Study On Three-phase Four-wire APF In Distribution Network With Unbalanced

Electricity is the main source of energy for industrial production and the main driving force for promoting the sustainable development of the economy and society.With unbalanced loads and the large number of non-linear power electronic devices connected to the distribution network,a great deal of imbalance and harmonic components penetrate into the power grid,resulting in great safety and reliability risks to the distribution network operation.A three-phase four-wire active power filter(APF),as a representative of AC-quality on-site power management devices,has the characteristic of improving power quality dynamically and efficiently.The APF reference current detection control when the grid voltage is distorted accurately and unbalanced nonlinear load is studyed.The main contents are as follows:The phase-locked loop technology for grid voltage is firstly studied in this paper.A new type of phase-locked loop is proposed,which can realize the accurate and fast detection of phase angle through the corresponding relationship between the commercial value of the sum for the absolute value of the grid voltage of each phase and the phase angle of detection,to provide the accurate phase angle to harmonic detection section.At the same time,the dual second-order generalized integrator is used to obtain the positive sequence component of the grid voltage,which is used as the input of the novel phase locked loop to achieve the accurate phase-locking of the voltage in case of unbalanced or distorted grid.The simulation and experiment verify the rapidity and accuracy of the new phase-locked loop.In order to solve the influence of unbalanced current and harmonic current caused by unbalanced nonlinear load in three-phase four-wire low voltage distribution network on the safety and reliability of distribution network,the ip-iq harmonic detection method is proposed.Based on the control theory,the use of improved second-order generalized integrator can effectively enhance the quality factor and dynamic characteristics of control system.Taking the LCLtype three-phase four-leg APF as the research object,the digital PI + repetitive controller is designed considering the fast dynamic response of the complex harmonic current and the non-static tracking.According to the characteristic of the three-phase four-leg,the implementation of the 3D-SVPWM modulation algorithm in the a-b-c coordinate system.Compare with the traditional α-β-γ coordinate system algorithm,it greatly reduces the complexity of the modulation algorithm.A simulation model of three-phase four-leg APF was built with Matlab/Simulink platform to verify the effect of three-phase four-leg APF to deal with the distribution network under three different working conditions.The main circuit electrical design and software design of the three-phase four-leg APF device were carried out,and the prototype was tested experimentally.Simulation and experimental results show that three-phase four-wire APF can effectively suppress the neutral current in the power grid and reduce the three-phase unbalance in the distribution network.