HenryOhm
Electrical
- Jun 22, 2005
- 58
All,
We have a 4160 island power system that is high-resistance grounded (broken delta PT tertiaries only on one common bus switchgear cubicle, not each cubicle set of PT's, alternators actually floating). The switchgear has vacuum breakers and so came with small surge "limiters" that were underrated (Uc < 4160). I believe these were just a standard offering from the manufacturer but that their ratings were based on an assumed grounded system, i.e. always seeing 2400V line-to-ground. They were also not true arresters I guess as they only had a roughly one thousand amp rating(?). These "arresters" were connected grounded wye. But, when these cubicle breakers open, they are disconnected from the high-resistance broken delta system and become effectively floating. The only return path would then be through one of the other two arresters or the same cubicle's PT primaries which are also connected grounded wye, so high impedance paths.
So, when these arresters see a voltage spike and go to suppress it, does the lack of a low impedance return path prevent them operating properly or reduce their effectiveness? I assume if the voltage spike is big enough, two arresters phase-to-phase will work in series to suppress the spike?
More importantly, we had a pretty ugly switchgear failure in a generator cubicle where the arresters definitely failed (one looked to have exploded), the start of the failure possibly related to them being underrated. Is there a way that one could fail over time but not present a real problem to the system until a second one of the three failed? These arresters are almost two decades old. They have been in a tropical-like environment where I believe the ambient would have derated them further and may also have played a role.
The failure mode I understand arresters to have is becoming a short to ground. Usually that failure avalanches and becomes obvious very fast until they rupture. But, without any sort of return path of significance in our case, and so no real current flow through them during normal operation other than perhaps capacitive, could a failed one just sit sort of unnoticed until a second one connected to it started to fail? We have a ground fault system but it may not be very sensitive so perhaps a "failed" arrester could become only a lower impedance path to ground?
We plan to change out all these "arresters" to ones with Uc > 4160 after ambient derating and with higher energy dissipation numbers. But, we hope to do this work during a shutdown in about six months. But, I'm worried if the failure mode may be such that we could have another arrester or two in the system that might be in a similar semi-shorted state but not apparent and just waiting for one of its grounded wye partners to fail also? We have almost two hundred of them in a number of 4160 switchgear lineups. Because of our operation, each arrester may have accumulated 10,000-20,000 spikes from the switching transients that may be produced with the opening of these vacuum breakers.
Thanks in advance for any and all help.
We have a 4160 island power system that is high-resistance grounded (broken delta PT tertiaries only on one common bus switchgear cubicle, not each cubicle set of PT's, alternators actually floating). The switchgear has vacuum breakers and so came with small surge "limiters" that were underrated (Uc < 4160). I believe these were just a standard offering from the manufacturer but that their ratings were based on an assumed grounded system, i.e. always seeing 2400V line-to-ground. They were also not true arresters I guess as they only had a roughly one thousand amp rating(?). These "arresters" were connected grounded wye. But, when these cubicle breakers open, they are disconnected from the high-resistance broken delta system and become effectively floating. The only return path would then be through one of the other two arresters or the same cubicle's PT primaries which are also connected grounded wye, so high impedance paths.
So, when these arresters see a voltage spike and go to suppress it, does the lack of a low impedance return path prevent them operating properly or reduce their effectiveness? I assume if the voltage spike is big enough, two arresters phase-to-phase will work in series to suppress the spike?
More importantly, we had a pretty ugly switchgear failure in a generator cubicle where the arresters definitely failed (one looked to have exploded), the start of the failure possibly related to them being underrated. Is there a way that one could fail over time but not present a real problem to the system until a second one of the three failed? These arresters are almost two decades old. They have been in a tropical-like environment where I believe the ambient would have derated them further and may also have played a role.
The failure mode I understand arresters to have is becoming a short to ground. Usually that failure avalanches and becomes obvious very fast until they rupture. But, without any sort of return path of significance in our case, and so no real current flow through them during normal operation other than perhaps capacitive, could a failed one just sit sort of unnoticed until a second one connected to it started to fail? We have a ground fault system but it may not be very sensitive so perhaps a "failed" arrester could become only a lower impedance path to ground?
We plan to change out all these "arresters" to ones with Uc > 4160 after ambient derating and with higher energy dissipation numbers. But, we hope to do this work during a shutdown in about six months. But, I'm worried if the failure mode may be such that we could have another arrester or two in the system that might be in a similar semi-shorted state but not apparent and just waiting for one of its grounded wye partners to fail also? We have almost two hundred of them in a number of 4160 switchgear lineups. Because of our operation, each arrester may have accumulated 10,000-20,000 spikes from the switching transients that may be produced with the opening of these vacuum breakers.
Thanks in advance for any and all help.