Study On Measuring Technology And Protection Technology Of Non-linear Residual Current

With the increasing of power electronic devices, residual current becomes more complicated when electric leakage faults happen. Therefore, it is necessary to solve new problems in protection theory and measuring technology of residual current. Based on the analysis of the development of residual current protection technology, this paper focuses on the study of measuring technology and protection theory of non-linear residual current, which has significant theoretical and application value.The dynamic characteristics of the current-drive and voltage-drive magnetic modulation residual current transformers are analyzed, and a simplified mathematical model is derived to demonstrate the relationship between the measured DC signal and the even order harmonic components of magnetic field in the magnetic core. The residual current detecting model of voltage-drive magnetic modulation transformer with a single magnetic core is built, and it is proved that the residual current can be measured precisely by detecting the excitation current. The analysis model of magnetic modulation current transformer considering magnetic hysteresis has been built. With the same difference between positive saturation threshold current and negative saturation threshold, whether magnetic hysteresis is considered will not influence the frequency of pulsed excitation voltage and the average excitation current. Therefore, it provides a theoretical foundation for the simplified engineering calculation. A self-adaptive control method of excitation voltage frequency is proposed to improve the detecting sensitivity of DC residual current and to reduce the power consumption of the exciting power supply.The new residual current protection methods have been studied. Waveform characteristics of pulsating DC residual currents are analyzed to identify different waveforms, such as half-wave, 90° wave, 135° wave and so on. Various thresholds are proposed for residual current with different waveforms to improve the accuracy and consistency of pulsating DC residual current protection. A residual current protection method based on residual variation is proposed. The protection models for both sinusoidal and non-sinusoidal residual current have been introduced, and the dead zone of residual current protection is overcome. Furthermore, the methods to reduce nuisance tripping are carried out, such as percentage-ratio restraint residual current protection and selective protection in IT power grid.The residual current protection controller is designed. Factors that affect the operating performance of the residual current transformer are analyzed, and the frequency of excitation voltage, average excitation current and detection range of residual current are the main characteristic parameters of the transformer. And several factors, such as varying number of turns, tuning amplitude of pulsed excitation voltage and reversing the exciting current, are taken to make the transformer meet the design requirements. Hardware and software design are carried out according to the range and requirements of residual current protection.The simulation and experimental study of residual current protection controller is conducted. A simulation model of voltage-drive magnetic modulation residual current transformer is built using Matlab/Similink, which includes B-H curve model with magnetic hysteresis taken into consideration, pulsed excitation voltage control circuit model and input-output model. The output characteristics of the residual current transformer are tested, including the dynamic process of excitation voltage and waveform of excitation voltage with input current. The comparison between experimental and simulation results is presented, which proves that the theoretical analysis and simulation model can be used to guide the design of residual current transformer. Furthermore, the dynamic process of excitation current considering magnetic hysteresis is more consistent with the actual changing process. Several tests are conducted, such as the operating characteristics of the residual current protection controller, telecommunicating function and EMC.