Dielectric loss refers to the sum of polarization loss and conductivity loss caused by transformer oil under the action of alternating electric field. Dielectric loss factor can reflect the insulation characteristics of transformer, the aging degree of transformer oil under the action of electric field, oxidation and high temperature, and the degree of pollution of polar impurities transformer and charged colloid in oil.

Measurement of capacitance and dielectric loss of circuit-breaker capacitor is a very important task to ensure safe and stable running of circuit-breaker. However, due to serious electric interference to on-site measurement, dielectric losses of more and more 500kV circuit-breaker capacitor exceeded the limit at 10kV test voltage according to the CSG trial standard. To address the above-mentioned problems, the paper proposes that methods of using different frequency power source and boosting test voltage be utilized to measure the capacitance and dielectric loss. Based on the above measuring methods, several common test schemes are put forward and application ranges are also analyzed followed by capacitance and dielectric loss measurement for some 500kV circuit-breaker capacitor at substation. Test results indicate that the dielectric loss of capacitor reduces as the voltage rises; the change in test voltage frequency can effectively shield the interference from on-site electric field.