Current Differential Protection for Wind Farm Transmission Line Based on Fault Region Fitting Coefficient
Hou Bing1, Li Yifan1, Qin Lansha1, Wang Xuliang1, Li Hui2,3
1. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources North China Electric Power University Beijing 102206 China; 2. State Grid Beijing Chaoyang Electric Power Supply Company Beijing 100022 China; 3. Beijing Huairou Laboratory Beijing 101400 China
Abstract：Doubly fed induction generators (DFIGs) are widely used in onshore wind farms. Fault currents in DFIGs are characterized by amplitude limitation and phase angle control, degrading conventional current differential protection based on synchronous power source characteristics. This paper proposes a differential protection scheme using fault region fitting coefficient. It constructs virtual restraint current based on double-end current amplitude coefficients to balance amplitude. An operating boundary model is built using complex domain current vector relationships, determining fault point distribution in the complex plane under internal and external faults. The scheme then constructs the fault boundary in the transient time-domain by deriving trends with transition resistance and restraint coefficient, and adaptively adjusts protection criteria. Firstly, a pilot differential protection scheme based on the fitting coefficient of the fault area is proposed using the transient current information after the fault. To eliminate the differential dead zone problem caused by the weak infeed current on the DFIG side, a virtual restraint current is introduced. Then, based on the differential equation containing the virtual current, a boundary model that can characterize the fluctuation characteristics of the transient current is constructed, and the distribution characteristics of the fault point on the complex plane under both internal and external faults are analyzed through the boundary model. Subsequently, the relationship between the basic parameters of the boundary model and the restraining coefficient, as well as the ratio of the current amplitudes at both ends, is derived using partial differential equations. On this basis, the critical tangential relationship between the transient fault point fitting curve and the boundary circle is utilized to obtain the solving equation for the adaptive fitting coefficient. This equation is used to adjust the restraint level of the differential equation, thereby locking the fault point within the operating region. Finally, the effectiveness of the proposed protection scheme under various fault conditions is verified using a real-time digital simulator (RTDS). The following conclusions can be drawn from the simulation analysis: (1) Compared to traditional models, the proposed protection scheme has low dependence on short-circuit levels. By introducing the ratio of the current amplitudes at both ends to regulate the restraining current, the scheme weakens the differential dead zone problem caused by the weak infeed current on the wind farm side, thereby improving the applicability of the current balance law during internal faults. (2) The proposed model can also simultaneously ensure high-speed operation and reliability. The protection operating equation is adaptively set based on transient electrical information, allowing for fault clearance using only 8 ms of fault current information during the transient time domain. It maintains high reliability even with a 300 Ω transition resistance. (3) The protection setting scheme employs a qualitative rather than quantitative analysis method, providing good adaptability for converter units using mainstream control strategies. Additionally, this method relies solely on current information, eliminating the need for additional voltage transformers, and thus offers high economic efficiency.
2025, Vol. 40 
