Shaheena Khanum
This paper describes a full modeling and control scheme of a grid-connected Permanent Magnet Synchronous Generator (PMSG) based wind energy conversion system. The main goal is to keep a stable DC-link voltage and to work at a unity power factor. The suggested system design consists of a wind turbine coupled to a PMSG, where the PMSG variable frequency output is rectified to a DC connection and then inverted to grid compatible AC. The mathematical model of the PMSG is created in the spinning dq-reference frame. This permits decoupled torque and flux control using stator voltage dynamics. The electromagnetic torque is proportional to the q-axis current. The mechanical dynamics of the turbine-generator system is described by the swing equation. The control system is hierarchical. The outer Proportional-Integral (PI) controller controls the voltage of the DC-link and generates the reference d-axis current which determines the active power flow. The reference q-axis current is adjusted to zero in order to keep the unity power factor and avoid reactive power exchange with the grid. These reference currents are then used as inputs to inner current control loops that calculate reference voltages for the pulse width modulation inverter. A three-phase Phase Locked Loop (PLL) synchronizes the generator and is used to monitor the grid phase angle and frequency. The active and reactive power consumption is calculated using the dq components in synchronism. The main contribution of this work is the systematic inclusion of the PMSG model, the DC-link regulation and the dq-axis current control into one framework. This allows the continuous transmission of active power with small variations of reactive power. Moreover, the proposed method offers an effective framework for harmonic analysis and system optimization in wind energy applications.