Research On Shunt Active Power Filter In Three-phase Four-wire System

At present,a variety of power electronic devices have been applied to production and life,improving production efficiency and people’s lives.But these non-linear load also produce a lot of harmonics,worsening the electricity surroundings.Active power filter has fast dynamic response speed and the grid impedance has little influence on the compensation performance.It not only can suppress the harmonics,but also has a good compensation effect on reactive power and unbalanced current,so it has attracted widespread attention.In this paper,the four-leg active power filter that is used in the three-phase four-wire system is the research object,and its harmonic current detection algorithm and control strategy are focused on emphatically.Finally,the simulation and experiment are carried out.There are many factors that can affect the compensation effect of active power filter,the detection speed and precision of harmonic current detection algorithm is one of the important aspects.In this paper,the principle of the commonly used pq detection algor:ithm and ip-ip detection algorithm are described and analyzed in detail.The problem whether to separate the zero sequence current component in the three-phase four-wire system was carried out by formula derivation and analysis.The control strategy is the core of the active power filter,which includes two parts:the control strategy of current loop and the control strategy of voltage loop.The current loop generates the PWM wave to drive the switching element to turn on and off by a certain control method and modulation method,make compensation current dynamic track the change of instruction current.In this paper,the PI controller and the repetitive controller are connected in parallel as the controller of the current loop,so as to ensure the system has good dynamic response speed and tracking performance.The voltage loop uses a certain control method to maintain the DC side voltage at a suitable value.The control method usually uses the PI control,but the PI control is less robust,and in the face of complex and variable loads,it is often short of the ideal control effect.In this paper,the fuzzy control and PI control are combined to get the parameters of the fuzzy self-tuning PI control method,the method can modify the parameters in real time according to the condition of the control object,and improve the control effect.In this paper,according to the theoretical analysis and design,MATLAB is used to build the simulation model and simulate in nonlinear load,the current changes before and after the compensation are compared,and the correctness of the theory is verified.Finally,the experimental platform is built.The experiment is carried out under the nonlinear load,and the experiment results show that the device has good ability of restraining harmonics and compensating unbalanced current.