## 2 Frequency Regulation Model of DFIG
### De-Loading Strategy for FR Reserve Capacity
#### 2.2.2 Division of Wind Speed Region  
For participating in down-regulation, the DFIG can reduce the power output directly through controlling rotor speed or pitch angle. However, DFIG cannot participate in up-regulation when operating in MPPT state since the power output has already reached the maximum value. In order to obtain up-regulation reserve capacity, the DFIG needs to operate in a de-loading state. In order to  
Figure 1: Change rule of DFIG (double-fed induction generator) blade power coefficient.  
Figure 2: Normal operation of DFIG (double-fed induction generator). (**a**) DFIG power output change when \(\beta=0^{\circ}\); (**b**) Normal operation trajectory of power output and pitch angle.  
realize the de-loading of DFIG, the rotor speed needs to deviate from the optimal speed. The suboptimal operation state of rotor corresponds to two rotor speeds under the same WS, i.e., the low-speed and over-speed suboptimal rotor speed. In the actual situation, the over-speed suboptimal trajectory is usually adopted in order to maintain DFIG's stable operation.  
However, in consideration of the operation limit of rotor speed, the de-loading level may not be satisfied if the power output regulation is only based on rotor speed control. In this situation, the pitch angle control will be activated to keep DFIG power output at a designated de-loading level. In order to realize the smooth transition between rotor speed control, pitch angle control, and their combination in the de-loading strategy, reasonable division of the WS region according to the operation characteristic of DFIG is important.  
The whole WS range is divided into three regions considering the operation characteristic of DFIG, including the start-up region, the FR region, and the cut-out region. In the start-up region, the power output of DFIG is too low for providing frequency support; in the cut-out region, the DFIG will exit the grid as the WS is too high. Therefore, the DFIG will only participate in FR in the FR region. Further, considering the operation limit of DFIG, the FR region should be divided into three regions, that is, the low, medium, and high WS region. The division principle of WS regions is elaborated below.  
Firstly, in order to ensure that the up-regulation capacity can be fully obtained, the WS at the lower boundary of low WS region should meet the requirement that the maximum point of DFIG power output can be reached when the rotor speed is at the lower limit. Secondly, when suboptimal rotor speed corresponding to designated de-loading level has reached the upper limit, the WS will be set as the boundary of low and median WS region, and it is where the pitch angle control will start to work. Then, the rated WS is set as the boundary of median and high WS region, where the upper limit of rotor speed equals to the optimal speed; thus, only pitch angle control will work on the de-loading in high WS region. Finally, the cut-out speed of DFIG \(v_{\text{cut-out}}\) is set as the higher boundary of high WS region. The division principle and corresponding calculated values are shown in Table 1, where \(d\%\) is the de-loading rate, which is set to be \(10\%\) in this paper [5] and can be adjusted according to the actual situation.