2388-MANUL-V10.00.00-UTP2 Operating Manual
Page 2 of 74
The most sensitive method of detection is using an airborne ultrasonic Microphone centred at 40 kHz. This
method is very successful at detecting partial discharge activity provided there is an air passage between the
source and the Microphone.
Electromagnetic Discharge Activity
When partial discharge activity occurs within high voltage switchgear, it generates electromagnetic waves in
the radio frequency range which can only escape from the inside of the switchgear through openings in the
metal casing. These openings may be air gaps around covers, or gaskets, or other insulating components.
When the electromagnetic wave propagates outside the switchgear, it produces a Transient Earth Voltage (TEV)
on the metal casing of the switchgear. The Transient Earth Voltage (TEV) is only a few millivolts, of short
duration having a rise time of a few nanoseconds and can be detected non-intrusively by placing a TEV probe
on the outside of the earthed switchgear whilst the switchgear is in service.
Ultra-High Frequency (UHF) Partial Discharge Activity
Electromagnetic waves produced by partial discharge activity within high voltage switchgear can also radiate
away from the asset through free space as Radio Frequency Interference (RFI). RFI due to partial discharge is
broadband in nature and can be detected across a range of frequencies. These radiated emissions can be
detected using a suitable antenna and radio receiver. Corona discharges around sharp points in air are common
at EHV voltages and must not be allowed to mask damaging partial discharges originating from problem plant
items. Corona discharges tend to be more of a problem when measuring below 200MHz. For this reason, the
UltraTEV Plus
2
UHF Receiver is designed to operate at higher frequencies (typically around 800 MHz). It has
the ability to be tuned up or down to move away from any local telecommunication transmissions which would
interfere with measurements.
Cable Partial Discharge Activity
When a partial discharge event occurs in a cable, there will be a resulting current pulse coupled onto the earthed
sheath of the cable. These pulses will propagate away from the discharge site in both directions. Once the pulse
reaches a change in impedance at a cable joint or termination, a partial reflection occurs. This results in the
pulses travelling back down the cable several times (depending on the cable length) as they decay away. The
current pulses can be detected non-intrusively using a High Frequency Current Transformer (HFCT) sensor
placed around the cable earth strap.
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