Research On Modulation Strategy Of Three-Level DSTATCOM

With the rapid development of modern industry, large amount of reactive power, harmonics and asymmetric current appear in the power system, which affect the power quality seriously. Due to its perfect performance such as high control precision, fast response speed and so on, a new generation of reactive power compensation device Distribution Static Synchronous Var Compensator(DSTATCOM) with full-controlled power devices attracts widespread concern of scholars at home and abroad in the technical field for grid dynamic compensation. The performance of DSTATCOM is closely related to its pulse width modulation strategy, so the study on the DSTATCOM modulation strategy has great significance on ensuring the reliable control of devices and stable operation of the equipment.This paper focuses on the three-level DSTATCOM space vector modulation strategy, for the inherent neutral-point potential balance of three-level NPC topology, a modified NTV~2(Nearest Three Virtual Vectors) modulation strategy is proposed. This strategy improves the neutral-point potential imbalance problem with poor recovery ability in the traditional method NTV~2, while eliminating the low-frequency oscillation of neutral-point potential, so that compensation effects of DSTATCOM devices can be guaranteed.The main circuit topology and modulation method of DSTATCOM have been analyzed and compared. The two-level triangular carrier wave modulation and hysteresis modulation are introduced briefly and the topology and working principle of three-level NPC are discussed in detail. Respectively, this paper illustrates the phase disposition PWM modulation and space vector modulation. Compared with carrier wave modulation, the space vector modulation has the high utilization of DC voltage and good output waveform quality. The implement principles of NTV(Nearest Three Vectors) modulation and NTV~2 modulation are analyzed in detail and simulation results obtained by building the three-level NPC inverter model verify that NTV~2 modulation has the better output waveform so that the low-frequency oscillation at neutral-point potential in three-level NPC inverter will be fully controlled. The neutral-point potential self-balance mechanism of three-level NPC inverter is analyzed in the time domain, then the neutral-point potential self-balance mechanism with NTV~2 modulation method is established based on the switching-cycle small signal model. Simulation results show neutral-point potential with NTV~2 modulation method has weak ability for self-balancing which needs additional active control strategy in practical applications.For the uncontrollable region of unbalanced neutral-point potential mentioned in NTV~2 modulation method, a modified NTV~2 modulation strategy based on multi-objective optimization control is proposed. Simulation results show that three-level DSTATCOM devices can not only realize the unbalance control of neutral-point potential but also implement its output compensation by the modified NTV~2 modulation method.