Ultra high frequency electromagnetic wave detector for diagnostic of metal clad switchgear insulation

Abstract

This paper describes a newly developed ultra high frequency electromagnetic wave (UHF wave) detector for tracking discharges in metal clad switchgear (MCS) and its performance. The tracking discharges included waves with frequencies of more than a few hundred megahertz and the attenuation of the UHF waves transmitted through a MCS enclosure was about IOdBpV. On the other hand, noise in the ultra high frequency range was relatively low. The simple UHF wave detector was tested in the laboratory and field.Keywork: metal clad switchgear, insulation, tracking discharge, diagnostic, ultra high frequency electromagnetic wave, detector, spectrum analyzer. INTRODUCT IONMetal clad switchgear (MCS) up to 35kV usually include circuit breakers, busbars, current transformers, voltage transformers, arresters, and feeder cable terminations. It is important to diagnose the insulation condition of MCS from the viewpoint of distribution system reliability. MCS sometimes deteriorates during operation and experiences failure after years of use in damp and polluted environments[ 11. Their failures often result from electrical tracking of the organic epoxy insulation which supports the phase conductors. This tracking, which occurs only under moist conditions, usually takes years before it causes a flashover. Most diagnostic techniques rely on detecting one or another of the effects produced, usually optical, acoustic, electrical or chemical. Partial discharges (PDs) are associated with the tracking. Acceptance of PD tests for the purpose of assessing the insulating condition of operating MCS requires the following conditions be met: there is no threat to the MCS being examind; the test cost is lower than the cost of outages and failures; there are no false indications of deterioration; there is an effective diagnosis before catastrophic failure; and the test can be implemented as part of an overall maintenance strategy. On-line PD tests have tended to give excessive numbers of false indications due to electrical noise. External noise sources include PDs from the power system, arcing from poor contacts and SM 371-5 PWRD A paper recommended and approved by the IEEE Power System Instrumentation &Measure-ments Committee of the IEEE Power Engineering Society for presentation at the IEEE/PRINTED IN USA 5welded joints, carrier communication systems, radio transmissions and so on. To differentiate between an actual PD within the MCS and noise, the characteristics of the PD and noise must be known. Several attributes of the PD and noise are helpful in drawing distinctions: phase position with respect to the ac cycle; the frequency spectrum; pulse magnitude and its distribution; polarity; repetition rate; and physical location.A metal clad monitor has been developed to monitor the PD activity continuously during normal service of the MCS [21. The discharge was detected by a 500kHz radio antenna and a 40kHz ultrasonic microphone mounted close to the electrical insulation.The present study develops a simple detector for tracking discharges in diag...