## 6 Case Study
### Four-Units Cluster Active Frequency Support Test  
Similar to the above cluster, four units in one wind farm are selected as a flexible scheduling cluster to support the wind farm power output and frequency support ability. The four units are included in a 100 MW wind farm connected in a IEEE standard 14-bus electric power system. The wind farm contains 20 wind turbines and each unit has a capacity of 5 MW. The entire wind farm is divided into five clusters according to the connection of electric power transmission lines. Each power transmission line is connected to four units which can be scheduled as a cluster. The topological structure and unit connection mode of the wind farm are set as in Figure 11, where each unit is connected by a 1.5 km electric power transmission line. The four units used in above simulations are connected to one power transmission line as a cluster. Another failure cluster is founded with four units of which the actual wind turbines are replaced by amplifiers with static gain of 1 to simulate the active power transient change of the wind farm. The wind farm controller superimposes two signals with equal amplitude but opposite direction on the basic active power set-point and sends them to the selected cluster and failure cluster, respectively, and the selected cluster acts as required to test the active power  
Figure 10: Four-units cluster robustness test—comparison between the proposed strategy and the conventional strategy under varying power demand, sub-figures a, b, c, d are the detailed comparison for wind turbine 1, 2, 3, 4 respectively.  
Figure 9: Four-units cluster robustness test—cluster power output under varying power demand.  
and frequency support ability for the wind farm. The other 12 conventional units are working by following a constant power set-point to avoid the unnecessary interference.  
The selected cluster is simulated under two types of test singles. The first active power set-point is the step change single to test the cluster active power and frequency support ability under a unit emergency cut out/start situation. Under the first condition, several units are selected to cut out or join up to provide a step change active power signal and the frequency response of the wind farm are plotted as in Figure 12. The cluster power set-point is the same as the above simulations.  
In Figure 12, the blue curves are the wind farm frequency response under the PD control strategy, while the orange one is the corresponding to the proposed strategy. Clearly, under the proposed strategy, the wind farm frequency response is much more stable than the conventional strategy. When the wind farm active power changes, the frequency is stabilized quickly at the initial stage. Once the power set-point step changed, the frequency under the proposed controller has a lower overshoot and faster adjusting time than the conventional controller. In addition, during the time range when the cluster power set-point remains still, the frequency response under the proposed  
Figure 11: Units connection structure in the tested wind farm.  
Figure 12: Wind farm exit node frequency response.  
controller has little fluctuation, which indicates that the cluster inner disturbance caused by natural wind turbulence and units' inertial difference can be significantly restrained by the proposed controller.  
Note that, in Figure 12, the wind farm frequency responses under different cluster control methods show a similar trend but not completely the same. This is due to the low frequency oscillation of power grid, which is still a challenge problem in the power system. The proposed strategy mainly focuses on unit active power control and optimization within a cluster concept, which cannot handle the low frequency oscillation problem.