A simplified PWM strategy for NPC three-level converter with unbalanced dc-links is proposed in this paper to achieve high quality line-to-line output voltages and to maximize the linear modulation range. The simplified strategy takes the direct output voltage modulation by calculating the special solutions of the voltage-second balance equations without detecting the position of the reference vector in the asymmetrical and complicated space voltage vector diagrams to reduce the calculation time. A novel solution based on the state transition is proposed to extend the maximum linear modulation index to 1.15. Furthermore, the asymmetric control of the split dc-link by the proposed PWM is implemented by adjusting the special solutions. Difference between the conventional SVPWM and the proposed strategy is conducted to illustrate the advantages of the simplified strategy. The effectiveness of the proposed modulation strategy is verified by simulation and experiment results.Index Terms-linear modulation range, NPC inverter, PWM, unbalanced dc-link.

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A novel simplified pulse width modulation(PWM) strategy for neutral point clamped (NPC) three-level converter is proposed in this paper.The direct output voltage modulation is applied to reduce the calculation time. Based on this strategy, several optimized control methods are proposed. The neutral point potential balancing algorithm is discussed and a fine neutral point potential balancing scheme is introduced. Moreover, the minimum pulse width compensation and switching losses reduction can be easily achieved using this modulation strategy. This strategy also gains good results even with the unequal DC link capacitor. The modulation principle is studied in detail and the validity of this simplified PWM strategy is experimentally verified in this paper. The experiment results indicated that the proposed PWM strategy has excellent performance, and the neutral point potential can be balanced well with unequal DC link captaincies.

As the number and capacity of photovoltaic (PV) power stations increase, it is of great significance to evaluate the PV-connected power systems in an effective, reasonable, and quick way. In order to overcome the challenge of PV's time-sequential characteristic and improve upon the computational efficiency, this paper presents a new methodology to evaluate the reliability of the power system with photovoltaic power stations, which combines intelligent state space reduction and a pseudo-sequential Monte Carlo simulation (PMCS). First, a non-aggregate Markov model of photovoltaic output is established, which effectively retains some time-sequential representation of the PV output. Then, the differential evolution algorithm (DE) is introduced into the sampling stage of PMCS to carry out an intelligent state space reduction (ISSR). By using the DE algorithm, success states are searched out and removed, thus the state space is reduced and formed with a high density of loss-of-load. Hence, unnecessary samplings are avoided, which optimizes the PMCS sampling mechanism and improves the computational efficiency. Finally, the proposed method is tested in the modified IEEE RTS-79 system. The results indicate that this new method has a better computational efficiency than the time-sequential Monte Carlo simulation method (TMCS) and pure PMCS. In addition, the effectiveness and feasibility of this method are also verified.

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