Research On Discrete Control Strategy Of Active Power Filter

With the progress of science and technology, especially for the development of electronictechnology, more and more power electronic devices have been used in power system. Theproblem of power quality is increasingly serious. Because of the nonlinear characteristics ofpower electronic devices, the harmonic is born. This is not only increasing the losses of theelectric energy but also cause the damage of all sorts of devices in the power system. As akind of device which can restrain harmonic effectively, active power filter is gradually appliedin the actual.Active power filter use the harmonic detection circuit to detect the harmonic currents.Then, get the compensation current through the instruction current arithmetic circuit. Thecompensation current is opposite in direction but equal to the size of the harmonic. Finally,inject the compensation current to the power grid to eliminate the harmonic. At present, manypeople who are studying on active power filter use the continuous model as the foundation.But in the actual, the computer can only handle the discrete signal, which will affect the actualcompensation effect of active power filter. With the development of control theory and digitalcontrol technology, it is meaningful to study the active power filter in a discrete way.According to the problems which have been shown above, this paper will start from theactual. First of all, establish the discrete switching affine model according to thecharacteristics of active power filter. This model is composed of multiple piecewise linearsystem, each subsystem is linear, and the whole system is nonlinear, which is closer to theactual system. During the process of modeling,“switching equilibrium point” where the stateof the discrete switched affine system has to be driven to is discussed. The main task is to finda control strategy making the discrete switching affine system stable. Secondly, study thestability of the discrete switching affine system through the Lyapunov quadratic stabilitytheory. After trying, a way to make the system stable is found. The states of the system aredriven to the zero under the control strategy. At last, the feasibility of the method has beenverified by the results of MATLAB simulation and the experiments. The result shows that thecontrol method which is proposed by this paper can compensate the harmonic currentsaccurately, and compensate the harmonics fast and effective even the nonlinear load is changed.