On a 35kV 3phase system, we have no neutral connection from the incoming delta/wye transformer. The wye is only connected to a high resistance ground system.
We have a current transformer (CT) for each phase for overcurrent protection. The return CT wires from each phase are jumpered together and are hooked into our 'neutral' current input on the protection relay.
This is a Siemens relay, type 7SJ641. The relay itself has two neutral current parameters, "measured" and "calculated". We are using measured.
While this seems to be OK as a balanced load doesn't produce much return on the common CT wire, we sometimes we see massive spikes in neutral current, when in reality, there is not that much current.
We have seen 1500A neutral current (for very short duration) but this is impossible with the high resistance ground limiting current to 400A.
Am I missing something in that thinking? Is this how our relay needs to be set-up?
I don't know if it change anything if I change the neutral parameter to "calculated," or if it would still calculate a similar current based on phase imbalance.
The large imbalance is due to the nature of our operations, and is expected from time to time.
I also am assuming the best possible resolution to this is to connect a CT to the high resistance ground system and input that into our protection relay. Is that correct thinking?
The reason this is a concern is that we are tripping the upstream breaker on neutral current since we have our settings so high to ride through the spikes we are seeing.
I have tried contacting Siemens, but they are incredibly slow to respond.
