Partial Discharge Energy Measurements on Electrical Machine Insulation When Energized at Frequencies between 0.1 Hz and Power Frequency

The paper describes a very-low-frequency (VLF) high-voltage test facility and includes the results of partial-discharge (corona) measurements undertaken on samples of electrical machine insulation. The samples were energized by means of a high-voltage generator, capable of producing alternating Voltages from 0.1 Hz up to power frequency. In order to eliminate spurious signals, partial-discharge measurements were undertaken using a pulse discrimination system (PDS). The output of the PDS was fed to a pulse height analyzer which enabled the distribution of discharge magnitudes to be displayed. This information could also be transferred to an x-y recorder to provide a permanent record. The pulse height analyzer also gave integrated values of both energy and discharge magnitude. The results demonstrate the variation of discharge-inception voltage with frequency and show how it is affected by the properties of the insulation under test. The inception voltage is constant over the range 0.1 Hz to 50 Hz for low-conductivity materials, but increases at lower frequencies when there is significant conduction across the void surfaces. If tests are undertaken at a specified level above the appropriate inception voltage, then the integrated discharge magnitudes and energy values have been found to be similar to those obtained at power frequency. The paper recommends the basis for a test procedure in which discharge energy/voltage measurements would be undertaken at a specified frequency within the range 0.1 Hz to 1 Hz, the exact value depending upon the type of insulation.