Clean mineral oil is vital to proper operation of a transformer/rectifier. The mineral oil provides not only a cooling medium but a dielectric medium to handle the voltage stress within the unit. The dielectric strength of the oil will decrease. One of the ways to remove any contamination, and restore the dielectric strength, is to filter the mineral oil. Although the filtering process described below will not remove moisture or degasify the oil, it is very effective for particulate removal.

 

Filtering should be done with a two stage filter. The first stage should use a Cellulose cartridge designed to remove particles 25 microns or larger. The second stage filter should be designed for removal of particles .5 microns or larger. A pump will also be required to circulate the mineral oil through the filters. The pump should be suitable for use with mineral oil. The flow rate of the pump should not exceed the maximum allowable flow through the filters. 

 

The hookup of the equipment is relatively simple. First you should de-energize the T/R set and properly ground the precipitator field for personnel safety. Next attach the intake side of the pump to the drain valve located at the bottom of the T/R set. The outlet of the pump will be run through the filter. Remove the T/R set access cover and place the filter outlet hose in a horizontal position just below the oil level. This horizontal position will help eliminate the formation of air bubbles as the oil is returned to the T/R set. Once the hose is in position reinstall the access cover as tightly as possible to reduce any further contamination.

Once the equipment is properly connected the pump should be run for about 8 hours to remove the contaminants from the fluid. It may be necessary to have some additional fluid available to prime the pump or serve as make up for the oil in the filtering system. It is very important that the high voltage coils are not exposed to air during the filtering process. Exposure of the coils to air may result in a failure upon re-energization of the T/R set. After the filtering is complete, remove the access cover and visually inspect the mineral oil. If particles can still be seen, continue filtering until all of the particles have been removed. Once completed, test the oil’s dielectric strength. The minimum SATISFACTORY level should be 28 KV using ASTM test method D877.

 

Once the filtering equipment has been disconnected, reinstall the T/R access cover. Next, pressurize the top air space above the oil with dry nitrogen to 4 psi. Let stand for 2-3 hours, then check for any decrease in the pressure level. The pressure test is performed to insure that the T/R is properly sealed and will not breathe. After the pressure test is completed, bleed off excess nitrogen to .5 psi. The process of applying the nitrogen blanket then bleeding off excess pressure can be repeated several times. This will essentially purge the top air space of any moisture.

The T/R set mineral oil should be tested on a annual basis to insure proper dielectric strength. Additional tests as can also be performed on the oil. They include neutralization number, interfacial tension, power factor, color, and moisture content.