Neutral Grounding Resistor Conductor Failure

[Fritzy92]

Posted this on another forum already, but figured I'd look for insight here as well. 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.

Here's how the grounding is configured at the transformer:

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.

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:
[ol 1]
[li]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.[/li]
[/ol]
[ol 2]
[li]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.[/li]
[/ol]

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:
[ol 1]
[li]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.[/li]
[/ol]
[ol 2]
[li]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.[/li]
[/ol]

Any ideas about what caused this?

 

[davidbeach]

I'm pretty sure you never want that transformer to be anything other than solidly grounded. If you want an NGR, that wye winding needs to have it's own neutral bushing and not be connected to the neutral of the other side. The two sides are not decoupled and there can be undesired interactions.

 

[waross]

In Canada an NGR is not allowed when there are line to neutral loads.
That line to line short may have raised the primary neutral potential quite a bit. That could cause excess current through the NGR.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

As mentioned, you cannot have 277 V loads if high-resistance grounded. This is not allowed by NEC. Agree with David - need separate neutral bushing for Xo terminal. Revert to solid grounding or get a new transformer and ditch the 277 V loads. These can be fed via a 480 to 480 V delta-wye isolation transformer.

 

[waross]

David & dpc
I had a thought.
Please tell me if it has any merit.
With a wye/delta transformer, any circulating current causes neutral current in the primary neutral.
A line to line fault will cause both phase angle errors and lower voltages on two phases, both of which lead to circulating currents and neutral currents.
If this transformer has a three legged core, could the phantom delta cause a serious neutral current in the event of a line to line fault?

On the damage to the cables. The cables exiting the far end of the flexible conduit appear undamaged.
I surmise that the NGR overheated and that the cable damage was caused by radiant heat from the resistor, not over current.
There may have been excess current from the primary side or the resistor may have had a short time rating that was exceeded.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

You do not have a high resistance grounded system because the wye-wye transformer connects the neutral to the 33 kV grounded system neutral, at least partially bypassing the NGR.

 

[waross]

" 33 kV grounded system neutral,"
Over the years I have seen a number of issues when the multiple safety grounds on the primary circuit were in bad repair.
In one case a large part of the primary neutral current was returning on residential grounding systems.
In another case, a primary insulator failed and dropped a primary conductor onto the cross arm. The current tracked to a secondary conductor and put a high standing voltage on the secondary circuit.
The high standing voltage arced to ground inside a building and started a fire.
At no time did any protection operate.
The circuit was eventually cleared manually back at the substation.
In another instance, a high ground current baked the ground dry and the resistance to ground and the touch potential of the grounded conductor rose high enough that a worker coming in contact with a supposedly grounded conductor was shocked unconscious.
The primary neutral should be grounded but.....

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

Please explain your notation. NGR has two sides, N and G, but your notation has G/N as if they are the same potential.

Are the 277 loads connected to the N side of the NGR?

What's the nature of the 33 kV source? Wye or delta? 3 or 4 wire? Grounded, ungrounded, impedance grounded?

 

[Fritzy92]

I'm pretty sure you never want that transformer to be anything other than solidly grounded. If you want an NGR, that wye winding needs to have it's own neutral bushing and not be connected to the neutral of the other side. The two sides are not decoupled and there can be undesired interactions.

That's my thinking too.

In Canada an NGR is not allowed when there are line to neutral loads.
That line to line short may have raised the primary neutral potential quite a bit. That could cause excess current through the NGR.

As mentioned, you cannot have 277 V loads if high-resistance grounded. This is not allowed by NEC. Agree with David - need separate neutral bushing for Xo terminal. Revert to solid grounding or get a new transformer and ditch the 277 V loads. These can be fed via a 480 to 480 V delta-wye isolation transformer.

I'm aware, I'll definitely be correcting this one way or another.

David & dpc
I had a thought.
Please tell me if it has any merit.
With a wye/delta transformer, any circulating current causes neutral current in the primary neutral.
A line to line fault will cause both phase angle errors and lower voltages on two phases, both of which lead to circulating currents and neutral currents.
If this transformer has a three legged core, could the phantom delta cause a serious neutral current in the event of a line to line fault?

On the damage to the cables. The cables exiting the far end of the flexible conduit appear undamaged.
I surmise that the NGR overheated and that the cable damage was caused by radiant heat from the resistor, not over current.
There may have been excess current from the primary side or the resistor may have had a short time rating that was exceeded.

Your theory about the NGR overheating and damaging the cables makes perfect sense to me.

Please explain your notation. NGR has two sides, N and G, but your notation has G/N as if they are the same potential.

Are the 277 loads connected to the N side of the NGR?

What's the nature of the 33 kV source? Wye or delta? 3 or 4 wire? Grounded, ungrounded, impedance grounded?

Sorry for the poor notion. The 33 kV source is 4-wire grounded wye. The 33 kV neutral is connected to the ground bar in the transformer primary compartment. The 277V loads are connected to the N side of the NGR.

There is an SEL-351 at the 33kV utility connection. Here's what the event records show from the utility fault.

Seems that the fault caused a shift in the center point of the high side winding, which caused current to flow in the NGR.

I believe our best course of action is to keep the system solidly grounded. However, I'll need to either convince the owner that the generator is adequately protected without an NGR, or I'll need to add extra protection for the generator. Do genset controllers typically provide adequate protection? I believe the genset size is 835 kW.

 

[stevenal]

So the grounded neutral 33 KV short circuited the NGR under normal conditions, and found a return path through the NGR during a long or never clearing primary fault. I agree it should be removed.

 

[wroggent]

The one-line shows 400A next to the resistor. What is the meaning?

Is there a jumper between the H0 and transformer enclosure/ground in the cable compartment or elsewhere?

 

[dpc]

If you have 277 V loads, you can't resistance ground the generator. It will need to be solidly grounded. If you remove the 277 V loads, it can be high resistance grounded.

Wait - your one line shows the resistor as 400 A. Is this correct? If so, that's low resistance grounding. You can't do that at 480 V. Either ungrounded, solidly-grounded or high-resistance grounded.

 

[stevenal]

The one-line shows 400A next to the resistor. What is the meaning?

Probably the current rating at 277 V. A bit more at 19.05 kV.

 

[Fritzy92]

The one-line shows 400A next to the resistor. What is the meaning?

Is there a jumper between the H0 and transformer enclosure/ground in the cable compartment or elsewhere?

400A is the NGR nameplate rating - 277V, 400A, 0.7 Ohms, 10 sec.

There is a single H0X0 terminal in the secondary compartment, which was connected to the "neutral" side of the NGR.

If you have 277 V loads, you can't resistance ground the generator. It will need to be solidly grounded. If you remove the 277 V loads, it can be high resistance grounded.

Wait - your one line shows the resistor as 400 A. Is this correct? If so, that's low resistance grounding. You can't do that at 480 V. Either ungrounded, solidly-grounded or high-resistance grounded.

Yup, that's what I thought. I'm definitely telling them to remove the NGR and go solidly-grounded.

 

[waross]

If you don't know it doesn't matter much but.
Has the transformer a three legged core?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Skipping ahead the primary and secondary neutrals are tied together, big no-no in HRG.


Fault or even load current on the secondary will cause the primary neutral to rise significantly in voltage, pushing current through the resistor to get back to the utility source. Resistor will get very hot if continuously passing this current or briefly in large amounts. Be happy it was not worse, with the current setup line to neutral loads can be damaged in the facility, and its possible to put 1000s of volts in the LV neutral. How this passed is beyond me but its a wreck and if you want my honest opinion that resistor needs go...

 

[Fritzy92]

If you don't know it doesn't matter much but.
Has the transformer a three legged core?

Not sure. Only info I have is what's on the nameplate.

Skipping ahead the primary and secondary neutrals are tied together, big no-no in HRG.


Fault or even load current on the secondary will cause the primary neutral to rise significantly in voltage, pushing current through the resistor to get back to the utility source. Resistor will get very hot if continuously passing this current or briefly in large amounts. Be happy it was not worse, with the current setup line to neutral loads can be damaged in the facility, and its possible to put 1000s of volts in the LV neutral. How this passed is beyond me but its a wreck and if you want my honest opinion that resistor needs go...

Thanks for the explanation, it's definitely helping me understand what caused current on the NGR.

The amazing part is this thing has been connected for the past 10 years or so, with no issues until now.

 

[waross]

It's amazing that no-one was ever shocked or at least got a tingle from the transformer enclosure.
With a ground fault on the 480 Volt secondary the transformer case would have a solid touch voltage of 277 Volts.
Your plant has been incredibly lucky.
The heating of the resistor is evidence that the primary circuit grounding is not effective at your plant.
A solidly grounded supply neutral would have effectively shorted across the resistor and shunted any neutral current/voltage to ground.

As for my question concerning the transformer core.
A three legged core forms a "Phantom Delta". A delta winding or a phantom delta may greatly increase the neutral/ground current.
The regulars here know that wye;delta issues are my pet topic. grin
Whatever type of core, you had enough current returning on the neutral to do a lot of damage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wroggent]

Also amazing that none of the loads connected to nuetral got fried.