An accurate transient analysis method for DFIG with crowbar protection under grid faults

Abstract

In order to protect the converters in a doubly fed induction generator (DFIG) based wind turbine during grid fault, a crowbar is the commonly used protection method to realize low voltage ride through (LVRT). Conventionally, the transient process of DFIG with the crowbar is analyzed in an analogous manner to a squirrel-cage induction motor (SCIM). But the crowbar resistance is usually large enough to change the quantitative relationship of internal resistance and reactance, so the analogous analysis would cause certain errors. For this issue, an accurate analysis method was proposed, where the mathematical model of DFIG with crowbar was transformed into a second-order differential equation in complex domain and then was calculated by solving the differential equation. According to the characteristic roots, there are two transient frequency components in the stator flux as well as in the rotor flux, and the transient process of stator flux would be slowed down by a larger crowbar resistance or a lower rotor speed. A case of a 1.5 MW DFIG was built in Matlab/Simulink and the simulation results verified its accurateness.