The UHF sensor provides a low cost,

The UHF sensor provides a low cost, easy to apply,
wideband PD detection method for the diagnosis of power
equipment. An experiment has been designed to measure prebreakdown
discharge currents in LN2 under AC voltages using
wideband, UHF and RFCT, sensors and the conventional IEC
60270 standard. The results confirm that the electromagnetic
waves radiated by charge movement during PD activity can be
captured by a UHF sensor with a good signal to noise ratio.
UHF measurement gives higher temporal resolution allowing
finer details of the discharge event, such as a pulse train,
when compared to conventional method. The information lost
in the time domain due to the limitation of narrow bandwidth
Fig. 8. RFCT peak-to-peak plotted against apparent charge during positive
discharges at 17.6kV, 19kV and 23kV.
sensing can aid understanding of the dynamics of charge
movement. Therefore, combination of UHF and IEC standard
measurement can provide a more powerful tool for PD testing
and diagnostics.
Results from the UHF sensor show that PD events in point
cathode consist of a train of single pulses with ascending
magnitude which cannot be resolved by the lower bandwidth
of standard capacitive coupling circuits. Meanwhile, during
positive discharges, only one single event is seen with much
higher magnitude than negative discharges. These results are
in line with previously published observations [3].
It is difficult to quantify PD magnitude from UHF measurement
due to the variability in the energy received by the
sensor. This is thought to be because of the chaotic behaviour
of charge dynamics during streamers, which results in the
variability of the UHF pulse shapes. Further study is required
to confirm this hypothesis.