Dynamic Phasor Finite-Element Modeling of a DFIG for Grid Connection Studies

Abstract

Cosimulation studies of electric power systems and electric machines have always been a challenge. In order to reduce the simulation time to a reasonable value, lumped-parameter electric machine models are commonly used in electric power system modeling software packages to avoid the heavy computational burden of more accurate modeling methods especially finite-element method (FEM) on the expense of less accuracy. The proposed technique in this article combines the dynamic phasor modeling technique for power system simulations with the FEM to accurately model the doubly fed induction generator while connected to the grid. The utilization of dynamic phasors enables adopting large simulation time steps resulting in a significant reduction in the simulation time compared to the conventional time-domain FEM modeling. The mathematical foundation of the proposed modeling method is presented including the generator's core saturation. Custom-written C++ codes have been developed by the authors to execute the new dynamic phasor FEM algorithm and the conventional time-domain FEM in order to fairly compare their accuracy and numerical performances. As the proposed method combines time and frequency domains, a unique capability of modeling the rotor movement can be achieved. The rotation can be represented by physically incrementing the rotor and airgap mesh as in regular time-domain solvers, by mathematically representing the rotation using the virtual blocked rotor method as in frequency-domain solvers, and the proposed method of combining the two aforementioned approaches. The three methods of modeling rotor rotation are discussed, and their simulation results are compared to give a guide to choose the proper method for the different modeling targets. The results show that the proposed dynamic phasor FEM is capable of producing comparable results to the traditional time-domain solver at a substantially reduced simulation time.