Abstract
The average switching model of modular multilevel converter (MMC) is built in this paper when the hot reserved strategy is adopted as a fault-tolerant control. When the MMC SM faults, the rest of the SMs cannot support the DC-link voltage, which results in interruption of the inverter. To tackle this issue, a novel fault-tolerant control strategy is proposed to bypass the SM under fault and re-regulate the SM capacitor voltage and carrier phase-shift angle to maintain the main components of circulating current, and reduce the Total Harmonic Distortion (THD) of grid connected current to enable the stable operation of the photovoltaic inverter. The maximum power tracking control is improved to solve the problem of long restoration time when faults occur, based on a constant voltage startup method combined with a fix-step incremental conductance method. Simulations in MATLAB/Simulink and experimental results have verified the feasibility and effectiveness of the proposed control strategy.
Highlights
modular multilevel converter (MMC) can increase power and boost voltage by connecting several modules in series [1,2,3,4]
A multi-loop control strategy based on a six-order dynamic model of the modular multilevel converter is presented [6], and the MMC is well controlled during normal operation
In order to verify the feasibility of the asymmetric fault-tolerant control strategy proposed in this paper, a model is built in MATLAB/Simulink (R2016b, MathWorks company, USA) to simulate the single and two SMs in phase A, respectively
Summary
MMC (modular multilevel converter) can increase power and boost voltage by connecting several modules in series [1,2,3,4]. In order to increase the utilization of redundant SMs, and avoid the problem of long capacitor charging time caused by putting redundant modules into operation as cold reserved, study of MMC fault-tolerant control strategy focuses more on hot reserved redundant SMs, at present. In [23], the faulty SM is located by the fault localization method, during which time period, the output voltage and current of the MMC are maintained smoothly, with no obvious distortion or overcurrent problems Another fault-tolerant approach was proposed in [24] through suppressing the fundamental circulating current component caused by the asymmetrical operation. SMs coordinate with cold reserved SMs and realize the stable operation of converter and ensure the symmetry of bridge legs This is not suitable for practical application. The constant voltage tracking method of maximum power point tracking (MPPT) cannot complete accurate tracking when the photovoltaic inverter fault, in this case, the improved MPPT control method, is proposed combined with the incremental conductance algorithm; it can realize stable and accurate fault tolerance of MPPT control, and the problem of the voltage support and fault voltage recovery of the DC side bus are solved
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