Some work using the SCADA data and the power curve proposed ways to monitor the WTG's performance with the power curve using certain confidence intervals[10] and [11]. The power curve of a WTG is the relation between the wind speed and the produce power, but can also be regarded as an indicator of the performance of the WTG. A change in the WTG's behavior will result in a change in its power curve [10], [12],[13] and [14].