Study On Damping Control Strategy Of Digital LCL Grid-connected Inverter

With the gradual exhaustion of natural resources,the renewable energy grid-connected technology has received unprecedented attention.As an important grid-connected device,grid-connected inverter is widely used in distributed generation systems.Therefore,it is of great significance to study the control strategy of grid-connected inverter into the power grid.According to the coordinate transformation principle,the mathematical models of the LCL grid-connected inverter were established in the stationary and rotating coordinate systems respectively.The harmonic distribution of the output voltage of the three-phase inverter bridge was analyzed,and the parameter selection method of the LCL filter was given.With regard to the resonance problem of LCL filter,passive damping and active damping control methods of LCL grid-connected inverters were studied respectively.And based on the system design performance indexes such as steady-state error,phase margin and amplitude margin,the constraints of system parameter selection were discussed.After the system parameters were determined,an LCL grid-connected inverter simulation model was built in Matlab/Simulink,and the feasibility of active damping and passive damping control strategies was verified.For the digital control system,due to the computation delay caused by sampling,storage and computation,etc.,the problem of control model change of grid-connected inverter was deeply studied.The discrete domain expression of the system loop gain was deduced.And based on this expression,the relationship between the pole distribution of the right-half of the system loop gain and the system parameters was analyzed.On the base of the above research,a kind of control strategy of grid-connected inverter based on capacitance current estimation was studied.But the estimate will produce delay when the capacitance current changes from the continuous domain to the discrete domain.In order to compensate the delay of capacitance current estimation,the least squares method is used to fit the fractional-delay filter to obtain a more accurate differential operator,thereby compensating for the delay in estimating the capacitance current.In addition,the S-function function in Matlab is used to build a digital control LCL grid-connected inverter simulation model,which verifies the feasibility of the control strategy for the grid-connected inverter system under digital control.According to the research needs,hardware circuit design was conducted,a grid-connected inverter experiment platform based on digital control was set up,and a software program was written.The feasibility of the the active damping grid-connected inverter control strategy based on capacitance current estimation is verified by the results of the experiment.