Solar turbine generator PMG lose 9

[newelecteng]

Hi all

We have solar gas turbine generator 3.1mw. We face yesterday PMG lose signal in HMI (the turbine reach it rated speed with 0 voltage and 0 excitation).
We check PMG output from generator side it does not give any voltage.
We check the PMG resistance it was 1.4omh (same as the other unit that are working).
We check the prepment magnatic inside it was able to grap screw driver.

Any help

Thanks in advance

 

[catserveng]

Most units this size have PMG fuses, have you checked those?

Have you checked the insulation resistance of the PMG stator?

With unit running at rated speed, check PMG voltage at generator connection box, most PMG's used on these units have a 240VAC nominal PMG output. If voltage present there then check at generator control panel where PMG voltage comes in, should be same value as measured at generator connection box.

In some packages they have a contactor or relay in the PMG circuit for fault shutdowns to kill power to the AVR or the CGCM (what you have varies with age and how it was ordered), sometimes these fail or teh output driving them fails.

If you have no or low PMG voltage (usually less than 50VAC) at the generator connection box you have a PMG or wiring problem in the generator excitation section of the generator. In most cases it is a wiring or connection issue, but something the PMG itself has failed.

Hope that helps, MikeL.

 

[waross]

Hi Mike. In a pinch, while waiting for parts, is it possible to power the AVR with three phase 240V, 60Hz from the grid, or is there a frequency issue with replacing the PMG source?
On small standby sets, I have replaced PMG AVRs with self powered AVRs while waiting for parts, but I doubt that there will be a suitable self powered AVR sitting around for that size of set. (Self powered = powered by the generator output)

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[newelecteng]

Thanks Mike we did the flowing

-Checked the PMG winding resistance and the reading was 1.31OHMs
-Megger the PMG insulation resistance with 250v, and the reading was >1GΩ.
-Checked the PMG cables continuity between the generator and the control panel and it was good.
-Checked all the rectifiers’ diodes and the surge suppressor all healthy.
-Checked the exciter field and the winding resistance and the reading was 17.4 OHMs.
-Checked the main winding continuity resistance and the reading was 0.06 OHMS.
-Megger the main winding insulation resistance with 2.5KV, and the reading were >20G

But we still have the problem

 

[newelecteng]

Mike we already check PMG voltage at generator connection box it read zero. There is no output from PMG.

 

[waross]

Mechanical Issues? Is the PMG turning with the shaft?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[newelecteng]

You mean the PMG rotor. Yup it rotate

 

[catserveng]

Did you disconnect the PMG leads going from the generator connection box to the turbine controls and recheck? A PMG lead or two grounded in the conduits between the generator and the controls will pull a PMG output down pretty far, newer PMG's don't have much extra power output. If you disconnect the leads to the gen and it still doesn't make voltage there, then you pretty much have a failed PMG, likely a stator, doesn't happen often but do get PMG's that look good by testing and just won't put out. But its pretty rare.

Also check if it has RFI Filters (Capacitors) in the PMG circuit at the PMG stator, this is pretty typical for KATO tail ends, and sometimes a failure in those makes it look like the PMG has failed by shorting it out.

On using separate Source for PMG power, depends on what the AVR is and if its using Diode Fault Monitoring, as that protection is using PMG frequency for a reference in the diode fault calculation. So if the unit has a Combined Generator Control Module (standard on a number of Solar packages) or a digital AVR like a DECS200 or similar, then using 50 or 60 Hz house power for the AVR may cause some nuisance problems. You can get around that changing the program in the AVR but usually not recommended, or allowed in some cases since in the CGCM its locked down in the PLC code.

Some more specific details about your unit could get you better answers, Solar turbines has a "standard" configuration for a lot of machines, but a large number go out with customized control packages depending on customer requirements.

MikeL.

 

[edison123]

Disconnect all the outgoing cables from the PMG stator.

Run the set at rated RPM.

Measure the PMG stator output voltage and frequency and check if they match the rated.

If they match, the problem is in the outgoing circuit.

If they do not, then likely causes are a) failed PMG stator winding b) significant loss of magnetism in the permanent magnets, which I have personally seen. Screw driver test is not proof of full magnetism.


PS: You might want to correct the typos in your first post while the correction window is still open.

Muthu

http://www.edison.co.in

 

[newelecteng]

Deras

We have CGCM for generator control.And we disconnect the PMG leads going from the generator connection box to the turbine controls we still have zero voltage output.
and we Checked the PMG winding resistance and the reading was 1.31OHMs which is good compare to other units.
And we did meger test on PMG stator which is good.

 

[newelecteng]

Muthu
We Checked the permanent magnetic strength by iron bar and compared with the stand by unit, found the same.

 

[waross]

I once saw an insulation fault that mimicked a zener diode.
The wire was shorted to ground to the point that we could hear the sound of the feeder cable expanding in the cable tray as the short circuit current heated up the feeder conductors.
We tested with a multimeter and read infinity Ohms.
Then we connected a megger. As we started to crank the megger slowly, the reading was in the meg=Ohm range.
At an estimated 3/4 of testing speed, the needle abruptly swung from almost full scale to zero Ohms to ground.
Try the megger on 500 or 1000 Volts.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TugboatEng]

Just to check the basics, you're checking PMG output in AC volts, correct? PMG outputs 3 phase AC.

 

[edison123]

A working rotating field + A working armature winding = Voltage

Solve the equation.

Muthu

http://www.edison.co.in

 

[catserveng]

Muthu, nice equation! Even a "weak" magnet on a PMG will produce some voltage at no load.

Tug, most of the "standard" Solar packages in that size had Kato tail ends, they all usually come with single phase reconnectable 120/240VAC. 3 Phase PMG's are common to some manufacturers, but still a large number are single phase. At least in most of the machines I see.

To the OP, I just worked on a 3MW Kato 12kV tail end last night, same complaint as your, no PMG voltage. Went thru the checks at the exciter connection box lifted the PMG leads and on the PMG side we still had zero volts. Meggered the leads, got a reading of over 200meg-ohms. Stator resistance at 1.7 ohms, all looked ok Pulled the exciter cover, traced the PMG leads back to coming out of the PMG stator. Found right were one of the PM leads came out of the stator, the insulation on the wire was broken where the varnish ended, and as we peeled it back and looked closer we found only 2 strands of the wire were intact, as we cleaned it back some more both of those stands were so brittle they broke. Fortunately the break was far enough away from the winding (not by much) we were able to make a field repair and get the unit back online.

Now on this machine it is in a coastal area, and the cooling air for the generator hits the outboard area of the PMG stator windings first, and in this case was obvious that corrosion was taking its toll on the PMG stator.

In your case you really haven't provided very much good information, pictures, details about which tail end you have, how its wired, etc. If you want good help, you need to provide good information. And when you do figure it out, please let us know to help improve the collective knowledge of the group.

MikeL.

 

[edison123]

Mike

Thanks, was a bit miffed with lack of details.

LPS for that interesting troubleshooting anecdote.

Muthu

http://www.edison.co.in

 

[mszahid]

Dear,

we are facing similar issue with one of solar turbines, could you please share the details how you resolved the issue. i have gone through almost all the testing and investigations to be carried out in the above trailing messages.

saad