High Impedance Busbar Differential Voltage Test

[DiscoP]

A test was recently performed on a high impedance bus differential scheme and we are struggling to understand the reason for the result.

The scheme is set to operate at around 300 Volts. A voltage source was used to raise the voltage on the diff circuit for the A phase, and it tripped at the expected value, drawing about 50mA of current which was also expected (Alstom MFAC relay)

However about 50 Volts was observed on the B Phase, and about 10 Volts on the C phase

Repeating the test on B phase had similar results (50 V on both A and C phases for 300 Volts), and on C phase (10V on A Phase, 50V on B Phase).

There was no ph-ph insulation problems detected with the megger, and no stray earths were found.

Does anyone have an explanation about what might be the cause of this. Our guess is capacitance between the phases ?

 

[DTR2011]

Have you tried hanging a lamp or low impedance meter on the adjacent phases? It could rule out the capacitance theory.

 

[submonkey]

Hi DiscoP,

Is it possible that the phases are interacting via earths/shorts
on the primary system? Remember that CTs work just like any other
transformer. Perhaps try adjusting the switching so that the primary
circuit so that the primary side of all the CTs is open circuit and
repeat your test.

I'd be extremely surprised if inter-wiring capacitance was sufficient
to cause the effect observed.

Thanks,
Alan

 

[DiscoP]

Thanks for your suggestions.

We did use an older analog meter with lower impedance and the voltages stayed the same. It may not be capacitance, we are at a loss.

I thought of primary earths too. The guys on site said they opened the links on all wires out to the CTs and the same affect was observed. Which I guess means it is not current going through the CTs to earth.

 

[submonkey]

Hi DiscoP,

That is a puzzle! Could it be leakage
through the metrosils? Maybe a wiring
error?

I think it would be worth properly
understanding before energisation.

Can you disconnect the pieces (relay,
metrosils, etc) and test separately?

Alan