| With the proliferation of nonlinear loads in the power system, harmonic pollution becomes a serious problem that affects the power quality in both transmission and distribution systems. Active power filter (APF) has been proved to be one of the most flexible and efficient methods for tackling power quality problems. So far, different current control strategies have served in APF to satisfy its fast response and accuracy requirements. Although the research on the APF control strategy has become a hot topic, there is a lack of comprehensive comparison and analysis about the harmonic compensation effects based on commonly used control strategies. To avoid the arbitrariness of current control strategy selection, and inspire better choice for current control strategy, this paper chooses three types of current control strategy applied in APF to compare.They are hysteresis control, deadbeat control and PR control strategy in αβ coordinates. The experimental comparison and analysis based on the APF prototype platform is made, to assess their diagnostic evaluation for harmonic compensation performance.This paper presents the main circuit structure of shunt APF, illustrating the basic principle of harmonic compensation. The ip-iq harmonic extraction algorithm based on the instantaneous reactive power theory is analyzed. As the critical factor affecting the performance of APF, the current tracking control strategy becomes the key research in this paper. It first illustrates the realization principle of the most widely used hysteresis control strategy, whose switching frequency is not fixed.The relationship between the switching frequency and the hysteresis bandwidth is under study. Against the disadvantage of hysteresis control, further study is about deadbeat control strategy, which has fixed switching frequency and could reduce the control delay. The principle of deadbeat control based on the average calculation and some related current prediction methods are introduced. On the basis of the above work, proportion parallel resonant control strategy in αβ coordinates is proposed in the paper. Aiming at the current compensation of specified5th and7th harmonics, a parallel PR controller model is designed in α/β coordinates. In the mean while, combining with LCL filter’s effective inhibition of high order harmonic, the proposed strategy could make LCL-type APF achieve zero steady-state error for the specified harmonic current compensation. The paper analyzes the impact of PR controller and LCL filter parameters on the control system. Simulations are carried out to complete verification of the theoretical analysis, and get the reasonable parameters. For the comparative experiments of the specified control strategies, a shunt active power filter experimental prototype based on DSP2&335control is developed. The paper offers the hardware and software design methods of the prototype in detail, and put forward a perfect soft-start program as well as with over-voltage and over-current protection when APF is connected with the grid.With this prototype, experiments are carried out to evaluate the performance of APF with the traditional hysteresis control, deadbeat control based on multi-prediction methods and the proposed PR control strategy in the αβ coordinates in dynamic and steady state. The paper makes a comparative analysis from multiple aspects such as error percentage, THD, low frequency THD, dynamic response, etc. Ultimately the respective strengths and weaknesses of three control strategies about harmonic compensation are summarized in the paper. |
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