The reliability of GIS is very high but any failure that occurs can cause extensive damage result and the repair times are considerably long. The consequential losses to system security and economically
can be high, especially if the nominal GIS voltage is 420 kV and above. In view of these circumstances, increasing attention is being given to diagnostic techniques for in-service maintenance undertaken to improve the reliability and availability of GIS. Recently considerable progress has been made in diagnostic techniques and they are now used successfully during the service life of the equipment. These diagnostic techniques in general focus on the GIS insulation system and are based on partial discharge (PD) measurements in GIS.
There are three main methods for in-service PD detection in GIS: -
the chemical method that rely on the detection of cracked gas caused by PD, the acoustic method designed to detect the acoustic emission excited by PD, and, the electrical method which is based on detection of electrical resonance at ultra high frequencies (UHF) up to 1.5
GHz caused by PD excitation in GIS chambers (UHF method). These three dielectric diagnostic methods cannot be used for the detection of poor current carrying contacts in GIS. This problem does not always produce partial discharges and at early stages it does not cause gas cracking. An interesting solution to use two techniques - the current unbalance alarm scheme and partial discharge monitoring was advised by A. Salinas from South California Edison Co. Unfortunately this way is complicated and very expensive. The investigations performed in Japan on standing alone SF6 breaker showed that joule heating of
the contact accompanied by released power of 1600 Watt produce temperature difference on the enclosure up to 7 degrees centigrade that could be detected by infra-red Thermal Imaging System. According to CIGRE Joint Working Group 33/23.12 Report, 11% of all GIS failures are due to poor current carrying contacts in GIS.
The Israel Electric Company (IEC) in seeking a solution to this problem have undertaken experimental work to examine the possibility of in-service diagnostic of poor contact problem in GIS via direct local heating detection, using a Thermal Imaging System.
The experiments were carried out on the part of the GIS with nominal SF6 pressure. The following aspects of the problem were examined:
- the range of power released in the defective contact that could give the practical temperature rise on the surface of enclosure;
- temperature distribution on the surface of enclosure;
- the influence of spacer type (with holes or without) on the heat transfer process;
- the influence of the length of SF6 tubes and there position (horizontal or vertical);
- the temperature difference between upper and lower parts of the tubes in horizontal position;
- practical use of the Thermal Imaging System for detecting poor contact problem in GIS.