Modelling and Validation of a Grid-Connected DFIG by Exploiting the Frequency-Domain Harmonic Analysis

Abstract

Wind Energy Conversion Systems (WECS) based on a Doubly-Fed Induction Generator (DFIG) represent the most common configuration employed in wind turbines. These systems involve injecting harmonic currents toward an electrical grid from a back-to-back power converter, potentially creating voltage distortions. To assess this phenomenon, a case study of a 3 kW DFIG-based wind turbine connected to the electrical grid is presented for analysis in the harmonic domain. Initially, a DFIG-based load flow analysis for determining the operating conditions is tackled at the fundamental frequency. Then, the modelling of a DFIG under steady-state operating conditions at harmonic frequencies is analyzed discussing its characteristics in the harmonic domain. The high-frequency harmonics in the output voltage of a pulse width modulation-driven inverter feeding the rotor windings of a DFIG and its connection to a three-winding transformer are also analyzed. This investigation produced a complete model of the DFIG connected to the electrical grid. The results demonstrated that although a considerable harmonic contribution up to the 25th order exists, it remains harmless since it is below 5%, according to the Std. IEEE 519.

Keywords

• Doubly-Fed Induction Generator (DFIG)
• back-to-back power converter
• harmonic analysis
• steady-state studies
• load flow calculation