Simulation Study On Active Frequency Drift Method Of Island Detection In Photovoltaic Grid-connected System

Island operation is easy to happen when the distributed photovoltaic system connects to the grid,which not only damages power grid and power equipments,but also endangers the safety of the operators.Detecting the island quickly and accurately is the key for the safe operation of the power grid.Based on the detailed analysis of two active frequency shift islanding detection methods of active frequency drift(AFD)and active frequency drift with positive feedback(AFDPF),an improved low harmonic AFDPF island detection method is proposed in this paper.Meanwhile,the proposed method is studied in the grid-connected model of the single-phase distributed photovoltaic system.The main works include:(1)The non-detection zone(NDZ)that evaluating the effectiveness of the islanding detection methods was deeply analyzed,and the grid-connected model of the single-phase distributed photovoltaic power generation system was built in the Matlab/Simulink simulation software,and the module parameters were debugged to make the inverter output current tracking the grid voltage well.Meanwhile,the NDZ of active frequency shift method is described in the Qf0×Cnorm Coordinate system,and the relationship between disturbance parameters of active frequency shift and NDZ was obtained.In addition,the relationship between the disturbance parameters of AFDPF method and the islanding detection time,the total harmonic distortion(THD)of the inverter current was also concluded.The detection condition of the method without NDZ is obtained.(2)A low harmonic active frequency drift algorithm with positive feedback with sign function was proposed in order to solve the problem that the chopper factor and the frequency positive feedback coefficient were same and invariable whether the frequency fluctuations were in the normal or abnormal range by the traditional method of the AFDPF.Mean while,the simulation model of islanding detection for distributed single-phase photovoltaic power generation system was established.Compared with the traditional method,frequency of public point voltage deviated from the grid frequency more quickly in proposed method,and the islanding detection time was shortened by 0.0603s,and the THD of inverter output current was reduced by 0.47%,and the detection was not affected by the response of the load characteristic.(3)In order to solve the problem that the effect of islanding detection was reduced due to the influence of inverter on the operation of multi inverter grid-connected operation,the application of active frequency shift method in multiinverter was analyzed.The islanding detection in the distributed photovoltaic system was simulated by using the proposed method of the improved active frequency shift under the two different conditions of the same disturbance and different disturbance for the inverter while the dual inverter using the new island detection method.The results show that the proposed method had good detection effect.The new method was suitable for all Qf0<2.5 loads under the multi inverters grid-connected with the grid.