Study On Some Key Technologies Of Grid-Connected Photovoltaic System

The grid-connected photovoltaic (PV) system which uses power electronics technology,digital signal processing and advanced control theory has been focused currently. It is one ofthe most effective and potential solutions to combat the energy crisis. In this thesis, invertertopologies, grid-connected current control methods and systematic reliability etc, all of whichare important to the performance of the grid-connected PV system, are discussed. At the sametime some new methods are proposed to improve the PV system performance. The maincontents of the thesis are as below:1. The thesis studied a new inverter, namely Z-source inverter. In the thesis the topology ofthe Z-source inverter is analyzed and the relationships of the all state variables arerevealed. Research is done on the best range of the shoot-through state in the nonidealcondition. The small-signal model of Z-source expatiates. The large-signal model of theZ-source inverter which is based on the dynamic phasor method is proposed firstly and itstime-invariant state-space equations are deducted. Compared to the small-signal model,the proposed large-signal model can reflect more accurate of its voltage and current. Thesimulation results of the small-signal model, the large-signal model and the actual circuitsare contrasted and analyzed.2. With the help of the pole and zero trajectories and the Bode diagram, the non-minimum-phase characteristic of Z-source inverter is studied in the thesis. According to thenon-minimum-phase characteristic presented in the DC-side and the functional demandsof the system, two constant-frequency sliding-mode controllers are proposed to guaranteethe system robustness. By using the controllers, the effects caused by thenon-minimum-phase characteristic are mitigated. The quality of grid-connected current isensured. The design process of the controllers is depicted. Also, a small-signal modelingmethod is employed to analyze stability of the close-loop system and its parameters arecalculated. The correctness and validity of the inverter and proposed controllers areproved by simulation and experimental results. Furthermore, the control method to theDC-side voltage of Z-source can also been used in other sorts of Z-source inverter and converter.3. The current control method plays an important role to quality of the grid-connected PVsystem. Using T filter, the quality of the grid-connected current can be improved.However, due to T filter the systematic ranks are increased, improper control arithmeticwill cause grid-connected current resonance. In this thesis, the filtering performance isparticularly analyzed when the damping is imposed in different location on the T filter.One kind of passivity-based control theory, namely Interconnection and DampingAssignment Passivity-based Control (IDA-PBC), is proposed to improve the quality ofgrid-connected current and to realize the global stability. This simple control strategy,which is based on energy balancing, has a clear physical meaning. Simulation andexperimental results of this method verify that the static and dynamics characteristics ofthe system are fine and global stabilities are realized.4. The grid-connected PV system would not operate normally if the conduction-interferences,which exist universally in the power system, come into the power-supply subsystem viathe power line. Meanwhile, it is necessary that the power-supply subsystem is isolatedfrom the power source. In order to improve the systematic reliability, a novelInterference-free Transformer (IFT) is proposed to suppress conduction-interference. Thefabrication of IFT is materialized through an air-gap shunt added to the magnetic circuit ofthis transformer, while an equivalent step-down inductor can be obtained between theprimary and the secondary windings. The inductor is connected with a parallelresonance-circuit in series to form a filter-network which makes the high-frequencydifference-mode (DM) interferences be attenuated. Two windings in opposite directionsare connected in series with the primary winding to suppress the common-mode (CM)interferences. A design procedure is proposed, and a prototype is built and analyzed. Thesimulation and experimental results validate the analysis and the optimizing method forthe design of the proposed transformer.5. Grid voltage information, such as the frequency and phase angle, is very important ingrid-connected PV system. There are some drawbacks in hardware phase-locked loop(PLL). This thesis proposes a novel software algorithm that can detects the frequency,phase and amplitude of the grid voltage simultaneously. In the start-up stage, both thegrid-voltage signal and its orthogonal signal are sent to a software phase control loop todetermine the phase and amplitude of the output. After the algorithm steps into theneighborhood of steady status, the algorithm adopts indirectly phase control method andfinally obtains stable the amplitude, frequency and phase of the input signal. The control method can meet the demands of the static state and dynamic, and also avoid to calculatethe sine value in real-time. This algorithm shows practical to be planted in MCUs andDSPs. Simulation and experimental results verify its validity.