The five-level active neutral-point-clamped (5L-ANPC) converter has a great application potential in high power variable-frequency drives (VFDs) with high and medium voltage. However, the traditional method for capacitor voltage buildup is not applicable to the capacitors inside the back-to-back (B2B) 5L-ANPC converter. To solve the problem, the topological features of the B2B-5L-ANPC converter are fully analyzed, and a capacitor voltage pre-charge strategy for this converter is also proposed. With this strategy, all the capacitors in the converter can be charged by controlling the on and off states of switch tubes in the bridge arm, no additional hardware is needed. Moreover, the structural features of the voltage booster circuit were examined in four stages, and used to derive the capacitor voltage buildup formula in each stage. In addition, the relationship between voltage buildup rate, equivalent pre-charge resistor and capacity of capacitor is identified. Finally, Simulation results from a B2B-5L-ANPC simulation model which is constructed according to the actual system parameters and experimental results from a scaled-down prototype are provided to support the proposed control strategy.

This paper investigates the asynchronous motors driven by a Three-Level Neutral-Point-Clamped Voltage Source Inverter (3L-NPC-VSI) and aims to achieve control without weight factors and reduce torque ripple. It puts forward a non-weighted two-stage Finite-Control-Set Model Predictive Control (FCS-MPC) strategy. First, a hierarchical optimization method is adopted to address the difficulty of setting weight factors in traditional FCS-MPC applications. The method offers stratified designs of three performance indices, voltage jump, common-mode voltage, and current tracking, obviating the need for weight factor setting and reducing the calculation load of predictions. Secondly, to further mitigate torque ripple, an optimal vector or vector combination is implemented at the current control layer by adhering to the principle of minimal current tracking error. During the selection of the optimal vector combination, the first vector of the combination is chosen to be the vector at the end of the present cycle. This ensures that there is at most one switch within each control period, reducing the switching losses of the two-stage FCS-MPC. Lastly, detailed simulation and experimental analyses are conducted to verify the feasibility and effectiveness of the proposed strategy.

The power grid has become the most important infrastructure system and dominates the economic and social developments in modern society. However, the power network often suffers various failures which can cause huge damages to the power grid. Hence, this paper proposes a community detection method to explore the critical community and identify the critical node, so as to study the vulnerability and resilience of the power grid subjected to malicious attacks. Meanwhile, this paper presents a comprehensive model to quantify the resilience characteristics considering performance level and change speed, and the IEEE 118 network is adopted to verify the feasibility and effectiveness of the proposed schemes. The results show that the attack strategies considering the community structure can cause severe damages to the power grid compared with the traditional methods, and Community Gravity-based Recovery (CGBR) has the better recovery ability among the six recovery strategies. Moreover, the results also demonstrate that Community Priority and Gravity-based Recovery (CPGBR) and CGBR can generate the most complete communities, and they can cause the least resilience damages on the number of surviving nodes (SN) subjected to the attacks on nodes 49 and 69 respectively, and Gravity-based Recovery (GBR) can result in the least resilience damages on the power supplied (PS). Hence, we can discover that there is a tradeoff between decreasing resilience loss and increasing network performance by different choices of recovery strategies. Moreover, the results show that the proposed methods can precisely identify critical nodes and rapidly recover network characteristics, therefore this study has great significances for decreasing resilience loss of the power grid subjected to malicious attacks.

Multilevel converters are widely used in medium-voltage power conversion systems. In this paper, a novel five-level converter is proposed. For a three-phase system, the converter requires 30 power semiconductor devices and 12 flying capacitors in total. The required blocking voltages of all the switches are the same and the converter is suitable for medium-voltage high-power applications. A method has been proposed to control the flying capacitors' voltages. The feasibility of the proposed topology and the control method have been validated through simulation in MATLAB/Simulink.