# Output Power Smoothing Control for a Wind Farm Based on the Allocation of Wind Turbines
## 3 Power Smoothing Control Strategy  
### Power Allocation of the PWTs, CWTs and the BESS  
The wind turbines of the wind farm were divided into CWTs and PWTs with different control targets in the proposed power smoothing control strategy of this paper. The PWTs adopted the MPPT control strategies by combing the rotor inertia based power smoothing strategy. The CWTs were in charge of the output power smoothing for the wind farm by giving the appropriate power according to the output power need and the right calculations. The BESS with small capacity was configured as the back support of the CWTs to satisfy the high power demanding of the grid. This paper focuses on the wind turbine allocation based power smoothing control strategy itself and then the quantity configuration of the CWTs and PWTs was not discussed due to the limited space. Here, in the wind farm shown in Figure 2, the wind turbines 1-6 were chosen to be the PWTs while the turbines 7-9 were the CWTs. The power allocation method of the WTs and the BESS for the proposed power smoothing control strategy is shown in Figure 4. All the WTs of the wind farm were controlled to output the maximum power if the grid demand power of the wind farm \(P_{given}\) exceeded the maximum capability of the WTs \(P_{\sum\text{max}}\). At this time, the demand power cannot be reached and the BESS will work in the discharging state as shown in the gray box of Figure 4. The discharge power of the BESS was obtained by \(P_{given}\) subtracting the output power of the all WTs. When the maximum output power of the wind farm was more than the demand power, the judgment will be continued. Then when the demand power was more than the sum of the maximum output power of the PWTs and the minimum output power of the CWTs, the BESS will be on the rest as shown in the yellow box of Figure 4 and the PWTs will be controlled to output the maximum power. The reference power of the CWTs can be obtained by \(P_{given}\) subtracting the output power of PWTs. Otherwise, the CWTs were controlled to output the minimum power and the process will continue by comparing the state of charge (SOC) of the BESS with the specified maximum value \(SOC_{\max}\). The BESS was on the rest when the real-time SOC exceeded the \(SOC_{\max}\), as shown in the yellow box of Figure 4. At this time the control target of the PWTs should be changed as the reference power of the PWTs were given by \(P_{given}\) subtracting the minimum output power of CWTs. Otherwise, the BESS will be in the charging state, as shown in the blue box of Figure 4 and the main judgment process will continue. The calculated charging power should be compared with the maximum charging power capability of the BESS \(P_{ess\_max}\). Figure 3: The wake effect model. (**a**) Jensen model principle; (**b**) wind turbine distribution structure. If the calculated charging power was beyond \(P_{\textit{ess\_max}}\), the reference charging power was set to the value \(P_{\textit{ess\_max}}\). Then the reference power of PWTs will be obtained according to \(P_{\textit{given}}\), \(P_{\textit{ess}}\) and \(P_{\textit{control\_min}}\). If the calculated charging power was less than \(P_{\textit{ess\_max}}\), the PWTs will be controlled to output the maximum power and the extra power was stored in the BESS. The red digital markers (1)-(3) in Figure 4 are referring to the general power allocation of CWTs and PWTs. The detailed reference power for each CWT and PWT can be obtained according to the wind speed relationship between each WT. Though the [PERSON] wake effect model is widely used in engineering due to its simplicity, it is not accurate enough in prediction over short-term period, as well as for wind farm application. Thus, the wind speed of each WT needed in the control process was not calculated according to the wake effect model. It can be achieved through the anemoscope simply. Alternatively, short-term wind speed forecasting can be utilized to get the wind speed, which will be studied by authors next.