In this study, the physics and effectiveness of active yaw control under various wind conditions are investigated systematically, based on wind tunnel experiments and a new analytical wind farm model. The power and wake velocity measurements of a three-row miniature wind farm reveal that the peak power gain (18%) is reached in partial-wake conditions, when the wind direction is misaligned with the turbine column by 2-4 degrees. In contrast, the power gain in the full-wake condition (5.4%) is a local minimum. For a single-column wind farm, the optimal yaw angle distribution always exhibits a decreasing trend from upstream to downstream, which can be associated with the secondary wake steering effect. Analytical model predicts that with increasing number of rows, both the peak power gain and the leading-turbine yaw angle increase asymptotically. The maximum value of the yaw angle is mainly determined by the cosine exponent of the thrust coefficient (p). With a typical value of p = 1.8, the maximum yaw angle value is approximately 30 degrees. Turbulence intensity and streamwise spacing have similar effects on active yaw control. When these two parameters increase, the relative power gain decreases monotonically. (C) 2021 Elsevier Ltd. All rights reserved.