Research On Key Techniques Of Multi-module Shunt APF

With the requirement of green and low-carbon development of our country, the grid harmonic problem has gained increasingly more attention recently. As one of the main effective equipment to deal with the grid harmonic problem, Active Power Filter (APF) becomes large-scale industrial application gradually. With the increasing requirements of high-capacity APFs in industries, multi-module parallel connected APF becomes an important developmental direction and research hotspot for its flexible capacity and high reliability. This dissertation focuses on some key technologies of the multi-module parallel connected APF.In order to ensure the synchronization of sampled signals of each module and avoid the single point of failure of the one step sampling strategy, a two steps sampling strategy of the modularized APF is designed in this thesis. The harmonic current detection method for unbalance system such as ip iq, band pass digital filter and Discrete Fourier Transform (DFT) are compared. A modified Discrete Fourier Transform (DFT) method is proposed based on this basis, which can be used for selective harmonic current compensation or unbalance systems. Compared with conventional DFT method, the computing efficiency and the required data space of the modified DFT are all improved. The effectiveness of the design method and related technology are verified by experiments.Each single APF module of the traditional N+X redundant system need a certain physical address and is difficult for industrialization. To solve this problem, based on a proposed numbering method, a centralized control and a master-slave control method are proposed respectively. The reliability of the N+X redundant APF system is analyzed. The current-sharing strategy and current limiting strategy are designed. At last, an N+1redundant shunt APF system with a great potential application value and high reliability is achieved. A redundant strategy which can make use of the undamaged bridge arm of the damaged modules is also given. Compare with the method that removes the whole module when it is damaged, the flexibility of system can be improved by this redundant strategy. The feasibility and practicality of the proposed strategies are verified by prototype experiments.To solve the resonance problem of the APF with LCL filter, a LC branch circuit is adopted in passive damping method, which can suppress resonance and reduce loss and heat of the conventional damping resistance. The dynamics with multivariable of the multi-modular system are given. An equivalent simplified single APF model is also deduced based on a reasonable assumption that the parameters of all the APF modules are the same. The stability of the modularized APF system is analyzed based on the consideration of the coupling of the grid side impedances.The modularized system is trial-constructed taking the design and realization of a three modular APF system as an example. The APF modules are designed to be a "drawer" type which is compact and easy to realize hot swap. In order to ensure the heat dissipation allowance of the "drawer" structure, the Computational Fluid Dynamics (CFD) simulation method of fluid-thermal coupling is given on the forced air-cooling condition, which can accurately simulate the actual temperature of the module. The research costs and circle are reduced by this simulation method. The implementation approach of the parallel control strategy, communication strategy and human-computer interface are designed. At last, the experiments of the three module parallel APF system are done. The experiments include the dynamic, steady state compensation on balance and all kinds of unbalance conditions and the temperature rise tests. The compensation experiments prove that the modularized system has a good performance. The results of the temperature rise tests are consistent to that of the simulations, which show that the introduced CFD method is feasible to analyse the temperature rise of APF modules.