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Turbogenerator failure
During start up (not connected to grid, no load) at about 60% nominal speed of turbogenerator, generator's frequency and voltage goes up to its nominal and turbine speed increase stops,also not responding to manual speed increase.

Hello to all,
I've an issue regarding turbogenerator's start up.

During start up (not connected to grid, no load) at about 60% nominal speed of turbo-generator, generator's frequency and voltage goes up to its nominal (60 Hz and 450 V) and turbine speed increase stops. also not responding to manual speed increase.

As I remember, the formula for frequency related to speed is f=pn/120. Generator nominal speed is 1800 rpm, so I've 4 pole generator.

So I can't wrap my mind around this thing, how can it be that generator speed (n) is just around 1100 rpm, but frequency and voltage is already at nominal.

Where the fault could be?
Thank you very much!

PJeff,

How is the turbine coupled to the generator?

> How is the turbine coupled to the generator?

By reductor

Having never heard of a 'reductor' between a prime mover and a generator, I presume that it is a reduction gear which reduces the RPM of the turbine to a lower speed so the generator can be operated at synchronous speed while the prime mover is operated at it's optimal speed.

Look, frequency is directly proportional to speed. If the turbine at your site is coupled by a reduction gear to the generator and you say it never reached rated speed (of the turbine or the generator) then there's no way the frequency could have been at rated--regardless of the number of poles of the generator.

I don't know where you were measuring the frequency and voltage, or if there was some relay(s) or indicating light(s) that tell the operator the unit is at rated frequency and voltage but there is a problem with the sensing for the relay(s) or the sensors used for the indication.

I was wondering if the turbine was not directly coupled to the generator through a reduction gear if it might have been possible for the generator rotor to be at a much higher speed than it should have been for the turbine speed. There are several manufacturers of smaller turbines that have what are called "free turbines" serving as the "power turbine" that are coupled to the generator rotors, usually directly coupled (no reduction gear).

But, we don't have enough information about when this problem started and about the configuration of the unit and its auxiliaries and frequency/voltage sensing.

Another thing: To a certain extent, generator voltage is a function of rotor speed. If excitation is held constant and speed increases or decreases then voltage will increase or decrease proportionally. You indicated the frequency and the voltage were at rated when the unit was below rated speed. I still maintain there's no way the frequency could have been at rated if the speed was less than rated, but it is possible that the excitation control may have thought the generator was at rated speed (when it wasn't) and increased the excitation to make generator terminal voltage equal to rated. Not likely, but a possibility.

Again, you have not provided sufficient information about the configuration of your machine, when this problem started (which is always the FIRST question to be asked when a problem like this develops), and what has been done to try to troubleshoot the problem and what the results of the troubleshooting were.

Has the unit been started and synchronized since this event?

I had something very similar when I thought I could calibrate a vintage TSI speed channel before roll. If generator frequency is at rated, then turbine speed is at rated. I would expect a speed calibration error

you stated it was a 4 pole generator at 1800 rpm with a reduction gear. What is your rated turbine speed?