Partial discharge

Can you be more specific? What type of particles are you talking about?

Well, there goes my dyslexia again! That's how I wound up selling my soul to Santa!

I'm not sure what magnitude of PD you are concerned about or the type of detector you are using. Measured partial discharge magnitudes rarely have absolute meaning. From a practical point of view, partial discharges occur (and can be measured) when some form of electrical activity within the system results in a rapid change of the electric field that causes a current to flow in a conductor that is connected to the measurement circuit. To be recognized as PD, the current must be large enough to be detected and must occur with a sufficient repetition rate to be recognized as something other than random noise.

PD can occur in solids, liquids and gases in ways which may not affect the short time dielectric strength or be detectable in routine or proof tests but they will eventually cause failure in most systems.

In oil-paper insulation systems, internal partial discharges can also generate explosive gases. Gas detectors are frequently used to confirm PD and to evaluate the internal condition of transformers.

If the internal PD in a power transformer are real and significant, the only effective means of reduction or elimination I am aware of would be reprocessing or reconditioning by draining the oil, vacuum treating and reimpregnating the core and refilling with degassified oil.

Enclose it in a container filled with a non-conducting fluid or inert gas.

Mark

Partial discharge occurs when the electic field strength between conductors or conductors and earth causes the dielectric(s) between them to ionise, but not break down initially - although it will in time. The only remedies are to improve the relative dielectric strength(s) of the dielectics(insulant(s) involved i.e. paper, oil, gas(if it is a dry type transformer). Obviously the cooling / insulant is the easier and cheaper to improve, but if the conductor insulation has aged, or the initial design had stress points which have led to this increased partial discharge over time, then the rectification is much more costly and time consuming. Before doing anything you should find out where the disharge points are as it is sometimes simply caused by the conductors connecting the windings to the bushings being too long and laying against earthed metal (transformer tank walls, transformer core etc.) when the bushings are pushed into the tank after termination of the internal conductors during site assembly.The actual location of the p.d. points is a low cost, non intrusive test and can be done at several points from outside the transformer when energised. By correllating the relative p.d. readings, and knowing the approximate location of windings, tap switches, bushings, connecting leads etc you can deduce the p.d. location(s). This will then give you a better idea of where and what is causing the p.d's.