Switch Yard and Its Equipments(2)

SWITCHYARD & ITS EQUIPMENTS & HVDC

BY-ELITE GROUPAmit joshi

UPES Dehradun

INTRODUCTION The switchyard is a junction connecting the

Transmission &Distribution system to the power plant.

Switchyard consists of the air insulated aluminium bus type and of high voltage SF6 –insulated dead tank circuit breakers arranged in a ring bus configuration.

Each circuit breaker are equipped with a no-load breaker, air insulated, disconnect switch on each side.

An isolating disconnect switch are installed in each generator transformer connection to the bus.

In switchyard, a power transformer is used to step up or step down the voltage.

Current and Voltage transformers are located at points within the switchyard to provide for metering and relaying.

Control, protection and monitoring for the switchyard will be located in the switchyard relay room of the electrical building.

All protection and circuit breaker control will be powered from the station battery-backed 220V DC system.

A grounding grid are provided to control step and touch potentials .

Lightning protection are provided by shield wires for any overhead lines- Lightning arrestors.

Interface with SCADA system are provided. The communication between the facility switchyard and the control building is facilitated.

Revenue metering are provided on the outgoing

lines, recording net power to or from the switchyard.

OVERVIEW

11 kV Bus –Bar

Lightning Arrestor Isolator

11 kV Circuit Breaker

Current Transformer

Power Transformer 11 kV/ 400 kV

Incoming Outgoing line Outgoing line Line 11kV 400 kV 400 kV

1) Bus Bar2) Insulators3) Lightening Arrestor4) Earthing System5) Control Panel6) Transformer 7) Circuit Breaker8) Arching horns9) SCADA Nodes10) ROW

Equipments used in Switchyard

Used to interconnect the loads and sources of electrical power

It connects incoming and outgoing transmission lines

Also connect generator and main transformer in power plant

Material used: Copper or Aluminium Size of bus bar to determine max. amount

of current passed

BUS BAR

Rigid bus-bars:used for low,md. And high voltages

Strain bus bars:used for high voltages

Insulated phase bus bars:used for mdium voltages

Sulphur hexaflouride bus bar:used for medium and high voltage system

TYPES OF BUS BAR

1500 ampere bus bar

Supported the poles and towers in such a way that currents from conductors do not flow to earth through these supports

Insulator

Pin type

Suspension type:design for 11 kv

Types of Insulators

Used on power system to protect the system from damaging effect of lightning

Lightning Arrestor

•It Does not Absorb the Lightning

•It Does not Stop the Lightning

•It Does Divert the Lightning to Ground

•It Does Clamp (limit) the Voltage produced by the Lightning

•It Only protects equipment electrically in parallel with it.

What exactly does a Lightning Arrestor Do?

Control panel mostly consists of meters and protective relays. The meters include ammeter, voltmeter, wattmeter, energy meter etc. The relays include fuse failure relay, auto reclose relay, check synchronizing relay, auxiliary relay and transformer relays like OLTC out of step, winding temperature alarm, oil temperature alarm. The trip indicators included are CB SF6 gas density low, CB Air pressure low, VT fuse fail alarm, CB pole disc trip, carrier signal received, back up protection, auto reclose lock out, control DC supply fails, distance protection inoperative, carrier out of service, distance protection trip etc.

Control Panel

Earthing is to be provided in substations due to following reasons:-

To provide a means to carry electric current into the earth under normal and fault conditions , without exceeding any operating and equipment limits or adversely affecting continuity of service .

To assure that a person in the vicinity of grounded facilities is not exposed to the danger of electric shock.

Earthing Systems

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 function is to interrupt continuity, to immediately discontinue electrical flow.

In switchyard High Capacity Circuit Breakers are used.

In 400/220KV substation Power Grid there is SF6 circuit breaker system.

Sulphur Hexafluoride Circuit Breakers

The SF6 is an electro-negative gas and has a strong tendency to absorb free electrons.

The contacts of the breaker are opened in a high pressure flow of SF6 gas and an arc is struck between them.

The conducting free electrons in the arc are rapidly captured by the gas to form relatively immobile negative ions.

This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc.

Advantages of using SF6 Circuit breaker Very short arcing time. Can interrupt much larger currents. Noiseless operation due to its closed gas

circuit. No moisture problem. No risk of fire since SF6 gas is non-

inflammable Low maintenance cost. No carbon deposits so that tracking

and insulation problems are eliminated.

Disadvantages of using SF6 SF6 breakers are costly due to high cost of

SF6. SF6 gas has to be reconditioned after every

operation of the breaker, so additional equipments are required.

Instrument Transformers

Current transformers, together with potential transformers (PT), are known as instrument transformers.

It controls excess and low current It also helps in steping up or down the

potential. Since the measuring instruments and

protective devices are designed for low voltages (generally 110V) and currents (about 5A). Therefore, they will not work properly if mounted directly on power lines.

Current Transformer

The current transformer is used to measure the very high current passing through the bus.

It step downs the current and measurements are taken in the control room the ratings of CT is based on the ampere

These CTs are connected to the control room through cables.

Voltage Transformer There is a step down transformer, which

step down the high voltage to a value that can be measured using the measuring instruments in the control room.

This has an additional core for the carrier communication.

The CVT are connected between phase and ground in parallel to the circuit.

Arcing Horns

Arcing horns are for the protection of the insulators in case of high voltage, which it cannot stand.

They are two metal rods fitted at the top most and bottommost parts of the insulator.

During high voltage insulators can't resist this and cracks may be developed. In order to avoid these arcing horns are provided. They conduct the high voltage to the ground and protect the insulator.

SCADA nodes SCADA stands for supervisory control and

data acquisition It basically stores data for current and give

alarm to Power Line Communication Carrier(PLCC) which further is transmitted to National Load Despatch Centre(NLDC) ,RLDC.

ROW(Right of Way) It is the path to access devices of a

switchyard Physical path IT equipments

HVDC

Aim of Electric power supply Supply of required amount of power to all

consumers over the entire geographical area at all the time continuously.

Supply energy at lowest cost. Maximum possible coverage of geographical

area. Maximum Security of supply and minimum

fault duration. Supply of power within the targeted limit of

frequency. (in case of AC supply) Supply within specified limits of voltages.

Why DC over AC? Power Transfer through an AC transmission

link is given by: Pac= ( |V1|.|V2|.sinδ)/X Reactive power comes in play. Power Transfer through the AC line can’t be

controlled easily, quickly and accurately. Losses are high as the reactive power is

high. Voltage drop will be more.

HVDC High-voltage, direct current

(HVDC) :bulk transmission of electrical power.

For long-distance distribution, HVDC systems are less expensive and suffer lower electrical losses.

For shorter distances, the higher cost of DC conversion equipment compared to an AC system may be warranted where other benefits of direct current links are useful

AC DC Ac

Synchronization of stations, as per frequency as well as voltage.

Interconnecting two same frequency line. Asynchronous DC linkage between the AC

connections.

Rectifier Inverter

Economic Consideration

+Ve for HVDC: Lesser conductors are used as compare to

AC. Lesser losses so better quality. Long distances covered. Simpler design of towers. HV so more security.

Economic Consideration

-Ve for HVDC: Not for Short distance. Higher Initial cost.

Economic Consideration(for overhead lines)

HVDC

AC DC AC

Equipments used in Converter stations:

1. Shunt Capacitors.2. Converters.(silicon controlled rectifiers,

Thyristors)3. Smoothing Reactor.

Rectifier Inverter

Transmission Models Monopolar: The line has one energized

conductor with the return path through the earth.

Bipolar: A bipolar transmission gives two circuits which are almost independent of each other. Bipolar mode has one conductor at a positive potential with respect to ground and a second conductor operating at negative potential of the same magnitude.

Back-to-back station is a plant in which both static inverters and rectifiers are in the same area, usually in the same building. The length of the direct current line is kept as short as possible.

coupling of electricity mains of different frequency.

coupling two networks of the same nominal frequency but no fixed phase relationship.

Advantages Of HVDC Economical for long distance bulk

transmission. Greater power per conductor and simpler

line construction. Ground return is possible. No charging current and skin effect. Less voltage regulation problem because

only IR drop is involved.

Advantages cont. Easy reversibility and controllability of power

at DC link. DC line is an asynchronous or flexible link it

can interconnect two rigid systems operating at different frequencies.

For a single DC line b/w 2 converter stations, circuit breakers are unnecessary since control of the converters can be used to block current flow during faulty conditions.

Disadvantages

Installation of complicated converters and DC switchgear is expensive.

Converters require considerable reactive power.

Harmonics are generated which requires filters.

Converters do not have overload capacity. Lack of HVDC circuit breakers hampers

multi terminal or network operation.

Disadvantages Cont. There is nothing like DC transformer which

can change the voltage level in a simple way.

Reactive power required by the load is to be supplied locally as no reactive power can be transmitted over a DC link.

Contamination of the insulators in polluted areas or along the sea coast. Pollution affects DC more then AC.

Present Scenario India has been a pioneer developer of HVDC

since 1990 when the 1000 MW Rihand-Dadri line was commissioned in UP.

Since then many 500 MW lines have come up.

The 2000 MW Talchar-Kolar link is the biggest so far and spans from states ; Orissa, AP, Tamilnadu and Karnataka.

Future Trends R & D work is under way to provide a better

understanding of the performance of HVDC links to achieve more efficient and economical designs of the thyristor valves and related equipments .

Controllers would be more & more microprocessor based which can be modified or upgraded without requiring hardware changes.

Future Trends In the near future, it is expected that fibre

optics system would be used to generate firing signals & direct light fired thyristors would be employed for HVDC converters.

THANK YOU !!