DEVICE AND METHOD FOR DETECTING A FAULT CURRENT IN A PHOTOVOLTAIC INSTALLATION, AND PHOTOVOLTAIC INVERTER COMPRISING THE DEVICE

Abstract

The disclosure is directed to a detection device for detecting a fault current (I.sub.fault) at a PV generator and/or at DC lines of a PV installation assigned to the PV generator. The PV generator has at least one first PV string and a second PV string, which are connected to a PV inverter of the PV installation via in each case two DC lines. In this case, the detection device has at least one current transformer and an evaluation circuit connected to the at least one current transformer. The current transformer can be used jointly by the first PV string and the second PV string, wherein a measurement signal of the jointly usable current transformer represents a difference between a first summation current (I.sub.sum,1) flowing from the first PV string towards the ground potential (PE) and a second summation current (I.sub.sum,2) flowing from the second PV string towards the ground potential (PE). The fault current (I.sub.fault), when it arises, is a part of the first summation current (I.sub.sum,1) and/or the second summation current (I.sub.sum,2). The disclosure also includes a PV inverter having a corresponding detection device and also a method for detecting a fault current (I.sub.fault).

Claims

1. A detection device configured to detect a fault current (I.sub.fault) at a PV generator and/or at DC lines of a PV installation assigned to the PV generator, wherein the PV generator has at least a first PV string and a second PV string, which are connected via in each case two DC lines to a PV inverter of the PV installation, wherein the detection device has at least one current transformer and an evaluation circuit connected to the at least one current transformer, wherein the current transformer comprises a current transformer jointly used by the first PV string and the second PV string, wherein a measurement signal of the jointly used current transformer represents a difference between a first summation current (I.sub.sum,1) flowing from the first PV string toward a ground potential (PE) and a second summation current (I.sub.sum,2) flowing from the second PV string toward the ground potential (PE), and wherein the fault current (I.sub.fault), when it occurs, is a part of the first summation current (I.sub.sum,1) and/or a part of the second summation current (I.sub.sum,2).

2. The detection device according to claim 1, wherein the jointly used current transformer comprises a jointly used toroidal current transformer through which the two DC lines of the first PV string and the two DC lines of the second PV string pass for detection of the measurement signal.

3. The detection device according to claim 2, wherein the two DC lines of the first PV string are arranged relative to one another such that, during a normal operation of the PV installation, a DC current (I.sub.Str,1) flowing therein passes through the toroidal current transformer in two opposite directions, and wherein the two DC lines of the second PV string are arranged relative to one another such that, during the normal operation of the PV installation, a DC current (I.sub.Str,2) flowing therein passes through the toroidal current transformer in two opposite directions, and wherein a DC line connected to a positive pole of the first PV string is arranged relative to a DC line connected to a positive pole of the second PV string such that, during the normal operation of the PV installation, the DC currents (I.sub.Str,1, I.sub.Str,2) in the DC lines assigned to the positive poles pass through the toroidal current transformer in opposite directions.

4. The detection device according to claim 3, wherein the first PV string comprises a combination of two PV substrings having DC lines, the DC lines of which are arranged relative to one another such that, during the normal operation of the PV installation, the DC currents in the DC lines assigned to the positive poles of the PV substrings pass through the toroidal current transformer in a same direction.

5. The detection device according to claim 4, wherein the first PV substring is configured such that a nominal power assigned thereto is at least 50% smaller than a nominal power of the second PV substring.

6. The detection device according to claim 1, wherein the current transformer is configured for a maximum current value which corresponds to a value of at most 50% of the greater of the two values from among a first leakage current (I.sub.leak,1) and a second leakage current (I.sub.leak,2), wherein the first leakage current (I.sub.leak,1) characterizes a current flowing from the first PV string toward the ground potential (PE) in normal operation of the PV installation, and the second leakage current (I.sub.leak,2) characterizes a current flowing from the second PV string toward the ground potential (PE) in normal operation of the PV installation.

7. The detection device according to claim 1, wherein the PV installation comprises a number of PV strings, wherein the number is greater than two, wherein the detection device has an equal number of toroidal current transformers as the number of PV strings connected to the evaluation circuit of the detection device, wherein for a detection of the measurement signal, each of the toroidal current transformers is passed through by the DC lines of two different PV strings, and wherein the DC lines of each PV string each pass through two different ones of the toroidal current transformers.

8. A method for detecting a fault current (I.sub.fault) on a PV installation, wherein a PV generator of the PV installation has at least a first PV string and a second PV string, which are connected via in each case two DC lines to a PV inverter of the PV installation, with a detection device that comprises at least one current transformer and an evaluation circuit connected to the at least one current transformer, wherein the current transformer comprises a current transformer jointly used by the first PV string and the second PV string, wherein a measurement signal of the jointly used current transformer represents a difference between a first summation current (I.sub.sum,1) flowing from the first PV string toward the ground potential (PE) and a second summation current (I.sub.sum,2) flowing from the second PV string toward the ground potential (PE), and wherein the fault current (I.sub.fault), when it occurs, is a part of the first summation current (I.sub.sum,1) and/or a part of the second summation current (I.sub.sum,2), the method comprising: detecting a measurement signal of the at least one current transformer, wherein the measurement signal represents a difference between a first summation current (I.sub.sum,1) flowing from the first PV string toward the ground potential (PE) and a second summation current (I.sub.sum,2) flowing from the second PV string toward the ground potential (PE); and signaling a fault current (I.sub.fault) when the detected measurement signal exceeds a first threshold value (S.sub.TH,1).

9. The method according to claim 8, wherein the signaling of the fault current (I.sub.fault,1) occurs only when the measurement signal exceeds the first threshold value (S.sub.TH,1) with an abrupt increase, which represents a change in a difference between the first summation current (I.sub.sum,1) and the second summation current (I.sub.sum,2) of at least 10 mA.

10. The method according to claim 8, further comprising: determining a resistive current component of the measurement signal detected by the at least one current transformer; and signaling the fault current (I.sub.fault) only when the measurement signal detected by the current transformer has an abrupt change in the resistive current component by at least 10 mA when the first threshold value (S.sub.TH,1) is exceeded.

11. The method according to claim 8, wherein the PV generator of the PV installation has a number of PV strings, wherein the number is greater than two, and wherein the detection device has an equal number of current transformers, further comprising determining a PV string causing the fault current by two of the PV strings in each case jointly using one of the current transformers for the detection of the measurement signal, and wherein each of the PV strings uses two of the current transformers in each case.

12. The method according to claim 8, wherein, in response to the signaling of the fault current (I.sub.fault), isolating the PV generator or only the PV string of the PV generator causing the fault current (I.sub.fault).

13. The method according to claim 8, wherein DC lines of a first group of PV strings and an equally sized second group of PV strings pass through a shared toroidal current transformer such that each of the PV strings of the first group of PV strings has a first summation current (I.sub.sum,1) that flows toward a ground potential (PE) and is at least approximately compensated by a second summation current (I.sub.sum,2) of a corresponding PV string of the second group flowing toward the ground potential (PE).

14. The method according to claim 8, wherein the two PV strings of a jointly used current transformer are similar to one another such that, in a normal operation of the PV installation, a difference between a first leakage current (I.sub.leak,1) flowing from the first PV string toward the ground potential (PE) and a second leakage current (I.sub.leak,2) flowing from the second PV string toward the ground potential (PE) falls below a second threshold value (S.sub.TH,2).

15. The method according to claim 14, wherein the second threshold value (S.sub.TH,2) corresponds to a value of 25% of leakage current taken from a maximum from among the first leakage current (I.sub.leak,1) and the second leakage current (I.sub.leak,2).

16. A PV inverter comprising an AC output for connection to an AC voltage network and at least two DC inputs configured to connect to at least two PV strings of a PV generator, the PV inverter further comprising a detection device, comprising at least one current transformer and an evaluation circuit connected to the at least one current transformer, wherein the current transformer comprises a current transformer jointly used by a first PV string and a second PV string, wherein a measurement signal of the jointly used current transformer represents a difference between a first summation current (I.sub.sum,1) flowing from the first PV string toward the ground potential (PE) and a second summation current (I.sub.sum,2) flowing from the second PV string toward the ground potential (PE), and wherein a fault current (I.sub.fault), when it occurs, is a part of the first summation current (I.sub.sum,1) and/or a part of the second summation current (I.sub.sum,2), wherein the detection device is configured to: detect a measurement signal of the at least one current transformer, wherein the measurement signal represents a difference between a first summation current (I.sub.sum,1) flowing from the first PV string toward the ground potential (PE) and a second summation current (I.sub.sum,2) flowing from the second PV string toward the ground potential (PE); and signal a fault current (I.sub.fault) when the detected measurement signal exceeds a first threshold value (S.sub.TH,1).

17. The PV inverter according to claim 16, wherein the PV inverter has a nominal power of at least 10 kW.

18. The PV inverter according to claim 16, wherein the PV inverter comprises a transformerless PV inverter.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0031] The disclosure is further explained and described below using exemplary embodiments illustrated in the figures. In the drawings:

[0032] FIG. 1 shows a PV installation having a PV inverter according to the disclosure in one embodiment;

[0033] FIG. 2 shows a detection device according to the disclosure in a first embodiment having two PV strings and a jointly used toroidal current transformer;

[0034] FIG. 3 shows a detection device according to the disclosure in a second embodiment having three PV strings and three toroidal current transformers;

[0035] FIG. 4 shows a flow chart of the method according to the disclosure for detecting a fault current in one embodiment.

DESCRIPTION OF THE FIGURES

[0036] FIG. 1 shows a photovoltaic (PV) installation 1 having an embodiment of a PV inverter 10 according to the disclosure. The PV installation 1 includes a first PV string 2.1 and a second PV string 2.2. Each PV string 2.1, 2.2 has a plurality of PV modules 3 connected in series. The two PV strings 2.1, 2.2 form a PV generator 8. The two PV strings 2.1, 2.2 are designed similarly, in one embodiment, identically, with regard to the number and type of the PV modules 3. In addition, the two PV strings 2.1, 2.2 are arranged close enough together that they are subject to at least similar ambient conditions with regard to irradiation and temperature. By way of example, the photovoltaic inverter 10 is configured as a so-called multi-string inverter. For this purpose, it has at least two DC inputs 11.1, 11.2 for DC lines 4.1, 5.1, 4.2, 5.2 of the two PV strings 2.1, 2.2 from photovoltaic modules 3. Each of the DC inputs 11.1, 11.2 is connected to a shared DC link circuit 14 via a separate DC/DC converter 13. The shared DC link circuit 14 is in turn connected to a DC side of a DC/AC converter 15. The AC side of the DC/AC converter 15 is connected to an AC output 12 of the PV inverter 10. Connected to the example three-phase AC output 12 of the PV inverter 10 in FIG. 1 is an alternating voltage (AC) network 20, which likewise has a three-phase design. The DC inputs 11.1, 11.2 are connected to the DC/DC converters 13 via a current transformer 31; a control circuit or unit 16 of the PV inverter 10 controls the switches of the DC/DC converters 13 and of the DC/AC converter 15 for the desired voltage conversion. The PV inverter 10 furthermore comprises a detection device 30 having the current transformer 31 and an evaluation circuit or unit 32 connected to the current transformer 31. The current transformer 31 is arranged between the inputs 11.1, 11.2 and the DC/DC converters 13 of the PV inverter 10 and, in one embodiment, is configured as a toroidal current transformer. In this case, a measurement signal of the current transformer 31 represents a difference between a first summation current I.sub.sum,1 flowing from the first PV string 2.1 toward the ground potential PE and a second summation current I.sub.sum,2 flowing from the second PV string 2.2 toward the ground potential PE. The evaluation circuit 32 is configured to further process, e.g., evaluate, the measurement signal measured by the current transformer. It is also connected, in terms of control technology and for communication, to the control circuit or unit 16 of the PV inverter 10.

[0037] The first PV string 2.1 has a first parasitic capacitance 6.1 in relation to the ground potential PE. The second PV string 2.2 has a second parasitic capacitance 6.2 in relation to the ground potential PE. Parasitic capacitances 6.1, 6.2 occur in PV modules 3 and are dependent on the ambient conditions, such as humidity, temperature, precipitation or the like.

[0038] A first leakage current I.sub.leak,1 flows via the first parasitic capacitance 6.1 from the first PV string 2.1 in the direction of the ground potential PE. A second leakage current I.sub.leak,2 flows via the second parasitic capacitance 6.2 from the second PV string 2.2 in the direction of the ground potential PE. The leakage currents I.sub.leak,1, I.sub.leak,2 are capacitive reactive currents which result from an AC potential being superimposed on a DC potential of the PV modules 3 in relation to the ground potential. The leakage currents I.sub.leak,1, I.sub.leak,2 together with the parasitic capacitances 6.1, 6.2 are dependent on the ambient conditions of the PV strings 2.1, 2.2, such as humidity, temperature, precipitation, or the like. In some cases, they can change significantly over time, even if rather slowly over time. However, they change in a similar manner for the similar PV strings 2.1, 2.2.

[0039] In the event of a fault, e.g., when a grounded person 7 produces a contact between one of the PV strings 2.1, 2.2 and the ground potential PE, in addition to the leakage current I.sub.leak,1, I.sub.leak,2, a fault current I.sub.fault flows toward the ground potential PE on the PV string 2.1, 2.2 on which the fault was caused. This is illustrated in FIG. 1 by way of example using the example of the first PV string 2.1.

[0040] The measurement signal of the current transformer 31 now represents the difference between the first summation current I.sub.sum,1 and the second summation current I.sub.sum,2. In this case, the first summation current I.sub.sum,1 comprises the sum of the first leakage current I.sub.leak,1 and, in the event of a fault as shown in FIG. 1, the fault current I.sub.fault. The second summation current I.sub.sum,2 comprises the sum of the second leakage current I.sub.leak,2 and the fault current I.sub.fault likewise present there in the event of a fault on the second PV string 2.2. For the case shown in FIG. 1 that no fault is present on the second string 2.2, the second summation current I.sub.sum,2 corresponds to the second leakage current I.sub.leak,2.

[0041] In the case of high-performance PV inverters 10 without galvanic isolation between the DC and AC sides, the vastly predominant part of the summation currents I.sub.sum,1, I.sub.sum,2 corresponds to the leakage currents I.sub.leak,1, I.sub.leak,2 which are caused by the parasitic capacitances 6.1, 6.2. For the protection of persons 7 against electric shock, sudden changes in the summation currents I.sub.sum,1, I.sub.sum,2, as can arise due to the flow of life-threatening currents through human bodies of persons 7, must now be detectable. Such resistive fault currents I.sub.fault are already life-threatening at current intensities which can be significantly below the usual current intensities of the non-hazardous capacitive leakage currents I.sub.leak,1, I.sub.leak,2. In order to now be able to measure a potentially occurring resistive fault current I.sub.fault with such low current intensities reliably and with little effort, it is proposed to dispense with a respectively separate measurement of a summation current I.sub.sum,1, I.sub.sum,2 with a leakage current I.sub.leak,1, I.sub.leak,2 which is in each case possibly high, but similar for both PV strings 2.1, 2.2. Instead, the current transformer 31 outputs as a measurement signal the difference, that is to say the difference between the summation currents I.sub.sum,1, I.sub.sum,2, and thus also the leakage currents I.sub.leak,1, I.sub.leak,2, between the two PV strings 2.1, 2.2. This difference is small according to the aforementioned prerequisite of the similarity of the PV strings 2.1, 2.2. A measurement signal that is usually continuously present, i.e., the measurement signal which is present in normal operation of the PV installation 1 without incidence of a fault, i.e., without a fault current I.sub.fault, is likewise low due to the resulting compensation of the summation currents I.sub.sum,1, I.sub.sum,2.

[0042] A change of the summation current I.sub.sum,1, I.sub.sum,2 in one of the PV strings 2.1, 2.2, for example when a person touches a live conductor, affects only that of the PV strings 2.1, 2.2 on which the fault has occurred and which is thus assigned to the fault. In the event of a fault, the remaining difference between the summation currents I.sub.sum,1, I.sub.sum,2 of the similar PV strings 2.1, 2.2 therefore increases significantly relative to the otherwise compensated and usually permanently present measurement signal. This effect is clearly discernible in the measurement signal of the current transformer 31. Accordingly, a correspondingly sensitive, favorable current transformer 31 can be selected, and the fault current I.sub.fault can be detected with reliable and favorable means.

[0043] FIG. 2 shows a first embodiment of a detection device 30 according to the disclosure such as can be arranged, for example, in a PV inverter 10 according to the disclosure of the PV installation 1 of FIG. 1. The detection device 30 contains a current transformer 31, and an evaluation circuit or unit 32 connected to the current transformer 31. The current transformer 31 in FIG. 2 is formed as a toroidal current transformer 33, which is passed through by the two PV strings 2.1, 2.2 of the PV installation 1. The pass-through is designed such that a DC current I.sub.Str,1 of the first PV string 2.1 flowing in the positive DC line 4.1 is compensated by a DC current I.sub.Str,1 flowing in the negative DC line 5.1 in the opposite direction through the toroidal current transformer 33 and usually of equal magnitude. The same applies to the DC currents I.sub.Str,2 of the second PV string 2.2, which flow in its positive DC line 4.2 and in its negative DC line 5.2. The DC currents I.sub.Str,1, I.sub.Str,2 are thus usually differential-mode signals that flow within the DC lines. These DC currents I.sub.Str,1, I.sub.Str,2 occurring as differential-mode signals in particular do not flow, at least not in normal operation of the PV installation 1 in which no fault current I.sub.fault is present, from the DC lines toward the ground potential.

[0044] In addition to the DC potential on the positive DC lines 4.1, 4.2 and on the negative DC lines 5.1, 5.2 of the two PV strings 2.1, 2.2, an AC potential is respectively superimposed in relation to the ground potential. The AC signal is present for topology-related reasons because the PV inverter 10 to which the PV strings 2.1, 2.2 are connected on the input side is connected on the output side to the AC grid. The AC signal, which is illustrated in FIG. 2 by means of the wavy lines, is usually a common-mode signal in relation to the ground potential. In conjunction with the parasitic capacitances 6.1, 6.2 of the two PV strings 2.1, 2.2, this results in a respective leakage current I.sub.leak,1, I.sub.leak,2 and thus a respective summation current I.sub.sum,1, I.sub.sum,2 that flows from each of the PV strings 2.1, 2.2 toward the ground potential PE. The passage through the toroidal current transformer 33 with regard to the summation currents I.sub.sum,1, I.sub.sum,2 is now such that the first summation current I.sub.sum,1 of the first PV string 2.1 passes through the toroidal current transformer in the opposite direction to the second summation current I.sub.sum,2 of the second PV string 2.2. Since the parasitic capacitances 6.1, 6.2 of the PV strings 2.1, 2.2 are similar, the result in normal operation of the PV strings 2.1, 2.2 (i.e., in the absence of a fault current) is the measurement signal of the toroidal current transformer 33 being compensated as much as possible. If a fault current now occurs in one of the two PV strings 2.1, 2.2, it is added to the leakage current of the faulty PV string and results in a significant measurement signal of the current transformer 33.

[0045] The principle described is also applicable to PV inverters 10 with more than two PV strings in that the DC lines of two similar PV strings in each case are guided in pairs in the manner illustrated in FIG. 2 through a current transformer 31, for example, a toroidal current transformer 33. In the case of a number n of PV strings, a number n/2 of current transformers 31, for example, toroidal current transformers 31, are thus required. Alternatively, however, the DC lines 4.1, 4.2, 5.1, 5.2 of any other even number of similar PV strings 2.1, 2.2 can be combined in this way, namely in that the summation current of one half of the PV strings is guided in the one direction and the summation current of the other half of the PV strings is guided in the opposite direction through the shared toroidal current transformer 33 as a current transformer 31.

[0046] In applications in which an even number of PV strings 2.1, 2.2 having a shared current transformer 31, as shown in FIG. 1 and FIG. 2, is monitored, locating an abrupt change of a summation current I.sub.sum,1, I.sub.sum,2 on one of the PV strings 2.1, 2.2 is possible only indirectly. Although an abrupt change in the entirety of the two PV strings 2.1, 2.2 can be detected, it cannot yet be detected as a result of the design, which of the two PV strings 2.1, 2.2 causes the abrupt increase and thus the fault current I.sub.fault. One possibility for determining the respectively faulty one of the PV strings 2.1, 2.2 is explained in more detail in connection with FIG. 3.

[0047] FIG. 3 shows a second embodiment of a detection device 30 according to the disclosure. In some points, the second embodiment is similar to the first embodiment of the detection device 30 according to FIG. 2, which is why, with regard to the commonalities, reference is made to the figure description of FIG. 2. In the following, the differences between the second and the first embodiment are primarily illustrated.

[0048] In contrast to the first embodiment, the second embodiment of the detection device 30 is configured for a total of three PV strings 2.1, 2.2, 2.3 and includes three toroidal current transformers 33.1, 33.2, 33.3, which are connected to the evaluation circuit 32. The first DC current I.sub.Str,1 of the first PV string 2.1 flows through the first toroidal current transformer 33.1 twice, once each in opposite directions. In addition, the first DC current I.sub.Str,1 of the first PV string 2.1 likewise flows through the third toroidal current transformer 33.3 twice, once each in opposite directions. The second DC current I.sub.Str,2 of the second PV string 2.2 flows through the second toroidal current transformer 33.2 twice, once each in opposite directions. In addition, the second DC current I.sub.Str,2 of the second PV string 2.2 likewise flows through the first toroidal current transformer 33.1 twice, once each in opposite directions. The third DC current I.sub.Str,3 of the third PV string 2.3 flows through the second toroidal current transformer 33.2 twice, once each in opposite directions. In addition, the third DC current I.sub.Str,3 of the third PV string 2.3 flows through the third toroidal current transformer 33.3 twice, once each in opposite directions. Each toroidal current transformer is therefore always flowed through by the DC currents of a respective pair of two of the PV strings 2.1-2.3. In this case, the pass-through is designed such that the leakage currents I.sub.leak,1, I.sub.leak,2 I.sub.leak,3, and also the summation currents I.sub.sum,1, I.sub.sum,2, I.sub.sum,3 for each pair of PV strings 2.1-2.3 with a shared toroidal current transformer 33.1-33.3 pass through the shared toroidal current transformer 33.1-33.3 in opposite directions. In this way, a respectively pairwise compensation of the summation currents I.sub.sum,1, I.sub.sum,2, I.sub.sum,3 results for each of the toroidal current transformers in this case as well.

[0049] With an embodiment shown in FIG. 3, it is possible to locate the PV string 2.1, 2.2, 2.3 that has actually triggered an abrupt change in the summation current. In this case, the toroidal current transformers 33.1, 33.2, 33.3 are used in such a way that the DC lines 4.1, 4.2, 4.3, 5.1, 5.2, 5.3 of two PV strings 2.1, 2.2, 2.3 each are guided through them in the manner of a “daisy chain” so as to bring about compensation.

[0050] Each of the PV strings 2.1, 2.2, 2.3 in FIG. 3 is connected in each case to two particular ones of the three toroidal current transformers 33.1, 33.2, 33.3 that are characteristic of it. An abrupt increase in the summation current I.sub.sum,1, I.sub.sum,2, I.sub.sum,3 on one of the PV strings 2.1, 2.2, 2.3 therefore produces a jump of the corresponding measurement signal in the two toroidal current transformers 33.1-33.3 that are respectively characteristic of it. By means of a comparison as to which two of the total of three toroidal current transformers 33.1-33.3 now exhibit an abrupt increase in the measurement signal, the PV string that has caused the fault can be deduced. In this case, the comparison of the measurement signals of the individual toroidal current transformers and the determination of the faulty PV string 2.1-2.3 can be performed via the evaluation circuit or unit 32.

[0051] The procedure described here can also be transferred to a configuration of the detection device 30 having more than three PV strings 2.1, 2.2, 2.3. In general, a number n of toroidal current transformers is also required for a number n of PV strings 2.1-2.n in order not only to indicate a fault in one of the PV strings 2.1-2.n but also to determine the respectively faulty one of the PV strings 2.1-2.n.

[0052] An inequality of the PV strings 2.1, 2.2, 2.3 considered in pairs and respectively passing through a toroidal current transformer can cause an overregulation of the respective toroidal current transformer 33.1, 33.2, 33.3. Conversely, too great an inequality of the PV strings 2.1, 2.2, 2.3 connected via a toroidal current transformer 33.1, 33.2, 33.3 to the PV inverter 10 can be assumed if the measurement signal of a jointly used toroidal current transformer 33.1, 33.2, 33.3 continuously detects a difference between the summation currents I.sub.sum,1, I.sub.sum,2, I.sub.sum,3 with high current intensity even in normal operation of the PV installation 1, i.e., if this current intensity exceeds a particular rated value as a second threshold value S.sub.TH,2. In such a case, however, appropriate measures can be initiated, e.g., a different grouping with respect to the jointly used toroidal current transformers 33.1-33.3 and/or, in some cases, a change of individual PV strings 2.1-2.3 with respect to the type and number of PV modules 3 assigned to the PV string 2.1-2.3.

[0053] FIG. 4 shows a flow chart of a method according to the disclosure for detecting a fault current according to one embodiment.

[0054] At S1, a measurement signal of the at least one current transformer 31 is detected. The measurement signal represents, for example, a difference between a first summation current I.sub.sum,1 flowing from the first PV string 2.1 toward the ground potential and a second summation current I.sub.sum,2 flowing from the second PV string 2.2 toward the ground potential. When using a detection device 30 which is configured according to FIG. 3 for more than two PV strings 2.1, 2.2, the measurement signal can additionally also include a difference between a first summation current I.sub.sum,1 flowing from the first PV string 2.1 toward the ground potential PE and a third summation current I.sub.sum,3 flowing from the third PV string 2.3 toward the ground potential PE and/or a difference between a second summation current I.sub.sum,2 flowing from the second PV string 2.2 toward the ground potential PE and a third summation current I.sub.sum,3 flowing from the third PV string 2.3 toward the ground potential PE.

[0055] At S2, a check is made as to whether the detected measurement signal exceeds a first threshold value S.sub.TH,1. If not (NO at S2), the method continues at S1. If so (YES at S2), an optional act S3 follows, in which a check is made as to whether the measurement signal changes abruptly. If this is the case (YES at S3), a check is made at optional act S4 as to whether the resistive component of the measurement signal changes abruptly. If this is the case (YES at S4), the method continues at S5. If the check at optional acts S3 or S4 is negative (NO at S3 or S4), the method continues back to S1. If optional acts S3 and S4 are not carried out, the method continues, after a check is made at S2, with act S5 if the detected measurement signal exceeds the first threshold value S.sub.TH,1.

[0056] At S5, a fault current I.sub.fault is signaled if the detected measurement signal according to the check at S2 exceeds the first threshold value S.sub.TH,1. A signaling can, for example, be an alarm signal generated within the evaluation circuit or device 32, which signal can trigger further measures. A signaling of the fault current I.sub.fault can also be initiated by the evaluation circuit or unit 32 via a signal transmitted by radio to an operator of the PV installation 1.

[0057] Act S5 is followed by an optional act S6, which can be carried out, for example, for an embodiment of the detection device 30 according to FIG. 3. At S6, the affected PV string 2.1, 2.2, 2.3 that has triggered the change in the measurement signal and thus the fault current I.sub.fault is determined. After determining the affected PV string 2.1, 2.2, 2.3, it can be isolated at S7 in order to avert possible hazards for persons 7. In this case, the PV installation 1, together with the remaining PV strings not affected by the fault current I.sub.fault, can continue to be operated.