future plan, long life (insulation life) and maximum power

efficiency [13].

2.2.1.6 Transformer Application

Transformers are usually four or two numbers in each substation with each two

working in parallel together. They are in parallel so that in case one break down the

power does not have to cut out because the load is then transferred to the other

transformer as shown in Figure 2.8 below . Transformers are connected in parallel in

almost all of the substations of the power network mainly for the following reasons:

Load shifting

Maintenance procedures

Reliability & Back up

More economic

In order to accomplish parallel operation, transformers must have the same

voltage ratio, tapping points in use (if the voltage ratio or tapping points are not the same,

circulating currents will be generated leading to possible overheating). Second Vector

diagram must be same, otherwise it will lead to line and phase voltages being intermixed

and insulation stressed. Finally impedance angle must be equal in order to avoid unequal

loading of transformers.

15

FIGURE 2.8 Parallel Operation of two Transformers

2.2.1.7 Transformer Losses

Transformer losses are specified into two types, which are load loss and core loss which

are normally given on transformer nameplates.

Load loss

Load loss, also known as copper loss, due to resistance of windings. Load loss is

proportional to square of load current which is load-dependent.

PCu loss α IL2

Total copper loss equal to

PCu = I12R1 + I22R2 or = I1 2R1eq = I22R2eq Watts (2.6)

Since resistance varies with temperature according to this formula:

R = Ro [1 + α (T - To)] (2.7)

16

Core loss

Core loss, also known as iron loss is the loss in core material (normally made of iron).

Core loss is constant for transformer and independent of load. There are two factors

contributing to core loss which are Hysterisis and Eddy current loss. The formula to

calculate the core loss from Hyterisis and Eddy current loss is:

H + E = (kh + kef) fBm2 W/m3 [for n = 2] (2.8)

The effect of frequency (harmonics) is significant on core losses as shown in Table 2.2

below.

TABLE 2.2 effect of frequency in core loss

Core loss type 50 Hz 60 Hz

Hysteresis 1.0 1.2 20% increase

Eddy current 1.0 1.44 44% increase

2.2.2 Cables

Cable is a device used to connect, carry and distribute electrical current [2]. The two

types of conductors normally used in power system are copper and aluminum due to their

high conductivity.

2.2.2.1 Types of Cables

There are two types of cables either single core (one phase single core usually stranded

aluminum or copper) or multicore (3 phases and neutral, often solid) which can be seen in

Figure 2.9-2.10. The size of the cables are different depending on the voltage, load,

distance, and current carrying capacity. If length of the circuit exceeds the capacity of a

cable, joints are used to connect the unit lengths.

17

FIGURE 2.9 single core XLPE cable FIGURE 2.10 three cores XLPE cable

2.2.2.2 Cable Insulation and Construction

The insulation in cables PVC, XLPE or EPR are used to prevent the flow or leakage of

current between conductor and earth. Older cables are oil-impregnated paper

insulated. The most common type of cables in Malaysia are Cross Linked

Polyethene Cables (XLPE) [14]. These cables have better mechanical, electrical and

thermal properties as shown in Figure 2.11 below the cable instruction. Each layer of

cable have its own function as shown in Table 2.3.

FIGURE 2.11 XLPE cable constructions

18

TABLE 2.3 Functions of each layer in the cable Construction [14]

Item Function

Conductor

To carry the current under the following conditions:

- Normal operation

-Overload operation

-Short-circuit operation.

To withstand pulling stresses during cable lay.

Internal semi-conductor

To avoid the strong electrical field generated by the

current at the interface between the internal semi-

conductor and the insulation to guarantee close contact

with the insulation.

To ensure a smooth electric field at the conductor.

Insulation To ensure that the cable is able to handle different

voltage levels during its service life.

External semi-conductor

To ensure close contact between the insulation and the

screen.

To avoid concentration of eclectic field at the interface

between the insulation and the external semi-

conductor.

19

Metallic screen

In order to offer:

1- An electric screen (to eliminate the electric field

outside the cable)

2- Radial waterproofing (to ensure that there is no

direct contact between the insulation and water)

3- Welded aluminum screen bonded

4- An active conductor for the capacitive to a PE

jacket and zero-sequence short-circuit current

combination of copper wires and

Outer protective sheath

To resist corrosion.

To apply mechanical protection

To reduce the cable’s ability on fires spreading.

2.2.2.3 Cable Loading

The size of the cables is different depending on the current carrying capacity. One of the

important coefficients is current carrying capacity. Current carrying capacity is defined as

the maximum amount of current a cable can carry before melting either the conductor

or the insulation [2]. Loading the cable more than it is rated will result in temperature

rise, which will affect the cable life due to deterioration of insulation. The best

percentage to load the cable is 50% - 80% in design. Temperature of cable

insulation or busbar system must be well below defined values to limit aging (except

MIMS cable) Maximum demand must be used to determine cable size. Due to some factors

such as ambience temperature, mutual heating due to bunching, harmonics and

future expansion cable must be de-rated to maintain lifetime end efficiency of cable.

20

2.2.3 Switchgear

Basic aim of switchgear is to take electrical power from main supply source and

distribute it to appropriate circuits within building or network. Switchgears are designed

in such way there is proper control of power flow (disconnections such as circuit breaker

or fuse) and proper electrical protection against the damaging effects of faults (relays)

and to prevent danger of electric shock or injury to personnel during normal or

abnormal operation. Switchgear contains number of panels; these panels can be feeder

switchgear (incomer or outgoing), transformer switchgear, and Bus coupler switchgear.

The main duty of switchgear is to de-energize the system in order to maintain

equipment as well as to clear faults downstream. The insulation of switchgear may

be a simple open-air isolator in the case of low voltages and it can also be insulated

by other substance for higher voltage levels. There are many types of switchgears

depending on the type of the insulation material, such as gas-insulated switchgear

(GIS) where the conductors and contacts are insulated by pressurized sulfur

hexafluoride gas (SF6). There is also oil and vacuum insulated switchgear.

Switchgears usually contain many compartments.

2.2.3.1 Main Compartments Used in Switchgear

A switchgear contains some components which help controlling the power flow

and to provide maximum safety and reliability of supply.

I. Isolators

Their main function is to isolate electrical circuit inside the switchgear. Unlike

circuit breakers, Isolators are not load disconnecting devices; meaning that they

cannot be closed or opened whenever there is load (current flow).

21

II. Earthing switches

Earthing switches are used to discharge the voltage on deadlines to earth [4]. It is

necessary to earth the conducting parts before maintenance to provide safe zone for

personal near the de-energized tools. Earthing ensures that an accidental re-

energizing does not cause an injury.

III. Circuit breaker (CB)

CB is a “switching and current interrupting device” [5]. Basic construction of a

circuit breaker contains a set of rigid and moving contacts. These contacts can be

separated by an operating mechanism. An arc is being generated due to the

separation of current carrying contacts. The arc is being extinguished by a suitable

medium such as dielectric oil, air, vacuum or SF6 gas.

IV. Instrument transformers

Current transformers (CT)

The only objective of current transformer is to step down the current to low amp in

order to be read by other monitoring devices. Hence, it is used in measuring and

protection. A magnetic circuit (generally made of an iron alloy) in the shape of a

toroid is surrounded by “n1” turns on the primary and “n2” turns on the secondary

(refer to Figure 2.12). The primary can be reduced to a simple conductor (1-phase)

passing through the toroid (n1 =1) (refer to Figure 2.13). Through the magnetic

field that surrounds the conductor, the flowing current can be calculated [15].

22

FIGURE 2.12 Wound Primary CT FIGURE 2.13 Ring CT

Potential or voltage transformers (VT or PT)

Similar to current transformer, the duty of the voltage transformer is to step down the

voltage to a suitable level in a way that the monitoring devices are able to read it. Also it

is used to feed the potential coils of relays. The primary winding is connected to the

measured voltage and the secondary winding to a voltmeter (refer to Figure 2.14).

FIGURE 2.14 Voltage transformer

The voltage transformer is connected to a very high impedance (used on an almost open

circuit). On the other hand, if Z is reduced, the current supplied is too high and the VT

will deteriorate.

V. Busbar:

Busbars are usually made of aluminum or copper conductor which is sustained by

insulators. The main duty of these busbars is to connect the costumers to the sources of

the electric power in the distribution system. Busbars used for very high range of current

carrying capacity normally laminated for very high ratings:

23

i. Improves heat loss by providing bigger surface area

ii. Limits increase of resistance due to skin effect (skin and proximity effect in AC

conductors due to eddy currents, lamination limits eddy currents)

VI. Protective Relays:

Protective Relays are “automatic devices which can sense the fault and send

instructions to the associated circuit breaker to open” [6]. Every part of the power system

is provided with a protective relay system and an associated switching device.

2.3 Power Flow

Power distribution system has only one main duty which is to supply a real and reactive

power to the load with the maximum possible demand. Power flow studies are mainly

concerned on determining the magnitude of bus voltage with its angle at all the buses,

real and reactive power flows (line flows) in different lines and the total losses

occurring during the transmission process in the power system. All these data help the

power system engineer to identify the transformer’s and cable’s operation loading, in

order to recommend suitable transformer and cable sizing. Moreover it helps the engineer

to identify the power factor of the network and whether the network needs power factor

correction capacitors.

2.3.1 Power Flow Solution

In order to handle power flow problem, it has to be assumed that the system is a balanced

system, as well as a single-phase model has to be used. With each bus there are four

quantities associated, real power P, reactive power Q, voltage magnitude |V| and phase

angle. The system buses are categorized into three types [7].

Slack bus: Also known as swing bus, is taken as reference where magnitude and

phase angle of the voltage are specified.

24

Load bus: At these buses the active and reactive power are specified, but, the

magnitude and phase angle of the bus voltage are unknown.

Regulated bus: These buses known as generator buses or voltage control. At each

bus the real power and voltage magnitude is specified. These buses used to limit voltage

drop.

The earliest methods on solving load flow problems for a considerably large

systems were based on the Gauss-Seidel method. But in the other hand this method has

relatively poor convergence characteristics. Hence, Newton method was developed to

improve the convergence of the Gauss-Seidel method, but it was initially thought to be

impractical for realistically sized systems because of computational problems with large

networks. The underlying problem for the iterative Newton method is the solution of a

matrix equation of large dimension. The flow of electricity through a network could be in

a steady state because a constant electricity flow was assumed. There are many other

methods used to calculate power flow such as Newton-Raphson and Runge-Kutta or by

using software. In the power flow study, it is necessary to solve the set of nonlinear

equations. In GAUSS-SEIDEL method, the iteration sequence is:

(2.9)

After the iteration solution of the bus voltage, next step is computing line flows and line

losses

Iij = yij (Vi – Vj) (2.10)

Thus power line flows are

25

Sij = Vi I*ij (2.11)

And power line losses are

SL ij = Sij + Sji (2.12)

2.3.2 Power Flow Analysis

A power flow analysis is useful to study contingency operating modes or system load

changes by using a model rather than the real system. Load flow studies are being

performed in order to determine the system voltages under various contingency

conditions, as well as to determine the equipment loading such as transformers and

cables. Moreover, these studies are usually used to find out if the system needs an

additional generator, capacitive, or inductive VAR support, or the placement of capacitors

and/or reactors to maintain system voltages within specified limits. It is requested to

perform load flow studies before any design in order to develop an optimum generating

strategies and system controls and is necessary when planning or expanding electrical

power systems. There are many other methods used to calculate power flow by using

software such as MATLAB, SINCAL or ETAP.

2.3.3 ETAP Software

ETAP (electrical power systems design and analysis software) is a utility that helps

electrical engineers in the process of designing, simulating, operating and optimizing

power systems. It provides complete interface of electrical software application which

provides an outstanding power monitoring, system optimization, real-time prediction and

energy management. It is also offered in ETAP a full set of Electrical Engineering

software solutions including load flow, fault analysis and reliability assessment. Its

functionality can be customized to fit the needs of any power system from small network

and a power grid.

26

2.4 Fault Analysis

Fault analysis [short-circuit analysis] are being carried out in order to identify the amount

of currents that flow in a power system during fault conditions (or supply bypasses the

load). Since growth of a power system often results in increased available short-circuit

current, the momentary and interrupting ratings of new and existing equipment on the

system must be checked to ensure the equipment can withstand the short-circuit energy

because short circuit current can lead to disastrous effects on equipment due to significant

thermal heating, electromechanical effects and fires [8]. Fault contributions from utility

sources, motors, and generators are taken into consideration. Short-circuit currents must

be calculated for every point in the network. The results of a fault study are also used to

selectively coordinate electrical protective devices.

2.4.1 Types of Fault

In the context of electrical fault calculations, a power system fault may be defined as

any condition or abnormality of the system which involves the electrical failure of

primary equipment, i.e. generators, transformers, busbars, overhead lines, cables

and all other items that operate at power system voltage. Electrical failure

generally implies one of two conditions or types of failure (sometimes both), namely

insulation failure resulting in a short-circuit condition or a conducting path failure

resulting in an open-circuit condition, the former being by far the more common type

of failure. Records show that 80% of faults are caused due to Phase-to-earth

connection, whereas 15% of breakdowns are caused by Phase-to-phase faults, but only 5% of

initial faults are three-phase [8].

Short-circuited phases

Faults of this type are caused by insulation failure between phase conductors or between

phase conductors and earth, or both. Figure 2.15 gives details of the various short-

circuited-phase faults.

27

FIGURE 2.15 Short-circuited-phase faults

The three-phase fault, which may or may not be to earth, is the only balanced short-circuit

condition and is the one used as the standard in determining the system fault levels or

ratings.

I. Three phase faults

The balanced fault is the severest fault and the simplest to determine. Hence, this is the one

normally used to determine the 'duty' of the system switchgear

Earth has no effect

Determines rating of plant

Maximum fault level

II. Phase-phase faults

Reasonably common fault caused by conductors clashing, ice dropping from top to middle

phase or insulation failure phase-phase fault equal to 87 % of three phase fault level.

28

III. Phase-earth faults

Most Common Fault Caused by insulation failure flows in the earth and causes earth fault

relays to mal-operate

2.4.2 Causes of faults in the electrical system

I. Overhead Lines:

Approximately 80% of faults on overhead lines are due to environmental causes such as

lightening, snow, pollution, and animals. A lightning stroke is a transient fault. Lightning

and pollution are a major cause of overhead line faults

II. Underground cables:

Faults on underground cables are caused by:

Third party damage

Deterioration of the solid insulation of the cables

Joint failure

Sealing end flashovers and failures

III. Transformers:

Fault conditions affecting transformers include:

Winding short circuits

Core faults

Bushing faults or flashovers

Cooler failure

Over-fluxing

Tap changer faults

29

IV. Substations

Faults in this category include:

Busbar faults

Switchgear faults

Faults on CTs & VTs

2.4.3 Methods of Fault Calculation

The information normally required from a fault calculation is the values of the currents

and voltages at stated points in the power system, when a given fault condition is imposed

on the system. Fault calculation is therefore, essentially a matter of network analysis and

can be achieved by a number of methods, i.e. mesh current or nodal-voltage methods,

network reduction techniques or simulation using a network analyzer. The choice of

method depends on the size and complexity of the circuit model and the availability of

computing facilities [23].

An essential part of power system analysis and fault calculation is the determination

of the equivalent system network, taking into consideration the system operating conditions

and the fault conditions. As stated earlier, faults can be subdivided into either balanced

(symmetrical) or unbalanced (asymmetrical) fault conditions, those cases could be

analyzed, normally by the method of symmetrical components. Both classes of fault are

analyzed by reducing the power system, with its fault condition, to an equivalent single-

phase network.

2.5 Protection of Sub-Station and Distribution System

In a power system consists of generation, transmission and distribution systems some

failure may occur somewhere in the system. When a fault occurs at any part of an electric

power system, it must be cleared quickly in order to avoid damage or to maintain the rest of

the system in a continuous operation, and that is where protection comes in. Usually it

comes to our head thinking of an electric power system in terms of its more effective parts

30

like big generating stations, transformers, high-voltage lines, etc. While there are some of

basic elements, but are necessary and fascinating components. Protective relaying is one of

these components. In protection schemes there is main and backup protection, which can be

applied on upstream part up to transmission system. But main protection for our project is

only in the distribution part (11Kv – 0.415Kv).

2.5.1 Function of Protection

Basically the function of protection can be specified in to two parts. First is the relay, which

work as brain, relay sense a fault current and send a signal to the interrupter

(muscles) to break the circuit within a specific time to prevent any damage to

equipment.

2.5.1.1 Relay

Usually, a relay is an electromechanical instrument that can be triggered by an electrical

current. They are used in many applications due to their high reliability as well as relative

simplicity and long life. Relay basic structure consists of a sensing unit and electric coil

[16]. Whenever the voltage or current exceed a specific limit, the coil activates the

armature, which operates either to close the open contacts or to open the closed contacts.

When a power is supplied to the coil, it produces a magnetic force that activates the switch

mechanism.

2.5.1.2 Components of Protection Schemes

Each power system protection scheme is made up from the following components:

1. Fault Detecting or Measuring Relays.

2. Tripping and other Auxiliary Relays.

3. Circuit Breakers or fuse.

4. Current Transformers.

31

2.5.1.3 Circuit Breaker vs. Fuses

Fuses

A fuse is an over current protection device; it possesses an element that is directly heated

by the passage of current and is destroyed when the current exceeds a predetermined

value. A suitably selected fuse should open the circuit by the destruction of the fuse

element, eliminate the arc established during the destruction of the element and then

maintain circuit conditions open with nominal voltage applied to its terminals, (i.e. no

arcing across the fuse element).

The majority of fuses used in distribution systems operate on the expulsion

principle, i.e. they have a tube to confine the arc, with the interior covered with de-

ionizing fiber, and a fusible element. In the presence of a fault, the interior fiber is heated

up when the fusible element melts and produces de-ionizing gases, which accumulate in

the tube. The arc is compressed and expelled out of the tube; in addition, the escape of

gas from the ends of the tube causes the particles that sustain the arc to be expelled. In

this way, the arc is extinguished when current zero is reached. The presences of de-

ionizing gases, and the turbulence within the tube, ensure that the fault current is not re-

established after the current passes through zero point. The zone of operation is limited by

two factors; the lower limit based on the minimum time required for the fusing of the

element (minimum melting time) with the upper limit determined by the maximum total

time that the fuse takes to clear the fault [17].

In distribution systems, the use of fuse links designated K and T for fast and slow

types respectively, depending on the speed ratio, is very popular. The speed ratio is the

ratio of minimum melts current that causes fuse operation at 0.1 s to the minimum melt

current for 300 s operations. For the K link, a speed ratio (SR) of 6-8 is defined, and 10-

13for a T link. Figure 2.16 below shows the comparative operating characteristics of type

200 K and 200 T fuse links. For the 200 K fuse a 4400A current is required for 0.1 s

clearance time and 560A for 300s, giving an SR of 7.86. For the 200T fuse, 6500A is

required for 0.1 s clearances, and 520A for 300s; for this case, the SR is 12.5 [17].

32

FIGURE 2.16 Time/Current Characteristics of Fuse Links: (a) 200K; (b) 200T Fuse Link

Circuit breaker

A circuit breaker is an automatically operated electrical switch designed to protect

an electrical circuit from damage caused by overload or short circuit. Its basic function is

to detect a fault condition and by interrupting continuity to immediately discontinue

electrical flow. Unlike a fuse, which operates once and then must be replaced; a circuit

breaker can be reset (either manually or automatically) to resume normal operation.

Circuit breakers are made in various sizes, from small devices that protect an individual

household appliance up to large switchgear designed to protect high voltage circuits

feeding an entire city.

All circuit breakers have common features in their operation, although details vary

substantially depending on the voltage class, current rating and type of the circuit breaker.

It must detect a fault condition; in low-voltage circuit breakers this is usually done within

the breaker enclosure. Circuit breakers for large currents or high voltages are usually

33

arranged with pilot devices to sense a fault current and to operate the trip opening

mechanism. The trip solenoid that releases the latch is usually energized by a separate

battery, although some high-voltage circuit breakers are self-contained with current

transformers, protection relays, and an internal control power source.

Once a fault is detected, contacts within the circuit breaker must open to interrupt

the circuit; some mechanically-stored energy (using something such as springs or

compressed air) contained within the breaker is used to separate the contacts, although

some of the energy required may be obtained from the fault current itself. Small circuit

breakers may be manually operated; larger units have solenoids to trip the mechanism,

and electric motors to restore energy to the springs.

The circuit breaker contacts must carry the load current without excessive heating,

and must also withstand the heat of the arc produced when interrupting (opening) the

circuit. Contacts are made of copper or copper alloys, silver alloys and other highly

conductive materials. Service life of the contacts is limited by the erosion of contact

material due to arcing while interrupting the current.

Finally, once the fault condition has been cleared, the contacts must be closed

again to restore power to the interrupted circuit.

2.5.2 Characteristic required of the protective system

I. Reliable: The protective equipment should not fail to operate in the events of

faults in the protected zone. It may be necessary to provide backup

protection to cover the failure of the main protection [9].

II. Fast: Protective system has to be fast in clearing the faults in order to

minimize the damage that may occur to the affected components.

III. Selectivity: Disconnection of equipment is restricted to the minimum necessary to

isolate the fault.

IV. Sensitive: Sensitive enough to operate under minimum fault condition.

34

2.5.3 Types of Protection

The main duty of the protection system in the distribution networks is to isolate the faulted

component only, while leaving as much component in the network as possible under

operation.

It is a usual practice to divide the protection scheme base into place of protection (zone).

I. Transformer protection

II. Feeder protection

III. Busbar protection

2.5.4 Transformer Protection

Transformer is one of the major components in distribution networks. It is required to

possess a high reliability in order to avoid disturbance in the network. Therefore, faults in

the transformer causes more sever disturbance in the distribution network comparing to

overhead line fault, which can be fixed faster. The various types of protection schemes for

power system transformers include:

2.5.4.1 Differential Protection

The differential protection is used to provide internal fault protection to equipment.

The principle of differential protection consists of comparing two currents (into and

out from) the same phase that are normally equal. If the current entering into the

transformer is not equal to the current leaving it, the differential protection is

operating.

A current differential relay provides restraint coils on the incoming current

circuits as shown in Figure 2.17. The restraint coils in combination with the

operating coil. If a current flow in the operating coil, a signal will be sent to CB to trip

instantaneously. A differential protection must possess an immediate response, when the

differential current exceeds the settings of the relay (20%, safety error), and only operate

for a fault within its zone.

35

FIGURE 2.17 Differential Protections.

The protection therefore must be stable concerning:

1. Inrush currents:

When a transformer is energized, there is a magnetizing inrush current, which

can be as high as ten times the full load current of the transformer. This high inrush current

lasts for only a few cycles. However, it can cause the differential relay to operate because it

has the appearance of an internal fault (current flows into but not out of the transformer).

2. Through fault currents.

3. Overfluxing of the transformer.

2.5.4.2 Restricted Earth Fault Protection

Restricted earth fault protection is often applied to transformers having grounded star

windings to provide sensitive earth fault detection for faults near the transformer neutral.

The inclusion of a stabilizing resistance encourages the circulating fault current to flow via

the magnetizing impedance of the saturated current transformer thus minimizing spill

current in the REF relay.

36

Restricted earth fault protection for each of the windings of a transformer can be

provided by connecting the CTs as shown in Figure (2.18) for delta and star connected

transformer windings.

FIGURE 2.18 Restricted Earth Fault Protections.

2.5.4.3 Earth Fault Protection

Earth fault protection is frequently applied to one or other of the transformer windings.

Over current relays which are connected to line CTs will operate for both phase-to-phase

and phase-to-earth provided the fault currents are above the relay setting which, in order to

allow emergency load transfers, is normally chosen to be between 20 to 80 % of the circuit

full load rating.

2.5.5 Feeder Protection (cables)

Cables are the intermediate components that provide the connection between various

components in the distribution networks. Line protection will take different considerations

in compare to other component such as transformers, busbars and generators. Although

differential protection is the ideal protection scheme for lines but it is very expensive to be

installed. The length of each cable can be several kilometers, hence it is difficult to compare

the current at both ends, and in order to solve this issue a costly pilot-wire circuit is

37

required. This expense may be justified but in general less costly methods are used [19].

The common methods of line protection are:

I. Time-graded over current protection.

II. Directional protection.

III. Distance protection.

IV. Unit Protection.

V. Earth Fault.

According to TNB’s Guidelines, distribution protection is to ensure distribution

network can operate within preset requirements for the safety of the public, staff and

overall network including equipment items. Therefore, the protection system should

be able to isolate faults on the network in a minimum time in order to minimize

damage the review of distribution protection guideline is shown in Table 2.4.

TABLE 2.4 Types of protection scheme used in TNB distribution system [19]

System Configuration Protection Scheme

Radial underground cable operated circuit

(11,22,33)

Over current protection

Earth-fault protection

Radial overhead line cable operated circuit

(11,22,33)

Over current protection

Earth-fault protection Additional of auto-

recloser

Parallel circuit (loop) Directional earth -fault protection

Over-current protection

38

Parallel interconnector

Pilot wire/fiber optic cable

Unit protection/earth-fault Protection as

backup

2.5.5.1 Time-graded Over Current Protection

The Over-current relays are connected to the CTs of the system to measure the fault

current. The relay picks up when the magnitude of current exceeds the current setting

which is 130% of the current load (rule of thumb), then it sends a signal to the C.B to trip

after certain time pass [10]. In this scheme which concerns with over current protection

time discrimination is included. In other words the time setting of relays is so graded that in

the event of fault, the smallest possible part of the system is isolated.

2.5.5.2 Directional Protection

To obtain discrimination, the circuit breakers on both sides should trip to disconnect only

the faulty cable which used in loop network.

2.5.5.3 Distance Protection

Both time graded over current and differential system are not suitable for the

protection of a very long and high voltage transmission line due to pilot wire system

becomes too expensive owing to the greater length of the pilot wires required. This

has led to the development of distance protection in which the action of relay depends

upon the distance (or impedance) between the point where the relay is installed and

the point of fault.

39

2.5.5.4 Unit Protection

The fundamental principle of Unit Protection is applied to the transmission line by

comparing the current entering the line at one terminal, with the current leaving line at the

remote terminal as shown in Figure 2.19. The relays at each end of the transmission line

compare data on the line current via fiber-optic communications link or pilot wire. If the

two ends are not equal, within a reasonable tolerance, then a fault condition is detected, and

the line is tripped.

FIGURE 2.19 Unit Protections

2.5.5.5 Earth Fault

When the fault current flows through earth return path, the fault is called Earth Fault. In the

other hand, faults where Earth is not involved are called phase fault. It is very important to

install earth fault protection because this type of fault is relatively frequent. It is more

sensitive than over current protection for earth fault currents. For earth faults several

combinations of schemes available depending upon length of the cable.

2.5.6 Busbar Protection

Busbars are important elements of electric power system and require the immediate

attention of protection engineers for safeguards against the possible faults occurring on

them. Busbar must be built to be electrically flexible and reliable enough to give a

40

continuous service. It must have an adequate capacity to carry all loads and robust

construction to withstand abnormal electromechanical forces. Busbars may be copper or

aluminum bars, which operates at constant voltage. The incoming and outgoing lines are

connected to the busbars.

Busbar protection needs careful attention due to the following reasons:

1. Fault level at busbars is very high.

2. The stability of the system is affected by fault in the bus zone.

3. The fault on busbar causes disconnection of power to a large portion of the system.

4. Any fault on busbar should be interrupted in shortest possible time, in order to avoid

damage to the installation due to heating of conductors.

2.6 Reliability and Quality of Supply

Reliability of a distribution system is evaluated in terms of the availability and quality of

power supply at each customer service entrance. Analysis of customer failure statistics

show that, compared to other portions of electrical power systems, distribution system

failures contribute as much as 90% towards the unavailability of supply to a load. These

statistics show how important the reliability evaluation of distribution systems can be.

The basic reliability indices normally used to predict or assess the reliability of a

distribution system consist of three reliability indices:

Load point average failure rate λ

Average outage duration r

Annual unavailability U

In order to evaluate the severity or significance of a system outage, using the three basic

indices mentioned above, two expanded sets of indices listed below must also be calculated.

The two expanded sets of indices include the number and average load of customers

connected at each load point in the system, and the customer interruption cost. The first set

is the system reliability index, which consists of:

41

I. System Average Interruption Frequency Index (SAIFI)

SAIDI =

=

(2.13)

II. System Average Interruption Duration Index (SAIDI)

CAIDI =

=

(2.14)

III. Customer Average Interruption Duration Index (CAIDI)

SAIFI =

=

(2.15)

2.6.1 Power Quality -TNB’S Limits on Quality of Supply

I) Voltage Regulation (at Customer’s Terminal):

MV of 6.6/11/22/33kV: ±5% of nominal voltage

LV of 230V & 400V: +10% & -6% of nominal voltage

II) Voltage Unbalance

Definition: negative phase sequence voltage, positive phase sequence voltage.

Causes: Unbalance phase impedance and loads.

Acceptable Unbalance Voltage level is <2%.

III) Loads Affecting Supply Quality.

Steel Making ARC furnaces, rolling mills, welding equipment, induction

furnaces, power semiconductors rectifiers, computers, railway traction, etc.

IV) Harmonics.

Caused by nonlinear loads such as rectifiers and other power semiconductors.

Acceptable limits based on the electrical regulation-MSIEC61000Series [9].

42

2.6.2 Effects of Harmonics in Distribution Components

Harmonics are multiple of the supply frequency of 50 Hz (for example 3rd harmonics is

150 Hz, 5th harmonics is 250 Hz and so on). These harmonic can affect distribution

network components.

Harmonics can affect transformer by significant increase in eddy loss as shown in

Table 2.2 result in higher operation temperatures (overheat) and reduce in service life time

due to deterioration of insulation.

Also cables can be effect in present of harmonics by skin effect (alternative current

tend to flow on outlet surface of conductor) result in overheat and reduce efficiency.

Second effect is overload of neutral line due to Triple harmonic. To maintain life time and

efficiency of components needs to de-rate the transformer and cable.

CHAPTER III

METHODOLOGY

3.1 Introduction

This chapter focuses on designing an 11KV distribution network, with a maximum demand

of 3MVA by using 10 load points. Moreover, it briefly explains the selection of network

components ratings and justifications. The objective of this chapter is to establish the

loading criteria for transformers and underground cables used in the distribution system for

planning purposes.

First of all, based on the maximum demand, the proper incoming voltage level was

determined. Then, the ratings of the transformers and cables were chosen based on the

maximum demand and consideration of load growth. Moreover, other factors such as power

factor, derating factor and diversity factor of load were also taken into considration.

The next step was to construct a low voltage network (11kV), and to select network

type based on the supply level. Analyze the network using load flow simulation via ETAP

software to determine the voltage drop and power losses at different contengincy

conditions.

After performing the power flow, fault analysis is needed to ensure that the network

components are able to withstand abnormal events and to determine fault currents and

voltages in oreder to choose an appropriate rating for the protection devices.

44

Finally, protctive devices will be chosen based on the fault analysis result. Circuit

breakers and fuses will be installed and coordinated in order to optimize their operation.

Moreover, reliablity assessment test will be performed to the network with the intention of

checking the quality of the supply at each load point. Figure 3.1 shows the methodolgy in a

flow chart form.

45

FIGURE 3.1 Flow Chart of the Project

Start

Literature review on key network components such as transformers and cables

Construct a low voltage network with 3 MVA load using ETAP software

Perform Power Flow

Meet the loading

criteria

Perform fault analysis

Install Protection devices

Perform reliability assessment test

Write report

End

NO

YES

Selecting components ratings and design approach

46

3.2 Design Criteria

Utility companies around the world are trying to balance between three important

factors in designing distribution networks. Startng off with the reliability which is

the “Capability of distribution system to continue operating after the occurrence of an

interruption” [1]. Secondly is the cost which means trying to build a distribution network

with reasonable prices. Finally is Safety which means that the operation of equipment has

to be managed in order to secure the continuity of supply. To satisfy these requirements,

equipments has to be designed, installed, commissioned, operated, and maintained in a

suitable manner within a management system that is effective in meeting the reliability

goals within budget target. Electrical engineers have to be able to forecast the voltages and

currents at all places within the circuit in order to design any electrical system.

3.3 Design Procedure

The design of a distribution system depends on the load characteristics. In order to

determine the load characteristic, the very first step is to calculate the maximum

demand from the diversity factor and Total Connected Load (TCL). The maximum

demand (MD) of a distribution system is defined as “the greatest of all demands

which have occurred during a specific period of time” [2]. It is important to

calculate accurate load estimation in order to plan a system that has the ability to meet the

current and future demand. however, substations, feeders, and cables have to be designed,

installed and commissiond to ensure compliance with regulations and prevent high load

factor.

3.3.1 Demand Estimation

Distribution networks have to be accurately designed to meet initial and future maximum

demand. In Malaysia, the demand estimations are based upon load declared by consumer

and TNB’s own information on load profile characteristics for various consumer classes.

Ranges of values are given as demand profiles which are known to vary according to

geographical location of consumers around the TNB service areas.

47

Fairly accurate assessment of individual and group demand of consumers are

critical to correct dimensioning of the network or facilities in meeting the initial and

future demand of consumers as imposed on the network [20].

According to TNB’s Electricity Supply Application Handbook, the maximum

demand for various types of customers varies with the location type of the premise as

shown in Table 3.1.

TABLE 3.1: Range of maximum demand (M.D) for domestic consumer sub-classes

Group Coincident Factor

Group coincident factor is applied in the computation of unit demand and group demand.

The typical values for coincident factors for different groups of consumers are shown in

Table 3.2 below

48

TABLE 3.2: coincident factor for different consumer group [20]

Consumer Groups Coincident Factors

Residential 0.9

Commercial 0.85

Industrial 0.79

Residential + Commercial 0.79

Residential + Industrial 0.87

Commercial + Industrial 0.79

Mixed Group 0.75

In this project, the designed distribution network was assumed to be located in a

countryside (Rural) area where 100% of the loads are domestic. Based on Table 3.1, the

maximum demand varies with the type of premise, the larger the premise the greater the

load.

3.3.2 Selection of Supply Voltage (Income-Intake)

The Maximum Demand (MD) determines the voltage level of the distribution network.

From the obtained voltage level, which is based on the range of MD, typical supply scheme

can also be specified. Figure 3.2 is quoted from TNB Electricity Supply Application

Handbook [4]. Since the MD load capacity of the network lays between 1000KVA and

5000KVA, the supply voltage is selected to be directly fed form TNB 11KV

switching station base on ESAH-MD and typical supply schemes [2].

49

FIGURE 3.2 ESAH-MD Supply Schemes [21]

In order to represent TNB’s supply as a utility in ETAP software, the parameters were

inserted as shown in Figure 3.3 below. By only choosing the utility’s maximum demand

and supply voltage, ETAP were able to fill up all the remaining parameters.

50

FIGURE 3.3 key-in utility’s parameters into the software

3.3.3 Distribution Network Type Selection

There are various types of networks used in distribution systems and each network has its

own characteristics. Selection of network depends on level of security supply (duration of

restoration). An adopted security level definition for TNB distribution systems is shown

below in Table 3.3.

51

TABLE 3.3 Level of Security in TNB [21]

In this project, the network was generally designed to facilitate an average restoration time of

less than 4 hours, which is level 3. Higher security level can be designed upon

customer’s request and all additional cost borne by the customer.

Level 1

In level one, the restoration time is less than 5 seconds, which means that the

connection of supply will use double circuit with SCADA system, one is considered

as the main feeder and the other one is the feedback feeder. Such connection has been

installed in Putrajaya and VIP customers.

Level 2

In level two, the restoration time is less than 15 minutes, which means that the

consumer is connected to loop network supply with SCADA system.

Level 3

In level three, the restoration time is less than 4 hours that means that the supply is

connected to loop network or short radial for less than 5 substations connected to it.

52

3.3.4 Components Selection (rating)

Ratings of network components have been selected based on the maximum demand as

well as TNB’s specifications to ensure that the device settings and ampere ratings are

best available to minimize the impact of faults. There are many factors affecting rating

of components such as harmonic, demand growth and ambient temperatures. All these

factors have to be taken into consideration.

3.3.4.1 Transformer Rating

The life span of a transformer is determined by the rate of deterioration of its winding

insulation. The IEC 354:1991 (formerly British Standard CP1010: 1975) indicates

how transformers may be operated in different conditions of ambient temperature and

services, without exceeding the acceptable limit of deterioration of winding insulation

through thermal effects caused by loading. Preventing the winding insulation of the

transformer from exceeding its acceptable limit of deterioration implies that the

normal life of the transformer is being maintained. The transformer would have a life

span lower than its normal life if the rate of deterioration of the winding insulation is

allowed to exceed 80% of the transformer loading.

The determination of transformer rating (i.e. kVA capacity) to cater for the

load of a consumer is based on the maximum demand (as declared by the consumer)

and also other considerations, notably economic (capitalization of losses, etc.). For

planning purpose, by assuming an ambient temperature of 300C, the maximum loading

capability of the ONAN & ONAF transformer (the type commonly used in the distribution

system) can be set at 90% of its capacity, since the IEC 354:1991 (formerly BS CP1010)

shows that with a 90% loading for 24 hours a day at this ambient temperature does not

cause the transformer to exceed the acceptable limit of deterioration of its winding

insulation [11].

53

Rating of transformers

Since we have different loads distributed all over the network as shown in figure 3.6, we

will have different transformer sizes for each load point. Starting with 200KVA loads, the

transformer was chosen with a capacity of 300KVA. This choice was based on “Tenaga

Nasional Berhad, LV Planning Guideline”. In this guidline, it was mentioned that to

choose an appropriate transformer size, we should consider the following:

Maximum demand of the load taking into consideration the expected growth in the

coming 15 years.

TNB’s standards for transformer ratings:

1. Maximum transformer loading is 85%

2. Coinsident of residintial load is 0.9

Hence, transformers should handle loads based on the following equation:

(4.1)

Table 3.4 shows a standerd transformer capacity for different demanded loads

TABLE 3.4: Number of substation required [13]

MD Transformer Capacity

Up to 85 KVA 1 substation @ 100KVA

Up to 250 KVA 1 substation @ 300KVA

Up to 425 KVA 1 substation @ 500 KVA

< 425 KVA Require more than on transformer connected in

parallel

54

Therefore, a 300KVA transformer was selected for a 200KVA load. Likewise for a

300KVA load, the transformer selected was with size 500KVA. Finally, to handle the

600KVA loads, two transformers were connected in parallel in order to maintain the

loading within the permissible limits as well as to improve the reliability of the network.

Transformers chosen were 300KVA and 500KVA. Transformer specifications were

chosen based on NEXANS data sheet which is attached in Appendix A, and here are the

most important specifications:

Rated power : 300KVA, 500KVA

Cooling type : ONAN

Frequency :50 HZ

Vector Group :Dyn-11

Primary Rated Voltage :11KV

Secondary voltage :0.415KV

These specification were inserted into ETAP as shown in Figure 3.4 which contains the

input data for all the transformers in the network.

55

FIGURE 3.4 Transformer’s input data

3.3.4.2 Cable Size

Electricity is distributed by the use of cables and busbars. Cables are used for full range of

current levels at low voltage (240/415V) and high voltage (11kV). In this

project, due to the use of three-phase system, the types of cables were chosen to be

multi-core with 3 phases and neutral, often solid section aluminum. The insulation type for

low voltage cable is either PVC or XLPE. The conductor size of cables must be

selected correctly according to the current carrying capacity, future expansion of load,

ambient temperature, installation methods and bunching effect.

ETAP software was used to figure out the current running in each cable. Cables were

installed in such way that cables are only loaded with 25% of its current carrying capacity.

Though TNB’s limit for the cable loading is up to 50%, this value was chosen to meet future

expansion. The types and sizes of the cables were chosen based on “Tenaga Nasional

Berhad’s Electricity Supply Application Handbook” and “XPLE Insulated Cables” which is

attached to Appendix B. These loading criteria were performed to all cables in the

network. Figure 3.5 below shows the detailed input data of each cable.

56

FIGURE 3.5 Cable input data

3.3.4.3 Protection Devices Selection and Rating

Protection devices form a very important part of the distribution networks. They

provide the security to network components and they increase the reliability at each

customer entrance. Distribution-level protection is based on a time-over current design.

Such design includes selection of equipment and settings, placement of equipment, and

coordination of devices to clear faults with the minimum impacts on customers. One of

the main priorities is to prevent further damage to utility equipment. Secondary goals are

reliability and power quality.

Distribution protection is not designed to have backup, although there must be

overlap between protective devices; an upstream device should operate for a fault if the

downstream protector fails. In addition, distribution protection is based on standardized

settings, equipment, and procedures.

In this design, the chosen cables are underground cables, which have a very small

number of failures comparing to overhead lines, but unlike over-headlines, when failure

occurs at the underground cable, it needs long time to be repaired. Hence, protective

devices used are circuit breakers and fuses due to their ability to sectionalize the faulted

57

parts of the network while the remaining network can operate without any interruption.

The protection devices rating will be chosen after performing the fault analysis. The

rating of these protection devices will be selected based on the fault analysis. This will

be further discussed in the results.

The next step is to build this network in ETAP software as shown in Figure 3.6.

Now it’s ready to perform power system using ETAP software to determine the voltage

drop and power losses at peak load.

58

FIGURE 3.6 One-line diagram of the network

CHAPTER IV

RESULTS AND DISCUSSIONS

4.1 Introduction

Network analysis is the process of finding the voltages across load, and the currents

through every component in the network. There are numbers of different techniques in

order to achieve it. Power system analysis can be expressed as one cycle that consists of

power flow analysis and fault analysis to insure that the system can withstand fault events

(network strength). Moreover, Protection settings should be set based on fault analysis.

Network analysis helps the engineers in terms of planning future expand, prevent the

network system from overloading, and to operate the network in economic mater before

starting to construct the network in the real life (site).

Power flow or load flow analysis is performed to determine the steady state

operating condition of a power system. Power flow study is the most frequently carried-

out and performed by power utilities and it is required to be performed at almost all the

stages of power system planning, optimizing, operating and controlling.

4.2 Maximum Demand Based on the Area’s Population

As discussed in the methodology, distribution networks have to be accurately designed to

meet initial and future maximum demand. In Malaysia, the demand estimations are based

upon load declared by consumer and TNB’s own information about load profile

characteristics for various consumer classes [20].

In this project, the designed distribution network was assumed to be located in a

countryside (Rural) area where 100% of the loads are domestic. According to Figure 4.1

below, the electricity consumption per capita in Malaysia has increased rapidly for the

60

last few years. In 2009 the electricity consumption per capita was 3,600KW but today it’s

expected to be around 4,280KW, which is almost 0.5KW/h for each person. Hence, for an

area with a total population of 5600 inhabitants, the maximum demand is 2800KW as

follows:

0.5KW/h × 5600 inhabitants = 2800KW (4.1)

Taking the power factor to be 0.85, the maximum demand of the area will be equivalent

to 3.3MVA as shown:

(4.2)

However, the actual power that will be delivered is only 3MVA due to the coincident

factor where 3.33×0.9 = 3MVA. Hence, the designed distribution network is with a

maximum demand of 3MVA.

FIGURE 4.1 Electricity consumption per capita for Malaysia [20]

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4.3 Power Flow

Power flow studies, commonly known as load flow, form the backbone of power system

analysis and design. They are necessary for planning, operating, economic scheduling and

exchanging of power between utilities. In addition, power flow analysis is required for

many other analyses such as stability and contingency analysis.

The main objective of power flow studies is to determine the voltage magnitude

with its angle at all the buses, real and reactive power flows (line flows) in different lines

and the transmission losses occurring in a power system. These values enable power

system engineers to identify over-loaded transformers and cables, recommend proper

transformer and cable sizing and assess the need for power factor correction capacitors.

A power flow study is useful to study contingency operating modes or system load

changes by using a model rather than the real system. Moreover, Load-flow studies

determine if system voltages remain within specific limits under various contingency

conditions.

62

4.3.1 Power Flow Simulation

FIGURE 4.2 Power flow results

63

4.3.2 Discussion

3.3.2.1 Load Flow Report

The one-line diagram shown in Figure 4.2 shows the power flow results but the load flow

report in Appendix C has an easier way of displaying the results. As shown in Figure 4.2

the power was divided into two equal quantities through bus101 and bus201, this division

is due to the identical loading criteria in both sides of the network. In each part of the

network, it can be noted that the power factor has never dropped below 85 % as shown in

Figure 4.3.This indicates that the network is reliable and economical, because a maximum

amount of real power is being transferred through cables, while a minimum amount of the

reactive power is being transferred.

FIGURE 4.3 Power Factor

4.3.2.2 Loading of Network Component

Using load flow analysis, each and every component in the network has been tested in

order to determine whether it is under or over loaded as well as to ensure that all

components are loaded within TNB’s specifications.

64

Transformer Loading

The design criteria states that the transformer loading should be maintained between 50%

and 80% in order to meet both TNB specifications and future demand. As shown in

Figure 4.4, the transformer’s loading has been maintained within 57%-70%. This loading

percentage satisfies the present needs and it will meet future plans. In the same time it

will insure a long life (insulation life) and maximum power efficiency. The importance of

keeping the transformer within this loading limit is to minimize the losses across the

transformer, which increases as the transformer loading increase. These losses can be in

the form of copper losses or core losses, which also results on forming Hysteresis and

Eddy current losses.

All these types of losses cause an increment of temperature of the transformer due

to the flow of current in the winding, which can cause deterioration of paper insulation

and oil, and hence reducing the transformer’s lifetime. Moreover, maintaining the

transformer loading within this limit also affected the voltage drop and the power losses

across the transformers. This will be further discussed in following sections.

FIGURE 4.4 Transformer loading

65

Cable Loading

Likewise, the loading in the design criteria of the cable states that the current running

through each cable should never exceed 25% of its current carrying capacity. Loading the

cable more than its current carrying capacity will result in temperature rise, which will

affect the cable life due to deterioration of insulation. As shown in Figure 4.5 below,

cable loading was maintained between 13% and 25% in order to meet the design criteria.

This will bring so many advantages to the network. First of all, the network will be able

to fulfill current needs of the customers as well as it will be able to satisfy future plans.

Second, power will be transferred through the cables with a minimum possible reactance

which will decrease the power losses and voltage drop across the cable. Finally,

temperature of the cable will always be at an acceptable value, which will guarantee long

life for the cable.

FIGURE 4.5 Cable loading

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4.3.2.3 Voltage Drop

Load flow results shown in Figure 4.2 as well as the branch losses summery reported in

Appendix C shows clearly the voltage drop at each branch. Voltage everywhere in the

network has never dropped below 5% of nominal voltage, which is the maximum

acceptable voltage drop. Major voltage drops in the network was across cables and

transformers. As it is represented in Figure 4.5 below, the voltage drop across each cable

in the network has never exceeded a percentage of 0.3% of the system. This voltage drop

can be considered very small compared to other distribution networks; this was achieved

by optimizing the cable loading, where the cable loading has never exceeded 25% as

discussed before. The voltage drop has a proportional relation with the amount of current

running through the cable.

FIGURE 4.5 Voltage drop across the cables.

Transformers in the other hand cause a noticeable voltage drop across the network. As

shown in Figure 4.6 below, voltage drop across each transformer can be as high as 2.2%.

That is due to the core and copper losses, which increases as the current flowing through

the transformer increase. The voltage drop across the transformer could be very high if

the loading of the transformer exceeded 85%, but in this design because the loading were

67

maintained between 60% and 70%, the voltage drops were acceptable since the voltage

drop has never exceeded the allowable percentage of 5%.

FIGURE 4.6 Voltage drop across the transformers.

4.3.2.4 Power Losses

The power difference across the two ends of each cable is represented by power losses

(the difference between the node at the start of the line and the end of it). In other words,

power loss is equal to the difference between “From-To Bus Flow” and “To-From Bus

Flow” stated in the branch losses summery report, refer to Appendix C.

For example, at the “From-To Bus Flow” of cable 1 power flow is 1.257MW and

0.692MVAR but at the “To-From Bus Flow” it shows -1.254MW and -0.704KVAR.

Hence the power losses and the cable are the difference between them, which is 3.2KW

and -11.8KVAR. The negative and positive signs at the branch losses summery report

attached on Appendix C have a special indication, where the negative sign indicates that

power is being delivered from the cable. Likewise the positive sign indicates that the

cable is receiving power.

68

Power losses across the cables occur because of their resistance. These losses are

represented as I2R. Hence, loading of the cable plays a major role on its losses. As it is

represented in Figure 4.7 below, power losses vary from one cable to another, which is

due to the cable loading, the higher the current running in the cable, the higher the losses.

FIGURE 4.7 Power losses across the cables

Furthermore, power is also lost across each transformer in the network as shown

in Figure 4.8 below. Transformer has two types of losses, load losses and core losses.

Load losses are also known as copper loss, due to resistance of windings. Load losses are

proportional to square of load current which is load-dependent.

PCu loss α IL2

In the other hand, core losses are also known as, iron losses which are the losses in core

material (normally made of iron). Core losses are constant for transformer, independent of

load. There are two factors contributing to core losses which are Hysterisis and Eddy

current losses.

69

Figure 4.8 Power losses across the Transformer.

The total power losses all over the network were calculated to be 44.5KW and -

14.8KVAR. These values represent 1.5% of the total power generated. This shows that

the network has a very high efficiency since there is only small amount of power being

lost.

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4.4 Fault Analysis

Fault analysis [short-circuit analysis] have been carried out in order to identify the

amount of currents that flow in a power system during fault conditions. Moreover,

performing fault analysis was important in order to ensure that the equipment can

withstand the short-circuit energy because short circuit current can lead to disastrous

effects on equipment due to significant thermal heating, electromechanical effects, and

fires.

Three-phase fault was applied to the network’s medium voltage buses because it is

the severest fault and the simplest to determine. Moreover, using this type of fault

eventually results in obtaining the rating of the circuit breakers as well as the fuses.

Furthermore, Analysis was performed when all MV busses are faulted because all the

protective devices planned to be installed in MV level only and that’s where most of the

dangerous breakdowns usually occur.

Short circuit analysis report attached in Appendix C and Figure 4.9 below helped

on choosing the appropriate settings for all fuses and circuit breakers connected to the

network. As represented below, fault current varies with the variation of the total load

connected to the bus. For example, the main bus has a steady state fault current of

63.1KA whereas bus104 has a steady state fault current of only 3.9KA. This huge

difference comes due to the different amount of power passing through the bus, the more

the bus is loaded, the higher the fault current.

71

FIGURE 4.9 Fault levels of MV buses

Choosing the settings of the protection devices is the most difficult and important part in

the protection devices installation process. The network’s security, quality and reliability

depend on the protection scheme applied to the network. There are so many values that

have to be taken into consideration such as the device’s making and breaking values, as

well as its capacity. The protection devices should provide the full protection for the

current network configuration, and it should be flexible enough for future’s development.

As shown in Table 4.3 below, the most important values were the making peak

and AC breaking. These values were chosen based on the short circuit analysis with an

increment of 15% in order to overcome overloading of the device, which may lead to the

breakdown of the device. Although at the main bus –for example- the peak fault current is

107 KA and the steady state fault current is 63.1KA, the chosen values for CB101 and

CB201 were 124KA as a making peak and 72.95KA as AC breaking which are 15%

higher than the original values, and the same procedure were done to all other circuit

breakers. By introducing this increment to the device capability, all the circuit breakers in

the network were managed to withstand the fault current under different contingency

conditions without breaking down.

72

For the sake of simplicity, all other values such as Ithr (short-time rated withstand

current in KA), Tkr (rated withstand time in sec) and MinDelay (minimum delay) were

set to default values given by ETAP.

TABLE 4.3 Circuit breaker input data

Rated KV Making Peak AC breaking

CB101 11 KV 124 72.95

CB201 11 KV 124 72.95

CB102 11 KV 46 30

CB202 11 KV 46 30

CB103 11 KV 46 30

CB203 11 KV 46 30

CB104 11 KV 13 9

CB204 11 KV 13 9

CB105 11 KV 19.34 13.25

CB205 11 KV 19.34 13.25

Unlike circuit breakers, fuses were much easier to set up. The simplicity comes from the

library provided by ETAP. By only choosing the rated voltage and the breaking current,

ETAP was managed to fill up all other values needed. The values inserted were based on

the fault analysis performed. Table 4.4 below shows the input data for each fuse

TABLE 4.4 fuses input data

Rated KV Breaking current

Fuse 101 11 KV 30 KA

Fuse 102 11 KV 9.0 KA

Fuse 103 11 KV 11.85 KA

Fuse 104 11 KV 17.7 KA

Fuse 105’ 11 KV 5.9 KA

Fuse 105’’ 11 KV 5.9 KA

Fuse 201 11 KV 30 KA

Fuse 202 11 KV 9.0 KA

Fuse 203 11 KV 11.85 KA

Fuse 204 11 KV 17.7 KA

Fuse 205’ 11 KV 5.9 KA

Fuse 205’’ 11 KV 5.9 KA

73

4.5 Reliability Assessment.

A stable and reliable electric power supply system is an inevitable pre-requested for the

technological and economical growth of any nation. Due to this, utilities must strive and

ensure that the customer’s reliability requirements are met and the regulators

requirements are satisfied at the lowest possible cost. It is known fact around the world

that 90% of the customer service interruptions are caused due to failure in distribution

system. Therefore, it is worth considering reliability assessments as it provides an

opportunity to incorporate the cost or losses incurred by the customer as a result of power

failure and this must be considered in planning and operating practices [22].

This study has been carried out in order to verify whether the network can supply

a reliable power or the network needs further improve. In distribution system planning,

reliability aspects are an important part of the decision base. Hence, to be able to assess

and simulate reliability is needed in the planning process. In reliability planning, several

issues are addressed

Fault statistics

Outage consequence assessment

Simulation tools and methods

4.5.1 Input Data

In order to perform the reliability assessment test to the network, reliability parameters of

each component in the network has to be inserted, such as active failure rate [λA], Passive

failure rate [λP] and Mean time to repair in hours [MTTR]. For the sake of simplicity,

ETAP default values were used for each component as shown in Table 4.1 below.

74

TABLE 4.5 Reliability input data

Type λA (f/yr) λB (f/yr) MTTR (Hour)

Cables 0.0046 0.0046 20.80

Transformers 0.0036 0.0036 67.00

Circuit Breakers 0.0030 0.0045 50

Fuse 0.0030 0.0045 100

4.5.2 Discussion

This result of the reliability assessment test depends mainly on the fault statistics, which

were chosen to be ETAP default values as discussed before. Figure 4.10 shows the

number of failures per year at each costumer entrance in the network. The results in the

figure show the difference in reliability from one costumer entrance to other. This

difference is due to the distance between the main utility and the customer, the longer the

distance the greater the interruption rate. Loads at the same distance such as L101 and

L201 have an equal interruption rate.

FIGURE 4.10 Average interruption rates

75

Figure 4.11 shows the small difference in the number of interruption converted into hours

per year. Although the difference in reliability from one load point to another depend

mainly on the distance from the utility, but loads L104 and L105 are almost at the same

distance, but load L105 has a better reliability. This is because of the transformers

configuration, where in load L104 there is only one transformer connected whereas in

load L105 there are two transformers connected in parallel. Hence, when one transformer

fails to operate the other transformer will continue supplying the load. This is why in the

main substation the designers usually choose to connect two small transformers in parallel

instead of one large transformer.

FIGURE 4.11 Annual outage duration

Affects of protection devices installation

The reliability of the network has increased dramatically with the installation of the

protection devices. As shown in Figure 4.12 below, the SAIDI of the network has

improved from 15 Hours/Costumer.yr to 5 Hours/Costumer.yr. This abrupt improve is

due to the functionality of the protection devices in sectionalizing the faulted sectors of

the network.

76

Without the protective devices, any fault will interrupt the entire network, which

will lead to very high interruption duration per year. Whereas after the installation of the

protective devices, when the fault occur, only the faulted area will be affected but the rest

of the network will remain functioning.

Although the network was designed as a radial network, the results of the

reliability assessment test proved that the network has an impressive reliability. Figure

4.13 below shows the detailed reliability results of the network.

FIGURE 4.12 Comparison of SAIDI

77

FIGURE 4.13 Reliability assessment test

CHAPTER V

CONCLUSION AND RECOMMENDATION

5.1 Conclusion

As a conclusion, the project was a Design Based Project with a primary focus on

designing a small distribution network with all requirements needed to build it. This

project helps to understand the main power distribution concept and most of the tools and

the devices used to perform this process starting from the substation (the source) to the

cables (the connector) to the distributor (consumer).

The cost of the project may be high due to the use of cables loaded only around

25% of their current carrying capacity and transformers loaded at 75% of their maximum

capacity. However, these features are needed to sustain the load growth and provide

highly secured and reliable supply even beyond the year 2020 considering the time value.

It is better to design the network at bigger scale earlier than spending high budget to

expand the network later.

The stated objective was fulfilled in this project. All modeling and power flow

simulation were done using ETAP software. All performances of all numerical methods

were obtained and also fault analysis was performed and protection devices were

installed. Further analysis was done in order to verify the network’s reliability.

79

5.2 Recommendation

In the future, this project can be further upgraded through the use of many advanced

approaches. A number of recommendations in order to increase the reliability of the

supply are as follows:

1) The Use of loop network

By introducing NOPs (Normally Open Points) to connect between the feeders, the

reliability of the network will rapidly improve. This is because each load will be supplied

from more than one feeder. That means if interruption occurs in the main feeder, the load

can be supplied from different feeder, hence the total interruption duration will decrease

and the reliability of the network will increase accordingly.

2) Higher voltages result in lower losses and give better reliability

The use of a higher voltage in transferring the electrical power leads to the decrease of the

losses across the cables. Since the losses across the cable are in the form of I2R, the

higher voltage will use a lower amount of current to transfer the same amount of

electrical power. Moreover, with higher voltage and same load density, the line can serve

more customers and run for a longer distance.

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83

APPENDICES

84

APPENDIX A

FIGURE A1 Transformer’s specifications

85

APPENDIX B

FIGURE B1 Cable’s specifications

86

APPENDIX C

FIGURE C1 Load flow report

87

FIGURE C2 Load flow report Cont.

88

FIGURE C3 Bus loading summary report

89

FIGURE C4 Branch loading summary report

90

FIGURE C5 Brunch losses summary report

91

FIGURE C6 Load point output report

92

FIGURE C7 Reliability indexes

93

FIGURE C8 Short Circuit summary report