Hybrid Generator Grounding 1

[Okpower]

I am checking a New 25 MW generator specifications and I am evaluating the options for ground fault protection. This generator will be installed in parallel with two ones that are already installed (since 10 years ago).
Is the Hybrid generator grounding protection the best for this fault? Do I have to consider the same type of protection that the old ones?

Thanks for your opinions

 

[ScottyUK]

What is "hybrid generator grounding"?

Normal practice for a generator of that size is to use a single phase grounding transformer with a resistive burden on the LV side sized to limit stator ground fault current to 5 or 10A in order to protect the stator core.

What did you have in mind?


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[Okpower]

Here is a short explanation:

When there is a ground fault in the generator stator windings or associated bus connections to the generator breaker, the ground (87GD) and possibly the phase (87G) differentials will operate to initiate a high-speed generator shutdown. As part of the shutdown, the ground interruption device in series with the low-resistance ground path is tripped, disconnecting the low resistance ground source. This reduces ground current from 200 to 400 Amps to less than 10 Amps during the coast-down period, greatly reducing ground fault damage.


Taken from :

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

 

[ScottyUK]

eugeniojbg,

I've never seen such a system in use on an HV generator in the UK. A 200 - 400A ground fault would cause severe damage to the stator core even if it was cleared quickly. UK practice, in common with many other countries, is to limit the ground fault to 10A or so as a maximum. It is simple to detect a ground fault using a modern relay from any of the major manufacturers: Areva, ABB, GE, Siemens, etc. There seems no reason risk causing unneccessary damage to a very expensive capital asset like a stator core, unless you have a situation with an embedded generator on an existing MV network such as that shown on the sketch in you linked to.

Can you describe your generator's connection a little more? I had assumed - perhaps wrongly - that this was a utility generator connected into the HV network via its own GSU transformer. 25MVA is quite large for an embedded generator.

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Sometimes I only open my mouth to swap feet...

 

[dpc]

I assume you're referring to the hybrid grounding scheme being recommended by IEEE for industrial facility generators.

I think the decision will depend pretty heavily on how the existing generators are grounded. If they are high-resistance grounded, I don't see much reason to do anything different. If they are low-resistance grounded, there might be some benefit in using the hybrid scheme on the new generator, but ideally, you would want to convert them all.

Probably should check with the insurance company. Insurers have been one of driving forces behind the move toward the hybrid system.

 

[davidbeach]

Scotty, In the US (and presumably elsewhere with US types of influences), utility generation has been high resistance grounded as you are familiar with and the generator connected to unit transformer so that there is no need for the generator to feed ground faults - trip on any fault. In industrial systems, on the other hand, the generator is often directly connected to the system without benefit of a unit transformer. To facilitate clearing of downstream ground faults, the generators are low resistance grounded, to supply the 200A to 400A mentioned so that detectable ground fault current flows. All is fine and good as long as there are no faults in the generator. As a large cohort of industrial generators installed in the 40's, 50's, and 60's started to age and develop internal faults, the insurance carriers saw that they were having to pay out much larger claims than expected. Testing and studies determined that most of the generator damage was occurring after the generator breaker tripped. As a means of limiting this damage, but still allowing the generator to furnish ground fault current to faults elsewhere on the system, the hybrid grounding system was developed. A low resistance grounding system is installed in parallel with a high resistance grounding system with a high speed switch on the low resistance side. On faults in the generator zone, the switch opens and the generator becomes high resistance grounded.

 

[ScottyUK]

Thanks David,

I'd realised what the scheme was from the diagram on the Beckwith website - the system is fairly unusual over here as far as I'm aware. The reasons behind the research are interesting - it's good to see insurers actually trying to reduce the risk rather than just loading premiums to cover it. I probably didn't put much emphasis on the "...unless you have a situation with an embedded generator on an existing MV network..." comment in my post. My bad. Embedded machines aren't really my field and they're subjected to some quite different conditions compared to the big central generating station machines I'm used to.

The 87GD looks like a 64 REF function from what I can work out between the diagram and the description. Am I somewhere near? The 87GD designation isn't something I've seen before. Presumably the 87G is a standard stator diff scheme?

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Sometimes I only open my mouth to swap feet...

 

[davidbeach]

Yep, the 87GD is similar to the 64 REF. I'm not sure I fully understand exactly what an REF is, so I can't say for sure what the differences are. GD=Ground Differential. 87G is the standard phase differential, G in this case being Generator.

 

[ScottyUK]

REF - restricted earth fault. Basically a form of diff scheme for Y-connected windings which employs a CT on each of the lines and one CT on the bond from the star point to earth, all connected in parallel with a single-phase relay. The MCAG14 is a typical Hi-Z REF relay - there's an example on the bottom of page 2. Sorry about the link - I tried to link directly to the .pdf but it doesn't want to play the game.

http://www.areva-td.com/static/html/TDE-AGF_eLibGrouper-eLibrary_Detail_1030012080034.html?#

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Sometimes I only open my mouth to swap feet...

 

[Okpower]

Ok, very interesting and useful all of your opinions. The generator as you suggest, will feed a "islanded" Gas Plant with some power exportation. The old generators are just low resistance grounded with typical ground protection. Now the issues to evaluate are :

- Is this system proved enough?

- Is the system very expensive? For sure this equipment will not have any insurance, so the project has to handle the risks. And maybe, we have to suggest the same instalation on the rest of the generators

 

[davidbeach]

Is REF always implemented using Hi-Z? If so, that may be the difference between REF and Neutral (of Ground) Differential, which is typically Low-Z.

 

[davidbeach]

eugeniojbg, You'll actually get more bang for your buck by putting the hybrid system on the old generators if you put it on less than all of the systems. In the grand scheme of things, the cost of the hybrid system is very low compared to the value of the generator (what is the cost difference -including replacement power - between a 2-3 month rewind and a 12-18 month restack and rewind?). Even if the hybrid system fails, you have no less reliability than if you didn't have it at all, so it adds no risk.

No insurance? Why would anybody spend that kind of money and not insure the equipment?

 

[rbulsara]

a star for david.

 

[ScottyUK]

David,

Old fashioned REF was pretty much always Hi-Z for stability reasons and normally required a dedicated set of CT's. The modern low burden uP-based relays allow a Low-Z scheme to be used and share the CT's used by the rest of the protection. Sounds like 64 and 87GD are very close cousins if not identical.


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Sometimes I only open my mouth to swap feet...

 

[davidbeach]

Thanks Scotty. Sounds like a Low-Z REF and a Low-Z 87-GD are pretty much the same thing. Don't know of (doesn't mean they don't exist) any Hi-Z 87-GD.