Thursday, May 15, 2014

Switchgear Equipment in a Substation

Introduction of Visited Substation

Substation Location:  Division-1, Electric Supply, Amberkhana, Sylhet
Distributed Area: Amberkana, Shahi-Eidgah, Dorga-Mohalla, Mirermoydan, SubidBazar,                                                       Patantula, Ahakalia, Airport, Baghbhari.


A substation may be defined as “assembly of apparatus which transfers characteristics of electrical energy from one to another for example from alternating current to direct current or from one to another.” Every electrical circuit needs a switching device and protective device. They are together called Switchgear. These are located in the substation.

Why we need Sub-station

For economical transfer electrical energy or to meet various demand of the load, we require sub-station.

Fig: an outdoor Substation

Schematic Power System Diagram

Fig: Schematic Power System Diagram

Why we need to know about Switchgear

All electrical equipment and circuits have to be protected by protective devices against damages arising abnormal conditions.

Switchgear equipment which is used in Amberkhana Substation

High & Low voltage switchgear panel

  Ø  Current Transformer
  Ø  Potential Transformer
  Ø  Circuit Breaker
                        o   Vacuum Breakers
o   SF6 Breakers
  Ø  Air break Switch
  Ø  Isolator 
  Ø  Oil switch
  Ø  Relays

Incoming & Outgoing switchgear

  Ø  Surge Arrestor
  Ø  Fuses

Current Transformer (CT)

A current transformer is an instrument transformer, used along with measuring or protective devices, in which the secondary current is proportional to the primary current (under normal conditions of operation) and differs from it by an angle which is approximately zero.

Fig: Current Transformer

Why we use C.T in a sub-station

Current T/F is used for protection and metering purposes. It steps down the current level of the line for following purposes.

   Ø  Measurement: ammeter, kw meter, kwh meter, power factor meter
   Ø  Control and protection: current when exceeds the set value operates the relay which senses the faulty condition.

 Potential Transformer (PT)

Potential Transformer or Voltage Transformer gets used in electrical power system for stepping down the system voltage to a safe value which can be fed to low ratings meters and relays. Commercially available relays and meters used for protection and metering, are designed for low voltage. 

Fig: Potential Transformer 

Why we use P.T in a sub-station

It steps down the voltage level of line for -
   Ø  Measurement: voltmeter, frequency meter , kw meter, kwh meter , power factor meter
   Ø  Control & protection: used in directional over current protection and under  frequency relay operation (48.8 Hz , df/dt relay)

Circuit Breakers

A circuit breaker is equipment which can open or close a circuit under all conditions, such as: no loads, full loads or fault condition.

Circuit breakers inter locking system at normal condition

When closing:
   Ø  Open earthlings switch
   Ø  Close isolator
   Ø  Then close circuit breaker

When opening
   Ø  First to open circuit breaker
   Ø  Next to open isolator
   Ø  Then the earthlings switch to close

Vacuum breakers

Vacuum circuit breakers are used mostly for low and medium voltages. Vacuum interrupters are developed for up to 36 kV and can be connected in series for higher voltages. The interrupting chambers are made of porcelain and sealed. They cannot be open for maintenance, but life is expected to be about 20 years, provided that the vacuum is maintained. Because of the high dielectric strength of vacuum, the interrupters are small. The gap between the contacts is about 1 cm for 15 kV interrupters, 2 mm for 3 kV interrupters.

Fig: Vacuum breakers

Advantage of Vacuum Circuit Breaker
   Ø  no oil is required for are quenching
   Ø  least maintenance
   Ø  compact size
   Ø  light in weight

Sulfur Hexafluoride (SF6) Circuit Breaker

Sulfur Hexafluoride (SF6) is an excellent gaseous dielectric for high voltage power applications. It has been used extensively in high voltage circuit breakers and other switchgears employed by the power industry. Applications for SF6 include gas insulated transmission lines and gas insulated power distributions. The combined electrical, physical, chemical and thermal properties offer many advantages when used in power switchgears. Some of the outstanding properties of SF6 making it desirable to use in power applications are:
   Ø  High dielectric strength
   Ø  Unique arc-quenching ability
   Ø  Excellent thermal stability
   Ø  Good thermal conductivity

Fig: Sulfur Hexafluoride (SF6) Circuit Breaker 

Air Breaker Switch

An "air break switch' is a switching device that uses air the dielectric. Air break switches can be single pole or 'gang operated', and can be operated manually (using either a handle/ratchet mechanism or an insulated 'hook stick' made either of wood or fiberglass). Air break switches are designed for switching under load, but there is often a noticeable arc associated with switching. Air break switches can be found either in substations or out on the distribution system - either pole top or in pad-mounted metal enclosures. Air-break switches at currents up to 750 kV, which are generally used at high-voltage power plants and substations. 
   Ø  It is operated on No Load Only.
   Ø  Rated Voltage :- 12 KV
   Ø  Normal Current :- 400 A

Fig: Air Breaker


   Ø  Air-break switches lie in the fact that they are fireproof and explosion proof.
   Ø  Have rapid connect and disconnect operation, and are relatively simple in design.


   Ø  The presence of equipment for the production and storage of compressed-air supplies


An isolator is a non load-breaking switch, and is provides a visible means of isolating a component, such as a circuit breaker, transformer, etc., from the high-voltage lines, whenever it is necessary to perform maintenance of that component. Normally, isolators come in pairs, with one on each side of the component to be isolated. Isolators are only opened after the load current has been broken using a circuit breaker, and must be closed before the circuit breaker is re-closed. To work on, say, a h.v. circuit breaker, the breaker must be tripped, the isolators on either side must be opened and locked off, temporary earths attached to either side of the circuit breaker- to-work card, detailing the maintenance work, must be issued to the crew by the supervising engineer.

Fig: Isolator

Why we use it in Sub-station

   Ø  Its prime purpose is operator safety.
   Ø  It is physical and visual disconnection of the circuit from power supply.
   Ø  Isolators are usually used on both sides of the C.B in order to repair or replace the C.B.
   Ø  It is used to separate any section from rest part of the power supply.

Maintenance of isolator

   Ø  oiling &greasing of moving parts
   Ø  check contact firmness
   Ø  check open / close trials
   Ø  Earthlings switch

Oil Switch

An oil switch is a high-voltage switch whose contacts are opened and closed in oil. The switch is actually immersed in an oil bath and contained in a steel tank. The reason for placing high-voltage switches in oil is that the oil will break the circuit when the switch is opened with high voltages. A separation of the switch contacts does not always break the current flow because an electric arc forms between the contacts if the contacts are opened in oil. The oil will quench the arc. Further more if an arc should form in the oil. It will evaporate part of the oil because of the high temperature and will partially fill the interrupters surrounding the switch contacts with vaporized oil. This vapor develops a pressure in the interrupters which assists in quenching or breaking the arc by elongating the arc.

Fig: Oil switch


Relay is a device when detects the fault supplies information to the breaker for circuit interruption.

Working principal of relay

  Ø  Under normal load current a small current flows in the relay operating coil so breaker remains unchanged.
  Ø  When fault occurs a large amount of current flows in the relay operating coil and it energized the trip coil of the breaker. Hence the breaker is open.

Fig: Scheme of Relay

Why we use it in Sub-station

   Ø  Reliability
   Ø  Sensitivity
   Ø  Selectivity
   Ø  Quickness
   Ø  Non interference with future extension
   Ø  Protection for transformer

Surge arrester

Lightning Arresters or Surge Arresters are always connected in Shunt to the equipment to be protected; they provide a low impedance path for the surge current to the ground. Wave trap is a parallel tuned inductor - capacitor 'tank' circuit made to be resonant at the desired communication frequency. It is the effort to utilize the same transmission line between two substations for the purpose of communications. At this communication frequency the tank circuit provides high impedance and does not allow passing through them & onto the substation bus & into transformers.
   Ø  Lighting arrestor is used for lightning surge protection.
   Ø  Lighting arrestor is used in line side as well as T/F HV & LV side.

Fig: Surge arrester


Fuse is a safety device. It is connected in series with the circuit and protects the electrical apparatus and equipment from damage, when excess current flows. It is one of the simplest protective devices and is used as circuit interrupting device under short circuit condition.

Fig: Fuse

Types of Fuse element

   Ø  Rewire able fuse
   Ø  Cartridge type fuse
   Ø  Drop out fuse
   Ø  High capacity H.R.C. fuse
   Ø  High voltage H.R.C. fuse

Drop Out Fuse Specification

   Ø  11 KV, 50 HZ.
   Ø  Rated Voltage :- 12 KV
   Ø  Rated Normal Current:- 100 A


   Ø  Training Institute for Chemical Industries(TICI)
o   Electrical Substation & Distribution System
o   Electrical Protective Devices & Electrical Symbols



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