High Voltage Tests
High voltage equipment is used to test different states of insulation integrity.
The intrinsic strength of a material is its ability to withstand an impulse. Most 'good' solid insulators have intrinsic electric strengths of 5 - 15 MV/cm. This is determined using the impulse generator.
On application of an ac voltage to insulation, a current is present. Heat is generated in the insulation due to its resistance and dielectric loss (dipole realignment). The ratio of the component of current in phase with the voltage to the component of current leading the voltage (capacitive) is the dielectric loss tangent, tand. d is known as the 'dielectric loss angle'. Typically in good dielectrics tand . 0.001. Measurement of the loss tangent and capacitance of an insulation is achieved using the Schering Bridge Test. It compares the insulation to a negligible loss (lossless), air or gas (N 2 ) filled standard capacitor.
Solid materials usually have imperfections such as cavities or voids filled with dielectric of intrinsic strength below that of the solid, eg. filled with gas or liquid. The combined effects imply that the cavity dielectric can break down causing erosion of the solid dielectric (discharge channels). Over an extended period of time this leads to permanent breakdown.
Several discharges occur across the cavity per half cycle of the applied waveform. The number of discharges increases with the applied voltage leading to faster breakdown of the dielectric.
The discharges are usually displayed on a 50Hz elliptical time base. They are measured in pico-coulombs. The apparatus to measure this is a partial discharge detector.
It is sometimes suggested that HVdc is used for testing HVac equipment as HVac overvoltages can accelerate ageing. In over-voltage testing, new equipment insulation is subjected to voltages well in excess of their working voltage. This is to detect gross faults in the insulation. However with HVac, this may initiate discharge channels from voids due to the high incidence of discharges during the test. Often this is greater than the number of discharges experienced in the rest of the insulation life time. There is thus a resultant lowering of the discharge inception voltage which accelerates the ageing of the insulation.