Research On Critical Issues For Distributed PV Grid-tied System

The structural adjustment in energy industry is being spread widely in the world,mainly due to the strengths of the new renewable energy,such as clean,pollution-free,and abundant,etc.It will offer substantial promotion for technological improvement,environmental protection and economic development in the long term.The distributed generation system(DGS)relying on the new renewable energy is permeating the classical electricity networks and smart grids and it has been a hot issue in the field of distributed generation.Recently,an energy utilization system of deep integration of new energy technologies and information technology,namely energy internet,has been a new focus concerned by both international academia and industry,and it also becomes a fresh forefront of research and an important subject following after the smart grid in energy industry.In energy internet,the DGS relying on renewable sources of energy will be one of the fundamental core units of the future energy internet.Given that high-gain DC converter,grid synchronization and islanding detection are three key technologies in DGS,this paper has undertaken further researches on above three aspects.The main results are as follows:For the fact that the current boost converters have a constrained step-up ratio,which cannot meet the requirement of high ratio in high-gain applications and have some problems such as high voltage stress on switching devices,high current ripple and low efficiency of the circuit,this paper here puts forward a novel high-gain interleaved boost converter based on a composite structure containing coupled inductors and switched capacitors.According to the low ripple approximation,the working principle of the propsed converter under different modes is analyzed detailedly.Furthermore,the functional relationship between the voltage gain of the suggested converter and the duty cycles of converter switching is derived by using the principle of volt-second balance.To ensure the propsed converter can meet the requirement of achieving MPPT and high-gain at the same time in the PV applications,a PWM open-loop controller based on dynamic sliding evolution algorithms for the novel converter is designed and realized,combing dynamic evolution theory with sliding control strategy.According to necessary simulations and experiments,the feasibility and availability of the suggested boost converter along with its control algorithm are verified.Considering the fact that grid synchronization technology can provide a necessary synchronous reference signal for the grid-connected inverters,a frequency adaptive PLL scheme for single-phase system under distorted grid conditions is proposed in αβ axis.Firstly a modified second-order generalized integrator-based quadrature signal generator(SOGI-QSG)with DC offset elimination and harmonics rejection is suggested then the adaptive complex filter is introduced in αβ axis with the purpose of eliminating the influence of harmonics and voltage fluctuation further.Finally a frequency adaptive controller with fast dynamic response is built,ensuring the PLL scheme have reliable and stable performance even in the frequency-varying conditions.The accuracy of the proposed PLL scheme is verified through simulations and experiments.A new islanding detection scheme combining with synchronization phase lock technology is presented in view of the deficiencies of current active islanding detection approaches,such as out of accuracy due to the injected high frequency harmonics and the even harmonics persistence.Firstly a simple frequency-adaptive PLL is constructed in αβ coordinates to capture the phase of the grid fast and accurately.Then the third harmonic injection by phase perturbation is provided and the criterion for selecting the appropriate disturbance coefficients has also been given.The sliding Goertzel filter with the high-speed computational property is applied to extract the third-harmonic component of the point of common coupling(PCC)voltage,with the intention of determining whether the system is in an islanding situation.Finally both feasibility and availability of the proposed method are verified through simulations and experiments.