Issues with auto control 1

[davva]

We have a ships brushless ac 440v 450kw 0.8pf generator which is experiencing issues in auto control, would appreciate any ideas for further fault finding.

In hand control can achieve rated output

In auto we can deliver 100% rated kw out of set in auto control

In auto above 50% rated kva the voltage drops off and avr seems to run out of steam.

Have changed out all avr cards with known working set but problem remains.

Had sort of ruled out checks on exciter side of machine because working in hand control but now starting to look at this also as running out of ideas, other than wiring/poor connection/dry joint issues.

Thanks

 

[ScottyUK]

What components are common to both modes? Obviously the machine itself, but what else? Does manual control share some of the AVR's components, e.g. the controlled rectfier at the AVR output, the PMG (if fitted), etc?

Can you get at the output of the controlled rectifier with an isolated scope? Have a look at the output waveform - you should see faint commutation spikes as each power switch starts to conduct. If they're not at evenly spaced 60º intervals then you may have one leg of the rectifier failed which effectively reduces the gain of the circuit.

Have you checked the rotating rectifier? A failed diode on the shaft will also reduce the effective gain of the AVR, so you need more field current to achieve a given output. Manual mode may allow you to boost the field current above what the AVR allows in auto, so you may be over-driving the field to achieve your rated output. Do you have any data on the machine, or maybe a set of Vee curves? That would aid you in determining the expected excitation current for a given operating point.

 

[davva]

Thanks scotty, yes i did wonder if we may be overdriving in field. Also thought about looking a waveform, but would not have been looking for faint communtation notches so will look at that. The avr output bridge rectifier was also swapped out with one from a known working set.

Apart from wiring (which departs to different circuits on hand/auto select switch) only common component is pm pilot exciter and the shunt field itself. The hand control just has trimmer pot and full wave diode rectifier in circuit


When you refer to 'one leg of rectifier down' do you mean rotating diodes, or the avr output bridge rectifier. Assume you mean the later, but wasn't sure if you were referring to a reflected commutation period of rotating rectifer. I.e. Looking at waveform could show up failed rotating diode

Thanks for guidance

 

[ScottyUK]

You guessed right - I meant the AVR rectifier. If you already chaged it then it's probably out of the frame anyway.

I think your next step is to put a metering shunt in the field circuit, or get a sensitive clip-on ammeter. Measure what the field is doing.

Do you have a manual for the AVR? Does it have an over-excitation limiter? The OEL coming in early would give you the symptoms your seeing. In large machines it is usually a supervisory controller which is active in manual as well as auto, not so sure about smaller ones and if it is only active in 'auto' it might offer some explanation.

 

[squeeky]

Is the machine running hot?. A rotor earth fault would give you these symptoms if near the end of the winding. It will work until the second fault then it's all over if you don'theset have rotor earth fault protection. If you down load the connection diagram for the ABB RXPPK protection relay, you could See the diagrams and set up some tests. Not inthe office and working off my tablet.

 

[ScottyUK]

squeeky,

I don't follow your logic - with a rotor fault the problem would be there with both AVR and manual control of the field, wouldn't it?

 

[squeeky]

The AVR would deliver up to OEL. As the demand continues to climb, it can no longer regulate as the sectional the rotor that is operational is at limit and there is no further compensation and thus the voltage drops. The symptom I had on a rotor1.6MVA, 400v unit was the over heating. The earlier comment on blown diodes would also hold true blur inky experience without the overheating.

 

[davva]

check out rotating diodes and were o.k., also changed out with other healthy machine. same issue resides.

see attached for configuration.

currently in process of checking compounding diodes and resistors. any other ideas greatly appreciated.

 

[waross]

I have to assume that the manual control may supply a higher voltage to the field than the AVR.
Have you checked the polarity of the series field circuit?
If either the shunt field or the series field has become reversed this could limit the output.
I quick check may be to disconnect the series field and do a test run. If the voltage is higher then that is the problem. If the voltage is lower without the series field then it was connected correctly.

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

 

[davva]

apparently no one has touched series field circuit, so assume this to be o.k. at moment, wire number check seems to tally.

replaced compounding diode assembly and no change

attached are sum results in auto for the fault gen versus one with no probs.

seems avr is firing at the front stop at 50% load.

no starting to take some meggar and resistance checks of exciter circuit as thinking there may be a fault in winding.

 

[GTstartup]

I think you have to conncentrate on the differences between Auto and Manual control. You can rule out anything between the AVR and field because it is common to both auto and manual.

Differences between Auto and Manual:

Auto is closed loop and needs voltage feedback to regulate, manual is open loop control and regulates field current, the voltage is what it is. So check that the feedback is coming back correctly from the generator PT.s

Automatic channel has limiters whereas in my experience, manual channel does not. Check that all limiters are not in action in auto control

Manual Channel and Auto channel may use a different set of thyristors/firing circuit etc.. If this is the case, inspect and test the thyristors on the auto channel, swap out any common components from manual to auto channel, see if anything improves.

 

[ScottyUK]

"You can rule out anything between the AVR and field because it is common to both auto and manual."

Can you? I'm not so sure, because you have a feedback loop around that path which may be masking some of the problem. The auto controller will compensate for any loss of gain in the AVR loop so far as it is able to.

Good point about the second rectifier, that is definitely worth looking at. Some large sets use a lower voltage source for the manual mode, especially if the AVR is a powerful one designed for field forcing in auto mode, so the two paths have quite different characteristics.

 

[davva]

Maybe i wasnt clear but the results taken were for two different generators. First results are for our problem generator in auto. The 2nd set for another generator generator which is ok in auto up to full load.

We wanted to also compare manual results for both sets as well but didnt get the chance to do this yet.

 

[davva]

turned out to be fault internal to hand/auto switch!!!

 

[ScottyUK]

Ha-ha, so often the fault is somethign simple - pleased you got it sorted out.

 

[squeeky]

That's relief. When you say hand auto, are you referring to A25 and A26, automatic voltage regulation or A27 and A28 Field Current regulation? I have searched the manuals for hand / auto and found nothing. I would private the feed back. Thanks.

 

[davva]

The series field D1 and D2 was being shorted by the hand/auto switch in the auto mode but not in hand mode. I attached the schematic in earlier post. You will see the switch does short the winding but should only do this in the off position (presume a safety feature to ensure finding is shorted during maintenance), the switch was going open circuit in hand but was shorting the series field in auto.