Generator AVR 1

[HamidEle]

I don't have much experience with generator but have to model the AVR and step-up transformer. The generator size is 40MVA, the step-up transformer is 13.8/260kv 50MVA. Trying to figure out if AVR is capable of maintaining 260KV bus @ 100%. The transformer has no on load tap changer.

 

[rhatcher]

HamidEle, maybe I do not understand your question. Are you trying to figure out if the AVR can allow the 40MVA generator to provide 100% output from the 50MVA transformer to the 260KV bus?

If so, I would say that the answer is probably no. This conclusion does not require a model to confirm.

The problem is not deciding if the AVR can provide enough excitation to overcome the voltage drop (output impedance) of the generator at 125% overload. The AVR possibly can provide this output. Most AVR's are not sized specifically to the generator requirements and have some excess output capacity to work with.

The problem is deciding if the 40MVA generator prime mover can supply 125% overload power. The prime mover probably cannot supply 125% power since most prime movers are sized specifically to the generator output. If the prime mover can supply 125% power, it will be for a short time rating.

 

[waross]

If this is a prime rated set, it will max out thermally at 44 MVA and the prime mover will max out at 35.2 MW
If this is a standby rated set, it will max out thermally at 40 KVA and the the prime mover will max out at 32 MW
If you want to know if the generator will maintain 100% voltage at 40 MVA, that is more an issue for the generator than the AVR.
Check your generator specs for the maximum voltage.

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

 

[ScottyUK]

The lack of an OLTC may be the limiting factor, although you may be able to bias the operating range a little by tapping up or down and allowing the AVR to compensate. Changing tap to increase the reactive capability at the top end of the range will also increase the lower end of the reactive capability.


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

 

[HamidEle]

I checked the generator specification. It says, the generator shall allow 100% voltage to be maintained at 90% speed. The voltage variation after instantaneous application of 1005 load shall not exceed -15% and recovery to 97% of rated voltage shall occur within 1.5 seconds.

 

[waross]

What is the voltage drop through the transformer? You can probably go about 10% over on the generator voltage to compensate for the transformer voltage drop.
If you need more than that, you may consider dropping the generator voltage and setting the transformer taps down accordingly. Be aware that dropping the voltage below rated will drop the KVA rating proportionally. eg. If the voltage is 10% low, the allowable KVA will be 10% below rated KVA.

NOTE:
It says, the generator shall allow 100% voltage to be maintained at 90% speed. The generator may allow this and it implies that the generator may be pushed to 10% over voltage,
BUT MOST AVRs ON DIESEL SETS WILL NOT ALLOW THIS.
As the speed/frequency drops so also does the inductive reactance of motors and transformers. This may be close to saturation for many devices.
The AVRs that I see on diesels have a feature called Under Frequency Roll Off. This allows a 3Hz drop in frequency and then starts to drop the voltage proportionately. 3 Hz is 55. An AVR with UFRO will allow a 5% speed drop. With a 10% speed drop, the UFRO feature will allow the first 5% speed drop and then drop the voltage 5% for the further 5% speed drop. (Regardless of the generator capability).
Check your AVR specs for UFRO and wait for Scotty to post with information on the use of UFRO on large sets.

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

 

[davidbeach]

40MVA probably isn't a diesel set.

Probably the only prime mover that could maintain full shaft power output at 90% speed would be a hydro machine, if even then. Steam turbine certainly couldn't, you'd fry it in a heartbeat. A gas turbine would probably stall as the compressor wouldn't be supplying anywhere near enough air for the combustion stage.

 

[rmw]

DavidBeach,

You are referring to a Frame type gas turbine where the turbine shaft rotates at generator speed, right?

With most aeroderivatives have the compressor spools independent of the power turbine driving the generator and typically they turn much faster than the power turbines. One GE Aero LM model cold end drive would be an exception, however.

rmw

 

[davidbeach]

If I think about it, I know that multishaft gas turbine machines exist. Never seen one, have no feel for them, and frankly didn't even think about one as I wrote my post above.

So, two power turbines? One to drive the compressor at a constant speed and one to drive the generator at what ever speed?

 

[rmw]

In the common aero machine, the power turbine is direct connected to the driven load and in the case of a generator it turns sync speed. (or for this strange islanded system of the OP, 90% of sync speed). The other spool or spools are associated with the jet engine that is essentially a gas generator that provides the hot fluid that drives the power turbine. The jet engine can be a multiple spool machine with spools turning at different speeds.

I am thinking of a 3 spool machine where some of the jet engine spools turn in the 10K range while a compressor spool (still a spool of the basic jet engine) turns 5K rpm. So that GT Generator operates at 3 different speeds, 3.6K, 5K, and 10K, all independent of the other. In 50 Hz, the 3.6K would be 3K.

rmw

 

[rhatcher]

Nice answer davidbeach! Hopefully hamidelec will reply with more information about the prime mover.

 

[waross]

My post was more about the UFRO feature on AVRs. As well as maintaining a safe V/Hz ratio the lower voltage tends to lower the load and aid in recovery from block loading. It would seem that turbines could benefit from load shedding when a block load drags down the speed. Is UFRO common on the AVRs used on larger sets.
Do you know if your AVR has UFRO Hamid?

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

 

[ScottyUK]

Bill,

There's normally a volt/Hz limiter which prevents over-fluxing of the machine. Similar function, different name.

Utility sets don't generally load-shed in an under-frequency event, except as a load dump when it disconnects from the system completely. Instead they'll follow the system frequency down and in the case of a gas turbine possibly try to correct it by over-firing the engine. It eats up hot parts life at a disturbing rate but is a useful way of gaining system reserve at short notice until a bigger set or a bigger station can pick up the shortfall. Transmission system operators normally pay for the privilege of having system reserve available, because otherwise the station operator would act to protect his machines by disconnecting at the time when the system is already in trouble: with a collapsing grid frequency the last thing the TSO needs is generation dropping off the system.


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

 

[waross]

Thanks Scotty.
Yours
Bill

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

 

[HamidEle]

Sorry for the late reply. This is a gas turbine and will be running in parallel with the grid. In the event of losing the utility supply, we are planning to do a load shedding. Our spec. does not call for UFRO feature, not sure if it is necessary to specify this. But we did specify a volt/Hz limiter. We will prabably never see 10% speed drop. Therefore, the requirement of maintanining 100% voltage at %90 speed might be overkilled.

 

[rmw]

I still remember from my college days when I worked in the campus power house, one afternoon when the boiler tripped and the operator got real flustered and couldn't get the boiler restarted.

As a student worker, they wouldn't let me touch anything (I had operated boilers in the Navy before returning to college but that mattered not) but they would let me stand in one corner and watch the water glass and keep them informed on the drum level.

The boiler tripped almost daily that summer at about the same time before they discovered that the sunlight falling on the FireEye mounting tube was causing heat wave refraction of the light rays from the flame on its way to the scanner, so I got to see this fiasco almost daily during my shift while the operators rotated shift every few days and only got to see it rarely.

So from my "station" I observed this old inept operator fail to restart the boiler on 2-3 attempts because he forgot to reset the manuals on the 2 gas fuel trip valves but he was a crotchety old guy and didn't like me anyway and I wasn't about to try to set him right. But when the lights began to waver -I mean flickering rhytmically indicating that the turbine was slowing way down I asked him "shouldn't you go call the City (municipal power plant) and tell them that we are coming to them? When we dumped load on them we commonly tried to give them some heads up.

He ran off to the phone and while he was gone, I reset the gas trips punched the start button and hastily retreated to my "station". When he returned, he saw that the boiler was on line and asked how that had happened. I said, "Well, I don't know, first that light came on (indicating the first light in the start up sequence - probably the purge light) then that one - the next one, then that one - pointing to the third one, and finally that one there and there it is. I didn't completely lie.

Unless he is reading this (he would have to be doing it from the grave) he nor they will never know how that boiler got relit. I was smarter than to tell.

Point of the story is that I have wondered many many times over a 30+ year career in the power industry what the frequency was when those lights started zoning in and out before he dumped our load on the city. I can't remember whether we were islanded or not, but we might have been. I nnow parts of the campus were always on the city and not on the campus power plant. I guess we must have been because the Municipal Power plant was on the huge multi-state "grid" so for the turbine to have slowed down that much, we would have had to drag the entire grid down - doubtful.

And if we were islanded, I have often wondered what the frequency mismatch did when he closed the breaker to connect us to the city.

Point being, you never know when a scenario like the some presented above might be real. The one I witnessed couldn't have been preplanned.

rmw