Abstract
In view of the complex calculation and limited fault tolerance capability of existing neutral point shift control algorithms, this paper studies the fault-tolerant control method for sub-module faults in modular multilevel converters on the basis of neutral point compound shift control strategy. In order to reduce the calculation complexity of shift parameters in the traditional strategy and simplify its implementation, an improved AC side phase voltage vector reconstruction method is proposed, achieving online real-time calculation of the modulation wave adjustment parameters of each phase required for fault-tolerant control. Based on this, a neutral point DC side shift control method is proposed to further improve the fault tolerance capability of the modular multilevel converter (MMC) system by compensating the fault phase voltage with non-fault phase voltage. By means of the compound shift control strategy of the DC side and AC side of the neutral point, an optimal neutral point position is selected to ensure that the MMC system output line voltage is symmetrical and the amplitude is as large as possible after fault-tolerant control. Finally, the effectiveness and feasibility of the proposed control strategy are verified by simulation and low-power MMC experimental system testing.
Highlights
In recent years, modular multilevel converters (MMCs) have found wide applications in the fields of medium and high voltage DC transmission, traction power supply, and high power AC drive, owing to their advantages of small output harmonics, high modularity and flexible structure
This paper proposes an SM fault-tolerant control strategy based on neutral point compound shift for the MMC system under Phase Disposition PWM (PDPWM) [19], in hope of solving the problems of complex calculation and limited fault tolerance capability of existing neutral point shift control algorithms
Compared with the current approaches, it has three main benefits: (i) The computational complexity of the fault-tolerant reconstruction adjustment parameters is reduced, and the implementation of the algorithm is simplified by optimizing AC side phase voltage vector reconstruction method; (ii) The proposed neutral point DC side shift control method further increases the number of single-arm faulty SMs that can be fault-tolerant, and improves the fault compatibility; (iii) The optimal neutral point position can be selected based on the compound shift control mechanism of the neutral point DC side and AC side so that the MMC system output line voltage is symmetrical and the amplitude is as large as possible
Summary
Modular multilevel converters (MMCs) have found wide applications in the fields of medium and high voltage DC transmission, traction power supply, and high power AC drive, owing to their advantages of small output harmonics, high modularity and flexible structure. Fault-tolerant control of SM faults in MMC systems is realized by optimizing the reconstruction of the AC side phase voltage vector and obtaining the algorithm of adjustment parameters. Compared with the current approaches, it has three main benefits: (i) The computational complexity of the fault-tolerant reconstruction adjustment parameters is reduced, and the implementation of the algorithm is simplified by optimizing AC side phase voltage vector reconstruction method; (ii) The proposed neutral point DC side shift control method further increases the number of single-arm faulty SMs that can be fault-tolerant, and improves the fault compatibility;. The effectiveness of the proposed strategy is verified by simulations and experiments
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