Guangtao Zhou

This paper investigates the operation of the T-type cascaded H-bridge (T²C-HB) converter following H-bridge submodule faults. Initially, through a detailed analysis of the fault, this paper clarifies that the reason for which voltage balance cannot be achieved under high output voltage conditions is the limited flexibility in selecting redundant switching vectors. In practical applications, to enhance output efficiency, converters are required to operate at high output voltages. However, a fault occurring under such conditions leads to distorted output currents. Existing fault-tolerant (FT) methods, due to the excessive modulation burden on the healthy phases, are only effective under low output voltage conditions. To enhance the reliability of converter operation, this paper introduces an innovative FT method that integrates harmonic injection with clamping techniques. This method enables the converter to rapidly return to normal operating state under nearly all output conditions when a submodule fault occurs. Simulation and experimental results validate the feasibility and effectiveness of the proposed method.

Hybrid multilevel converters have gained more popularity due to their unique superiority. Thus, the reduced switch hybrid multilevel (RSHMC) converter is adopted in this article which consists three H-bridge cells and a Vienna rectifier. The RSHMC converter combines the advantages of these two topologies and can output higher voltage level by adopting fewer switching devices compared with other multilevel converters. However, the Vienna rectifier of the RSHMC converter has the inherent current zero-crossing distortion (CZCD) problem, which will increase the total harmonic distortion of the grid-side current. Thus, a modulation method based on phase disposition-pulsewidth modulation is proposed to eliminate the CZCD of the RSHMC converter by selecting redundant switching states. Besides, in order to ensure the normal operation of the RSHMC converter, the proposed modulation method can also realize the voltage balance control of the floating capacitor and the neutral point which is also realized by selecting the switching states to charge or discharge. Consequently, the RSHMC converter can operate normally with balanced voltage and without CZCD after adopting the proposed modulation method. Finally, the feasibility and correctness of the proposed modulation method are verified by simulation and experimental results.

Hybrid multilevel converters have gained more popularity due to their unique superiority. Thus, the T ² C-HB converter is adopted in this article which consists a T-type converter (T ² C) and three H-bridge (HB) cells. This hybrid converter combines the advantages of these two topologies and can output the same voltage level by adopting fewer switching devices compared with other multilevel converters. However, due to the high switching frequency of the HB switches, they may fail in practice which will cause serious deterioration in the output waveform and compromise the reliability of the entire system. Thus, a fault-tolerant control method based on floating capacitor voltage balance is proposed in this article. First, based on the level-shifted pulse width modulation (LS-PWM) method, the fault-tolerant control is accomplished by modifying the three-phase reference voltage. Second, a strategy which bypasses the HB equivalently when the absolute value of output voltage exceeds the peak value of the dc-link voltage is proposed to balance the floating capacitor voltage. Besides, the three-phase reference voltage is modified again utilizing the similar idea in order to balance the line voltage. Consequently, the line voltage remains unchanged during postfault operation and the floating capacitor voltage can restore balance without excessive fluctuation. Finally, the feasibility and correctness of the proposed method are verified by simulation and experimental results.

Brushless DC motor (BLDC) is mostly used in UAVs and the load is a piston engine with changing torque. The traditional “three-stage” sensorless control method is prone to failure under variable load. In this paper, under the MATLAB/Simulink environment, the simulation model of BLDC starting control system was established, the mathematical relationship among phase current (ia ), reverse electromotive force (Ea ), electromagnetic torque (Te ), load curve (TL ) and speed (N) was compared and analyzed. Finally, the reason why the traditional “three-stage” control method is easy to fail under the variable load environment is explained from the mechanism level. Then a compound “three-stage” starting control strategy is proposed, and the simulation model of the control strategy is built in MATLAB/Simulink environment. The simulation results show that the starting performance of the strategy under variable load environment is obviously better than the traditional “three-stage” control method.