| Recently, the power electronics is widely used in the power system. The pollution of the power system is caused by the broad application of the power electronics. The power quality is influenced by the harmonic harm, such as increasing the line and transformer losses, damaging the device for protecting and measuring. Active power filter (APF) which provides an excellent solution for the dynamic harmonics is widely used.The principle of APF is as follows: the measured harmonic is used to calculate the reference compensation current, of which the amplitude is the same as the harmonic and the direction is contrary to the harmonic. The harmonic is compensated by the compensation current which traces the reference compensation current. In this process, APF is influenced by the two factors. Firstly, the compensation current is delivered to the power system via an ac-side inductor. The precise compensation current is depended on the reasonable range of the inductance value. So, the design of the inductance plays an important role. Moreover, the control strategy of APF is one of the most important factors. It has two main functions: for one thing, it ensures that the reference compensation current is calculated precisely. For other thing, it ensures that the compensation current traces the reference precisely. Whether the error between the compensation current and the reference is large or the compensation current hardly traces the reference, it would reduce the compensation effect.In view of the two factors mentioned above, the nonlinear dynamics is used. The design method of ac-side inductance based on the switched system theory considering the practical engineering index is proposed. Meanwhile, the switched control strategy is also proposed. The reason why the topology structure of APF is always changing is that the on or off state of the power module of APF is controlled by PWM signals. Hence, APF is a typical switched system consists of piecewise linear subsystems. Above all, according to the switched characteristics of APF, the switched system model is established. Since the nonlinearity of APF, it is hard to obtain the analytic solution expression of the model. Therefore, the numerical solutions of this model are calculated by the time-domain analysis. Then, the relationship between the tracing index and the ac-side inductance is obtained by the numerical solutions. According to the known index, the reasonable range of the ac-side inductance is determined. By comparing the range with that obtained by the traditional design method, the effectiveness and rationality of the method is confirmed.Moreover, the design method of the switched control strategy for active power filter is proposed. Based on the switched model mentioned above, the switched system error model is established. After that, the switched path, which makes the system asymptotically stable, is obtained. Under the action of the switched path treated as the control input of the system, the state variable of the switched system error model converges to the original point so that the harmonic suppression is achieved. It is worth to point out that a common quadratic Lyapunov function or the functions for every subsystem can be designed to determine the switched path. By comparing with the traditional control strategy, the switched control strategy is more reasonable. Meanwhile, the switched control determined by the Lyapunov functions for every subsystem is more reasonable than that determined by the common Lyapunov function. Finally, the experimental platform, whose control algorithm is designed based on the Lyapunov functions for every subsystem, is constructed. The effectiveness and rationality of the proposed approach is confirmed by the experimental result. |
PDF Full Text Request