Neutral Grounding Resistor Conductor Failure

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Sunny_92

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Electrical Engineer
Alright guys, this one has me stumped. A customer of ours had the conductors that connect to their neutral grounding resistor burn up. The 480V system is configured as a high resistance grounded system that consists of loads and generation (landfill cogeneration). General one-line below.

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Here's a few photo's of the failed equipment. As you can see, the conductors burned up near the resistor. The resistor seems to have not been severely damaged though.

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I'm having trouble determining what what could have caused this. No other equipment damage was found and no faults could be located on the system. What could have caused the conductors to heat up enough to ignite? Does this look like it could have been a lightning strike?

Sequence of potentially related events:
  1. At the beginning of Sept. there was a phase-phase fault on the 33kV utility system very close to where it connects to this system in question. The customer's generator was running at the time (exporting power to the grid) and tripped offline. Utility crews fixed the issue on their side. After that, the customer's genset wouldn't run, and it was found that a 12V actuator wasn't functioning. They replaced the actuator and everything went back to running normally.
  2. About 2 weeks later, maintenance personnel noticed a burnt smell and damage at the NGR. A testing company was brought in to test the generator, transformer, and cables, and reported no issues found.

Right now, the system is energized with the neutral solidly grounded at the transformer. Everything seems to be running fine, but the customer has not brought the generator back online out of fear of possible damage.

A few interesting facts:
  1. The system was originally designed & installed as a solidly grounded system. There are "neutral" conductors run from the transformer to the distribution equipment, and some 277V lighting loads exist.
  2. At some point a second generator was added, which was supplied by a foreign manufacturer. The manufacturer of the second generator demanded that an NGR be installed, so it was, but apparently not properly. However, the second generator has since been permanently removed from the system.

Any ideas about what caused this?
 
Were the neutrals you mentioned in #1 of the "interesting facts" still connected in addition to the NGR at the time of this failure?

Is there ground fault and NGR monitoring equipment?
 
Craigv said:
Were the neutrals you mentioned in #1 of the "interesting facts" still connected in addition to the NGR at the time of this failure?

Is there ground fault and NGR monitoring equipment?
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Yes, the neutrals were and still are connected.

No, there is no ground fault or NGR monitoring.
 
Sunny_92 said:
Any ideas about what caused this?
Click to expand...

Is the utility 33kV side a grounded wye?

I had one case where the utility supplied a transformer with an internal H0-X0 jumper. This caused the customers neutral grounding resistor to have to deal with the utility's neutral currents, such as miles of capacitive coupling, resulting in currents in excess of the NGR's design.
 
jim dungar said:
Is the utility 33kV side a grounded wye?

I had one case where the utility supplied a transformer with an internal H0-X0 jumper. This caused the customers neutral grounding resistor to have to deal with the utility's neutral currents, such as miles of capacitive coupling, resulting in currents in excess of the NGR's design.
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Sort of...here's how it's connected. Having the resistor between the 33kV side center point and the 33kV utility neutral/ground doesn't seem right to me. I'm guessing this is the root of the issue, but I'm still having trouble understanding what exactly would cause current on the NGR from the utility system.

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I'm a bit confused by the "NEUTRAL DISCONNECT" shown on the transformer nameplate. I don't see such a thing inside the wiring compartments.

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The "neutral disconnect" is internal and if it's connected, as Jim Dunbar mentioned in his example, that PoCo fault situation may have caused the NGR to be subjected to excessive current. With no NGR monitoring there's no way to know what current was being imposed on it. The phase-to-phase fault would cause a huge imbalance, which would have been returned via the neutral. If your xfmr neutral is connected to the utility side, it may have seen some huge current.
 
I had to blow the photograph up to look closely at the resistance bank. It looks like the resistance and cable have been subject to prolonged heating. The oxidisation of the cable like that doesn’t occur in one transient fault.

Personally I’d tell the PoCo to sort their own earthing for the 33kV star primary and not rely on the companies NER. I would also include temperature monitoring for the NER cabinet.

Given the choice, I prefer liquid NER’s to resistance banks.
 
Craigv said:
The "neutral disconnect" is internal and if it's connected, as Jim Dunbar mentioned in his example, that PoCo fault situation may have caused the NGR to be subjected to excessive current. With no NGR monitoring there's no way to know what current was being imposed on it. The phase-to-phase fault would cause a huge imbalance, which would have been returned via the neutral. If your xfmr neutral is connected to the utility side, it may have seen some huge current.
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Makes sense.

Tony S said:
I had to blow the photograph up to look closely at the resistance bank. It looks like the resistance and cable have been subject to prolonged heating. The oxidisation of the cable like that doesn’t occur in one transient fault.

Personally I’d tell the PoCo to sort their own earthing for the 33kV star primary and not rely on the companies NER. I would also include temperature monitoring for the NER cabinet.

Given the choice, I prefer liquid NER’s to resistance banks.
Click to expand...
Well the transformer is owned by the customer, so we can't whine to the utility on this one. I also doubt the customer will like the idea of replacing the transformer, which I believe would be required to isolate the primary and secondary and ground the high side winding. I'm thinking our best option is to keep it solidly grounded. Just need to make sure the generator is adequately protected doing so. Does the genset controller provide adequate protection or should additional means be employed? I believe the gen size is 835 kW.
 
Sunny_92 said:
A few interesting facts:
  1. The system was originally designed & installed as a solidly grounded system. There are "neutral" conductors run from the transformer to the distribution equipment, and some 277V lighting loads exist.
Click to expand...

Are you allowed L-N loads in a resistance grounded system? I seem to remember reading that you couldn't have L-N loads, but if a neutral conductor is pulled from X0 I don't see a technical reason why it wouldn't work, and you should still have ground-fault protection. Is the limitation by code only?
 
cpickett said:
Are you allowed L-N loads in a resistance grounded system? I seem to remember reading that you couldn't have L-N loads, but if a neutral conductor is pulled from X0 I don't see a technical reason why it wouldn't work, and you should still have ground-fault protection. Is the limitation by code only?
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It's not allowed by code. It would technically work, but there's a few good reasons why it's a bad idea. However, I'll let someone smarter than I explain those reasons.

Tony S said:
If the transformer belongs to the customer then they should have a say in the method of earthing their system.

Connecting both MV and LV N→E together and then relying on the NER is just asking for trouble.
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I'm definitely going to have them convert to solidly-grounded. Why on earth low resistance grounding was ever installed like this amazes me. Thank you for all the input.
 
If the NER is required for the LV side of the transformer keep it as it is and move the MV star point to a separate earth point.

In the UK we don’t use MV star point earthing other than at the origin of the supply and then a NER is used.

I find it fascinating how supply systems differ from country to country. Whether one system is better than another, I don’t know, I’m not high enough up in the pecking order.
 
Sunny_92 said:
It's not allowed by code. It would technically work, but there's a few good reasons why it's a bad idea. However, I'll let someone smarter than I explain those reasons.


I'm definitely going to have them convert to solidly-grounded. Why on earth low resistance grounding was ever installed like this amazes me. Thank you for all the input.
Click to expand...

It handles ground fault detection better, but really should be used with a monitoring system.
 
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