Synchronous generator cannot produce enough voltage

[eeprom]

Hello,
I can use some advice on a generator problem. I have a synchronous generator 10MW, 13.8kV. About a month ago, the plant had a shutdown, and afterwards they couldn't get on line because the generator couldn't build enough voltage. We got to about 9.4kV. The rotor was pulled out and checked out. No problems have been found in the rotor. The field current used run at 117A to get the stator voltage to 13.8kV. Now we need 150A to get to 9.4kV. Has anyone encountered such a problem, and can you give me a head start on what to look for?
thanks

 

[waross]

I took it as a conventional DC generator exciter feeding the rotor via slip rings. That was a pretty standard configuration on older machines. But that guess and $2 will get me a cup of coffee.

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

 

[yonrico]

First you need to determine if the AVR is operating
correctly.

1.You may want to check the output from the Permanent
Magnet Generator supplying your AVR.

2.Check if the AVR is reading the same voltage as the
busbar. If they are the same then the AVR is fine.
If not the same replace the AVR.

 

[eeprom]

The rotor has two sets of coils. The rotor's two coils are stacked on the rotor so they act as one field.
Don't get too wrapped up in this. Think of one winding consisting of two pieces of wire, connected in parallel. The stator is a two pole synchronous. Under normal operating conditions, the field current is around 120A and the stator voltage is 13.8kV.

The AVR?? It's not that complicated. The AVR could be in another state, it doesn't matter. While the rotor voltage/current were measured at 243V/146A, the stator voltage got to a little over 10kV. So, unless the rotor's two winding were wired with opposing fields, then the rotor was producing a field. The rotor speed was verified at 3600 rpm. So, knowing this much, what would cause the stator voltage to be so low?

 

[rbulsara]

I would ask the people who touched this generator/exciter during the shutdown. What did they do and touched. The coil wiring did not change on its own during the shutdown, right?

Rafiq Bulsara

http://www.srengineersct.com

 

[ScottyUK]

eeprom,

The AVR design matters if it is a rotating rectifier type because a failed diode or diodes on the shaft effectively reduces the gain of the system, so a given excitation current produces a smaller output. If it's a slipring machine then that's something which can be discounted. That's why people are trying to establish what the machine configuration is.

If this is a slipring machine then I'd be looking (and praying) for an instrumentation fault, then shorted turns on the rotor. If there's a rotating rectifier then add that between the previous two guesses in the order of likelihood and desirability.


----------------------------------

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

 

[waross]

If your rotor was originally wired with the two coils in series and has been reconnected with the two poles in parallel that may explain the high exciting current and low output voltage.

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

 

[burnt2x]

If the generator is unloaded (unsaturated), the field current to generator voltage should be linearly related. If in the past it was 117 amps to 13,800 volts, 150 amps should be around 17,692 volts at the generator terminals. You're getting about half the voltage expected with your field amps! Something is wrong with your rotor windings. Can you get data on your units vibration prior to shutdown and the units vibration with that low voltage condition? You may be having rotor winding faults

 

[GTstartup]

Factory data includes rotor resistance. Get a ductor (DLRO) and measure slip ring to slip ring - compare, this will be a good start to narrow down where the problem is

 

[rmw]

Does your rotor have any kind of ground fault detection?

rmw

 

[edison123]

eeprom

How do you know that there are two sets of coils ? This is a cylindrical rotor, in which you cannot physically see the rotor coils. And in a two pole machine, they cannot be connected in parallel since that that would require a lot of complicated connection arrangement not warranted for this capacity of generator.

You have misunderstood how the windings are connected and if they are connected in opposite,as you claim, you'll get zero voltage from the stator.

You're yet to post the generator and exciter details despite many prompts here. If you want proper answers, you should post full details instead of telling us "don't get too wrapped up in this".

 

[wolf39]

eeprom:

The two pole rotor has two sets of coils. Both sets of coils are connected in series but the first coil is wound in opposite direction to the other in order to get one north pole and one south pole.

Seems you mix-up the phase voltage with the terminal voltage (rated terminal voltage is 13800 volts, the phase voltage is 7967 volts). You better check the P.T. arrangement (check this for all three phases). The other possibility is a mix-up between series and parallel connection of the stator winding in case there are two separate circuits. The last possibility I can think of is that the calibrated shunt in the excitation circuit is defect. If the cross section is reduced (broken connection within the shunt) the respective voltage drop increases and simulates a higher excitation current.

It would help the forum to know what kind of work was performed on the generator and excitation equipment between plant shutdown and re-commissioning. We also would be interested to hear from you in case the problems were solved.

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[eeprom]

Thank you all for your help. Well, the problem most likely has been found. I took a pair of control transformers (480/120) and hooked them up in an open delta, just as the sensing PTs are. I wired the high side of this into a welding socket so that I could simulate a 3 phase 120V circuit, as would normally be seen by the voltmeter. Once I energized the control transformers, I went upstairs to the panel voltmeter. Phase a-b read normally. The other phases were way off. I went back down to the transformers to check their secondary voltages. 120 on all. The current draw was less than 1/4 amp per phase.

I checked the panel switch to see if it had a big voltage drop. It was okay. Then I traced out the circuit as shown on the three line. I had already checked all the fuses, so I started looking for breaks in the circuit. After some time, I found a wire pulled out of terminal block. And that was it. I re-connected it and all voltages were normal again. Someone had recently installed a new metering device on that circuit, and in doing so put too may wires into a terminal block and one of those wires didn't stay connected.

 

[waross]

Thank you for the closure, eeprom. It's nice to know what the problem was.

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

 

[rmw]

Funny how wild the conjecture can get when in reality the problem is so simple.

rmw

 

[edison123]

Some times, small things = big hassles. Good to know the problem got resolved.

Bill got it right in his 30th July post.

Have you double checked your voltage measuring accuracy?