Dedicated Unit Switchgear Bus to HT Motors Like BPF in Generation Utilities

^{Please don't shout (all-caps).}

It's bitchin' man!

I can't read all caps, it gives me a headache.

The idea of separate unit auxiliary switchgear bus for large motor loads such as BFP in power plants is not new. NTPC has been following this since early 1980s in their 500MW units.

A typical 500MW unit has two steam turbine driven BFPs and one motor driven BFP. The motor driven BFP is about 10MW and is connected to separate 11kV bus. The Unit Auxiliary transformer connected to generator terminals has two secondary windings, one of which is 11kV and is mainly meant to power the BFP.

Dear Sir,

Thank you very much for the reply.I do accept your comment.I've seen schematics with 11MW MDBFP connected to 11kv BUS.But this MDBFP is fed from Station switchgear.But what is the advantage we are getting by feeding MDBFP from UAT instead of from ST?Kindly clarify.

Too much lingo, not enough info...

I like that

Under unitised concept, all the unit auxiliaries (essential for running of the generating unit) are supplied directly from the generator terminals through unit auxiliary transformer.

BFP is the largest motor in a coal based steam power plant.

In normal case (when the motor is connected to common auxiliary bus), the voltage dip at the switchgear bus during motor starting needs to be limited to 85% rated so as not affect the other running loads connected to the same switchgear. When the BFP is supplied from a separate switchgear, it is possible to allow the voltage to dip to much lower value (starting torque requirement of BFP driven through fluid coupling is pretty low).

This helps in optimising the unit auxiliary trafo as well as the motor specifications and also enables use of standard designs instead of custom made ones.

It is not a new idea, it is a function of the design philosophy for the power plant and can be based upon a number of criteria including reliability, operational flexibility, cost, etc.

Reliability: essentially the Main Boiler Feed Pump (MBFP) is connected to its power source with the fewest possible components. Fewer components means less can go wrong, and in a plant with designed-in redundancy in its auxiliaries, a trip in an auxiliary circuit will not remove the MBFP from service thus allowing the plant to operate at reduced output instead of going off-line.

Operational flexibility: The plant can be isolated from the grid during an emergency (islanding) without shutting down. Protective relaying can be set to sever the connection to the grid while not tripping the plant auxiliaries essential for continuing operations, this allows for faster grid restoration using this plant as a power source for other plants.

Cost: by connecting this motor as close to the source as possible it will reduce the overall cost of the switchyard and the station auxiliary switchgear since they will not have to carry the MBFP's power requirements.

A base load generating station has many design considerations that go beyond that of simply running all the time. I'm sure there are a few other "old-timers" out there that can add to this.