| ABSTRACT:With the increasing concern with the supply of fossil fuels and the environmental issues in recent years, the use of renewable energy has been gaining widespread acceptance. Distributed generation is an effective method of using renewable energy sources. As a result, more and more renewable energy sources are connected to the grid through inverters. The integration of distributed generation brings an important protection problem in the detection of islanding.Grid-connected inverter is the core of distributed generation system connecting to power grid. This paper chooses the three-phase voltage source inverter as representative for research, and focuses on the three-phase inverter’s mathematical model and its double-loop control strategy. A simulation model of a three-phase grid-connected inverter based on constant power control was built using PSCAD/EMTDC. Then, the simulation model of the test circuit for islanding detection was built.As for the basic theories of islanding detection, the voltage/frequency detection method was analyzed. The non-detection zone (NDZ) of this method in ΔP×AQ coordinate system was analyzed in detail. According to Norton principle, the voltage harmonics detection method was analyzed, and this method does not have the NDZ. However, it may mal-operate during the false islanding and the conventional test circuit is not available for testing voltage harmonics. Meanwhile, the power disturbance anti-islanding detection method is studied. The condition which the power disturbance value meets the detection requirement was identified, and the impact of the traditional power disturbance method on the power quality was mainly caused by current harmonics during disturbance switching.Currently, in the study of the islanding phenomenon, the linear RLC load model is commonly used, but it cannot accurately reflect the problems caused by the non-linear load. In order to better assess the actual performance of islanding detection, a circuit with rectifier load for the islanding test is proposed based on the conventional test circuit. Then, the impact of the rectifier load on the voltage/frequency detection method is analyzed, and the voltage harmonics method is tested. The simulation results show that the rectifier load has caused the NDZ to deviate, and the degree of deviation is related to the parameters of the rectifier load. The results also show that the proposed circuit can accurately reflect the actual response characteristics of voltage harmonics when islanding occurs, and the voltage harmonics is related to the resistor of the rectifier load.Based on the conventional test circuit for islanding detection, an adaptive power disturbance (APD) method is proposed. This method calculates the minimum power disturbance value required to effect islanding detection. Voltage harmonics is used to initiate the APD, thus a novel hybrid anti-islanding strategy is produced. This strategy can detect islanding effectively, alleviating the NDZ. Also, it does not mal-operate during the false islanding and reduces the frequency and the disturbance value of the power disturbance as much as possible, so that the impact on the power quality is very much reduced. Considering the impact of the nonlinear load, the proposed method was amended to ensure the reliability of the detection. Finally, the anti-islanding strategy for microgrid of multi-distributed generators was studied and the anti-islanding control logic for microgrid was given. The simulation results in the conventional anti-islanding test circuit, the test circuit with the nonlinear load and the microgrid test scenario verified the validity and reliability of the proposed strategy. |
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