Comprehensive Study On SPWM And SVPWM Correlation In Single-phase Three-level NPC Converters

Multilevel converters are widely used in medium and high voltage power conversion applications because of their excellent performance.Among them,singlephase three-level Neutral-Point-Clamped(NPC)converters are well studied and applied in distributed power grids because of their simple circuit structure and good current-voltage harmonic characteristics.In this paper,the I-type topology and T-type topology SPWM and SVPWM modulation strategies for single-phase three-level NPC converters are studied,and the equivalence of the two modulation strategies without and with the neutral point voltage balance control is investigated in depth.The article firstly introduces the basic principles of single-phase three-level I topology and T topology,describes their steady-state and dynamic operating characteristics,discusses the influence of neutral point voltage imbalance on the converter performance,and makes a theoretical analysis of the causes.It also reveals the principle of single-phase three-level neutral point voltage compensation,which is to use the redundant pairs of positive and negative small vectors to generate the same line voltage while the neutral point current has opposite polarity to achieve the balanced control of the neutral point voltage.For single-phase three-level SPWM and SVPWM modulation strategies,the SPWM and SVPWM are proved to be equivalent without the neutral point voltage balancing control strategy after rigorous mathematical analysis and derivation from the PWM signal duty cycle perspective;it is also proved that the SPWM with zerosequence injection for neutral point voltage balancing control and the SVPWM with the complementary characteristics of positive and negative vectors for neutral point voltage The SPWM with zero-sequence injection for neutral point voltage balancing control is also shown to be equivalent to the SVPWM with positive and negative small vector complementarity.Simulation results of two modulation methods for single-phase three-level I-type and T-type circuit topologies are also presented to further demonstrate the equivalence between the two;thus revealing the nature of different modulation methods in terms of dispersion and convergence.