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A brief introduction to essential substation components

This article presents brief introduction to substation. The substation components are discussed. Also the position, working and purpose of each component of substation is presented briefly.

Purpose, Position and function of essential substation components


There are many definitions of substation however most common is” A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions”.

Substation components

  • Transformer (main component)
  • Rest of components are Auxiliary like CB, CT, PT, lightning Arrester etc.

Figure below shows the typical grid station arrangement with minimum number of substation components. There can be more components in the sophisticated substation however the components described in this article are essential and needs to be in every substation.

general arrangement and substation components


Among substation components , Transformer is the main and most expensive one. It is used to change the voltage level for transmission or distribution purpose. It consists of two coils which are electrically isolated but magnetically coupled. One is called primary winding and second is called secondary winding. Between two windings there is iron block called core (in large transformers).

Alternating current in primary winding setup flux, which travels through the core and induce voltage in secondary winding. To vary the voltage on secondary side, the important factor is turn ratio which is the ratio of number of coils in primary winding to the secondary winding. By varying the turn ratio, the voltage level can be varied.

V1/V2 = n1/n2

The function of transformer is to step up and step down the voltage level. Normally based on voltage level, substations are named. For example 132kV Rawat substation, 220kV  Ghatti substation etc.

If the voltage on the secondary side needs to be varied manually; a movable connection from secondary or primary winding is taken which is called transformer tapping. For further details refer to all about transformer tapings.


Types of transformer

On the basis of cooling arrangement, transformer are divided into following

  • Oil Filled Self-Cooled Type
  • Oil Filled Water Cooled Type
  • Air Blast Type

On the basis of core there are two types of transformer

  • Core- Type Transformer
  • Shell-Type Transformer

On the basis of winding types of transformers are

  • Two winding T/F
  • Three winding T/F
  • Auto transformer

Circuit Breakers

Among safety substation components, Circuit breaker is the main component. It is the back bone of electrical safety system. Its basic function is to interrupt the continuity of electrical power in faulty condition. It isolates the faulty part of the network from the healthy part by opening its contacts.

Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.

Circuit breakers are connected with the relays. Relays senses different abnormal condition and gives trip command to the breaker. One of the important properties of circuit breaker is that they have the ability to break very high currents and quench arcs that are produced as a result. Different quenching media are used for this purpose.

Position of circuit breaker:

At least one breaker is installed for the protection of each line on both sides of transformers. However the number vary with the complexity and bus bar scheme.

Types of circuit breaker based on arc quenching media

  • SF6 circuit breaker
  • Oil circuit breaker
  • Vacuum circuit breaker
  • Air Circuit breaker

On the basis of operating mechanism, types of circuit breaker are

  • Hydraulic circuit breaker.
  • Pneumatic circuit breaker.
  • Spring operated circuit breaker.

Bus bar and bus bar scheme

A metallic or copper strip that have a specific voltage and frequency which serves as a connection point for the incoming and outgoing line in substation is called bus bar.

In a substation, there is a copper strip that runs from one end of switch yard to another and having a specific voltage and frequency is called bus bar.

The purpose of bus bar is

  • Providing connection points to incoming lines and outgoing lines.
  • Increases the reliability of substation that is if one incoming is out of service, the whole substation can be energized by energizing the bus using any other incoming line.

 Position in substation

In each substation, there are two bus bars. One is on the HT (high tension or high voltage) side and other is on LT (low tension, low voltage) side.

Incoming lines are terminated on HT busbar. Also transformer incoming  is connected to HT bus bar.  Outgoing lines are connected on LT bus bars. For example in 132kV/11kV substation, there will be 132kV bus bar and 11kV bus bar. The voltage transformers are connected on Bus bars.

Components are arranged in definite manner in a substation such that they provide power with maximum reliability with the provision that power can be switched off manually in normal condition and automatically in faulty condition is called bus bar scheme.

There are a number of bus bar schemes. Most prominent are as follows

  • Single breaker single bus bar scheme
  • Single breaker Double bus bars scheme.
  • Double breaker double bus bar scheme.
  • Main and transfer bus.
  • Ring bus or Mesh bus bar scheme.
  • Breaker and a half (1 1/2 breaker) bus bar schemes

For further details click all about bus bar schemes


A current transformer is an instrument transformer which steps down the current. The current in the secondary winding of CT is proportional to the current in the line.  Since very high current flows in the lines, which can neither be measured directly nor used with any protection devices. Therefore the current is step down, which is then used for protection as well as metering purpose

Purpose in substation

  • In metering, they are used in ammeters, power meters, energy meters etc.
  • In protection, their output is given to the protective relays. Most common are over current, short circuit, earth fault, reverse power etc.

Position in Substaion:

CTs are connected in series in substation. Most commonly they are connected on incoming line, both side of transformer and outgoing feeders.

CT sensing components of substaionIt must be noted that protection CTs are different from measuring CTs. Main difference lies between the saturation level of their core. Measuring CTs are saturated earlier than Protection CTs.

Potential Transformer

Another type of instrument transformer which reduces the voltage level in substation is called potential transformer or voltage transformer. It is uneconomical to design measuring as well as protective equipment to work on very high voltages. Therefore using instrument transformers, voltage is brought to low and safe level which is then given to protective relays and measuring instrument.

The purpose of Potential transformer is

  • Reducing line voltage for measuring purpose. Meters for very voltage level become uneconomical due to sophisticated insulation requirement.
  • Reduces voltage for protection purpose.

Position in Substaion: These substation components  is usually connected to the bus bars and it is always connected in parallel.For further click voltage transformers


Isolator is a mechanical switch which is used to isolate a portion of network. Unlike circuit breaker, it is off load switch and it is manually operated. It is a visual switch that is it is clearly visibly that Isolator is on or off. It is also named as Disconnector switch by some engineers. They are normally connected on incoming lines, bus bars and both sides of transformers.

The basic purpose of isolator is

  • To isolate those parts or components of substation which needed maintenance or repair.
  • To provide visual inspection of connection and disconnection of switch.
  • It is mechanically interlocked with circuit breaker. It means it cannot be open until breaker is not open. Similarly breaker cannot be closed until Isolator is open.11-KV-Isolator

Position in Substaion: It is located on each bus bar, incoming lines and both sides of transformer.

In some cases, an earthling arrangement is coupled with isolator switch. That assembly is called disconnector earthling switch.

 Earthing Switch

Earthing switches are safety devices used to open or to close a circuit when there is no current through them. Grounding must be conducted at both the upstream and downstream sections of the device under maintenance. This is accomplished by earthing switches.

earth switch

The purpose of earthing switch in substation is

  • Provide protection along with isolator. Isolator cannot remove static charges. So for complete protection, system or component is isolated as well as earthed on both ends.
  • It is mechanically coupled with isolator. Earthing switch cannot be closed if isolator is closed (connected). Similarly isolator cannot be on (closed) if earth switch is intact (closed).

Position in Substaion: It usually accompanies isolator. It is connected at each incoming line, at HT bus bar and both sides of T/F.

 Lightening Arresters

A lightning arrester / surge arrester  is a protection device which provides protection against lighting and voltage surges. It consists of high voltage terminal and ground terminal. In normal condition, it offers very impedance (infinity) for current. However during fault, the medium between high terminal and ground is either ionized or offers less impedance path towards ground. As a result high voltage surges are gounded saving electrical equipment.

  • Provide protection from voltage surges
  • Provide protection against lightning.
  • It is accompanied with corona ring which provide protection against corona.
  • Arching horns are also joined with surge arresters which serve as second line defence against lighting.

lightning protection components of substation

Position in Substation: It is connected across expensive substation components like transformers.


Relays are an intelligent substation components which differentiates between normal and abnormal condition. When relay senses faulty condition, it gives a trip command to circuit breaker. Circuit breaker isolates faulty part of system from rest of system. Different types of relays are used substation. A specific number is assigned to each relay by ANSI international. These are universal numbers and always points to that specific relay. Most common are

  • Instantaneous Over current relay (50)
  • Inverse time over current relay. (51)
  • Differential relay. (87)
  • Under voltage relay. (27)
  • Frequency relay. (81)
  • Earth fault relay.(64)


For Other small substation components  click here

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About Syed Noman ud din

Syed Noman ud din is an Electrical Engineer and working in Industry from last 3 years. He writes technical articles for electrical and electronic engineers. He has also published several research publications in renowned international journals.

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