PHOTOVOLTAIC POWER CONDITIONING UNITS

• US 20120081933A1
• Filed: 11/15/2011
• Published: 04/05/2012
• Est. Priority Date: 11/01/2011
• Status: Active Grant

• Current Assignee: Tesla Inc.
• Original Assignee: Enecsys Limited
• Inventors: Garrity, Paul
• Granted Patent: US 8,526,205 B2
• Time in Patent Office: Days
• US Class Current: 363/37
• CPC Class Codes: H02J 2300/24 of photovoltaic origin
• CPC Class Codes: H02J 3/381 Dispersed generators
• CPC Class Codes: H02J 3/44 with means for ensuring correct phase sequence
• CPC Class Codes: H02J 3/46 Controlling of the sharing of output between the generators, converters, or transformers
• CPC Class Codes: H02M 7/4807 having a high frequency intermediate AC stage
• CPC Class Codes: Y02E 10/56 Power conversion systems, e.g. maximum power point trackers

Abstract

We describe a photovoltaic (PV) panel power conditioning circuits, in particular for a PV panel with multiple sub-strings of connected solar cells. The power conditioning unit comprises a set of input power converters, one connected to each sub-string, a shared dc link to provide a common dc bus for the set of input power converters, and a common output power conversion stage coupled to the shared dc link to convert power from the shared dc link to ac power for a mains power supply output from the power conditioning unit. Local conversion of the sub-strings facilitates control of the power available from the panel and optimum energy harvesting, as well as local maximum power point tracking (MPPT) adjustment.

Claims

1. A photovoltaic (PV) power conditioning unit for delivering power from a PV panel to an ac power supply output, the PV panel comprising a string of series-connected solar cells with a least one electrical tap connection on said string to define a set of series-connected sub-strings of said solar cells, the power conditioning unit comprising:

a set of input power converters, one for each said sub-string, each having a pair of dc input connections for connection to either end of a said sub-string, each of said input power converters having a power output;

a shared dc link to provide a common dc bus for said set of input power converters, wherein said power output of each said input power converter is coupled to said shared dc link to provide power from a said sub-string to said dc bus;

an energy storage capacitor, coupled to said shared dc link, to store power from said PV panel for delivering to said ac power supply output; and

a common output power conversion stage coupled to said shared dc link to convert dc power from said shared dc link to ac power to provide an ac output from said power conditioning unit.

2. The photovoltaic power conditioning unit of claim 1, wherein a said input power converter is a voltage-increasing dc-to-dc power converter.

3. The photovoltaic power conditioning unit of claim 2, wherein a said input power converter comprises a dc-to-ac converter, wherein said dc-to-ac converters of said input power converters for said sub-strings are synchronised, wherein secondary windings of said transformers are connected in parallel.

4. The photovoltaic power conditioning unit of claim 3, wherein said dc-to-ac converters drive a shared power coupling transformer, wherein said power output of an input power converter comprises a winding of said shared power coupling transformer, and wherein each said input power converter is coupled to said shared dc link by a common output winding of said shared power coupling transformer.

5. The photovoltaic power conditioning unit of claim 1, wherein a said input power converter comprises a dc-to-ac converter, wherein said dc-to-ac converters of said input power converters for said sub-strings are synchronised such that each operates at a successive relative phase offset of (180°

/n) or (360°

/n) where n is a number of a said input power converters, to reduce a voltage ripple on said dc link.

6. The photovoltaic power conditioning unit of claim 1 further comprising:

a sensor to sense one or both of a voltage on a sub-string and a current provided by a said sub-string; and

a bypass controller coupled to said sensor to detect shading of a said sub-string and, responsive to said detection, to control a said input power converter connected to said shaded sub-string to reduce or stop power conversion by said input power converter from said shaded sub-string.

7. The photovoltaic power conditioning unit of claim 6, wherein a said input converter comprises a dc-to-ac converter comprising a set of switches, wherein said bypass controller is configured to control said switches to switch on, responsive to detection of said shading, to provide a bypass current path for said shaded sub-string through the input power converter for the shaded sub-string.

8. The photovoltaic power conditioning unit of claim 6, wherein said power outputs of said input power converters are connected in series, wherein said bypass controller is configured to increase a voltage gain of one or more unshaded said input power converters to compensate for said reduced or stopped power conversion from said shaded sub-string.

9. The photovoltaic power conditioning unit of claim 1 further comprising an output power converter MPPT control system, wherein said output power converter MPPT control system is configured to control said common output power conversion stage to maximise said dc power drawn from said shared dc link.

10. The photovoltaic power conditioning unit of claim 1, wherein each of said input power converters has a respective associated sub-string MPPT controller to provide separate MPPT control for a said sub-string of said PV panel.

11. The photovoltaic power conditioning unit of claim 10 further comprising:

a sensor to sense one or both of a voltage on a sub-string and a current provided by a said sub-string; and

a bypass controller coupled to said sensor to detect shading of a said sub-string and, responsive to said detection, to control a said input power converter connected to said shaded sub-string to reduce or stop power conversion by said input power converter from said shaded sub-string, wherein a said sub-string MPPT controller includes said bypass controller.

12. The photovoltaic power conditioning unit of claim 1, wherein each said input power converter comprises:

a set of power converters having respective inputs and outputs connected in parallel; and

a power level controller to selectively enable operation of power converters of said set of power converters responsive to a detected level of power being drawn from a corresponding sub-string to which the set of power converters is connected.

13. The photovoltaic power conditioning unit of claim 1 further comprising:

a sub-string sensing system; and

at least one input power converter controller coupled to said sub-string sensing system to control a level of power conversion of one or more of said input power converters responsive to a sub-string voltage sensed by said sub-string sensing system.

14. The photovoltaic power conditioning unit of claim 13, wherein said sub-string sensing system comprises a voltage-programmed current source having a current programming input coupled to receive a voltage signal from a said sub-string, and having a programmed current output to output a programmed current dependent on a level of said voltage signal, and wherein said programmed current output is coupled to a current-to-voltage converter to convert said programmed current to a voltage dependent on said programmed current for input to said input power converter controller.

15. A photovoltaic (PV) power conditioning unit for delivering power from a PV panel to an ac power supply output, the PV panel comprising a string of series-connected solar cells with a least one electrical tap connection on said string to define a set of series-connected sub-strings of said solar cells, the power conditioning unit comprising:

a sensing circuit to sense a voltage or current from one or more of said solar cells, said sensing circuit comprising;

a sense input to receive a voltage signal from said one or more solar cells dependent on said sensed voltage or current; and

a voltage-programmed current source having a current programming input coupled to said sense input, and having a programmed current output to output a programmed current dependent on a level of said voltage signal, wherein said programmed current output is coupled to a current-to-voltage converter to convert said programmed current to a voltage output from the sensing circuit dependent on said programmed current.

16. The photovoltaic power conditioning unit of claim 15, wherein said voltage input is coupled to said current programming input of said programmed current source via a full-wave rectification circuit.

17. The photovoltaic power conditioning unit of claim 15 further comprising a current mirror coupled between said programmed current output of said programmed current source and said current-to-voltage converter.

18-20. -20. (canceled)

21. A photovoltaic power conditioning unit for extracting power from a PV panel and providing an ac output, the PV panel comprising a set of series-connected sub-strings of solar cells, the power conditioning unit comprising:

at least one power converter to extract power from a said sub-string for said ac output;

a sensing system to detect that a current source representing said sub-string is starting to reverse; and

a current bypass system coupled to said sensing system to, responsive to said detection, bypass said sub-string in which said current source is starting to reverse.

22. The photovoltaic power condition unit of claim 21 further comprising a set of said power converters, each to provide power from a respective said sub-string to said ac output, and each having a said sensing system and a said current bypass system, wherein said current bypass system comprises one or more semiconductor switching devices connected across the input of a said power converter.

23. The photovoltaic power condition unit of claim 22 further comprising an MPPT controller for each said power converter to control the power converter to maximise power extracted from a respective said sub-string.

24-26. -26. (canceled)