Video: PXDP-II

Video transcript

What partial discharges are, how the PXDP-II picks them up on cables, switchgear and machines, and how to read what it shows. A partial discharge is a small breakdown in an insulation that does not bridge the conductors, so nothing trips. Repeated every cycle, it slowly eats the insulation away. Three families: internal discharges, in voids and cracks, typical of cable joints; surface discharges, on damp or dirty insulators; and corona, around sharp points in air, heard mostly as ultrasound.

It reads in decibels, and in picocoulombs once calibrated. 60 decibels of dynamic range, 5 picocoulombs of sensitivity, 300 kilohertz to 70 megahertz of bandwidth, and 7 and a half hours on its batteries. On top, SENSOR, where every probe goes in, and REF, a reference pulse for self-tests. Below the display, the menu keys, backlight and power. The speaker helps tell discharges from noise. One instrument, many sensors:

capacitive and differential sensors for joints, TEV and a bi-phase coupler for switchgear, HFCT clamps for earth returns, acoustic sensors, and a sniffer for machines. Normal mode shows several cycles and the peak level. Peak-angle mode locks to the mains and averages the peak every 3 degrees: random noise fades, while discharges pile up at fixed angles. That is what tells them apart. On a cable joint, a self-test first: the sensor around the reference module on REF must read within its marked tolerance.

Then the band-pass filter goes on SENSOR, the sensor lead onto it, and the sensor, on its insulated rod, around the joint. A joint is read at 4 points, end to end, and an elbow at two, low and high. The display gives a level in decibels and the speaker a sound: discharges crackle in a repetitive way. In a noisy substation, two sensors work in differential: positions 1 and 2 read each sensor, position 3 subtracts them. Shared noise drops by 10 decibels or more.

A discharge under one sensor does not drop. In metal-clad switchgear, an internal discharge sends transient voltages to the outside of the enclosure. The 20 decibel attenuator goes on SENSOR, the probe lead onto it, and the TEV probe holds by its magnet on the cell. The bi-phase coupler uses the capacitive outlets of the voltage indicators. First the magnetic ground module on the cabinet, clipped to the earth post. Then red on line one, black on line two,

and the output through the filter to SENSOR. A discharge also sends current pulses down to earth. The HFCT clamp opens around the earth return of the cable, with no contact with live parts. The band-pass filter goes on SENSOR, and the coaxial cable joins it to the clamp. For rotating machines, the corona sniffer scans the stator winding. The earth goes first: the ground cable from REF to the earth of the machine. Then the probe on SENSOR. Swept along the slots, the level rises over a defect.

Corona and surface discharges also make ultrasound. The acoustic module sits on top of the instrument. The contact probe goes on enclosures, and the parabolic dish, with its laser pointer, finds sources on overhead lines from a distance. Off line, it becomes a test system in picocoulombs. The object is fed from a discharge-free source through a capacitive coupler. The calibrator injects 200 picocoulombs to set the scale, then the voltage is raised step by step.

Levels are relative, so each site has its own normal value. On joints, about 20 decibels above normal is suspect; on elbows, 10. Measure once a year, in the same order, and follow the trend in the PC software. Four rules. Sensors never touch live parts: only insulation or earthed enclosures. Treat every terminal as live. No explosive atmospheres. And ask the operator before using the indicator outlets. It is used for quality control of medium and high voltage equipment, to check joints and elbows when a network is built or repaired,

to follow insulation ageing, and as a safety check before live work. Partial discharge detection, from Amperis.