A few motors tripped at the same time!!

[JLSN]

Hi,

We had a power system (15 kV) with a a neutral current limited by a resistor to 1400 amps for single phase to ground faults.

When we had a feeder cable failure in the system, a few motors connected at that voltage level tripped at the same time. When you have this kind of fault, the two "healthy" phases will have 15 kv to ground voltage, will the faulted one will have zero voltage to ground.

Now, this motors have surge capacitors downstream a zero sequence CT (inside the motor connection box). All the motors tripped with the same function 50G, which have a setting of 10 amps primary, but it also have zero cycles of delay.

In my opinion, and according to IEEE-242 it is not recommended to have 50G to avoid mis-operations, but what do you think happened?

Thanks

JL

 

[lansford]

Given the size of the grounding resistor, the voltage to ground under ground fault condition, will not behave as you describe. If it was an ungrounded system and a strong ground fault occurred, it would. If it was solidly grounded there would be some shift of the neutral depending on several conditions. Your system would have some shift between the two cases.
Given the total distributed capacitace on your system,it would be better to add a few cycles delay to the 50G.
JIM