In-situ Insulation Test of 400 kV GIS

To guarantee the insulation strength of gas insulated substations (GIS), a number of different voltage waveforms, e.g., switching, lightning and ac have to be applied to the GIS after installation. Because of very huge dimensions of GIS for nominal high voltages, it is not possible to carry out these tests in the factory and parts of the whole system have to be,delivered and put together to build the complete GIS. As the result, all insulation tests have to be performed on site. Even if different parts of the system are tested in the factory, because some of the problems occur during the transportation and installation, the insulation strength of the whole GIS can be degraded. In this paper, a novel test set-up and the measurement results of a 400 kV GIS have been installed for the Mobarakeh steel industries, Isfahan, Iran, are presented. This system has a length of about 100 m, which corresponds to a total capacitance of about 10 nF per phase. Because of this relatively large capacitance, the power ratings of the test voltage sources have to be very high. For achieving such a high power, a two step cascade voltage transformer (each 800 V/300 kV with a maximum output current of 2 A) fed through an autotransformer enhanced with a number of inductors to compensate the capacitive current and to minimize the input current of the test transformers have been used to apply the necessary 515 kV to perform the ac tests of the whole GIS. The measurements carried out on the system showed that the first two phases passed the test successfully; however the third phase could not withstand the applied voltage because of the pollution near one of the spacers. After replacing the faulty spacer, the insulation strength of the third phase has been recovered.