System and method for reducing harmonic currents by current injection

A method for reducing harmonic currents when power is converted with power conversion means between a three-phase alternating current power system having a base fundamental frequency and a direct current power system defined by a voltage across opposed first and second terminals and receiving a direct current through said terminals, said method comprising:

• providing current modulating means connected between said direct current power system and said power conversion means, and impedance means connected between said current modulating means and said three-phase alternating current power system;
• operating said current modulating means to receive a first superimposed current comprising the direct current and a first sinusoidal current at a selected harmonic frequency of the fundamental frequency with an amplitude thereof proportional to the direct current, and to provide a second superimposed current to an output comprising the direct current and a second sinusoidal current at the selected harmonic frequency with an amplitude thereof proportional to the direct current;
• 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 sinusoidal current;
• and
• injecting said injection current into the three-phase alternating current power system.

A method for reducing harmonic currents when power is converted with power conversion means between a three-phase alternating current power system having a base fundamental frequency and a direct current power system having first and second terminals and receiving a direct current through said terminals, said method comprising:

• providing current modulating means connected between said direct current power system and said power conversion means, and impedance means connected between said current modulating means and said three-phase alternating current power system;
• operating said current modulating means to provide a sinusoidal injection current at a selected harmonic frequency of the fundamental frequency with an amplitude proportional to the direct current;
• and
• injecting said injection current through said impedance means into each separate phase of the three-phase alternating current power system.

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 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 current from the power conversion means and modulating a first and second superimposed sinusoidal currents with the direct current at a selected harmonic frequency of the fundamental frequency, with an amplitude proportional to the direct current;
• 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 each sinusoidal current;
• and
• means connected between said means for combining and said multiphase alternating current power system, said means providing a substantially low impedance for the injection current injected into said multiphase alternating current power system and a higher impedance for a current having the base fundamental frequency.

An apparatus for reducing harmonic between a three-phase alternating current power system having a base fundamental frequency and a direct current power system having terminals and having a direct current through said terminals comprising:

• a power converter connected between the three-phase alternating current power system and the direct current system for converting power between the two systems;
• current modulating means connected between said direct current power system and said power converter for providing a sinusoidal injection current synchronized and in phase with the alternating current power system and at a selected frequency and amplitude proportional to the direct current;
• and
• coupling means connected between said current modulating means and said three-phase alternating current power system for injecting a portion of said injection current through said coupling means into each phase of the three-phase alternating current power system.