Differential Protection of Main Transformer of Doubly-Fed Wind Farm Based on Current Synthesized Vector Trajectory Characteristics

During the fault ride-through process of the doubly-fed wind farms, the short-circuit current provided by the wind farm side exhibits frequency deviation, which leads to errors in the Fourier algorithm based phasor extraction. Additionally, the second harmonic restraint element may act incorrectly, leading to longterm blockage of the differential protection. This paper proposes a novel time-domain protection scheme based on the characteristics of the current synthesis vector trajectory. Based on the intrinsic differences between internal faults and external faults of transformers, two current synthesis vectors are constructed accordingly, and corresponding protection schemes are designed according to the trajectory characteristics of each synthesis vector. A simulation model of the control and protection of the delivery system of doubly-fed wind farms is constructed in PSCAD/EMTDC. Experimental results show that the proposed scheme can identify various internal faults and is not affected by the interference of magnetizing inrush current, current transformer (CT) saturation accompanying external faults and CT measurement errors accompanying external faults. The proposed scheme can operate in 15ms at the fastest and the average operation time is less than a fundamental frequency cycle.