Generator Breaker Failure Protection exception

[odlanor]

3.4 Generator Breaker Failure Protection
Breaker failure protection shall be provided for all relay-initiated generator trips with the exception of anti-motoring.
WHY the exception??

 

[pwrengrds]

Likely because for this fault you generally have some time before damage will occur.

 

[davidbeach]

Though I wouldn't create that exception. All protection trips should initiate breaker failure protection.

 

[ScottyUK]

A steam set will be quite happy motoring if condenser vacuum is maintained. If vacuum is lost then the LP stage of the machine will be damaged due to windage heating of the blades, and this happens pretty quickly. A gas turbine will suffer over-cooling of the power turbine, which will eat up a lot of hot parts life, which equates to a lot of money. It sounds like the exception was written to suit steam sets, not gas turbines. From a plant operator's point of view I'd want that machine off the bars as fast as possible: a turbine is a significant capital asset to place at risk, not to mention the potential for lost revenue while it is being repaired.

If the machine exported into a double-busbar substation and other circuits could be transferred to the alternate busbar prior to dropping the first bus then that might be acceptable if it could be achieved quickly. 'Quickly' would be minutes, not hours, and that would require a pre-determined emergency switching plan to be in place.

An integrated generation / transmission utility might decide the risk of wrecking a machine was less than the wider impact of tripping a grid substation when the BFP operates. With a fragmented utility market like that in the UK the plant operators will want act to protect their assets and leave the transmission system operator deal with the fallout. It can get contractually complicated when a substation is in shared ownership and the upstream BFP is implemented using breakers owned by the other party with a different set of priorites.


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

 

[odlanor]

davidbeach,
see reference:
PJM Relay Subcommittee
Protective Relaying Philosophy and Design Standards
Revision: 03
Effective Date: June 1, 2003

 

[m3ntosan]

ScottyUK,

Some power station operators are using this method to take the gen set out from the system, trip the turbine and wait for the motoring protection to trip the generator, so I am assuming there’s no reason to clear a busbar if the CB fails, assuming the turbine is already out and motoring is not a problem for generator.

When would you expect motoring to occur?

Apart from when turbine tripped and generator is still connected to the system or in the very unlikely situation of a system fault causing an unstable gen set to exceed 180 degrees power angle and go motoring for half a cycle situation when overspeed protection should operate.



May you grow up to be righteous, may you grow up to be true...

 

[ScottyUK]

m3ntosan,

I would expect motoring to occur IF any of the many prime mover protections operated to trip the unit and closed off steam at the main stop valves AND the generator circuit breaker failed to open when commanded.

The risk is not to the generator but to the turbine. On a steam set the LP stage - where the blading is longest - has very high tip velocity and windage can quickly heat the tips to the point where they lose their mechanical strength.

Reverse power relays on steam sets are notoriously difficult to set because they are trying to detect the very small power required to maintain the machine spinning. With condenser vacuum established the motoring power is a tiny fraction of machine rating. If the machine is still excited and exporting reactive power then it is quite possible for small phase shifts in the instrument transformers to make some part of the reactive power to appear as active power, which will confuse the relay by giving a false indication of power export.

A gas turbine will have a large load due to the braking effect of the compressor and will trip the relay.


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

 

[odlanor]

thank you guys
I only understand hidroelectric.
I am with david