A Diagnosis Strategy for Open-Circuit Submodule Faults in MMCs under Nearst Level Modulation Suitable for Different Powers

With the increasing voltage and power level of modular multilevel converters (MMC), the number of its submodules (SMs) also increases, and the issue of its reliability is highlighted. The failure of IGBT in any SM can affect the normal operation of MMC and even further lead to the damage of other components, which eventually lead to the collapse of the whole system. Thus, it is necessary to detect the fault and locate the malfunctioning SMs accurately to improve its reliability. MMC’s SM open-circuit diagnosis strategy can be divided into two categories: hardware-based and software-based. Compared with the hardware-based strategy, the software-based strategies need no extra hardware, have low computational burden. However, it is difficult to set the empirical threshold of existing software-based strategies between different operating powers, which hinders its further development. To solve this issue, a diagnosis strategy for open-circuit SM faults in MMCs suitable for different operating powers is proposed. The SM switching function is predicted based on the number of SM inputs and the position of SM capacitor voltage sorting arrangement and then the corresponding SM capacitor voltage is predicted. On the basis, the fault is detected and located by judging whether the absolute error between the predicted value and the actual value of the SM capacitor voltage exceeds a threshold. Moreover, to further eliminate the effects of errors and prevent the existence of fault misdiagnosis situations, a time flag is used in both the detection process and location process. In the proposed strategy, to avoid the effects of misdiagnosis near the arm current-over-zero point, a current-over-zero filtering part is proposed. Based on the mathematical deduction, the principle of setting threshold is given. It is indicated that the proposed strategy eliminates the need to manually set the threshold when the MMC operating power changes, which greatly reduce the difficulty of setting the threshold compared with the existing strategies. At the same time, theoretical analysis verifies that the proposed strategy is not only suitable for NLM modulation strategy, but also can be extended to other MMC modulation strategies after adjusting the prediction method of SM switching function. 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 result verify that the proposed strategy can accurately diagnose two types of MMC SM open-circuit faults at different operating powers. It is observed that the absolute error of a Type I malfunctioning SM is high when the arm current is negative and the absolute error of a Type II malfunctioning SM is high when the arm current is positive, which verifies the correctness of the theoretical analysis. Before the fault occurs, the absolute error will not exceed the threshold so that misdiagnosis will not occur during normal operation. After the fault occurs, the absolute error exceeds the threshold, the system can quickly detect the fault and then locate the remaining malfunctioning SMs. Moreover, the proposed strategy integrates the advantages of existing strategies, including no extra hardware, low computational burden, fast diagnosis (＜20 ms), and can cope with the cases of multiple SM failures in the arm. © 2024 Chinese Machine Press. All rights reserved.