GT Running Spare on the Islanded Power System

[sykimk]

I have N+1 gas turbine generator configuration on my islanded system and 1 generator will be a running spare (i.e. on-line but No power generation – Client’s expectation). If one of running generators is tripped-off by a fault in this operating condition, a running spare will start providing generation to support load demands. For your reference, this islanded power system has 3 sets of 12MW GTGs are running and 1-idential running spare and total plant loads are approx. 33MW. i.e. Total 4 identical GTGs are on the system.

On this situation, I think I need to shed some loads since the spare generator CANNOT follow the required load demands due to GT’s limited step load change (assuming 10 ~ 20%) although it was already on-line without power generation. I think all GTGs may be tripped off by GT’s overload protection and system frequency will continuously decay and it will also result in generator trip by underfrequency relay without a proper load shedding system under this operating condition. In order to avoid this situation, I think that the running spare must share the load demands with all other running generators. That is, 4-GTGs must in operation with load sharing mode.

Did I understand correctly? Any of your advices will be invaluable and please also advise if I missed some aspects of GT responses following disturbance.

Thanks in advance.

 

[itsmoked]

Even if they are sharing if one trips off the others are hit with 25% load steps. Is that still too much?


Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

These are probably aero machines so I'm not sure, but a 25% step load on a frame machine would be around the value where it would cause an irrecoverable speed drop and trip. We were nervous about steps of 20% and were relatively happy at 15% of rating, but that was from sync idle, not a part-loaded condition. You would need a good model of the machine and governor control loop to be sure, or you could play safe and dump the non-critical loads and bring them back in on in a sequenced start. If you have loads like cooling tower fans your process will withstand them being off for a few tens of seconds until you can restart them.


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

 

[waross]

With three machines running at 11 MW each, if one machine trips, you will have 2 machines running at 16.5 MW each, an overload of 137.5% and a step load of 37.5% until the idling machine picks up its share of the load.
With four machines running at 8.25 MW, if one machine trips you will have three machines running at 11 MW each, a loading of 92% and a step load of 23%.
Keith, the machines are loaded to a total of 33 MW not the maximum 36 MW. It drops the step load expressed as a percentage of full rated load.
Control will be a smooth natural response to extra loading, there is no need to detect a trip and respond by advancing the governor setting on the idling machine.

Bill
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"Why not the best?"
Jimmy Carter

 

[sykimk]

Waross,

I appreciate your comments but it is a bit unclear for me.
As far as I know, GT will have a permissible overload capability with short duration (e.g. 120% for 3 seconds). In this case, I am wondering whether the idle machine can pick up its share of the load within 3 seconds for example. If not, all running GTGs will be tripped off due to GT overload protection.

Would you please advise a bit more in details so that I can make sure whether the idle machine needs to be shared with all loads or can be connected as a running spare (on-line but no generation)?

I think the best operating case would be that the idel machnie needs to be shared with all other three GTGs to minimze step load change impact. Do I understand correctly?

Thanks in advance.

 

[waross]

Hello sykimk:
Short duration overload capacity is not the same as step load capacity. The scenario you have presented shows a step load of 46% and a simultaneous overload of 137.5% (Sorry I made a mistake in the previous post. The situation is even worse than originally assumed)
I'm a diesel guy. Scotty is one of our GTG guys.
Any comments Scotty?

"I think the best operating case would be that the idel machnie needs to be shared with all other three GTGs to minimze step load change impact. Do I understand correctly?"
Yes.

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

 

[ScottyUK]

Without any data on the machines themselves my opinion is that your best chance for keeping your three remaining units in service when the fourth trips is to have them all sharing load rather than having one at sync idle. Sync idle is the most sensitive state to having a step load from the work we did as part of a black start facility. Once you have a bit of load on the set it's behaviour is more stable as further step loads are added. If this is a CHP or CCGT then the steam quality from the idling machine will probably be awful (low temp and low superheat) so that's another reason not to do this.

Bill is correct that short term overload capability is different to a step load. A big step load may well result in a compressor surge and stall condition which is bad news for the engine. You may have bigger problems than a trip to deal with if that happens, so best avoid it.


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

 

[rcw retired EE]

Check the operating fuel costs. A gas turbine requires a lot of fuel to run at synch speed due to the power drawn from the shaft by the compressor stages. I don't have the figures with me now, but 50-70% of full load fuel consumption is what I recall. A CTG has much higher fuel usage at no load than a diesel engine generator.

Look at the fuel usage of the total plant with one unit idling. It may be higher than four units at 60% load each. Use that arguement to help justify operating all four units.