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Yet another gen grounding question
4

Yet another gen grounding question

Yet another gen grounding question

(OP)
My client has a system with two 2M DG sets, 12.47kV on a paralleling board. Each is grounded via an 18 ohm resistor. We are proposing to add a third - the gear is set up for eight ultimately. My concern - 18 ohms seems like a rather low resistance value, esp. for what could be eight units in parallel. Should I be concerned for the potential damage if there is a winding ground fault? I'm trying to decide if we should recommend increasing the grounding resistance.

GF protection is 87G and 51N. Windings are 2/3 pitch.

Thanks.

RE: Yet another gen grounding question

2
As fast as your protection can get the generator breaker open, your generator is then grounded by only one 18 ohm resistor.  A resistor that small, assuming that the resistor in the neutral-ground connection rather than on the secondary of a transformer in the neutral-ground connection, isn't really doing a whole lot for the generator.  Serious generator protection would have about 720-1500Ω of resistance and the generators connected through unit transformers or a pair of generators on one transformer.  But that low resistance is really common in industrial installations.

What you may want to consider is using one resistor between a common neutral bus and ground so that you have the same ground resistance and the same ground fault current regardless of the number of running machines.  If you have the condition having anywhere from 18Ω to 2.25Ω you will have a difficult time setting your ground fault protection out on the system.

RE: Yet another gen grounding question

Hello.
From my point of view, David's idea is good and right solution.
One common resistor between common neutral bus and ground point.
Regards.
Slava.

RE: Yet another gen grounding question

I agree with David and Slava.

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

RE: Yet another gen grounding question

(OP)
Thanks guys. This is an industrial installation for standby use. Reliability and maintainability are of great importance, almost more so than protection of the units.

One concern I have with the single resistor approach has to do with maintenance. With a single resistor, units that are shut down for maintenance may still be energized through the neutral unless there is a neutral disconnect (which we do not currently have).

I had also thought about changing to a high-resistance approach which might allow me to use a 100% stator protection relay. But I'm not sure what other problems that may cause.

RE: Yet another gen grounding question

Hello Alehman.
Please pay attention: your generators are 2/3 pitch.
100% stator ground fault protection aren't applicable.
Best Regards.
Slava

RE: Yet another gen grounding question

2
You may want to check the IEEE (Pulp and Paper Group, I think) recommendations for industrial generator grounding.  There has been a working group headed by Louie Powell working on this issue for several years.   This link should take you to a Beckwith Electric page describing the hybrid grounding system that the working group came up with.  

http://www.beckwithelectric.com/powerlines/powerlines-40.html#1

High-resistance grounding is best for generator protection, but will lower availability because it is difficult if not impossible to selectively coordinate for a ground fault when there are multiple generators.  Lower resistance allows for selective coordination using overcurrent relays, but increases generator stator damage.  

With today's modern relays, 50 A should be plenty to two or three levels of overcurrent selectivity.  I agree that the resistance values in use now at this site are much too low.

Another issue with multiple grounding resistors to be aware of is the potential for circulating harmonic currents.  This generally is not a big deal to the generators, but the resistors need to have adequate continuous rating to deal with it.  This can be a concern when 10-sec resistor ratings are used.

One question - does this 12.47 kV system connect directly to  a grounded utility 12.47 distribution system?

RE: Yet another gen grounding question

Dave, good document, thanks.

Now, I would like ask one Q's, please don't angree.
Hybrid, intresting system with additional logic for high speed device with low resistance current disconnection capability, etc..
Please excplain me, what is a difference, if I don't add high resistance part and only disconnect low-resistor in fault time ( of course after main breaker open and only in caes of ground fault).
BTW, current decay will possible tens seconds (from one our local report).
Regards.
Slava

RE: Yet another gen grounding question

recorder, 17sec after trip.

RE: Yet another gen grounding question

(OP)
Slava - thanks. I discussed this with Schweitzer (relay mfg). They could not provide definite answer without knowing the harmonic content. That's a really ugly picture.

dpc - Thanks for the link. That will be my homework tonight. As you can tell I'm not a generator protection expert. As to your question - the generator bus is connected through transfer switches (made of 3-pole breakers) to the distribution buses. The transfer switch can be configured to operate with a brief closed transition. As best I can tell, the neutrals are not connected; everything is 3-wire. Loads are all DY transformers. I haven't looked at the downstream protection yet. Everything except the 3rd generator itself is existing, including protection relays.

Industrial installations with individual, fairly low value grounding resistors seem to be common, at least in the U.S. I agree that the resistance values are rather low. Is there a way to decide how much is too much fault current from the point of view of the generators?

RE: Yet another gen grounding question

Here's a PowerPoint presentation by Louie Powell discussing tradeoffs of various generator grounding methods and stator damage issues.  There's a little bit on the hybrid grounding method at the end.  

RE: Yet another gen grounding question

Dave, thanks a lot for both of links.
I will continue to learn this issue. very, very intresting.
Best Regards.
Slava

RE: Yet another gen grounding question

I've seen quite a few generator installations where they were very careful with the generator grounding, but totally ignored the transformer and utility ground sources that could flow into the generators.  

RE: Yet another gen grounding question

Usually the utility will try to work with you on the power source configuration. The problem I've seen is that the people who are asking for interconnect don't know what to ask for. They normally will ask for the least cost instalation.

If you want to control the utility source fault one of the best ways is to use a delta-delta transformer (or a wye delta), followed by a zig-zag transformer. That way you have control of the zero sequence source.

RE: Yet another gen grounding question

cranky108, if you want to control the type of transformer connection you will generally have to take service at primary (the distribution voltage) and supply the transformer your self, or you will have to have a 1:1 transformer between your secondary service and the generator.  Utilities will generally not supply special transformers.

RE: Yet another gen grounding question

I work for a utility and I know the answer is, it depends on the customer.
But in general for small customers, they don't have much of a choice, and the fault currents are usually very low.
For larger customers they normally do take primary service.

The issue for the middle size customers, is it depends.

RE: Yet another gen grounding question

Hello.
Additional Q on the issue.
Waht is a common practic in US:
Installed zigzg in Main trafo zone or with separate CB.
What is a action in case of zigzag CB trip.
Regards.
Slava

RE: Yet another gen grounding question

Zigzag grounding transformers are virtually extinct in the US.  I've put one in in 35 years and I've only seen a few others, most of them relics.

As far as zigzag CB trip - you NEVER want to trip the zigzag off the line.  If you trip it, it should be the last thing on the system to go.  

RE: Yet another gen grounding question

(OP)
dpc, thanks for the link to that presentation. One or two of the articles referenced therein were also very helpful.

The jist I get is that for external ground fault contribution less than 1200 to 1600 A or so, the damage from the internal contribution is typically greater than that from external sources. Above that level of external fault current, external sources may become the greater contributor. All dependent on clearing and decay times of course.

RE: Yet another gen grounding question

I think that the damage level will depend on the method of local grounding used.  If you have ground differential, the external fault should be gone within 6 to 8 cycles.  But there are a lot of smaller generators in service out there without ground diff.  

The main impetus behind the new IEEE Working Group recommendations was the complaints of the insurance companies who were getting tired of paying to restack stators on generators that were protected based on the current IEEE protection recommendations.  The low resistance grounding methods, while a big improvement over solid grounding, were still allowing significant stator damage.  

I recently did some relay settings for a new 13.8 kV system at a large plant with lots of large motors on it.  Their grounding resistor is sized for 1000 A.  I recommended that this be reduced to 100 A, but the design consultant said that 1000 A was their "standard", although they couldn't really explain why.  Old ways die hard in this business.

I think just increasing the resistance for the low-resistance grounding method would be a big improvement for many installations.  But this also means maybe replacing relays, CTs, etc.     

RE: Yet another gen grounding question

(OP)
Yea, I was assuming 87 protection. Not to get off on another topic, but my client has a propensity to connect large (comparable to one generator kVA) transformers downstream of the ATS, so they sometimes end up with long delays on generator 87 to avoid nuisance trips. Not good.

Quote:

Old ways die hard in this business.
  

RE: Yet another gen grounding question

Hello Alehman.
What is delay on 87G? Only right choice of CT's avoid nuisance trip. Don't agree with any delay on 87 protection
( if it's possible)
Dave, 1000A it seems so much.
Up today in my area used  300-500A ( resistors for 10sec only), but I think your 100A solution is more better. For newer relay isn't problem.
Yerstaday, we check some 22kV SWG with newer relays. On the 400/5A CT for curios we inject primary 4,4.1,4.2A. Relay show those currents 1:1.
Regards.
Slava

RE: Yet another gen grounding question

(OP)
CT's are another item I'm looking at on this project. The existing drawings are not clear. One source says the existing are ANSI class C20, which seems very low. Obviously need to do a burden calculation once I know for sure.

RE: Yet another gen grounding question

C20, I don't remember correlation between ANSI and IEC CT types. I see, you have,very "fun" project and customer.
Actally once, we need organizaied FAQ with correlation between ANSI and IEC for CT, VT and wiring size.
Regards.
Slava

RE: Yet another gen grounding question

C20?  That not a CT; thats a current sensor. winky smile

A generator is not the place for low quality CTs.  For slavag, a C20 CT will support a standard burden of 0.2Ω at 100A secondary without saturation, but there is also an assumed X/R in that and on a generator it is highly likely that the X/R will be considerably higher.

My thoughts:

Available ground fault current is much too high, but unfortunately all too common.

Hybrid grounding as mentioned by dpc could save you a restack, but at 2MW may not be considered economically viable.

Generator differential with good CTs doesn't need to wait for anything else, but you have to know that the CTs can never saturate.

Full differential (6 CTs) is better than ground differential (4 CTs) and the more ground resistance (otherwise a good thing) the more poorly ground differential will work, particularly with the breaker open.

For the single resistor between a common neutral bus and ground, you could add a neutral disconnect switch, key interlocked with the generator breaker.  That way when a generator is off-line for servicing you could disconnect it from the neutral once you have also locked out the breaker.

With what I've learned over the years, for that size generator, if starting a new installation, I'd buy the less expensive 480V generator and use those savings (and probably a bit more) on the purchase of a unit transformer.  There is a 12.47kV generator installation I designed once that I was sorely disappointed when the project was canceled.  The longer it's been since then the more I'm glad it was never built, because it would have had solidly grounded generators.

I'm now on the other side of the table and it is my job to tell customers that want to connect their generation to our system that it has to look effectively grounded at the point of common coupling.  Above 480V (or 600V where that is used) solidly grounded should never be used, low impedance grounded should be avoided if at all possible (much easier for new installations than existing) and high resistance grounded coupled to a unit transformer is the gold standard.  The hybrid grounding systems are a vast improvement over the low resistance grounded systems.  

RE: Yet another gen grounding question

(OP)
Thanks David. I agree on the CT's, but I must go through the exercise of proving it to my client.

RE: Yet another gen grounding question

Alehman, David.
Maybe for the maintanance used individual neutral switch for each generator?
Regards.
Slava

RE: Yet another gen grounding question

Quote 'Above 480V (or 600V where that is used) solidly grounded should never be used, low impedance grounded should be avoided if at all possible (much easier for new installations than existing) and high resistance grounded coupled to a unit transformer is the gold standard.  The hybrid grounding systems are a vast improvement over the low resistance grounded systems.'

In USA and Canda every large hydropower generator I worked with used a transformer and resistor for grounding the generator neutral. In Europe and Asia I have never seen a transformer used - only a resistor. Why is this?

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