## 1 Introduction  
Emerging environmental concerns and attempts to curtail the dependence on fossil fuel resources are bringing renewable energy resources into the mainstream of the electric power sector. Among the various renewable resources, wind power is the most promising from both technical and economic standpoints. The new global total for wind power at the end of 2015 was 432.9 GW, which represents a cumulative market growth of more than 17% [1]. By 2030, wind power could reach 2110 GW and supply up to 20% of the global electricity [2]. This large penetration of wind power into the existing grid has introduced some vulnerabilities to the power grid. In order to maintain the stability of the power system and ensure smooth operation, low voltage ride through (LVRT) requirements have been imposed around the world [3]. In the event of a fault, LVRT mandates that wind farms (WFs) stay connected to the grid in order to support the grid in the same manner as conventional synchronous generators (SGs). Most wind turbines are constructed using fixed-speed wind turbines with squirrel cage induction generators (FSWT-SCIGs). SCIGs have some advantageous characteristics, such as simplicity, robust construction, low cost, and operational simplicity [4]. However, FSWT-SCIGs are connected directly to the grid and have no LVRT capabilities during voltage dips [4]. Moreover, the FSWT-SCIG requires a large reactive power in order to recover air gap flux when a short circuit fault occurs in the power system. If sufficient reactive power is not supplied, the electromagnetic torque of the SCIG decreases significantly. As a result, the rotor speed of the SCIG increases significantly and can make the power system unstable [4]. Reactive power compensation is a major issue, especially for FSWT-SCIGs. A capacitor bank is usually used to meet the reactive power compensation requirement of an SCIG. However, the SCIG requires more reactive power during fault conditions than in the steady state, and the capacitor bank is not able to supply more reactive power during transient conditions. A static synchronous compensator (STATCOM) [5], superconducting magnetic energy storage (SMES) [6], and an energy capacitor system (ECS) [7], for example, are installed in WFs with FSWT-SCIGs in order to improve the LVRT capability during a fault condition. However, the overall system cost increases. On the other hand, variable-speed wind turbines with doubly fed induction generators (VSWT-DFIGs) have some advantageous characteristics, such as light weight, higher output power and efficiency, lower cost, variable-speed operation, and smaller size. In addition to the lower power electronic converter rating required by the DFIG, compared to permanent magnet synchronous generators (PMSGs) [8], the recent price upsurge of permanent magnet materials has given the DFIG another advantage over the PMSG [9]. In addition, the DFIG has better system stability characteristics than the SCIG during fault conditions, because of its capability for independent control of active and reactive power delivered to the grid [10]. By taking advantage of DFIG reactive power control, it is possible to stabilize the SCIG in a WF. Thus, reactive power compensation can be implemented at lower cost. The partial converter is connected to the rotor terminal of the DFIG via slip rings. The converter consists of a rotor-side converter (RSC) and a grid-side converter (GSC). As reported in previous studies [10; 11; 12], various control strategies can be adopted for both the RSC and the GSC. However, the design procedure of the RSC is very crucial because it is controlling active and reactive power delivered to the grid. Some auxiliary hardware circuits have been used to help the DFIG to improve the LVRT requirement. For example, the rotor crowbar circuit is used in the rotor terminals to isolate the RSC from the rotor circuit [13; 14]. However, the rotor crowbar circuit converts the DFIG to a simple induction machine, which absorbs reactive power from the grid.