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| Open Circuit Fault Diagnosis Strategy of MMC Sub-Module Based on Generalized Capacitor Voltage Unbalance |
| Wu Hong, Wang Yue, Liu Yi, Li Pengkun, Li Runtian |
| State Key Laboratory of Electrical Insulation and Power Equipment Xi'an Jiaotong University Xi'an 710049 China |
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Abstract Modular multilevel converters (MMC) have features of modular construction, redundancy configuration, and power scalability. The MMC topology is composed of a large number of series-connected submodules (SMs), and reliability is one of its important issues. The faults of the insulated gate bipolar transistor (IGBT) can generally be divided into short-circuit faults and open-circuit faults. The short-circuit fault is destructive, but now the gate driver is always integrated with short-circuit protection. In contrast, the open-circuit fault will not immediately damage the SM and may remain undetected for a long time. MMC's SM open-circuit diagnosis strategy can be divided into hardware-based and software-based categories. The software-based strategies have the advantages of no additional hardware and fast diagnosis time. However, it often requires an empirical threshold, which is difficult to set when the MMC operates at different power.
Therefore, an open-circuit fault diagnosis strategy for MMC sub-modules based on generalized capacitor voltage unbalance degree (GCVUD) is proposed. The fault is detected by judging whether the GCVUD of the SM with the largest capacitor voltage in the arm exceeds the threshold. The remaining faulty SMs are located until the GCVUD of one SM is within the threshold according to the capacitor voltage sorting results. Moreover, to further eliminate the effects of errors and prevent fault misdiagnosis situations, a counting flag is used in the detection process and location process.
In this strategy, the threshold setting determines the accuracy and rapidity directly. The threshold shall be greater than the GCVUD to ensure no misdiagnosis during normal operation. On the other hand, the threshold shall be less than the minimum of the corresponding GCVUD in the case of two types of open-circuit faults. The type II fault of SM in rectifier mode is selected for setting the diagnostic threshold. The quantitative relationship between the diagnostic threshold and the operating power is given by calculation and deduction.
A detailed real-time digital simulation model of the MMC-HVDC system is built in a hardware-in-the-loop platform. The power-stage model is emulated by the target machine, and the control system is realized by the DSP TMS320F28335 control board. The experimental results verify that the voltage characteristics of the faulty SM are related to the type of fault and the MMC operating mode. The SM capacitor voltage under the type I fault will remain unchanged. The SM capacitor voltage will increase continuously in inverter mode but will keep an abnormal balance in rectifier mode. The GCVUD in the type II fault in rectifier mode is smaller than in other cases when the operating power is equal, and its calculated value is close to the actual value. The proposed strategy can accurately diagnose two types of MMC SM open-circuit faults at different operating powers. Before the fault occurs, the GCVUD will not exceed the threshold, so misdiagnosis will not occur during normal operation. If the fault occurs, the GCVUD exceeds the threshold, and the system can quickly detect the fault. Then, the number of faulty SMs and their serial numbers can be determined. In addition, the proposed strategy can cope with the cases of multiple SM failures in the arm and complete the diagnosis process within one fundamental frequency period.
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Received: 23 May 2022
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2023, Vol. 38 
