Research On The Islanding Detection Of Three-Phase Grid-Connected Inverter With Active Negative-Sequence Current Disturbing

As one of the key interfaces between the power grid and the renewable energy sources such as solar energy and wind energy etc, the islanding detection has already become one of the necessary functions for the grid-connected inverter, based on the requirements for operation of the power grid, maintenance of the distributed system and safety of the electrical equipment etc. As traditional method of single-phase islanding detection has been practiced, a new method of islanding detection will be put forward, which based on the three-phase grid-connected inverter and the distributed power generation system. This thesis focuses on the method of islanding detection of three-phase grid-connected inverter for the distributed power generation system, which possesses the advantages of high efficiency, high reliability and high flexibility of islanding detection and protection.Based on the three-phase power generation system, the output current of the three-phase inverter can be analyzed into positive-sequence, negative-sequence and zero-sequence. Considering the characteristics of negative-sequence disturbances do not bring any problem when the grid is normal; the negative-sequence of the inverter current will change obviously, and the output voltage achieve the value of the islanding protection when the grid is abnormal. A new method of islanding detection of three-phase grid-connected inverter with active negative-sequence current disturbing will be researched and expounded in this thesis. The results of theoretical analysis and MATLAB simulation illustrate that this method can detect the islanding phenomenon fast and without dead-zone within 3 cycles. Considering the principle of negative-sequence detection, a new negative-sequence extractor was designed, which can extract negative-sequence flexibility and detect negative-sequence with high efficiency. Regard to the experimental verification, 3 kVA prototype experimental device of islanding detection of three-phase grid-connected inverter was designed, which can afford experimental verification and performance analysis for the method of islanding detection of three-phase grid-connected inverter with active negative-sequence current disturbing, and experiments illustrate that the negative-sequence of inverter current approximately inexistence and seemly disturbances do not bring any problem when the grid is normal; this method can detect the islanding phenomenon within 3~4 cycles when the grid is abnormal. At the same time, through the research of islanding detection based on double three-phase converters system , the islanding detection of double three-phase converters system with negative-sequence current injection has been discussed simply.