Broken delta ground fault detection

[UPMICHIGAN]

Hello

I am trying to understand why I am detecting voltage on my broken delta and why sometimes this voltage rises high enough to operate my relay and trip my generator on ground fault

My system is a 4160 volt ungrounded delta and we use a Wye grounded/broken delta PT arrangement for ground fault detection

A couple questions I have

There are absolutely no line to neutral loads, but will different line to line voltages seen by the wye/broken delta cause zero sequence voltage to be detected? For example if AB volts is different then AC volts is different then BC volts does that mean A-GND is different then B-GND is different then C-GND?

I’m confused why I have any voltage to ground at all in the broken delta to begin with and why it sometimes rises too high.

Am I right that under normal conditions 0 volts on broken delta?

Even if line to line voltage is different that doesn’t necessarily mean line to ground voltages aren’t equal, correct?

Could my problem not be voltage magnitude difference but voltage angle difference?

Thanks

 

[jim dungar]

You have voltage to ground because you have things like 'coupling capacitance' and 'leakage current', this is why ground fault detection is required.

Do you know the magnitude of the current?
Is this a new system or has it been running for awhile?
Is the wye point of your PT connected to ground?
What protective relay are you using and does it have a trip record?

 

[UPMICHIGAN]

You have voltage to ground because you have things like 'coupling capacitance' and 'leakage current', this is why ground fault detection is required.

Do you know the magnitude of the current?
Is this a new system or has it been running for awhile?
Is the wye point of your PT connected to ground?
What protective relay are you using and does it have a trip record?

I do not know the magnitude of current. Generally I think of “ungrounded “ systems as having nearly 0 current and this is why we detect voltage on the broken Delta

We are using a basler 59N and monitoring the voltage across the broken delta. The theory as I understand it is zero sequence currents circulate in the delta thus can me measured by monitoring the “break” in the delta. Full line to ground faults should give rise to 208 volt on the broken delta

Currently we always have about 6 volts and 59N is set to picked 10 volt. My question is, is the ever present 6 volts normal conditions on most broken deltas under “no fault “ conditions or should I be closer to 0 volts?

Does imbalance in line to line voltage necessarily mean imbalanced line to ground voltage?

Thanks

 

[jim dungar]

Yes, it is possible that your ungrounded MV distribution cables have enough coupling capacitance/charging current to cause a 6V output on the delta side.

You may want to search for information on medium voltage charging current and how to estimate it, such as:

Charging Current and Maximum Cable Length - ELEK Software

Charging current in power cables is 10-20 times larger than for overhead lines. The maximum length of cables is determined by their capacitance.

elek.com

 

[UPMICHIGAN]

Yes, it is possible that your ungrounded MV distribution cables have enough coupling capacitance/charging current to cause a 6V output on the delta side.

You may want to search for information on medium voltage charging current and how to estimate it, such as:

Charging Current and Maximum Cable Length - ELEK Software

Charging current in power cables is 10-20 times larger than for overhead lines. The maximum length of cables is determined by their capacitance.

elek.com

Fantastic! Thank you for helping me