Closed Delta 2

[Viceng]

I am dealing with an ungrounded system which is utilizing a closed delta VT configuration to sense a SLG fault.

My question: The HV side of the VT is Wye solidly grounded - Does this configuration not introduce a ground reference into an otherwise completely ungrounded system?

I would imagine that this VT doesn't come into play on calculations since it is not a source and hence will not contribute fault current but I was hoping for some clarification.

Thank you.

 

[davidbeach]

Closed delta or broken delta? If all you have is the three windings and a CT to measure circulating current in the delta it would be a closed delta and a decent ground source for the ungrounded system; probably couldn't withstand a continuous ground fault on the system. If you have the three windings, but one connection of the delta is broken and a resistor inserted, you have a broken delta connection. Yes, it will source ground fault current to the wye side, but that current will be very low compared to the system charging currents and can be safely ignored.

No such thing as an "ungrounded system", just lots of systems with that name that are actually high impedance capacitively grounded systems. Resistance grounded systems are far more manageable and can provide all of the benefits of an "ungrounded system" without the severe negative aspects.

 

[dpc]

I believe this is more typically referred to as a "broken delta", at least in the US.

The zero sequence impedance will be high due to the impedance in the secondary. The voltage relay used for ground fault detection will have a fairly high impedance. Current can't flow in the primary unless it can flow in the secondary (except for magnetizing current), so this makes it a weak grounding source.

But, a "ballast" resistor should be used on the delta side of the PTs in parallel with the voltage relay to reduce neutral shift and possibility of ferro-resonance.

 

[Viceng]

My apologies, yes, broken delta

Thank you for the responses

"Current can't flow in the primary unless it can flow in the secondary" - really clarifies it for me.

"No such thing as an "ungrounded system", just lots of systems with that name that are actually high impedance capacitively grounded systems" - Good point.

Much appreciated.

 

[busbar]

 
One side note—VT primaries must be rated for full ø-ø voltage, [and not ø-g] else in an SLG fault two of the primaries will go into saturation—with likely winding damage. VT-primary fuses shouldn’t be counted on to protect VTs in this situation.