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Undrvoltage settings of LV Changeover Breaker

Undrvoltage settings of LV Changeover Breaker

Undrvoltage settings of LV Changeover Breaker

(OP)
I am an electrical engineer in a combined cycle power plant. I am nowadays involved in the undervoltage settings calculations of MV and LV Buses Changeover breakers.
The existing undervoltage settings of the LV and MV incoming breakers and the Changeover (Bus Coupler) breakers are as follows:
MV incoming breakers: 80% of nominal voltage with 3 sec time delay and 20% on nominal voltage with 0 sec time delay.
Auto Bus Transfer Device for MV Bus Coupler: 80% of nominal voltage with 0.3 sec time delay.
LV incoming breakers: 80% of nominal voltage with 3 sec time delay and 20% on nominal voltage with 0 sec time delay.
LV Bus Coupler: 80% of nominal voltage with 1 sec time delay and 20% on nominal voltage with 0 sec time delay.

The problem is that the Auto Bus Transfer Device for MV breakers takes about 80ms each time while manual changeover from MV incoming breaker to MV Bus Coupler and it takes about 380ms while auto changeover from any incoming breaker to Bus Coupler. And During this changeover, LV incoming breaker opens and its bus coupler closes, which seems unnecessary. Also, it seems also not feasible to include some time delay in LV side changeover breaker, because all loads will trip in case of opening of LV incoming breaker on the actuation of any protection.
But I want to put a time delay of 0.1 sec for the LV Changeover Breaker undervoltage setting to avoid unwanted tripping during manual changeover of MV Incoming Breakers.
I want to know what settings will be optimum here. Also, Is the behavior of Auto Bus Transfer Device is normal or not while manual changeover from both MV incoming breakers to a MV icoming breaker and MV Bus Coupler (First Break and then Make operation.





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