Research On Grid-connected And Seamless Transfer Techniques Of Distributed Generation Systems

Due to the higher requirement of power quality and the fossil energy crisis, distributed generation systems have become very popular. As the interface between distributed sources and grid, power electronics equipments are critical to the power quality injected into the grid. In this paper, grid-connected technology and two-mode operation technology are studied based on a three-phase grid-connected inverter.The mathematical model of three-phase grid-connected system with LCL filter is established firstly. There is complicated coupling between d coordinate and q coordinate under the synchronization reference frame, and no coupling betweenαcoordinate andβcoordinate under stationary frame. Then the main characteristics of LCL filter are illustrated. LCL filter is equivalent to combination of low frequency part and high frequency part which is easier for analysis.According to the damping principles, most grid-connected strategies based on LCL filter are classified into three types including: adding resistor to increase impedance of LCL filter, inhibit resonant excitation sources and compensate the resonant component in the LCL filter. The three strategies are compared in terms of stability, robustness, sensor number and et al. The experimental results verified the analysis.In this paper, the two-current-loop control strategy is analyzed in detail. Furthermore, a specific design method for the two-current-loop controller is firstly proposed to determine the controller parameters which can make the tuning procedure easier. Pole placement and pole-zero cancellation methods which are rather conventional are used in this method. However, the proposed method provides a way to get the parameters of the two-current-loop controller which can maximize the use of the two feedbacks and get good control performance. Practicality of this design method is discussed and experimental results are also provided to verify the feasibility and correctness of the design method.A novel multi-loop control scheme is proposed to realize the two-mode operation of an LCL-based three-phase inverter. The control scheme consists of three parts, GCM controller, SAM controller and seamless transfer controller. GCM and SAM controllers are both dual-loop controllers, however only three control loops are used in all. Seamless transfer between two modes is also realized in the scheme. Design of the control scheme is discussed in detail. Finally the proposed scheme is verified on a laboratory prototype.Since the PI regulator under stationary frame can not track the reference command without error, PR regulator is adopted here. The two implementation methods based on different operators (q operator andδoperator) are compared in this paper detailedly. It is also proved with mathematical tools that it is better to realize PR controller with delta operator than with shift operator.