Non-Intrusive Detection of Partial Discharge

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A partial discharge will emit energy in the form of electromagnetic (radio, light, heat); acoustic (audio and ultrasonic); and gases (ozone and nitrous oxides).  The most practical methods for non-intrusive testing are based on the detection of the radio frequency part of the electromagnetic spectrum and ultrasonic emissions.

Acoustic emission from partial discharge activity occurs over the whole acoustic spectra.  Audible detection is possible but depends on the hearing ability of the individual.  Using an instrument to detect the ultrasonic part of the acoustic spectra has several advantages.  Instruments are more sensitive than the human ear, are not operator dependent and operating above the audible frequency are more directional.

The most sensitive method of detection is using an airborne ultrasonic microphone centered 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.

When PD activity occurs within high voltage switchgear it generates electromagnetic waves in the RF 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 also impinges on the metal cladding of the switchgear producing a transient voltage in the external metal cladding of the switchgear.  The Transient Earth Voltage (TEV) is a few millivolts to a few volts and lasts only a short time with a rise time of a few nanoseconds.  The PD activity may be detected non-intrusively by placing a probe on the outside of the switchgear while the switchgear is in service.

Keeping in our tradition of stocking innovative testing solutions, Protec Equipment Resources has added the EA Technology UltraTEV Plus+ to our ever expanding inventory of rental equipment.  The UltraTEV Plus+ detector has been specifically developed to enable electromagnetic and ultrasonic activity to be detected in a single simple to operate test set.