This article proposes an effective stepwise nontransient method for entering and exiting the active short circuit (ASC) of permanent magnet synchronous motor (PMSM) to protect the safety of vehicle's drive system when fatal faults occur in the electric vehicles (EV) powertrain system with high motor speed. It is found that excessive transient current and overshoot voltage inevitably occur when ASC enters and exits directly, which may cause the demagnetization of permanent magnet and failure of motor controller unit (MCU). The two-phase short circuit is utilized as a transition step before entering and exiting three-phase ASC according to the rotor magnet position, and almost no transient current and overshoot voltage exist during the whole short-circuit period. Then, there are no constraints of ASC for system design and the system material cost can be reduced. This approach is useful for function safety implement of MCU for EV application with fast attenuation rate of transient current, high robustness, and limited resources requirement. The method with effective suppression of transient current and overshoot voltage is verified by the experimental results from a 15 kW PMSM test platform.

DC transformers are essential for DC grids to interconnect different DC networks. This paper proposed a high voltage DC transformer, which consists of two diode-clamped modular multilevel converters (DCM2C) front-to-front connected with a medium frequency transformer. The DCM2C is prominent for its simplified voltage balancing control using clamping circuits. However, a voltage controller and corresponding measuring circuits are still needed for each converter arm. With the existing quasi two-level modulation technology, this paper proposed a voltage self-balancing method of the DCM2C when applied to the DC transformer. This approach is beneficial for reducing the overall cost of the system with no voltage sensor requirements. Meanwhile, the power loss of the clamping circuit is kept low with the clamping inductor. Both mathematical analysis and experimental results are conducted to verify the effectiveness of the proposed converter.

The modular multilevel converter (MMC) has been proven to be an effective solution for grid-tied applications with medium/high voltage ratings. The MMC contains a large amount of floating capacitors, which require the voltage balancing control. The voltage balancing control complexity is normally increased with the number of floating capacitors. As a result, the computation burden of the controller as well as the corresponding hardware cost will be high. This paper proposed a novel Z-type MMC, which can achieve capacitor voltage self-balancing without any balancing control. Therefore, the overall control of the converter is significantly simplified. Both mathematical analysis and experimental results have confirmed the effectiveness of the proposed method.

A power decoupling control for wind power converter based on series-connected modular multilevel converter (SCMMC) and open-winding permanent magnetic synchronous generator (OW-PMSG) is proposed in this article. The power converter based on SCMMC and OW-PMSG achieves medium voltage direct conversion and transmission and has higher dc-bus voltage compared with conventional MMC, which makes it a promising solution for copper consumption reduction of cable, low power transmission loss, and the elimination of the step-up converter. The proposed power decoupling control realizes independent power control of each phase in the wind power converter by decoupling the negative-sequence and zero-sequence current and guarantees the phase dc-link voltages balance of the SCMMC and the robustness to restrain the imbalance power disturbance. Instantaneous three phase dc-link voltages balance of the SCMMC is achieved especially when the imbalance of phase dc-link voltages occurs. The feasibility of the wind power converter based on SCMMC and OW-PMSG and the proposed power decoupling control is verified through simulations and a scaled-down laboratory prototype.

Aim to charge all submodule (SM) capacitors of the full-bridge modular multilevel converter (FB-MMC) to their nominal voltage values smoothly and effectively, this article proposes a closed-loop soft startup strategy by utilizing the negative output voltage of FB-SM, which completely suppresses the potential inrush currents at the startup process. Based on the analysis of the precharging process of FB-MMC with load, when bypassing the current limiting resistors, the inrush currents are induced owing to the presence of load. To solve this issue, an improved startup strategy is further proposed by intentionally controlling the dc voltage to zero. With this strategy, the main ac contactor is closed with zero current, which is beneficial for extending the service life of mechanical switches. Besides, the overall control system is integrated and optimized because separately designed startup control is not required, thereby reducing the computational burden of digital controllers. Moreover, the SM capacitors can be charged rapidly by a ramp shaped charging current, and equal charging of all SMs during startup process can be guaranteed with the voltage balancing control algorithm. Simulation and experimental results of three-phase FB-MMC rectifier interfaced with grid and 5 kV dc are provided to validate the feasibility and effectiveness of the proposed startup strategy.

This paper proposes a voltage adjustable capacitor isolated solid state transformer (ACISST) to link the medium voltage direct current (MVDC) distribution and low voltage direct current (LVDC) distribution. Compared with traditional isolated bidirectional dc/dc converter (IBDC), the proposed ACISST doesn't contain high frequency transformer in base modules (BMs). The galvanic isolation between the primary side and the secondary side of the ACISST is realized by the isolation capacitor as quasi isolation. The elimination of the high frequency transformer eliminates the core losses of magnetic elements, improves the conversion efficiency to a certain extent and reduces the weight of the ACISST. The voltage adjustable unit realizes the secondary voltage regulation of stable control when the primary side voltage fluctuates in an acceptable range or load power varies. Bidirectional power flowing is one of the features of ACISST, which is based on the bidirectional power flowing characteristic of BM and simplified dual active bridge (SDAB). Simulation and experimental results are presented to validate the theoretical analysis.