Standby power supply with load-current harmonics neutralizer

• US 5,343,079 A
• Filed: 12/15/1992
• Issued: 08/30/1994
• Est. Priority Date: 02/25/1991
• 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: Quinn, Conor A., Peterson, Maximilian R., Mohan, Ned
• Primary Examiner(s): Sterrett, Jeffrey L.
• Application Number:
  US07/991,091
• Time in Patent Office: 623 Days
• Field of Search: 307/46, 307/64, 307/66, 307/105, 363/40, 363/44
• US Class Current: 307/105
• CPC Class Codes: H02J 9/062 for AC powered loads
• CPC Class Codes: Y02B 70/30 Systems integrating technologies related to power network operation and communication or information technologies for improving the carbon footprint of the management of residential or tertiary loads, i.e. smart grids as climate change mitigation technology in the buildings sector, including also the last stages of power distribution and the control, monitoring or operating management systems at local level
• CPC Class Codes: Y04S 20/20 End-user application control systems

Abstract

A standby power supply system is provided for supplying normal AC power to a critical load from an AC power source during normal operating conditions, and for supplying emergency AC power to the load during failure of the AC power source. During normal operation, the standby power supply system actively neutralizes undesirable harmonic components in the input current drawn by the load. The standby power supply system includes a power conversion device having a DC side coupled to a back-up power source and an AC side in parallel with the load and the AC source. A harmonic distortion sensor senses a harmonic distortion current component of a load current drawn by the load during normal operating conditions. A controller is responsive to the harmonic distortion sensor for causing the power conversion device to produce a harmonics neutralizing current to substantially neutralize the harmonic distortion current component produced by the load. A method is also provided for supplying AC power to a load during normal and emergency operating conditions.

Claims

1. A method of operating a power converter positioned in shunt with a three-phase four wire AC system including three-phase conductors and a neutral conductor, the power converter including a DC power storage system, the method comprising the steps of:

sensing distorted line-to-neutral voltage signals on said four wire AC system to generate voltage distortion signals;

identifying a DC power level signal corresponding to said DC power storage system;

processing said voltage distortion signals and said DC power level signal to obtain gate signals that force said power converter to produce harmonic neutralized three-phase sinusoidal currents on said three-phase four wire system.

2. The method of claim 1 further including the step of processing said source voltage sensor signals to yield first, second, and third voltage distortion component signals.

3. The method of claim 2 further including the step of deriving first, second, and third charging voltage signals from said DC power level signal.

4. The method of claim 3 further including the step of adding said first, second, and third voltage distortion component signals with said first, second, and third capacitor charging voltage signals, to produce first, second, and third voltage controller reference signals.

5. The method of claim 4 further including the steps of

sensing first, second, and third inverter signals;

and

combining said first, second, and third voltage controller reference signals and said first, second, and third inverter signals, to produce a plurality of gate signals for said power controller.

6. The method of claim 3 further including the step of combining said first, second, and third voltage controller reference signals to produce a neutral controller voltage reference signal.

7. The method of claim 6 further including the steps of:

sensing first, second, and third current signals; and

combining said first, second, and third inverter signals to yield an aggregate inverter signal.

8. The method of claim 7 further including the step of combining said neutral controller voltage reference signal and said aggregate inverter signal to produce gate signals for said power controller.

9. An apparatus for neutralizing load current harmonics in an electrical load, comprising:

a power converter having an AC side and a DC side, with the AC side for coupling in parallel with a three-phase AC power source providing power to the load;

a capacitor bank coupled to the DC side of the converter, the converter supporting a DC charging voltage across the capacitor bank with power drawn from the AC source;

a harmonic current sensor for sensing a harmonic distortion current component of a load current drawn by the load, the harmonic current sensor including;

a converter current sensor for generating a converter current sensor signal in response to an output current of the AC side of the converter;

a load current sensor for generating load current sensor signals in response to the load current;

voltage sensors for generating source voltage sensor signals in response to line-to-neutral voltages of the three-phase AC source; and

a capacitor bank voltage detector for generating a capacitor bank voltage signal in response to a DC voltage across the capacitor bank; and

a controller responsive to the harmonic current sensor for causing the power converter to generate a harmonic neutralizing current, the neutralizing added to the load current to cancel the harmonic distortion current component, wherein the controller includes;

a regulator for comparing the capacitor bank voltage signal with a preselected reference value signal to generate an amplitude adjustment signal;

a multiplier for multiplying the source voltage sensor signals by the amplitude adjustment signal to generate a capacitor bank charging voltage signal;

a notch filter for removing a fundamental frequency component signal representative of a fundamental frequency component of the load current from the load current sensor signal to generate a distortion current component signal representative of the harmonic distortion current component;

a first summer for summing together the distortion current component signal and the capacitor bank voltage signal to generate an inverter current reference signal;

a second summer for summing together the negative value of the inverter current reference signal to generate a neutral voltage reference signal;

a third summer for summing together the negative value of the inverter current reference signal with the inverter current sensor signal, and for comparing the neutral voltage reference signal with the inverter neutral voltage signal, to generate the control signal.

10. An apparatus for neutralizing load current harmonics in an electrical load, comprising:

a power converter having an AC side and a DC side, with the AC side for coupling in parallel with an AC power source providing power to the load, the AC power source comprising a three-phase four wire service including three phase conductors and a neutral conductor, the power converter including plural switching devices arranged to generate an AC side output current comprising three inverter phase currents to supply the respective phase conductors;

a harmonic current sensor for sensing a harmonic distortion current component of a load current drawn by the load;

a switching frequency filter for filtering any switching frequency current components from the AC side output current, with the filter having a neutral junction;

a DC capacitor bank coupled to the DC side of the converter, with the capacitor bank having two capacitor sections with a junction therebetween;

a neutral return conductor coupling together the capacitor bank junction, the filter neutral junction, and the neutral conductor; and

a controller responsive to the harmonic current sensor for causing the power converter to generate a harmonic neutralizing current, the neutralizing current added to the load current to cancel the harmonic distortion current component.

11. An apparatus according to claim 10 wherein:

the converter supports a DC charging voltage across the capacitor bank with power drawn from the AC source;

the harmonic current sensor comprises;

a converter current sensor for generating a converter current sensor signal in response to all output current of the AC side of the converter;

a load current sensor for generating load current sensor signals in response to the load current;

a voltage sensor for generating a source voltage sensor signal in response to a line-to-neutral voltage of the AC source; and

a capacitor bank voltage detector for generating a capacitor bank voltage signal in response a DC voltage across the capacitor bank;

the controller comprises;

a regulator for comparing the capacitor bank voltage signal with a preselected reference value signal to generate an amplitude adjustment signal;

a multiplier for multiplying the source voltage sensor signal by the amplitude adjustment signal to generate a capacitor bank charging voltage signal;

a notch filter for removing a fundamental frequency component signal representative of a fundamental frequency component of the load current from the load current sensor signal to generate a distortion current component signal representative of the harmonic distortion current component;

a first summer for summing together the distortion current component signal and the capacitor bank charging voltage signal to generate an inverter current reference signal; and

a comparator for comparing the inverter current reference signal with the inverter current sensor signal, and for comparing the neutral voltage reference signal with the inverter neutral voltage signal, to generate the control signal.

12. An apparatus for neutralizing load current harmonics in an electrical load comprising:

a power converter having an AC side and a DC side with the AC side for coupling in parallel with a three-phase four wire AC power source providing power to the load;

a capacitor bank coupled to the DC side of the converter;

wherein the converter supports a DC charging voltage across the capacitor bank with power drawn from the AC source;

a harmonic current sensor for sensing a harmonic distortion current component of a load current drawn by the load, the harmonic sensor including;

a converter current sensor for generating a converter current sensor signal in response to an output current of the AC side of the converter;

a load current sensor for generating load current sensor signals in response to the load current;

a voltage sensor for generating source voltage sensor signals in response to individual line-to-neutral voltages of at least two phases of the AC source; and

a capacitor bank voltage detector for generating a capacitor bank voltage signal in response to a DC voltage across the capacitor bank; and

a controller responsive to the harmonic current sensor for causing the power converter to generate a harmonic neutralizing current, the neutralizing current added to the load current to cancel the harmonic distortion current component.

13. An apparatus for neutralizing load current harmonics in an electrical load, comprising:

a power converter having an AC side and a DC side, with the AC side for coupling in parallel with three phase conductors of an AC power source providing power to the electrical load, wherein the power converter comprises an inverter having at least six switching devices responsive to a control signal for producing an inverter output current, the inverter including a DC charging voltage across a capacitor bank with power drawn from the AC source;

a harmonic current sensor for sensing a harmonic distortion current component of a load current drawn by the electrical load, the harmonic current sensor including;

three voltage sensors for generating source voltage sensor signals in response to the line-to-neutral voltage of the three phase conductors of the AC source;

a capacitor bank voltage detector for generating a capacitor bank voltage signal in response to a DC voltage across the capacitor bank;

three inverter current sensors for generating inverter current sensor signals in response to the inverter output current; and

three load current sensors for generating load current sensor signals in response to the load current; and

a controller responsive to the harmonic current sensor for causing the power converter to generate a harmonic neutralizing current, the neutralizing current added to the load current to cancel the harmonic distortion current component.

14. An apparatus according to claim 13 wherein:

the inverter has at least eight switching devices arranged to generate an inverter output current comprising three inverter phase currents to supply the respective phase conductors and a neutral current to supply the neutral conductor; and

the apparatus further includes a switching frequency filter for filtering any switching frequency current components from the inverter output current.

15. An apparatus for delivering sinusoidal currents to an electrical load, comprising:

a power converter having an AC side and a DC side, with the AC side coupled in parallel with an AC power source providing power to the electrical load with a three-phase four wire system including three-phase conductors and a neutral conductor, and the DC side coupled to a DC storage system;

a plurality of voltage sensors for generating source voltage sensor signals corresponding to distorted line-to-neutral voltage signals of the three-phase conductors of the AC source;

a DC storage system voltage detector for generating a DC voltage signal in response to a DC voltage across the DC storage system; and

a controller responsive to said source voltage sensor signals and said DC voltage signal to cause the power converter to generate harmonic neutralized three-phase sinusoidal currents on said three-phase four wire system.

16. The apparatus of claim 15 further including means for processing said source voltage sensor signals to yield first, second, and third voltage distortion component signals.

17. The apparatus of claim 16 further including means for deriving first, second, and third DC charging voltage signals.

18. The apparatus of claim 17 further including means for adding said first, second, and third voltage distortion component signals with said first, second, and third DC charging voltage signals, to produce first, second, and third voltage controller reference signals.

19. The apparatus of claim 18 further including

first, second, and third inverter current sensors that provide first, second, and third inverter signals;

and

first, second, and third current mode controller devices that respectively receive said first, second, and third voltage controller reference signals and said first, second, and third inverter signals, to produce a plurality of gate signals for said power controller.

20. The apparatus of claim 18 further including means for combining said first, second, and third voltage controller reference signals to produce a neutral controller voltage reference signal.

21. The apparatus of claim 20 further including

first, second, and third inverter current sensors that provide first, second, and third inverter signals;

and

means for combining said first, second, and third inverter signals to yield an aggregate inverter signal.

22. The apparatus of claim 21 further including a fourth current mode controller that receives said neutral controller voltage reference signal and said aggregate inverter signal and produces gate signals for said power controller.