voltage regulators and neutral grounding resistors 1

[qstorm]

We doing upgrade works (additional disconnect switches, power circuit breakers and neutral grounding resistors)on a 2x25 MVA substation. The existing transformers are 69 kV delta-13.8 kV wye with a solidly grounded neutral. Prior to our work, three single-phase wye-connected automatic voltage regulators (AVR) were installed on the 13.8 kV side (the transformers don't have on-load tap changers). The neutral point is also grounded. Now when we installed the neutral grounding resistor (NGR), it was observed that there was a circulating current on the neutral which would cause the NGR to heat up and eventually trip the protection circuit on the incomer. We suspect that this is being caused by phase unbalance on the AVR or that the AVR should have been connected delta in the first place. What remedial measures can you suggest?

 

[busbar]

If permitted in the regulator instruction book and equipment nameplates, it may be necessary to isolate the step-regulator S0L0 wye point and connect it to the high side of the grounding resistor. Verify that the regulators are fully 95 or 110kVBIL rated, and not be required to be solidly grounded {id est, having graded insulation}.

Reconnecting the regulators in delta may not be possible with a nominal 8kV {versus 13.8} nameplate rating. Also, in the full-delta configuration, the respective set of L1 voltages may be undesirably phase shifted with respect to the S1 bushings. Verify it from the literature and equipment labels, but it is common to use an open-delta configuration on ø-ø–connected regulators, but the capacity is reduced to 57.7% of the wye [three-unit] kVA capacity. Also verify that existing “13.8kV” surge protection is appropriate for the changed (now resistance-grounded) configuration.

 

[stevenal]

Seems to be an application issue here. In my experience, NGRs are used where minimal unbalance occurs. Even if below trip, unbalance current in the resister means lost power. Single phase regulators, on the other hand, are used where unbalanced loads are expected. What is the thinking here?