Major Training Report

POWER GRID CORPORATION OF INDIA POWER GRID CORPORATION OF INDIA POWER GRID CORPORATION OF INDIA POWER GRID CORPORATION OF INDIA

LIMITEDLIMITEDLIMITEDLIMITED

Grid Structure & Operation and 765kV/400 kV

Switchyard Design

VOCATIONAL TRAINING REPORTVOCATIONAL TRAINING REPORTVOCATIONAL TRAINING REPORTVOCATIONAL TRAINING REPORT

(15/05/2011 TO 12/06/2011)(15/05/2011 TO 12/06/2011)(15/05/2011 TO 12/06/2011)(15/05/2011 TO 12/06/2011)

Submitted To: - Submitted By:-

Mr. RaMr. RaMr. RaMr. Rameshwar meshwar meshwar meshwar Siddharth GuptaSiddharth GuptaSiddharth GuptaSiddharth Gupta Manager MANIT- BHOPAL

Power Grid, Gwalior 3RD

YEAR,

ENERGY ENGINEERING

ii

ACKNOWLEDGEMENT

Presenting the vocational training report today remains an unparallel event

for me as it recapitulates all toils and efforts and thanks to everyone who

made it possible for me to achieve something which appeared to be

impossible one.

Wherever and whatever I present today has been made possible by the

underlying efforts of my training in charge at POWER GRID, Gwalior and

I am grateful for their constant support during my training period.

Last but not the least I wish to thank all those noble hearts who directly or

indirectly helped me to complete my vocational training and provided me

with their valuable time to give some precious information.

iii

POWER GRID CORPORATION OF INDIA POWER GRID CORPORATION OF INDIA POWER GRID CORPORATION OF INDIA POWER GRID CORPORATION OF INDIA

LIMITEDLIMITEDLIMITEDLIMITED

Power Grid Corporation of India Limited (POWERGRID) is a Navratna

state-owned electric utility company headquartered in Gurgaon, India. Power

Grid wheels about 51% of the total power generated in India on its

transmission network. Power Grid has a pan India presence with around

82,045 Circuit-km of Transmission network and 135 nos. of EHVAC &

HVDC sub-stations with a total transformation capacity of 91,945 MVA. The

Inter-regional capacity is enhanced to 22400 MW. Power Grid has

consistently maintained the transmission system availability over 99.00%

which is at par with the International Utilities.

The 765/400/220 kV Sub Station at Gwalior was commissioned in 2007. It

has four 765 kV single circuit transmission lines charged at 400 kV, two of

them between Power Grid Gwalior & Power Grid Bina and two between

Power Grid Gwalior and Power Grid Agra. It also has two 220 kV double

circuit transmission lines with one double circuit line between Power Grid

Gwalior & MPEB Malanpur and one double circuit line between Power Grid

Gwalior & MPEB Mahalgaon. SIL for 400 kV line is 510 MW and line can

safely loaded upto 1100 MW. Once charged to 765 kV, SIL will increase to

2300 MW.

The company is already building two 765 kV single circuit transmission line

between Power Grid Gwalior & Power Grid Jaipur and two 765 kV single

circuit line between Power Grid Gwalior & Power Grid Satna. Another 765

kV single circuit transmission line between Power Grid Gwalior & Power

Grid Bina is proposed.

iv

Table of ContentsTable of ContentsTable of ContentsTable of Contents

Chapter No. Title Page no.

About Power Grid, India iii

1) Grid Structure & Operation 1

2) Load Scheduling 3

2.1 Load Scheduling 3

2.2 Scheduling & Dispatch Procedure 4

2.3 Revision of Schedule 4

3) Switchyard Design 6

3.1 One and a Half Breaker Arrangement 6

3.2 Double Main & Transfer Bus Bar 8

3.2.1 Working of Double Main & Transfer

Bus Bar

8

3.3 Choice of Bus Bar Scheme 10

3.4 Connecting the Transformers 11

4) Switchyard Components 13

4.1 Bay 13

4.2 Reactor 13

4.3 Circuit Breaker 14

5) 4.3.1 Technical Specifications of CB 14

4.3.1.1 800 kV CB 15

4.3.1.2 420 kV CB 15

4.3.1.3 225 kV CB 16

4.4 Current Transformers 16

4.4.1 Technical Specifications of CT 16

4.4.1.1 400 kV CT 16

4.4.1.2 225 kV CT 16

APPENDIX

APPENDIX I 17

APPENDIX II 18

1

Chapter 1

GRID STRUCTURE & OPERATION

Power Grid has been accorded the status of Central transmission Utility

(CTU) by the Government of India. The company has divided the

transmission network of the country into 8 regions namely Northern Region -

I, Northern region – II, Western Region – I, Western Region – II, Eastern

Region – I, Eastern Region – II, North Eastern Region, Southern Region - I &

Southern Region - II. This division is however internal to the organization.

For the purpose of monitoring grid operations, the power system has been

divided into five regions and REGIONAL LOAD DISPATCH CENTERS

have been established in each region for the integrated operation of power

system in the concerned region. These RLDCs are

i. Northern Region Load Dispatch Center

ii. Eastern Region Load Dispatch Center

iii. Southern Region Load Dispatch Center

iv. North Eastern Region Load Dispatch Center

v. Western Region Load Dispatch Center

A NATIONAL LOAD DISPATCH CENTER is established at the national

level for optimum scheduling and dispatch of electricity among the Regional

Load Dispatch Centers. Both the RLDCs and NLDC are setup and operated

by the POSOCO a wholly owned subsidiary of Power Grid Corporation of

India Limited, in its capacity as the Central Transmission Utility.

The functions of RLDCs are: -

a) Responsible for optimum scheduling and dispatch of electricity within

the region in accordance with the contracts entered into with the

licensees or the generating companies operating in the region.

b) To monitor grid operations.

2

c) To keep accounts of quantity of electricity transmitted through the

regional grid.

d) Exercise supervision and control over the inter-State transmission

system.

e) Responsible for carrying out real time operation for grid control and

dispatch of electricity within the region through secure and economic

operation of the regional grid in accordance with the Grid standards and

Grid code.

f) Facilitating transactions of power under short-term and long term open

access for inter/intra regional exchanges following regulations and

procedures issued by the Central/State Electricity Regulatory

Commission and Central Transmission Utilities.

g) Coordinating and issuing drawl schedules of State Power Utilities from

all Central Generating Stations and dispatch schedules of Central

Generating Stations.

A STATE LOAD DISPATCH CENTER is established at state level to ensure

integrated operation of the power system in a State. The establishment of

SLDC is mandated by section 30 & 31 of Indian Electricity Act 2003.

Functions of SLDCs are: -

a) Responsible for optimum scheduling and dispatch of electricity within

a State in accordance with the contracts entered into with the licensees

or the generating Companies operating in that State.

b) To monitor grid operation.

c) To keep accounts of the quantity of electricity transmitted through State

grid.

d) To exercise supervision and control over the inter-State transmission

system.

e) Responsible for carrying out real time operation for grid control and

dispatch of electricity within the State through secure and economic

operation of the State Grid in accordance with the Grid standards and

State Grid Code.

3

Chapter 2

Load Scheduling

2.1 LOAD SCHEDULING (as per WESTERN REGION LOAD DISPATCH CENTER)

� For the purpose of scheduling each day would be divided into 96

blocks of 15 minutes duration each and for each block WRLDC would

intimate each SLDC the drawl schedule and to each ISGS the

generation schedule in advance.

� The net drawl schedule of any state would be the sum of the ex-power

plant schedules from different ISGS and any bilateral exchange agreed

with other constituent state in Western region or any other region less

estimated transmission losses.

� The generation schedule to each ISGS shall be the sum of the

requisitions made by each of the beneficiaries, restricted to their

entitlements and subject to the maximum and minimum value criteria

or any other technical constraint as indicated by WRLDC.

� IEGC mandates the grid frequency operation in the band of 49.50 Hz to

50.20 Hz, however for the safety and security of the system the normal

range of desirable frequency is 49.7 to 50.2. The state shall initiate

action to restrict the drawl of its control area from the grid whenever

the frequency falls to 49.7 Hz and do not under drawl whenever the

frequency is above 50.2 Hz provided that when the frequency is higher

than 50.2 Hz, the actual net injection shall not exceed the scheduled

dispatch for that time block. Also when the frequency is greater the

50.2 Hz the ISGS may back down (at their own discretion) without

waiting for an advice from RLDC to restrict the frequency rise.

4

2.2 SCHEDULING AND DISPATCH PROCEDURE

1. By 1000 hrs everyday each ISGS shall advise WRLDC the station-wise

ex-power plant MW and MWh capabilities foreseen for the next day

i.e. between 0000 to 2400 hrs of the following day, at 15 minutes

interval

2. The above information shall be compiled by WRLDC and the MW and

MWh entitlements available to each state during the following day at

15 minutes interval shall be intimated by WRLDC to states by 1100

hrs.

3. After receipt of the information in regard to the availability from

different ISGS, all the states shall review such availability vis-à-vis

their foreseen demand and their own generating capability, including

the bilateral exchanges if any. By 1500 hrs the SLDCs would advise

WRLDC their requisition in each of the ISGS along with the bilateral

exchanges they intend to have with the other state / states and the

estimates of demand / availability in their own states

4. By 1700 hrs WRLDC shall convey to each ISGS the generation

schedule i.e., ex-power plant dispatch schedule and to each state the net

drawl schedule i.e. the schedule at the periphery of the state (after

deducting the apportioned estimated transmission losses).

5. The SLDC/ ISGS may inform the modifications/ changes to be made if

any in the above schedule to WRLDC by 2200 hrs.

6. On receipt of such information and after consulting with the concerned

constituents if required, the WRLDC shall issue the final generation /

drawl schedule to each ISGS/SLDC by 2300 hrs.

2.3 REVISION OF SCHEDULE

1. In case of forced outage of a unit, WRLDC will revise the schedules on

the basis of revised declared capability. The revised schedule will

5

become effective from the 4th time block, counting the time block in

which the revision is advised by the generator to be the first one.

2. In the event of a situation arising due to bottleneck in evacuation of

power due to transmission constraint, WRLDC shall revise the

schedule which shall become effective from the 4th time block,

counting the time block in which the transmission constraint has been

brought to the notice of WRLDC as the first one.

3. Revision of declared capability by generator(s) and requisition by

beneficiary(ies) for the remaining period of the day will also be

permitted with advance notice. Revised schedules / declared capability

in such cases shall become effective from the 6th time block, counting

the time block in which the request for revision has been received in

RLDC to be the first one.

4. If, at any point of time, RLDC observes that there is need for revision

of the schedules in the interest of better system operation, it may do so

on its own and in such cases, the revised schedules shall become

effective from the 4th time block, counting the time block in which the

revised schedule is issued by RLDC to be the first one

6

Chapter 3

Switchyard Design

A switchyard has various components like CT, PT, Relay KIOSK, Isolators

etc. A detailed list of all switchyard components is given in Appendix – II.

Any transmission line originates or terminates at a Bus-bar. One bus bar is

usually connected to more than one transmission lines depending upon its

power handling ability. Different types of bus-bar designs are used based on

requirement. Some of the commonly used bus bar arrangements are One and

a half breaker arrangement, Double Main & Transfer Arrangement, ring

main Arrangement, Mesh Arrangement and Single Bus Bar Arrangement

(with or without Bus sectionalization). The choice of the type of bus bar

arrangement depends on

i. System voltage

ii. Provision of extension with the load growth

iii. Economy keeping in view the needs and continuity of supply

iv. Maintenance possibility with interruption of supply

v. Protection during faults.

In the 400/220 kV switchyard of Power Grid Gwalior, One and a half

Breaker Arrangement is used for 400 kV transmission line and Double Main

& Transfer Arrangement is used for 220 kV Transmission line. Both types of

bus bar arrangements are explained below.

3.1 One and a Half Breaker Arrangement

This type of arrangement needs three circuit breakers for two circuits. The

number of circuit breaker per circuit comes out to be 1�

� , hence the name.

This circuit is preferred in those stations where power handled is large. A

schematic diagram for a one and a half breaker scheme is as shown below.

7

Fig 3.1

CB – Circuit Breaker

One and a Half Breaker Bus bar Arrangement scheme

Circuit 1 Circuit 1

CB 1

CB 2

CB 3 CB 3

CB 2

CB 1

Circuit 2 Circuit 2

Bus 1

Bus 2

8

It is clear that three circuit breakers are used in one dia between the two bus-

bars, Bus 1 and Bus 2 for two circuits emerging out of it. Two such dia are

shown in the figure. Following advantages are associated with this type of

bus bar arrangement.

1. The supply is not interrupted in the event of fault on a bus as either of

the bus can be used to maintain supply and keep the feeders (or

transmission lines) charged.

2. The supply is not interrupted in the event of any fault on a circuit

breaker

3. Possibility of addition of circuits is always there.

The CT and PT are not shown for the sake of clarity.

3.2 Double Main and Transfer

This arrangement is quite frequently used where load and continuity of

supply justifies additional cost. Generally, this system has two main bus-bars

and one transfer bus-bar. However at Gwalior sub-station, two transfer bus-

bars have been used for saving area. Both transfer bus-bars are electrically

connected to each other. Two bus bars are used to increase redundancy.

3.2.1 Working of Double Main and Transfer Bus-Bar arrangement

The two main bus-bars are electrically connected to each other through a bus

coupler. They can be connected or disconnected from each other at will,

depending upon the system requirements and contingencies. Under normal

conditions both the bus-bars remain charged. Two bus-bars are used to

increase redundancy. This scheme provides for one transfer bus. To save area

and to accommodate more feeders, two transfer bus-bars can be used but they

are electrically connected and treated as one for all purposes. Such an

arrangement is present in the switchyard of the Power Grid Gwalior’s

substation.

A single line diagram for the Double main and transfer arrangement is given

on the next page

9

Fig 3.2

CB – Circuit Breaker

Double Main & Transfer Bus bar Arrangement scheme

Transfer

Bus

Coupler

CB

CB CB

Bus 2

Bus 1

Transfer Bus

CB

Feeders /

Transmission

Line

10

As shown in the figure, each feeder comes with only one circuit breaker,

unlike the One and a half arrangement where effectively each feeder had two

circuit breakers. In case a fault occurs on the breaker associated with a feeder,

the continuity of the supply could still be maintained by transferring the

feeder to the transfer bus. For this, firstly the transfer bus is charged by

closing the TBS or the Transfer Bus Coupler and then closing the isolator

connecting transfer bus and the feeder. One transfer bus is used for all the

feeders. However, only one feeder at a time can be put on the transfer bus.

The designing does not permit more than one feeder to be put on the bus at a

time.

3.3 Choice of Bus –Bar Scheme

As already explained above, the choice of bus – bar scheme depends on

various factors like system voltage, protection, redundancy and economy. At

the Gwalior sub – station, the 400 kV line are connected to the one and a half

breaker bus – bar while the 220 kV line are connected to the double main and

transfer bus – bar.

One and a half breaker arrangement is more reliable as each circuit feeder has

effectively two circuit breakers. In can one has some fault or has to be taken

into maintenance, the arrangement would remain equally effective and power

handling capability would remain same. One breaker with each dia can be

safely taken out of service. However the cost is very high as more circuit

breakers are being used. This is the cost of increased protection and ability to

maintain the continuity of supply under faulty conditions. The cost of a 400

kV line tripping and ultimately going out is very high as one such line

normally handles 500 – 600 MW or power. All power would be lost

otherwise.

220 kV line is connected to double main and transfer bus – bar. This

arrangement is more economical than the one and a half scheme as it requires

only one circuit breaker with each circuit. In the event of a fault in any

breaker, the circuit associated with it can be connected to the transfer bus.

11

However only one circuit at a time could be connected to the transfer bus. It

gives reduced protection and restoring supply might take longer in the event

of any fault if it extends to more than one circuit and all circuits except one

would go out of service.

3.4 Connecting the Transformers

The transformers are connected between the bus bars. The power rating of the

transformers depends upon the power to be handled in the bus bars. Using a

transformer with power rating much higher than the average power flowing

through it would lower the power factor.

A total of three 3-ϕ transformers are installed at Gwalior sun-station. All

three are 315 MVA, 400/220 kV, 50 Hz transformers.

A complete switchyard diagram of the 400/220 kV substation is given on the

next page. Two buses are connected via two 315 MVA, 400/220 kV, 50 Hz

transformers for voltage and current transformation.

13

Chapter 4

Switchyard Components

The switchyard at PGCIL, Gwalior substation is different from the typical

substations which have two or three storey control rooms with relay panels

occupying most of the floor space. Control room is more compact than ever

with its size limited to a 7m X 7m room.

Relay panels are housed in KIOSK installed at different panels in the

switchyard.

4.1 Bay

A transmission line when enters in a switchyard in connected to a bay. A bay

is basically a collection of isolator and wave trap connected in series and

CVT, LA, earth switch connected in parallel. In sequence starting from the

transmission line’s last tower and going towards the switchyard, they lie as

follows: LA, CVT, WT, earth switch, and isolator.

LA comes first to protect the switchyard components from being damaged

from the sudden voltage or current surge. Then comes the CVT which, on

high voltage lines, are mostly used for the transmission of communication

signals. They send and receive these high frequency signals. WT are used for

filtering out the high frequency signals from the current as they may be

outside the range of the switchyard components which are mostly designed to

operate at the frequency of or around 50 Hz. Earth switch comes next to earth

the line, if necessary. Isolator is the last component of the bay and is used to

isolate the line from the bus bar.

4.2 Reactors

The reactor is connected to the transmission line to compensate the capacitive

power due to its own capacitance. The load connected to these lines is

otherwise inductive in nature. The rating of the power is decided by the

14

amount of reactive power flowing in the transmission line. For economic

reasons, half of this reactive power is compensated by the reactors. Here the

reactor is connected to the line and not to the bus bar, as shown in the

switchyard SLD. All this data regarding power flow, amount of reactive

power, power factor is calculated during the designing of lines. These

parameters are greatly influenced by the local conditions. For example,

industrial loads generally consume more reactive power then the residential

loads.

4.3 Circuit Breaker

The Circuit Breakers used here are SF6 type circuit breakers. These breakers

have to adhere to certain condition like fault clearing time, switching time,

maximum fault levels to be handled etc. The circuit breakers and accessories

shall conform to IEC: 62271-100, IEC: 62271-01 and other relevant IEC

standards. A circuit Breaker is made of several components like SF6 gas,

insulator, contacts, fuses, sensors, cables, air pipes and support structure.

Each of these components must separately confirm to certain standards of

structural, chemical and electrical stability for the circuit breaker to qualify as

a whole.

Certain conditions for regulation of the SF6 gas based CB’s quality are

1. The design and construction of the circuit breaker shall be such that

there is a minimum possibility of gas leakage and entry of moisture.

There should not be any condensation of SF6 gas on the internal

insulating surfaces of the circuit breaker.

2. In the interrupter assembly there shall be an absorbing product box to

minimize the effect of SF6 decomposition products and moisture.

3. Each Circuit Breaker shall be capable of withstanding a vacuum of

minimum 8 mill bars without distortion or failure of any part.

4.3.1 Technical Parameters of Circuit Breaker

15

The ratings of circuit breaker to be used depend largely on the system

voltage.

4.3.1.1 800 kV Circuit Breaker

Sr. No. Parameter Value

1 Rated continuous current (A) at design ambient

temperature.

3150

2 Rated short circuit current breaking capacity at

rated voltage

40 kA

3 Symmetrical interrupting capability (kA rms) 40

4 Short time current carrying capability for one

second (kArms)

40

5 Operating mechanism or a combination of

these

Spring

6 Trip coil and closing coil voltage 220 V DC

7 Maximum allowable switching overvoltage

under any switching

condition

1.9 p.u.




temperature.

2000 / 3150


rated voltage

40 kA / 50 kA / 63kA

3 Symmetrical interrupting capability (kA rms) 40 / 50 / 63


second (kArms)

40 / 50 / 63

5 Operating mechanism or a combination of these Pneumatic/spring


7 Maximum allowable switching overvoltage

under any switching

condition

2.3 p.u.

16




temperature.

1600/2500


rated voltage

40 kA / 50 kA

3 Symmetrical interrupting capability (kA rms) 40 / 50


second (kArms)

40 / 50

5 Operating mechanism or a combination of these Pneumatic/spring


4.4 Current Transformers

4.4.1 420 kV CURRENT TRANSFORMERS


1 Rated Primary Current 2000/3000

2 Rated short time thermal current for 1 sec. 40 kA/50kA/63kA

3 Rated dynamic current kA (peak) 100 /125/157.5

4.4.2 225 kV CURRENT TRANSFORMERS


1 Rated Primary Current 1600 A

2 Rated short time thermal current for 1 sec. 40 kA for 1 sec./50 kA

for 1 sec

3 Rated dynamic current kA (peak) 100 / 125

17

APPENDIX APPENDIX APPENDIX APPENDIX –––– IIII

ABBREVATIONS

1. CTU – Central Transmission Utility

2. CVT – Capacitive Voltage Transformer

3. ISGS – Inter – State Generating Stations

4. IEC – International Electromechanical Council

5. IEGC – Indian Electricity Grid Code

6. LA – Lightening Arrester

7. NLDC – National Load Dispatch Center

8. PU – Per Unit

9. RLDC – Regional Load Dispatch Center

10. SLD – Single Line Diagram

11. SLDC – State Load Dispatch Center

12. TBS – Transfer Bus Coupler

13. WRLDC – Western Region Load Dispatch Center

14. WT – Wave Trap

18

APPENDIX APPENDIX APPENDIX APPENDIX –––– IIIIIIII

Switchyard Components

1. Circuit Breaker (SF6 type)

2. Current Transformer (both dead tank and

3. Potential Transformer

4. Wave Trap

5. Isolators

6. Inter Connecting Transformer (ICT)

7. Shunt Reactors

8. Lightning Arrestors

9. Capacitive Voltage Transformer

10. Relay KIOSK

11. Series capacitors

12. Power Transformer

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