Analytical Method for DFIG Transients During Voltage Dips

Electromagnetic transients resulting from voltage dips not only present challenges for low-voltage ride-through (LVRT) of doubly fed induction generators (DFIGs) in wind turbines (WTs) but also introduce complications for protective relays in power systems. Analysis of the voltage-dip response is a fundamental requirement for methodologies targeting LVRT performance and is also useful for power system development. To date, it has not been feasible to take into account the interval between voltage dip and crowbar triggering, and, consequently, the accuracy of the analytical results has been compromised. In this paper, a concept of equivalent fault source is presented with which the transient process during voltage dips is decomposed into one response to the equivalent stator fault source and the other response to the equivalent rotor fault source. The former results from dips in the supply voltage, while the latter arises from variations in both the rotor resistance and the control voltage produced by crowbar activation. This makes it feasible to incorporate the interval, i.e., the triggering delay, into the analytical solution. Additionally, this decomposition approach allows for the physical mechanisms behind the individual transient components to be more clearly understood. With the explicit analytical formulas, some intrinsic characteristics of the transients are identified. The comparative accuracy and advantages of this analytical method over the conventional approaches is demonstrated through simulations using a 2-MW DFIG for commercial WTs and further verified experimentally on a laboratory-scale test rig.