Neutrals on grid connected generators 2

[electrouw]

Hi,

I have read every single thread related to generator neutral grounding and not grounding, third harmonics, circulating currents etc. I've read through Chapter 19 of Westinghouse T&D.
I'm not sure if it is my inability to understand, the different regulations or simply different site setups, but I can not get a grip on how I should go about the following.

The installation is as follows:

It is important to note that the generators are always connected to the grid so they are never operating in island mode. Each generator has only one circuit breaker connecting straight to the 415V LV bus. This LV bus is connected to the LV side of the star/delta 1500kVa transformer. On this same LV bus, 1200kVa worth of industrial equipment is also connected (mostly 3phase load), including some power factor correction gear with reactors blocking the 5th and 7th harmonics.

First only the 500kW generator were commissioned and worked fine with no excessive neutral currents. The neutral is directly connected to the neutral bus in the main switch room, which is directly earthed.

When installing the second generator (600kW) the problem started. This generator is connected exactly the same way as the first one. It's neutral is also connected to the neutral bus in the switch room. Both generators are set on PF control. On this I might also mention that the PF on the one 500 was set to 0.93 while the PF on the 600kW was set to 0.88. Can this have an affect on the problems I'm having?

The problem is the neutral current and third harmonics.

Since the installation of the second genset we've had capacitors blowing up, bus bars melting and fuses blowing out of the panels.

I have measured and am convinced that the neutral currents (same as phase currents in magnitude) are due to the third harmonics.

From reading all the threads I've seen heaps of different opinions but it could be due to slight variations on the specific sites.

Can I please get help on how to solve this one. I've seen solutions like zig zag transformers, disconnecting one of the generators' neutrals, NERs, neutral current eliminators, harmonic filters etc etc.

Help will be much appreciated.

 

[davidbeach]

One generator wants to hold the generation bus at a pf of 0.93 and the other at 0.88 and you are surprised at circulating currents? Set them both in reactive droop mode and let them find their own operating point. Voltage droop at the same time would also probably be a good thing.

 

[wayne440]

"...the PF on the one 500 was set to 0.93 while the PF on the 600kW was set to 0.88. Can this have an affect on the problems I'm having?..."

As "davidbeach" pointed out, yes. If it were my plant, it would also affect who ever set it up that way, if that person could be found.

 

[RRaghunath]

I think you need to keep one of the generator neutral switches open, considering that the generator neutrals are directly connected to the neutral bus (that is connected to ground). This prevents circulating third harmonic currents and related problems.
Earth fault clearance in the system should not be a concern as the generator whose neutral is earthed can supply the required currents for protection operation.

 

[PHovnanian]

The presence of third harmonic currents can be verified quite easily with a clamp-on CT and a suitable power analyzer (or oscilloscope) on various neutrals.

Do these problems occur with only the second unit (600 kW) on line? If so, suspect problems (third harmonic, etc.) with that unit alone. If problems are only seen with both units on line, I'd go with davidbeach's suspicion: It may have something to do with the interaction of the power factor controls of both units.

 

[waross]

One of your generators may have a damaged winding. The heat generated by the circulating currents caused by the PF settings may have damaged a winding with the result that heavy neutral currents are now flowing.


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

 

[pwrtran]

First only the 500kW generator were commissioned and worked fine with no excessive neutral currents. The neutral is directly connected to the neutral bus in the main switch room, which is directly earthed.

When installing the second generator (600kW) the problem started.

The problem is implied above - you have dissimilar generators that have dissimilar instantaneous voltage output waveforms. When you parallel them the circulating current is created due to the instantaneous voltage difference between 2 units and thus generated third harmonics.

 

[Ceast]

What is your total N current as a % of full load?

If the units are paralleled to the grid they sould be fine running in PF mode.

What type of regulators? Is there a droop CT input?

Have you tried running your 600kw paralleled with the 2nd unit off?

 

[Ceast]

There are wye generators connected to a wye / delta GSU?

 

[electrouw]

Ceast,

When you are saying they should be fine running on PF mode, does is make a difference if the PF's are set to different values? a few people seem to think that is the problem. The first thing I want to clear up is the neutral connections. Do I isolate them or not?

 

[electrouw]

the total N current at full load is around 80% of the phase current. very high.
I'm not sure what type of avr's are in there
I have tried runnin the 600kW paralleled on its own. Neutral current are definitely there.

 

[pwrtran]

electrouw

Your problem is not due to running different PF control it is probably inherent with the generator - the way the alternator is built and you can do nothing about it.

If you have high neutral current @ 60Hz, it is caused by unbalanced load. If the most of the neutral current is 180Hz, which is in your case, it is from paralleling dissimilar generators. Running only one generator the problem will disappear.

Check the generator spec to see if they are both 2/3 pitch.

 

[rbulsara]

What pwrtran said in the preceding post..

If the gens are running in parallel with utility, running them at different power factor is not an issue. The issues may be more due to incompatible pitch of the new generator winding (non 2/3).

As for the neutral currents, You can check this by disconnecting the neutral of the new generator from the system. This will make the new gen ungrounded but it should not pose any issue as the other gen and the utility are grounded. If it solves the issue, you may choose to run the system in this configuration or ground the new gen through a resistor. Ungrounded or resistance grounded gen will not serve L-N loads. You may need to review protection or monitoring and revise as necessary.

As for blowing fuses and melting buses the issue may be the resonance due to power factor correction caps and reactors. I would look to not use them if not really required or do a detailed analysis.


Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

That seems to be a lot of current for unmatched waveforms. I would expect uneven wave forms to cause a relatively small circulating current at no-load or light load, but that the circulating current would be swamped by the load current as the load increases.

Something is not right. Have you checked to be sure that the first generator has it's neutral connected? Despite what the prints show, this problem may have presented when the first gen-set was commissioned and been solved by disconnecting the neutral connection.

Wye:delta transformers may be problematic when the neutral is connected to the wye point.
The grid will feed a delta:wye transformer bank with no issues, but when the generators feed the grid through a wye:delta connection, problems may result. Any voltage unbalance or phase angle error on the grid may result in large currents circulating in the delta and in the neutral of the wye side.

The neutral of the first generator may have been disconnected but not properly documented.

Been there, done that. (Not exactly but similar. Wye:delta issues.)


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

 

[electrouw]

Thanks for all the inputs. I think the attached drawing will get us back to my question.please have a look

 

[pwrtran]

You may leave only one generator neutral connected and the other float so that there is no loop for zero sequence and triplex harmonics. The generator with floating neutral will not compensate your L-N load.

 

[waross]

I would check the phase to phase and phase to neutral voltages from the grid with the generators disconnected. Particularly the phase to neutral voltages. One phase to neutral voltage higher or lower than the others on a delta:wye transformer is an indication of unequal primary phase to phase voltages and the attendant phase angle errors.
This is not much problem for your plant, the motors may run a little hot. However, the generators do not see the delta:wye connection that the grid sees, the generators see a wye:delta transformer (bank). As well as circulating currents between the generators and the transformer you will also be causing circulating currents in the delta HV winding of the transformer. Does the transformer temperature seem a little high for the loading?
I heard you say that the generators will never be islanded, but what about unforeseen occurrences? Is it possible for any type of fault or failure to trip out the grid supply but leave the generators islanded long enough to possibly damage line to neutral connected equipment?
Now if the grid fails but the delta:wye transformer remains connected to the generators, the transformer will stabilize the neutral and avoid the damage that may result from a floating neutral. A wye:delta bank is sometimes used to create a ground reference or an artificial neutral.
I have seen a lot of issues with wye:delta transformer banks and your symptoms are text-book examples of wye:delta issues.
A distant fault on the grid may cause neutral currents, neutral damage and generator breaker tripping.

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

 

[electrouw]

waross,

firstly thanks for always providing some insight and possibilities.
1) I dont have access to the transformers (utility owns it) so Im not sure whether they are running hot.
2) I have vector shift and ROCOF protecting an island situation. Since commissioning, every time there was a grid failure or even just a quick switch the generators were immediately disconnected. The chances of that protection failing is still always there. Question, lets say an island mode does occur, what will happen if the L-N load is still connected to the N on the transformer (unenergised) but the phases are still being energised by the generator?

As for your concerns on the L-L and L-N voltage, they are perfectly balanced, I confirmed it today on site.

The big neutral currents only become and issue once the 600kW generator is connected. For starters I think I should just remove it's neutral and see how that goes.

 

[waross]

In island mode, the generators will see a wye:delta transformer arrangement. This arrangement may be used in place of a zig-zag transformer to generate an artificial neutral. As long as the transformer is connected it will stabilize the neutral and avoid a "floating neutral" issue. You may consider resistance grounding the generator neutral.
But, possible causes:
Is it possible that the utility has a tap changer or automatic voltage regulation that may be hanging up on one phase? I am thinking possibly a tap changer is operating in response to the 600 kW generator picking up some load, but one phase is sticking.
You may have bad windings in the generator.
Try disconnecting the neutral.
Try checking the L-L and L-N voltages on the generator with no load.

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

 

[pwrtran]

It is a good question why a relatively small third harmonic voltage caused by distortion waveforms out from two paralleled dissimilar generators can cause a huge third harmonic current on neutral?

I still would explain that the triplex flux jump through the high flux resistant air gap to complete the loop, thus result a very small third harmonic impedance Z3rd. So, a relatively small third harmonic voltage can cause a huge third harmonic circulating current on neutral!