Study Of Very Fast Transient Overvoltage In GIS And Its Influence On Electronic Transformers

The in-situ installation of electronic transformers directly places signal acquisition circuits in substations with complex electromagnetic environment, breaking the traditional boundaries between primary equipment and secondary equipment. Because signal acquisition electronic circuits work long in the complex electromagnetic environment, electromagnetic interference (EMI) occurs frequently during the operation of electronic transformers in recent years. Especially the very fast transient overvoltage caused by disconnector switching operations, could have a strong electromagnetic interference impact on electronic transformers'acquisition circuits through conduction, radiation, or a combination of both, which leads to strike, crash, signal distortion and protection misoperation of acquisition circuits. Therefore, it has important theoretical significance and practical engineering application value to study the effect of VFTO conducted interference and electromagnetic radiation on reliability of GIS electronic transformer and the interference suppression.In this paper, we study the generation mechanism and characteristics of very fast transient overvoltage (VFTO) generated by disconnector switching operations in GIS, and further study the effects of conduction interference and electromagnetic radiation on the electronic transformer, and put forward the measures against electromagnetic interference of GIS electronic transformer. The main research works are as follows:By analyzing the VFTO waveforms and frequency characteristics of GIS with different structures, sizes and different electrical connections, the main factors influencing the frequency characteristics of VFTO are determined. On this basis, the VFTO frequency calculation method for different structures, sizes and different electrical connections of GIS is put forward, and the corresponding formula of VFTO frequency is deduced. Firstly, the transmission line model of the equivalent test circuit of the disconnector operating the short bus is designed, then the VFTO frequency calculation model is given, and the calculation formula of VFTO dominant frequency is deduced according to the calculation model. Using the same method, this paper also deduces the calculation formulas of VFTO dominant frequency in four kinds of different GIS electrical connection modes, which are short busbar earthing on the load side of the disconnector, short busbar earthing via capacitors on the load side, short busbar with series capacitor on the load side and branch busbar on the power supply side. The VFTO dominant frequency is calculated by EMTP software and VFTO frequency calculation formula. The calculated results are in good agreement with the measured results of VFTO experiment based on UHV GIS, which provides a basis for studying the VFTO characteristics of GIS.After comparing main time-frequency analysis methods of analyzing VFTO waveform, the generalized S-transform is chosen to analyze the time-frequency spectrum of VFTO measured waveforms. From the analysis of generalized S-transform, we can see that the different frequency components of VFTO have different local characteristics, that is, the amplitude and duration of frequency components are different, and the dominant frequency component has the highest amplitude and the longest duration. Based on the generalized S-transform method, we also compare and analyze VFTO frequencies under these situations-opening and closing operations of disconnector, repeated strike, different measuring point locations, and different structure parameters GIS and arrester installation, which provides a theoretical basis for studying the influence of VFTO conducted interference on electronic transformer and the interference suppression.The characteristic that the capacitive voltage divider ratio of GIS electronic voltage transformer is changing with frequency is analyzed. Under the high-frequency signal conditions, the capacitive voltage divider ratio declines sharply than the power-frequency condition. The VFTO conducted interference output from capacitive voltage divider still has high amplitude up to more than ten thousand volts, which has the over-voltage impact on the electronic transformer acquisition circuit, and may lead to damage to the acquisition circuit. In order to verify the VFTO conducted interference output from the capacitive voltage divider, the VFTO conducted interference of the capacitive voltage divider of the GIS electronic transformer was measured by using the 500kV GIS isolation switch operation test platform of China Electric Power Research Institute. The opening and closing operation tests were carried out 50 times, and the results show that the amplitude distribution of the VFTO conducted interference output from capacitive voltage divider is 9.45kV?14.17kV.The structure of GIS electronic transformer and the electromagnetic radiation characteristics of VFTO are studied. This paper establishes the calculation model of 220kV GIS electronic transformer, and simulates the electric field radiation intensity and the magnetic field radiation intensity of the VFTO inside the metal collecting box of the electronic transformer using the transient electromagnetic field calculation software. The results show that the electromagnetic field generated by VFTO can penetrate the terminal block disc of electronic transformer and radiate into the metal acquisition box, and form strong electromagnetic interference to the acquisition circuit installed inside the metal acquisition box. the transient electric field intensity reaches up to 10kV/m, and the transient magnetic field intensity reaches up to tens of kA/m, while the current EMC test standard specifies that the maximum electric field strength is lOV/m and the maximum magnetic field strength is 1kA/m, that is, the simulation result is far beyond the current EMC test standard.Aiming at the problem that the VFTO conducted interference has an overvoltage impact on the acquisition circuit, the suppression of the multi-layer coaxial capacitor parallel structure is proposed. This method increases the capacitance of the low-voltage side and the high-frequency divider ratio of the capacitive voltage divider, and reduces the VFTO conducted interference amplitude of the capacitive voltage divider, realizing the suppression of the VFTO conducted interference of the capacitive voltage divider. The VFTO conducted interference suppression of 220kV GIS electronic transformer's four layer coaxial capacitor parallel structure is simulated, and the calculation proves that four layer coaxial capacitor parallel structure can effectively restrain the VFTO conducted interference of the signal input-lines, reducing the amplitude of VFTO conducted interference to less than 1kV.At the same time, the influence of multi-layer coaxial capacitor parallel structure on the measurement error of electronic transformer is also studied. The results show that, after using the multi-layer coaxial capacitor parallel structure, the ratio difference of capacitive voltage divider is negligible relative to the power frequency, while the angular difference is beyond the standard limits, but we can amend it through the phase compensation.Aiming at the problem that the VFTO electromagnetic radiation interference may cause damage to the acquisition circuit of the acquisition box, it is suggested that the diameter of the terminal plate should be reduced and the connection length between the metal acquisition and the GIS shell should be lengthened to suppress the VFTO electromagnetic radiation interference. Simulations of different diameters of the terminal plate and different connection lengths between the metal acquisition and the GIS shell are carried out by transient electromagnetic simulation software. The simulation results show that, firstly, when the diameter of the terminal block disc is reduced from 80mm to 40mm, the VFTO radiation electric field interference decreases from 2.93kV/m to 0.13kV/m, and the VFTO radiation magnetic field interference decreases from 25.74kA/m to 1.28kA/m. Secondly, when the connecting length of the metal acquisition box and the GIS shell is increased from 30mm to 90mm, the VFTO radiation electric field interference decreases from 2.93kV/m to 0.21kV/m, and the VFTO radiation magnetic field interference decreases from 25.74kA/m to 0.87kA/m.The results of this paper can effectively improve the EMC of electronic transformers, and its operation stability and reliability, so as to provide theoretical guidance for electronic transformers'VFTO interference suppression, relevant test methods, and the design and manufacture of products.