Sub-Synchronous Range of Operation for a Wind Driven Double-Fed Induction Generator

Abstract

In this paper the operation of a double-fed wound-rotor induction machine, coupled to a wind turbine, as a generator at sub-synchronous speeds is investigated. A novel approach is used in the analysis, namely, the rotor power flow approach. The conditions necessary for operating the machine as a double-fed induction generator (DFIG) are deduced. Formulae describing the factors affecting the range of sub-synchronous speeds within which generation occurs are deduced. The variations in the magnitude and phase angle of the voltage injected to the rotor circuit as the speed of the machine changes to achieve generation at the widest possible sub-synchronous speed range is presented. Also, the effect of the rotor parameters on the generation range is presented. The analysis proved that the generation range could increase from sub-synchronous to super-synchronous speeds, which increases the amount of energy captured by the wind energy conversion system (WECS) as result of utilizing the power available in the wind at low wind speeds.

Sizing of Converters Interfacing the Rotor of Wind Driven DFIG to the Power Grid

“…It is well known [3] that the power flowing in the rotor of the DFIG has three components; namely, the mechanical power input to the rotor from the wind turbine P m , the electromagnetic power transferred from rotor to stator P g , and the slip power P s which is the electrical power transferred between the rotor and any external electrical source or load (e.g. converter) via the wound rotor slip rings.…”

“…From the DFIG rotor side, following the rotor power flow approach as given in ref. [3], maximum air gap power transferred from the rotor to the stator of the DFIG at any slip s is expressed as…”

Comparing the Quality of Power Generated from DFIG with Different Types of Rotor Converters

“…These results are obtained at both sub and supersynchronous speeds, since the double fed induction machine is known for its operation as a generator even at sub-synchronous speed at specific values of injected rotor voltage magnitude and phase angle [1,[5][6][7]. Results are compared and conclusion of optimum system trading off control complexity and cost is given.…”