PHASE-LOCKED LOOP

1 . A phase-locked loop for estimating a phase angle of a three-phase reference signal, wherein the phase-locked loop comprises:

means for calculating an estimated first state and an estimated second state of a model of an unbalanced three-phase system at a fundamental frequency of the reference signal on the basis of the reference signal and an estimated fundamental frequency, wherein the model comprises a first state representing a sum of a positive and a negative sequence component of the reference signal at a harmonic frequency, and a second state representing a difference between the positive sequence component and the negative sequence component;

means for calculating a fundamental positive sequence component of the reference signal on the basis of the estimated first state and the estimated second state;

means for calculating a direct component and a quadrature component in a rotating reference frame synchronous with the estimated phase angle on the basis of the fundamental positive sequence component and an estimated phase angle;

means for determining an estimate of an amplitude of the fundamental positive sequence component on the basis of the direct component; and

means for determining estimates of the estimated fundamental frequency and the estimated phase angle on the basis of the quadrature component.

2 . A phase-locked loop according to claim 1 , wherein the phase-locked loop comprises:

means for extracting harmonic contents of the reference signal at least at one harmonic frequency other than a fundamental harmonic frequency of the reference signal on the basis of the reference signal, the estimated fundamental frequency, and the model of an unbalanced three-phase system, and

means for compensating for the reference signal on the basis of the extracted harmonic content.

3 . A phase-locked loop according to claim 1 , wherein the phase locked loop comprises:

means for calculating a fundamental negative sequence component of the reference signal on the basis of the estimated first state and the estimated second state of the model of an unbalanced three-phase system.

4 . A phase-locked loop according claim 3 , wherein the means for determining estimates of the estimated fundamental frequency and the estimated phase angle comprise a controller minimizing the magnitude of the quadrature component.

5 . A phase-locked loop according to claim 4 , wherein the controller comprises a PI controller and the estimated fundamental frequency is obtained directly from an integrating part of the PI controller.

6 . A phase-locked loop according to claim 2 , wherein the means for extracting the harmonic contents and the means for compensating for the reference signal are configured to be disabled if harmonic distortion does not exceed a set limit.

7 . A method for estimating a phase angle of a three-phase reference signal, the method comprising:

calculating an estimated first state and an estimated second state of a model of an unbalanced three-phase system at a fundamental frequency of the reference signal on the basis of the reference signal and an estimated fundamental frequency, wherein the model comprises a first state representing a sum of a positive and a negative sequence component of the reference signal at a harmonic frequency, and a second state representing a difference between the positive sequence component and the negative sequence component;

calculating a fundamental positive sequence component of the reference signal on the basis of the estimated first state and the estimated second state;

calculating a direct component and a quadrature component in a rotating reference frame synchronous with the estimated phase angle on the basis of the fundamental positive sequence component and an estimated phase angle;

determining an estimate of an amplitude of the fundamental positive sequence component on the basis of the direct component; and

determining estimates of the estimated fundamental frequency and the estimated phase angle on the basis of the quadrature component.