Research On The Anti-Islanding Method For Inverter-Based Distributed Generation Of PV System

With the exploitation of global fossil energy, energy problem is becoming more and more serious. According to statistics, oil resources will be exhausted in around2050, the countries which rely on traditional energy will suffer a loss. Therefore, developing renewable energy, reducing the use of fossil energy and improving the use of renewable energy such as solar energy becomes a key factor in solving the problem of energy crisis for most countries around the world. Photovoltaic power generation is an important way of utilization of solar energy, the study of photovoltaic (PV) grid inverter has become the key point in the research of PV system. The inverters should be equipped with islanding detection scheme, for this reason this thesis focuses on PV system islanding detection method based on the inverter.The frequency-shift techniques and the grid impedance detection method are selected as a representative anti-islanding scheme in this paper. The paper gives the analysis of detection performance of these two methods as well as their advantages and disadvantages, and puts forward a new algorithm. Firstly, this paper studied the photovoltaic system control strategy. Secondly, this paper deeply analyses the frequency drift islanding method and explains the reasons conducting the NDZ of the islanding detection and the related parameters from the aspect of physical property. Thirdly, this paper analyses the performances of the frequency drift islanding method in LVRT and proposes the improved algorithm. At last this paper analyzed islanding detection method based on grid impedance and proposed a new impedance island detection algorithm. Specific innovation points are as follows:(1) Adaptive island detection based on SFS algorithm is proposed, which uses the change of system frequency proposes the adaptive setting of cf0and can reduce the harmonic distortion rate (THD) at grid connected.(2) The reason leading to the error of PLL is analyzed and the improved method of SPLL is used in the model;(3) Proposed a new islanding method based on grid impedance variations of double threshold and hierarchical use capacitance, implementing the accurate and rapid detection, and can effectively avoid the false island miscalculationThe PV system model is set up in the simulation environment of MATLAB/Simulink. The new algorithms were verified in the simulation mode. Results show that the proposed method can quickly detect islanding and has less impact on power grid power quality compared with the original algorithm.