Design of EMI Filter for Microcomputer Protection Device of Switchgear Based on Integrated Electromagnetic Disturbance Conduction Model

In scenarios such as offshore wind farms and distributed intelligent power distribution networks, the stochastic fluctuations of new energy output, electric vehicle charging and discharging, and energy storage equipment cause the power grid to frequently suffer from electromagnetic transient impact. To ensure the safety of key equipment such as transformers and power electronic devices, it is necessary to quickly interrupt faulty devices or lines. As a result, high-voltage switchgear circuit breakers frequently operate, causing a large amount of electromagnetic interference, leading to a serious threat to secondary equipment. The source of electromagnetic interference during the interruption of high-voltage switchgear is the vacuum circuit breaker, and the disturbed devices are intelligent electronic devices. The electromagnetic interference mainly has three transmission paths, involving devices including the vacuum circuit breaker and its distributed capacitance, current transformers, secondary cables, intelligent electronic device (IED), and ground grid. To accurately analyze the impact of the disturbance voltage generated by the breaker's interruption on the IED's port, it was necessary to study the complete process of electromagnetic interference conduction. However, in actual power systems, the switchgear can operate under various conditions, making systematic electromagnetic interference tests difficult. Therefore, the electromagnetic transient simulation could be used to analyze the characteristics and conduction process of the breaker’s interruption. The paper took the KYN61-40.5 high-voltage switchgear as an example and established the integrated electromagnetic disturbance conduction model through the finite element simulation and sweep frequency methods. The test and simulation results showed that the simulated wave amplitude, dominant frequency, and duration of the breaker's primary side voltage were 342 V, 312 kHz, and 59.2 μs, respectively. The maximum relative error compared with the experimental results was 6.0%, and the waveform shape was quite close. The amplitude, dominant frequency, and duration of the simulated disturbance voltage of the microcomputer protection device current measurement port were 7.8 V, 282.2 kHz, and 47.1 μs, respectively, with the maximum relative error of the amplitude being 9.8%. The experimental and simulation results verified the accuracy of the integrated electromagnetic interference conduction simulation model. The EMI filter is a device used to suppress electromagnetic interference and can effectively enhance the electromagnetic compatibility of sensitive devices such as IEDs. The low-pass EMI filter designed in this paper was composed of a 3rd order constant K filter and two m-derived filters in series, which has a cut-off frequency of 100 kHz and notch frequencies of 125 kHz and 310 kHz. The disturbance voltage amplitude dropped by 30 dB at the notch frequencies. The parameters of each component were determined according to the design methods of the constant K filter and m-derived filters, and the EMI filter was built by typical inductors and capacitors. The electromagnetic interference waveform was input into the EMI filter, and the disturbance voltage amplitude dropped from 7.1 V to 0.43 V, a relative decrease of 93.94%, and the duration dropped from 50.8 μs to 14.52 μs, a relative decrease of 71.42%. This shows that the EMI filter designed in this paper has a good suppressive effect against the electromagnetic interference caused by the interruption of the circuit breaker, meeting the design requirements. The research results of this paper can provide a reference for the analysis of the electromagnetic interference conduction process and the improvement of the electromagnetic compatibility level of the IEDs of the high-voltage switchgear. © 2024 China Machine Press. All rights reserved.