Abstract
Each arm of modular multilevel converter (MMC) consists of a large number of sub-module (SM) units. However, it also increases the probability of SM failure during the long-term system operation. Focusing on the fault-tolerant operation issue for the MMC under SM faults, the traditional zero-sequence voltage injection fault-tolerant control algorithm is analyzed detailed and its disadvantages are concluded. Based on this, a novel fault-tolerant control strategy based on phase disposition pulse-width modulation (PD-PWM) is proposed in this paper, which has three main benefits: (i) it has carrier and modulation wave dual correction mechanism, which control ability is more higher and flexible; (ii) it only needs to inject zero-sequence voltage in half a cycle of the modulation wave, which simplifies the complexity of traditional zero-sequence voltage injection control algorithms and much easier for implement; (iii) furthermore, the zero-sequence voltage can even be avoided injecting under the symmetrical fault conditions. Finally, the effectiveness of the proposed control strategy is verified with the simulation and experiment studies under different fault conditions.
Highlights
Compared with the traditional two-level converter, modular multilevel converter (MMC) enjoys the advantage of modular design [1,2], which allows MMC good scalability and facilitates the improvement of voltage level, as well as other advantages as many output levels and good harmonic characteristics
Aiming at this problem and for better deal with the multiple SM fault conditions, under the background of the redundant SMs equipped with hot reserve scheme and only bypassing the faulty SMs when SMs malfunction, a novel fault-tolerant control strategy is proposed in this paper
Compared to the current approaches, it has three main benefits: (i) it has carrier and modulation wave dual correction mechanism, which control ability is more higher and flexible; (ii) it only needs to inject zero-sequence voltage in half a cycle of the modulation wave, which simplifies the complexity of traditional zero-sequence voltage injection control algorithms and more easier for implement; (iii) the zero-sequence voltage can even be avoided injecting under the symmetrical fault conditions
Summary
Compared with the traditional two-level converter, modular multilevel converter (MMC) enjoys the advantage of modular design [1,2], which allows MMC good scalability and facilitates the improvement of voltage level, as well as other advantages as many output levels and good harmonic characteristics. With injecting the zero-sequence voltage to modify the modulation waves, the converter line-voltages in the grid side are ensured balanced to maintain the system stable operation under SM faults The advantage of these methods in [15,21] is that they do not need to adjust the SM capacitor voltages in fault arms. When the upper and lower arm occur SM faults simultaneously, multiple zero-sequence voltages need to be calculated, which increase the complexity of the control method Aiming at this problem and for better deal with the multiple SM fault conditions, under the background of the redundant SMs equipped with hot reserve scheme and only bypassing the faulty SMs when SMs malfunction, a novel fault-tolerant control strategy is proposed in this paper.
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