Alternator Issues post bad weather/unit shutdown

[ekuegler]

Hi all,

Can you explain the effects of a large imbalance between phases R-Y, R-B = 0.38 milli ohm Y-B = 4 ohm when a generator is ductored, IR/PI good.

Star bar disconnected, IR between phases good. Line to line ductor all 3 phases identical. Only when star bar connected is there an issue when ductoring.

Unit didn't trip on any protection type, severe weather to cut a long story short caused Shutdown and consequently seawater/lube oil to enter alternator enclosure/ submerge stator 1/4 way up, now drained down awaiting chemical clean.

Can this unit be ran once cleaned with the large imbalance as above? Could interturn failire be a cause?Surely the diff protection would have operated in this instance....?

Any thoughts on alternate tests to carryout to pin point route cause. Strange situation given it ran with no known issues/alarms prior to unit shutdown due the plant ESD.

 

[ekuegler]

Regards running the unit in this condition, I would say no. Any opinions or expert advice greatly accepted.

 

[ekuegler]

Assume with this imbalance if online the NPS protection would disconnect the generator set..?

 

[davidbeach]

Just to make sure I understand your numbers - R terminal to neutral to Y terminal and R terminal to neutral to B terminal are both 0.38 milli ohm while Y terminal to neutral to B terminal is 4 ohms? If so, you have a test set up problem. There is nothing in the 4 ohm path that isn't also part of a 0.38 milli ohm path.

 

[ekuegler]

Yes that's correct, R-Y,R-B = 0.38 milli ohm, Y-B = 4 ohm, regards test set up we have experimented with 2 different ductors giving identical results, standard ductor test carried out.. So elimination of a test unit abnomaly. We get an imbalance with the star bar connected between above phases, but with it disconnected the line to line ductoring resulted in all 3 phases reading identical resistances.

This we cannot understand, even when bolting together instead of using the star bar we get the abnomally as above, so ruling any issue with the star bar out.

Could an /insulation failure/short on 1 phase give rise to such an anomally?

Any thoughts.....!

 

[ekuegler]

*interturn/insulation

 

[rasevskii]

If the stator was actually partly submerged in seawater then it is likely more or less ruined, or will fail rather quickly after you start up the unit. You do not say what is the voltage rating or MVA size of the unit. The insulation will be severely compromized. Do a megger reading first at 500V on each phase and see what you get. If it is a small unit, get it to a rewind shop and follow their advice.

Corrosion of the stator iron will occur quickly also of the copper conductors. If the unit was submerged for a long time in salt water (days or more) then it will be possibly unrepairable.Not to mention the simeltaneous damage to the rotor. If it is a brushless unit even more bad news...

rasevskii

 

[ekuegler]

Hi there,

My apologies - 11kV, 60Hz, 10.5MW, rated, IR/PI tested at 5Kv. Insulation results suprisingly good, in ggig range. As you mention and as we have concluded the fact that the stator has been submerged in seawater/lube oil is a concern and will inherently impact on the sets integrity. It is a brushless type, submersion was on the lower stator part, rotor IR at 14 G ohm.

It is suggested that a chemical clean be carried out but how sure or advantageous this is given it was submerged for 3 days I don't no. Insulation breakdown is my concern and will a chemical clean remove all the impregnated interfaced mix.

Putting this part aside what is the consensus on running the unit with this large an imbalance/ possible effects or damage....! Rotor heating...! Again we have opinionated a no run response to our shore based team. Your thoughts would be appreciated.

 

[rasevskii]

Amazing that you get such remarkably high Mohm readings under these conditions. Did you dry out the unit by heaters, or run it on short circuit with low manual excitation? This is a way of drying it out.

Also the air passages in the stator and the spaces between the laminations must still be full of water/oil that will be driven out as the unit is operated.

The best procedure would be to comtact the OEM or an established winding repair company for an opinion.

Also the rotating rectifier assembly and exciter field and armature may be affected.

IMHO it would not be advisable to consider running the unit on voltage until at least the OEM opinion is available.

Others here may comment?

rasevskii

 

[rasevskii]

On your ductor readings, if there is a bolted joint in series with the circuit on one test but not on the others, that joint may account for the high reading. (corroded)

The IR also may look good at megger test voltages (5KV) but at full voltage there may an issue (fault) at the coil end(s) due the semiconducting tape/paint being salted over or otherwise no longer effective.

rasevskii

 

[ekuegler]

We had toyed with the idea/proposed running it in short cct mode, left in the sure teams hands as to a systematic approach forward, I.e plan to strip and rewind ashore or in situe or if they feel any additional testing is required not that there is any other. Will wait and see I expect.

We were also surprised with the IR/PI results given no treatment in terms of drying out using heaters/short cct operation.

The units ACH obviously incurred damage due to submersion and are inactive. Excitor assembly already backloaded given size.

In few words it's not looking good.

Best regards and thankyou for your posts.

 

[ekuegler]

We have eliminated to the greatest extent all possible high resistance causes, I.e ductored from line side coil connection to the star point side corresponding cable, we thought that the crimps could be the cause but on removal no change in results or very very minute. Exhausted I think the lot.

 

[rasevskii]

You mean that the unit was running when the submersion had occurred? In that case there can be a lot of damage somewhere maybe not easily seen. Inside the winding... Since I believe that you have a resistance earthed system, there may have been an arcing fault that cleared itself. The fault current may not have been high enough to operate the diff protection. The earth fault protection should have operated.

BTW what is an ACH? Is this a Diesel? A lot of other equipment must have taken a hit also.

Enjoy your life on the platform and good luck!

rasevskii

 

[ekuegler]

Lol, cheers.

ACH -anti con heaters/space heaters.

Unit was running prior to ESD which caused unit trip and coincidently due to severe weather caused the lube oil cooler to rupture, oil/sewater migrating to the alternator enclosure via the unit DE/NDE bearings. Yes other cooler failures due to bad weather occurred.

We run on an unearthed system so no theoretical solid earth. Earth fault monitoring is implemented. No record of any elect protection operation, so assume migration of oil/water occurred after unit shutdown due to platform ESD.

Thanks again

 

[ScottyUK]

It is possible that you are seeing the results of a stray current path through parts of the machine. That would certainly upset the ductor and if the current path happens to be through the neutral tiebar then it might explain the weird results you are seeing. Try swapping the positions of the test probes so the injected current goes the other way through the windings - does the reading change at all?

What sort of ductor are you using - a high current set or a little 1A / 10A type? Can you get a reasonably sensitive tong tester around the neutral bar?


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