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The capacitor condition monitoring is an important issue for the reliable operation of modular multilevel converters (MMCs) in high-voltage applications. The numerous capacitors make the monitoring for the capacitor condition complex and computation-consuming. This paper proposes a hierarchic capacitor condition monitoring strategy to assure the reliability of high-voltage MMCs. In the strategy, the switching times of the switching signals are counted to detect the condition of the capacitor in each submodule (SM). Then, the subsequent capacitance calculation method is only conducted to the abnormal capacitors indicated by the above condition detection algorithm. Meanwhile, the variations of the measured and reference capacitor voltages are employed to calculate the capacitance in the proposed method. The condition detection algorithm shrinks the scope where the abnormal capacitors locate, which avoids calculating the total capacitances in MMCs. In the proposed capacitance calculation method, the complex operations are further saved. Both the two measures reduce the computation burden effectively. The implementation of the whole monitoring strategy is simple and fit for high-voltage MMCs. Moreover, the strategy has no adverse influence on the normal operation of MMCs. The effectiveness of the proposed strategy is verified by the simulation studies with the professional tool Matlab/Simulink. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) in high- voltage applications contain large numbers of power switches, which increases the fault probability inside MMCs. This paper proposes a detection method for the faulty arm containing switch open-circuit failures in high-voltage MMCs to improve the reliable operations of MMCs. The proposed method uses the periodicity of the deviation between the maximum and minimum capacitor voltages to provide the detection reference. To yield a reliable reference, the repetitive signal generator is employed. Then, the fault is identified by the difference between the referenced and measured capacitor voltage deviations. An auxiliary criterion is also designated to avoid the false detection caused by the mutations in MMCs. The proposed method can reduce the difficulty in the selection of thresholds. Moreover, the proposed method is robust in high-voltage MMCs with inferior capacitor voltage balancing performance, which ensures detection rapidity and accuracy. Finally, the feasibility of the method is verified by simulation results in Matlab/Simulink. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is one of the most attractive topologies for high-voltage and high-power applications. Reliability is an important challenge for MMCs due to large amounts of power switches in the phase leg of the MMC. To enhance the reliable performance of MMCs, an effective and accurate fault detection and localization strategy for MMCs with the nearest level modulation is proposed in this paper. According to the general characteristics of the MMC, the capacitor voltage of the faulty submodule (SM) is higher than the healthy ones’. Based on the sorting algorithm, the faulty SM will have the maximum voltage and it will be identified by the proposed fault detection method. The auxiliary monitoring is supplemented to the detection algorithm for the cases of faults occurring in multiple SMs. The faulty SM localization strategy is based on the differences of capacitor voltages. The faulty switches are located by the distinctions of capacitor voltage in different current intervals. The proposed strategy could be applied under the cases of single or multiple SMs failures and identifies three kinds of the power switches faults in the faulty SM. The validity of the proposed strategy is verified by the simulation results in MATLAB/Simulink. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC)-based multiterminal HVdc (MTdc) transmission is a favorable option for asynchronously interconnecting several weak ac systems. This paper investigates the potential resonance of such an MMC-MTdc system excited by background harmonics in any of the MMC's ac systems and the undesirable transfer of these harmonics to the remote ac systems. First, the harmonic transfer rule across one MMC is established using phase leg voltage analysis and it is then used to develop an equivalent circuit model of the MMC-MTdc system. This model is used to investigate the mechanism of resonance excited by ac-side background harmonics. Finally, a control strategy is proposed to mitigate this resonance. Analytically calculated results from the derived model and the proposed control strategy are validated via electromagnetic transients simulation of a five-terminal MMC-MTdc network. [ABSTRACT FROM PUBLISHER]

This paper investigates interconverter interactions in multi-infeed systems that include both rectifiers and inverters. Performance indices used earlier for multi-inverter systems are extended to include the rectifier-inverter mix. These indices can be used to predict the maximum available power (MAP) and power stability limits of the converters as well as transient phenomena, such as transient overvoltages (TOV) and inverter commutation failures (CF ).The results show that the presence of a rectifier in the neighborhood of inverters is effectively similar to an increase in the inverters' short-circuit ratio in that the inverters achieve a higher MAP, are less susceptible to commutation failure, and have smaller TOV. [ABSTRACT FROM AUTHOR]