Way back in 2009, I posted thread238-234338 here about failure of surge arresters on a 4.8 kV ungrounded delta system owned by my client. I received some excellent feedback at the time and thought I would post an update.
The arreters were replaced in 2009 with new IEEE C62.11 station class type, rated 9 kV, 7.65 kV MCOV, 63 kA relief rating.
They lasted until yesterday, when fuses on two phases opened. This is a main-tie-main feed with two utility circuits and arresters connected on the load side of the main switches. Utility crews were first on the scene and opened the main switch with the blown fuses and closed the tie, re-energizing the arresters. There was an immediate explosion in the arrester compartment of the switchgear and the door of that compartment blew open.
The utility crew removed the failed arresters and instructed facility personnel that they should not be replaced because they now have a policy prohibiting arresters at the service point on their system. They also said that there is a ground fault on their system somewhere (else) that they have not been able to locate.
I surmize that the arrester(s) failed due to overvoltages resulting from a fault on the utility's system and that they do not want customers to have arresters because they fail causing ground faults on the system.
I hesitate to leave a system without arresters, especially knowing that overvoltage events are not uncommon on this system. In further discussion with the utility, they conceded that it would be acceptable to connect arresters at individual transformers downstream of the service point. One transformer already has 6kV distribution class arresters that have not faulted thus far.
Looking for suggestions or experience with arresters on ungrounded systems.
Thanks,
Alan
Alan
ôThe engineer's first problem in any design situation is to discover what the problem really is.ö Unk.
The arreters were replaced in 2009 with new IEEE C62.11 station class type, rated 9 kV, 7.65 kV MCOV, 63 kA relief rating.
They lasted until yesterday, when fuses on two phases opened. This is a main-tie-main feed with two utility circuits and arresters connected on the load side of the main switches. Utility crews were first on the scene and opened the main switch with the blown fuses and closed the tie, re-energizing the arresters. There was an immediate explosion in the arrester compartment of the switchgear and the door of that compartment blew open.
The utility crew removed the failed arresters and instructed facility personnel that they should not be replaced because they now have a policy prohibiting arresters at the service point on their system. They also said that there is a ground fault on their system somewhere (else) that they have not been able to locate.
I surmize that the arrester(s) failed due to overvoltages resulting from a fault on the utility's system and that they do not want customers to have arresters because they fail causing ground faults on the system.
I hesitate to leave a system without arresters, especially knowing that overvoltage events are not uncommon on this system. In further discussion with the utility, they conceded that it would be acceptable to connect arresters at individual transformers downstream of the service point. One transformer already has 6kV distribution class arresters that have not faulted thus far.
Looking for suggestions or experience with arresters on ungrounded systems.
Thanks,
Alan
Alan
ôThe engineer's first problem in any design situation is to discover what the problem really is.ö Unk.
