Analysis Of Photovoltaic Grid-connected System Under Weak Power Grid

With the increasingly prominent environment problems,the development of clean and efficient energy has become consensus.The solar-based photovoltaic power generation have been greatly developed world wide.In China,the distribution of solar energy resources and electricity consumption determine the development of photovoltaic power generation into two directions: distributed photovoltaic power generation in microgrid which is absorbed into local distribution network,and large scale photovoltaic power plant in remote areas which is connected to power grids via long distance transmission.Both modes above appear a weakening of the relationship between the photovoltaic power and the power grid,which is called as weak power grid.In this case,the impedance of the gird should not be neglected and the background harmonics at point of common coupling can be very serious,which may make the outputs of the grid-connected inverters not to meet the requirement of the grid and more worse may cause instability of the generation system endangering the safe operation of the grid.This thesis focuses on the effect of the weak power gird on the grid-connected photovoltaic inverters and the adaptability of the grid-connected photovoltaic inverters to the weak power grid.The relevant research and analysis are carried out as below.Firstly,photovoltaic power generation system and weak power grid are briefly introduced and the mathematic model of photovoltaic generation system under the weak power grid is established.Secondly,photovoltaic generation system under the weak power grid is analyzed revealing that the impedance of the weak power grid greatly affects the point of common coupling,the filter characteristics,dynamic response and system stability.Hence,the impedance-based stability criteria is proposed,through which the influence of the weak power grid on the number of inverters connected to grid and system stability are analyzed.Finally,the methods to improve the adaptability of photovoltaic grid-connected inverters to weak power grid are discussed from two aspects of LCL filter design and impedance reconstruction.In the aspect of LCL filter design,the reason why the traditional filter design is not suitable for the weak power grid is revealed and a new filter design method considering the impedance of the weak power grid is proposed.In the aspect of impedance reconstruction,methods of changing the equivalent output impedance of the grid-connected inverter system and equivalent grid impedance are given through adding specified feedforward path in control strategy or additional capacitive device at point of common coupling,which increase the phase margin enhancing the robustness of the photovoltaic generation system.