Transformer Rectifiers for Electrostatic Precipitators Longevity and Risk Management by Oil Filtering and De-Gassing

Looking at the picture below,  you could be forgiven for thinking that this was simply a matter of the diode stack inside the transformer rectifier getting old and failing.

High Voltage Diode Boaard

At face value, of course, you would be correct. The diode board has failed, but there is much more than meets the eye going on here.

The real question though becomes why has the diode board failed?

Having spent multiple years going through transformer rectifiers and observing these failures, it has become increasingly apparent that aging transformer rectifiers are no longer as dependable as one might hope they will be.

• ·       The cooling and insulation fluid is degrading.
• ·       The transformer and rectifier are experiencing degradation.

The high voltage components, diode stacks and the high voltage resistors start to behave just like the precipitators they serve, and the electrostatic attraction associated with these components starts to pull particulate out of the fluid.

This crud collides with the transformer mechanical surfaces, diodes stacks and High voltage resistors, causing contamination.

Once this contamination builds up around the diodes and the resistors  themselves, it is only a matter of time before local overheating and arc-over of the devices shuts the system down on undervoltage or over current situations.

Contaminated Oil and Surfaces

Of course, at this point confusion prevails!

If the transformer lid is not removed or an oil sample is not analyzed, it’s not immediately clear as to whether the precipitator, the rectifier or the controls are at fault.

Valuable time is wasted as well as spares replacement for components that are perfectly healthy.

Having said all this, all is not lost. There are a few ways of mitigating this level of disaster without having to have the transformer rectifier completely rebuilt at short notice.

Obviously, a good long term plan is to have sufficient spares to accommodate failures of this nature. But this is both impractical and expensive. You need time to budget for this.

There have been companies that have simply drained the oil and replaced it with new oil. Once again at face value, this is a reasonable approach.

Where it starts to fail is that simply replacing the oil does not address the problem of the contaminants associated with degradation of the old oil.

If there are any buildups on the diode stacks or on the high voltage dividers and other locales within the transformer rectifier. These remain there and as such, the risk of failure has not really been significantly changed.

An alternative solution presents itself.

This is where we are finding the most success with at this point.

We hook up an oil filtration unit to filter and circulate the oil, thus getting rid of particulate and the buildup of gases in the equipment.

    

Recirculation System                                               Re-circulation, Filtration and Degassing in progress

We use the circulation system as a vacuum and pick up particulate that is lying around the tank. Removing it from within the cooling ducts of the transformer rectifier and off the surfaces of the mechanical transformer. 

      
Cleaned Oil and Surfaces

 Allowing the fluid to circulate multiple times Throughout the system ensures that as much of the particulate from the oil degradation is removed.

We then inspect the diode stacks and high voltage rectifiers to see if they are worthy of leaving in place or should be replaced as part of this program.

Obviously, the best thing to do would be to replace them with new units.

The oil would then get sent away to be evaluated. If it does not pass…..

Then at this point, the oil can be drained and replaced since at this point in time, the system would have been thoroughly cleansed, removing the majority of the particulate matter that has built up within the system, leaving you with the transformer that, while not new ,at least has a much lower risk of failure in the short term...