If a partial discharge occurs in the phase to earth insulation of an item of high voltage plant, a small quantity of charge is transferred capacitively to the earthed metal cladding. An electromagnetic wave is generated at the discharge site which propagates away in all directions and by escaping through an opening in the metal cladding, such as a gasketed joint, can be detected on the outer surface as a Transient Earth Voltage (TEV). The TEV has a nanosecond rise time and an amplitude which varies widely from millivolts to volts.
In dry termination cable boxes, surface discharges have been found which are less than 50pC and emit very low levels of electromagnetic signal. Provided a direct air passage is available through the skin of the cable box the airborne Ultrasonic signal generated by these surface discharges is detectable with an Ultrasonic detector.
Often, but not always, a problem will only be detected by one of the techniques. TEV is successful at identifying when there is a leakage to earth and is suitable for compound and oil insulated components. Generally there will not be any accoustic emissions present. Ultrasonic, on the other hand typically detects problems between phases, terminations and switch tank spouts are likely candidates. Ultrasonic will also show up problems with Air Switches, Insulators and Bushings in Outdoor structures.
- Voltages upto 33kV
- Indoor Metalclad switchboards
- Indoor and Outdoor Ring Main Units
- Partial Discharge Locator and ultrasonic survey recommended every 2 years
- Partial Discharge Continuous Monitoring recommended every 4 years
- Measurement of Discharge in dB
- Higher Accuracy
- Wider Bandwidth
- Double probe accurately locates the position of Discharge
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- Continuous Monitoring for a set period
- Data stored on floppy disc for upto 2 months
- Analysis of data using standard PC
- Monitor 8 zones at a time
- Aerials negate external interference
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- Provides spot measurements
- Locates low level surface discharges
- Suitable for use on all air insulated sytems
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