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Generator with Burned Neutral Ground Bond and Wye - Wye Transformers

Generator with Burned Neutral Ground Bond and Wye - Wye Transformers

Generator with Burned Neutral Ground Bond and Wye - Wye Transformers

(OP)
Greetings,

I have a system where we are burning the neutral to ground bonds on two 480V Y / 277 with a two-thirds pitch natural 1400kW gas generator alternator. (I have attached a single line and a screenshot of the transformer data.) Each generator has a step-up transformer that is 480Y/13.2kV Y. The XOHO on the transformer is bonded internally. Each transformer is connected to a medium voltage generator main breaker. We created a new service entrance breaker that creates a UM/ GM pair. We were able to run one generator for about 100 hours before we started seeing unbalance voltages on the high side of the transformer. Two line-to-line phases were showing 13.1kV and the third was 14.1kV. The voltage imbalance was preventing us from synchronizing to the utility. The cause of the voltage imbalance was the burned-up neutral. After taking measurements on the other generator, the neutral to ground bond was showing 400-500 amps at 25% loading.

This system operates both as a backup system, and in extended parallel with the utility. The utility required as part of the interconnection agreement the use of the grounded wye- grounded wye transformers, and I do not have hope that I will be able to swap a delta-wye transformer. I have been told transformers that are grounded wye - grounded wye with solid grounding at the generator are subject to high harmonic currents and that could be causing the issue. We haven't seen any evidence of damage on the grounds.

In reading through some of the IEEE books, grounded wye- grounded wye can't be considered a separately derived system. Therefore, connecting neutrals from the generator to the transformer would not be a true separately derived system.

There currently is no neutral running between the transformer and the generator. I wouldn't expect harmonics to be the issue if there is no neutral. I would expect currents to flow through the grounds, but there is no indication of damage at the grounds. There is a grounding wire connected from the generator, the paralleling breaker, to the transformer. There is no indication of damage on this wire either. Several methods have been proposed for solving the problem:

  • Remove the ground at the generator, and pull neutrals back to the transformer
  • Place a neutral grounding resistor at the neutral to ground bond to reduce the current
  • Replace both transformers with a single grounded wye - grounded wye transformer and have the common connection on the low voltage side
  • Check if harmonics are coming from the facility causing the neutral to ground bond to burn up. From there do impedance grounding to eliminate the harmonics
I am leaning towards the first bullet point, but I don't have a good sense it will solve the problem. My questions are:

What could be the cause of the neutral to ground bond burning up?
What would be good points to check in the system for root cause?
What would be a good corrective action to fix the problem?

One line, grounding, XFMR data: https://files.engineering.com/getfile.aspx?folder=...

RE: Generator with Burned Neutral Ground Bond and Wye - Wye Transformers

1400 KW = 1556 KVA
1556 KVA = 518 KVA per phase.
Full load current = 515,000 KVA / 277 Volts = 1872 Amps per phase, FLC
Current at full load = 1872 Amps x 25% = 468 Amps.
You have 400 Amps to 500 Amps on the neutral?
Look for an open phase.
Possibly an open winding, possibly an open connection, possibly a blown fuse.
The open may be anywhere from a failed generator winding to a primary fuse.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Generator with Burned Neutral Ground Bond and Wye - Wye Transformers

For my curiosity may I ask you,
1)Can you please confirm whether all your three generators are having 2/3 pitch or one is 2/3 & the other(s) is 5/6 pitch?
2)When G1 is already synchronized through 52-UM to the utility and running and 52-GM is already closed,
how do you synchronize G2 and or G3?
3)Are 52-G2 & 52-G3 are having sync facility?

RE: Generator with Burned Neutral Ground Bond and Wye - Wye Transformers

(OP)
1. All generators are two-thirds pitch.
2. All generators synchronize across the low voltage circuit breakers. They then ramp up to load. Right now we are only running one generator at a time for peak shaving.
3. All generators synchronized at the time of commissioning. Both 52-G2 and 52-G3 can synchronize to the utility.

Update from a field test, we were seeing a constant 400-500 amps on the neutral bond across different load levels. IT looks like there are high amounts of harmonics in the system. We have harmonics testing this week to see how bad the harmonics get.

RE: Generator with Burned Neutral Ground Bond and Wye - Wye Transformers

It looks like you have found the root cause. May be there are 3H coming from the utility to circultae in your neutral.
The prime reason for 2/3 pitch design is to reduce/ eliminate third harmonics (3H). Accordingly they can be paralled.
Going for 2/3 pitch will increase 5H&7H but they are line harmonics. Also 2/3 pitch will decrease the zero seq impedance
but I think it should have been taken care during ground fault protection and MCC withstand.
Pl. update the forum with your findings.

RE: Generator with Burned Neutral Ground Bond and Wye - Wye Transformers

The zero-sequence impedance of generators is usually low. This can result in the generator feeding unbalanced loads on the utility system and having very high ground currents during ground faults on the utility system. You should consider running the generator ungrounded. If you do, there should be a transfer trip or other means to prevent the utility system from being operated ungrounded if the feeder trips and the generator keeps running.

RE: Generator with Burned Neutral Ground Bond and Wye - Wye Transformers

(OP)
A further update on this site.

We did discover high third-order harmonics coming from the utility by using a power quality meter. In turning over this design to a power quality expert, they came back and pointed out that our wye-wye system needed neutrals to be code compliant. I have been looking through the NEC for specific requirements of a neutral with an on-site generator. I am getting lost in the code references. It seems sections 250.26 and section 250.30 would be most applicable for issues with running a grounding conductor (neutral) from the wye of the transformer to the wye of the generator.

There is currently no neutral coming from the utility at 13.2kV, and we don't have a neutral on the low voltage wye side of the transformer.

Are there other code sections that this system wouldn't be compliant with?

RE: Generator with Burned Neutral Ground Bond and Wye - Wye Transformers

As I mentioned, you have found 3H coming from the utility into your system.
Since your system is Wye>Wye>Wye from the utility to generator, it looks like
a "sink" for 3H. Therefore, unless you change the transformer to DELTA/wye grounded
to block the 3H coming from the Utility, I donot think you can avoid the problem.
Also it looks like the 3H & other harmonics are coming from a nearby consumer
connected on the same feeder. Therefore, you sholud measure the harmonics levels
at the common coupling point (PCC) and if they are higher than the IEEE-519 limits,
you should submit a case to the Utility.

I am not aware of bonding the neutrals of the generator & transformer. But it will
worson the circulating 3H currents unless you block at the PCC.

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