Resistance grounded transformer question
Ive got a 3 phase 12kV to 480V transformer that’s delta to wye. It has a neutral grounding resistor on the secondary with a 346 ohm resistor.
In a direct line to ground fault it should only pull a maximum of 277/346 (0.8A) of current. This is also stamped on the box as a sanity check.
The ground fault protection is set for 500mA but is adjustable. I have seen a couple of times now where it has been pulling over 1A of current. I know because I cranked up the protection relay to 1A and it still tripped. I double checked the resistance and confirmed they were still 346 ohm. And I checked with an amp clamp to confirm it wasn’t the protection relay.
This system does also feed multiple vfds. I suspect that it must be getting higher voltage to ground through one of the vfds dc bus via the breaking choppers, which usually run up to 700v. Or possibly the motor cables during a rapid regen breaking generating a higher voltage.
Does this seem possible? Does anyone have much experience with this type of grounding? Are there other things that could cause this higher amperage to be pulled?
Thank you!