Reactive power compensator and method of controlling the same

1. A reactive power compensator for compensating for reactive power of a power system, the reactive power compensator comprising:

a plurality of phase clusters each including a plurality of cells, wherein the plurality of phase clusters comprises a first-phase cluster, a second-phase cluster, and a third-phase cluster; and

a controller configured to control the plurality of phase clusters,

wherein the controller includes:

a phasor transformation unit configured to perform phasor transformation with respect to voltage values and current values of the first-phase cluster, the second phase cluster, and the third-phase cluster to generate a phasor diagram;

a negative sequence cosine vector calculator configured to calculate first-phase to third-phase negative sequence cosine components on the generated phasor diagram;

a center-of-gravity component calculator configured to calculate a center-of-gravity component based on the calculated first-phase to third-phase negative sequence cosine components; and

a zero-sequence component calculator configured to calculate a zero-sequence component based on the calculated center-of-gravity component,

wherein the controller is configured:

to generate an offset signal including the zero-sequence component; and

to compensate for energy errors between the plurality of phase clusters based on the generated offset signal.

2. The reactive power compensator according to claim 1 , wherein, when the plurality of phase clusters has a star connection topology, the offset signal is a zero-sequence voltage.

3. The reactive power compensator according to claim 1 , wherein, when the plurality of phase clusters has a delta connection topology, the offset signal is a zero-sequence current.

4. The reactive power compensator according to claim 1 , wherein the phasor diagram is generated based on positive sequence components and negative sequence components of the voltage values and positive sequence components of the current values.

5. The reactive power compensator according to claim 4 , wherein each of the first-phase to third-phase negative sequence cosine components is located at a point where each of first-phase to third-phase negative sequence voltage vector values of the voltage values meets each of first-phase to third-phase positive sequence current vector values of the current values.

6. The reactive power compensator according to claim 1 , wherein the center-of-gravity component is located at a center of the first-phase to third-phase negative sequence cosine components.

7. The reactive power compensator according to claim 1 , wherein distances from the first-phase to third-phase negative sequence cosine components to the center-of-gravity component are equal.

8. The reactive power compensator according to claim 1 , wherein, when a distance value from an origin which is crossing of an x axis and a y axis of the phasor diagram to the center-of-gravity component is a first distance value, the zero-sequence component includes a second distance value that is origin-symmetrical to the first distance value, and a phase of the second distance value.

9. The reactive power compensator according to claim 8 , wherein the second distance value is located in an opposite direction to a direction from the origin to the center-of-gravity component, and is twice the first distance value.

10. The reactive power compensator according to claim 8 , wherein the second distance value is a zero-sequence voltage and the phase of the second distance value is a zero-sequence phase.

11. The reactive power compensator according to claim 1 , wherein the controller further includes a command value generator that performs control to generate a command value based on the offset signal and to switch the plurality of phase clusters according to the command value.

12. The reactive power compensator according to claim 1 , wherein the phasor transformation unit makes at least one of positive sequence voltage vector values of the voltage values coincident with a horizontal axis (x axis) to generate the phasor diagram.

13. The reactive power compensator according to claim 12 , wherein the phasor transformation unit determines the locations of the other positive sequence voltage vector values, first-phase to third-phase negative sequence voltage vector values and first-phase to third-phase positive sequence current vector values based on the at least one of the positive sequence voltage vector values coincident with the horizontal axis.

14. A method of controlling a reactive power compensator for compensating for reactive power of a power system, the method comprising:

performing phasor transformation with respect to voltage values and current values of a plurality of phase clusters each including a plurality of cells to generate a phasor diagram;

generating an offset signal on the phasor diagram;

generating a command value, to which the offset signal of a zero-sequence component is applied; and

controlling the plurality of phase clusters using the command value to compensate for energy errors between the plurality of phase clusters,

wherein the generating of the offset signal includes:

calculating first-phase to third-phase negative sequence cosine components on the generated phasor diagram;

calculating a center-of-gravity component based on the calculated first-phase to third-phase negative sequence cosine components; and

calculating the zero-sequence component based on the calculated center-of-gravity component.

15. The method according to claim 14 , wherein each of the first-phase to third-phase negative sequence cosine components is located at a point where each of first-phase to third-phase negative sequence voltage vector values of the voltage values meets each of first-phase to third-phase positive sequence current vector values of the current values.

16. The method according to claim 14 , wherein the center-of-gravity component is located at a center of the first-phase to third-phase negative sequence cosine components.

17. The method according to claim 14 , wherein, when a distance value from an origin which is crossing of an x axis and a y axis of the phasor diagram to the center-of-gravity component is a first distance value, the zero-sequence component includes a second distance value that is origin-symmetrical to the first distance value, and a phase of the second distance value.

18. The method according to claim 17 , wherein the second distance value is located in an opposite direction of a direction from the origin to the center-of-gravity component, and is twice the first distance value.

19. The method according to claim 18 , wherein the second distance value is a zero-sequence voltage and the phase of the second distance value is a zero-sequence phase.

20. The method according to claim 14 , wherein the generating of the phasor diagram further includes making at least one of positive sequence voltage vector values of the voltage values coincident with a horizontal axis (x axis) to generate the phasor diagram.

21. The method according to claim 20 , wherein the generating of the phasor diagram further includes determining locations of the other positive sequence voltage vector values, first-phase to third-phase negative sequence voltage vector values and first-phase to third-phase positive sequence current vector values based on the at least one of the positive sequence voltage vector values coincident with the horizontal axis.

22. The method according to claim 14 , wherein, when the plurality of phase clusters has a star connection topology, the offset signal is a zero-sequence voltage.

23. The method according to claim 14 , wherein, when the plurality of phase clusters has a delta connection topology, the offset signal is a zero-sequence current.

24. The method according to claim 14 , wherein distances from the first-phase to third-phase negative sequence cosine components to the center-of-gravity component are equal.