## 1 Introduction
As a kind of existing energy storage resources, electric vehicle (EV) can take the place of additional energy storage equipment in assisting WT to participate in FR without extra cost, as well as improve system frequency response on both load and source side at the same time owing to its dual role. In Ref. [11], a charging and discharging control strategy for EV cluster to participate in secondary FR of a two-area interconnected system containing WTs is put forward. An optimal charging/discharging scheduling strategy of EVs is proposed in [12] to improving the frequency stability of micro-grid with penetration of wind power. However, instead of being one of the FR participators, wind power model is only taken as random disturbance of power generation considering its intermittent characteristics in these mentioned researches, which will increase the pressure of system FR. Therefore, under the background of large-scale wind power integration, it is necessary to appoint a part of WTs in the grid as auxiliary frequency regulation units, sharing FR pressure with EV so as to stabilize system frequency. In Ref. [13], the coordination of EV cluster and WT in system FR is realized by hierarchical control algorithm, in which EV cluster participate in FR in the lower layer, while WT only participates in generation dispatching plan in the upper layer. The coordinated control strategy of WT's pitch angle and EV based on model predictive control is proposed in Ref. [3] for micro-grid FR, but the control imposed on WTs is used for smoothing its power output, so only EV cluster responds to the frequency deviation signal. Two coordinated FR methods for EV and WT based on centralized and distributed control, respectively, are analyzed in Ref. [14], but in the proposed control structures, EV and WT output power for FR separately without the allocation of demand active power between them, which means that the FR tasks of EV and DFIG are not coupled with each other, which may cause the system to demand power that cannot be effectively fulfilled or the waste of power output. Moreover, in the most relevant researches, the influence of FR reserve capacity of DFIG or EV on the power output limit is not considered. Therefore, it is necessary to study the allocation method of system FR task considering the FR capacity so as to achieve the mutually coupled coordination of DFIG and EV in FR. For the sake of solving the problems mentioned above, the main contributions of the paper are listed as follows:  
* Through reasonable dividing of WS regions and operation point selection of pitch angle, a modified de-loading strategy of DFIG for FR reserve capacity is proposed, which realizes the continuous change of rotor speed or pitch angle and guarantees expected FR capacity in every WS zone. * Combining rotor speed and pitch angle control, the FR model of DFIG for both up-regulation and down-regulation under continuously varying WS in whole WS range is established. * In the premise of meeting the driving demand of EV owners, based on the real-time allocation strategy of system FR task considering the real-time predicted FR capacity of DFIG and EV cluster, the coordinated control strategy of DFIG and EV in system secondary FR is presented. The remainder of the paper is organized as follows. Section 2 presents the de-loading strategy and FR model of DFIG based on the combination of rotor speed and pitch angle control. Then, Section 3 elaborates the FR control strategy of EV cluster considering the driving demand of vehicle owners. The coordinated FR strategy of EV and DFIG based on the real-time allocation of system FR task is described in Section 4. The analysis of simulation results is shown in Section 5. Finally, conclusions are drawn in Section 6.