Elliptical-Curve-Based Method for Precise Calculation of Maximum Rotor EMF in DFIG Under Grid Faults

The doubly-fed induction generator (DFIG) is prone to losing control due to high rotor EMF during severe grid faults. Therefore, calculating the maximum rotor EMF of DFIG under different grid faults is the theoretical basis for various low voltage-ride through (LVRT) strategies. In traditional calculation methods, the amplitudes of the positive sequence, negative sequence, and transient dc components of the EMF are summed simply, but each component may not reach its maximum value simultaneously, leading to errors. In this article, the electromagnetic processes of the DFIG are investigated for quantifying the rotor EMF magnitude during grid faults. Then the vector trajectory of the rotor EMF is plotted by using the elliptical curve, and the analytical expression of the rotor EMF is obtained. By finding its extremum point, the maximum value of the rotor EMF under faults is calculated. Through the proposed method, the voltage boundary on the rotor side of DFIG under faults is precisely determined, which can provide reference for DFIG transient analysis and enhanced excitation converter design. At last, the accuracy of the proposed calculation method is experimentally validated through a 1.5-kW DFIG, with errors less than 2% under all fault conditions.