Abstract
Detection of partial discharges (PDs) is widely used as a condition assessment tool for
high voltage equipment. Application of low frequency test voltage is often preferred in
the case of test objects with a large capacitance. The question addressed here is how
results from PD-measurements performed at low frequencies correspond to that
occurring at 50 Hz power frequency. Different theoretical models for void voltage were
examined and compared to experiments performed on laboratory samples of
mica/epoxy, including embedded cylindrical voids. All test objects were preconditioned
at 10 kV and 50 Hz for 5 min before partial discharge inception voltage (PDIV) testing
by stepwise increasing the test voltage from 0 to 10 kV. The PDIV test was first
completed at 300 Hz before being repeated at decreasing frequencies down to 0.1 Hz.
The temperature was varied in the range of 20° to 155°C. The results at high
frequencies showed that a pure capacitive model fits well to the measurements.
Measured dielectric response in mica/epoxy explained the decreasing PDIV at low
frequencies and high temperatures. A high PDIV was measured at a combination of
low temperatures and low frequencies. This was correlated with a reduced void
resistance of the electrically stressed void sidewalls caused by the PD activity during the
preconditioning period. This indicates that the effect of PD by-products decays faster at
higher temperatures. Values of PDIV are, therefore, expected to be dependent on both
temperature and frequency.
high voltage equipment. Application of low frequency test voltage is often preferred in
the case of test objects with a large capacitance. The question addressed here is how
results from PD-measurements performed at low frequencies correspond to that
occurring at 50 Hz power frequency. Different theoretical models for void voltage were
examined and compared to experiments performed on laboratory samples of
mica/epoxy, including embedded cylindrical voids. All test objects were preconditioned
at 10 kV and 50 Hz for 5 min before partial discharge inception voltage (PDIV) testing
by stepwise increasing the test voltage from 0 to 10 kV. The PDIV test was first
completed at 300 Hz before being repeated at decreasing frequencies down to 0.1 Hz.
The temperature was varied in the range of 20° to 155°C. The results at high
frequencies showed that a pure capacitive model fits well to the measurements.
Measured dielectric response in mica/epoxy explained the decreasing PDIV at low
frequencies and high temperatures. A high PDIV was measured at a combination of
low temperatures and low frequencies. This was correlated with a reduced void
resistance of the electrically stressed void sidewalls caused by the PD activity during the
preconditioning period. This indicates that the effect of PD by-products decays faster at
higher temperatures. Values of PDIV are, therefore, expected to be dependent on both
temperature and frequency.