Research On Harmonic Characteristics Of Grid-connected Photovoltaic Inverter System

To satisfy the needs of development of social energy,the government encourages the development of photovoltaic and other new energy generation.At the same time,supportive policies are formulated one after another,the permeability of the photovoltaic has been greatly improved.However,with the increasing scale of photovoltaic power generation,on the one hand,more and more grid inverter power electronic devices are connected to the electricity grid and cause harmonic problem in power grid.What's more,the photovoltaic power is greatly influenced by environmental condition such as light and temperature,exacerbating the volatility of input power and then causing the power grid voltage fluctuations.On the other hand,in order to improve the power grid power supply,the trend of inverter parallel operation is more and more obvious and bring in more aggravated resonance phenomenon in power grid what causes the power quality of the power grid further deteriorated.In the meantime,with the support of government policies,household pv has started to be put into use.In the foreseeable future,the popularization level of household pv will be greatly improved.Under the condition of multi-photovoltaic grid-connection,it is increasingly important to precisely locate and define the harmonic source in the power grid,define the harmonic responsibility,and establish a set of standardized and accurate reward and punishment system.Firstly,this thesis introduces the mathematical model of photovoltaic cells,analyzes its output characteristics and then chooses the suitable maximum power point tracking control method.This thesis also introduces the working principle of three level inverter and the modulation method,gives out the grid-connection control strategy and establishes a MATLAB model of photovoltaic grid to research about the influence of the light and temperature conditions for the effect of the harmonic.This thesis compares the new LLCL filter and the traditional LCL,L type filter to analyze its filtering performance.In view of the LLCL type inverter,single inverter output impedance model is established and bases on this,the paper establish equivalent model of the LLCL type inverter and derive the transfer function of grid-connected current.The parallel resonance mechanism of multi-LLCL inverter is described in the paper and the effects of the number of grid-connected inverters,grid impedance and inverter parameters on the resonance characteristics of multillcl inverter are simulated and analyzed respectively.In order to solve the resonance problem of a single LLCL grid-connected inverter,this thesis proposes eight passive damping methods and an active suppression strategy based on filter branch capacitance feedback and conducts simulation analysis in turn.For the parallel resonance problem of multi-LLCL inverter,this thesis adopts the global suppression strategy of adding suppression units at the PCC location,including R-type,RC type and active type methods and analyzes the advantages and disadvantages of the three methods.In this thesis,the method of harmonic source location is studied and a harmonic source location scheme under multiple photovoltaic parallel networks is proposed which provides a certain foundation for the relevant departments of the power grid to define the harmonic responsibility and establishes the reward and punishment mechanism of harmonic.Finally,a 14-node distribution network model is established in PSCAD to prove the accuracy of the proposed positioning strategy.