Required Voltage Rating for Black Start Generator 2

[bxny]

My question pertains to a combined cycle power plant, one on one (gas turbine generator, steam turbine generator). The gas turbine generator will have a static frequency converter for starting. The plant will have black start capability, provided by diesel generator. Gas turbine generator output voltage is approx. 16.5kV. Question: What is the appropriate output voltage rating for the black start diesel generator?

 

[waross]

16.5 kV?
Generator Step Up transformer?
What capacity?
Generally lower voltages are easier and safer to service.
As a very rough rule of thumb that may be violated at will, what voltage will give you about 1000 amps or less?

Bill
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"Why not the best?"
Jimmy Carter

 

[bxny]

16.5kV is the output of the gas turbine generator.
There is a step-up transformer, but it is out of the picture. There is a generator breaker between it and the gas turbine generator. Between the gas turbine generator and the generator circuit breaker are the taps to the static frequency converter.
The black start generator is to provide startup power when the grid connection is lost or not available.

 

[rbulsara]

Would it not depend on the voltage of the auxiliary devices you need to start up such as lighting, pumps, fans or a starting motor?

List the loads you need to energize to allow a black start.

 

[BSEE1980]

Black start gens can be any voltage, common ones inlude 480V, 4160v, and maybe even higher.
I would also suggest you condsider your loads: motor loads can have high inrush currents and static inverters often don't handle inrush currents well.
Why do you need a static inverter? Hz conversion? Why not just have a aux power tap & CB off the gen leads to feed station power? You will probably need a xmfr, but that is cheaper than an inverter.

 

[davidbeach]

Any voltage you want, some are more practical than others. My guess is that you would probably look at 480V and 4160V. 480V would be more likely if the aux load is less than 1.5MW or is all served at 480V. 4160V would be a good choice above 2.5MW or if your motor bus is 4160V. Various trade-offs need to be considered.

 

[waross]

If the frequency convertor has not yet been purchased, can you consider two generators?
One would be for the station auxiliaries. The second, possibly with a step up transformer, would be the stating generator.
If the diesel generator is direct connected to the turbine generator before the diesel generator is started, the diesel generator may take the place of the frequency convertor. The generator need only be sized for accelerating the turbine generator.
The cost of two generators may be less than the cost of one large generator and a static convertor.
There have been discussions in these fora regarding motor/generator starting with a direct connected generator accelerating from a standstill. The diesel generator emulates a Variable Frequency Drive As the Automatic Voltage Regulator produces an output voltage closely proportional to the frequency/speed as the diesel generator accelerates.

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

 

[BJC]

Go through the start-up sequence. What are the minimum loas starts, when do they occur and what voltage are they?
Mark up a set of oneline drawing with the minimum loads for starting the G-T only.
Plan to sequence the start-up loads so the generator sees as few big jumps as possible.
Look at the voltages and pick the one with most
The minimun is what ever it takes to bring up the G-T and energize the Plant aux transformer. After the Gas Turbine is producing you can bring on the steam system.

 

[bxny]

Thankyou, all, for your input!
I do understand about determining which loads are required for starting up the plant, and going from there.
What I am looking for is anyone that is knowledgeable about or has direct experience with these exact circumstances - i.e. static frequency converter (sfc) starting of a gas turbine generator (rated ~ 200MVA) under black start conditions.
The sfc is part of the gas turbine generator package. Although, I do not have the exact specifications for this unit, on other units I've seen, the sfc is comprised of a transformer, with other components - converters, inverter - connected to its load/low voltage side. Rating is about 5000kva, 4160V.
My question is based on having no experience with providing black start capability with this type of configuration - medium voltage input requirement for sfc, etc., do I need one or two diesel generators, 100% vs. 50% each, what should their output voltage be, etc..

 

[rbulsara]

Whats the hurry? Wait for the right information. Google on "black start" and you get a ton of information. Beyond that engage experienced consultants.

 

[rcw retired EE]

Sounds like you have a 200 MVA gas turbine with a static start that uses the generator as a synchronous motor to spin the gas turbine up to running speed. The static starter is probably powered from the plant's auxiliary bus (6.9 kV, 4.16 kV, 11 kV) through a dedicated transformer with drive output to the generator about 4.16 kV, variable frequency. The drive controls are interconnected with the exciter to get the correct excitation on the generator rotor.

Your blackstart voltage should match your plant auxiliary bus voltage.

Typical power use for the staic starter is 5-8 MW. Typical MVA is 7-11 MVA for this size of generator. The load does not last long, but it has a large peak.

Try to find a diesel generator that can handle 11 MVA output with high harmonics. It's not easy.

I have used six 2.5 MVA machines in parallel to power the 6.6 kV plant auxiliary bus and run the static starter. They were specially designed to handle the high harmonic load. They also powered the plant auxiliaries and acted as standby/emergency generators. It was cheaper than other options.

The Teeside power plant in England used a GE LM 6000 gas turbine (40 MVA +/-) as a black start generator that also doubles as a peaking unit. The LM 6000 has a motor to roll the turbine through a hydraulic drive, about 200-350 kW. A small diesel handles that starting load and runs the plant auxiliaries. (I have started an LM6000 with a 500KVA diesel but it was tight.) The diesel gets the lights on and powers enough auxiliaries to start the LM6000 which then energizes the plant's MV bus and powers the first static start unit. The diesel is synchronized to the MV bus and shut down.

Black start is very expensive and doesn't buy that much. But if it is needed, try this method. (Or hire me.)

 

[ScottyUK]

Teesside's LM6000 is 56MVA, and the diesel set is 800kVA. Nice summary - were you involved in that project? I have so many questions about the quality of the auxiliary plant on that unit...


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

 

[bxny]

Thanks, rcwilson, for the informative input. Yes I am talking about an ~200MVA gas turbine generator. I am assuming that the high harmonic load is from the variable frequency drive electronics. Is that correct?

 

[NCTHAI]

For black start DG Set used on Gas Turbine
- Voltage rating should be equal to you SFC / LCI input transformer voltage.
- Power Rating should be equal to Start up power Required + auxiliaires load.
- You do not need full 16.5 KV voltage as input to SFC / LCI.
- Voltage rating will also depend on auxiliary voltage rating.
- Having LV DG set is not recommneded. MV is preferable.
- You may need intermediate trasnformer to match SFL / LCI voltage ratings and auxiliary load rating.
- Most preferable way is to have DG voltage rating equal to your station SWGR rating, In black start conditions use dead bus closing of gen set to stn swgr then distribute to aux load or SFL / LCI as desired.

Hope this helps!!

Regards,

 

[BSEE1980]

So does the inverter use the gen as the starting motor or does it run a separate motor for starting? I have seen both ways, but it makes a big difference in the inverter (LCI vs regular VFD) and the time you have to start.

The amortisseur (starting) windings need to support it: most gens don't have any amortisseur windings at all. Sync motors do, but they must be sized correctly for high inertia loads (like gear boxes and turbines). Thus you could use an LCI, but then you must do sequencing to get the field on at the right time.

Be Careful, or you could end up with one very expensive boat anchor!

I think the suggesting to hire a consultant / engineering firm might be the right way to go.

Steve

 

[rcw retired EE]

Scotty - I was not involved with Teeside. Several engineers from my clients and our former owners used to brag about it. I was going to do that LM6000 design but it went to a different division/office.

Steve- If the turbine uses a static start with the generator to roll the turbine, it is a package design provdied by the turbine/generator supplier. Not a lot of risk. Most vendors use the Load Commutated Inverter (LCI) technology. "LCI" has become the slang term for gas turbine starting system.

 

[ScottyUK]

Hi rcwilson,

Did your current / former employer have the initials S&S?


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

 

[bxny]

Hello rcwilson,
Would you tell me the name of the manufacturer of the diesel generators mentioned in your post of 25 Feb 09 22:46?
Thanks,
bxny

 

[rcw retired EE]

Scotty - I didn't work for S&S, just an EPC design build power plant engineering firm.

bxny- We had Cummins diesel generators for blackstart of a GE 9FA machine. Other suppliers like Cat also had a good offering. The parallel operation controls seemed to work well.