Transformer turns ratio tests using ref capacitor method 1

[afroeeng]

I recently had a contractor doing ratio tests on our transformers but they were using the reference capacitor method instead of the conventional voltage method. They basically used a capacitor of known size in series with one winding while applying high voltage (11KV) on the other winding (HV). The ratio of the ref capacitor/measured capacitor was the calculated turns ratio N/n. I simply don't get the concept, could someone please explain this relationship.

 

[CanadianKid]

The principle of using a reference capacitor with a HV Insulation PF Test Set (Doble is your term) is not comlex.

The first step has the test set measuring the reference capacitor and recording the capacitance value (many times, one records this value once and imprints it on the capacitor for various temperatures).

The second step has the test set applying high voltage to the H side of the transformer and applying the X (secondary)transformer voltage to the reference capacitor. The reduced voltage to the reference capacitor results in a reduced effective capacitance as seen on the Test set (Doble). One then records the second capacitance.

The first capacitance reading (step 1) is divided by the second reading (step 2)and the effective ratio of the transformer is known.

For example, if a transformer is 1:1, then the voltage seen by the reference capacitor for both tests is equal, and the 1:1 ratio is acheived.