Test procedures for large power transformers

Partial discharge measurement

A partial discharge in an insulating medium is a localized electrical discharge, which does not bridge the electrodes of the insulation structure. The field strength of a weak part of the dielectric may exceed the strength, which causes a breakdown.

It is, however, to be observed that the weak parts mentioned may form a small portion of the insulation structure only. The remaining whole insulating gap can, therefore, withstand voltage stresses corresponding even to the test voltage, and the breakdown remains partial.

Due to the sinusoidal variation of the applied voltage the electrical field strength increases again after the discharge has been extinguished. When the field strength reaches its critical value, a new discharge occurs.

Thus discharges take place repeatedly (Figure 1).

The situation is enlightened by the simple analogue circuit of a cavity (Figure 2). C_a is the capacitance of the whole insulating gap, the spark-gap and the capacitance C_c represent the cavity and the capacitance C_b represents the dielectric in series with C_c.

When the voltage U_c across C_c has increased enough, the spark-gap ignites. The capacitance C_c disharges and the voltage difference across the cavity vanishes within 1…1000 ns.

The discharge magnitude or apparent charge q and the voltage U_c are related by the following equation:

q = C_b × U_c

The discharge gives rise to a current pulse, which causes a fast voltage charge at the terminals of the transformer. This change can be measured by means of a capacitive voltage divider and a pulse transformer.