Research On The Modeling Of The Online Over-voltage Monitoring System And The Numerical Inversion Calculation Methods

In the modern power systems, the phenomenon of over-voltage often occurs because of the lightning strike, failure, resonance, switching operation, etc. As soon as running across the over-voltage, the electrical equipments during operation are likely to generate breakdown, discharge, flashover, explosion accident and so on. These accidents cause not only economic loss of power companies but also serious effect on the security and stability of the power network. Therefore, monitoring the change of the over-voltage of the power system is of great importance to ensure the security and stability of the power network.However, there are limitations of the existing on-line over-voltage monitoring devices in the modern power systems. Contraposing this problem, this paper presents a new method for on-line over-voltage monitoring based on the T-parameters of the devices in the substation, which could conquer the above certain limitations. This method modeled the potential transformer (PT), secondary cables and voltage divider as several two-port networks in cascade. Adoption of this T-parameters model, the over-voltage of the high-voltage port of the PT was derived inversely from the voltage signal of the secondary device by means of specific numerical calculation methods such as Numerical Inverse Laplace Transform (NILT) and time domain recursive convolution (TDRC).Applying the principle, the over-voltage on-line monitoring system was put up in the laboratory and the two-port transmission parameters model of the PT, secondary cables and voltage divider was built based on the frequency domain measurements. Through the analysis of frequency domain measurement data of secondary cables and voltage divider, the cascading model of PT, secondary cables and voltage divider was simplified. By means of NILT and TDRC, the over-voltage was calculated and the simplified model was validated.In the application of NILT, this article used Vector Fitting instead of Pade approximation to approximate the partial fraction summation of the exponential function, which could simplify the acquisition of the poles and residue. At the same time, a method in allusion to the arbitrary waveforms source for using NILT was proposed in the study, which could conquer the limitation of the source in application to NILT. Through the analysis of frequency domain measurement data of secondary cables and voltage divider, the cascading model of PT, secondary cables and voltage divider was simplified as the PT only, which could inversely derive the over-voltage of the high-voltage port of the PT from the voltage signal which was amplified from the signal of the secondary port of the voltage divider. The establishment of the simplified model provides a solid foundation for the further practical application of the monitoring method in this paper.The comparison between the calculated results and measured data indicates that the modified NILT method is feasible and the simplified model is available. Comparing the results calculated by NILT with ones by TDRC, it shows that NILT is better than TDRC in reducing the numerical oscillation of the simulation effectively.