Classification of Substations

Classification of Substations

AU: May-12 There are several ways of classification of substations which are as shown in the Fig. 8.2.1.

Most of the types of substations as shown are self explanatory.

The static substations are stations from where power is transmitted to various load centres. They are either step up or primary substations or secondary substations which fed power directly to load and remaining power is transmitted. The converting substations change a.c. power to d.c. power with suitable equipments for giving supply to traction, welding or electroplating.

The extra high voltage transmission substations come under the category of transformer substations which change voltage levels. They receive power at some voltage and delivery of power is made at some other voltage. Transformer is their main component. High voltage substations operate with voltage level between 11 kV and 66 kV. Extra high voltage substations operate in the voltage range of upto 400 kV. The voltage ranges above 400 kV are handled by ultra high voltage substations.

The distribution substations convert voltage levels to low value for the use of electrical power to consumers. The distribution is carried by 3 phase, 4 wire system with line to line voltage of 400 volts and line to phase voltage of 230 volts. The grid substations are those from where bulk power is transmitted from one point to other in the grid. Any disturbance in these substations may cause failure to the grid.

The industrial substations supply power to the individual industrial consumers who need bulk power.

For improving the power factor of the system, power factor correcting substations are installed. As the transmission line is loaded, voltage decreases while it increases when load on the line is lowered. Thus due to inductance and capacitance of line, large variations in voltage and power factor occur. Thus at the receiving ends, synchronous condensers or other power factor improving devices are installed.

The substations which change the supply frequency is known as frequency changeover substation. In industrial applications frequency other than supply frequency is required which can be obtained from these substations.

According to constructional features, the substations are divided as indoor, outdoor, pole mounted or underground.

In indoor substation, the equipments of substation such as circuit breakers, switching fuses, instruments etc are installed indoor for having better economy. The voltage handled by these substations is upto 11 kV.

These substations consist of series of open and enclosed chambers or compartments. The main equipment of given installation is arranged in these compartments. These are further subdivided as substations of integrally built type where apparatus is installed on site. The cell structures are constructed of concrete. Substations of composite built type contains assemblies which are prefabricated but installed on site. The compartments of such substation take form of metal cabinets. The unit type factory fabricated substations are built in workshop or factory and transported to the site. Only incoming and outgoing power circuits are to be made. Fully enclosed metal clad cabinets are used.

For voltages beyond 66 kV, outdoor substations are used as the clearances required between conductors and space required for other apparatus is so large that indoor installation proves to be uneconomical. It is further divided into pole mounted or foundation mounted substations.

The pole mounted substation is meant for distribution. It is the cheapest form of substation as no building work is involved. These substations are erected for mounting distribution transformers upto 250 kVA capacity. All the equipments are outdoor type and are mounted on H-type pole or 4 pole structure.

The incoming line (normally 11 kV) is supplied to the transformer through gang isolator and fuses. The mechanically operated triple pole switch is used for switching on or off of H.T. (High Tension) line. The lightning arrestors are installed to protect the substation against lightning strokes. The oil circuit breaker automatically isolates the transformer in case of fault. This substation is erected in thickly populated location. The maintenance cost of these substations is low.

The foundation mounted substations are built entirely in open and in such substations all the equipments are assembled into one unit usually enclosed by a fence for safety reasons. Substations for primary and secondary transmission and for secondary distribution (Above 250 kVA) are foundation mounted outdoor type. As the equipments required are heavy, the site selected for these substations must have good access for transportation.

Underground substations are installed where there is shortage of land or land price is very high. The design of these substations requires critical considerations. Its size should be as minimum as possible with reasonable access to personnel and equipments. It should have provision for emergency lighting, good ventilation, indication of excessive rise in temperature to disconnect H.T. supply and protection against fire.

In addition to above types there are special types of substations which are as mentioned below :

1) Mining substations - These are underground and are required for very special purposes.

2) Mobile substations - This is also a special purpose substation required mainly for temporary requirements such as for construction purposes.

3) Cinematograph substations - It is used to meet specific requirements.

Review Questions

1. Explain in brief about the classification of substation.

2. Explain different types of substations.