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]

Is it turning fast enough? Have you double checked your voltage measuring accuracy? If this is brushless did you check the rotating diodes?
If this is a static exciter do you have a blown diode?

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

 

[Pikoplatt]

Did you perform pole drop tests and rotor insulation-to-ground test on the rotor to check for turn-to-turn shorts and grounds? If you have a double-ground (at two locations) on the rotor winding, you will require more field current to generate a given voltage.

 

[edison123]

Pikoplatt

If there is a double ground on the rotor, he cannot run the machine at all. It would short part of the rotor winding leading to severe magnetic unbalance and vibrations. I agree with your turn shorts though (ac pole drop test should confirm that) along with Bill's speed, diodes and measurement issues.

eeprom

How do you measure the rotor current ? Do you have sliprings on the rotor (DC exciter or static excitation ?) What checks did you run on the rotor ?

 

[Yuma]

You could use an oscilloscope to see the field current waveform, and it will tell you if you have a non-working rotating diode or a blown fuse of a static diode - in case this were the problems...

 

[rbulsara]

blown or non-working diode= shorted diodes perhaps more than open.

Rafiq Bulsara

http://www.srengineersct.com

 

[Pikoplatt]

edison 123 Quote:"If there is a double ground on the rotor, he cannot run the machine at all."

The location of the double grounds can be within the same pole, just a few turns apart, so only a few turns are shorted out. I agree, if grounds involve large portions of the same pole winding or more than one pole winding, there will be significant vibration.

 

[eeprom]

Okay, first of all I can read a DC current on the rotor, so the exciter is probably working. And despite the exciter, if I have a field current of X amps, I should be able to look at the generator curves to establish what the stator voltage should be.

I like the idea of checking out the voltage measuring device. I don't think that has been looked into.

 

[edison123]

The fact that you're generating voltage is proof that your exciter working. But it's not working at its rated capacity (due to diode failure). This is assuming you have ruled other rotor faults.

 

[waross]

Check your speed measuring device also.

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

 

[GTstartup]

Is it static excitation or brushless? Seems like it is static because you are giving actual Field Current and not just exciter current, which would be much lower. I would measure the field current with an independent instrument, say a clamp on DC current probe at the cables to the slip ring (or at the cables to the exciter if it's not a static excitation system. Also I recommend a DLRO on the rotor to compare against the factory resistance value

 

[eeprom]

The exciter is an old style DC coupled generator with brushes. The exciter was able to supply 247V and 144A. So the exciter works. If the current is flowing through the rotor, then there must be a field. The stator windings have not been checked yet. The most stator voltage created so far has been 10,100V at a field current of around 255A. The rotor speed has been measured with two independent tachs, and the frequency was measured at 60Hz, so there is no speed problem.

 

[waross]

Taking the over excitation into account, that's about what you would get for a correct voltage if you were measuring phase to wye point instead of phase to phase.


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

 

[eeprom]

Yes, I have thought of this also. But the wiring has been in place (and working correctly) for 20 years. I've checked the PTs for blown fuses. All circuitry is good. But I have considered something else recently...This rotor has two sets of windings. I don't know how this could happen, but if one set of windings is wired opposite the other set, the magnetic fields would oppose. The generator would still build some voltage, but not enough. I don't have access to the rotor to determine even how the rotor windings are attached to the slip rings. But this is easy to check and worth looking at.

 

[rmw]

Quote: " two sets of windings ". Is this a 2 pole or 4 pole machine?

rmw

 

[edison123]

It would be better if you posted all the generator nameplate details and exciter nameplate details along with photos of "two sets of rotor windings". I have not seen or heard of two sets of rotor windings.

 

[rmw]

Two sets of windings. I thought about it for a while before I asked the above. A 2 pole has one set of windings on one side of the rotor and one just like it on the other side as I know you know, but to some that could be 2 sets of windings. A four pole, on the other hand, if you laughed out loud at the above (as I did when I initially thought that), would then truly have 2 sets of windings. It is still ambiguous to me as stated.

rmw

 

[GTstartup]

Agree with edison, this is getting silly, 17 posts in and we still have to read between the lines to find out if it is static or brushless excitation

 

[rmw]

Quote eeprom: "The exciter is an old style DC coupled generator with brushes." I think the one thing we do know is that is static excitation, although I admit it did take 11-12 posts to get that far.

rmw

 

[ScottyUK]

I'm still not sure what this machine is. That description sounds like a PMG as a source of excitation power, but whether it's feeding a slipring machine or a rotating rectifier is still anyone's guess.


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

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