Research On Islanding Effect Analysis And Detection Method Of Distributed Photovoltaic Grid-Connected System

The energy issue is a major strategic issue concerning the national economy and people's livelihood.It has always played a very important role in China's economic and social development.In recent years,China's energy industry has achieved remarkable results,and the production and consumption of clean energy has been greatly improved.Among them,solar photovoltaic power generation has got the fastest growing,and in the future many of new photovoltaic power generation projects will be centralized on distributed ones.As the installed capacity and the scale of grid-connected operation of distributed photovoltaic power generation continue to expand,the impact of photovoltaic power generation systems connected to the main power grid and local loads is also deepening.The unplanned islanding self-circulation system formed by the distributed power supply failure with the grid-connected power system will cause damage to the power equipment and pose a threat to personnel safety.As an essential part of new energy grid connection,islanding detection is of great significance for ensuring the safe and stable operation of distributed PV grid-connected systems.First of all,the thesis analyzes the causes,mechanism and hazards of islanding effect in the distributed photovoltaic grid-connected system,and provides a theoretical basis for further research on the islanding detection method.After analyzing and comparing various types of islanding detection methods and detection blind zone of various methods in different coordinate systems,and for the initial truncation coefficient and feedback coefficient of the traditional Active Frequency Drift with Positive Feedback?AFDPF?method in islanding detection can only take a fixed value,which results in problems such as large detection blind zone,long detection time and high total harmonic distortion,the improvement of traditional AFDPF detection method is proposed.Firstly,by introducing the symbol function,the problem of inconsistent disturbance direction caused by different load conditions can be solved.Then,the feedback based on the cubic power function is used to reduce the influence of strong disturbance caused by large frequency difference on the system power quality.Then,the fuzzy control algorithm is used to fuzzify the disturbance coefficient,so that it can be adaptively adjusted to establish a new truncation coefficient.At the same time,the Q_f0×C_norm coordinate system is used to analyze the blind zone of the improved AFDPF method and evaluate the effectiveness of the method.In the thesis,two sets of distributed grid-connected system islanding detection models for the improved AFDPF method based on single inverter and multi-inverter are built on Matlab/Simulink simulation platform,and comparative analysis of simulation results from improved AFDPF detection methods based on single inverter and multi-inverter systems has been done.Finally,a combined islanding detection method combining improved ADPPF method and passive method is proposed.The combined detection method is simulated and verified in multi-machine system.The results show that the improved islanding detection method can effectively shorten the islanding detection time,speed up the detection speed,improve the detection efficiency,and has a good universality.