System for reducing harmonics by harmonic current injection

• US 5,499,178 A
• Filed: 02/10/1994
• Issued: 03/12/1996
• Est. Priority Date: 12/16/1991
• Status: Expired due to Fees

• Current Assignee: Regents of The University of Minnesota (University of Minnesota)
• Original Assignee: Regents of The University of Minnesota (University of Minnesota)
• Inventors: Mohan, Ned
• Primary Examiner(s): Sterrett, Jeffrey L.
• Application Number:
  US08/194,701
• Time in Patent Office: 761 Days
• Field of Search: 307/105, 363/39, 363/40, 363/41, 363/44, 363/84, 363/87, 363/89, 363/95, 363/96, 363/98, 363/129, 363/132, 363/137
• US Class Current: 363/39
• CPC Class Codes: H02J 3/01 Arrangements for reducing harmonics or ripples
• CPC Class Codes: H02J 3/1842 wherein at least one reactive element is actively controlled by a bridge converter, e.g. active filters
• CPC Class Codes: H02M 1/12 Arrangements for reducing harmonics from ac input or output
• CPC Class Codes: Y02E 40/20 Active power filtering [APF]
• CPC Class Codes: Y02E 40/40 Arrangements for reducing harmonics
• CPC Class Codes: Y02P 90/40 Fuel cell technologies in production processes

Abstract

A circuit is provided for generating a current that is a harmonic of the frequency of the utility system and which is of a selected amplitude and phase to reduce the total harmonic distortion of a system that converts alternating current to direct current, or vice versa. The DC signal is sampled and by controlling switches, a sinusoidal current at the desired harmonic is generated, and is provided to a neutral of a transformer that injects this current into the multiphase AC system used by the utility in a very low cost, easily controlled, and highly effective manner. In a second embodiment, the injection current is provided to a common junction of a magnetic device.

Claims

1. A harmonic reducing system for reducing harmonic currents generated from power conversion means connected between a multiphase alternating current power system having a base fundamental frequency and a direct current power system having terminals carrying a direct current, said power conversion means operating at the base fundamental frequency, said harmonic reducing system comprising:

current modulating means connected between said direct current power system and said power conversion means, said current modulating means being connected to receive at least a portion of the direct current carried through the terminals and modulating said at least a portion of the direct current with first and second superimposed sinusoidal currents at a selected harmonic frequency of the fundamental frequency greater than the fundamental frequency, wherein the first and second superimposed sinusoidal currents have an amplitude proportional to the direct current to reduce the selected harmonic currents; and

means connected to the current modulating means for combining said first and second superimposed currents to form a sinusoidal injection current at the selected harmonic frequency, said injection current having an amplitude substantially equal to twice the amplitude of the first superimposed sinusoidal current; and

coupling means connected between said means for combining and said multiphase alternating current power system for injecting a portion of said injection current through said coupling means into each phase of the multiphase alternating current system, the coupling means including a set of windings connected in a zigzag arrangement having a neutral connected to the means for combining for receiving the injection current.

2. The harmonic reducing system of claim 1 wherein the selected frequency includes the third harmonic frequency.

3. The harmonic reducing system of claim 1 wherein the current modulating means regulates a voltage across the terminals of the direct current power system.

4. The harmonic reducing system of claim 1 wherein the power conversion means comprise an inverter to transfer power from the direct current power system to the multiphase alternating current power system.

5. The harmonic reducing system of claim 1 wherein the coupling means includes a second set of windings connected in a wye arrangement.

6. The harmonic reducing system of claim 1 wherein the power conversion means comprise a rectifier to transfer power from the multiphase alternating current power system to the direct current power system.

7. The harmonic reducing system of claim 1 wherein the coupling means comprises a transformer having primary windings connected to the multiphase alternating current power system and secondary windings connected to the power conversion means, the set of windings being connected to the secondary windings.

8. The harmonic reducing system of claim 1 wherein the coupling means further includes a capacitor connected between the neutral of the set of windings and the means for combining.

9. The harmonic reducing system of claim 1 and further comprising;

second power conversion means connected between the multiphase alternating current power system and the direct current power system;

second current modulating means connected between said direct current power system and said second power conversion means, said second current modulating means being connected to receive at least a second portion of the direct current carried through the terminals and modulating said at least a second portion of the direct current with third and fourth superimposed sinusoidal currents at a selected harmonic frequency of the fundamental frequency greater than the fundamental frequency, wherein the third and fourth superimposed sinusoidal currents have an amplitude proportional to the direct current to reduce the selected harmonic currents; and

second combining means connected to the second current modulating means for combining said third and fourth superimposed currents to form a second sinusoidal injection current at the selected harmonic frequency, said second injection current having an amplitude substantially equal to twice the amplitude of the third superimposed current; and

second coupling means connected between said second combining means and said multiphase alternating current power system for injecting a portion of said second injection current through said second coupling means into each phase of the multiphase alternating current system.

10. The harmonic system of claim 9 wherein the second coupling means includes a second set of windings having a second neutral connected to the second combining means and receiving the second injection current.

11. The harmonic reducing system of claim 9, wherein each current modulating means comprises a first current shaping modulator and a second current shaping modulator connected to the direct current power system, and wherein the first mentioned means for combining comprises a first junction point connected between the first and second current shaping modulators of the first-mentioned current modulating means, the first-mentioned coupling means being connected to the first-mentioned current modulating means at the first junction point;

and wherein the second combining means comprises a second junction point connected between the first and second current shaping modulators of the second current modulating means, the second coupling means being connected to the second current modulating means at the second junction point.

12. The harmonic reducing system of claim 9 wherein the selected frequency includes the ninth harmonic frequency.

13. The harmonic reducing system of claim 11 wherein the terminals comprise a first terminal and a second terminal and wherein each of the first current shaping modulators are connected to the first terminal and wherein each of the second current shaping modulators are connected to the second terminal.

14. The harmonic reducing system of claim 11 wherein the terminals comprise a first terminal and a second terminal and wherein the first current shaping modulator of the first-mentioned current modulating means is connected to the first terminal, the second current shaping modulator of the first-mentioned current modulating means is connected to the first current shaping modulator of the second current modulating means, and the second current shaping modulator of the second current modulating means is connected to the second terminal.

15. The harmonic reducing system of claim 1 wherein the coupling means comprises a transformer being primary windings connected to the multiphase alternating current power system and the set of windings are secondary windings and are connected to the power conversion means.

16. The harmonic reducing system of claim 15 wherein the primary windings comprise delta connected windings.

17. A harmonic reducing system for reducing harmonic currents generated from power conversion means connected between a multiphase alternating current power system having a base fundamental frequency and a direct current power system having terminals carrying a direct current, said power conversion means operating at the base fundamental frequency, said harmonic reducing system comprising:

current modulating means connected between said direct current power system and said power conversion means, said current modulating means being connected to receive the direct current carried through the terminals and modulating said at least a portion of the direct current with first and second superimposed sinusoidal currents at a selected harmonic frequency of the fundamental frequency greater than the fundamental frequency wherein the first and second superimposed sinusoidal currents have an amplitude proportional to the direct current to reduce the selected harmonic currents; and

means connected to the current modulating means for combining said first and second superimposed currents to form a sinusoidal injection current at the selected harmonic frequency, said injection current having am amplitude substantially equal to twice the amplitude of each sinusoidal current; and

coupling means connected between said means for combining and said multiphase alternating current power system for injecting a portion of said injection current through said coupling means into each phase of the multiphase alternating current system, the coupling means comprising;

magnetic core means forming a closed magnetic path;

a plurality of windings wound around a portion of the magnetic core means, wherein a first end of each winding is connected to a phase of the AC system; and

connection means for connecting a second end of each winding to form a common junction, the common junction receiving the injection current.

18. A harmonic reducing system for reducing selected harmonic currents generated from power conversion means connected between a multiphase alternating current power system having a base fundamental frequency and a direct current power system having terminals carrying a direct current, said harmonic reducing system comprising:

first current modulating means connected between said direct current power system and said power conversion means, said current modulating means being connected to receive at least a portion of the direct current carried through the terminals and modulating said at least a portion of the direct current with first and second superimposed sinusoidal currents at a selected harmonic frequency of the fundamental frequency, wherein the first and second superimposed sinusoidal currents have an amplitude proportional to the direct current to reduce the selected harmonic currents; and

first combining means connected to the first current modulating means for combining said first and second superimposed currents to form a first sinusoidal injection current at the selected harmonic frequency, said first injection current having an amplitude substantially equal to twice the amplitude of the first superimposed sinusoidal current;

first coupling means connected between said means for combining and said multiphase alternating current power system for injecting a portion of said injection current through said coupling means into each phase of the multiphase alternating current system, the coupling means including a transformer having wye-connected windings with a neutral connected to the first combining means for receiving the injection current;

second power conversion means connected between the multiphase alternating current power system the direct current power system;

second current modulating means connected between said direct current power system and said second power conversion means, said second current modulating means being connected to receive at least a second portion of the direct current carried through the terminals and modulating a third and fourth superimposed sinusoidal currents with the second portion of the direct current at a selected harmonic frequency of the fundamental frequency, wherein the third and fourth superimposed sinusoidal currents have an amplitude proportional to the direct current to reduce the selected harmonic currents;

second combining means connected to the second current modulating means for combining said third and fourth superimposed currents to form a second sinusoidal injection current at the selected harmonic frequency, said second injection current having an amplitude substantially equal to twice the amplitude of the third superimposed current; and

second coupling means connected between said second combining means and said multiphase alternating current power system for injecting a portion of said second injection current through said second coupling means into each phase of the multiphase alternating current system, the second coupling means including a second transformer having wye-connected windings with a second neutral connected to the second combining means for receiving the second injection current.

19. The harmonic reducing system of claim 18 wherein each current modulating means comprises a first current shaping modulator and a second current shaping modulator connected to the direct current power system, and wherein the first combining means comprises a first junction point connected between the first and second current shaping modulators of the first current modulating means, the first coupling means being connected to the first current modulating means at the first junction point;

and wherein the second combining means comprises a second junction point connected between the first and second current shaping modulators of the second current modulating means, the second coupling means being connected to the second current modulating means at the second junction point.

20. The harmonic reducing system of claim 19 wherein the terminals comprise a first terminal and a second terminal and wherein each of the first current shaping modulators are connected to the first terminal and wherein each of the second current shaping modulators are connected to the second terminal.

21. The harmonic reducing system of claim 19 wherein the terminals comprise a first terminal and a second terminal and wherein the first current shaping modulator of the first current modulating means is connected to the first terminal, the second current shaping modulator of the first current modulating means is cormsated to the first current shaping modulator of the second current modulating means, and the second current shaping modulator of the second current modulating means is connected to the second terminal.

22. The harmonic reducing system of claim 18 wherein the selected frequency includes the ninth harmonic frequency.