Research On Overvoltage Online Monitoring System Based On The Potential Transformer

Due to the atmosphere lightning and switching operations, the electrical equipment in the substations are influenced by lightning overvoltage and switching overvoltage. These overvoltages may cause damage to the equipment’s insulation, power failure and other accident. Monitoring overvoltage online for safe operation of the grid is very important. Potential transformer is an essential device in the power substation. It can be used to achieve monitoring overvoltage. Because of the high amplitude of lightning overvoltage and switching overvoltage, the iron core of potential transformer is in a strong magnetic field environment. Excitation characteristics and iron loss characteristics reach saturation in a very short time. The iron core shows a nonlinear characteristic, so the distortion of the secondary current and voltage happens. An improved model of10kV potential transformer is used based on the original models in this paper. No-load tests, scattering parameters measurements experiment and impulse voltage test are carried out. The parameters of the iron core part are calculated by the no-load tests data and the parameters of other resistances, inductances and capacitances in the model are calculated by the scattering parameters. A constrained optimization algorithm is used to optimize the parameters of the model based on quasi-Newton algorithm. The input and output characteristics between calculation and experiment are compared, achieving a good match. In order to restore the true primary voltage waveform, a time domain inversion calculation method is proposed based on the Kirchhoffs law, getting the primary voltage waveform exactly. By modeling the potential transformer, the secondary cables and capacitive voltage divider as a whole two-port network, the comparison between the calculated results and measured data verify the feasibility of this method. Because of containing noise in secondary waveform and containing oscillation in primary waveform, the methods of de-noising and filter are used. Also, the EMTP simulation circuit models under overvoltage in35kV power substation are built. By verification, the method of inversion calculation is valid.