Research On Islanding Detection Methods Of Grid-connected Photovoltaic Power System

Nowadays, the energy and environmental crisis is becoming more and more serious. The key to sustainable development is the utilization of renewable energy. Among the different ways of utilizing new energy, photovoltaic (PV) system has become the research focus for its particular advantages. As the increasing of grid-connected PV system, a special phenomenon that is islanding must be considered. Islanding phenomenon will cause serious danger to system and staff. So the inverter must have the ability of islanding detection to make sure the safety of system and grid. This paper focuses on three-phase grid-connected inverter and researches the islanding detection methods on the foundation of grid connection control. An improved reactive current disturbance method with frequency positive feedback is proposed and the validity is proved by simulation and experimental results. The details are as follows:First, this paper introduces the working principle of three-phase grid-connected inverter. The grid voltage oriented control strategy is applied and the active current and reactive current can be decoupled based on the system model in dq coordinate reference frame. Then the three-phase digital phase-locked loop is analyzed to track the phase and frequency of grid voltage. Simulation results in Matlab/Simulink realize the unit power factor grid-connected operation.Second, the mechanism and detection standards of islanding are introduced. Several kinds of passive and active islanding detection methods are summarized and compared with each other. To improve the performance of the conventional methods, an improved reactive current disturbance method with frequency positive feedback is proposed. The working principle and parameter design are discussed, which proves that this method has faster detection speed and smaller non-detection zone (NDZ) than traditional reactive current disturbance method. Also, this paper analyzes the influence on islanding detection caused by multi-inverters parallel operation. Then, the validity of proposed method in multi-inverters system is researched. Compare the NDZ of different parameters and analyze parameters influence to the effectiveness of islanding detection for multi-inverters. Simulation results validate the theoretical analysis.Finally, a three-phase inverter grid-connected control system based on TMS320F28335DSP is designed, including hardware circuit and software program. An experimental platform is built up and the grid-connected current control and islanding detection are both tested on it. Experimental results verify the effectiveness of theoretical analysis.