Corona Discharge activity in the slot section and exits of High Voltage windings

This phenomenon has been a well-documented subject for many years and hundreds if not thousands of papers have been written addressing and explaining this problem. With this in mind I will not try to describe the fault in length but I should state that almost all high voltage ‘F’ class windings can suffer from this type of damage and if found some action is required and should be taken.

The basis for the problem, in this instance, is the damaged protective conducting outer layer of the high voltage coil in an area where the coil exits the slot and just before it comes into contact with the overhang graded tapes.

This damage ‘Potential Discharge’ (PD) was noticed during a previous inspection to a customer's cogeneration facility, which also noted some ingress of this damage into the slot section. This evidence leads to the fact that the PD damage is continuing to spread, at an unknown ratio to the running hours of the machine, and with time this can only cause serious and costly down time for the machine.

There are several reasons why this type of damage can occur and depending on who gives the explanation (OEM/others) the descriptions may vary but the outcome will always be the same. Without the protective outer coat of conductive varnish/tapes the build-up of the potential, due to the ionization of the air, will reach such a state of charge that it will jump the air gap between coil and the earthed magnetic core to discharge itself, this can be likened to a small explosion. This will, without doubt, damage the main wall insulation of the coil. If these conductive varnish/tapes are kept in a good and non-damaged condition the discharge is managed and a high potential is not allowed to build up and the insulation is protected.

There are several and distinctive stages to PD damage but from the photos seen this appears to be the onset of the first stages and therefore due to the expertise of the inspection team that first identified the damage a scheduled repair can be undertaken and managed to either retard this damage, stop it progressing further or eliminating it completely.

Picture 1 shows the damage to the machine in mention and as found in the last inspection of 2014.

Picture 2 shows the progression of PD from another machine and what could happen to the machine if the issue is not addressed.

Picture 3 shows a coil that has been severely damaged by corona discharge and consequently failed.

Picture 1

This photo shows the Cogeneration's facility generator damage. The white colouring that can be seen is the backing and holding glass tape after all the conductive tape, it was originally holding, has burnt away. Similar damage has also been noticed travelling inside the core section and along the axial length of the coil for a short distance.

Picture 2

In this photo burning damage can clearly be seen and as indicated by the arrows. This is the progression of PD within the slot portion of the high voltage coil. The top layer of insulted felt is badly damaged which is a result of the PD activity between the coil sides and magnetic core. At this point in time we do not think this type of damage has occurred too far down the slot on the cogeneration's facility machine.

Picture 3

Due to severe PD damage to the sides of a high voltage coil the insulation has broken down to the magnetic core resulting in a complete stator rewind and the shutdown of the generator for many months. The arrow is pointing to the blow hole and failure point in the insulation. The darker coloured stripes are the areas of coil adjacent to the air cooling vents and have not been effected by the discharge, all the other protective tapes have been completely destroyed.

The above photos have shown PD from its mildest form to its most destructive, from the start of slot exit damage to the complete degradation of the insulation of the coil sides and subsequent failure.

The above photos are designed to show the destructive force PD can become if not addressed and not make customers assume their machines will end up in this way. It has already been stated that we believe the cogeneration's facility machine is in the early stages of PD damage and therefore what are the options for repair.
 

Repair Option 1

A first stage repair which would consist of the removal of the three end wedges from the effected coils. This would allow a further and fuller inspection and define an ending point of the ingress of damage to the slot section.

From there the side of the coils could be packed out with a ‘vetrinite’ conductive packer and conductive varnish to re-establish the required contact area between the high voltage coil side and the magnetic core and reduce the air gap.

New wedges would be entered into place before the slot exit areas were repaired using conductive varnish and graded varnish, all areas would then be sealed using epoxy resin.

This type of repair has been carried out on numerous occasions and been successful but it is not a guaranteed repair and there is a possibility at some stage in the future the same problem may return. However is should also be stated this type of repair carries the least risk of damaging other parts of either the existing winding system or magnetic core pack.

Repair Option 2

The complete removal of the damaged coils and replacement with new coils would certainly return those areas back to a state of a new machine but this type of repair can carry more risk than option 1.

Whenever removing damaged coils in a generator or motor and replacing them with new coils, as a partial rewind, it has to be understood that the remaining coils will still remain in the in the same condition as found. Also the physical operation of removing coils from a working system has to be done in such a way that it does not diversely effect the other original coils that are not being replaced. This option will also require more time to accomplish than option 1.

The above two options are the only two practical ones to recommend otherwise we are going into the realms of a complete rewind and/or stator replacement, both of these will be extremely expensive and time consuming and not necessary at this point in time. The only other option is to take no action but this is condemning the machine to more severe and intrusive repair procedures in the future and possible failure so it is not a real option.

In Conclusion

Irrespective of which option is chosen it is imperative that the winding engineers employed to carry out such work are fully skilled and experienced in this type of work, they should also have a full knowledge of the Brush type designs incorporated in the cogeneration's  facility generator.

On a final note it should be stated that the generator windings could be susceptible to this type of damage in areas other than the ones already identified and is a dilemma for all customers. This damage has no recognised time scale of appearing and could show up many years in the future at any point where a high potential is present and therefore regular inspections should be carried out. This is most important when we take into consideration that the damage found on this machine was effecting the top half coils. If it had been found that bottom half coils had been damaged then only option one would be available as to replace damaged bottom half coils it would require the removal of nearly all the top half coils and in that instance it would be better to either request a rewind or a new stator.

On the cogeneration's facility generator the damage has been identified on the top half coils so either option 1 or 2 can and should be recommended dependent on the customer’s requirements in respect to down time allowed and other aspects relevant to him.

It should be recommended that a winding inspection should be carried out whenever the customer has a shut down for one day or more on a basis of no more than a one to two year gap.

Richard Pipes
Senior Winding Engineer