An Improved Active Frequency Offset Based On Positive Frequency Feedbackisland Detection Study

The increasingly depletion of non-renewable energy sources such as coal,oil and natural gas and the rapid growth of electricity load have effectively promoted the rapid development of distributed power generation systems containing renewable energy.The integration of a large number of distributed generation units reduces the stability of the system.Therefore,the research on system integration and islanding operation has become a research hotspot for distributed generation.Unplanned islanding will endanger the lives of power maintenance personnel and will affect the stable operation of the power system,thus it is of importance to accurately and quickly detect the islanding state of the system.Firstly,this thesis focuses on the principle and related standards of the islanding detection method at home and abroad.The research on the passive harmonic detection methods includes voltage harmonic detection method,over/under voltage detection method,over/under frequency detection method,frequency change rate detection method and voltage phase mutation detection method,and the research on the active detection methods includes synovial frequency deviation method and active frequency deviation method.Several evaluation coordinate systems for detecting the effectiveness of the islanding algorithm are studied.After comparison,the coordinate system description method of the load characteristic parameter is selected to reflect the blind zone of the islanding detection algorithm.A comparative study of the active frequency deviation method and the frequency deviation method with positive feedback is employed in the thesis,in which the detection principle and the distribution of blind zone are studied and the similarities and the differences of the two methods are compared.The improvement thoughts and effects of the active frequency deviation detection method with positive feedback are studied to find out the shortcomings of the detection and thereby providing solutions for the improvement of the detection method.Secondly,an improved algorithm for active frequency deviation detection with positive feedback is proposed.With respect to the problems of large detection blind zone,slow detection time and large harmonic distortion rate existing in the original algorithm,the reasons of the above problems are studied based on the mathematical principle of improving algorithm.Considering the normal fluctuation of the power grid,the system reduces the influence on the grid current when no islanding are detected,reduces the distortion rate of the current,and speeds up the detection speed when islanding occurs.Based on the mathematical principle,the working principle of the improved algorithm and the detection blind zone are studied,and the feasibility of the improved algorithm is illustrated.Lastly,the active frequency deviation method,the active frequency deviation method with positive feedback and the improved active frequency deviation method with positive feedback are simulated by Matlab/Simulink simulation software,respectively.The rapidity of the detection algorithm and the total distortion of the grid-connected current harmonics are compared.The simulation results show that the detection time of the above three detection methods is 0.2s,0.1s,and 0.06s in the same situation,and the total harmonic distortion of current is 4.33%,4.29%,and 3.63%,respectively.The proposed improved algorithm improves the speed of island detection and reduces the total harmonic distortion rate of the current.