PHOTOVOLTAIC POWER CONDITIONING UNITS

• US 20120081934A1
• 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, Mumtaz, Asim
• Granted Patent: US 8,472,220 B2
• Time in Patent Office: Days
• US Class Current: 363/37
• CPC Class Codes: H02J 2300/24 of photovoltaic origin
• CPC Class Codes: H02J 2300/26 involving maximum power point tracking control for photovoltaic sources
• CPC Class Codes: H02J 3/381 Dispersed generators
• CPC Class Codes: H02J 3/40 Synchronising a generator for connection to a network or to another generator
• CPC Class Codes: H02M 3/285 Single converters with a plurality of output stages connected in parallel parallel operation of a plurality of converters in dc distribution networks H02J1/10
• CPC Class Codes: H02M 5/458 using semiconductor devices only
• CPC Class Codes: H02M 7/46 using discharge tubes without control electrode or semiconductor devices without control electrode
• 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 system comprising a PV panel with multiple sub-strings of connected solar cells in combination with a power conditioning unit (microinverter). The power conditioning unit comprises a set of input power converters, one connected to each sub-string, and a common output power conversion stage, to provide power to an ac mains power supply output. Integration of the micro-inverter into the solar PV module in this way provides many advantages, including greater efficiency and reliability. Additionally, embodiments of the invention avoid the need for bypass diodes, a component with a high failure rate in PV panels, providing lower power loss and higher reliability.

Claims

1. A photovoltaic (PV) panel system comprising:

a PV panel in combination with a power conditioning unit for providing ac power from the PV panel,

wherein the PV panel comprises a string of series connected solar cells having first and second connections to either end of said string and having at least one tap connection between first and second connections to define at least two sub-strings of said string, each sub-string having an electrical connection at either end,

wherein said power conditioning unit comprises;

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

a common output power conversion stage coupled to said a set of input power converters to provide an ac output from said power conditioning unit.

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

3. The photovoltaic panel system 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 panel system 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, 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 panel system 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 panel system of claim 1, wherein said sub-strings lack bypass diodes, wherein the power conditioning unit further comprises:

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 (PV) panel system 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 (PV) panel system of claim 6 further comprising a temperature sensing system to sense a temperature of a said sub-string, wherein said bypass controller is configured to control power conversion from said shaded sub-string responsive to said sensed temperature of said sub-string.

9. The photovoltaic (PV) panel system 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.

10. The photovoltaic (PV) panel system of claim 1 comprising:

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, wherein said common output power conversion stage receives dc power from said shared dc link to convert to ac power; and

an output power converter MPPT control system configured to control said common output power conversion stage to maximise said dc power drawn from said shared dc link.

11. A photovoltaic (PV) panel system 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.

12. The photovoltaic (PV) panel system as claimed in claim 11, wherein said sub-strings lack bypass diodes, wherein the power conditioning unit further comprises:

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.

13. The photovoltaic (PV) panel system of claim 12, wherein a said sub-string MPPT controller is configured to control power drawn from a respective sub-string to sweep over a power range to determine a shape of an I-V curve for the sub-string, to identify shading of the respective sub-string to reduce or stop said power conversation from said shaded sub-string.

14. The photovoltaic (PV) panel system 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.

17. The photovoltaic (PV) panel system of claim 1, wherein said power conditioning unit comprises a circuit on a circuit board mounted within an environmentally sealed enclosure on said PV panel, wherein said circuit board is connected within said enclosure to said first and second connections and to said at least one tap.

15-16. -16. (canceled)

18. A method of providing power from a photovoltaic (PV) panel, the PV panel comprising a plurality of sense-connected sub-strings of solar cells, each said sub-string comprising a plurality of series-connected solar cells, the method comprising:

providing a set of input power converters, one input power converter for each said sub-string;

supplying power from each said sub-string to a common dc bus using said set of input power converters; and

converting power from said dc bus to an ac power output from said PV panel.

19. The method of claim 18 further comprising:

detecting when an individual said sub-string is shaded; and

bypassing the input power converter for said shaded sub-string responsive to said detecting.

20. The method of claim 18 further comprising:

controlling said supplying of power from each said sub-string to said common dc bus using a plurality of sub-string MPPT control loops, one for each of said set of input power converters; and

controlling said converting of power from said dc bus to ac power using at least one ac power output MPPT control loop.

21. A photovoltaic (PV) panel system comprising a PV panel in combination with a power conditioning unit for providing ac power from the PV panel,

wherein the PV panel comprises a string of series connected solar cells having first and second connections to either end of said string and having at least one tap connection between first and second connections to define at least two sub-strings of said string, each sub-string having an electrical connection at either end,

wherein said power conditioning unit comprises a circuit on a circuit board mounted within an environmentally sealed enclosure on said PV panel, wherein said circuit board is connected within said enclosure to said first and second connections and to said at least one tap,

wherein said power conditioning unit further comprises:

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 from said shaded sub-string.

22. The photovoltaic (PV) panel system of claim 21, wherein said power conditioning unit comprises a set of input power converters, one input power converter for each said sub-string,

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.

23. (canceled)