General Overview

The assembly of apparatus used to change some characteristics of electric supply is called a substation. Substations are important part of power system. The continuity of supply depends on the successful operation of substation.

A substation may be called upon to change voltage level or improve power factor or convert A.C. power into D.C. power. According to the service requirement, substations may be classified into:

i)            Transformer Substation: which change the voltage level of electric supply

ii)           Switching Substation: they are meant for performing switching operation of power lines without transformation of voltage level.

iii)            Power factor correction Substation: these types of substations include the synchronous condensers required for improving power factor

iv)            Frequency changer Substation: sometimes power at frequency different from normal frequency is required for connecting two networks operating at two different frequencies. The substations converting normal supply frequency to other frequency is called frequency changer substation.

v)             Converting Substation: the power generated, transmitted and distributed employing alternating currents. For some purpose such as battery charging, electric traction, electric welding; D.C is necessary. Substations converting A.C to D.C or vice versa are known as converting substations.

Transformer substations can be classified into:

1. Step up substation: where the stepping - up of voltage takes place. It is usually located after the plant.
2. Bulk power substation: receives the power from transmission system (132 or 220 or 500 KV) and transforms it to subtransmission systems (66KV).
3.  Distribution substation: receives power from the subtransmission circuits and transforms it to primary feeder voltage (11 or 22 KV).

Another classification is based on where the substation equipment will be housed. This classification is:

i.      Outdoor substation: it needs more protection against pollution than the indoor substation

ii.     Indoor substation: it has less trouble with pollution flashover on insulators but is more expensive than outdoor substation.

iii.    Gas Insulated Substation (GIS).

The advantage of GIS:

• Due to insulation (sulpher hexafluoride SF6), the area of the GIS plan is much less than the area of conventional type, hence it is very suitable for the high population regions where the cost of land is very expensive. Also the clearance between each unit and other will decrease hence the area decreases.
• There is a possible leakage of the gas, but its acceptable if occurs in range 1% for one kit and 5% for all kits in substation annually.
• All parts of substation inside depth which is filled by SF6, and there is a manometer each distance to measure the pressure inside the ducts if it decreases than that certain limit and steps out the range then it will close the tripping coil circuit of C.V. hence, the C.B. is opened to protect the system.

Yet the GIS type is more expensive than other types and needs continuous check that the SF6 level is within the acceptable ranges.

Anyway, the following points are the requirements of a good substation:

§  It should be located at a proper site. It is better to be located at the load center as much as possible.

§  Circuits are designed so that failure chances become small.

§  In case of fault; protection switchgear should work correctly

§  Fire extinguishers are installed

§  Reactors to limit the short circuit current are used

§  It should be easily operated and maintained.

§  It should involve minimum capital cost.