Stean turbine drive induction generator

[dougreno]

We have a 1.275kW induction generator driven by a stm turbine. It continually trips on undervoltage, sync speed is 3600 and we run very close to that about 3610 with a let down pressure at 175#. When it trips it is hard to restart, ie it is difficult to get it back above 3600 rpm. Couple of questions, this is a 4160kV application.

1. When should the breaker close automatically at some point above 3600RPM?
2. What shoud the set point speed of the turbine be as a driver to allow the motor to be used as a generator? Currently that set point is 3610.

3. If more information is required please ask, while i am not an electrical controls guy, i can get the information easily and have plenty of experience with mechanical aspects of turbines.

Thanks you in advance for you help.

 

[cranky108]

I don't think I can answer your questions, but I might be able to provide some information.

Keeps tripping on undervoltage. You understand that the VAR requirments for an induction generator are somewhat close to that of an induction motor.

Also it is possible to close in the generator at 3600, but you should expect a little power delivered as at this point the Generator/motor at the same speed will have no field in the windings. The field is developed from transformer action when the generator/motor is moving faster or slower than the system.

Large sync. Generators are brought on a little faster than the system, and when closed they line up with the system. Then the steam is increased to pickup load.

Something simular is you can close at near 3600, then increase steam to pick up load.

 

[cranky108]

We have (or can have) simular issues with our induction hydro units (we have two). The difference we have is we operate at a lower speed.

 

[eeprom]

The breaker should close according to the generator phasing and the generator frequency. In general your generator should be going slightly above 3600. I think that 3610 is too fast. 3601 would make me more comfortable.

A little overview of how this should happen...

The governor controls the output of the generator by opening/closing the steam rack. The governor will typically control the generator in speed mode until the generator is on line. Once on line, the governor has no control over speed because the generator is in synch with the grid. That tiny little generator couldn't go faster than the grid if it wanted to.

At the moment that the generator goes on line, it should (might) be acting as a generator, producing a small amount of load. It might also be motoring. If there is any fluctuation in the grid, your generator may start motoring, in which case your reverse power relay should trip.

Once the generator is on line, the governor should be switched from speed control to power control. You will give a set point of kw to the governor, and the steam rack will control accordingly.

I hope this helps.

EE

 

[davidbeach]

Induction generator. I'd frankly be surprised if it really worked at only 3610. It needs to run at some slip speed above synchronous. 3610 would be like a motor running at 3590; doesn't really happen if there is any load. Any real generation will push the speed higher and higher.

 

[rasevskii]

Sorry, eeprom has it wrong. He is thinking of a synchronous machine.
For this induction generator, as dpc has said, 3610 is too low. set it a bit higher like about 2 or 3% that would be around 3670 rpm at least. Maybe that is why the UV is tripping, there is too high an inrush. Also it may be the UV time lag is too short.

If this is an old machine that was working well before, what has changed now so that there is now a probleM?

rasevskii

 

[davidbeach]

dpc? I don't see any posts by dpc. ;-)

 

[dpc]

I generally try to avoid induction generators - they never seem to deliver on their supposed simplicity and low cost. Bill (waross) will have better answers than I will.

I'm not quite sure why you would even need to trip an induction generator on undervoltage other than as a means of external fault detection. You certainly don't need to trip the turbine unless it is going out on overspeed after the generator is disconnected. What are the settings on the undervoltage trip? Normally over/under voltage protection just opens the generator breaker and does not trip the 86G or 86T lockout relay.

You should be able to close the generator breaker once you get to about 3600 rpm, but there will be a significant current transient that occurs as the motor's magnetic field builds up. This is different than a synchronous machine. You can expect transient current of maybe 3 to 4 times rated current for a short period of time. The no-load magnetizing current once the inrush dies down should be about 25% of rated current. The slip at rated load should be similar to slip at rated load as a motor. You might try loading it up a bit faster after you put it on line. Sitting at no load for an extended period can create some unintended situations in terms of protection.

I'm wondering if your system has some type of switched cap bank or other form of excitation support that is not operating properly. Induction generators can have fairly poor power factors in the generator region and this could be another reason the voltage is dipping.

Lots of generalities and few specifics, I'm afraid. I'd take a look at your undervoltage settings. They may just need to be de-sensitized a little.

 

[eeprom]

Oops...Induction, not synchronous. Sorry.

 

[waross]

An induction generator doesn't have much control over voltage. For many applications I would get rid of the under voltage trip.
An exception may be a voltage control system of switched or variable capacitors to increase the voltage. If that is the case, find out what has failed in the capacitor system and fix it.


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

 

[iop995]

If is a grid connected system (and seem to be for this power) you can start-up in motor mode (is very usefull for machine magnetisation) and very close to sync speed machine machine move in generator mode and a reactive current path from grid will assure magetisation current. Induction generator control is somewhat more difficult than synchronous (which have an build-in AVR and a strong DC excitation) but for grid connected systems, sync speed is kept by grid and governor controll power feed to grid. In insulated systems is much more complicate to control output voltage/frequency of an induction generator because there not source for magnetisation current and must switch various cap banks for that. For details check "SEIG" on search engines.