Switch Yard and Its Equipments(2)

SWITCHYARD & ITS EQUIPMENTS & HVDC BY-ELITE GROUP Amit joshi UPES Dehradun

INTRODUCTION 

The switchyard is a junctionconnecting 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 SCADAsystem 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

 

 

line

Incoming 400 kV

Outgoing line Line11kV

400 kV

Outgoing

Equipments used in Switchyard 1)Bus Bar 2)Insulators 3)Lightening Arrestor 4)Earthing System 5)Control Panel 6)Transformer 7)Circuit Breaker 8)Arching horns 9)SCADA Nodes 10)ROW 11) 12) 13)

BUS BAR 

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 

TYPES OF 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 

1500 ampere bus bar

Insulator 

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

Types of Insulators  



Pin type



Suspension type:design for 11 kv

 

 

Lightning Arrestor   

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

What exactly does a Lightning Arrestor Do?  

•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.

Control Panel 



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.

Earthing Systems 

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. 

Circuit Breaker 

 

A circuit breaker is an automaticallyoperated 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. 



SulphurHexafluoride 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 noninflammable  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 

AC

DC Rectifier

Inverter



Synchronization of stations, as per frequency as well as voltage.  Interconnecting two same frequency line.  Asynchronous DC linkage between the AC connections. 

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

  

Rectifier

Inverter

Equipments used in Converter stations:



1.Shunt Capacitors. 2.Converters.(silicon controlled rectifiers, Thyristors) 3.Smoothing Reactor.

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 RihandDadri 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 !!



