Contents I. Introduction
Nowadays, variable-speed wind turbines are the most promising renewable resources that are being deployed in several countries. Among the variable speed wind turbines (WT), those with doubly-fed induction generators (DFIG) have the biggest market share [1]. DFIG WTs have two particular characteristics: 1-their energy source, i.e. the wind power, is intermittent, and 2-they facilitate fast and flexible control through power electronic converters. The latter is usually employed to capture the maximum power from the wind; and therefore, the extracted wind power is a function of wind speed and the WT rotor speed [2]. This is performed by the maximum power point tracking (MPPT) strategy in normal wind speeds [3], [4]. To prevent the WT from mechanical damages, the WT rotor speed and the extracted power are confined in wind speeds greater than the cut-out speed [5]. So long as the penetration level of WTs in power grids is rather low, MPPT is the most appropriate control approach. However, increasing the WTs penetration level tackles several issues for the system [6]. Not participating in the grid inertial response and not providing active operating reserve are two significant issues that need attention. The latter is a permanent problem and requires a perennial production margin, but the former is temporary and makes trouble when a frequency event like a sudden load or generation change occurs. Hence, hereafter these problems are referred to as temporary and permanent problems, respectively.
