Generator 100% Earth Fault Protection

[kssschsekhar]

Dear Folks,
With reference to the attached protection schematic diagram of 153.5MVA@0.8 lag Generator, 100% E/F protection was not considered. With my experience, such higher rating machines require 100% E/F protection. If someone knows, please discuss general / standard criteria of installing 100% E/F protection for such higher rated machines.

Advanced Thanks for your participation...

 

[davidbeach]

No machine "requires" 100% stator ground fault protection. It is always an economic decision. I would have arranged for 100% protection, but the amount of protection is up to the person doing the protection design. The insurance underwriter may also have something to say about it.

 

[kssschsekhar]

Thanks David for your reply. As per my understanding 100% E/F protection is required for higher rated machines with the following reason.

Quote
An undetected fault near the neutral end of the generator will bypass the grounding transformer and the ground-fault relaying. If a second ground occurs in the same phase near the generator terminal it will be undetectable and unrestricted by the grounding impedance. The generator differential relay would not operate for this fault, because current at each end of the winding would be the same. This extremely high magnitude fault would persist until it evolved into a phase-to-phase fault that is detectable by the differential relay.

Unquote

With refer to above discussion, seems 100% E/F protection is required for higher rated machines.

 

[davidbeach]

I'm not saying that it wouldn't be a good idea, and I'd certainly include it if I were doing the protection design. But "required"? Required says that there is a governing code or standard, with some enforcement mechanism, in place to make that "requirement". It's all economics, and I happen to believe that 100% stator ground fault protection is a good investment, particularly since the use of a decent generator relay makes it available for free, but I'm not aware of any "requirement".

 

[slavag]

Hi.
A better use 100%-yes, requerements-no, maybe some local code, but not some standard.
We have a lot of applications with 95% protections on the 200+ MVA, but..on the 64S voltages principle, not current.
Im also don't see 78-pole slip, are also better, but not requrements . Same for dead-mash protection.
BTW, 24-overflux function is connected to VT on the high side of trafo, is also not so good, preferd on the generator voltages.

Best Regards.
Slava

 

[ScottyUK]

I agree: it is a very good idea to use a 100% stator E/F relay, but it's unlikely to be 'required' either by code or in order to run the machine.


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If we learn from our mistakes I'm getting a great education!

 

[kssschsekhar]

Thanks a Lot for your Great partcipation. Also thanks to Slavag for identifying additional protection requirements.

 

[slavag]

Hi Kssschsekhar.
Aren't protection requirements and not some standard or code.
But, Im not agree with such application.
Supplier used redandancy on the terminal base, but connect all protection ( generator and transfromer) in series to same CT's, isn't good style of design.
And for the stub used only one CT before GCB, imho, you need additional CT after GCB for include GCB and naybe VT's in zone of generator 87G ( only one of relays).
Isn't redandancy for my pinion.
Best Regards.
Slava

 

[GTstartup]

I think you may have missed a subtle point of David's post. Most newer relays have these functions built in and just need to be set and enabled by a few mouse clicks. If that is the case in your situation then I believe it is "required" by common sense.

 

[slavag]

Yep, GT this is a point.
G60 have this functionality, why not use it. Why connect only current from neutral point of generator.
For additional, possible use also 64REF functionality.
Best Regards.
Slava