Question

Assuming that we have a 66/11-kV primary substation with three 66/11-kV, 20MVA (rating of each transformer), power transformers, then the installed capacity is 60MVA and the firm capacity is 48MVA. I'd like to know what is the difference between installed and firm capacity. Furthermore, how was the firm capacity calculated from the installed capacity? What I know is that the firm capacity is calculated as follow: Firm Capacity = 20 + 20 + 4 + 4 = 48MVA, and that the equation (n - 1)x120% is used, where n is the no. of transformers.

Please reply as soon as possible.

Answer 1

Installed capacity- Installed capacity is also known as Name plate capacity. It refers to the maximum amount of energy a power plant is able to produce in ideal conditions. It is measured in MVA.

Firm capacity- Firm capacity means the amount of energy a power plant can produce with high degree of certainity at a given time.

The net amount of power produced by a power plant is practically less than its installed capacity due to some reasons like, maintenance of plant equipments, shutdown due to minor or major failure, running plant at undercapacity and unavailabity of fuel.

So, the garunteed power produced by the power plant is less than its maximum capacity which is defined by the factor known as firm capacity.

The firm capacity differs from plant to plant and it is determined through saveral probabilistic methods where the data is gathered from the study of plant or the substation.

Take the given no. of transformers in a substation is N of MVA.

The Firm capacity is (N-1) MVA which symbolises that even if one transformer fails we have N-1 transformers working in our substation.

The firm transformation capacity of the sustation beacomes (N-1) x MVA.

In your example,

Three 66/11-kV, 20MVA power tranformers.

The formula to calculate the installed capacity is,

I.C. = MVA1 +MVA2 + MVA3

where MVA1, MVA2 and MVA3 are the nameplate MVA rating of each transformer.

I.C.= 30MVA +30MVA + 30MVA = 90MVA.

The formula to calculate the firm capacity (which depends on given plant and its observbations) is,

% of each transformer capacity.

F.C = (3-1) x 1.20 x 20MVA =48 MVA.