High neutral currents when synchronized with the grid.

[electrouw]

Hello there.

I have an issue that has been there for a while, but I dont think I can ignore it any more since some caps from the power factor correction gear blew up a while back. Another problem which can be related to this is the current from the generators jumping radically between 900A and 600A in two seconds.

Setup:

One 625kVA and one 750kVA generator connected to a bus on the LV side of a wye/delta, 0.415kV/11kV 1500kVA transformer. Both generators only operate in parallel mode.
Both generators have their neutrals connected to the busbar neutral, which is in its turn earthed. Also connected on the LV bus are a couple MCC's with a total maximum load of around 1000kVA. I must also mention that some large VSD's are fed from these MCC's.

I have recently measures the I,V and harmonics directly above the feeder isolator that comes from the 1500kVA transformer.Just to be spefific, the measurement was taken before the busbar branches out to the generators and MCC's.

Attached is a graph of my findings.I will attach both a THD and V/I graph for the period of interest.
Take note of Friday 07:27. Before that instance, the generators were connected to the grid, and the neutral current was the same magnitude as the A and C phases. As soon as the GCB tripped, the power went up (more power needed to be supplied by the grid) but the neutral current went right down to zero.

1)If there are harmonics present due to the non-linear load, shouldnt it stay present regardless of the generators coming on and off the grid?

2) Can the harmonics be generated by the generators?

3) Is the generators somehow uncovering the existing harmonics in the system?

I think most of the issues are related to harmonics, but please help me to figure out if there is a way of minimising these effects and if not, what can be done. Filters? Transformers?

Please share some thoughts on my issues and if you need any more info let me know as I might have left out key information which could in fact point to the issue.

 

[rbulsara]

Just some hints, I do not intend to solve this over Internet.

1. Power factor correction capacitors on a generator system are odd and generally not a good idea.

2. The high neutral currents are most likely (I have not looked at your attachments) third harmonic currents, that circulates between the utility and generators. The alternator winding pitch has more to do with it than anything else. 2/3 pitch are generally preferable when paralleling with the utility (at least in the USA in my experience).

Subject to more scrutiny of your system, solutions could vary from making adjustments to settings to see if it helps to replacement of the alternators with proper pitch!



Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Wye:delta transformer? Is the wye on the Hi side or the Lo side?
Is the wye point connected to the appropriate system neutral?
Are you able to check the utility side voltages, line to line and line to neutral (if applicable).
Are you able to check the utility phase angles?
Are your phase currents balanced?

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

 

[vandal06]

How is your system earthed?

 

[electrouw]

rbulsara
As for the power factor correction gear, it was installed in the plant before the generators. When you say that it is most likely harmonics caused by the alternator winding pitch, do you expect these harmonics to be present in an islanding situation with a non-linear load as well? We have had these generators on a load bank (big heaters) and there was no sign of harmonics.It only seems to become a problem once connected to the grid.


waross
1)The wye side of the 1500kVa transformer is on the Lo (0.415kV) side.
2)The wye point is connected to the appropriate system neutral.
3)Do you refer to checking the 11kV voltages on the Hi side of the transformer? I do not have access to the transformers but I can probably arrange to get it checked. (neutral not applicable)
4)As for the phase angles I can try to get that checked out as well.
5)I don't know if you had a look at the attachments, but it does appear that the phase currents aren't very well balanced.

vandal06
The following is the case for both the generators.
There is a separate earth spike connected to the exposed metal of the skid and the alternator housing. The neutral terminal on the wye connection of the alternator is connected to the neutral bus bar on in the MSB (main switch board). The neutral does not pass the circuit breaker nor does is go through an isolator like the three phase cables.

The MSB has a large earth grid. There is an MEN connecting the earth grid to the neutral bus. The neutral bus is also connected to the neutral terminal on the wye side (Lo) of the 1500kVa transformer.

Is this enough? Could there be an earthing problem causing these high neutral conductor currents?

 

[rbulsara]

electrouw;
Only when paralleled with utility. In is-landed mode with multiple parallel generators, can also generate some circulating currents, but those are result of exchange of vars and does not appear to be the case in what you described.

I would look to disconnect PFCC when on generator.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

If you do not have access to the high voltage side you may check the voltages and phase angles on the secondary. The loads must be balanced and the generators must be offline.
Unbalanced phase to neutral voltages will cause neutral currents as the generators try to correct the voltage errors.
Unbalanced phase to phase voltages often indicate phase angle errors and neutral currents will result as the generators try to correct the errors.
Don't be surprised if you see both conditions.

As soon as the GCB tripped, the power went up (more power needed to be supplied by the grid) but the neutral current went right down to zero.

A good indication of grid balance issues.
These issues may result if the utility is using single phase voltage regulators somewhere upstream of you. It is possible that the 900V to 600V jump may be caused by the operation of a single phase voltage regulator upstream of you.


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

 

[electrouw]

waross

you mean the 900A to 600A jumps in the phase current?

 

[waross]

Sorry for the typo. Yes, jumps in current, not voltage.
I was thinking; an abrupt change in the voltage in one phase such as may be caused by the operation of a single phase voltage regulator responding to an unbalanced load elsewhere on the system.
An abrupt change in the voltage of one phase will often cause a greater percentage change in the current.

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

 

[RRaghunath]

There is another possibility - Resonance.
When you have combination of transformer, PFCC and the harmonic loads, there is always a possibility of resonance occuring in the system.
Operation of the system at slightly higher voltage than rated makes the situation worse.
In this case, it could be that the voltage at MSB is higher (than rated) when both generators are running causing resonance conditions that disappeared when one of the generators tripped (lowering the voltage). Even slight variation in voltage can make a big impact on harmonics.
This can be verified by actual simulation.
As rbulsara mentioned, it is a good idea to disconnect the PFCC when ever generators are in service.
Further, ensure the individual VSDS's have their own input filters.

 

[electrouw]

waross
I checked the phase to neutral as well as the phase to phase voltages on the LV side. They all seem very well balanced.

raghun
I will check if there is any difference with the PFCC switched off. If there is, what will be the solution? The site really need the power factor correction.

thanks for the replies so far guys!

Could this issue be related to the neutral not having the right impedance? I keep reading into earthing transformers on the neutral.

 

[RRaghunath]

eletrouw,
You say "The site really need the power factor correction", Why is it so!!
Are the generators not capable of providing VARs to correct the power factor of the system!!
The capacitors are justified / required only when the generator(s) is off.
In fact, capacitors connected at MSB could pose problem for the stability of the generator(s), as the generators are forced to run on leading power factor mode.

 

[waross]

Hi electrouw;
It is possible for line to neutral voltages to be balanced while the line to line voltages are unbalanced and the phase angles are unbalanced.
The cause may be miles away. Imagine a distribution line running for some distance, picking up a lot of single phase loads. At some distance out from the power transformer the voltage variation due to voltage drops becomes excessive. A bank of voltage regulators is installed, the voltages are corrected and the line may run for more miles before more correction is needed.
But in a rural area the loads tend to be single phase. Ranchers and petroleum producers may be running large single phase pumps. If one phase is more heavily loaded, one voltage regulator will operate to correct that phase. Now the line to neutral voltages are correct but the line to line voltages and the phase angles are not equal. The voltages must be equal, not just close. The current is limited by the impedance of the ground loop. The pu impedance of the ground loop may be 5% or 10% or 20% so that the ground current may be 20 or 10 or 5 times the voltage error that is causing the ground current.
If the phase to phase voltages are equal then we must consider harmonics.
You may be able to scope the grounding conductor and get a millivolt trace by connecting one scope lead to the neutral point in the panel and the other scope probe on the other end of the grounding conductor where it connects to the ground grid. unbalanced voltage problems will show a 60 Hz current. Harmonics will show higher frequencies.


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

 

[electrouw]

waross,

I did measure the harmonic distortion and it was very clear that there was a lot of 3th order harmonic distortion.

could this neutral currents be circulating currents? I must say that the PF control isn't set to the same value on the 2 generators.