## 4 The Coordinated FR Strategy of DFIG and EV
### The Procedure of Coordinated FR Strategy  
The detailed procedure of DFIG and EVA coordinating to participate in system secondary FR are shown below.  
**Step 1:** According to the WS environment, whether the DFIG can be connected to the grid safely or not will be confirmed. If the security of the DFIG connection can be ensured, the current WS region is determined by the WS, and then the DFIG is controlled to operate in the corresponding de-loading state. At the same time, EVs are connected to the grid according to daily driving rules. Under the dispatch of EVA, EVs in the grid are divided into discharging group, charging group, and energy demand group; then, EV's controllable energy for up-regulation and down-regulation can be calculated in real time.  
**Step 2:** Considering the FR capacity and the type of FR, the system FR task \(P_{task}\) will be allocated to DFIG and EVA according to the proposed real-time allocation strategy.  
**Step 3:** Following the FR task allocated by the control center, both the DFIG and EVA output power to participate in FR.  
DFIG: Based on the allocated FR task, the DFIG starts to output power by following the commands from the rotor speed and pitch angle controller. When \(P_{task}^{wind}\) is accomplished, DFIG stops regulating rotor speed and pitch angle, and the actual power output of DFIG is \(P_{a}^{wind}\). If \(P_{task}^{wind}\) has not been accomplished when the rotor speed, pitch angle, or output power has reached the operation limit, the DFIG stops regulating its operation state and remains at current power output \(P_{a}^{wind}\).  
EVA: Based on the allocated FR task, the EVA instructs the EVs inside the cluster to participate in FR. If the FR capacity of EVA is larger than the FR task of EVA, i.e., \(|R^{ev}|\geq\left|P_{ev}^{task}\right|\), EVs in the FR service group will be sorted by the credibility \(\rho_{i}\) of each EV, and then the EV with a lower credibility will be given higher priority to output power for FR until the FR task is accomplished; if \(|R^{ev}|<\left|P_{ev}^{task}\right|\), all the EVs in the FR service group will output power for FR at the same time. The actual power output of EVA is \(P_{a}^{ev}\).  
**Step 4:** If the system FR task can be accomplished by the actual FR power output of DFIG and EVA, i.e., \(\left|P_{a}^{wind}+P_{a}^{ev}\right|\geq\left|P_{task}\right|\), the conventional unit will not participate in FR. If the actual FR power output of DFIG and EVA cannot fulfill the system FR task, i.e., \(\left|P_{a}^{wind}+P_{a}^{ev}\right|<\left|P_{task}\right|\), the shortage will be compensated by the power output of a conventional unit. The FR task allocated to the conventional unit is shown in (21):  
\[P_{task}^{unit}=P_{task}-\left(P_{a}^{wind}+P_{a}^{ev}\right) \tag{21}\]  
Then, considering the limitation of ramp rate, the conventional unit output power according to the allocated FR task.  
In summary, the flow chart of DFIG and EVA coordinating to participate in system FR is shown in Figure 7, where the up-regulation and down-regulation capacity are set to be positive and negative, respectively, for both EVA and DFIG.