In this work, the surge current ruggedness (the ability to withstand the surge current until failure happens) in the third-quadrant is investigated for evaluating the reliability of the SiC MOSFET. A dynamic surge test platform (DSTP) is developed, and the surge trajectories are observed in detail. It is found that the source-drain current is mainly carried by the body diode of SiC MOSFET during the surge event, regardless of the gate-source voltage is positive or negative. The gate-source terminals shorting and three-terminals shorting are two surge failure modes of the SiC MOSFET, which depend on the shocking strength of the surge power. Furthermore, the oscillation induced by dynamic avalanche effect (DAE) is observed in the surge test, which expands the understandings about the negative effect of surge event. This work is aimed to give the useful knowledge about the surge current ruggedness, and some practical suggestions based on the experimental results are given out to improve the system reliability.

Low frequency transmission technologies can significantly enhance power transfer capacity and distance of alternating current transmission by reducing transmission frequency. This paper proposes a dual bipolar modular AC-AC converter (DBMAC) topology for the frequency conversion between low-frequency (15~20 Hz) and grid-frequency transmission (50 Hz). The operation principle is elucidated, showing the principal of the frequency decoupling. The mathematical models of DBMAC are developed and the control strategies are given for both input and output sides. The simulation is conducted and its results verify the effectiveness of the proposed DBMAC.

This paper proposes a non-invasive fault location method for modular multilevel converters (MMC) considering light load conditions. The prior-art fault location methods of the MMC are mostly developed under full load conditions. However, it is revealed that the faulty arm current will be suppressed to be unipolar when the open-circuit fault happens on the submodule switch under a light load. This leads to the capacitor voltage of the healthy and faulty submodules rising or falling with the same variations, increasing the difficulty of fault location. The proposed approach of injecting the second-order circulating current will rebuild the bipolar arm current of the MMC and enlarge the capacitor voltage deviations between the healthy and faulty SMs. As a result, the fault location time is significantly shortened. The simulations are carried out to validate the effectiveness of the proposed approach, showing that the fault location time is reduced to 1/6 compared with the condition without second-order circulating current injection.