If a transformer bushing is rated @ 135pF then after it has been in service for a year or so, you find that its capacitance has increased to 270pFafter tests. what would be the cause? Isn't it suppose top decrease at least?
You did not mention what testing was done to measure the bushing capacitance - was it Doble power factor or dissipation factor test? Is the 135pF the nameplate capacitance? Usually test results are expected to be within about 5-10% of nameplate.
Several factors can influence the capacitance value when tested. The capacitance and the leakage current in the bushing are proportional. Anything that can increase the leakage current (such as air humidity, moisture on the bushing surface, moisture or contamination in the test tap) can affect this.
Were the tests done at the same ambient temperature and transformer oil temperature? If the first test was done on a new transformer at normal ambient (say 20C) and the second done when the transformer was just deenergized (maybe 40C or 50C) that can influence the results. There are adjustment factors that can be used to compensate for this.
Was the same test voltage used, and the same test connections? A higher voltage can cause a higher reading, if the paper/foil capacitor layers leak more current. Tests should be done at the same voltage each time to be comparable.
These are some of the things you need to consider to evaluate whether this bushing is acceptable or not.
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Bushing Capacitance
Re: Bushing Capacitance
