Analysis Of Micro - Grid Fault Based On Inverter Control Characteristic

In order to solve the problem of power supply, a large number of micro grid power generation technology using clean energy has been put forward and received a lot of attention. The micro grid can enter the island operation in order to ensure the stability of the important load when necessary and be highly efficient in using new energy sources at the same time. However, due to the applying of a large number of power electronic devices, the running characteristics of the micro grid system in a fault or abnormal condition are mainly affected by the corresponding control strategy. Therefore, the analysis of the mathematical characteristics of the control strategy of the inverter in micro grid has great importance for the safe and reliable operation of the whole micro grid system.This thesis firstly introduces the basic principle of the PQ control strategy when the micro grid is in grid connected operation. From the point of view of mathematical analysis, the characteristics of PQ control inverter under fault condition are studied also in this thesis. According to the inverter capacity and characteristics of double closed loop control strategy, this thesis derives that the PQ control inverter has two limit failure states which are voltage saturation state and current saturation state; puts forward two boundary conditions for the two states and the mathematical model. With the help of the PSCAD/EMTDC, the grid connected micro grid system is established and verifies the correctness of the mathematical model through simulation. Then this thesis analyzes the basic principle of the VF control strategy of micro grid in the isolated island operation. According to the limit of inverter capacity and the features of control strategy, it is pointed out that the VF control inverter has two working conditions in fault status which are the voltage steady state and the current steady state. Based on the analysis above, the thesis proposes the boundary condition and mathematical model of VF inverter under fault condition. By using the PSCAD/EMTDC, the micro grid simulation system is built and the correctness of the mathematical model is verified though simulation.When the micro grid system switches to the island operation, the load gap problem will occur due to the limitation of the system capacity. It is pointed out that the fault statue analysis of the micro grid is similar to the statue analysis of the load gap problem. Based on the analysis of fault characteristics of the PQ and VF control strategies, the output current of the VF control inverter is considered as basis to judge whether the system power is balanced or not. Furthermore, the algorithm and the control strategy of load reduction in the island operation are proposed, which can recover the power balance and make ensure that the system reduces the loads as few as possible at the same time. Using the PSCAD/EMTDC, the effectiveness of the load reduction algorithm and control strategy is verified through simulation.