Classification of sub-stsations:

There are several ways of classifying sub-station.  However the two most important way of classifying them are.

• According to service requirement
• According to constructional features

According to service requirement:

          According to service requirement sub-station may be classified into.

Transformer sub-station

         Those sub-station which change the voltage level of electrical supply are called TIF s/s.

Switching sub-station :   

          These sub-station simply perform the switching operation of power line.

Power factor correction S/S : 

          These sub-station which improve the p.f. of the system are called p.f. correction s/s. these are generally located at receiving end s/s.

Frequency changer S/S :

           Those sub-stations, which change the supply frequency, are known as frequency changer s/s.  Such s/s may be required for industrial utilization

Converting sub-station :

              Those sub-station which change a.c. power into d.c. power are called converting s/s ignition is used to convert AC to dc power for traction, electroplating, electrical welding etc.

Industrial sub-station:

             Those sub-stations, which supply power to individual industrial concerns, are known as industrial sub-station.

According to constructional features :

             According to constructional features, the sub-station are classified as :

1)Outdoor Sub-Station :-

             For voltage beyond 66KV, equipment is invariably installed outdoor. It is because for such Voltage the clearances between conductor and the space required for switches, C.B. and other equipment becomes so great that it is not economical to installed the equipment indoor.

2)Indoor Sub-station :-

             For voltage upto 11KV, the equipment of the s/s is installed indoor because of economic consideration.  However, when the atmosphere is contaminated with impurities, these sub-stations can be erected for voltage upto 66KV

3)Under ground sub-station :-

                In thickly populated areas, the space available for equipment and building is limited and the cost of the land is high.  Under such situations, the sub-station is created underground.The design of underground s/s requires more careful consideration.

• The size of the s/s should be as minimum as possible.
• There should be resonable access for both equipment & personal.
• There should be provision for emergency lighting and protection against fire.
• There should be good ventilation

4) Pole-mounted sub-station :-

                    This is an outdoor sub-station with equipment installed overhead on H.pole or 4-pole structure.  It is the cheapest from of s/s for voltage not exceeding 11KV(or 33KV in some cases).  Electric power is almost distributed in localities through such sub-station. The 11KV line is connected to the T/F (11KV/440V) through gang isolator and fuses.  The lighting arresters are installed on the H.T.  Side to protect the sub-station from lighting strokes.  The T/F step down voltage to 400 V, 3 phase, 4 wire supply.  The voltage between any two lines is 400 V & between line & neutral is 230V.  The oil ckt breaker installed on the L.T. side automatically isolates the mounted sub-station.  T/F are generally in the event of fault generally 200KVA T/F is used.

There are mainly three types of sub-stations. 1.step up/down substations-where stepping up or stepping down of voltage has done. 2.power factor improvement sub station-where power factor is improved by installing capacitor banks or synchronous condensors. 3.switching sub stations- Where switching action like switching on or off has done for maintenance purposes for other reasons.