Substation Design Guidelines

Consider open loop MV network as an example

The rating of substation equipment as well as the connections must be decided.

Selection of substation equipment

Switchgear

1. Selection of transformers
2. Ratings of circuit breakers
3. Ratings of switch disconnecters (Load-break switches)
4. Ratings of isolators (Disconnecters)

Measurement

1. Characteristics of voltage transformers (VTs)
2. Characteristics of current transformers (CTs)

Selection of Transformers

1. Transformer size/s must be selected according to the maximum expected load and possibility of future expansions.
2. The size of transformer may be selected from power ratings given below to supply present and future loads.

Commonly available ratings for substation transformers

Rated powers:

25 kVA, 50 kVA, 100 kVA, 250 kVA, 400 kVA, 630 kVA, 800 kVA, 1000 kVA, 1250 kVA, 1600 kVA, 2000 kVA

Primary voltages (line-to-line):

6 kV, 7.2 kV, 10 kV, 12 kV, 22 kV, 24 kV, 31.5 kV, 33 kV, 34.5 kV, 35 kV, 36 kV

Secondary voltages (line-to-line): 380 V, 400 V

Example:

Substation Design

Consider that you would like to choose a transformer to supply power to a factory which requires maximum of 270 kVA of power at 400 V on the LV side and no expension is considered for near future.

The power will be provided by connecting the factory to 33 kV MV voltage level. Choose the transformer.

Solution:

Powers:

25 kVA, 50 kVA, 100 kVA, 250 kVA, 400 kVA, 630 kVA, 800 kVA, 1000 kVA, 1250 kVA, 1600 kVA, 2000 kVA

Primary Voltages (line-to-line): 6 kV, 7.2 kV, 10 kV, 12 kV, 22 kV, 24 kV, 31.5 kV, 33 kV, 34.5 kV, 35 kV, 36 kV

Secondary Voltages (line-to-line): 380 V, 400 V

Selected transformer size for the factory:

400 kVA, 33 kV/400 V