# Fault Studies and Distance Protection of Transmission Lines Connected to DFIG-Based Wind Farms
## 1 Introduction  
In addressing the problems of environmental pollution and energy shortages, the world has been focusing on the development and use of wind energy, and large-scale wind farms are being increasingly integrated into the power grid. Wind turbines based on doubly fed induction generator (DFIG) have been widely used in existing wind farms [1; 2] due to the benefits including flexible power control, high efficiency, and small converter capacity [3; 4; 5]. However, DFIG is sensitive to grid disturbances, especially to voltage sags. The grid voltage sags cause a large transient overcurrent in the DFIG rotor circuit [6; 7]. As a result, the DFIG is disconnected from the grid to avoid damage to the rotor side converter (RSC). With the increasing popularity of wind power, the grid codes [8; 9; 10] require the wind farms to remain connected to power grid during grid-voltage sags to ensure system stability. Therefore, low voltage ride through (LVRT) techniques are proposed to protect the converter, meaning that the DFIG does not need to be disconnected from the grid during disturbances [11; 12]. A commonly adopted solution to protect the DFIG converter during voltage sags is to use a crowbar circuit to isolate the RSC from the rotor circuit [13; 14]. Many studies have been performed to investigate the DFIG fault characteristics considering LVRT capability. Due to the influence of power electronic devices, the DFIG fault characteristics are different from the traditional synchronous generator. The behavior of the DFIG during three-phase voltage sags was analyzed [15]. The authors divided the magnetic flux of the machine into forced flux and natural flux to help understand the causes of the overcurrent that appear during a fault. The physical behavior of the DFIG was presented and later developed with analytical solutions for the generalized DFIG equations operating under symmetrical fault conditions [16]. Comparing the conventional induction machine with DFIG, the short-circuit current expression of a crowbar-protected DFIG and the maximum value of the short-circuit current were outlined [17; 18]. The DFIG operation under asymmetrical voltage sags and why such sags are more harmful than symmetrical sags were explained [19]. In addition, the DFIG fault current characteristics under non-severe fault conditions were studied [20; 21; 22]. Under non-severe faults, the crowbar protection is not activated and the DFIG fault current characteristics are considerably different than that under severe fault conditions. Notably, the fault characteristics of DFIG with LVRT capability are influenced by the control strategy. A detailed analysis was completed by considering the influence of the controller on short-circuit parameters, such as time constants, initial symmetrical short-circuit current, and the peak short-circuit current [23]. Hence, many researchers have attempted to improve the DFIG control strategy to meet the grid code requirements [24; 25; 26]. DFIG pose new problems and challenges for the traditional relaying protection of the power grid. Although the aforementioned DFIG fault characteristics have been scrutinized in many publications, the negative impacts of DFIG on the protection of the transmission system, and particularly distance protection, have not received sufficient attention. Large-scale DFIG-based wind farms are generally integrated into power grids with transmission lines, and distance protection is usually used as either the main or the backup protection for the transmission line [27]. Refs. [28; 29] indicated that traditional distance protection and directional elements are unreliable when a balanced fault occurs because the fault current frequency of DFIG significantly deviates from the nominal frequency. The impacts of grounding configurations on ground protective devices used in DFIG-based wind energy conversion systems were studied in [30; 31].