Generator Field Failure. 2

[aolalde]

We have four Synchronous Generators Diesel Engine driven and a Static Reactive Power Module 20 MVAR, all synchronized to the utility power line. The four generators are identical size and manufacturer; 15 MVA, 12 MW, 400 RPM, 13.2 kV. 60 HZ, field Voltage 267 VDC, and current 231 DCA. The generators are brushless excited, with a rotating 6 diode rectifier and AC 59 kW, 26.7 HZ, 3-phase armature. The main rotor field has 18 poles one circuit series, the rotor outside diameter is 10.5 Foot.

The four generators under normal conditions provide 10.5 MW and 6 MVAR each one.( Very conservative load).
After 7 years in service, generator #1 failed. We found 3 of the 17 rotor field series jumpers fused.
Three poles were re-wounded, the jumpers were replaced and the unit reconnected in line. It last 2 days and the very same type of failure developed in the field connections again.
This time all 18 poles were rewound and all field connections replaced. The unit ran five days and developed the very same type of failure. Four jumpers making the interpole connection were totally melted and six pole windings were grounded. This time one pole shoe had five amortisseur winding bars totally fractured, each pole shoe has 12 bars. The rotor paint shows signs of overheating.
The stator winding does not shows signs of any damage and it test OK, insulation resistance, PI and comparison surge test.The rotor failure has been similar but involving different random poles.
The electric records during the days in operation show, that this generator #1 was generating an average of 10.5 MW but only around 3.9 MVAR and for the last 1.5 hours it worked as induction generator, taking reactive power from the line bus.
Questions:
1- Has somebody experienced a similar problem?
2- Why the series connections are melted but not the pole windings?
3- In theory when one jumper in the series connection melts, the field circuit is opened, what feeds the current to melt three more jumpers?
Any comments will be appreciated.

 

[aolalde]

Scotty:

One corner of the laminated pole face, close to the rotor rim shows signs of flashover. That flashover was between the last field turn and the pole side lamination very close to the rim and it jumped over insulation 2.5 mm thick ( 0.098”). The jumpers did not leave a clear path of flashover but it seems to be against the rotor rim body.

 

[ScottyUK]

Thinking aloud here -

If the rotor is reasonably clean, then the flashover required some significant voltage to initiate it. That would require two conditions: an existing fault to ground on the rotor, then either a second ground fault at the location of the burn or opening of the the rotor circuit to initiate the arc.

It seems we're discussing flashover damage from a relatively brief arcing fault rather than more severe damage which would probably have occured if the rotor formed part of the path which carried current for long enough to melt the jumpers.

From the damage to the amortisseur winding, is it possible that this machine has pole-slipped at some point? It seems unlikely that a diesel prime mover would withstand that kind of shock without some signs of damage. Had the broken bars actually broken, or had the end brazes suffered thermal failure?

I'm a little out of my depth with slow-speed salient pole machines because I'm used to looking at high speed turbogenerators which are quite different animals. If I'm adding confusion I apologise.


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[aolalde]

Two poles had fractured damping bars. The brazing joint seems in good condition. The fracture occurred right at the edge of the inner face of the short circuit ring ( for this case the ring is made of individual copper plates interconnected with bolted copper jumpers.

 

[petronila]

Hello aolalde,

Did you take some pictures? can you send ?

Thanks

Petronila

 

[petronila]

Hello aolalde,

Taking acount your last comments That seems to indicate arcing of the high voltages induced in the rotor
poles during starting. In units With self-excitation, there should be something to do the the discharge.( a thyristor on the rotating rectifier).In a conventional rotating 6 diodes Rectifier(3+ and 3-) mounted on two separate plates (1 + and 1 -), there should be a thyristor across the 2 plates, to act as the discharge protection for the rotating fields.Without such protection, at the moment of starting, the motor behaves as a transformer with the stator winding as the primary, and the rotating poles as the secondary. With all poles connected in series, the turn ratio is too high. There could be several thousand volts built up in the rotor during starting. It discharges across to ground wherever the windings are close to ground potential.Perhaps there is something missing, or a failed component, on the rotating rectifier package.

Best Regards,

PETRONILA

 

[edison123]

Aren't we talking about diesel generators ?

 

[ScottyUK]

Petronila,

The rotating rectifiers on the brushless excitation system fitted to large turbo alternators have a standard six-pulse configuration, sometimes with two or more parallel diodes per limb in the bridge, but no thyristors and no clever tricks. I suspect that those fitted to a low-speed machine are not so far different.

Edison,

Yes.

Aolalde,

Is it possible that the flashover to the core at the jumper was a result of the broken jumper moving due to rotational forces? I ask this because we may be trying to incorporate it in the root cause explanation when it is actually a residual effect of the problem which caused the jumper to fail. Sometimes the residual effects add a lot of confusion when assessing the underlying cause.

The ground faults on six rotor bars: where did they occur, and what was the failure mechanism of the insulation? Were the fault locations burned from carrying large current or were the faults so solid that they would have had significant current-carrying capacity? Instinct is telling me that these ground faults were the underlying cause and that most of the other damage is more-or-less traceable through progressive failure of the rotor.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[edison123]

Scotty,

I posed the last question since petronila seems to have misread the equipment as motor (and I don't agree with his comments about starting as motor and high turns ratio affecting the field winding, since I have seen many synchronous motors started as induction motors without any problems).

As for discharge protection, I have seen them in small, LV generators and definitely none in large beasts connected to the grid.

Now coming to aolalde's poser, for me it seems to be a classic case of both asynchronous operation and loss of field operation along with the root cause of thin (2 mm thick) pole jumpers failing due to centrifugal forces. The symptoms he tells are exactly the same covered in the IEEE 492 Std (of which I have an official copy with its usual caveats about not reproducing its contents without their 'permission').

 

[electricpete]

I see where you mentioned IEEE492 (1999). I looked at paragraph 7.6.5 regarding loss of field.

I see where they talk about rotor overheating. The cause is not exactly clear to me.

Machine speed increases and acts like induction generator. I guess I can see that results in some increase in centrifugal force but I have a hard time understanding the role of centrifugal force in this scenario.

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[aolalde]

Thanks to everybody for interest and comments.
I am almost convinced that high voltage was induced in the pole field windings, but when? The ease to explain condition is when the generator worked as induction generator for more than one hour. I am convinced that most of the damage to the amortisseur winding and to the field poles was done during that time.
The speed has to be increased but we do not have a record of the over-speed, we know that 10.5 MW were generated under asynchronous condition but the slip could be 3%, 5%, 10%? I do not know.
Rotor centrifugal forces at 13,194 Ft/min normal synchronous 400 rpm are not small. When the generator worked over-speeding it could bend those jumpers.
My problem is to find the trigger that initiated the failure.

 

[motorspert]

You mention in your original post that the generators are diesel driven. I assume they are directly coupled (no flexible coupling). A diesel imparts considerable pulsating torques on the shaft system and the inertia of the generator rotor is used to damp some of this pulsation out.

An effect of the pulsations is to shake the poles which could cause the connectors between the poles to suffer from stress fractures - copper "work hardens" very quickly. Are the connectors the same as the original ones? I would check the size, material and shape - some connectors are design to be very flexible to cope with these mechanical and thermal stresses. I assume the diesl has been checked over for misfiring etc?

This phenomena is not unusual and would explain one failure, not sure how to explain the other two simultanopus failures. Did all of the non-failed connectors look ok?

The brushless exciter shouldnt have caused this - although the fact that this machine failed to deliver the full VARS suggest another problem with the excitation system.

Hope tjis helps

Richard

 

[ScottyUK]

As a possible source for the high voltage, what if a field conductor suffered a sudden open circuit failure? The stored magnetic energy in the rotor iron has to go somewhere and according to Lenz's Law it will show up as voltage.

I'm not sure if the damage attributed to HV flashover looks like a relatively short duration event, or if it has been a persistent fault. A persistent fault would rule out my theory above. Perhaps you can comment?


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[petronila]

Hello electricpete,

I looked too the IEEE 492 Paragraph 7.6.5 and This bit about operating as an induction generator is only true when the load is 'over-hauling' - meaning it can drive the rotor.For example: A pump motor pumping against a high head, or a conveyor.

Regards

PETRONILA

 

[electricpete]

petronila - what load are you talking about. This is a generator. Upon loss of excitation the sync generator will cease absorbing torque. If prime mover torque does not go to zero (or no-load friction/winding value), then the machine will accelerate. Machine operating above sync speed will act like induction generator.

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[motorspert]

Scotty
yes, a sudden open circuit leads to a very high induced voltage in the field, the exact value depends on the turns ratio between the rotor and stator and the slip.(percentage speed the rotor is above/below synchronous speed)

But yes, induced voltages can cause flashover

 

[edison123]

aolalde,

I agree with motorspert about torque pulsations in DG's. Usually DG's have a flywheel to smooth out torque pulsations. Does yours has one ?

I would look at some sort of anchoring the pole jumpers to the rotor rim to strengthen them.IMHO, the root cause was mechanical failure of the jumpers with collateral damages. I would also recommend loss of field protection to prevent such failures.

What is the present status of the machine ? Is it under repair / investigation ?

 

[electricpete]

The generator has always been generating low vars. That is the first symtpom seen so presumably closest related to the root cause.

It seems to me the exciter system (either rotating or stationary portion) is suspect and should be very carefully checked.



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[electricpete]

Is there any record of vibration spectra when the machine was running?

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[motorspert]

electricpete,
I am not sure the low VARS is related as the generator ran for 7 years like this, I suspect the AVR has been set up that way, maybe the original commissioning records give a clue.

If all generators are identical then a look at the machine logs should give an indication, a table of exciter current, voltage, current and PF would enable the generators to be compared. There is a slight possibility that the exciter is running on a single phase.

 

[aolalde]

I agree with your comments Electricpete. I am considering to recommend replacement of the exciter and rectifier bridge to ensure the field is properly excited.

In addition,loss of field protection will be another recommendation.

Actually all 18 poles are in our shop for test and rewinding of defective ones.

We do not have yet a solid root cause of failure.