The external signal, used to modify the generator torque (or the active power) reference, can be a function of the grid frequency or a pre-determined value. Based on the external signal employed to emulate the inertia, mainly two different approaches can be identified among the proposed approaches so far: Governor-Inertia Controller , , , , , , , , , , , , ,  and  and Step over Production (SOP) approach , , , ,  and . The Governor-Inertia Valspodar in essence a Proportional-Derivative (PD) controller with grid frequency as an input. In the SOP, the active power reference of the WT is increased for a predetermined duration to balance the supply demand mismatch. After the inertial support period, the active power reference is reduced below its optimum value to regain the kinetic energy. The magnitude and the duration of the injected power can be adjusted to achieve the desired inertial response. The major challenge, here, is the fine-tuning of the magnitude and duration of macronutrients action to improve the inertial response.