Study Of IGBT Open-Circuit Fault Detection Methods For Multi-Terminal VSC-HVDC System

Multi-terminal flexible DC power transmission(Voltage Source Converter based Multiterminal High Voltage Direct Current Transmission(VSC-MTDC))has the characteristics of flexible operation and easy expansion.Has a good application prospect.The power device is the core device in the VSC of the voltage source converter station,and its open-circuit failure will cause system current distortion and voltage and power fluctuations.Not only will it cause irreversible damage to the power device,but the secondary failure caused by it will seriously affect the safe and reliable operation of the system.Unlike short-circuit faults,which have obvious short-circuit current characteristics,open-circuit faults in power devices are somewhat concealed and difficult to detect directly.This article takes the parallel VSC-MTDC as the research object,and researches the non-intrusive open-circuit fault detection method of power devices based on VSC current characteristics to provide technical support for ensuring the safe operation of the VSC-MTDC system.The main research contents of the paper are as follows:First,by analyzing the working principle of the VSC-MTDC system,the thesis establishes a VSC mathematical model.According to the typical operating mode of the VSC-MTDC system,VSC control strategies of constant DC voltage,constant active / reactive power,and constant AC voltage / frequency are designed respectively,and the current response of the AC and DC sides of the VSC after the open circuit of the power device is theoretically analyzed The characteristics are verified by simulation,which provides a theoretical basis for studying the open circuit detection method of VSC-MTDC system.Second,using the DC component of the VSC AC-side current as the open-circuit fault feature of the power device,the conventional direct average current method,the Park vector method,and the normalized DC component method were designed and implemented.The detection performance of VSC under different operating modes and different operating conditions is studied.The performance of open-circuit fault detection of power devices under VSC-MTDC system power adjustment,DC voltage fluctuation,and power flow inversion is compared and analyzed.The characteristics of DC components are also analyzed.The selection of thresholds is discussed.Third,the DC component characteristics are sensitive to the system operating conditions and the detection speed is slow.Based on the basic theory of empirical mode decomposition,an open-circuit fault detection method for VSC-MTDC system based on instantaneous frequency characteristics was proposed.The modal decomposition of the AC-side current of the VSC is used to extract the instantaneous frequency characteristics of the intrinsic mode function(IMF)for open-circuit detection of power devices.The simulation results show that the IMF instantaneous frequency characteristics have better dynamic detection performance and can identify the occurrence time of the open fault of the power device.Fourth,in order to further improve the real-time performance of open-circuit fault detection in the VSC-MTDC system,it is proposed to use the transient high-frequency oscillating current generated by the switching of the power device to perform open-circuit detection.By analyzing the switching excitation process and frequency spectrum characteristics of IGBT power devices,the formation mechanism of high-frequency switching oscillation response current in VSC was revealed.The compression characteristics of the power devices were reduced and reconstructed using compression sensing theory,and verified by experiments.Finally,a high-speed open-circuit fault detection method for power devices based on the characteristics of the switch oscillation current is proposed,and preliminary verification is performed in a small-capacity experimental prototype system.The results show that the IGBT open-circuit fault can be accurately detected in one switching cycle.