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EA Technology
7
Introduction
Non-Intrusive Detection of Partial Discharge Activity
General
Partial discharges are electric discharges that do not completely bridge the electrodes. The
magnitude of such discharges is usually small however; they do cause progressive
deterioration of insulation that may lead to eventual failure.
Non-intrusive partial discharge detection provides a means for identifying these potential
sources of insulation failure that result not only in loss of supply to customers but can also
endanger staff.
A partial discharge emits energy in the following ways:
Electromagnetic:
Radio
Light
Heat
Acoustic:
Audio
Ultrasonic
Gases:
Ozone
Nitrous oxides
When the partial discharge event occurs in a cable, there will be a resulting voltage pulse
coupled into the earth sheath of the cable. These pulses will propagate away from the PD
side in both directions. Once the pulse reaches a change in impedance, this will cause a
partial reflection. This results in the pulses travelling back down the cable several times
(depending on the cable length) as they decay away.
A measurement is taken using the Radio Frequency Current Transformer (RFCT) at one end
of the cable. This will reveal a pattern of pulses, where the first pulse is the direct pulse from
the PD event. The second pulse is the reflected pulse from the far end. If the PD site is at
the far end of the cable, then the direct pulse and the reflection will be very close to each
other, or perhaps superimposed on each other. Conversely, if the PD site is at the near end
of the cable, the direct pulse will be received first, and then there will be a longer gap as the
pulse travelling away will have to travel nearly twice the length of the cable until it is
detected.
CableData Collector Operating Manual
Page 7 of 54
1365/L/01/1
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