Electrical Substation Equipment

For transferring of electrical power from generating unit to distributing units various types of electrical equipment are required. The equipment like bus bars, isolator, power transformer etc., are assembled together in the electrical substation through which consumers get electrical supply. The main equipment required for the substation installations is explained below in details:

Lightning Arrestor

Lightning Arrestor is the first member of the electrical substations. It protects the substation equipment from transient high voltage and also limits the duration and amplitude of the flow of current. The Lightning arrestor is connected between line and earth, i.e., in parallel with the equipment under protection at the substation.The lightning arrester diverts, the current of the surges to the earth and hence protects the insulation and conductor of the system from damage. The lightning arresters are several types and they are classified on the basis of the duties they perform.

Power Transformer

Power transformers are used for stepping up the voltage for transmission at generating station and for stepping down the voltage for further distribution at main step-down transformer substations. Usually naturally cooled, oil immersed type two winding, three phase transformers, are used for rating up to 10 MVA. The transformer for rating more than 10 MVA, are usually air blast cooled. For very high rating, the force oil, water cooling and air blast cooling may be used.

Such type of transformer operated at full load, and it is disconnected at light load hours. The power transformers are arranged in banks and can be thrown in parallel with other units. Thus, the efficiency of the power transformer is maximum at full load (i.e., with iron loss to full load copper loss ratio 1:1).

Instrument Transformer

Instrument transformer is used to reduced high voltages and currents to a safe and practical value which can be measured by conventional instruments (normally range is 1A or 5A for current and 110 V for voltage). It is also used for actuating the AC type protective relay by supplying the current and voltage by the current and potential transformer. Instrument transformers are classified into two types

  • Current Transformer – A current transformer is a device for the transformation of current from a higher value to a lower value. It is used in parallel with AC instruments, meters or control apparatus so that the meter or instrument coil cannot conveniently be made of sufficient current carrying capacity.
  • Instrument Transformer –  A voltage transformer may be defined as an instrument transformer for the transformation of voltage from a higher value to the lower value.


It is one of the most important elements in an electrical power substation. It is a type of conductor carrying an electrical current to which many connections are made. In order words, bus-bar is a type of electrical junction in which the incoming and outgoing of electrical current take place.

When the fault occurs in the bus- bar, then all the circuit equipment connected to that section must be tripped out to give complete isolation in the shortest possible time e.g. (60ms) so that the damage is avoided to the installation due to heating of conductors.

Wave Trapper

It is placed on incoming lines for trapping the high-frequency wave. The high-frequency wave which is coming from the remote substation disturbs the waves of voltage and current. Wave trapper trips the high-frequency wave and diverts them to the telecom panel.


It is a type of switches which is employed only for isolating the circuit when the current has only been interrupted. The isolator is called disconnected switches operates under no load condition. They are not equipped with arc-quenching devices. They do not have any specified current breaking capacity or current making capacity. In some cases, it is used for breaking the charging current of the transmission line.

Circuit Breaker

The circuit breaker is a type of electrical switches which is used for opening or closing of electrical circuit whenever faults occur in the system. It consists of two moving contacts which are normally closed. Whenever the fault occurs in the system, the relay sends the tripping command to the circuit breaker and hence their contacts are moved apart. Thus, the fault occurs in the system becomes clear.


In electric power stations and large capacity substations, the operation and automatic control circuits the protective relay system, as well as emergency lighting circuits, are supplied by station batteries. Station battery is assembled of a certain number accumulator cell depending on the operating voltage of the respective DC  circuit.

Storage batteries are two types lead acid battery and acid-alkaline batteries. Lead acid battery is most commonly used in power stations and substations because their voltage is high and very cheap low voltage

Capacitor Bank

Capacitor bank consists capacitors connected either in series or parallel. It stored the electrical energy in the form of electrical charges. Capacitor bank draws leading current which increases the power factor of the network and also the power transfer capability of the system increases.


The switch yards, houses transformers, circuit breakers and switches for connecting and disconnecting the transformers and circuit breakers. It also has lightning arrestors for the protection of the power station against lightning strokes.

Indicating and metering Instruments

Ammeters, Voltmeters, Wattmeters, kWh meters, KVARH meters, power factor meters reactive volt-amperes meters are installed in substations to control and maintain a watch over the current flowing through the circuits and over the power loads.

Carrier-current Equipment

Such equipment is installed in the substations for communication, relaying, telemetered or for supervisory control. The equipment is suitably mounted in a room known as carrier room and connected to the high voltage power circuit.


It protects the power system component against abnormal conditions such as faults. The relay is a sensing device which senses the fault, then determines its location and finally, it sends tripping commands to the circuit. The circuit breaker after getting the command from the relay disconnects the faulted element. Relays protect the equipment from damage and hence subsequent hazards like fire, the risk to the life are reduced by removing the particularly faulted section.


It is used in generating stations and substations to fix and insulate the bus bars systems. They may be subdivided into post and bushing type. A post insulator consists of porcelain body and their cap is made up of cast iron. It is directly bolted to the busbars by means of bus-bar clamps. A bushing or through insulator consists of porcelain-shell body, upper and lower locating washes used for fixing the position of bus-bar or rod in a shell.


  1. Richard Reply

    This is a properly detailed definition of the equipment found in a sub-station. Thank you for sharing these electrical power terms for researchers.

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