Research On Control Strategies Of Single-Phase Grid-Connected Photovoltaic Inverter Systems

As the energy and environmental problems have become increasingly prominent,the clean solar energy has been paid more and more attention.Grid-connected photovoltaic power generation will be the main form of solar energy utilization in the future.With the improvement of the performance of digital signal processor chips,it is possible that some advanced control strategies can be applied to grid-connected photovoltaic inverter systems.This thesis concerns with control strategies of single-phase grid-connected photovoltaic inverter systems,and the research goal is to improve the control performance.Aiming at the existing problems in single-phase grid-connected inverters,the thesis gives the thorough research and investigation about synchronous reference frame phase-locked loop and current loop control.Proper work of single-phase grid-connected inverters requires an accurate detection of phase angle of grid voltage.Synchronous reference frame phase locked loop(SRF-PLL)methods are widely used for estimation of the phase angle of the grid voltage.The main difference between the methods is the orthogonal system generator(OSG).The DC offset component is usually entered in the input signal via the measurement path of the grid voltage.Undesirable introduced DC offset will have a great influence on the performance of the PLL.To eliminate the influence of the DC offset component,this paper presents an improved PLL structure based on second order generalized integrator.The key component of the proposed PLL is orthogonal system generator with a closed control loop for DC offset.The favourable steady-state performance and the good dynamical performance of the proposed method are validated through the Simulation and experimental results.PI regulators in rotating reference frame are widely used for current regulation of three-phase systems.Since traditional stationary-to-rotating reference-frame transformation requires two orthogonal signals,such transformation cannot be directly applied to single-phase systems as variables in single-phase system contain only one component.Common approach is to create virtual quantities orthogonal to existing physical ones by phase shifting the physical ones a quarter of fundamental period.However,current delaying usually deteriorates system dynamic response.This paper presents a new stationary-to-rotating reference-frame transformation strategy based on variable step-size least mean square(LMS)algorithm via analysis of relationship between AC and DC quantities.This strategy does not need virtual orthogonal quantities.Adding forecast link into the proposed strategy,system dynamic response speed is improved significantly.Simulation and experimental results verify its effectiveness.