leggomyeggolas
Electrical
- Feb 25, 2021
- 3
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:
[ul]
[li]Remove the ground at the generator, and pull neutrals back to the transformer[/li]
[li]Place a neutral grounding resistor at the neutral to ground bond to reduce the current[/li]
[li]Replace both transformers with a single grounded wye - grounded wye transformer and have the common connection on the low voltage side[/li]
[li]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[/li]
[/ul]
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:
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:
[ul]
[li]Remove the ground at the generator, and pull neutrals back to the transformer[/li]
[li]Place a neutral grounding resistor at the neutral to ground bond to reduce the current[/li]
[li]Replace both transformers with a single grounded wye - grounded wye transformer and have the common connection on the low voltage side[/li]
[li]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[/li]
[/ul]
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: