Photovoltaic power conditioning units

• US 9,520,803 B2
• Filed: 04/04/2013
• Issued: 12/13/2016
• Est. Priority Date: 11/01/2011
• Status: Active Grant

• Current Assignee: Tesla Inc.
• Original Assignee: SolarCity Corporation (Tesla Inc.)
• Inventors: Garrity, Paul, Mumtaz, Asim
• Primary Examiner(s): PATEL, RAJNIKANT B
• Application Number:
  US13/856,575
• Publication Number:
  US 20130294126A1
• Time in Patent Office: 1,349 Days
• Field of Search: 363/95, 363/127, 363/56.01, 363/65, 363/16, 363/17, 363/20, 363/24, 363/26, 363/21.06, 363/37, 363/43, 363/56.03, 363/98, 307/45, 307/64, 307/66, 307/82, 307/77, 307/10.01, 307/9.01, 323/266, 323/299, 323/906, 323/907, 323282-288, 323/234, 323/318
• US Class Current: 1/1
• 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 the string and having at least one tap connection between first and second connections to define a plurality sub-strings of the string, each sub-string having an electrical connection at either end, and

wherein the power conditioning unit comprises;

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

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

2. The PV panel system of claim 1, wherein each input power converter in the set of input power converters is a voltage-increasing dc-to-dc power converter.

3. The PV panel system of claim 2, wherein each input power converter further comprises a dc-to-ac converter, wherein the dc-to-ac converters of the input power converters for the plurality of sub-strings are synchronized.

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

5. The PV panel system of claim 4, wherein the dc-to-ac converters of the input power converters for the plurality of sub-strings are synchronized, such that each input power converter operates at a successive relative phase offset of (180°

/n) or (360°

/n) to reduce a voltage ripple on the shared dc link, wherein n is a number of the input power converters.

6. The PV panel system of claim 1, wherein the plurality of sub-strings lack bypass diodes, wherein the power conditioning unit further comprises:

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

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

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

8. The PV panel system of claim 6 further comprising a temperature sensing system to sense a temperature of the particular sub-string, and wherein the bypass controller is configured to control power conversion from the shaded sub-string responsive to the sensed temperature of the particular sub-string.

9. The PV panel system of claim 6, wherein the power outputs of the input power converters are connected in series, wherein the bypass controller is configured to increase a voltage gain of one or more unshaded input power converters to compensate for the reduced or stopped power conversion from the shaded sub-string.

10. The PV panel system of claim 1 further comprising:

a shared dc link to provide a common dc bus for the set of input power converters, wherein the power output of the input power converters are coupled to the shared dc link to provide power from the sub-strings to the dc bus, wherein the common output power conversion stage receives dc power from the shared dc link to convert to ac power; and

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

11. The PV panel system of claim 1, wherein each of the input power converters has a respective associated sub-string MPPT controller to provide separate MPPT control for each of the plurality of sub-strings of the PV panel.

12. The PV panel system of claim 11, wherein the plurality of 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 the sub-string; and

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

13. The PV panel system of claim 12, wherein each sub-string MPPT controller is configured to control power drawn from a 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 sub-string to reduce or stop the power conversation from the shaded sub-string.

14. The PV panel system of claim 1, wherein each input power converter further 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 the set of power converters responsive to a detected level of power being drawn from a sub-string connected to a corresponding input power converter.

15. The PV panel system of claim 1 further comprising a sub-string sensing system and at least one input power converter controller coupled to the sub-string sensing system to control a level of power conversion of one or more of the input power converters responsive to a sub-string voltage sensed by the sub-string sensing system.

16. The PV panel system of claim 15, wherein the sub-string sensing system comprises a voltage-programmed current source having a current programming input coupled to receive a voltage signal from the sub-string, and having a programmed current output to output a programmed current dependent on a level of the voltage signal, and wherein the programmed current output is coupled to a current-to-voltage converter to convert the programmed current to a voltage dependent on the programmed current for input to the input power converter controller.

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

21. The PV panel system of claim 1, wherein a bypass diode and a switching device are coupled in parallel between the first and second connections of each sub-string, the switching device coupled to be activated by a bypass controller of the power conditioning unit.

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 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 sub-string;

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

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

19. The method of claim 18 further comprising:

detecting when an individual sub-string is shaded; and

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

20. The method of claim 18 further comprising:

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

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