Potential Transformer

• Potential transformer is the step-down transformer with voltmeter in secondary side. It is same as the power transformer but rating is small. 

• The potential transformer are employed for voltage above 400 V to feed the potential coil of indicating and metering device and relay. 

• These transformer make ordinary low voltage instrument suitable for measurement of high voltage and insulated them from high voltage. 

• The primary winding of the potential transformer is connected directly to the power circuit between two phase or between a phase and ground depending on the rating of transformer and its application. 

• In the secondary winding various indicating and metering device and relay are connected. The primary winding has large number of turns and the secondary winding which has a much smaller number of turns is  coupled magnetically through magnetic circuit to the primary winding. The ratio of primary to secondary voltage is known as Turn ratio. 

• The theory of operation of Potential transformer is essentially same as that of power transformer. The main difference between a potential transformer and power transformer is due to fact that the land current.

Outdoor Substation: its type and site selection

Outdoor Substation are erected outside in open air. So for the protection of outdoor substations lighting arr-esters are installed over the High Tension line to protect the transformer form the surges. Earthing must be done in two or more phases. High tension fuse unit is installed for protection of high tension side and to control low tension side iron clad low tension switch of suitable capacity with fuses is installed.

This can be sub-divided into two types;

1. Pole Mounted Substations:

They are erected for distribution of power in localities. They can be 1 stout pole or H- pole and 4 pole structures with suitable platforms employed for transformers of capacity up to 25KVA, 125 KVA, and above 125KVA ( up to 250KVA) respectively.

Pole Mounted Substations are step down substations i.e. distribution transformer of capacity 250KVA where all the equipment is mounted on poles. This substation is used whenever a high voltage line is running and from it a small  load such as a village or a small locality or factory is to be supplied. This substation are quite simple and cheap as no building for housing as in indoor substation. The equipment is required but because of increase in number of transformers, total KVA is increased, no load losses increases and the cost per KVA is thus more.

2. Foundation Mounted Substations:

Foundation Mounted Substations are normally for voltage 33 KV and above. This is recommended because large air clearances is required for above 33 KV.  This substation are either situated near a power station i.e. Step-Up Substation or near a large load center i.e. Step-Down Substation. In this substation all equipment lies open in the air. This is usually enclosed by a fence from the point of view of safety.

Site Selection:
Following points should be keep in mind while selection of site for Outdoor Substations:

a. There must be easy access for heavy transport.
b. The clearances and spacing are not only to be governed by the operating voltage but also from the consideration of encroachment from outside. 

Classification of Substation on the basis of Nature of Duties

As we know that Substation serve as sources of energy supply for the local areas of distribution in which these are locates. Substation can be divided into three parts on the basis of their Nature of Duties. They are briefly describes below:

 1. Step- Up or Primary Substations;

Primary Substations are usually associated with generating stations. As we are familiar that the generation of electric power is basically in low voltage but we have to transfer electric power in High Voltage due to many reasons like low voltage loss, high efficiency, high reliability and many more. So in this substation generated voltage which is usually low i.e 3.3 kv, 6.6 KV, 11 Kv or 33 Kv is steeped up to primary transmission voltage so that huge blocks of power can be transmitted over long distance to the load centers economically.

2. Primary Grid Substations:

Primary Grid Substations are located at suitable load centers along the primary transmission lines. The primary transmission voltage is stepped down to different suitable secondary voltage. The secondary transmission lines are carried over to the secondary substations situated at the load centers where the voltage is further stepped down to sub- transmission or primary distribution voltages.

3. Step-Down or Distribution Substations;

Distribution Substation are located at the load centers where the sub-transmission or primary distribution voltage is stepped down to secondary distribution voltage (400/240 V ). This substation feed the consumers through distribution network and service lines.

Classification of Substation in Hydro Power Plant

Substation serve as sources of energy supply for the local areas of distribution in which these are located. Their main functions are to receive energy transmitted at high voltage from the generating stations reduce the voltage to a value appropriate for local distribution and provide facilities for switching. Some are:
 1. Simply Switching Stations
 2. Converting Substation
 3. Points for safety devices
 4. Feeders
 5. Synchronous Condensers
 6. Street lighting equipment

Classification of Substation can be done by different criteria, which are describes below :

1. According to Nature of Duties
        i. Step-Up or Primary Substation
       ii. Primary Grid Substation
      iii. Step- Down or Distribution Substations
2. According to Service Rendered
        i. Transformer Substations
       ii. Switching Substations
      iii. Converting Substations
3. According to Operating Voltage
        i. High Voltage Substation
       ii. Extra High Voltage Substation
      iii. Ultra High Voltage Substation
4. According to Importance
        i. Grid Substations
       ii. Tower Substations
5. According to Design
       i. Indoor Substations
      ii. Outdoor Substations
              a. Pole Mounted Substations
              b. Foundation Mounted Substations.