Wake effect

A turbine extracts energy from the air, so the flow leaving it is slower and more turbulent than the flow arriving. Downwind machines sitting in that wake see reduced wind speed and higher turbulence, which costs them output and adds fatigue loading. The wake recovers with distance, but in a dense array it may not recover fully before reaching the next row.

Array losses of several percent are normal and are designed for; the problem is that wakes are strongly direction-dependent. The same turbine can be unaffected on one wind direction and deeply waked on another, so a machine that looks like a chronic underperformer in a headline figure may simply sit downwind of the prevailing direction.

That is why wake has to be modeled rather than averaged away. Attributing a loss correctly means separating what the array geometry explains from what the equipment explains - otherwise a waked turbine gets investigated for a fault it does not have, while a genuine yaw misalignment hides inside an expected array loss. Comparing each machine against its own matched-condition power curve is what keeps the two apart.