Field measurements and modelling of high frequency transients during disconnect switch operations in EHV Substations

Abstract

The article describes the complete procedure carried out in order to determine the cause of high levels of gases detected in chromatographic determinations performed in Current Transformers (CT) oil. These analyses of gases are normally included in routine maintenance work. The obtained results could evidence some possible damage in the machine insulation. According to the electrical stresses that Current Transformers are typically subjected to, it was presumed that the cause of the possible damaged could be the high-frequency transients that take place during air disconnect switch operations. Therefore, with the aim of determining the presence of such electrical stress, field measurements were performed. A specially designed measuring system was utilized to accomplish this goal. Taking into account the difficulties normally encountered when measuring currents flowing through Current Transformers to ground, especially in terms of accuracy, it was decided to perform voltage measurements. Then, by determining the real frequency response of the CT in the range of interest, it was possible to achieve such currents by computer simulations. In order to achieve accurate results in the simulations, it was necessary to develop an appropriate electrical model of this type of transformers in the analyzed frequency range. Once the model was carefully adjusted, computer simulations were performed. As a result, both shape and magnitude of such currents were also achieved. Finally, one of the Current Transformers with high levels of gases was taken out of service. Then, it was completely disassembled so as to confirm the suspected damage in the insulation. The results yielded in the inspection of the insulation are commented at the end of this work.