The Research On Control Strategy Of Active Power Filter

Over the recent years, power quality has been given attention due to the intensively use of power electronic controlled applications in all branches of industry. As a result, harmonics were generated from the power converters or non-linear loads that caused the power system operate with low power factor, low efficiency, voltage and current distortions, and increased losses in transmission and distribution lines. Conventionally, passive LC filters have been employed to eliminate line-current harmonics and to improve the power factor. However, the harmonic problem still persists because of its inability to compensate random frequency variations in currents, tuning problems and parallel resonance. Therefore, the better alternative method is by active power filters (APF).The paper firstly makes a brief introduction about the application of APF in different scene. We also analysis the characteristics of different loads and the harmonic generation mechanism, and describes the good compensation effect of APF to pave the way for the depth description about APF.Then the paper describes the theoretic bases of the proposed APF. The mathematic model in ABC three-phase stator coordinate of the two-level APF is set up. Using coordinate transformation, the mathematic model in d-q two-phase rotating coordinate is established. To control the waveform shape of output current, this paper presents a prediction based deadbeat control scheme. In this scheme, the predictive harmonic current value is obtained by the predictor. The predictor can achieve better prediction of the harmonic current with same sampling frequency, thus improves the overall performance of the system.In d-q two-phase rotating coordinate, this paper divides the control system of this APF into the outer-loop DC voltage controller and the inner-loop current controller. In the outer-loop controller, the DC voltage is regulated by the PI controller using active command current. The inner-loop current controller is divided into the detection module of calculating harmonic command current and the control module of tracking the command current. The method for detecting harmonic currents based on the instantaneous reactive power theory is adopted in the detection module. The method for tracking the command current and the two-level SVPWM modulation strategy is adopted in the control module. Finally, an experiment prototype of the proposed APF with two-level inverter is made on the basis of theoretic analysis and simulation study. The effective experimental results prove that the APF can achieve good compensation effect. These works lay a sound foundation of APF with two-level inverter for future industry application.