Research On Control Technology Optimization Of Three-phase PV Grid-connected System

The photovoltaic(PV)power generation,as the effective method of solar energy application,has been widely applied and rapidly developed for the past few years.And the grid-connected PV system has been the PV generation mode with the highest proportion of installed capacity.The increase of installed capacity promotes the sustainable development of PV conversion technology,and the PV inverter has been a union category of power converter.In recent years,the control technology research of grid-connected PV system at home and abroad,such as output current control,islanding detection methods and maximum power point tracking(MPPT)algorithms,are all the critical techniques of grid-connected PV generation,which are also the main research contents of this dissertation.The three-phase single stage grid-connected PV inverter is used as the control object,and the improved control methods of the above three aspects of control technology are all presented in this dissertation,which are based on the optimization study of relevant practical control methods.The main research work of this dissertation is summarized as follows:(1)The output current quality of grid-connected inverters is affected by grid voltage disturbance or uncertainty of inverter circuit parameters.According to the disturbance observer control principle,the model of current control based on disturbance observers is established to design current disturbance observing controllers.This method makes the variation of grid voltages and DC voltages as external disturbance,and improves the anti-interference ability of current control.In addition,compensation control is involved to reduce the difference of circuit parameters when active power is changed.This method can improve the dynamic response speed of current control and decrease output current harmonics.The results of simulations and experiments show that this method effectively restrains current harmonics caused by unbalanced grid voltages or voltage distortion,and improves the output current quality in low power conditions.When the input active power changes rapidly,both the response speed and quality of currents are improved by this method,and which can indirectly reduce the tracking time of MPPT for PV inverters.(2)According to the voltage magnitude trend of islanding condition,and the pulse width modulation control principle of grid-connected inverter,an active islanding detection method based on modulation index directional shift control is proposed.The modulation index,as the voltage control signal of pulse width modulation,can change the voltage magnitude in the islanding mode.The modulation index directional shift control method can rapidly decrease voltage magnitude to detection threshold,which achieve the islanding goals of non-detection zone and no negative influence to power quality.Compared with the conventional islanding detection method by voltage magnitude,this method improves the stability of detection speed in different active power condition which is verified by simulation and experimental results,and avoids the ineffectiveness of tradition method in maximum active power mode.(3)For the control characteristics of multi-inverter grid-connected PV systems,the islanding detection method based on modulation index directional shift control is further analysised in comparison to the conventional islanding method by voltage magnitude.Due to the time-dependence characteristics of PV array,the active current of each inverter is usually different when the islanding happens.As a result of that,the difference between the active current shift values of multiple inverters is more obvious with the increase of inverter capacity,and which will make the time delay,even failure of islanding detection.The real-time change of active power in PV system has been considered in the design of modulation index directional shift algorithm,therefore the above problem of islanding detection can be improved in theory.According to the simulation results and experimental results using the experimental setup with two parallel-connected PV inverters,in comparison with the conventional method by active current shift,the proposed islanding method makes the preferable synchronization of modulation index shift values from each inverter,and is more suitable to the multi-inverter grid-connected PV systems.(4)A new MPPT method with dynamic voltage error compensation is proposed,which can improve the dynamic tracking ability under two varying external conditions,rapid insolation variation and grid voltage fluctuation.Different from insolation variation,the dynamic error of tracking voltage caused by grid voltage fluctuation is regularly ignored by MPPT methods.According to the different current increment changes under the insolation variation and the grid voltage fluctuation,the reasons of dynamic voltage errors are analyzed and distinguished.And two corresponding modes of variable step size are designed in the tracking algorithm to regulate the voltage deviation from maximum power points.Compared with the incremental conductance(INC)method,the effectiveness and the improved dynamic tracking performance of the proposed MPPT method are verified by simulation and experimental results under the above two varying external conditions.