Dynamic Control of the Brushless Doubly Fed Induction Generator Under Unbalanced Operation

The brushless doubly fed induction generator (DFIG) (BDFIG) shows commercial promise for wind power generation because of its lower maintenance needs compared to the widely used DFIG. From power grid codes, installed wind turbines are sometimes required to be capable of operating under unbalanced grid conditions. This paper presents a novel controller for BDFIG-based wind generators operated with asymmetrical voltage supplies. Two synchronous reference frames, the positive and the negative, are introduced for applying a vector control scheme to separately control two sequences arising from the unbalanced voltage. Different control strategies are discussed for reducing the consequent oscillations. It is demonstrated that the proposed controller shows a significant performance improvement over the conventional controller and is effective to cope with potentially unbalanced conditions.

Nomenclature

PW and CW

Power winding and control winding.

, , and

Voltage, current, and flux vectors.

Arbitrary vector.

, , and

Voltage, current, and flux scalars.

and

Active and reactive powers.

Angular frequency of the arbitrary rotating reference frame.

, , and

Angular frequencies of the PW, the CW, and rotor.

and

Angular positions of the PW flux frame and rotor.

and

Pole pair numbers of the PW and the CW.

, , and

Resistances of the PW, the CW, and rotor.

, , and

Self-inductances of the PW, the CW, and rotor.

and

Coupling inductances between stator windings and rotor.

Leakage constant between the PW and rotor, i.e., .

VUF

Voltage unbalance factor.

and

Real and imaginary parts of vector .

Differential operator, i.e., .

Complex conjugate of vector .

subscripts 1, 2, and

PW, the CW, and rotor.

and

Forward- and backward-sequence components.

and

Rotating frame axis.

,

Stationary axis of the PW, normally simplified as , .

,

Stationary axis of the CW.

superscripts + and −

Positive and negative synchronous reference frames.