The Control And Experimental Research Of Four-leg Active Power Filter With Unbalanced Nonlinear Loads

With the rapid increase of electricity load, more and more unbalanced nonlinear loads appeared in three-phase four-wire system, which will bring some power quality problems, such as harmonic pollution, lack of reactive power and three-phase imbalance. Shunt four-leg active power filter is one of the effective measures to solve these problems. This paper will analyze and research the control scheme and experimental verification is also given.Harmonic current detection is a very important link. Testing effect and delay time of this part have a close relationship with compensation effect of four-leg active power filter. For this purpose, this paper proposes a harmonic current detection algorithm based on generalized integrator, which has more simple operation and less delay time. And there is no need to design complex phase-locked loop algorithm. So it can improve detection efficiency of the algorithm. The simulation results show that in three working conditions, harmonic distortion rate of three-phase fundamental positive sequence active current detected by the harmonic current detection algorithm based on generalized integrator is under 2.0%. And this algorithm has smaller detection delay(15ms).In current loop control part of four-leg active power filter, considering the influence of grid frequency fluctuation, the improved PR controller based on stationary coordinate is adopted Compared to the traditional PR controller based on dq0 coordinate, its algorithm structure is more simple. And it can adapt to the grid frequency fluctuation. The classical PI controller is used in voltage loop. Besides, a detailed design process of parameters is given. In the end, the simulation results prove that in three working conditions, the double loop control strategy of four-leg active power filter can make harmonic distortion rate of three-phase power source current and the load current unbalanced degree below 5.0%. And system power factor can be improved above 0.95. In addition, with the load mutation, the designed four-leg active power filter in this paper has faster dynamic response speed(20 ms).Finally, hardware experiment platform of four-leg active power filter is set up in the Laboratory. And related parameters are designed and appropriate components are selected. Using digital signal processor TMS320F28335 as control chip, control program of four-leg active power filter is written. The result of the experiment validate that the designed four-leg active power filter not only has better compensation effect but it also has less dynamic response time.