Caculation for Breaker Capacity Selection for Transformer Incoming and Outgoing

What is the fault level?

What is the rating of the transformer?

Already full load current of transformer on primary side and secondary side is available you may have to select the switch gear which can arry the required current and with stand the fault level.Other considerations like temperature,outdoor/indoor ,Vaccuam/SF6 also to be taken into .

Calculation for Breaker capacity selection

main points to be known for choosing breaker for incoming and outgoing of transformer are Breaker making and breaking capacity level. for making capacity u all ready given current values usesame only and for breaking capacity find fault current

fault level=1.1*V(line voltage)/(1.7*Z)

where z=impedence of transformer

breaking capacity of transformer should be greater than fault level

hi

for that it is very important to have knowledge of rating of transformer and fault level at various point in the system, nomally highest fault level at the terminal of transformer is calculted by Fault MVA = Rating of trans./ % impedance

and from that fault current too,

for 66 kv system you could have SF6 breaker outdoor and for 11 kv, vaccum breaker indoor.

whereas all other parameters of the breakere of 66 and 11 kv are standardised by companies,

regards

divyesh patel