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1MW VSD failure

1MW VSD failure

1MW VSD failure

(OP)
We experienced a 1MW variable speed drive failure recently.
The drive is connected to 6.6kV swithboard and 24-pulse type. The input power supply transformer is epoxy cast type and is the one failed. It resulted in 6.6kV Y&B phase fuse failure (6.6kV fuse backed latched Vacuum contactor) and happened when switched-on. The make is Ansaldo, France. Investigation is on-going.
Any thoughts welcome!!

RE: 1MW VSD failure

Epoxy cast transformers are known to have problems with voids in the resin. If the field strength in these voids exceeds ozone inception kV/mm, then ozone will produced and accumulates over time. Ozone affects the organic insulation material and it fails sooner or later. Switching on usually means transients and those transients will find the insulation fault.

There are several reasons for the elevated kV/mm level. One is if there are higher frequency harmonics. The high frequency increases capacitive leakage currents and that, in its turn, increases ozone production. The other reeason is the overvoltage that occurs when you switch open the vacuum contactor. If you do not have a voltage limiting device (arrester or RC combination) the overvoltage will almost allways be destructive over time.

I have had the latter phenomenon in quite a few places. The first one was around 1974 with the first vacuum contactors used for MV motors and the latest was a couple of years ago. Surge arresters helped in all cases.

The former phenomenon was not so easy to find. Normal harmonics analysis disn't show anything abnormal. But using an oscilloscope with FFT revealed high levels of non-harmonic resonance voltage on the 6.6 kV side. That HF voltage had the same effect and produced ozone in the voids. Three transformers exploded from that reason before we could find the reason. The Italian manufacturer is now defunct and we changed to Geafoil insulation. Those transformmers are still working very satisfatorily.

In both cases, it took 9 - 15 months before the damage occured.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: 1MW VSD failure

(OP)
Thanks skoggsgurra for sharing your experience.
The subject drive is a new one and is still under commissioning. It is true the 6.6kV feeder to the converter trafo is switched by Vacuum contactor.
But, the dry type converter trafo is provided with Surge arresters at its 6.6kV terminals (and the surge arresters do not seem to have suffered any damage, testing of SAs is yet to be done).

RE: 1MW VSD failure

I think that you can exclude that particular failure mode then. Did it work for any time at all? Or did the failure occur on first switch on? Any traces of heat? Or did it just 'explode'?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: 1MW VSD failure

(OP)
It was ON for three days under soaking.
Then it was switched-off for making cable connection from the inverter output to the motor.
After the connection, when switched-on, the failure occurred (thyrister firing is off).
The failure was in the middle (vertically speaking) of the winding and shows rupture of winding insulation.

RE: 1MW VSD failure

I think that you need to have a look at the surge arresters. Their connections - especially the 'cold' side - and their protection level.

It is quite common that the transformer is damaged at switch off because of the fast action of the vacuum bottles. But nobody notices until the next switch on when the damage causes a flash over or other breakdown.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: 1MW VSD failure

(OP)
Thanks. I agree.
I will let you know once the surge arrester is tested.

RE: 1MW VSD failure

Waiting eagerly! smile

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: 1MW VSD failure

(OP)
There is one set of surge arresters in the 6.6kV switchgear cable compartment as well. The site reported that both sets of surge arresters are intact and IR value is good.
Further, if the failure is related to voltage surge, you would expect the damage at the line end and not in the middle of the winsdings as is the case here.
Could it be winding failure due to quality issues (the transformer is still new and under commissioning as stated earlier)!!
The 24-pulse VSD package is from Ansaldo, France and the transformer is from Trafomec, Perugia, Italy.  
Has anyone got experience with the above makes!

RE: 1MW VSD failure

The dry resin transformers I had a bad experience with were made by Gonella. When I look at the Trafomec transformers, they look very similar. Same resin, same external connection arrangement. I think that you should make a discrete investigation to see if there is a connection between old Gonella and Trafomec. Drawings, tools, fabrication methods, personnel etcetera.  

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: 1MW VSD failure

I attended the failure of a dry type transformer, where the winding body had exploded sending fragments of resin through the steel casing with nice clean punched holes.  This was thought to be a result of moisture absorption in the humid climate.

RE: 1MW VSD failure

Did the resin as such absorb moisture? Or were there voids where moisture crept in? In both cases it is a quality issue.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: 1MW VSD failure

I have had 2 experiences of cast resin transformer failures.

One was put down to a certain lot of insulating material employed in between the turns. The transformer was repaired and mica based insulation material used instead in the new coils and no further problems. The transformer manufacturer should be able to trace lots of materials and advise where they have been used in yr equipment and cross-check with other customers.

The second was with a 12 pulse transformer that was in parallel with another for a 24 pulse drive.
One unofficial comment to me (not from the transformer manufacturer though) was that as the transformers had extended delta and/or zig-zag windings to get the appropriate phase displacements, (I can't remember which now) the likely cause of the failure was a manufacturing problem within the windings connections.

Damage is definitive in a cast resin transformer so the actual coils will not be repairable and it might not be possible to really determine what happened. How long the fault lasted will determine collateral damage perhaps to the core?

RE: 1MW VSD failure

(OP)
Thanks to KiwiMace, Drivesrock for sharing your experiences.
I guess it could be design / manufacturing competency as the transformer for a 24-pulse VSD is much complicated.
It could also be material / manufacturing / quality problem if moisture ingress in to the cast resin insulation had really happened.
Only a detailed investigation could bring out the fact. A 3rd party specialist is expected for the investigation.

Skogsgurra,
You mentioned Gonella make transformers. I could not find their website. However, it seemed that both Trafomec and Gonella are Italian companies.
 

RE: 1MW VSD failure

The Gonella transformer production was closed because of bad transformer quality. The transformers shown on the Trafomec website look identical to the Gonella transformers. That is why I think that manufacturing has been taken over by Trafomec. The fact that both are of the same nationality may support that thinking.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: 1MW VSD failure

I never got an official verdict on the voids vs the resin itself, but I would think voids, given the way the chucks came off.  

RE: 1MW VSD failure

I suggest that you also check your vacuum contactor for contact bounce.
On high fault rated contactors , high speed fault clearance is sometimes achieved by the use of very powerful springs.
Bleed off Air piston type damping cylinders are used to dissipate excess energy at the end of the stroke .
I've experienced that sometimes the size of the piston bleed off orifices are incorrectly sized for the altitude (denser air)& contact bounce occurs.
This causes high frequency high voltage transients & consequent weakening & damage to transformer windings, insulation,capacitors & motors.
You need to perform oscillographs of the action (movement) of the contact mechanisms when opening, to establish if this is your problem.   
   

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