Research On LCL Active Power Filter Based On Kalman Filtering

The harm of harmonic pollution to the power system is huge.Active power filter has been widely used due to its own flexibility and good compensation effect.Aiming at the problem of low accuracy of traditional APF compensation,taking LCL three-phase three-wire parallel APF as the research object,the main research content include harmonic current detection link,compensation current tracking link and LCL output filter.According to the characteristics of extended Kalman filter in nonlinear discrete systems,an improved EKF filtering method is proposed,which is applied to the harmonic detection link.The characteristic harmonics are filtered while the non-characteristic harmonics are filtered,and the effect of APF harmonic compensation is improved by improving filtering ability of the harmonic current.By analyzing the working principle of the LCL type three-phase three-wire parallel APF,the ip-iq harmonic current detection method is applied to detect the harmonic current,the synchronous coordinate system software phase-locked loop synchronization technology is adopted to provide the phase information required for the harmonic detection,the PI control technology is adopted to ensure the stability of the DC side capacitor voltage,repeated control eliminates the periodic error of the command current,and the hysteresis control ensures the real-time tracking of the compensation current.An improved EKF filtering method is proposed to replace the traditional low-pass filtering link to achieve the improvement of harmonic detection accuracy.The nonlinear system is linearized,based on the EKF combined with Kalman filtering method to deal with colored noise,the state expansion method is used to process the harmonic current signal as the process noise of the system,and the mathematical model of improved EKF filter method is formed to improve the accuracy of harmonic current detection.The parameters of LCL output filter are set to satisfy transfer function modulus and resonance frequency constraint conditions.The parameter design results show that the lower harmonic has no attenuation and the higher harmonic produced by the inverter has a strong attenuation.The active damping method based on proportional feedback of capacitance current is used to suppress the resonance peak,and the feedback coefficient is set in combination with the resonance peak and phase lag.Matlab is used to build the simulation model for comparative simulation experiments with the conventional low-pass filter on the filtering level,APF compensation effect and LCL type APF compensation effect.The results show that the improved EKF filtering method caneffectively reduce the current distortion rate of the power grid,the filtering effect of non-characteristic harmonics is obvious,the overall filtering level and the APF compensation effect are far better than that of the Butterworth filtering method.The constant load and sudden load conditions both improve the filtering level and the APF compensation effect.