Research On The Key Issues Of The Four-Leg PV-AF System

In recent years, with the rapid development of technology and economy, themass utilization of non-renewable energy has brought severe problems ofenvironmental pollution and ecological deterioration, which has a negative impact onthe social development and human health. Thus, much attention has been paid to therenewable energy, such as solar energy, tidal energy, wind energy, etc. Photovoltaic(PV) power generation system is an important form of utilization of solar energy andone kind of distributed power generation systems. Besides, photovoltaic powergeneration and active filter (PV-AF) system broadens the function of PV powergeneration system. As the active power is injected into the grid, harmonics in the gridcan be suppressed effectively. Thus PV-AF system has broad development andapplication prospect. In this dissertation, key technology for three-phase four-wirelow-voltage four-leg PV-AF system has been studied.Based on the analysis of conservative power theory (CPT) harmonic currentdetection method, an improved CPT harmonic current detection method based on gridvoltage harmonic elimination has been proposed in distorted grid voltage conditions,by extracting the fundamental component of voltage at the point of common coupling(PCC), which inhibits the influence of voltage harmonic components on the currentcalculation. Simulation and experimental results verify the rationality and feasibilityof the proposed method.The maximum power point tracking (MPPT) technique is the key to improve theenergy conversion, maintaining the operation efficiency of four-leg PV-AF system. Inthis dissertation, different kinds of MPPT methods are compared. The oscillationnearby the maximum power point (MPP) is analyzed while adopting the perturb andobserve method. The method judging whether the MPP is nearby the MPP has beenstudied and an improved MPPT method has been proposed. Experiments andsimulation show that the proposed method can track the MPP quickly and accurately.As for the control strategy of four-leg PV-AF system, the outer voltage loop isadopted for controlling DC voltage and tracking the MPP. The inner current loop isused to track the fundamental grid current and the harmonic current reference value,which is vital to the entire system controlling. In this dissertation, a novel current loopcontrol strategy based on compound controller with PI (Proportional Integral)controller and H_∞repetitive controller has been proposed. PI controller ensures the dynamic performance of the system. The internal model makes smaller steady-stateerror. Robust control effectively enhances the anti-interference performance of system.The parameters for the compound controller have been designed and the digitalimplementation for the controller has been conducted by discretization. The influenceof the discretization on the system performance has been studied in the discretedomain. Simulation and experiments have been implemented on the proposedcompound controller under different conditions. The results show that the systemowns decent dynamic performance, smaller steady-state error, good robustness andlow voltage ride through ability.For that the existence of negative impedance in the system will affect thestability of the system, the small-signal impedance modeling for the four-leg PV-AFsystem has been studied. Via different control strategies, the research about theinfluence of impedance mpedance characteristics on system stability has beenconducted comparatively. On the premise of guaranteeing the system performance,the impact of the negative impedance on system stability has been reduced throughthe optimization of the controller parameters, which has been verified.The main circuit parameters are the key factors affecting the performance of thefour-leg PV-AF system. In this dissertation, the parameters for the main circuit offour-leg PV-AF system have been designed. The devices are selected. Experimentalplatform of5kVA three-phase four-leg PV-AF system have been set up.