The Research Of Anti-islanding Strategy In The Grid-connected PV System

Solar photovoltaic power generation is the use of photovoltaic arrays photovoltaic effect ofsunlight change to electricity. Solar photovoltaic Despite has the many advantages to the power gridcaused by the photovoltaic power generation system and network security risks have to consider theisland effect is often an extremely important issue need to be addressed in the photovoltaic powergeneration system and network, solitary island effect （Islanding effect） refers to the distributedpower supply connected to the grid, large grid stop to some of the load, the distributed power supplystill to this part of the load due to various reasons, this part of the load referred to as the local loaddistributed power systems, if not timely and effective way to detect grid off state and within thestipulated time and the grid is disconnected, there is likely to endanger the personal safety of theentire grid and maintenance staff. Anti-islanding （Anti-islanding） strategy is particularlyimportant.The object of this article with small single-phase photovoltaic system, grid-connected inverterselection of voltage type, the input side of the parallel capacitor, the output current control. Invertermain circuit uses a transformer of the two structures, the front-end circuit DC/DC Converters andpost-stage DC/AC circuit, the front-end circuit PV array output DC voltage Upv boost to the Ud, theUd and then by a stage phase full-bridge inverter circuit inverter220V/50Hz AC. The test circuituses the international popular EU test circuit to determine the instantaneous change of the electricalparameters of the public point of detection and network islanding operation status.This paper focuses on active frequency shift method （active frequency drift: of AFD）islanding detection method. To influence changes in the frequency of common point voltage byincreasing or decreasing of the inverter output current frequency, and find a solution strategy for theinadequacies of the AFD detection. Positive feedback in order to enable fast response time, imposedon the disturbance signal cf. The test load RLC parallel resonant load to simulate the reality of theload, the study found that the nature of the load of AFD, especially the capacitive load detectionfails, the capacitive load voltage lag the current phase angle, which makes load slower than theresistive voltage zero-crossing moment came and passed to the common point voltage andfrequency disturbances therefore partially offset, completely offset, the test fails. In view of thissituation, in order to better detect the islanding operation status, the paper on the AFD method someimprovements, both positive and negative direction of the disturbance applied on the inverter outputcurrent frequency, this method for two-way positive feedback active frequency offset shift method（bidirectional positive feedback active frequency drift: BPFAFD）.Finally, using Matlab simulation to verify the BPFAFD detection validation results for thedifferent nature of the load detection results are good, and can detect islanding within0.5seconds.