Residual Voltage Transformer (RVT)

Very interesting question though!

I feel that the output voltage of a residual VT is not affected by the load connected to the primary bus. As long as there is no earth fault, RVT Secondary will not have any voltage. The theory is simple:

Supposing that you have a 110V rated RVT Secondary,

i) Ungrounded System or Grounded System - No ground fault - No secondary voltage across the Open Delta RVT Secondary

ii) Ungrounded System - Ground fault in one phase in the primary bus -RVT Secondary voltage will be 3 times the Phase Voltage of the RVT Secondary rated voltage

iii) Solidly Grounded System - Ground fault in one phase in the primary bus - RVT Secondary Voltage will be the phase voltage of the RVT Secondary rated voltage.

Your question is not completely understood, have seven RVTs failed in service?

If the RVTs are failing it maybe that your earthing system & RVT loading & connection are the problem.

"Introduction to Instrument-Transformers" by Brian D Jenkins, George Newnes Ltd publication 1967, Chapter 21, the end of section on Types of VTs- Various Fault & other abnormal conditions may help in your predicament.

Also see CR4 discussion on "Tarriff Metering on Earthed Voltage Transformers-Fig2" for Inversion vector diagram & better understanding of what maybe taking place.

Instruform