Substation Overview

In this article, we will discuss an electrical substation overview, including types of substations, AIS & GIS substations, HVDC and HVAC substations.

Electrical Outlet Box
Electrical Outlet Box

The electrical power generation from Hydro, Thermal, Nuclear, Renewable and other generating stations has to be evacuated to load centers. The generation voltage is limited to 11/13 kV due to the limitation of the rotating machinery.

The voltage at generating station needs to step up to higher voltage in order to transmit power to far located load centers. Again, at the load centers, the voltage is stepped down to lower voltages for onwards transmission and distribution. The transmission of power at a higher level reduces the line loss on account of ohmic loss(I2R).

There is a need of controling the load centers for power dispatch from powerhouses to ultimate consumers. For this, a number of Transformation and switching stations are created.  These stations are generally known as substations.

Thus, the Sub-Stations are fitted with the following equipment.

  • Switching devices (breakers, isolators, switches, and contactors.)
  • Control and measuring instruments
  •  Indicating, regulating, and protective devices

Need of Substation

Substations are needed in between long primary transmission lines. The main purposes of the substations are the followings.

  • To avoid switching surges.
  • For easy segregation of faulty zones.
  • For enabling effective protection of the system in the network.
  • To tap and provide different lines extended to different load centers without any step-down facility at the switching stations. 

Functions of Substation

Transform- A substation is able to transform voltage from one voltage level to another voltage level.

Dispatching- Establishment of economic load distribution and load shedding to maintain synchronism.

Control -Controlling the exchange of energy along with Voltage Control: reducing the reactive power flow by compensation of reactive power or by tap-changing.

Measurement -Measuring the voltage, current, and other parameters with the help of CT & PT/ CVT for metering purposes.

Protection-Protection of transmission system along with fault analysis.

Communication-Speech as well as Data transmission via Power Line Carrier/ OPGW for the purpose of network monitoring, control, and protection.

Operation Criteria

  • Ease of operation and control
  • Ease of testing the electrical installation
  • Safety
  • Flexibility
  • Reliability

Types of Substations

In accordance with the service

  • Transformer Sub-station(Step-UP and Step Down)-Depending on the voltage level it can be a Transmission Substation (Above 33 kV) and a Distribution sub-station (Below 33 kV).
  • Industrial Sub-Station-Bulk supply or industrial substation is generally a distribution substation but they are dedicated to one consumer only.
  •  Switching Sub-station -The substations use for switching the power line without disturbing the voltage is known as the switching substations.
  •  Converting Sub-station -In such types of substations, AC power is converted into DC power or vice versa or it can convert the high frequency to lower frequency or vice versa.
  • Frequency Changing Sub-station -This substation is also known as a Back-to-Back substation where two different frequencies of the system are synchronized in the HVDC application.
  • Mobile Sub-Station-For big construction purposes this substation fulfills the temporary power requirement during construction work. 

According to design

  • Indoor Type: The substations constructed under a roof are called indoor-type substations. Generally, 11 kV and sometimes 33 kV substations are of this type.
  • Outdoor Type: Outdoor-type substations are constructed in the open air. Nearly all 132kV, 220kV, and 400kV substations are outdoor-type substation. However, nowadays special GIS (Gas Insulated substations) are constructed for extra high voltage systems which are generally situated under the roof.

According to Mounting

Pole Mounted Substation

  • Pole-mounted substations are the type of distribution substation. It is constructed on two poles, four poles, and even more pole structures, depending on the power-delivering capacity of the substation.
  • In this type of substation, fuse-protected distribution transformer-250 KVA are mounted on poles along with electrical isolator switches.
  • All the equipment is the outdoor type and mounted on the supporting structures of high tension distribution lines.

Foundation Mounted Substation

The heavy equipment can not be mounted on the poles. For heavy equipment, a foundation-mounted substation is used. In the foundation-mounted substation all the equipment are mounted on the foundation and then the substation is embedded by the fence for safety purpose.

Classification of Substations by Operating Voltage

  • HV Substations – Operating voltages between 11 kV and 66 kV.
  • Extra High Voltage Substations – Operating voltages between 132 kV and 400 kV.
  • Ultra High Voltage – Operating voltage is more than 400 kV

Comparison between AIS & GIS

GIS is Gas Insulated substation, and AIS is air insulated substation.


Installing a GIS substation is faster than installing its AIS. GIS systems are compact and have less weight than AIS.


GIS technology is easier to care for and monitor on a regular basis as they offer front instead of rear access.

Arc flashes are rare in Gas Insulated Substations because all the switching elements are fully insulated. The connections parts of GIS are fully insulated and therefore there is almost no chance of touching the live parts with the earth.


GIS switchyards require significantly less maintenance.


With a GIS system, the clearance needed for phase to ground or phase to phase for all equipment is much less than that of an AIS. The GIS only needs one-tenth of the clearance compared to the clearance of AIS.

Environmentally Protected System

GIS  is a good choice to use in hilly areas or desert locations, as it can be enclosed in a building that is environmentally protected from harsh conditions.


  • Alternating current (AC) is the driving force in industries and utilities, but for the long transmission line (more than 400 miles) HVAC transmission is more expensive than that of direct current (HVDC). Because of the frequency, AC transmission line control is more complicated.
  • DC transmission does not have these limitations, which has led to building long HVDC transmission lines such as Talcher –Kolar 500KV HVDC line.
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