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One of our GT Unit has tripped during following condition,
1. Islanding was put OFF for work on islanding scheme on customer side in Mark VI HMI.
2. As a further precautions against toggling (state transition) of Grid Tie Breaker (L52TL) and House Tie breaker (L52TL_House) during the job signals were forced ("1" or current state)in service condition in both Units.
3. During work on islanding these signals toggled several times without affecting GT’s operation.
4. On getting clearance we planned unforcing of the signals in same state, still with islanding still OFF.
5. To be on safer side before unforcing all signals at a time. We choose to unforce one breaker status one at a time.
6. After checking above step. We kept Unit 2 House tie breaker (L52TL_house) unforced and L52TL signals still forced and islanding still OFF.
7. Next, Unit 2 L52TL is unforced in same state ("1"). Islanding still OFF.
During this, Islanding triggered sensing Unit 2 Grid Tie breaker (L52TL) Status Change, which was not there physically.
8. Based on the SOE summary report, L52TL toggles from state "1" to state "0" and back to normal state "1" after 64ms or state transition occurs.
We have conducted series of investigation and repeat the exercise for force/unforce not critical signal L30CC1 which is in alarm state.
1. While doing the exercise, we prepare the trend of L30CC1, L30CC1_ALM and L30FORCED_AL to verify the signals behaviour while using force/unforce.
2. It toggles 3times out of 50 to 60 times of force/unforce with the same state "1", state transition occurs.
3. It happend only if the signal is in state "1" not in state "0".
Why did was state transition occur during forcing/unforcing digital signal @ the same state "1" ?
1. Islanding was put OFF for work on islanding scheme on customer side in Mark VI HMI.
2. As a further precautions against toggling (state transition) of Grid Tie Breaker (L52TL) and House Tie breaker (L52TL_House) during the job signals were forced ("1" or current state)in service condition in both Units.
3. During work on islanding these signals toggled several times without affecting GT’s operation.
4. On getting clearance we planned unforcing of the signals in same state, still with islanding still OFF.
5. To be on safer side before unforcing all signals at a time. We choose to unforce one breaker status one at a time.
6. After checking above step. We kept Unit 2 House tie breaker (L52TL_house) unforced and L52TL signals still forced and islanding still OFF.
7. Next, Unit 2 L52TL is unforced in same state ("1"). Islanding still OFF.
During this, Islanding triggered sensing Unit 2 Grid Tie breaker (L52TL) Status Change, which was not there physically.
8. Based on the SOE summary report, L52TL toggles from state "1" to state "0" and back to normal state "1" after 64ms or state transition occurs.
We have conducted series of investigation and repeat the exercise for force/unforce not critical signal L30CC1 which is in alarm state.
1. While doing the exercise, we prepare the trend of L30CC1, L30CC1_ALM and L30FORCED_AL to verify the signals behaviour while using force/unforce.
2. It toggles 3times out of 50 to 60 times of force/unforce with the same state "1", state transition occurs.
3. It happend only if the signal is in state "1" not in state "0".
Why did was state transition occur during forcing/unforcing digital signal @ the same state "1" ?
