Micro-grid Harmonic Suppression Method Based On Virtual Impedance

Micro-grid is increasingly concerned with the ability of coordinating the contradiction between the grid and distributed generation system. However, the output voltage and current in micro-grid may be distorted to produce a great deal of harmonic components due to the access of non-linear loads and micro-sources in micro-grid, application of a large number of power electronic equipments as well as harmonic penetration of high voltage power grid. The existence of harmonics will bring a lot of harm to micro-grid, and even endanger the safety and stability of micro-grid. It is one of the important problems in micro-grid research to restrain the micro-grid harmonic and ensure the micro-grid power quality.In this thesis, a micro-grid harmonic suppression method based on virtual impedance is proposed to solve the micro-grid harmonic problems. Immune particle swarm optimization(IPSO) algorithm is used to optimize the micro-grid filter parameters so that micro-grid voltage harmonics are suppressed and the micro-grid voltage harmonic distortion rate is reduced.The main contributions are listed as follows:1. For the photovoltaic micro-grid based on conventional droop control system, the photovoltaic, inverter interface, power control, droop control, as well as voltage and current double loop control are modeled, and the harmonic problems produced in micro-grid system are analyzed.2. Micro-grid Filter parameter optimization scheme is proposed based on IPSO for the harmonics problems of the photovoltaic micro-grid system. Firstly, the corresponding objective function and fitness function are established and the constraints of LC parameters are provided.Secondly, by using the self-regulation ability, the immune algorithm is introduced into the particle swarm optimization algorithm to improve the global search capability and avoid the premature convergence problem of particle swarm optimization algorithm. Micro-grid simulation system is invoked in IPSO to calculate the optimal filter parameters by Matlab programming.Finally, compared with the simulation results obtained by empirical value method, the effectiveness and superiority of the proposed method are verified in the aspect of harmonic suppression.3. On the basis of the filter parameters optimization, micro-grid harmonic suppression method is proposed based on virtual impedance. Firstly, based on the fundamental power droop control in the micro-grid conventional control system, the harmonic reactive power is computed through the power controller. And harmonic reactive power droop control module is designed by using the droop relationship between harmonic reactive power and the command harmonic voltage. Secondly, according to the basic principle of virtual impedance, virtual harmonic impedance module is designed to realize the solutions of related harmonic current and virtual harmonic impedance, and obtain the virtual harmonic voltage. Virtual harmonic impedance module is combined with harmonic reactive power droop control module to obtain the harmonic compensation voltage. Finally, the proposed method is realized through the Simulink tool in islanding and grid connected state. The case analyses show that the proposed method could effectively suppress the micro-grid harmonics, reduce the voltage distortion rate and enhance the micro-grid power quality.