Method for activating a multi-string inverter for photovoltaic plants

• US 8,212,409 B2
• Filed: 02/09/2009
• Issued: 07/03/2012
• Est. Priority Date: 03/22/2008
• Status: Active Grant

• Current Assignee: SMA Solar Technology AG
• Original Assignee: SMA Solar Technology AG
• Inventors: Bettenwort, Gerd, Juchem, Ralf, Victor, Matthias, Müller, Tobias
• Primary Examiner(s): KAPLAN, HAL IRA
• Application Number:
  US12/322,916
• Publication Number:
  US 20090236917A1
• Time in Patent Office: 1,240 Days
• Field of Search: 307/82
• US Class Current: 307/82
• CPC Class Codes: H02J 1/102 being switching converters H02J1/108, H02J1/12 take precedence
• CPC Class Codes: H02J 2300/24 of photovoltaic origin
• CPC Class Codes: H02J 2300/26 involving maximum power point tracking control for photovoltaic sources maximum power point systems in particular G05F1/67
• CPC Class Codes: H02J 3/381 Dispersed generators
• CPC Class Codes: H02M 1/007 Plural converter units in cascade push-pull DC/DC converters with pre-regulator H02M3/3374; DC-AC converters following a DC-DC stage including a high frequency transformer H02M7/4807; DC-AC converters following a DC-DC conversion stage generating periodically varying voltages H02M7/4826
• CPC Class Codes: H02M 3/1584 with a plurality of power processing stages connected in parallel
• CPC Class Codes: H02M 7/48 using discharge tubes with control electrode or semiconductor devices with control electrode
• CPC Class Codes: Y02E 10/56 Power conversion systems, e.g. maximum power point trackers

Abstract

A method of activating a Multi-String inverter for photovoltaic generators (1a, 1b) of a photovoltaic plant (6), the Multi-String inverter incorporating on the input side a separate DC-DC converter (2a, 2b) for each generator string (photovoltaic generator) (1a, 1b) and each output of the DC-DC converters (2a, 2b) being connected in parallel and to an input of a DC-AC converter (3) and the DC-AC converter (3) being connected with an alternating current mains (4) for feeding into the mains aims at improving efficiency. This is achieved in that one or several electrical variables, namely input current, input voltage and/or input power are measured at each DC-DC converter (2a, 2b) and at least one of the DC-DC converters (2a, 2b) changing its operating condition as a function of this measurement when a limit value and/or a range is exceeded in such a manner that its power loss is reduced so that the energy yield of the photovoltaic plant (6) is increased.

Claims

1. A method of controlling a Multi-String inverter for a plurality of photovoltaic generator strings of a photovoltaic plant, said Multi-String inverter incorporating one separate DC-DC converter for each photovoltaic generator string on its input side, each output of said DC-DC converters being connected in parallel and with an input of a DC-AC converter and said DC-AC converter being configured to be connected to an alternating current mains for feeding into the mains, comprising:

measuring an input current, input voltage and/or input power at each DC-DC converter, thereby generating measured input parameters; and

changing an operating condition of at least one of the DC-DC converters based upon a comparison of the measured input parameters of at least two of the DC-DC converters.

2. The method as set forth in claim 1, wherein changing the operating condition of at least one of the DC-DC converters comprises altering a switching of a switch element of the at least one DC-DC converter in such a manner that its input voltage corresponds to its output voltage.

3. The method as set forth in claim 2, wherein changing the operating condition of at least one of the DC-DC converters occurs when the at least one DC-DC converter has a highest input voltage of all the DC-DC converters and when the input voltage thereof is above a mains peak voltage.

4. The method as set forth in claim 2, wherein changing the operating condition of at least one DC-DC converter comprises deactivating at least two DC-DC converters when two input voltages of the at least two DC-DC converters lie within a predetermined voltage range.

5. The method as set forth in claim 2, wherein changing the operating condition of at least one DC-DC converter comprises deactivating at least two DC-DC converters when input voltages at the at least two DC-DC converters are both above a mains peak voltage.

6. The method as set forth in claim 4, further comprising activating the at least one of the DC-DC converters when the input voltage of the at least one DC-DC converter differs by less than a predetermined amount from a mains peak voltage, when a power ratio of the photovoltaic generator strings of the at least two DC-DC converters is above a limit value and/or when a defined time has passed after the at least one DC-DC converter has been deactivated.

7. The method as set forth in claim 1, further comprising detecting partial shadowing of the photovoltaic generators as a function of operating conditions of at least two of the plurality of DC-DC converters.

8. The method as set forth in claim 1, further comprising closing, in an inactive operating condition of a respective DC-DC converter, an additional switch for bridging a semiconductor switch, a diode and/or a choke in in the respective DC-DC converter.

9. The method as set forth in claim 1, wherein each DC-DC converter is configured as a boost converter.

10. An inverter, comprising:

a plurality of DC-DC converters, each of the DC-DC converters comprising a pair of input terminals configured to couple to a photovoltaic module, and a pair of output terminals coupled together in parallel to form a single pair of DC-DC converter output terminals;

a DC-AC converter having a pair of input terminals coupled to the single pair of DC-DC converter output terminals, and having a pair of output terminals configured to couple to a grid; and

a controller configured to selectively control one or more of the DC-DC converters based on input voltages at the input terminals of at least two of the DC-DC converters and a grid voltage at the output terminals of the DC-AC converter.

11. The inverter of claim 10, wherein the controller selectively controls one or more of the DC-DC converters by altering an operating condition of the one or more DC-DC converters.

12. The inverter of claim 11, wherein altering the operating condition comprises deactivating the one or more DC-DC converters, wherein a voltage at the single pair of DC-DC converter output terminals and a voltage at the input terminals of the one or more DC-DC converters are the same.

13. The inverter of claim 10, wherein the controller is configured to deactivate one of the plurality of DC-DC converters if a voltage at the input terminals thereof is greater than a voltage at the input terminals of another of the plurality of DC-DC converters, and the voltage at the input terminals is greater than a peak grid voltage at the output terminals of the DC-AC converter.

14. The inverter of claim 13, wherein a deactivation of the one of the DC-DC converters comprises controlling an operating condition thereof so that a voltage at the single pair of DC-DC converter output terminals and a voltage at the input terminals of the one of the DC-DC converters are the same.

15. The inverter of claim 10, wherein the controller is configured to deactivate two of the plurality of DC-DC converters if the two DC-DC converters each have an input voltage at the respective input terminals that is greater than a peak grid voltage at the output terminals of the DC-AC converter, and if a difference between the voltages at the input terminals of the two converters is less than a predetermined threshold for a predefined period of time.

16. The inverter of claim 15, wherein a deactivation of the two DC-DC converters means controlling an operating condition thereof so that a voltage at the single pair of DC-DC converter output terminals and a voltage at the input terminals of the two DC-DC converters are the same.

17. The inverter of claim 10, further comprising a capacitor coupled across the single pair of DC-DC converter output terminals.

18. An inverter, comprising:

a plurality of DC-DC converters, each of the DC-DC converters comprising a pair of input terminals configured to couple to a photovoltaic module, and a pair of output terminals coupled together in parallel to an intermediate circuit;

a DC-AC converter having a pair of input terminals coupled to the intermediate circuit, and having a pair of output terminals configured to couple to a grid; and

a controller for controlling at least a first DC-DC converter of the plurality of DC-DC converters, wherein the controller is configured to deactivate a first DC-DC converter when a first voltage at the input terminals of the first DC-DC converter exceeds a peak voltage of the grid and the first voltage exceeds voltages at the input terminals of each other of the plurality of DC-DC converters.

19. The inverter of claim 18, wherein during deactivation the first voltage at the input terminals of the first DC-DC converter is substantially equal to a voltage of the intermediate circuit.

20. The inverter of claim 18, wherein during deactivation the inverter is configured to control a working point of a photovoltaic module coupled to the input terminals of the first DC-DC converter via the DC-AC converter.

21. The inverter of claim 18, wherein the controller is further configured to activate the deactivated first DC-DC converter when the first voltage differs from the peak voltage of the grid by less than a predetermined amount.

22. The inverter of claim 18, wherein the controller is further configured to deactivate a second DC-DC converter of the plurality of DC-DC converters, when the first DC-DC converter is deactivated, when a difference between the voltages at the input terminals of the first and the second DC-DC converters falls below a predetermined threshold.

23. The inverter of claim 22, wherein the controller is further configured to activate at least one of the first and second DC-DC converters when the input voltage of the at least one DC-DC converter differs by less than a predetermined amount from a peak voltage of the grid, when a power ratio between photovoltaic modules coupled to the first and second DC-DC converters is above a limit value and/or when a predetermined time has elapsed since one of the first and second DC-DC converters has been deactivated.

24. The inverter of claim 18, wherein the controller is further configured to deactivate a second DC-DC converter of the plurality of DC-DC converters, when the first DC-DC converter is deactivated, based on a comparison between a power loss from operating the second DC-DC converter and a power loss from operating photovoltaic modules coupled to the input terminals of the first and second DC-DC converters at a common working point.

25. The inverter of claim 18, further comprising an additional switch arranged to bridge at least one of a diode, a choke, and a semiconductor switch of the first DC-DC converter.