Key Techniques Research On Operation Of Multi-Modular Shunt APFS

With the growing industrialization of our country, the harmonic current in the power grid has been to a degree that can not be ignored. Active power filter(APF), which can be integrated as a brandnew power electronic equipment which can effectively compensating harmonics, is widely used in industrial, commercial and civil fields. When the capacity of harmonics is large, multi-module parallel connected APFs have become the most popular product in the market for the capacity configuration flexibility, high reliability, and mass production facility with lower cost, etc.Several harmonic current detection and closed-loop control methods are analyzed and compared in detail. In order to realize the accurate separation of the specified frequency harmonic current in the multi-module system, a sliding-window iterative discrete Fourier transform(DFT) is used. For the close-loop current controlling, "PI+repetitive control" strategy is presenred as the PI strategy can not offer enough precision when tracking a AC signal and the deadbeat strategy relies too much on the mathematical model of the control system. Then the simulative and experimental results is presented, which validate the dynamic and steady-state performance of the single APF module.The single APF module of the multiy parallel system uses LCL filter as its output current filter, so the resonant problem of the LCL filter must be taken into account. To get rid of it, an improved active damping control strategy which feeding back the current of capacitor branch circuit in LCL filter is selected to reduce the negative influence of sampling delay and high switching frequency current which go through the capacitor branch. In addition, the equivalent mathematic model of the multiy parallel system and the constraints for the number of module are also presented to ensure the feasibility and effectiveness.In order to improve the operation reliability, this dissertation studys the intelligent harmonic reference sharing and restricting control strategy. For the reference sharing, it uses unified distribution which is equipped with only one set of current transformers. Meanwhile, this method can update the real-time current sharing coefficient according to the number of running modules. But it may lead to current reference surpass the safety threshold. So, a harmonic current reference compound limiting control strategy applying to sliding window recursive discrete Fourier transform is designed. Last but not least, an improved closed-loop phase locking strategy and voltage feedforward strategy is designed to eliminate the influence of the voltage distortion.