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Synchronous generator cannot produce enough voltage

Synchronous generator cannot produce enough voltage

Synchronous generator cannot produce enough voltage

(OP)
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  

RE: Synchronous generator cannot produce enough voltage

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

RE: Synchronous generator cannot produce enough voltage

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.

RE: Synchronous generator cannot produce enough voltage

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 ?

RE: Synchronous generator cannot produce enough voltage

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...

 

RE: Synchronous generator cannot produce enough voltage

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

Rafiq Bulsara
http://www.srengineersct.com

RE: Synchronous generator cannot produce enough voltage

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.

RE: Synchronous generator cannot produce enough voltage

(OP)
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.

RE: Synchronous generator cannot produce enough voltage

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.

RE: Synchronous generator cannot produce enough voltage

Check your speed measuring device also.

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

RE: Synchronous generator cannot produce enough voltage

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

RE: Synchronous generator cannot produce enough voltage

(OP)
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.

RE: Synchronous generator cannot produce enough voltage

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

RE: Synchronous generator cannot produce enough voltage

(OP)
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.

RE: Synchronous generator cannot produce enough voltage

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

rmw

RE: Synchronous generator cannot produce enough voltage

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.

RE: Synchronous generator cannot produce enough voltage

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

RE: Synchronous generator cannot produce enough voltage

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

RE: Synchronous generator cannot produce enough voltage

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

RE: Synchronous generator cannot produce enough voltage

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!
 

RE: Synchronous generator cannot produce enough voltage

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

RE: Synchronous generator cannot produce enough voltage

  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.
   

RE: Synchronous generator cannot produce enough voltage

(OP)
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?   

RE: Synchronous generator cannot produce enough voltage

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

RE: Synchronous generator cannot produce enough voltage

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!
 

RE: Synchronous generator cannot produce enough voltage

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

RE: Synchronous generator cannot produce enough voltage

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

RE: Synchronous generator cannot produce enough voltage

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

RE: Synchronous generator cannot produce enough voltage

Does your rotor have any kind of ground fault detection?

rmw

RE: Synchronous generator cannot produce enough voltage

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".

 

RE: Synchronous generator cannot produce enough voltage


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
WWW.HYDROPOWER-CONSULT.COM


 

RE: Synchronous generator cannot produce enough voltage

(OP)
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.


 

RE: Synchronous generator cannot produce enough voltage

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

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

RE: Synchronous generator cannot produce enough voltage

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

rmw

RE: Synchronous generator cannot produce enough voltage

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

Bill got it right in his 30th July post.

Quote (waross):

Have you double checked your voltage measuring accuracy?

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