Study On The Topology And Circulating Current Of Multi-module APF Parallel

With the rapid development of China's economy,the era of knowledge economy dominated by high-tech has arrived,and information users who are sensitive to power supply quality occupy an important position in economic development.Harmonic suppression,reactive power compensation and solving three-phase current imbalance problem can be achieved simultaneously by Active Power Filter(APF).APF as a common harmonic suppression device,has fast response and good tracking performance.With the continuous development of industry,higher requirements are put forward for APF compensation capacity.Modular APF has become an important solution with its high modularity,flexible expansion and other advantages.In order to meet the expansion demand of harmonic compensation device,the topology structure,stability analysis and circulation suppression measures of modular APF are studied in this paper.The mathematical model of APF is deduced.Commonly used parallel redundancy control strategies and current limiting protection strategies in APF multi-module parallel system are compared in detail.In order to meet the compensating requirements of high voltage,large capacity and high reliability of power grid,a modular APF based on PCC isolated is proposed in this paper.The proposed APF is suitable for different voltage levels,and can flexibly adjust according to capacity requirements.The proposed topology's working principle is elaborated in detail.The advantages and disadvantages of using carrier phase shifting technology or nearest level approximation algorithm are compared and analyzed.Finally,the stability of the conventional modular APF system and the proposed APF system are analyzed.The analysis results show that the stability of the proposed APF multi-machine parallel system is no longer affected by the power grid or the parallel number,which simplifies the design of the controller.There is no circulating current problem in the multi-APF parallel compensation device under ideal conditions,because the output current and phase of all APF units are synchronized.However,circulation will be formed due to the inconsistency of parameters and different switching times in practice.In order to analyze the specific influence of system parameters on the circulation,the multi-APF parallel system is modeled and the circulating current generation mechanism and paths under different switching states are analyzed.The influence of common carrier phase shifting method on circulating current and system performance is discussed.The idea of differential control is introduced to suppress circulating current in parallel system,and its working principle is introduced.The feasibility and correctness of the above analysis are verified by simulation and experiment.