عملکرد DFIG (ماشین‌های الکتریکی تغذیه دوسویه) در زمان اتصال به شبکه در وضعیت ولتاژ نامتعادل شبکه

ABSTRACT Operation of Grid-Connected DFIG (doubly fed induction generator) Under Unbalanced Grid Voltage Condition

THE installed capacity of wind power in Europe reached approximately 33,600 MW until the end of 2004. The demand for connecting large-scale wind parks to the power grid is still on the rise. The increasing size of wind turbine and wind park resulted in new interconnection rules or grid codes. Today, there is a need to control wind power, both in active and reactive power, and to be able to stay connected with the grid when grid faults happen. The doubly fed induction generator (DFIG) has the largest world market share of wind turbine concepts since the year 2002 [1], because of its ability to provide variable speed operation and independent active and reactive power control in a cost-effective way. As DFIG’s stator is directly connected with ac grid, it has poor low-voltage ride through (LVRT) capability due to the poorly damped flux oscillations during grid voltage dip. Many studies have been conducted on the LVRT capability of DFIG [3]–[6], [9], with most of them focused on the behavior and protection of DFIG under symmetrical fault. During the symmetrical fault, transient overcurrent in the rotor is identified as the most severe LVRT problem of the DFIG, because the rotor-side voltage source converter (VSC) is very sensitive to thermal overload. Thus, the active crowbar protection is designed to short-circuit the rotor under symmetrical grid voltage dip, both to protect the rotor-side VSC and to damp out the oscillations faster. In reality, however, unsymmetrical fault happens much more frequently than symmetrical fault. Furthermore, it is identified that the unbalanced voltages can occur in a weak power grid even during normal operation [7], [8], [16], [17]. Under the unbalanced grid voltage, the most severe operation problem of DFIG may not be the transient overcurrent, but the large electric torque pulsation that causes wear and tear of the gearbox, and large voltage ripple in the dc link of back to-back VSC that may decrease the lifetime of the dc capacitance. Literatures [10], [11] define the instantaneous active and reactive power that can be used to design and operate the grid connected VSC under unbalanced grid situations [12]–[15]. Literatures [16], [17], [19] on the control system of DFIG under unbalanced grid voltage condition [16] used a feedforward loop on the classical-field-oriented current (FOC) controller to limit torque ripple, which was simple and robust. But its performance depended on the filter. In [17], the grid VSC was controlled as a STATCOM and supplied reactive power to compensate the unbalanced grid voltage. Its performance depended on the current ratings of grid VSC and impedance between generator terminal and fault location. However, a theoretical analysis of DFIG under unsymmetrical fault is still missing, and a systematic approach to limit both torque pulsation and dc voltage ripple is required. In this paper, the behavior of DFIG under unbalanced grid voltage is thoroughly analyzed, and a dual-sequence FOC controller is proposed. The rotor VSC is controlled to limit the torque pulsation, and the grid VSC is controlled to limit the dc voltage ripple. A dynamic model of DFIG is made in MATLAB/Simulink. Simulation results prove the effectiveness of the proposed control system.