Power line conditioner using cascade multilevel inverters for voltage regulation, reactive power correction, and harmonic filtering

• US 6,075,350 A
• Filed: 04/22/1999
• Issued: 06/13/2000
• Est. Priority Date: 04/24/1998
• Status: Expired due to Fees

• Current Assignee: Lockheed Martin Energy Research Corporation (Martin Marietta Corporation)
• Original Assignee: Lockheed Martin Energy Research Corporation (Martin Marietta Corporation)
• Inventors: Peng, Fang Zheng
• Primary Examiner(s): BERHANE, ADOLF D
• Application Number:
  US09/296,980
• Time in Patent Office: 418 Days
• Field of Search: 323/205, 323/207, 363/39, 363/40, 363/43, 363/95, 363/97, 363/71, 307/105
• US Class Current: 323/207
• CPC Class Codes: H02J 3/1814 wherein al least one reactive element is actively controlled by a bridge converter, e.g. unified power flow controllers [UPFC]
• CPC Class Codes: H02J 3/1857 wherein such bridge converter is a multilevel converter
• CPC Class Codes: Y02E 40/10 Flexible AC transmission systems [FACTS]
• CPC Class Codes: Y02E 40/20 Active power filtering [APF]

Abstract

A power line conditioner using cascade multilevel inverter used for voltage regulation, reactive power (var) compensation and harmonic filtering, including the control schemes for operating the cascade inverter for voltage regulation and harmonic filtering in distribution systems. The cascade M-level inverter consists of (M-1)/2 H-bridges in which each bridge has its own separate DC source. This new inverter (1) can generate almost sinusoidal waveform voltage with only one time switching per line cycle, (2) can eliminate transformers of multipulse inverters used in the conventional static VAR compensators, and (3) makes possible direct connection to the 13.8 kV power distribution system in parallel and series without any transformer. In other words, the power line conditioner is much more efficient and more suitable to VAR compensation and harmonic filtering of distribution systems than traditional multipulse and pulse width modulation (PWM) inverters. It has been shown that the new inverter is specially suited for simultaneous VAR compensation and harmonic filtering.

Claims

1. A power line conditioner operative for:

detecting a harmonic component in current flowing in an electric power line;

computing a control voltage for extracting the harmonic component from the current flowing in the electric power line;

computing a set of triggering phase angles for producing a quasi-square wave approximation of the control voltage;

receiving a direct-current power source feedback signal representative of a continually obtained fully-charged voltage level in a plurality of direct-current power sources of a cascade multi-level inverter;

based on the direct-current power source feedback signal, computing a direct-current phase angle offset for continually obtaining a desired fully-charged voltage level in the direct-current power sources;

driving the cascade multi-level inverter to produce an output voltage corresponding to the quasi-square wave approximation of the control voltage adjusted by the direct-current phase angle offset; and

supplying the output voltage to the electric power line to substantially extract the harmonic component from the current flowing in the electric power line.

2. The power line conditioner of claim 1, wherein the cascade multi-level inverter comprises a separate direct-current power source for each of a plurality of cascade levels.

3. The power line conditioner of claim 2, wherein the cascade multi-level inverter comprises a separate cascade leg of series-connected full-bridge inverters for each of a plurality of phases of the electric power line.

4. The power line conditioner of claim 3, wherein each leg of the cascade multi-level inverter is connected in parallel with a corresponding phase of the electric power line.

5. The power line conditioner of claim 3, wherein each leg of the cascade multi-level inverter is connected in series with a corresponding phase of the electric power line.

6. The power line conditioner of claim 1, further operative for filtering the output voltage before supplying the output voltage to the electric power line.

7. The power line conditioner of claim 1, further operative for detecting the harmonic component in the current flowing in the electric power line by:

obtaining a load current signal for the electric power line; and

filtering the load current signal to remove a fundamental frequency.

8. The power line conditioner of claim 1, wherein:

the quasi-square wave approximation of the control voltage comprises a plurality of quasi-square component waves;

each quasi-square component wave defines a duty cycle for a level of the quasi-square wave approximation of the control voltage; and

the power line conditioner is further operative for rotating the duty cycles among the cascade levels of the cascade multi-level inverter.

9. The power line conditioner of claim 1, wherein the current flowing the electric power line comprises a plurality of phase currents, further operable for:

computing a phase control voltage magnitude and angle for each phase;

computing a phase control voltage waveform corresponding to the phase control voltage magnitude and angle for each phase; and

driving the cascade multi-level inverter to produce a phase output voltage that approximately follows the phase control voltage magnitude and angle for each phase while continually obtaining the desired fully-charged voltage level in the direct-current power sources of the cascade multi-level inverter.

10. The power line conditioner of claim 1, wherein the current flowing the electric power line comprises a plurality of phase currents, further operable for:

computing a multi-phase control voltage magnitude representative of an instantaneous combination of a phase voltage and angle for each phase;

determining a modulation index corresponding to the multi-phase control voltage magnitude;

looking up a predefined set of triggering phase angles for each phase corresponding to the modulation index; and

driving the cascade multi-level inverter to produce a phase output voltage corresponding to the triggering phase angles for each phase while continually obtaining the desired fully-charged voltage level in the direct-current power sources of the cascade multi-level inverter.

11. The power line conditioner of claim 1, further operable for computing the control voltage by:

receiving a current feedback signal representative of a current flowing into the power line conditioner;

receiving a voltage feedback signal representative of a terminal voltage at the point where the power line conditioner supplies the output voltage to the electric power line;

computing an error signal by comparing the current feedback signal to the harmonic component of the current flowing in the electric power line;

computing the control voltage by combining the voltage feedback signal with the error signal adjusted by a gain; and

controlling the gain to minimize the error signal.

12. The power line conditioner of claim 1 further operative for:

detecting a reactive component in the current flowing in an electric power line;

computing a dual-purpose control voltage for simultaneously extracting the harmonic component and the reactive component from the current flowing in the electric power line;

computing a set of triggering phase angles for producing a quasi-square wave approximation of the dual-purpose control voltage;

driving the cascade multi-level inverter to produce a dual-purpose output voltage corresponding to the quasi-square wave approximation of the dual-purpose control voltage adjusted by the direct-current phase angle offset; and

supplying the dual-purpose output voltage to the electric power line to substantially extract the harmonic component and the reactive component from the current flowing in the electric power line.

13. The power line conditioner of claim 12, further operative for detecting the reactive component in the current flowing in the electric power line by:

obtaining a line voltage signal for the electric power line; and

computing a phase angle between the line voltage and load current signals.

14. A power line conditioner comprising:

a terminal for connecting the power line conditioner to the electric power line;

a current sensor for producing a load current signal representative of the current flowing in the electric power line;

a current feedback signal for representing the current flowing into the power line conditioner;

a voltage feedback signal for representing the voltage at the terminal;

a multi-level cascade inverter having a series-connected full-bridge inverter including a separate direct-current power source for each cascade level;

a direct-current feedback signal for representing a continually obtained fully-charged voltage level in the direct-current power sources of the multi-level cascade inverter; and

a control system for;

receiving the load current signal, the direct-current feedback signal, the current feedback signal, and the voltage feedback signal,

detecting a harmonic component in the current flowing in the electric power line,

computing an error signal by comparing the current feedback signal to the harmonic component of the current flowing in the electric power line,

computing a control voltage by combining the voltage feedback signal with the error signal adjusted by a gain,

based on the direct-current power source feedback signal, computing a direct-current phase angle offset for continually obtaining a desired fully-charged voltage level in the direct-current power sources, and

driving the multi-level cascade inverter to produce an output voltage at the terminal that approximates the control voltage adjusted by the direct-current phase angle offset to substantially extract the harmonic component from the current flowing in the electric power line while continually obtaining the desired fully-charged voltage level in the direct-current power sources.

15. The power line conditioner of claim 14, further comprising a filter for smoothing the output voltage connected between the multi-level cascade inverter and the terminal.

16. The power line conditioner of claim 14, wherein the cascade multi-level inverter comprises a separate cascade leg of series-connected full-bridge inverters for each of a plurality of phases of the electric power line.

17. The power line conditioner of claim 16, wherein each leg of the cascade multi-level inverter is connected in parallel with a corresponding phase of the electric power line.

18. The power line conditioner of claim 16, wherein each leg of the cascade multi-level inverter is connected in series with a corresponding phase of the electric power line.

19. The power line conditioner of claim 14, wherein the control system detects the harmonic component in the current flowing in the electric power line by filtering the load current signal to remove a fundamental frequency.

20. The power line conditioner of claim 14, wherein:

the output voltage comprises a quasi-square wave approximation of the control voltage defined by a plurality of quasi-square component waves;

each quasi-square component wave defines a duty cycle for a level of the quasi-square wave approximation of the control voltage; and

the power line conditioner is further operative for rotating the duty cycles among the cascade levels of the cascade multi-level inverter.

21. The power line conditioner of claim 14, wherein the current flowing in the electric power line comprises a plurality of phase currents, and the control system is further operable for:

computing a phase control voltage magnitude and angle for each phase;

computing a phase control voltage waveform corresponding to the phase control voltage magnitude and angle for each phase; and

driving the cascade multi-level inverter to produce a phase output voltage that approximately follows the phase control voltage magnitude and angle for each phase while continually obtaining the desired fully-charged voltage level in the direct-current power sources of the cascade multi-level inverter.

22. The power line conditioner of claim 14, wherein the current flowing the electric power line comprises a plurality of phase currents, and the control system is further operable for:

computing a multi-phase control voltage magnitude representative of an instantaneous combination of a phase voltage and angle for each phase;

determining a modulation index corresponding to the multi-phase control voltage magnitude;

looking up a predefined set of triggering phase angles for each phase corresponding to the modulation index; and

driving the cascade multi-level inverter to produce a phase output voltage corresponding to the triggering phase angles for each phase while continually obtaining the desired fully-charged voltage level in the direct-current power sources of the cascade multi-level inverter.

23. The power line conditioner of claim 14, wherein the control system is further operative for:

detecting a reactive component in the current flowing in an electric power line;

computing a dual-purpose control voltage for simultaneously extracting the harmonic component and the reactive component from the current flowing in the electric power line;

computing a set of triggering phase angles for producing a quasi-square wave approximation of the dual-purpose control voltage;

driving the cascade multi-level inverter to produce a dual-purpose output voltage corresponding to the quasi-square wave approximation of the dual-purpose control voltage adjusted by the direct-current phase angle offset; and

supplying the dual-purpose output voltage to the electric power line to substantially extract the harmonic component and the reactive component from the current flowing in the electric power line.

24. The power line conditioner of claim 23, wherein the control system is further operative for detecting the reactive component in the current flowing in the electric power line by:

obtaining a line voltage signal for the electric power line; and

computing a phase angle between the line voltage and load current signals.

25. A method for conditioning electric power flowing in an electric power line, comprising the steps of:

receiving a load current signal representing the current flowing in the electric power line;

receiving a current feedback signal representing the current flowing into a power line conditioner connected to the electric power line at a terminal;

receiving a voltage feedback signal representing the voltage at the terminal;

receiving a direct-current feedback signal representing a continually obtained fully-charged voltage level in direct-current power sources of a multi-level cascade inverter within the power line conditioner;

detecting a harmonic component in the current flowing in the electric power line;

computing an error signal by comparing the current feedback signal to the harmonic component of the current flowing in the electric power line;

computing a control voltage by combining the voltage feedback signal with the error signal adjusted by a gain;

based on the direct-current power source feedback signal, computing a direct-current phase angle offset for continually obtaining a desired fully-charged voltage level in the direct-current power sources; and

driving the multi-level cascade inverter to produce an output voltage at the terminal that approximates the control voltage adjusted by the direct-current phase angle offset to substantially extract the harmonic component from the current flowing in the electric power line while continually obtaining the desired fully-charged voltage level in the direct-current power sources.

26. The method of claim 25, further comprising the step of smoothing the output voltage.

27. The method of claim 25, further comprising the steps of:

producing a quasi-square wave approximation of the control voltage defined by a plurality of quasi-square component waves in which each quasi-square component wave defines a duty cycle for a level of the quasi-square wave approximation of the control voltage; and

rotating the duty cycles among the cascade levels of the cascade multi-level inverter.

28. The method of claim 27, wherein the current flowing in the electric power line comprises a plurality of phase currents, further comprising the steps of:

computing a phase control voltage magnitude and angle for each phase;

computing a phase control voltage waveform corresponding to the phase control voltage magnitude and angle for each phase; and

driving the cascade multi-level inverter to produce a phase output voltage that approximately follows the phase control voltage magnitude and angle for each phase while continually obtaining the desired fully-charged voltage level in the direct-current power sources of the cascade multi-level inverter.

29. The method of claim 27, wherein the current flowing in the power line comprises a plurality of phase currents, further comprising the steps of:

computing a multi-phase control voltage magnitude representative of an instantaneous combination of a phase voltage and angle for each phase;

determining a modulation index corresponding to the multi-phase control voltage magnitude;

looking up a predefined set of triggering phase angles for each phase corresponding to the modulation index; and

driving the cascade multi-level inverter to produce a phase output voltage corresponding to the triggering phase angles for each phase while continually obtaining the desired fully-charged voltage level in the direct-current power sources of the cascade multi-level inverter.

30. The method of claim 29, further comprising the steps of:

detecting a reactive component in the current flowing in an electric power line;

computing a dual-purpose control voltage for simultaneously extracting the harmonic component and the reactive component from the current flowing in the electric power line;

computing a set of triggering phase angles for producing a quasi-square wave approximation of the dual-purpose control voltage;

driving the cascade multi-level inverter to produce a dual-purpose output voltage corresponding to the quasi-square wave approximation of the dual-purpose control voltage adjusted by the direct-current phase angle offset; and

supplying the dual-purpose output voltage to the electric power line to substantially extract the harmonic component and the reactive component from the current flowing in the electric power line.

31. A computer readable medium storing computer-executable instructions for performing the method of claim 25.

32. A computer readable medium storing computer-executable instructions for performing the method of claim 30.

33. A device operable for performing the method of claim 25.

34. A device operable for performing the method of claim 30.