# Output Power Smoothing Control for a Wind Farm Based on the Allocation of Wind Turbines
## 4 Simulation Results
The DC link voltage can remain constant as shown in Figure 12f with little fluctuation around 5 V in the steady state. The maximum energy function of the MPPT power, the output power of WTs and output power of the wind farm is shown in Figure 12g. It reflects that the maximum energy function of the output power of wind farm was slightly less than the MPPT output power. The energy loss mainly happened in the PWTs by adopting the kinetic energy based power smoothing control. The power smoothing function is shown in Figure 12h, from which it can be seen that the value of output power by WTs was  
Figure 11: Time histories of the PWTs in the wind farm under the power smoothing operation mode. (**a**) Rotor speed of PWTs (1–3). (**b**) Torque of PWTs (1–3). (**c**) Output power of PWTs (1–3). (**d**) Rotor speed of PWTs (4–6). (**e**) Torque of PWTs (4–6). (**f**) Output power of PWTs (4–6). obviously smaller than that of MPPT control. Then, it can be summarized that the proposed method can realize the good performance of power smoothing. The power smoothing effect was better when the BESS is joined in as shown in Figure 12h. In some situations, the grid needs the wind farm to have the ability to output the specified power to meet the balance of the grid and loads. Thus, the third working mode of the wind farm was to output the determined power. The step-changed power demand (6 MW, 4 MW, 8 MW, 6 MW, 7 MW) for the wind farm was given in the simulation. The reference demand power was given to test the effectiveness of the proposed method in both cases of power increasing and power decreasing. Besides, the maximum reference demand power is limited by the sum of the maximum output power of wind turbines and the BESS. The simulation results of this case are shown in Figure 13. Being the same as  
Figure 12: Time histories of the CWTs and the wind farm under the power smoothing operation mode. (**a**) Output power of CWT (WT7). (**b**) Output power of CWT (WT8). (**c**) Output power of CWT (WT9). (**d**) Power of the BESS. (**e**) Total output power of the wind farm. (**f**) DC link voltage. (**g**) Maximum energy function. (**h**) Power smoothing function. that in the second working mode, the CWTs were given the reference power calculated according to block diagram Figure 4 after 1 s. The simulation results of PWTs were the same as that shown in Figure 11 and not given here. The output power of CWTs is demonstrated in Figure 13a-c, from which it can be seen that the CWTs outputted constant minimum power 200 kW between 2 s to 4 s due to the low grid demand power. The power of the BESS is shown in Figure 13, and it can be found that the BESS was working in the charge, discharge, and rest working modes according to the different power grid demands. The output power of the wind farm can follow the given step-changed demand power as shown in Figure 13e. As displayed in Figure 13f, the DC link voltage basically remained at the reference value, but with acceptable mutation due to the sudden power changes. The new wind speed condition of CWTs was given in the simulation to increase persuasion of the proposed power smoothing strategy, while the wind speed of the PWTs was the same as that in Figure 9. The wind speed of CWTs is shown in Figure 14a, from which it can be seen that the shape of downstream WT was different from the upstream WT. Then, the rotor torque of each CWT had different shapes, as demonstrated in Figure 14b. The actual output power of the three CWT can match the reference given power well as shown in Figure 14c-e. Figure 14f shows the output power of the wind farm and it can be observed that the output power was smoothed by comparison with the reference MPPT power. Figure 13: Time histories of the wind farm under the specified power given operation mode. (**a**) Output power of CWT (WT7). (**b**) Output power of CWT (WT8). (**c**) Output power of CWT (WT9). (**d**) Total output power of the wind farm. (**e**) Power of the BESS; (**f**) DC link voltage.