Yet another gen grounding question 4

[alehman]

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.

 

[davidbeach]

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 720-1500[Ω] 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 18[Ω] to 2.25[Ω] you will have a difficult time setting your ground fault protection out on the system.

 

[slavag]

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.

 

[waross]

I agree with David and Slava.

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

 

[alehman]

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.

 

[slavag]

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

 

[dpc]

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?

 

[slavag]

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

 

[slavag]

BTW, this generator after stator ground/earth fault.
25MW 6.6kV

 

[slavag]

recorder, 17sec after trip.

 

[slavag]

Sorry. some problem with my i-net.

 

[alehman]

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?

 

[dpc]

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.

 

[dpc]

And here's the link: (Doh!)

http://www.ewh.ieee.org/r1/schenectady/feb15_2006_slides.pdf

 

[slavag]

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

 

[dpc]

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.

 

[cranky108]

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.

 

[davidbeach]

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.

 

[cranky108]

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.

 

[slavag]

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