Research On Compensation For Three Phase Balance Of Single-phase Non-linear Load

The connection of large-capacity single-phase non-linear loads in grid caused voltage unbalance, fluctuation and flicker at public access point, which can affect power quality. The compensation of these loads is of broad and practical significance for three phase balance and eliminating the negative impact to the grid.The first problem for reactive power and harmonic compensation is current instruction extraction. d-q detection method based on instantaneous reactive power theory can only extract positive sequence component in positive sequence rotating coordinate system, and extract negative sequence component in negative sequence rotating coordinate system. In this paper, based on the relationship between positive sequence component and negative sequence component sequence of single-phase load fundamental wave, an improved algorithm for positive sequence component, negative sequence component and harmonic current separation in positive sequence rotating coordinate system is proposed. Only in single phase will the current positive and negative sequence component have specific relationship, so the improved algorithm can only be used in single phase system.In this paper, the selected compensator topology is four-quadrant converter with LCL type filter. Therefore, there are two issues for controller: LCL filter resonance damping and the command current track. A capacitive current feedback method for resonance damping is adopted, but in order to reduce the effect of one beat delay the digital control brings, the sampling time should move as near as possible to the PWM update time, thus improving the damping effect and improving the stability margin. Because of the movement of sampling time, the control delay is less than one beat, the ordinary z-transform discretization will be invalid. First order inertia or Pade approximation have error for delay modeling at low switching frequency. In this paper, a revised z transformation method and state-space method are adopted to establish the precise mathematical model when the delay time is less than one beat, which build the foundation for resonance damping and controller design. For multiple harmonic instruction tracking, PI controller does not have sufficient bandwidth, and PR controller needs multiple parallel, thus greatly increased the complexity of control system. Therefore, this article uses PI + repetitive control strategy, the current instruction can be tracked accurately.The DC voltage design principle is proposed. For unbalanced load balancing compensation, the three-phase voltages are different, the maximum voltage can be obtained by theoretical analysis. Thus, for DC voltage design, the maximum voltage for fundamental negative sequence and reactive power compensation is calculated firstly, and then consider the voltage utilization in modulation, some margin for harmonic compensation is left.Finally, 500 kVA four-quadrant converter simulation system and experiment prototype are build, the system can be stable and normal, which demonstrates the effectiveness of the proposed scheme.