Partial Discharge Recording For 6.6KV, 11KV, 22KV & 33KV Switchgear and Transformer Cable Compartment

How do we test?
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.

Why test using both TEV and Ultrasonic methods?
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.

TEV Principle of Operation

The Instrument:


 
Partial Discharge Monitor (PDM)

• Continuous Monitoring for a set period
• Data stored on floppy disc for upto 52-days
• Monitor 8 zones at a time
• Aerials negate external interference

Ultrasonic Detector

• Provides spot measurements
• Locates low level surface discharges
• Suitable for use on all air insulated sytems

Types of Switchgear

• Up to 66kV
• Indoor Metalclad switchboards
• Indoor and Outdoor Ring Main Units

Periodic Testing

• Partial Discharge Recording and ultrasonic survey interval from every three month; 6-month or even one year depend on the condition of the switchgear.

Contact our application engineers for further details click here.