Research On A Transformerless Hybrid Active Power Filter And Its Control Strategies

Hybrid Active Power Filters (HAPFs) are suitable to compensate a large number of current harmonic sources in power system. Therefore, study on the main circuit design and control strategy for HAPF has important theoretical and practical significance. Several key techniques of 20kVA three-phase three-wire laboratory experimental transformer-less HAPF with LC output filter, such as system configuration, design of main circuit and current controller as well as other practical problems are studied in this thesis.Firstly, this thesis summarizes the presented basic configurations of transformer-less HAPF based on analyzing their equivalent circuit for harmonics under line current feedback control. Furthermore, a comprehensive comparison of all configurations is made in terms of filtering performance, required ratings and costs.LC filter is gradually widely used in transformer-less HAPF for its good impedance characters and filtering performances, while design of a proper LC filter is difficult because it is bound up with many parameters. An LC filter design principle is proposed taking such critical factors as ratings of APF, filtering performace, bypass effect for switching ripples into account, which is explicit and suitable for practical application.In order to improve the precision of the current control loop, constinuous and discrete state-space averaging mathematical models of transformer-less HAPF are constructed in the a-b-c stationary frame and the synchronous frame separately in this thesis.The filtering performance of transformer-less HAPF relies heavily on the current loop controllers. Since traditional line current feedback controller cannot achieve good compensation accuracy at the fifth harmonic frequency, the fifth harmonic current feedforward and feedback controller are added to help the former controller. However, because a lot of different harmonic components are contained in the reference and feedback signals and the bandwidth of conventional Proportion (P) controller is limited, steady-state error always exists in the output current. To solve this problem, a double-loop composite controller composed by a P inner loop and repetitive controller external loop is put forward to improve the output current waveform and the design method is given in detail. Theoretical analysis, together with simulink simulation and experimental waveforms all certify that this composite controller could not only suppress current harmonics very well, including both odd and even harmonic current, but also presents excellent dynamic performances.