Dynamic Phasor Finite Element Modeling of Grid-Connected DFIG Considering Winding Space Harmonics

Abstract

This paper presents a new technique for quantifying the winding Magneto-Motive Force (MMF) space harmonics of wind-driven Doubly-Fed Induction Generator (DFIG) for power quality assessments. Instead of following the commercial electromagnetic transient programs in modeling electric machines using lumped-parameters models, the paper uses a combined Dynamic Phase modeling and Finite Element Method (FEM) to do the job. Dynamic Phase Modeling uses the principle of Moving Average Technique to model the targeted signal by complex-valued Fourier coefficients varying from a certain time-window to the next allowing for shifting-back the signal frequency and as a result, a larger simulation time-step can be used. Modeling the state variables of the interconnected FEM equations and the power system’s circuit equations as Dynamic Phasors allows for making use of the FEM’s precision while maintaining an economical computation requirements. The technique has the ability to efficiently model core nonlinearity and stator/rotor MMF space harmonics. The main speed-dependant characteristic frequencies of the stator/rotor winding space harmonics as reflected in the stator current time-harmonic content have been simulated using both the conventional time-domain FEM and the new Dynamic Phasor FEM under changing speed-command to test the validity and computational superiority of the new technique for modeling winding space harmonics. Custom-written C++ codes of the two techniques have been used for the simulation process and the results show the capability of the new Dynamic Phasor FEM technique to produce a comparable accuracy to the conventional one while reducing the simulation time to 15%.

Keywords

• DFIG
• dynamic phasor modeling
• finite element modeling
• power quality
• space harmonics
• wind energy