| MMC(Modular Multi-Level Converter)has been widely used in current DC transmission projects,which is closely related to the safety of power systems.Therefore its operational reliability needs high requirements.In order to improve its long-term operation reliability,it is important to monitor the status of the core component-IGBT(Insulated Gate Bipolar Transistor)module online.In some working conditions such as the MMC rectification mode,the FRD(Fast Recovery Diode)in IGBT module is more likely to age than the IGBT.Therefore,it is essential to on-line monitor the state of the FRD in the IGBT module.The temperature dependence of the forward characteristic curve of the FRD was studied through theory and experiment and the quasi-TCP(Temperature Compensation Point)current was defined.Based on this,a method for on-line monitoring of the aging state of FRD bonding wires in IGBT modules was proposed.This method does not need the junction temperature measurement,but only needs to monitor the forward voltage under the quasi-TCP current of the FRD forward characteristic curve,so as to evaluate the health status of FRD and the whole IGBT module.In order to verify the effectiveness of the method,the difference of the output/forward characteristic curves of IGBT and FRD chip with different number of bonding wires are obtained by experiment.Compared with the measurement results of IGBT chip,it is found that the method has higher sensitivity when applied to FRD.Based on this method,the characteristics of different on state voltage drop monitoring circuits are analyzed and a new circuit topology is proposed.Based on this circuit,an on-line monitoring system of IGBT module in MMC converter valve was designed and built.The system meets the on-line monitoring requirements of IGBT module of MMC converter valve through the test of withstand voltage,transient performance and temperature drift test. |
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