EA Technology UltraTEV Plus
2
Operating Manual 2388L015
Page 6 of 66
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.
4.4 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.
4.5 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 Radio Frequency Current
Transformer (RFCT) sensor placed around the cable earth strap.
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