SWITCHYARD LAYOUTS SWITCHYARD LAYOUTS & &

EQUIPMENTEQUIPMENT

Prashanth PittaPrashanth Pitta7244 7244

What is a Switchyard (Sub Station)?It is a switching station which has the following credits :-

Main link between Generating station and Transmission system, which has a large influence on the security of the supply.

Step-up and/or Step-down the voltage levels depending upon the Network requirement.

Switching ON/OFF Reactive Power Control devices, which has effect on Quality of power.

Switching Schemes

1)Single sectionalized bus2)Main and transfer bus3)Sectionalised Main bus with transfer bus4)Sectionalised double main and transfer bus5)Double main and Transfer Bus.6)Double Bus Scheme with single breaker.7)Ring bus8)One and a half breaker scheme9)Double bus, double breaker scheme

I/C Feeders

O/G Feeders

Bus-bar

CB

Isolators

Sectionalized Single Bus-bar system

MAIN & TRANSFER BUS-132 KV SYSTEM AT RSTPS

RING BUS SCHEME

This is not a commonly used arrangement at present. The ring bus arrangement has the advantage of requiring only one breaker for each circuit and any one breaker can be taken out for service for overhaul without de-energising any one of the lines.

A failure of a bus insulator will interrupt service to a single line only. The disadvantage of this arrangement is that it is necessary to trip two circuit breakers to isolate a faulted Line which makes the relaying quite complex.

G1 G2

CBCB

I/C Supply

O/G feeder

Bus

CB

Ring Bus system

O/G feeder

ONE AND HALF BREAKER SCHEME

DOUBLE BUS AND DOUBLE BREAKER SCHEME

Line-2

Line -3Line -1

BUS-I

BUS-II

CBCB

ISOLATORISOLATOR

Comparison of Configurations

Configuration Reliability Cost Area requirement

Single bus Least Least ( 1.0) Least

Main and transfer Least Moderate (1.16) Low area

Double bus, single breaker

Moderate Moderate (1.78) Moderate

Double bus, double breaker

High High (2.0) Greater

Breaker-and-a-half High Moderate (1.57) Greater

Switching schemes/Bus bar arrangement at RSTPS:-

400 KV System:

One and half breaker scheme is provided for 200 MW Generator feeders, 400KV outgoing lines and Tie Transformer feeders.

Double bus double breaker scheme is provided for 500 MW Generator feeders.

132 KV System:

Main and transfer bus scheme is provided

Switchyard equipments and Components:-

Power Transformer (ICT’s)

Switch Gears

Circuit Breaker

Isolators/ Disconnectors

Instrument Transformer (Current Transformers).

Voltage Transformer / Capacitor Voltage Transformers)

Surge Arrestors

Post Insulators and supporting structures

Wave Traps

Reactive Power compensating device

Conductors / Auxiliary Items

I. Transformer:-

Depending up on the function, Transformer and its auxiliaries may

included in switchyard are

Generator Transformer

Inter Connecting Transformer

– Auto Transformer

– Two / Three winding transformer

Single phase / three phase type

Cooling Arrangement (ONAN,OFAF, etc.)

Tap Changer

– On Load Tap Changer

– Off Circuit Tap Changer

II. Circuit Breaker:-

Circuit Breaker is a mechanical device capable of

making, carrying and breaking currents under normal circuit conditions and

making, carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those of short circuit.

Circuit Breaker Classifications:-

I. Depending on Interrupting medium:-(Bulk oil, minimum oil, Air blast, SF6, Vacuum)

II. Depending on Operating mechanism:- Pneumatic Hydraulic Spring Combination of above

CIRCUIT BREAKERS IN RSTPS

MAKE VOLTAGE RATING(KV)

OPER:MEC: ARC EXT: MEDIUM

No. of Breakers

AEG 400 CENTERALISED AIR COMPRESSOR SYSTEM

SF6

10 Nos.

NGEF 220&400 INDIVIDUAL AIR COMPRESSOR SYSTEM

SF6

1 No. (220KV)

BHEL 220&400 HYDRAULIC SF6

21 Nos.

ABB 220&400 INDIVIDUAL AIR COMPRESSOR SYSTEM

SF6

8 + 2

BHEL 132 MOTORISED

SPRING OPERATION

OIL

5 Nos.

TYPE OF SYSTEM

AEG

(COMMON COMP:)

(BAR)

NGEF

(INDIVIDUAL COMP:)

(BAR)

BHEL (HYD) (BAR)

ABB (INDIVIDUAL

COMP:)

(BAR)

OPERATING PR:

37 37 325 31.5

A/R LOCK OUT

34 34 300 30

OPERATION LOCK OUT

30 31 253 23

SAFTEY VALVE OPEN

42 42 ------- 33

CIRCUIT BREAKERS IN RSTPS -Operating parameters

Auto Reclosure of Circuit Breaker:-Auto Reclosure of Circuit Breaker:-

This is only used in EHV Lines.This is only used in EHV Lines.In EHV lines 90% of faults are by lightening, birds or tree branches.In EHV lines 90% of faults are by lightening, birds or tree branches.Fault will occur and can be cleared by opening CB’s at both ends.Fault will occur and can be cleared by opening CB’s at both ends.Helpful in transient faults.Helpful in transient faults.Circuit breakers will close as soon as the arc is extinguished.Circuit breakers will close as soon as the arc is extinguished.Gives continuity in service.Gives continuity in service.High speed single shot auto reclose.High speed single shot auto reclose.Min time for de ionizing is 0.2S, so the time designed for safe arc is 0.3S.Min time for de ionizing is 0.2S, so the time designed for safe arc is 0.3S.CB’s at both ends should close simultaneously to avoid loss of CB’s at both ends should close simultaneously to avoid loss of synchronism.synchronism.

III. ISOLATORS :

Isolator is an off load device provided in conjunction with circuit breaker to disconnect the equipment or the section, which is to be isolated from all other live parts.

The Isolators provided in the switchyard are of central break type.

The operation of isolators can be done from control room (remote) or local. Motorised operation for opening & closing of isolator is provided, however isolators can also be opened & closed manually in the event of non availability of motorised operation.

IV. EARTH SWITCH :

Earth switch is mounted on the isolator base on the line side or breaker side depending upon the position of the isolator.

The earth switch usually comprises of a vertical break switch arm with the contact, which engages with the isolator contact on the line side.

Earth switch is required to discharge the trapped charges on the line or equipment ( under shut own ) to earth for maintaining safety.

Earth switch can be operated only from local either by electrical operation or manually.

V. CURRENT TRANSFORMER ( CT )

Current Transformers are provided to step down the current to low values (1A/5A) suitable for measuring, protection and control Instruments.

Current Transformers also isolate measuring and protective devices from high system voltage.

CTs in the switchyard consist of five secondary cores. Core 1 & 2 are used for bus bar protection, 4 & 5 are for main 1 & 2 protection and core 3 is for measuring instruments.

•Primary voltage is applied to a series of capacitors group. The voltage across one of the capacitor is taken to aux PT. The secondary of the aux PT is taken for measurement and protection.

Secondary voltages (110 V AC) for metering purpose.

VOLTAGES FOR PROTECTIVE RELAYS

VOLTAGES FOR SYNCHRONIZING

DISTURBANCE RECORDERS AND EVENT LOGS

OVERFLUX RELAYS

PLCC

VI. CAPACITIVE VOLTAGE TRANSFORMER:-

VII. Wave Traps:-

Wave trap is used for Protection of the substation equipment from high frequency waves and communication between the Substations

WaveTrap

WaveTrap

Transmission Line

To control roomof S/S-2

To control roomof S/S-1

S/S-1 S/S-2

* Wave trap is used for Protection of the transmission line and communication between the Substations.

* VHF signal is transmitted from one end to the another through the same power line.

* Sends inter-trip signal to the other end CBs so that fault can be isolated at the earliest time.

VIII. SHUNT REACTOR:-

Long lines when lightly loaded, the receiving end voltage raises, due to Ferranti effect.

Shunt Reactors produce lagging MVAR there by control the receiving end voltages during lightly loaded conditions.

Shunt reactors also limit the short circuit fault levels.

Therefore, shunt reactors are provided on both the ends of Nagarjuna Sagar Line – 1 & 2, as the length of these lines being about 267 Km.

IX. SURGE / LIGHTNING ARRESTERS :

Surge Arresters are provided to ground the over voltage surges caused by switching and lighting surges.

Surge Arresters provide leakage path to the ground whenever the system voltage raises above the specified value.

They are equipped with surge monitors which measures the leakage currents and a counter to record the number of surges taken place.

X. BUSBAR :-

Bus bar is an Aluminium having wall thickness of 0.4” rated for 3000 A.

All incoming and outgoing feeders are connected in a schematic way to enable smooth operation and maintenance of equipment with out any interruption to the system.

The optimum power and distance over which the following voltages The optimum power and distance over which the following voltages Adopted are as given below:Adopted are as given below:

MW Kms

66 KV 10-13 50-60

132 KV 40-50 100-120

220 KV 120-150 200-250

400 KV 500-600 500-650