Temporary Increase in Fault Level

You have to work out the consequences and chances of a major fault in those 5 seconds against the consequences of several power outages during the switching procedure. I've all ways done the switching live because of the chaos an outage would cause.

Please provide the Z of 40 MVA transformer and the rupturing rating of 6.6 kv switchgear. Without checking the capacity of switch gear and cables closing the bus coupler will be risky

Why 5 seconds? During momentary parallelling (say for 1 sec) when both the incomers and the bus coupler will remain closed, you can trip the selected incomer through a 'trip selector' switch. This will ensure uninterrupted power supply to both the bus sections while taking one incoming feeder for maintenance.

As a good engineering practice the breaking capacity of the 6.6 kV circuit breakers should be in line with the 6.6 kV bus fault level considering parallel operation. However, some people take calculated risk considering the rare possibility of occurrence of fault during momentary parallel operation.

@ramvinod

%Z is 12 % and Isc making is 63kA and Isc breaking is 25kA

@debata

Trip selector is like taking the Bus coupler close contact to trip a pre-selected Incomer or something else ??

Yes, you are right. The NO contact of the bus coupler breaker is used in the tripping circuit of the pre selected incomer.

This is a normal practice. However, to find the ACTUAL RATINGS OF THE CIRCUIT BREAKERS, the fault levels on the 6.6kv bus-bars for two options( paralleling and non paralleling of the transformers) will have to be used.

Once you are able to determine the CURRENTS flowing during through the CIRCUIT BREAKERS during fault conditions, this can then be used to consequently compute the SHORT TIME CURRENT and FAULT LEVEL (in MVA) on each CIRCUIT BREAKER(220kv or 6.6kv).

These FAULT LEVEL computations are necessary to determine correct EQUIPMENT SPECIFICATIONS.This is needed so that damage to equipment during fault conditions could be prevented.

Momentary Paralleling is NOT allowed in India, if the Breakers & the Bus bars are not sized for the combined short circuit contribution from both the sources. I have had problems in the past.