  The estimation of the rotor effective wind speed is used in modern wind
turbines to provide advanced power and load control capabilities. However, with
the ever increasing rotor sizes, the wind field over the rotor surface shows a
higher degree of spatial variation. A single effective wind speed estimation
therefore limits the attainable levels of load mitigation, and the estimation
of the Blade Effective Wind Speed (BEWS) might present opportunities for
improved load control. This letter introduces two novel BEWS estimator
approaches: A Proportional Integral Notch (PIN) estimator based on individual
blade load measurements, and a Coleman estimator targeting the estimation in
the non-rotating frame. Given the seeming disparities between these two
estimators, the objective of this letter is to analyze the similarities between
the approaches. It is shown that the PIN estimator, which is equivalent to the
diagonal form of the Coleman estimator, is a simple but effective method to
estimate the BEWS. The Coleman estimator, which takes the coupling effects
between individual blades into account, shows a more well behaved transient
response than the PIN estimator.
