Generator AVR
Generator AVR
(OP)
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.





RE: Generator AVR
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.
RE: Generator AVR
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
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"Why not the best?"
Jimmy Carter
RE: Generator AVR
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If we learn from our mistakes I'm getting a great education!
RE: Generator AVR
RE: Generator AVR
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
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"Why not the best?"
Jimmy Carter
RE: Generator AVR
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.
RE: Generator AVR
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
RE: Generator AVR
So, two power turbines? One to drive the compressor at a constant speed and one to drive the generator at what ever speed?
RE: Generator AVR
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
RE: Generator AVR
RE: Generator AVR
Do you know if your AVR has UFRO Hamid?
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator AVR
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!
RE: Generator AVR
Yours
Bill
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator AVR
RE: Generator AVR
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