Research On Control Strategy Optimization Of Three-phase Three-wire Shunt Active Power Filter

With an increasing of power electronic converters in grid,harmonic pollution has become a vital issue.As a new and efficient power quality regulator,active power filter(APF)has been widely used to achieve real-time dynamic harmonic suppression.From the perspective of comprehensive harmonic compensation performance,APF based on the traditional current inner loop and voltage outer loop control strategy is difficult to bring satisfactory effect.In this thesis,these two strategies applied in three-phase three-wire shunt APF are optimized and improved.Firstly,architecture and fundamental of three-phase three-wire shunt APF are illustrated,and its mathematical model in time domain is established for designing the parameters of the main circuit.For command current extraction algorithm,synchronous coordinate transformation method based on instantaneous reactive power theory is used to extract harmonic or negative sequence fundamental current.Then,merits and demerits of several typical current inner loop control strategies are analyzed to lay a theoretical basis for the improvement of deadbeat control.Secondly,the command current tracking control and command current prediction algorithm of traditional deadbeat are improved.Aiming at the unsatisfactory steady-state command current tracking accuracy of traditional deadbeat,an improved tracking control method is formed by estimating the sampling value of compensation current in the next control period.On this basis,the error repetitive correction is added into the improved deadbeat channel to reduce the periodic current tracking error.To solve the problem of inaccurate prediction caused by grid frequency fluctuation,a frequency adaptive algorithm is proposed with real-time equivalence and adjustment of the integer-order and fractional-order prediction delay beat achieved by Lagrange Interpolation Polynomial.Then,considering the slow dynamic response of traditional prediction,the response delay can be reduced by 50% through transferring the prediction coordinate system.Simulation or hardware in the loop experiments is designed with different conditions to verify the effectiveness of the new tracking strategy and prediction.Finally,ripple mechanism of APF DC side voltage is analyzed theoretically to deduce the expression between output current and ripple value under steady-state condition,at the same time,the ripple regularity of DC side voltage is summarized.Aiming at the weakness of slow response speed and large overshoot in DC side voltage with PI,a combination of fuzzy control and PI is adopted to make PI parameters can be adjusted in real time.Simulation results verifies the proposed ripple regularity of DC side voltage and indicates the good dynamic and steady performance of the employed fuzzy-PI control strategy.