Research On Neutral-point Voltage Balance Of Three-Level NPC APF And Design Of Human-machine Interface

With the rapid development of electric power industry, a large number of non-linear loads especially the power electronic equipments have been widely used in electricity generation and power utilization terminals, which has aroused serious power quality problems and affected the secure and stability of the power system. As a kind of power quality compensation equipment, shunt active power filter(APF) is able to eliminate harmonic currents, compensate reactive currents and solve grid unbalance problem. APF has a lot of advantages, such as fast dynamic response, high precision steady-state compensation, and flexible compensation manners and so on.As the demand for high voltage and high power increasing, three-level active power filter has aroused more attention. Compared with two-level APF, three-level APF has a lot advantages, such as higher output voltage, lower total harmonic distortion and so on. Now three-level topology industrial applications have given priority to neutral-point-clamped(NPC) converter, but the diode type NPC converter has the inherent problem of neutral-point voltage imbalance. This paper is focused on neutral-point voltage balance strategy and human-interface for three-level APF using neutral point clamped converter.Firstly, a detailed introduction of the NPC shunt APF topology is presented and the continuous-time model for APF is built. The general SPWM and SVPWM algorithms for three-level APF are analyzed in details.In order to balance the neutral-point voltage, this paper proposes two neutral-point voltage balance strategies for NPC shunt APF:one is based on dynamic double modulation wave; the other is based on simplified SVPWM angle-prediction. The dynamic double modulation wave strategy is based on zero sequence voltage injection and the reference voltage modulation wave is made up of two modulation waves. A detailed double modulation wave mathematical model is deduced and the modified factor k is adopted to adjust the double modulation wave. As three-level vector sector space can be considered equivalent to six small two-level sector spaces, three-level SVPWM can be simplified to two-level SVPWM through coordinate transformation and modifies the reference voltage angle to change the operation time of each small vector and reduce the neutral-point voltage low-frequency oscillation.Finally, an electrical quality monitor system for APF is designed in this paper, which can monitor the operation of APF based on DSP and PS-LCD.