Performance Analysis And Design Method Of LCL Filter In Grid-connected Photovoltaic Power Generation System

In recent years,with the depletion of fossil fuels and the deterioration of the environment,in order to meet the demand for energy,the reform of traditional energy structure is imperative.Therefore,renewable energy such as wind energy and solar energy has been vigorously developed.The interface between the DC bus and the grid in the PV system requires an intermediate power electronics bridge to convert the electrical energy into the corresponding voltage level and type.As the Grid-connected inverter is the most important interface device,it has direct impact on the PV generation system and the operation of the network.In this paper,the three-phase grid-connected inverter system and the PV generation system have been modeled.The control strategy of the grid-connected inverter and the parameters design of LCL filter have been analyzed.In this paper,the two-stage grid-connected PV system using three-phase three-wire system.The pre-stage includes PV arrays and Boost converters,and the back-end circuit includes three-phase inverter based on LCL filter.At the very beginning,the engineering model of PV cell has been established.The boost converter was controlled by conductivity incremental method to achieve the voltage boost up and the maximum power point track.Then the mathematical model of the three-phase inverter based on LCL filter has been established,and the traditional parameters design method has been introduced based on the analysis of the filtering characteristic of LCL filter.In the case of grid voltage balanced,in order to achieve the current balance and unity power factor,the voltage and current double closed-loop control strategy has been established.And through the simulation to verify the grid-connected PV system can be stable and operate safely.In order to meet the harmonics standard,the alternate side of the inverter usually connected in series with a passive filter to reduce the high frequency harmonics.Compare with the traditional L-type filter,the LCL filter has low volume,weight and cost,that is why it was widely used in grid-connected PV system.But because it is a third-order system,it has inherent resonance point.If it has not been controlled,the high frequency current near the resonant frequency will produce oscillation,so the mechanical stress of the switching device,the system loss,the measurement error will increase.This will cause the inverter to fail to work properly,and even damage the inverter.The design of the LCL filter plays an important role in the filtering performance of the system.Although the traditional design method has simple theory,but the calculation is complicated and the parameters need to be matched with each other.It usually need to try again and again,and may not be able to get the optimal value.Some scholars have tried to adopt the optimization algorithm to select the parameters,but the principle is complex and not applicable in the engineering application environment.In this paper,the influence of the LCL parameters and the influence of the parameters on the power quality which inject to the grid were analyzed to give a reference basis for engineering application parameter selection.And a more suitable parameters selection method was given.