Neutral over voltage in Wye grd - Wye grd transformer

[Zylar]

Hi all,

We have a wye grd - wye grd 12kV - 480V transformer. The utility lines are 12kV 3 wire and the neutral of the primary windings are grounded through a potential transformer (PT) at the transformer pole. The neutral of the secondary is grounded the main 480V panel. This is a generator facility, so the output of the primary PT is connected to a 59N relay per utility's requirement in order to detect ground fault on the utility side and disconnect the generator, so it does not feed into the fault.

However, we are seeing significant neutral voltage on the PT. We measured 3,000V from the neutral of the wye on the primary side to ground. On the 59N relay, we are seeing ~30V which comprises of almost pure third harmonic voltage.
The L-L voltages on the 480V side looks normal around 460-470 V with 2% VTHD but the L-N voltages on the 480V side shows very distorted waveform with 40% VTHD, we are seeing about 120V of third harmonic voltage on 277V L-N.
This is all when the transformer is unloaded and generator cannot run because it keeps tripping intermittently from 59N.

Anyone has any ideas what could be causing such significant third harmonic voltage and neutral overvoltage?
Utility says they are not detecting any ground fault on the 12kV incoming lines. We checked the grounding on both primary and secondary side of the transformer and everything looks normal.
Thank you.

 

[7anoter4]

The transformer-on the measurement time-is energized from 12 kV Utility line-I suppose.

 

[waross]

The utility lines are 12kV 3 wire and the neutral of the primary windings are grounded through a potential transformer (PT) at the transformer pole.

It may be more accurate to rephrase this statement:
"There is no local ground on the 12 kV system."
A PT is used to measure the neutral shift in regards to the true neutral position.
A neutral shift is normal with an ungrounded system.
It may be caused by untransposed primary lines, unbalanced loads or a combination of the two.
In the event of a phase to ground fault, the voltage across the PT will rise to line to neutral voltage.
In the event of a low impedance ground fault the generator will feed into the fault and increase the fault current.
The neutral voltage will rise to about 6.9kV.
In the event of a higher impedance fault to ground, the fault current will be limited by the fault impedance.
In this case the generator may share the fault current but it will not increase the fault current.
Once you are connected and loaded the P.U. harmonics may drop.

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

 

[Zylar]

Thanks for the response.
We can't figure out where the zero sequence harmonic voltage are coming from. There is no corresponding harmonic current. We tried to completely isolate the generator by opening the main disconnect switch, lifted the neutral and ground cables and just energize the transformer only without any load and the neutral voltage shift is still here. Maybe there is an undetected high impedance ground fault on the 12kV system?
You are correct, there is no local ground on the 12kV. It is a 12kV 3 wire service.
Regardless, this is an old legacy system from 1980, the utility now says this configuration is no longer allowed per G.O. 95 regulation (Wye gnd - Wye gnd transformer on a 12kV 3 wire ungrounded system). Most likely we will re-connect the transformers to Delta - Wye gnd. Thank you

 

[waross]

If you have a spare distribution transformer, try connecting it from neutral to ground and then loading it in steps.
The results will give you an indication of the impedance of the issue causing the problem.
If your transmission line is not properly transposed you will have uneven line capacitance from each phase to ground.


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

 

[jghrist]

I suspect that the utility lines are not ungrounded, but have a ground at the HV/MV substation. The 3rd harmonic voltage may exist on the utility system, coming from the HV/MV transformer magnetizing current or possibly other sources if there are any wye connected loads on the system. Have the utility measure harmonic voltages on their lines without your transformer connected.

 

[VTer]

I believe the 3rd harmonic voltage is coming from the magnetizing current of the core. Are you sure that the secondary neutral is grounded?

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[cuky2000]

It appears that the transformer is under overexcitation caused by saturation of the steel core due to the high harmonic level in combination with potential ferroresonance associated with the PT.
There are a few suggested options to study the possibility of reducing the neutral harmonics as follow:

1) The PT act as a neutral grounding transformer (NGT). Adding the proper size of the resistor will convert the system in high resistance grounding and possibly mitigate the neutral overvoltage.
2)Adding a 3rd and 5th harmonic filter could reduce the Voltage harmonics
3) Adjust the transformer taps reducing the voltage
4) Delta transformer can block the flow of zero-sequence harmonic current.
5) A broken delta vs. single PT scheme is more commonly used to detect neutral overvoltage using 59N relays
6) The 59N, neutral overvoltage relay to monitor the zero sequence.

The following sketch is our the interpretation of the original post added to illustrate the discussion.
We hope this help.

 

[mvcjr]

I suppose the point of synchronization is on the 480-V side.

If the synchronization point was on the 12-kV side, there would probably be no problem synchronizing to the utility. There would be no neutral shift on the transformer windings since these neutrals are tied to the generator neutral.