A novel sensorless control scheme is proposed for an interior permanent-magnet synchronous motor drives at very low speed based on sliding-mode observer (SMO) in this study, which substitutes a sigmoid function for the sign or saturation function with a variable boundary layer, overcomes the time delay caused by the low-pass filter and the chattering problem used in the conventional SMO. In this study, an adaptive SMO is employed to estimate the rotor speed and extended electromotive force, a three-phase soft phase-locked loop technology is used to estimate rotor position because of its strong robustness since under voltage phase unbalance or polluted and variable-frequency environment. The global asymptotic stability of the proposed adaptive SMO is verified using Lyapunov stability analysis with considering motor parameter variations. Experimental results are presented to verify the principles and to demonstrate the effectiveness of the proposed method at very low speed from 2 to 17 r/min.

-In the process of virtual inertia control (VIC), the frequency regulation capability of the directly-driven wind turbine with permanent-magnet synchronous generator (D-PMSG) on wind farm is related to its rotor kinetic energy and capacity margin. This paper proposes the method for assessing the D-PMSG frequency regulation capability and defining its coefficient according to the operating state of wind power generators. In addition, the calculating method of parameters in VIC is also discussed according to the principles of primary frequency regulation and inertia response of synchronous generators. Then, by introducing the capability coefficient into the proportion-differential virtual inertia control (PD-VIC) for power coordination, a coordinated virtual inertia control (C-VIC) strategy is developed, with the consideration of the difference in frequency regulation capability between wind power generators. The proposed control method can not only give full play to the frequency regulation capability of wind power generators, decrease the movements of the pitch angle control system but also bring some self-coordination capability to different wind power generators thus to avoid a secondary drop in system frequency. The simulations and experiments prove the proposed method to be effective and practicable.

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