Method for determining a characteristic curve of a photovoltaic (PV) string, DC/DC converter, and photovoltaic system suitable for carrying out the method

Abstract

The disclosure relates to a method for determining a characteristic curve for a photovoltaic (PV) string of a photovoltaic system having an inverter which is connected to the photovoltaic string and to a power supply network. The photovoltaic string includes a series connection of a plurality of photovoltaic modules, in which series connection at least one of the photovoltaic modules is integrated into the series connection of the photovoltaic modules via a DC/DC converter. The at least one DC/DC converter operates the photovoltaic module assigned thereto in a first operating mode M1 at a maximum power point by varying, over time, a conversion ratio of output voltage (U.sub.Out) to input voltage (U.sub.In), and operates the photovoltaic module in a second operating mode M2 with a conversion ratio of output voltage (U.sub.Out) to input voltage (U.sub.In) that is constant over time. The method includes operating the at least one DC/DC converter in the second operating mode M2 in response to a current signature for the current (I.sub.Str) through the photovoltaic string, determining the characteristic curve by varying the current I.sub.Str or the voltage U.sub.Str of the photovoltaic string by the inverter, and detecting values assigned to one another for current I.sub.Str and voltage U.sub.Str of the photovoltaic string in the second operating mode M2 of the DC/DC converter.

Claims

1. A method for determining a characteristic curve for a photovoltaic (PV) string of a PV system having an inverter, which on an input side thereof is connected to the PV string and on an output side thereof is connected to a power supply network, wherein the PV string comprises a series connection of multiple PV modules, in which at least one of the PV modules is integrated into the series connection of the PV modules via a DC/DC converter, wherein the DC/DC converter of the at least one of the PV modules is suitable and configured to operate the PV module assigned thereto in a first operating mode M1 by temporal variation of a conversion ratio of output voltage (U.sub.Out) to input voltage (U.sub.In) at a maximum power point, and, in a second operating mode M2, to operate the PV module assigned thereto with a temporally constant conversion ratio of output voltage (U.sub.Out) to input voltage (U.sub.In), comprising: operating the DC/DC converter of the at least one of the PV modules in the second operating mode M2 in response to a current signature for a current (I.sub.Str) through the PV string, which current signature is produced by the inverter, determining a characteristic curve by varying the current 1st or a voltage U.sub.Str of the PV string by the inverter and detecting values assigned to one another for current I.sub.Str and voltage U.sub.Str of the PV string in the second operating mode M2 of the DC/DC converter.

2. The method according to claim 1, wherein the detection of the values assigned to one another for current (I.sub.Str) and voltage (U.sub.Str) for determining the characteristic curve is carried out by a measurement unit arranged within the inverter.

3. The method according to claim 1, wherein the current signature comprises the PV string being operated with a current I.sub.Str through the PV string below a limit value I.sub.LV so that I.sub.Str<I.sub.LV for a first time period t.sub.1.

4. The method according to claim 3, wherein the DC/DC converter is operated in the second operating mode M2 for a second time period t.sub.2.

5. The method according to claim 4, wherein, after the second time period t.sub.2 has elapsed, operating the DC/DC converter in the first operating mode M1 or temporarily in a third operating mode M3, wherein in the third operating mode M3 a power draw of the PV module assigned to the DC/DC converter is suppressed.

6. The method according to claim 1, wherein the current signature produced by the inverter is triggered in a time-controlled or event-controlled manner.

7. The method according to claim 1, wherein all of the multiple PV modules of the PV string are integrated into the series connection of the PV modules via a respective DC/DC converter, and wherein, in the second operating mode M2, all DC/DC converters within the PV string are operated with the same temporally constant conversion ratio of output voltage (U.sub.Out) to input voltage (U.sub.In).

8. The method according to claim 7, wherein all DC/DC converters within the PV string are operated with the temporally constant conversion ratio of the value 1 in the second operating mode M2.

9. The method according to claim 1, wherein all of the multiple PV modules of the PV string are integrated into the series connection of the PV modules via a respective DC/DC converter, and wherein the conversion ratio of output voltage (U.sub.Out) to input voltage (U.sub.In) in the second operating mode M2 for all DC/DC converters within the PV string is selected such that the voltage U.sub.Str of the PV string is within a permitted range for an input voltage of the inverter.

10. The method according to claim, 1 wherein the current signature contains an individual identifier of individual DC/DC converters of the PV string, wherein only the individual DC/DC converters are set to operate in the second operating mode M2, and the remaining DC/DC converters of the PV string are set to operate in a third operating mode M3 that suppresses a power draw of the assigned PV module, or wherein only the individual DC/DC converters are set to operate in the third operating mode M3 that suppresses a power draw of the assigned PV module, and the remaining DC/DC converters of the PV string are set to operate in the second operating mode M2.

11. A DC/DC converter having an input for connecting a PV module and an output for connecting the DC/DC converter to further PV modules of the series string of PV modules, wherein the DC/DC converter is configured to: in a first operating mode M1, operate the PV module assigned thereto at a maximum power point by a temporal variation of a conversion ratio of output voltage (U.sub.Out) to input voltage (U.sub.In) of the DC/DC converter, and in a second operating mode M2, operate the PV module assigned thereto at a temporally constant conversion ratio of output voltage (U.sub.Out) to input voltage (U.sub.In) of the DC/DC converter, and assume the second operating mode M2 in response to detection of a current signature contained in a current 1st flowing via the output of the DC/DC converter.

12. The DC/DC converter according to claim 11, wherein the DC/DC converter is configured as a buck converter.

13. A photovoltaic (PV) system comprising: a PV generator having at least one PV string comprising a series connection of multiple PV modules, an inverter connected on an input side thereof to the PV string and on an output side thereof to a power supply network, wherein the inverter is configured to: produce a current signature in the PV string in response to a trigger signal, and subsequently vary, in particular within a second time period t.sub.2, a voltage U.sub.Str of the PV string in order to determine a characteristic curve of the PV string, a measurement unit configured to detect values assigned to one another for current I.sub.Str and voltage U.sub.Str of the PV string, wherein in the at least one PV string, at least one of the PV modules is integrated into the series connection of the PV modules via a DC/DC converter, wherein the DC/DC converter is configured to: in a first operating mode M1, operate the PV module assigned thereto at a maximum power point by a temporal variation of a conversion ratio of output voltage (U.sub.Out) to input voltage (U.sub.In) of the DC/DC converter, and in a second operating mode M2, operate the PV module assigned thereto at a temporally constant conversion ratio of output voltage (U.sub.Out) to input voltage (U.sub.In) of the DC/DC converter, and assume the second operating mode M2 in response to detection of the current signature contained in a current I.sub.Str flowing via the output of the DC/DC converter.

14. The PV system according to claim 13, wherein all of the multiple PV modules of the PV string or all PV modules of the PV generator are connected to the series connection of the PV modules via a DC/DC converter.

15. The PV system according to claim 13, wherein the measurement unit is arranged within the inverter.

16. The PV system according to claim 13, wherein the measurement unit comprises or is connected to a communications unit for communicating the current values 1st and voltage values U.sub.Str.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The disclosure is shown below with reference to figures. These show:

(2) FIG. 1 a first embodiment of a PV system according to the disclosure;

(3) FIG. 2 a second embodiment of a PV system according to the disclosure; and

(4) FIG. 3 a flowchart of a method according to the disclosure.

DETAILED DESCRIPTION

(5) FIG. 1 shows a first embodiment of a PV system 1 according to the disclosure. The PV system 1 comprises a PV generator 4, which is formed by a PV string 3. Within the PV string 3, several PV modules 2 (here: three by way of example) are connected to one another via a series connection. Each of the PV modules 2 is integrated into the series connection of the PV modules 2 via a DC/DC converter 5 according to the disclosure. For this purpose, each of the DC/DC converters 5 has an input 5.1 to which the PV module 2 assigned to the DC/DC converter 5 is connected, and an output 5.2. The PV string 3 is connected to an input 11.1 of an inverter 11. In a first operating mode M1, the DC/DC converters 5 are designed to operate, via temporal variation of the conversion ratio of output voltage U.sub.Out to input voltage U.sub.In, the PV module 2 assigned thereto at its respective MPP operating point, independently of the further PV modules 2 of the PV string 3. In addition, in a second operating mode M2, they are designed to be operated with a temporally constant conversion ratio of output voltage U.sub.Out to input voltage U.sub.In. They are furthermore configured to detect a current signature within a current I.sub.Str flowing via their output 5.2 and to adopt the second operating mode M2 in response thereto.

(6) In FIG. 1, the inverter 11 is formed as an example of a single-stage, three-phase inverter. Alternatively, however, it is also possible for the inverter 11 to have a different number of phases at its output 11.2 and/or to have a multi-stage form (see FIG. 2). The inverter 11 includes a DC/AC converter 8 having an input-side intermediate circuit 7, and a control unit 9 for controlling the DC/AC converter 8. The inverter 11 also includes a measurement unit 6 connected to the input 11.1, an evaluation unit 10, and a communications unit 12. For feeding in regeneratively generated energy, the output 11.2 of the inverter 11 is connected to a power supply network (PSN) 15. The measurement unit 6 is designed to detect several electrical parameters, e.g., a current I.sub.Str and a voltage U.sub.Str of the PV string 3, and to transmit the detected values to the evaluation unit 10. The evaluation unit 10 is configured to evaluate a characteristic curve, e.g., an IU characteristic curve or a PU characteristic curve, from the detected values. It is also able to store the characteristic curve and to compare a currently measured characteristic curve to a previously determined characteristic curve. FIG. 1 shows the evaluation unit 10 as a separate component. Alternatively, however, it is also possible for the evaluation unit 10 to be an integral part of the measurement unit 6 or of the control unit 9. Alternatively, as shown in FIG. 2, it may also be arranged outside of the inverter. The communications unit 12 is designed to communicate the detected values of current I.sub.Str and voltage U.sub.Str, as well as further characteristic curve data, to an external unit, for example, a portal reachable via the Internet. In this way, a current state of the PV system 1 can also be queried by other electronic devices capable of accessing the Internet. The communications unit 12 is also configured to receive a trigger signal from an external communications device, e.g., a smartphone, and to initiate, based upon the trigger signal, an impress of the current signature onto the current I.sub.Str through the PV string 3.

(7) Impressing the current signature onto the current I.sub.Str in the PV string 3 can be achieved in one embodiment by means of an inductively or capacitively operating coupling unit (not shown in FIG. 1) present in any case in the inverter 11. Such coupling units are used, for example, to generate a keep-alive signal in order to leave the PV generator 4, in normal feed mode of the PV system 1, in an activated operating mode, in which power is drawn from the PV generator, and, in the event of danger (discontinuation of the keep-alive signal), to transfer it, by opening a separating unit close to the generator or closing a short-circuit unit close to the generator, into a safe state, in which a power draw from the PV generator is suppressed. Alternatively, however, it is also possible for the DC/AC converter 8 to operate the PV string 3 at an operating point close to no-load during a first time period t1, and thus to produce a current I.sub.Str below a limit value I.sub.LV in the PV string 3.

(8) In FIG. 2, a second embodiment of the PV system 1 according to the disclosure is shown. In many aspects, it is similar to the first embodiment already shown in FIG. 1. Therefore, only the differences from the first embodiment are explained below, while, with respect to the same features, reference is made to the description of FIG. 1.

(9) The PV system 1 illustrated in FIG. 2 includes a PV generator 4 having several (here: two) PV strings 3. Each of the PV strings 3 is connected to a separate input 11.1 of a two-stage inverter 11. In their structure, the PV strings 3 are similar to the PV string 3 of the PV system 1 shown in FIG. 1. Each of the PV modules 2 is integrated into the series connection of the PV modules 2 via a DC/DC converter 5. However, the numbers of PV modules 2 as well as of the DC/DC converters 5 of the two PV strings 3 of the PV generator 4 may differ from one another. In FIG. 2, the inverter 11 is formed as a so-called multi-string inverter, in which each of the inputs 11.1 is connected to a common DC intermediate circuit 7 via a DC/DC converter 13. The common DC intermediate circuit 7 is connected to an input of the DC/AC converter 8. Each of the DC/DC converters 13 is configured to vary a voltage U.sub.Str or a current I.sub.Str of the PV string 3 connected thereto in order to determine a characteristic curve. In addition, each of the DC/DC converters 13 is configured to impress a current signature onto the PV string 3 assigned thereto. This can take place, for example, by the DC/DC converter 13 operating the PV string at its no-load voltage for a first time period t1. For this purpose, a control unit 9 of the inverter 11 is connected in terms of control technology to both the DC/AC converter 8 and the two DC/DC converters 13. In contrast, each of the DC/DC converters 5 of the PV string 3 operating as power optimizers is configured to detect the current signature in the current flowing via its output 5.2 and to assume the second operating mode M2 in response to the detected current signature. In order to detect current values I.sub.Str and voltage values U.sub.Str, each of the inputs 11.1 is connected to a measurement unit 6. For communicating the detected current values I.sub.Str and voltage values U.sub.Str, the measurement units 6 are connected to a communications unit 12, which transmits the values to an external evaluation unit 10. The external evaluation unit 10 can, for example, be an Internet-based portal, which aggregates data of several PV systems in order to compare them to one another for quality statements. Here as well, the inverter 11 is connected, via an output 11.2, to a power supply network 15.

(10) FIG. 3 shows a flowchart of a method according to the disclosure as can be carried out, for example, with the PV system 1 of FIG. 1 or the PV system 1 of FIG. 2.

(11) In a first act S1, the PV system 1 is in normal feed mode, and the DC/DC converters 5 within the PV string 3 operate in the first operating mode M1. The PV modules 2 respectively assigned to the DC/DC converters 5 are here operated at a maximum power point (MPP). In act S2, there is a query as to whether the inverter 11 has received a trigger event. Such a trigger event can, for example, be transmitted via an external communications device, e.g., a smartphone, to the inverter 11 via its communications unit 12. Alternatively, however, the trigger event may also be generated in a time-controlled manner, e.g., by means of an electronic clock implemented within the inverter 11. If there is no trigger event, the method jumps back to act S1. On the other hand, if a trigger event is present, the method jumps to act S3, in which a current signature is produced by the inverter 11 in the PV string 3. In the illustrated case, the PV string 3 is operated during a first time period t.sub.1 at an operating point close to no-load. For the first time period t.sub.1, the inverter 11 thus generates a current I.sub.Str through the PV string 3 below a limit value I.sub.LV, with I.sub.Str<I.sub.LV. The DC/DC converters 5 in the PV string 3 detect the current signature in the current I.sub.Str flowing via their output 5.2. In response, they assume, at act S4, the second operating mode M2 with a constant conversion ratio of output voltage U.sub.Out to input voltage U.sub.In, and maintain it for a second time period t.sub.2. At act S5, the inverter 11 varies the voltage U.sub.Str or the current I.sub.Str of the PV string 3 during the second time period t.sub.2, either via the DC/AC converter 8 (in case of the PV system in FIG. 1) or via one of the DC/DC converters 13 (in case of the PV system in FIG. 2). During the variation, values assigned to one another of current I.sub.Str and voltage U.sub.Str of the PV string are detected by the measurement unit 6. After the second time period t.sub.2 has elapsed, the detection of the values of current I.sub.Str and voltage U.sub.Str is completed. In addition, in act S6, the detected values of current I.sub.Str and voltage U.sub.Str are transmitted to the evaluation unit 10, which evaluates therefrom, at act S7, the characteristic curve of the PV string 3 and, possibly, further characteristic values characterizing the PV string 3. Optionally, the characteristic values, as well as the detected values of current I.sub.Str and voltage U.sub.Str, can be transmitted to a further external evaluation unit, for example an Internet-based portal. The method subsequently jumps back to act S1, at which time the DC/DC converters 5 are operated again in the first operating mode M1.