�TIPS ENGINEER ZONE ��Substation Component

TIPS ENGINEER ZONE

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• What is a Switchyard ?

It is a switching station which has the following credits :

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

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

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

SWITCHYARD EQUIPMENTS

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• Equipments commonly found in switchyard :
• Lightening arrestor
• Current transformer
• Voltage transformer
• Power transformers / I.C.T.
• Bus bar and clamp fittings
• Support structure
• Isolators
• Circuit Breaker
• Wave traps
• Earthing switch

• Functions of various equipment :

• Transformers :

- Transforms the voltage levels from higher to lower level or vice versa, keeping the power constant.

• Circuit breakers :

- Makes or automatically breaks the electrical circuits under Loaded condition.

• Isolators :
• Opens or closes the electrical circuits under No-load conditions.
• Instrument transformers :
• For stepping-down the electrical parameter (Voltage or Current) to a lower and safe value for Metering and Protection logics.
• Earth switch :
• Used to connect the charged body to ground to discharge the trapped charge to have a safe maintenance zone.

Functions of various equipment :

• Lightning arrestors :

- Safe guards the equipment by discharging the high currents due to Lightning.

• Overhead earth wire :
• Protects the O/H transmission line from Lightning strokes.
• Bus bar :
• Conductors to which a number of circuits are connected.
• Wave Traps/Line traps :
• Used in PLCC circuits for Communication and Protection of Transmission lines
• Reactive Power control devices :
• Controls the reactive power imbalance in the grid by switching

ON/OFF the Shunt Reactors, Shunt Capacitors etc.,

• Current Limiting Reactors :
• Limits the Short circuit currents in case of faulty conditions.

Switchyard One line Diagram

Transfer Bus 400 KV

Main Bus II

Main Bus I

CB

CB

CB

GT 20.5/400KV

Gen Bay

Feeder Bay

Bus

Isolator

Transfer Bus

Bay

• Bus Switching Schemes :

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Bus Bar Schemes

• Single Sectionalised bus
• Main & Transfer bus
• Sectionalised Main bus with Transfer bus
• Sectionalised Double Main & Transfer bus Breaker Schemes
• Ring bus
• One and Half breaker
• Double bus Double breaker

* Single Sectionalized Bus-bar system :

I/C Feeders

O/G Feeders

Bus-bar

CB

Isolators

* Main & Transfer Bus-bar system :

I/C Feeders

Transfer Bus

CB

Isolators

Main Bus

Bus Coupler

* Ring Bus system :

I/C Supply

O/G feeder

Bus

CB

* One and Half Breaker scheme :

Tie CB

Main 1

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Main 2

Feeder 1

Feeder 2

• What is a Switchgear ?

“The apparatus used for Switching, Controlling and Protecting the Electrical Circuits and equipment”.

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• Need of Switchgear :

* Switching during normal operating conditions for the purpose of Operation and Maintenance.

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* Switching during Faults and Abnormal conditions and interrupting the fault currents.

Basic relay circuit scheme

Supply from source

Equipment

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RELAY

Battery

Trip Coil of Breaker

CB

CT

Circuit Breakers

• Main Parts of a Circuit Breaker :

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• Fixed Contact

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• Movable Contact

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• Operating Mechanism

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• Arc extinguishing medium

• Fault clearing process :

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During any Fault…..

• Fault impedance will be low, so fault current will increase and relay senses this increase in current.
• Relay contacts closes and sends trip signal to circuit breaker and the trip coil of the circuit breaker will get energized.
• Operating mechanism of the circuit breaker will operate and separate the contacts.
• Arc will be initiated between the contacts and it is extinguished by suitable methods.

• Arcing phenomenon :

- When a fault occurs, heavy current flows through the contacts

of the circuit breaker before they are opened by the protective system.

• At the instant when the contacts begin to separate, the contact area decreases rapidly and current density (I/A) increases and hence rise in temperature.

-The heat produced is sufficient to ionise the medium between the contacts. This ionised medium acts as conductor and an arc is struck between the contacts.

• The potential difference between the contacts is very small and is sufficient to maintain the arc.
• The current flow depends upon the Arc resistance.

is capable

• Few definitions :

Breaking Capacity Max fault current at which a CB

of breaking a circuit.

Making Capacity Max current a CB can withstand if it closing on

existing Short circuit.

Restriking Voltage –After the arc has been extinguished, the voltage across the breaker terminals does not normalize instantaneously but it oscillates The transient voltage which appears across the breaker contacts at the instant of arc being extinguished.

Recovery Voltage –Power frequency voltage which appears across the breaker contacts after the arc is finally extinguished and transient oscillations die out.

• Events/Timings during fault clearing process :

Fault clearing Time

Relay time

Circuit breaker Time

Instant

Of Fault

Closure of

Trip Circuit

Final arc Extinction

Circuit breaker Time

Closure of

Trip Circuit

=

+

=

=

to

to

Relay time

• Various types of CBs :

1. Miniature CB
2. Air Break CB
3. Air Blast CB
4. Oil CB
5. SF6 CB
6. Vacuum CB

Bulk Oil CB

Minimum Oil CB

• Air Break CB :

• Air Blast CB :

ABCB- Principle of arc quenching

• Bulk Oil CB :

• Minimum Oil CB :

• SF6 CB :

1. Op mechanism
2. Interrupter
3. Support
4. Op rod
5. Linkage
6. Terminals
7. Filters

8. Puffer cylinder
9. Nozzle
10. Fixed position
11. Fixed contact
12. Moving contact
13. Gas inlet

* Inert gas with high dielectric strength.

• Colour less and odour less.
• Non-toxic and non- inflammable.
• Sf6 is blown axially to the arc, hence it removes the heat by axial convection and radial dissipation. As result the arc dia reduces and comes to zero at current zero.
• Gas pressure in the chamber is at 5 ksc.
• SF6 is filled at a pressure of 12 ksc in the tank and maintained by means of an individual or a common compressor.
• The decomposition products of arcing are not explosive hence no chance of fire.

Advantage of SF6

• Vacuum CB :

Advantage of vacuum CB

• Used up to 66 KV.
• Vacuum is of the range of 10ˉ6 to 10ˉ8 torr.
• Vacuum is highly dielectric, so arc can’t persists.
• Separation of contacts causes the release of metal vapour from the contacts, the density of vapour depends on the fault current.
• At current zero the vapour emission will tends to zero and the density will becomes zero and dielectric strength will build up and restriking will be prevented.
• No emission to atmosphere, hence pollution free.
• Non- explosive and silent operation.
• Compact size.

Disadvantages

* High initial cost.

* Surge suppressors (R or RC combination) are to be connected at load side for limiting switching over-voltage while switching low pf loads.

MOVING PORTION

(Mechanism cover removed)

MECHANISM AND ITS OPERATION :

GEAR BOX

MOTOR

CLOSING

COIL

TRIPPING COIL

MECHANISM

SHAFT

CLOSING SPRING

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LIMIT

SWITCH

TRIPPING SPRING

Vacuum CB Operation

• ISOLATE

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• TEST

On Off

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• SERVICE

On Off

SAFETY INTERLOCKS:

The various interlocks provided on the breaker ensure that-

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(A) VCB Truck cannot be racked in inside the panel

unless:

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1. Door is closed .
2. Breaker is in tripped condition.
3. Plug & socket are engaged properly.

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(B) VCB Truck can not be racked out from panel unless:

1. Door is closed.
2. Breaker is tripped .

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• Door cannot be opened unless the breaker is in tripped condition .

Breaker can be closed only in ‘SERVICE’ or in ‘ TEST’ position.

Comparison of insulating methods for CBs

features for different types of CBs

AUTO RECLOSURE

• Over 80% of the faults are transient in nature.
• Designed to carry out one or two fast closing operations followed by automatically locked open.

AUTO RECLOSURE

FEATURES OF ISOLATORS

• OPERATES UNDER NO LOAD CONDITION
• INTERLOCKED WITH BREAKERS AND EARTHSWITCHES
• MOTOR DRIVEN AND HAND DRIVEN
• LOCAL AS WELL AS REMOTE OPERATION POSSIBLE
• ISOLATES SECTIONS FOR MAINTENANCE
• USED TO SELECT BUS BARS
• SELECT CT FOR BUS BAR PROTECTION

400kV Isolator

Electric Arc in an isolator

Isolator

FEATURES OF EARTH SWITCHES

• USED TO GROUND SECTIONS REQUIRED FOR MAINTENANCE
• GROUND INDUCTION VOLTAGES
• INTERLOCKED WITH BREAKERS AND ISOLATORS
• CAN OPERATE FROM LOCAL ONLY
• MOTOR DRIVEN AS WELL AS HAND DRIVEN
• SAFETY DEVICE

Source

CB

Earth S/W

Isolator

Isolator

Load

Lightning Arrestors

• Purpose :

- To discharge the high voltage surges in the power system due to lightning to the ground.

• Apparatus to be protected :
• Overhead lines………Earth/Ground wires
• HV equipment………LAs

* Substation…………...Lightning Masts, Earth wires

• Types :

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Rod gap LA :

Insulator

Equipment

body

Rod gap

• Gap length is such that

the break-down occurs at 80% of the spark voltage

• After the surge, the arc in the gap is maintained by the normal supply voltage.

400kV Surge Arrester

Metal Oxide surge arrestor

• The gapless arresters are based on metal oxide (MO) resistors, which have an extremely non-linear V/I characteristic and a high energy-absorption capability.
• They are known as metal oxide surge arresters, MO arresters for short.

Current Transformers

• Primary always connected in series with the line, thus the primary current is not affected by the burden of CT secondary cores
• Primary is adequately insulated from the secondary which is generally at ground potential.
• Unlike other transformers, primary winding current in CT is independent of the secondary winding load. The primary winding is designed to have very low impedance (i.e. one turn) & has negligible effect on the main current

• The voltage across the secondary is very small. It is minimum when the secondary is short circuited and maximum when open circuited.

Wave Trap

Wave Trap

• Coupling capacitor with line matching units – These are high pass Filters ( carrier frequency 50 KHZ to 500 KHZ ) pass carrier. Frequency to carrier panels and power frequency parameters to switch yard.

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Wave Traps.- Low pass filter when power frequency currents are passed to switch yard and high frequency signals are blocked.

Wave Trap

Transmission Line

Wave

Trap

Wave Trap

• 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.

To control room

of S/S-2

To control room

of S/S-1

S/S-1

S/S-2

36 KV SF6 Gas Insulated Switchgear

245 KV GIS

SPACE COMPARISON

ISOLATOR/EARTH SWITCH

245KV CIRCUIT BREAKERS

BUS PT

CT,ISOLATOR,EARTH SWITCH

�TIPS ENGINEER ZONE ��Substation Component

TIPS ENGINEER ZONE

www.tipsengineerzone.in

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