Dear @Sangwon.Lee,
I’m guessing the slight asymmetry between positive and negative yaw error is a result of wind shear. Even if you’ve set the wind shear to zero in the simulation, there is likely some shear across the rotor anyway as a result of shaft tilt, tower deflection, or floater heel.
FYI: The following more recent forum topic is likely of interest to you: Power loss with yaw misalignment.
Best regards,