Modeling And Harmonic Characteristics Research Of Active Distribution Network

As more and more distributed generation (DG) connected to distribution network, the traditional passive distribution network with unidirectional flow is becoming bidirectional active distribution network (ADN) integrating many distributed generation device. The introduce of DG inevitably brings a series of power quality issues, due to that DG can not directly connect to the grid, but have to convert energy to sinusoidal alternating current by power electronic devices. Generally, the power electronic devices use pulse width mode, the switching process of which will bring a lot of harmonics to the grid since the existing filter can not completely eliminate harmonics. Therefore it is of great importance to study the harmonic characteristics of ADN for analyzing and managing harmonics. The main topic of this paper is to establish model of ADN and to study its harmonic characteristics.Firstly, the paper introduces the development of avtive distribution network and the research status of harmonic analysis at home and abroad. An overview of harmonic problems of ADN including the propagation of harmonics, harmonic amplification and the harm of harmonic is introduced. Meanwhile, the power system harmonic evaluation criteria at home and abroad is illustrated based on the theory of power system harmonics.Secondly, the paper builds the topology and mathematical models of various components in avtive distribution network, mainly including DG model, transformer model, transmission line model and load model, among which DG model is focused on, containing Detail model, Average model and equivalent harmonic model for studying different harmonic prolems. The correctness of DG model is verified through MATLAB/Simulink simulation. Based on Detail model, the harmonic characteristic of DG converter with SVPWM is analyzed by simulation.Then, under the ideal grid conditions, through the establishment of the equivalent circuit, the harmonic propagation and superposition characteristic of ADN in the case of different point of connection and different number of DG is analyzed in theory. According to the modeling particularity of high order harmonic and low order harmonic, the simulation results of equivalent model connected to grid is compared with that of Detail model, which verifies the rationality and effectiveness of equivalent model. The theory and simulation come to a consistent conclusion. The harmonic voltage distribution characteristic under island operation is preliminarily studied. Under non-ideal grid conditions, the influence of grid low order harmonic on the output current harmonics of DG is studied. The superposition of harmonic voltage produced by multiple paralleled DG at point of connection is studied by simulation combined with example. The harmonic resonance problem caused by DG with LC filter at point of connection is verified, based on which, the paper proposes several measures to avoid resonance.At last, this work is summarized and prospects for the harmonic analysis and research direction of ADN.