Protective circuit for a photovoltaic (PV) module, method for operating the protective circuit, and photovoltaic (PV) system having such a protective circuit

Abstract

The disclosure relates to a protective circuit for a photovoltaic (PV) module that includes an input having two input terminals for connecting the PV module, an output having two output terminals for connecting further PV modules of a series circuit comprising PV modules, a first switch for connecting one of the input terminals to one of the output terminals, and a controller configured to control the first switch, wherein the protective circuit further includes a series circuit including a first diode and an energy store, wherein the series circuit is arranged in parallel with the input of the protective circuit. The protective circuit also includes a second diode, which connects an output terminal of the protective circuit to a midpoint of the series circuit including the first diode and the energy store, and wherein a series circuit including the second diode and the energy store is connected in parallel with the first switch. The disclosure also relates to a method for operating a protective circuit according to the disclosure, and to a photovoltaic (PV) system including a series circuit comprising PV modules.

Claims

1. A protective circuit for a photovoltaic (PV) module, comprising: an input having two input terminals configured to connect to the PV module, an output having two output terminals configured to connect to further PV modules of a series circuit comprising PV modules, a first switch configured to connect one of the input terminals to one of the output terminals, and a controller configured to control the first switch, wherein the protective circuit further comprises a series circuit comprising a first diode and an energy store, wherein the series circuit is arranged in parallel with the input of the protective circuit, wherein the protective circuit further comprises a second diode, which connects an output terminal of the protective circuit to a midpoint of the series circuit comprising the first diode and the energy store, and wherein another series circuit comprising the second diode and the energy store is connected in parallel with the first switch.

2. The protective circuit as claimed in claim 1, wherein an anode of the first diode is connected to an anode of the second diode via the midpoint of the series circuit comprising the first diode and the energy store, or wherein a cathode of the first diode is connected to a cathode of the second diode via the midpoint of the series circuit comprising the first diode and the energy store.

3. The protective circuit as claimed in claim 1, wherein an anode of the first diode is connected to the input terminal assigned to a positive pole of the PV module, or wherein a cathode of the first diode is connected to the input terminal assigned to a negative pole of the PV module.

4. The protective circuit as claimed in claim 1, wherein the energy store comprises a capacitor, a supercapacitor and/or a rechargeable battery.

5. The protective circuit as claimed in claim 1, wherein the first switch comprises a MOSFET.

6. The protective circuit as claimed in claim 1, wherein the protective circuit further comprises a bypass path in parallel with the input of the protective circuit, wherein the bypass path comprises a third diode, which is connected by its anode to the input terminal of the protective circuit that is assigned to a negative pole of the PV module, and by its cathode to the input terminal of the protective circuit that is assigned to a positive pole of the PV module.

7. The protective circuit as claimed in claim 6, wherein the bypass path comprises a second switch, which is configured to be driven by the controller.

8. The protective circuit as claimed in claim 7, wherein the second switch is a MOSFET, and wherein the third diode of the bypass path is formed by an intrinsic diode of the MOSFET.

9. The protective circuit as claimed in claim 1, wherein the controller comprises a microcontroller (C).

10. The protective circuit as claimed in claim 1, wherein the protective circuit comprises a communication unit configured to receive an external signal for driving the first switch and/or the second switch.

11. The protective circuit as claimed in claim 10, wherein the communication unit is configured to enable power line communication via a line connected to the output terminal, and wherein the protective circuit comprises a path of low impedance for a power line signal in parallel with the output of the protective circuit, wherein the path comprises a series circuit comprising an inductance and a capacitor.

12. The protective circuit as claimed in claim 9, wherein the protective circuit comprises at least one of the following components: a first current sensor configured to detect a charging current flowing in the series circuit comprising the first diode and the energy store to the energy store or flowing away from the energy store, a second current sensor configured to detect an input current flowing between the PV module and one of the input terminals of the protective circuit, a third current sensor for detecting an output current flowing between one of the output terminals of the protective circuit and the further PV modules connected to the protective circuit, a first voltage sensor configured to detect a charging voltage dropped across the energy store, a second voltage sensor configured to detect an input voltage present between the input terminals of the protective circuit, a third voltage sensor configured to detect an output voltage present between the output terminals of the protective circuit, and a fourth voltage sensor configured to detect a switch voltage dropped across the first switch.

13. The protective circuit as claimed in claim 12, wherein the controller is configured to drive the first switch and/or the second switch depending on at least one detected value from: the charging current, the input current, the output current, the charging voltage, the input voltage, the output voltage, and the switch voltage.

14. A method for operating a protective circuit that comprises: an input having two input terminals configured to connect to the PV module, an output having two output terminals configured to connect to further PV modules of a series circuit comprising PV modules, a first switch configured to connect one of the input terminals to one of the output terminals, and a controller configured to control the first switch, wherein the protective circuit further comprises a series circuit comprising a first diode and an energy store, wherein the series circuit is arranged in parallel with the input of the protective circuit, wherein the protective circuit further comprises a second diode, which connects an output terminal of the protective circuit to a midpoint of the series circuit comprising the first diode and the energy store, and wherein another series circuit comprising the second diode and the energy store is connected in parallel with the first switch, the method comprising: supplying the controller of the protective circuit with electrical energy from the energy store, wherein the protective circuit is operated in a first operating state as long as the input voltage U.sub.in present at the input terminals lies within a first value range, wherein in the first operating state the energy store is charged via the input terminals with the first switch permanently closed; wherein the protective circuit is operated in a second operating state as long as the input voltage U.sub.in present at the input terminals lies within a second value range, wherein in the second operating state the energy store is charged via the output terminals by repeated temporary opening of the first switch; and wherein the protective circuit, given the presence of predefined boundary conditions, is additionally operated in a third operating state, in which the first switch is permanently open, wherein in the third operating state the energy store is charged via the input terminals.

15. The method as claimed in claim 14, wherein the protective circuit further comprises a bypass path in parallel with the input of the protective circuit, wherein the bypass path comprises a third diode, which is connected by its anode to the input terminal of the protective circuit that is assigned to a negative pole of the PV module, and by its cathode to the input terminal of the protective circuit that is assigned to a positive pole of the PV module, wherein the bypass path comprises a second switch, which is configured to be driven by the controller, and wherein during the operation of the protective circuit in the second operating state the second switch is permanently closed in order to reduce a power loss.

16. The method as claimed in claim 15, wherein in the third operating state of the protective circuit the input voltage at the input terminals is kept below a second limit value by closing of the second switch, and wherein the second switch is repeatedly temporarily opened for the purpose of charging the energy store.

17. The method as claimed in claim 14, wherein the predefined boundary conditions whose presence results in the protective circuit being operated in the third operating state comprise a signal received by the protective circuit or an absence of a signal previously received by the protective circuit.

18. The method as claimed in claim 14, wherein a repeated temporary opening of the first switch in the second operating state and/orprovided that the protective circuit comprises the second switchthe repeated temporary opening of the second switch in the third operating state takes place upon an undershooting of a lower threshold value of the charging voltage dropped across the energy store and lasts in each case until the charging voltage dropped across the energy store exceeds an upper threshold value.

19. The method as claimed in claim 14, wherein a repeated temporary opening of the first switch in the second operating state and/or a repeated temporary opening of the second switch in the third operating state are/is clocked driving with a duty factor, wherein the duty factor is determined depending on a charging voltage, dropped across the energy store.

20. The method as claimed in claim 14, wherein a transition of the protective circuit into the third operating state comprises at least one of the following acts: operating the first switch for a first time duration in linear operation regulated depending on the switch voltage dropped across the first switch, operating the first switch for a second time duration in linear operation at a temporally rising switch voltage dropped across the first switch, shifting an operating point of the PV module connected to the protective circuit or of all PV modules of the series circuit comprising PV modules in the direction of an open circuit voltage of the PV modules via a photovoltaic PV inverter connected to the series circuit comprising PV modules, and bringing about an associated power reduction of the series circuit comprising PV modules.

21. The method as claimed in claim 15, wherein a transition of the protective circuit into the third operating state comprises: bringing about a power reduction of the series circuit comprising PV modules, before an opening of the first switch, wherein bringing about the power reduction comprises closing the second switch of the protective circuit or closing the second switches of a plurality of protective circuits of PV modules within the series circuit comprising PV modules.

22. The method as claimed in claim 21, wherein closing the second switch of the protective circuit or closing the second switches of a plurality of protective circuits of PV modules within the series circuit comprising PV modules takes place in such a way that the second switch respectively to be closed, upon the transition from the open to the closed state, is operated during a third time duration in linear operation with a temporally decreasing ohmic resistance, wherein the output current is detected, and wherein the first switch is opened if a magnitude of the output current undershoots a current threshold value.

23. A photovoltaic (PV) system comprising a PV inverter with at least one series circuit comprising PV modules that is connected to the PV inverter, wherein at least one of the PV modules comprises a protective circuit that comprises: an input having two input terminals configured to connect to the PV module, an output having two output terminals configured to connect to further PV modules of the series circuit comprising PV modules, a first switch configured to connect one of the input terminals to one of the output terminals, and a controller configured to control the first switch, wherein the protective circuit further comprises a series circuit comprising a first diode and an energy store, wherein the series circuit is arranged in parallel with the input of the protective circuit, wherein the protective circuit further comprises a second diode, which connects an output terminal of the protective circuit to a midpoint of the series circuit comprising the first diode and the energy store, and wherein another series circuit comprising the second diode and the energy store is connected in parallel with the first switch.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The disclosure is explained and described in further detail below on the basis of preferred exemplary embodiments illustrated in the figures.

(2) FIG. 1a shows a first embodiment of the protective circuit according to the disclosure;

(3) FIG. 1b shows a second embodiment of the protective circuit according to the disclosure;

(4) FIG. 2a schematically illustrates a current profile during the first operating state on the basis of the example of a third embodiment of the protective circuit;

(5) FIG. 2b schematically illustrates a current profile during the second operating state on the basis of the example of the third embodiment of the protective circuit from FIG. 2a;

(6) FIG. 2c schematically illustrates a current profile of the protective circuit during the third operating state on the basis of the example of the third embodiment of the protective circuit from FIG. 2a;

(7) FIG. 3 shows temporal profiles of switching states of the first and second switches and corresponding temporal profiles of individual electrical variables during the second operating state on the basis of the example of the protective circuit from FIG. 2a.

(8) FIG. 4 shows temporal profiles of switching states of the first and second switches and corresponding temporal profiles of individual electrical variables during the third operating state of the protective circuit from FIG. 2a.

(9) FIG. 5 shows a flow diagram of a method according to the disclosure for operating the protective circuit from FIG. 2a.

DESCRIPTION OF FIGS

(10) FIG. 1a shows the construction of a first embodiment of a protective circuit 1 according to the disclosure. The protective circuit 1 comprises an input having two input terminals 2a, 2b for connecting a PV module 10 and an output having two output terminals 3a, 3b for connecting further PV modules 10 of a series circuit comprising PV modules 10. In this case, individual, if appropriate also all of the further PV modules 10 of the series circuit can likewise comprise a protective circuit 1 of this type. An input terminal 2a assigned to a positive pole of the PV module 10 is directly connected to a first output terminal 3a of the protective circuit, while an input terminal 2b assigned to a negative pole of the PV module 10 is connected to a second output terminal 3b via a first switch S.sub.1. The first switch S.sub.1 is driven via a controller 9, which is symbolized by means of a dashed arrow. The controller 9 is supplied via an energy store, here illustrated in the form of a capacitor 5. The capacitor is interconnected in series with a first diode 4, wherein the series circuit comprising first diode 4 and capacitor 5 is connected in parallel with the input of the protective circuit. Furthermore, the protective circuit comprises a second diode 6, which is connected anodally to the second output terminal 3b and cathodally to a midpoint 12 of the series circuit comprising first diode 4 and capacitor 5. In this case, the first diode 4 is oriented with regard to its forward direction such that its cathode is connected to the cathode of the second diode 6. The protective circuit 1 furthermore comprises a communication unit 11, which is able to receive a communication signal. In this case, the communication unit 11 is configured in such a way as to receive the communication signal in a wired manner, e.g. by means of power line communication via lines connected to the output terminals 3a, 3b. As an alternative thereto, however, the communication unit can also be configured to receive the communication signal wirelessly, e.g. by radio.

(11) Various current and voltage sensors are connected to the controller 9, which connections are likewise symbolized by dashed arrows. A first current sensor 7.1 measures a charging or discharging current I.sub.ES flowing via the capacitor 5 as energy store. A second current sensor 7.2 measures an input current I.sub.in flowing from the PV module 10 via one of the input terminals 2a, 2bhere the input terminal 2a assigned to the positive pole of the PV module 10. A third current sensor 7.3 measures an output current I.sub.out flowing via one of the output terminals 3a, 3bhere the first output terminal 3ato the adjacent PV modules 10 of the series circuit comprising PV modules 10 that is connected to the protective circuit on the output side. A first voltage sensor 8.1 measures a charging voltage U.sub.ES dropped across the capacitor 5 as energy store, the charging voltage serving as a measure of the state of charge of the energy store. A second voltage sensor 8.2 measures an input voltage U.sub.in present at the input terminals 2a, 2b. In the case of a normal state of the connected PV module 10, the input voltage is positive, i.e. U.sub.in=U(2a)U(2b)>0, and usually corresponds to the MPP voltage of the PV module 10 connected on the input side. In a shading state of the corresponding PV module 10, the input voltage U.sub.in is negative, i.e. U.sub.in=U(2a)U(2b)<0. A third voltage sensor 8.3 measures the output voltage U.sub.out present at the output terminals 3a, 3b. In the first operating state of the protective circuit 1, in which state the first switch S.sub.1 is permanently closed, the input terminals 2a, 2b of the protective circuit 1 are connected with low impedance to the assigned output terminals 3a, 3b and the input voltage U.sub.in is substantially equal to the output voltage U.sub.out. A fourth voltage sensor 8.4 measures a switch voltage U.sub.S1 dropped across the first switch S.sub.1. The controller 9 comprises a microcontroller C and is designed to drive the first switch S.sub.1 depending on at least one detected parameter from the group: charging current I.sub.ES, input current I.sub.in, output current I.sub.out, charging voltage U.sub.ES, input voltage U.sub.in, output voltage U.sub.out and switch voltage U.sub.S1 or a combination thereof. The controller 9 is likewise designed to drive the first switch S.sub.1 depending on the communication signal received by the communication unit 11.

(12) An external bypass path 13 in the form of a bypass diode inherently connected to the PV module 10 is connected to the protective circuit 1 in accordance with FIG. 1a on the input side in parallel with the PV module 10. This is symbolized by a rectangle drawn in a dashed manner around both components in FIG. 1a. In this embodiment, the protective circuit 1 does not comprise a bypass path 13 as an integral part of the protective circuit 1 itself.

(13) FIG. 1b shows an alternative second embodiment of the protective circuit 1 according to the disclosure. The latter corresponds in many details to the first embodiment already described under FIG. 1a, and in this regard reference is made to the corresponding explanations of the description concerning FIG. 1a. At this juncture, rather, a description is therefore given of the differences between the second embodiment and the first embodiment.

(14) In contrast to the protective circuit 1 in accordance with FIG. 1a, in the embodiment in accordance with FIG. 1b, the first switch S.sub.1 is arranged between the input terminal 2a assigned to the positive pole of the PV module and the corresponding first output terminal 3a. The series circuit comprising capacitor 5 and first diode 4 here is connected in parallel with the input of the protective circuit 1 in a manner interchanged in its order. In this case, however, the flow direction of the first diode 4 relative to the polarity of the PV module 10 connected to the protective circuit 1 is unchanged. In both casesFIG. 1a and FIG. 1bthe flow direction of the first diode 4 enables the capacitor 5 to be charged via the input terminals 2a, 2b given a sufficiently high positive input voltage U.sub.in, but prevents the undesired discharge of the capacitor via the input terminals 2a, 2b given a negative input voltage U.sub.in<0. In FIG. 1b, the second diode 6 is connected anodally to the anode of the first diode 4 via the midpoint 12 of the series circuit comprising first diode 4 and capacitor 5, and cathodally to the first output terminal 3a. In both casesFIG. 1a and FIG. 1ba series circuit comprising capacitor 5 and second diode 6 is connected in parallel with the first switch S.sub.1 and thus enables an electrical path to which a current previously flowing via the closed first switch S.sub.1 can commutate upon the opening of the first switch S.sub.1.

(15) In addition to FIG. 1a, the protective circuit 1 in accordance with FIG. 1b comprises two additional input terminals 15a, 15b led out to a housing of the protective circuit 1. The additional input terminals 15a, 15b can be connected to a wire bridge, for example, if, in the maintenance situation, the PV module 10 with the bypass diode as bypass path 13, the bypass diode being inherently assigned to the PV module 10, is removed, in order to ensure an electrical path from the first output terminal 3a via the first switch S.sub.1 to the second output terminal 3b even when the PV module 10 is not present.

(16) FIG. 2a shows a current profile during the first operating state of the protective circuit 1 on the basis of the example of a third embodiment of the protective circuit 1. For the sake of clarity, the illustration here shows only the voltage sensors 8.1, 8.2, 8.3, 8.4 but not the current sensors 7.1, 7.2, 7.3. In addition, the communication unit 11 is not illustrated, for the sake of clarity. Notwithstanding this, the components not illustrated in FIG. 2a can nevertheless be present, however. The third embodiment, too, has commonalities with the first embodiment in accordance with FIG. 1a, and with regard to the commonalities reference is made to the corresponding explanations under the description concerning FIG. 1a.

(17) In contrast to FIG. 1a, the embodiment in FIG. 2a now comprises a bypass path 13 as an integral part of the protective circuit 1. For this purpose, the bypass path 13 is arranged for example in a common housing together with further components of the protective circuit 1. The bypass path 13 comprises a third diode 14 and a semiconductor switch, here a MOSFET, as second switch S.sub.2. In the case of the MOSFET, the third diode 14 can be embodied by a body diode of the MOSFET. It is likewise possible, however, for the bypass path 13 also to comprise a separate third diode 14, particularly if the semiconductor switch as second switch S.sub.2 is not a MOSFET. The first switch S.sub.1 is also embodied as a MOSFET in this embodiment. Both the first switch S.sub.1 and the second switch S.sub.2 are drivable by the controller 9. In the third embodiment of the protective circuit 1, as also in the previous embodiments, a path of low impedance for an alternating current signal as communication signale.g. a power line signalcan be connected in parallel with the output terminals. The path is not illustrated in FIG. 2a for reasons of clarity. The low-impedance path comprises a series circuit comprising inductance and capacitor and ensures that a communication signal arriving at one output terminal 3a, 3b can pass through the protective circuit 1 to the respective other output terminal 3b, 3a even with the first switch S.sub.1 open. In this case, the low-impedance path is of low impedance only for the alternating current signal, i.e. the communication signal, while it is of high impedance or nontransmissive for a direct current.

(18) In the first operating state, the PV module 10 connected to the protective circuit 1 is in a normal state and is operated at the MPP, maximum power point, but at least in the vicinity thereof. The input voltage U.sub.in here is positive, U.sub.in>0, and is substantially equal to the MPP voltage. The first switch S.sub.1 is permanently closed and thus provides a low-impedance path between the second output terminal 3b and the input terminal 2b assigned to the negative pole of the PV module 10. The second switch S.sub.2 is permanently open in the first operating state. In a current profile assigned to the first operating state, a current 20 flows via the second output terminal 3b, the first switch S.sub.1, via the input terminal 2b assigned to the negative pole, the PV module 10, via the input terminal 2a assigned to the positive pole of the PV module 10 and via the first output terminal 3a. In this case, a small partial current 21 flows via the series circuit comprising first diode 4 and energy store, here in the form of the capacitor 5and charges the latter. In the first operating state, the capacitor 5 is charged almost continuously by the partial current 21 provided that the magnitude of the input voltage U.sub.in allows this. For the protection of the consumers, in particular the controller 9, connected to the energy storehere the capacitor 5for the purpose of the supply, the energy store can furthermore comprise means for voltage stabilization or for voltage limiting, such as e.g. zener diodes, series regulators or shunt regulators. These are not illustrated in FIG. 2a for reasons of clarity.

(19) FIG. 2b illustrates the current profile during the second operating state, which is assumed for example during a shading state or a maintenance state of the PV module 10 connected to the protective circuit 1 on the input side. The basis of the illustration is the third embodiment of the protective circuit 1 as already shown in FIG. 2a.

(20) In the shading state of the PV module 10, the input voltage U.sub.in is negative, U.sub.in<0. In this operating state, the second switch S.sub.2 is permanently closed and thus provides a low-impedance connection between the input terminals 2a, 2b via the bypass path 13. The first switch S.sub.1, too, is predominantly closed from a temporal standpoint. It is repeatedly temporarilyi.e. momentarilyopened only for the charging of the energy store, here the capacitor 5, and is then closed again. In the open state, a current 22 previously flowing via the first switch S.sub.1 commutates to the series circuit comprising second diode 6 and energy store, here in the form of the capacitor 5, the series circuit being connected in parallel with the first switch S.sub.1. From the standpoint of this short time period a partial current 23 thus results, which charges the capacitor 5 as energy store. The partial current 23 can be the entire or just a fraction of the current 22 flowing previously via the first switch S.sub.1, depending on the extent to which the first switch S.sub.1 was opened, i.e. what ohmic volume resistance arose across it. In order to differentiate between the two current paths, in FIG. 2b the partial current 23 is illustrated in a dashed manner, while the current 22 is illustrated in a solid manner.

(21) FIG. 2c illustrates the current profile during the third operating state of the protective circuit 1, which is assumed for example during a safety or hazard situation. Here, too, the basis of the illustration is the third embodiment of the protective circuit 1 as already shown in FIG. 2a. The third operating state is assumed by the protective circuit 1 only under predefined boundary conditions, and is therefore signaled to the protective circuit 1 in a suitable manner. This signaling can be effected for example by means of absence of a power line signal that previously was regularly present at the output terminals 3a, 3b of the protective circuit 1 and received by the communication unit 11.

(22) During the operation of the protective circuit 1 in the third operating state, the first switch S.sub.1 is permanently open and an energy emission of the PV module 10 from the input terminals 2a, 2b to the output terminals 3a, 3b is prevented. In addition, the second switch S.sub.2 of the protective circuit is predominantly closed from a temporal standpoint and thus short-circuits the PV module 10 connected to the input. In this way, a redundant safety arises since an energy emission of the PV module 10 is prevented firstly on account of an input voltage being almost 0 V, U.sub.in0 V, and also by means of a disconnection between input and output of the protective circuit. In the third operating state, a short-circuit current 24 of the PV module 10 flows via the input terminals 2a, 2b and the second switch S.sub.2. Since the capacitor 5 as energy store slowly discharges on account of the energy consumption of the controller 9, it must be charged again. The capacitor 5 is charged in the third operating state by the second switch S.sub.2 being repeatedly temporarily opened. While the second switch S.sub.2 is open, a partial current 25 previously flowing via the second switch S.sub.2 then flows via the series circuit comprising first diode 4 and capacitor 5 back to the PV module 10. The capacitor 5 as energy store is charged again by means of the partial current 25. The rise in the input voltage U.sub.in, the rise occurring during the open state of the second switch S.sub.2, and also the time duration of the open state of the second switch S.sub.2 can be kept at very small values here. Moreover, opening of the second switches S.sub.2 of a series circuit comprising PV modules 10 with corresponding protective circuits 1 can be effected in a manner temporally offset with respect to one another. The voltage that occurs even in the case of a fault situation of the first switches S.sub.1 overall at the ends of the series circuit comprising the PV modules 10, i.e. the string voltage, can be minimized as a result. The risk of a disconnection not being properly carried out in the hazard situation can also be reduced again as a result. In FIG. 2c the partial current 25 is illustrated in a dashed manner, while the current 24 flowing in the short-circuit case is illustrated in a solid manner.

(23) In the embodiments in FIGS. 1a, 1b, 2a, 2b and 2c, the energy store is illustrated by way of example as a capacitor 5. As an alternative or in addition to the capacitor 5 illustrated, however, the energy store can also comprise a supercapacitor and/or a rechargeable battery.

(24) FIG. 3 illustrates temporal profiles of switching states of the first switch S.sub.1 and the second switch S.sub.2 and resultant temporal profiles of individual electrical variables during the second operating state of the protective circuit 1. Specifically they are: a temporal profile 33 of the switch voltage U.sub.S1, dropped across the first switch S.sub.1, a profile 34 of the charging voltage U.sub.ES dropped across the capacitor 5 as energy store, and a profile 35 of the output voltage U.sub.out. The descriptions concerning FIG. 3 and FIG. 4 are given by way of example taking account of the third embodiment of the protective circuit 1 in accordance with FIG. 2a.

(25) During the second operating state of the protective circuit 1, the second operating state identifying shading or a maintenance state of the PV module 10, the second switch S.sub.2, as illustrated in the corresponding profile 32, is closed throughout. The first switch S.sub.1 is closed predominantly from a temporal standpoint, i.e. over a long time duration 38, and is open only during a short time duration 37. In this case, in contrast to the illustration shown, the opening of the MOSFET as first switch S.sub.1 need not be effected completely. Rather, it is sufficient if the first switch S.sub.1 is opened only to an extent such that on account of the rising ohmic resistance between its drain and source contacts, a sufficiently large partial current 23 of the current 22 is passed via the series circuit comprising second diode 6 and capacitor 5, the series circuit being arranged in parallel with the first switch S.sub.1. In the short time duration 37, therefore, a raised value of the switch voltage U.sub.S1 can be observed, while the switch voltage U.sub.S1 falls to a small and usually negligible value during the longer time durations 38, within which the first switch S.sub.1 is completely closed. During the short time durations 37, the capacitor 5 is charged by the partial current 23 flowing via it, and this results in a rise in the profile 34 of the charging voltage U.sub.ES dropped across the capacitor 5. In the longer time durations 38, by contrast, a continuous fall in the charging voltage U.sub.ES results on account of the consumption of the controller 9 and, if appropriate, further consumers connected to the capacitor 5. This results in the sawtooth-like or triangular profile 34 of the charging voltage U.sub.ES as illustrated in FIG. 3. The temporal profile 35 of the output voltage U.sub.out of the protective circuit 1 is fixed at a negative value of very low magnitude within the longer time durations 38, during which the first switch S.sub.1 is closed, on account of the second switch S.sub.2 being closed throughout. It is only within the short time durations 37, during which the first switch S.sub.1 is open, that the output voltage U.sub.out approximately mirrors the inverse value of the charging voltage U.sub.ES dropped across the capacitor 5. On account of a relatively low volume resistance of the MOSFET as second switch S.sub.2, at the latter only a low power loss is to be dissipated and an outlay for cooling the second switch S.sub.2 can be kept low and simple, if not even be entirely obviated. The same applies also to the first switch S.sub.1 embodied as a MOSFET, given correspondingly short time durations 37 and/or switch voltages U.sub.S1 kept low.

(26) FIG. 4 illustrates temporal profiles 41, 42 of switching states of the first switch S.sub.1 and of the second switch S.sub.2 and resultant temporal profiles of individual electrical variables during the third operating state of the protective circuit 1. Specifically, FIG. 4 schematically illustrates the temporal profile 43 of the input voltage U.sub.in, the profile 44 of the charging voltage U.sub.ES dropped across the capacitor 5 as energy store, and the profile 45 of the output voltage U.sub.out.

(27) In the third operating state, the PV module 10 is disconnected and an energy emission thereof is suppressed. That is, the energy flow from the PV module to the output of the protective circuit is prevented. The disconnection is effected here in the present case by means of two redundant measures, namely a first switch S.sub.1 being completely open throughout in order to disconnect the output from the input of the protective circuit 1cf. temporal profile 41and a predominantly closed second switch S.sub.2, which additionally short-circuits the PV module 10 in the closed statehere during the long time durations 48. The second switch S.sub.2, as illustrated in the corresponding temporal profile 42, is opened only repeatedly temporarily, i.e. over short time durations 47. During these time durations 47, a partial current 25 is generated via the series circuit comprising first diode 4 and capacitor 5 as energy store, the partial current charging the capacitor 5 again. Accordingly, here, too, the result is a sawtooth-like or triangular profilesimilar to FIG. 3of the charging voltage U.sub.ES dropped across the energy store. The output of the protective circuit 1 is free of voltage throughout, as also illustrated in the profile 45 of the output voltage U.sub.out.

(28) FIG. 5 illustrates a flow diagram of a method according to the disclosure for operating the protective circuit 1 on the basis of the example of a PV system comprising a series circuit comprising a plurality of PV modules 10. In this case, each of the PV modules 10 of the series circuit comprising PV modules is connected to a protective circuit 1. The protective circuits 1 are connected via their output terminals in order to form the series circuit comprising the plurality of PV modules 10. A respective output terminal of the two outer protective circuits 1 is connected to a DC input of a PV inverter, which is in turn connected to a power supply grid in order to feed in generated electrical power. By way of example, the method is illustrated under the assumption that each of the protective circuits 1 is designed in accordance with FIG. 2a.

(29) The method starts at S50, e.g. in the morning at a time of day stipulated by sunrise. The first switch S.sub.1 and the second switch S.sub.2 are completely open at this time. Act S51 then involves checking whether an external signal, a so-called keep-alive signal, is received by the communication unit 11 of the protective circuit 1, whereby an infeed-ready state of the PV system is signaled. In this case, the keep-alive signal can be impressed as an external signal for example by a PV inverter in the context of power line communication on DC connection lines of the series circuit comprising the PV modules 10. If the keep-alive signal is not present (NO), it can be assumed that an infeed-ready state of the PV system is not present. In this case, at a later point in time a check is repeatedly made to establish whether the keep-alive signal is received at S51. If the keep-alive signal is not detected a number of times, then this can be interpreted as a termination condition and the method can be ended prematurely. By contrast, if a keep-alive signal is detected at S51 (YES), then at S52 the input voltage U.sub.in of the protective circuit 1 is measured by means of the second voltage sensor 8.2, which corresponds to the open circuit voltage of the PV module in this switch state of the first switch S.sub.1 and of the second switch S.sub.2. Act S53 involves checking whether the input voltage U.sub.in is greater than or equal to a third limit value U.sub.3, U.sub.inU.sub.3. If this is not the case (NO), then it can be assumed that sufficient irradiation is not yet present, and the method jumps back to S51, which is repeated after a waiting time has elapsed. By contrast, if the input voltage U.sub.in is greater than or equal to the third limit value U.sub.3 (YES at S53), U.sub.inU.sub.3, then in a step S54 the first switches S.sub.1 of the protective circuits are closed. Thus the PV modules 10 of the series circuit comprising PV modules 10 are connected to one another and a current flow from a PV module 10 to the adjacent PV module 10 within the series circuit comprising PV modules 10 can take place. The second switch S.sub.2 is completely opened provided that it is not already completely open anyway.

(30) Act S55 then involves checking once again whether the keep-alive signal is also still being received. If the keep-alive signal is also still present as external signal (YES), that is to say the PV system is still ready for infeed, an energy emission of the PV module 10 connected to the protective circuit 1 can take place.

(31) For the energy emission, the method then jumps to S56, in which the input voltage U.sub.in of the protective circuit 1 is once again detected. For reasons of measurement accuracy, this detection advantageously takes place in a state of the respective protective circuit 1 in which the second switch S.sub.2 is open. In the illustrated case, the value ranges characterizing a magnitude of the input voltage U.sub.in are chosen such that the first value range directly adjoins the second value range. In this case, the first value range is characterized by positive values of the input voltages U.sub.in>0), while the second value range is characterized by negative values of the input voltages U.sub.in(U.sub.in<0). Accordingly, a subsequent act S57 then involves monitoring whether the input voltage U.sub.in detected last is positive, i.e. whether U.sub.in>0 holds true. If this is the case (YES), then a normal state of the corresponding PV module 10 can be assumed and the protective circuit 1 is operated in the first operating state at S58. The first operating state identifies MPP operation of the PV module 10 connected to the protective circuit 1. In the first operating state, the first switch S.sub.1 is closed throughout, while the second switch S.sub.2 is open throughout. The energy storehere the capacitor 5is charged via the input terminals 2a, 2b. By contrast, if the input voltage is negative (NO at S57), i.e. U.sub.in<0 holds true, then the method jumps to S59, in which the protective circuit is operated in the second operating state. In this case, the second operating state identifies a shading state of the PV module 10 connected to the protective circuit 1. Here the second switch S.sub.2 is completely closed in order to minimize the power loss. The first switch S.sub.1 is likewise predominantly closed from a temporal standpoint, but is repeatedly temporarily opened for the purpose of charging the energy storehere the capacitor 5.

(32) By contrast, if at S55 the keep-alive signal is not received by the communication unit 11 of the protective circuit 1 (NO), then this signals a safety or hazard situation of the PV system. In this case, the method changes to S60, in which the protective circuit 1 is operated in the third operating state, identifying the safety or hazard situation. In this operating state, the PV module 10 connected to the protective circuit 1 is disconnected and an energy emission of the PV module is prevented. The disconnection is carried out here in a redundant manner by virtue of the first switch S.sub.1 being operated such that it is open throughout and, in addition, the second switch S.sub.2 being operated such that it is predominantly closed from a temporal standpoint. The second switch S.sub.2 is repeatedly temporarily opened only for the purpose of charging the energy store.

(33) Act S61 involves repeatedly or constantly checking whether a termination condition for ending the method is met. If this is not the case (NO), then the method jumps back to S55, in which receiving the keep-alive signal signals whether the PV system is also still infeed-ready or whether a safety or hazard situation is present. Via the loop in the flow diagram from act S61 back to act S55 the method is also able to change from one operating state to a respective other operating state. In this case, the loop is iterated until the termination condition at S61 is met (YES). The method is then ended at S62. The termination condition can be defined for example by the reaching of a specific time of day at which it can be assumed that sunset has taken place. As an alternative thereto, the termination condition can also comprise repeated occurrence of a keep-alive signal not having been received.