Stefa
Electrical
- Sep 18, 2002
- 7
Hi friend,
I have faced the problem of generator protection setting of back up underimpedance protection for generator protection as follows;
1)First theory: I have found one relay engineer set this 21 mho function having two zones setting as followings,1st zone set to cover in excess of unit transformer short circuit impedance and set time delay to coordinate with primary unit transformer protection relay and/or breaker failure protection.2nd zone set to back up for transmission line zone 1 distance relay with coordination time with zone 1 relay.
2)Second theory: Some relay engineer set zone 1 to cover the sum of 50% of the maximum load impedance of unit transformer and 100% of unit transformer short circuit impedance for 1st zone and have time delay setting approx. 1.5 s to discriminate with zone 3 transmission line distance protection time delay(approx 1 s).The 2nd zone is set at 90% of unit transformer short circuit impedance and have time delay setting to discriminate with gen differential relay operating time approx 40 ms.
3)Third theory:Some relay engineer set zone 1 to cover 70% of unit transformer short circuit impedance to cover for Low side bushing flashover only and have time delay setting approx 0.5 s to coodinate with gen differential relay.
The question is that;
A) which theory is correct ?
B) In second theory; the load impedance is taken into account for setting while other theories do not take into account,Do I need to take load impedance into account,if so,how much % setting is appropriate ? It seems to be that,in 2nd theory, the 1st zone is set to cover transmission line protection zone 3 reach while zone 2 he set in order to cover unit transformer only but set at only 90% of its short circuit impedance,is this setting is correct as normally zone 1 should be set to cover shorter than zone 2 as in 1st theory.Could anyone suggest me.
C) Can anyone suggest me the advantage or disadvantage of each theory and which one I should go on relay setting.
Than you very much for any suggestion.
I have faced the problem of generator protection setting of back up underimpedance protection for generator protection as follows;
1)First theory: I have found one relay engineer set this 21 mho function having two zones setting as followings,1st zone set to cover in excess of unit transformer short circuit impedance and set time delay to coordinate with primary unit transformer protection relay and/or breaker failure protection.2nd zone set to back up for transmission line zone 1 distance relay with coordination time with zone 1 relay.
2)Second theory: Some relay engineer set zone 1 to cover the sum of 50% of the maximum load impedance of unit transformer and 100% of unit transformer short circuit impedance for 1st zone and have time delay setting approx. 1.5 s to discriminate with zone 3 transmission line distance protection time delay(approx 1 s).The 2nd zone is set at 90% of unit transformer short circuit impedance and have time delay setting to discriminate with gen differential relay operating time approx 40 ms.
3)Third theory:Some relay engineer set zone 1 to cover 70% of unit transformer short circuit impedance to cover for Low side bushing flashover only and have time delay setting approx 0.5 s to coodinate with gen differential relay.
The question is that;
A) which theory is correct ?
B) In second theory; the load impedance is taken into account for setting while other theories do not take into account,Do I need to take load impedance into account,if so,how much % setting is appropriate ? It seems to be that,in 2nd theory, the 1st zone is set to cover transmission line protection zone 3 reach while zone 2 he set in order to cover unit transformer only but set at only 90% of its short circuit impedance,is this setting is correct as normally zone 1 should be set to cover shorter than zone 2 as in 1st theory.Could anyone suggest me.
C) Can anyone suggest me the advantage or disadvantage of each theory and which one I should go on relay setting.
Than you very much for any suggestion.