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Substation theory and practice

An electrical substation is a subsidiary station of an electricity generation, transmission and distribution system where voltage is transformed from high to low or the reverse using transformers. Electric power may flow through several substations between generating plant and consumer, and may be changed in voltage in several steps.

Power substation guides, research papers and studies
Power substation guides, research papers and studies (on photo: Ghana Aboadze 330kV substation)

A substation that has a step-up transformer increases the voltage while decreasing the current, while a step-down transformer decreases the voltage while increasing the current for domestic and commercial distribution. The word substation comes from the days before the distribution system became a grid.

The first substations were connected to only one power station where the generator was housed, and were subsidiaries of that power station.

Equipment in substation

Substations generally have switching, protection and control equipment and one or more transformers. In a large substation, circuit breakers are used to interrupt any short-circuits or overload currents that may occur on the network. Smaller distribution stations may use recloser circuit breakers or fuses for protection of distribution circuits. Substations do not usually have generators, although a power plant may have a substation nearby.

Other devices such as power factor correction capacitors and voltage regulators may also be located at a substation.

Substations may be on the surface in fenced enclosures, underground, or located in special-purpose buildings.

High-rise buildings may have several indoor substations. Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions.

Where a substation has a metallic fence, it must be properly grounded (UK: earthed) to protect people from high voltages that may occur during a fault in the network.

Earth faults at a substation can cause a ground potential rise. Currents flowing in the Earth’s surface during a fault can cause metal objects to have a significantly different voltage than the ground under a person’s feet; this touch potential presents a hazard of electrocution.

Browse guides and papers

Note that not all documents in this section are free to download. Navigate through sub-pages to discover all documents.

Safety operations on medium voltage switchgear

There are five types of hazards associated with the operation of electrical equipment: electrical shock, electrical burns, fire and explosion, heat build up and mechanical hazards. Let’s briefly describe each… Read more

Jun 08, 2013

Safety operations on medium voltage switchgear

Commissioning of HT Electrical System

The Insulation-resistance shall be taken with all the winding earthed, expect, the one being tested. While checking the value, external line, cable and lighting arresters shall be isolated from the H.T… Read more

Apr 24, 2013

Commissioning of HT Electrical System

Power System Earthing Guide

Consideration of appropriate safety criteria (usually an allowable shock voltage) is required for all electrical assets that form part of the network. Consideration should be made for substations (both inside… Read more

Apr 06, 2013

Touch and step voltages around a substation

Fixed or withdrawable MV switchgear?

During the evaluation of purchasing of a new medium voltage switchgear the following main aspects are reviewed: Safety, reliability and economics. In the early part of the 20th century, medium voltage… Read more

Mar 24, 2013

Guide to Grounding Systems

The general purpose of earthing system is to protect life and property in the event of 50/60 Hz faults (short-circuit) and transient phenomena (lightning, switching operations). The question of how… Read more

Jan 23, 2013

Practical guide to electrical grounding systems and applications


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    • Igwe Geoff
      Aug 05, 2021

      i thank you for your desire to extend knowledge.
      Please i am working on an estate that has 64Nos 500kva 11/415kv transformers.
      I divided the transformers into four radial circuits.

      One of the circuits has 26Nos 500kva transformers. Can i use 3 x150mm2 aluminium conductor to connect them considering that cureent for 26Nos 500kva is 679A

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