DE102020115757A1 - DEVICE AND METHOD FOR DETECTING A FAULT CURRENT IN A PHOTOVOLTAIC SYSTEM, AS WELL AS PHOTOVOLTAIC INVERTER WITH THE DEVICE - Google Patents

Abstract

The application describes a detection device (30) for detecting a fault current (IFehl) on a PV generator (8) and / or on the PV generator (8) assigned DC lines (4.1 - 4.3, 5.1 - 5.3) of a PV System (1), wherein the PV generator (8) has at least a first PV string (2.1) and a second PV string (2.2), which each have two DC lines (4.1, 5.1, 4.2, 5.2) with a PV inverter (10) of the PV system (1) are connected. The detection device (30) has at least one current transformer (31) and an evaluation unit (32) connected to the at least one current transformer (31). The current transformer (31) is designed as a current transformer (31) that can be used jointly by the first PV string (2.1) and the second PV string (2.2), a measurement signal from the jointly usable current transformer (31) being a difference between one of the first PV string (2.1) against the earth potential (PE) flowing first total current (ISum, 1) and a second total current (Isum, 2) flowing from the second PV string (2.2) against the earth potential (PE), and where the Fault current (IFehl), if it occurs, is a component of the first total current (ISum, 1) and / or the second total current (Isum, 2). The application also describes a PV inverter (10) with a corresponding detection device (30), as well as a method for detecting a fault current (IFehl).

Description

Technical field of the invention

The invention relates to a device and a method for detecting a fault current in a photovoltaic system (PV system). The fault current can in particular be a relatively small resistive current, which can occur in addition to a relatively large capacitive leakage current that is always present during normal operation of the PV system. The invention also relates to a photovoltaic inverter (PV inverter) with such a device.

State of the art

A photovoltaic system (PV system) can comprise a large number of electrical components, in particular photovoltaic (PV) modules, which are distributed in a decentralized manner over a large area. A group of PV modules that is grouped string by string, i.e. one below the other in the form of a series connection, is also called a photovoltaic string (PV string). A photovoltaic (PV) generator of a photovoltaic (PV) system can have one or more photovoltaic (PV) sub-generators, which are connected in parallel to each other via a separate DC / DC converter with a common DC voltage intermediate circuit (DC DC link) of a PV inverter are connected. Each of the PV sub-generators can have one or more PV strings connected in parallel to one another. Due to the design, the PV modules of a PV system always have an electrical capacity in relation to their surroundings, in particular in relation to their mostly grounded mounting. This capacity is not absolutely necessary for the function of the PV system, but inevitably results from the mechanical structure of the PV modules. It is therefore often referred to as "parasitic capacitance" or "leakage capacitance". The parasitic capacity of the PV system usually increases with the size of the PV generator assigned to it, which is why a powerful PV generator also has a correspondingly large parasitic capacity. In addition, the parasitic capacitance is dependent on ambient conditions and increases further, for example in the event of rain, due to an associated moist surface of the PV modules and / or a changed dielectric constant of the air due to increased air humidity.

In normal operation of the PV system, the PV modules of the PV generator usually have, in addition to a direct voltage (DC voltage) between their connections, an alternating voltage (AC voltage) with respect to the earth potential ( PE ) on. The AC voltage of the PV modules is particularly, but not exclusively, pronounced in PV systems with a transformerless PV inverter. The AC voltage of the PV modules compared to the ground potential depends on the individual circuit topology of the PV inverter and usually has a frequency that corresponds to a frequency of the AC network or an integral multiple of the frequency of the AC network to which the PV inverter is connected on the output side. Due to the AC voltage of the PV modules and their parasitic capacitance in relation to the earth potential, there is always a more or less strong leakage current of the PV generator compared to the earth potential, even during normal operation of the PV system.

If it comes due to an error, e.g. B. a defective line insulation, to a contact of a grounded person with a live component of the PV generator, for example the defective line, then there is - due to the direct contact usually abruptly - an additional fault current to the ground potential. Since a fault current above a value of approx. 30 mA can be hazardous to people, the normative requirement is to reliably detect such a fault current and, if such a fault current is detected, further measures, for example switching off and / or short-circuiting the PV generator, in particular the relevant one Initiate the PV sub-generator.

Due to the ever increasing nominal power of PV systems, the parasitic capacities of the assigned PV generators or PV sub-generators and thus the capacitive leakage currents that are always present during normal operation of the PV system also increase. The threshold value of 30 mA, for example, assigned to the fault current, however, remains constant and, if necessary, can still be reduced due to stricter normative restrictions. Any fault current that may be present can therefore be significantly smaller compared to the capacitive leakage current that is always present in the PV system. The detection of the fault current is therefore becoming increasingly complex and expensive due to the low signal-to-noise ratio and the associated sensitive measurement systems. It is therefore desirable, especially in larger PV systems, to be able to detect a potentially occurring fault current reliably and yet inexpensively, especially when the potentially occurring fault current is small compared to the capacitive leakage current that is always present during normal operation of the PV system.

From the pamphlet EP 2372857 A1 a method for determining a fault current component of a differential current is known, the differential current being composed of a leakage current component and a fault current component. In which In the procedure, the differential current is recorded as the sum of the current across the AC lines carrying the current from an alternating current generator. An electrical signal that is dependent on voltages on the alternator in relation to the earth potential is multiplied by a scaling factor with regard to a capacitance in relation to the earth potential. Finally, the electrical signal scaled in this way is subtracted from the differential current as a measure for the leakage current component.

Object of the invention

The invention is based on the object of specifying a device and a method with which even small fault currents can be reliably detected within the usually high leakage currents of a PV system. The device / method should be able to be implemented simply and inexpensively. It is also an object of the invention to provide a PV inverter suitable for carrying out the method.

solution

The object of specifying a device for detecting a fault current in a PV system is achieved according to the invention by a device with the features of independent claim 1. The object of specifying a method for detecting a fault current in a PV system is achieved according to the invention by a method having the features of independent claim 6. The object of showing a PV inverter according to the invention is achieved according to the invention with the features of the independent claim 14. Advantageous embodiments of the detection device are reproduced in the dependent claims 2 to 5, and advantageous embodiments of the method in the dependent claims 7 to 13. Advantageous embodiments of the PV inverter are listed in the dependent claims 15 and 16.

Description of the invention

A detection device for detecting a fault current on a PV generator and / or on the DC lines of a PV system assigned to the PV generator has at least one current converter and an evaluation unit connected to the at least one current converter. The PV generator has at least a first PV string and a second PV string, which are each connected to a PV inverter of the PV system via two DC lines. The current converter is designed as a current converter which can be used jointly or jointly by the first PV string and the second PV string. A current transformer is a measuring device that converts a current into a measurement signal that can be processed further. A measurement signal from the current transformer represents a difference between a first total current flowing from the first PV string towards ground potential and a second total current flowing from the second PV string towards ground potential. If a fault current occurs, it is in particular a component of the first and / or the second total current. The fault current can therefore also be detected via the difference between the first total current and the second total current.

Such a detection device has the advantage that, as a rule, during normal operation of the PV system, there is always an earth potential ( PE ) The leakage current flowing from two PV strings is largely compensated for, and can also be extinguished under certain circumstances. The first total current includes, in particular, the sum of a first leakage current of the first PV string flowing against ground potential and - if it occurs - a fault current on the first PV string that also flows against ground potential. The second total current comprises in particular the sum of a second leakage current of the second PV string flowing against ground potential and - if it occurs - a fault current on the second PV string that also flows against ground potential. The first PV string and the second PV string are preferably the same in terms of both the number of PV modules and their type, but at least have a similar structure. In addition, the PV strings are preferably adjacent, ie arranged spatially close to one another. In this way, there are similar environmental conditions at the first and the second PV string. The two PV strings are therefore designed to be similar in terms of their parasitic capacitances, so the first leakage current can largely be canceled out, or at least significantly reduced, by forming the difference with the second leakage current.

The same also applies in the opposite case, namely that the second leakage current can also be largely compensated for by forming the difference with the first leakage current and, under certain circumstances, can even be canceled. In this way, a background signal of the leakage currents which is usually present during normal operation of the PV system and which superimposes the fault current that may occur is reduced as far as possible. If a fault current occurs in one of the total currents in addition to the leakage current that is always present, the fault current is more pronounced from the background signal, the more compensating the difference between the leakage currents of the first and second PV string is. In any case, a measurement signal representing the fault current is more significant than would be the case without the formation of the difference between the total currents. Since the background signal from the current transformer is significantly reduced, the current transformer can be designed for a small nominal current that also detects small fault currents. The usually high background signal or noise signal, ie the entire and usually much larger leakage current of a PV string, is not measured, at least only to a significantly reduced extent, due to the difference between the two total currents. In addition, the same changing environmental conditions, which usually have a similar effect on the parasitic capacitances of the neighboring PV strings, can be at least partially compensated for.

Advantageous embodiments of the invention are specified in the following description and the subclaims, the features of which can be used individually and in any combination with one another.

In one embodiment of the detection device, the jointly usable current converter is designed as a jointly usable forward converter. A forward converter has at least one coil through which a magnetic field of the current-carrying conductor passes through it in order to detect a current flowing through a conductor. The coil can at least partially surround the current-carrying conductor at one point along its circumference. The forward converter can also be used to detect a combination of currents flowing in several conductors at a time. This is the case when the current-carrying conductors are surrounded jointly by the forward converter, that is to say the current-carrying conductors jointly pass through the forward converter. Here, the magnetic fields that are generated by the currents flowing in the multiple conductors also penetrate the at least one coil of the forward converter at the same time. Currents whose magnetic fields penetrate the at least one coil of the forward converter in the same directions are added to one another, while currents that penetrate the at least one coil of the forward converter in opposite directions are subtracted from one another. The subtraction of the currents is the case, for example, when the currents flowing in the two conductors pass through the flow converter in opposite directions. In the embodiment of the invention, the flow converter for detecting the measurement signal is now penetrated by the two DC lines of the first PV string and the two DC lines of the second PV string. The two DC lines of the first PV string are oriented relative to the two DC lines of the second PV string in such a way that the total currents flowing in them, during normal operation of the PV system, are essentially due to the first leakage current flowing to ground potential and the second leakage current flowing to ground potential are formed, which pass through the forward converter in opposite directions. In this way, the forward converter as a current transformer offers a simple form of forming the difference between the first and second total currents, the difference being formed by interspersing the forward converter with the first leakage current flowing from the first PV string against ground potential and that from the second in opposite directions with regard to the current directions PV string against earth potential is realized second leakage current.

In a further development of the detection device, the two DC lines of the first PV string are arranged with respect to one another in such a way that, during normal operation of the PV system, a DC current flowing therein between the first PV string and the PV inverter converts the forward converter into two to one another interspersed in opposite directions. The two DC lines of the second PV string are also arranged in such a way that, during normal operation of the PV system, a DC current flowing in the DC lines between the second PV string and the PV inverter converts the flow converter to each other interspersed in opposite directions. 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 in such a way that, during normal operation of the PV system, those between the respective PV strings and DC currents flowing to the PV inverter in the DC lines assigned to the positive poles pass through the forward converter in opposite directions. In this way, both a DC current of the first PV string flowing in the positive DC line is compensated for by the DC current of the first PV string flowing in the negative DC line in the measurement signal of the forward converter. In addition, a DC current of the second PV string flowing in the positive DC line is also compensated for by the DC current of the second PV string flowing in the negative DC line in the measurement signal of the forward converter. The DC currents flowing between the PV strings and the PV inverter each represent an operating current of the first PV string or the second PV string. These DC currents are also always present during normal operation of the PV system. In addition to the DC currents, however, the first leakage current of the first PV string flowing towards ground potential is also compensated for by the second leakage current of the second PV string flowing towards ground potential, and vice versa. This is the case because the first leakage current of the first PV string flowing towards ground potential passes through the forward converter in the opposite direction as the second leakage current of the second PV string flowing towards ground potential.

In one embodiment of the detection device, the current transformer is designed for a maximum value which corresponds to a value of a maximum of 50%, preferably a maximum of 25%, particularly preferably a maximum of 15% of the larger of the two values from the first leakage current and the second leakage current. The first leakage current identifies a current flowing from the first PV string against the ground potential during normal operation of the PV system and the second leakage current a current flowing from the second PV string against the ground potential during normal operation of the PV system. In this context, normal operation of the PV system refers to an operating state in which the PV system is in operation and no or almost no fault current flows. This embodiment enables a particularly cost-effective design of the current transformer.

In one embodiment, the detection device is designed and set up, not just a fault current I _miss to detect on one or more PV strings, but also to determine that PV string of the PV system in which the fault current I _miss occurs. For this purpose, the PV system, on which the detection of the fault current I _miss by means of the detection device, a number n, with n> 2, PV strings. In addition, the detection device has an equal number n, with n> 2, of flow converters, which are connected to the evaluation unit of the detection device. To detect the measurement signal, each of the flow converters is penetrated by the DC lines of two different PV strings. In addition, the flow converters are implemented in such a way that the DC lines of each PV string pass through two different flow converters. The combination of the flow converters penetrating the respective PV string then characterizes the PV string.

This embodiment offers the advantage that not only can a fault current in the PV system be detected, but the PV string assigned to the fault current can also be determined. Specifically, each PV string implements a specific combination of two different flow converters with each of its DC lines, which is individual for the respective PV string. A fault current of a specific PV string therefore leads to a corresponding measurement signal from precisely those flow converters that are penetrated by the corresponding PV string. As a result, the PV string whose DC lines pass through both flow converters that detect a fault current is therefore very likely also the PV string that can be assigned to the fault current.

• - Detektion eines Messsignals des zumindest einen Stromwandlers der Detektionsvorrichtung, wobei das Messsignal eine Differenz eines von dem ersten PV-String gegen das Erdpotential fließenden ersten Summenstroms und eines von dem zweiten PV-String gegen das Erdpotential fließenden zweiten Summenstroms repräsentiert;
• - Signalisierung eines Fehlerstroms, wenn das detektierte Messsignal einen ersten Schwellwert übersteigt.

A method according to the invention is used to detect a fault current in a PV system with a detection device according to the invention. A PV generator of the PV system has at least a first PV string and a second PV string, which are each connected to a PV inverter of the PV system via two DC lines. The method has at least the following steps:

• - Detection of a measurement signal of the at least one current transformer of the detection device, the measurement signal representing a difference between a first total current flowing from the first PV string against the ground potential and a second total current flowing from the second PV string against the ground potential;
• - Signaling of a fault current when the detected measurement signal exceeds a first threshold value.

The method gives rise to the advantages already explained in connection with the detection device.

In one embodiment of the method, the fault current is only signaled when the measurement signal exceeds the first threshold value with a sudden increase. The sudden increase in the measurement signal can represent a change in the difference between the first total current and the second total current of at least 10 mA, advantageously of at least 20 mA. Such an embodiment offers the possibility of recognizing a fault current caused by a person in a more targeted manner and of differentiating it from other effects which cause slowly, possibly creeping changes in the total currents. This makes use of the effect that a change in a total current usually occurs suddenly when a person comes into contact with a live component. As a result, the measurement signal also changes abruptly. Slow changes in the difference between the two total currents, for example due to unequal ambient conditions of the PV strings and the associated slow changes in the leakage currents of the two PV strings, are masked out and not signaled. A slow change is to be understood in particular as a change that extends over many seconds to minutes.

In one embodiment of the method, a resistive current component of the measurement signal detected by the at least one current transformer can be determined. Optionally, the fault current can only be signaled if the measurement signal generated by the current transformer is at If the first threshold value is exceeded, there is a sudden change in a resistive current component by at least 10 mA, advantageously by at least 20 mA. This determination of the resistive current component also enables a more targeted detection of a fault current caused by a person. In contrast to the leakage current, which occurs via a parasitic capacitance and thus represents a reactive current, the fault current flowing due to personal contact is usually a resistive current. In a further development of the method, for. B. determine a ratio of the resistive and thus ohmic component to the capacitive component via a phase relationship between the current flowing to earth potential and the AC voltage of the PV modules causing this current.

In one embodiment of the method, the PV generator of the PV system has a number n, with n> 2, of PV strings. In addition, the detection device has an equal number n, with n> 2, of flow converters. The DC lines of two different PV strings pass through each of the forward converters. In addition, the DC lines of each PV string pass through two different flow converters. This embodiment offers the advantage that, by means of the method, not only can a fault current in the PV system be detected, but also the PV string of the PV system assigned to the fault current can be determined. Each of the PV strings, including the PV string assigned to the fault current, namely passes through two different flow converters with each of its two DC lines. A fault current in a specific one of the PV strings therefore leads to a corresponding measurement signal in precisely those current transformers that are penetrated by the specific PV string. The PV string whose DC lines pass through both flow converters that detect a fault current is therefore very likely the PV string that can be assigned to the fault current.

In one embodiment of the invention, the entire PV generator can be activated in response to the signaling of the fault current. As an alternative to this, however, it is also possible that, in response to the signaling of the fault current, only the PV string of the PV generator causing the fault current is released, while the remaining PV strings or PV sub-generators not affected by the fault continue to be operated. In this way, the failure of a feed-in tariff associated with activation can be minimized. By activating the PV generator causing the fault current or the PV string causing the fault current, the fault current of the PV generator or the corresponding PV string is suppressed. The activation z. B. be done in that the PV string in question is disconnected from the PV inverter, possibly also short-circuited. A short circuit of the PV string without disconnecting the PV inverter from the AC grid should only occur if the PV string in question has a reverse current diode or is connected to the DC link of the PV inverter via a DC / DC converter is.

In one embodiment of the method, DC lines of a first group of PV strings 2.1-2.k and an equally large second group of PV strings 2.k + 1 - 2.n can enforce a common forward converter so that each of the PV -Strings 2.1-2.k of the first group has a first total current flowing against the ground potential, which is at least approximately compensated by a second total current flowing against the ground potential of a corresponding PV string of the second group 2.k + 1 - 2.n. In this embodiment, one forward converter can be used for 4, 6, 8 or more even-numbered PV strings at the same time. This can enable a cost-effective solution for the detection device as well as the method.

In an advantageous method, the two PV strings of a jointly used current transformer are similar in such a way that, during normal operation of the PV system, a difference between the first leakage current flowing from the first PV string against the ground potential and that from the second PV string against the Ground potential flowing second leakage current falls below a second threshold value. Used together means that the measurement signal detected by the current transformer depends on the difference between the currents associated with these PV strings. The combined PV strings should advantageously be selected so similarly as possible with regard to their parasitic capacitances that the second threshold value corresponds to a value of 25% based on a maximum of the first leakage current and the second leakage current.

A photovoltaic (PV) inverter according to the invention with an AC output for connection to an alternating voltage network (AC network) and at least two DC inputs for connecting at least two PV strings of a PV generator preferably has a detection device described above and / or is designed and set up to carry out the method described above. The PV inverter can be designed as a single-phase PV inverter or a multi-phase, in particular three-phase, PV inverter. The PV inverter can be designed in a single stage and in particular be free of a DC / DC converter arranged between one of the DC inputs and a DC / AC converter of the PV inverter. Alternatively, however, it is also possible that the PV inverter is designed in several stages and has a DC / DC converter connected upstream of a DC / AC converter of the PV inverter. In a further embodiment, the PV inverter can be designed as a so-called multi-string PV inverter, in which each of the two PV strings is connected to a common DC intermediate circuit via a separate DC / DC converter.

According to an advantageous embodiment, the PV inverter has a nominal power of at least 10 kW. With such a design, especially for large PV systems with the correspondingly designed PV inverter, the advantages of the method and the detection device are particularly important.

In a further embodiment, the PV inverter is designed as a transformerless PV inverter. With such a transformerless PV inverter, the leakage currents flowing from the PV strings against the earth potential can be particularly pronounced. The method and the detection device can therefore be used particularly advantageously in such PV inverters.

Figure list

• 1 eine PV-Anlage mit einem erfindungsgemäßen PV-Wechselrichter in einer Ausführungsform;
• 2 eine erfindungsgemäße Detektionsvorrichtung in einer ersten Ausführungsform mit zwei PV-Strings und einem gemeinsam genutzten Durchflusswandler;
• 3 eine erfindungsgemäße Detektionsvorrichtung in einer zweiten Ausführungsform mit drei PV-Strings und drei Durchflusswandlern;
• 4 ein Ablaufdiagramm des erfindungsgemäßen Verfahrens zur Detektion eines Fehlerstroms in einer Ausführungsform.

In the following, the invention is further explained and described with the aid of the exemplary embodiments shown in the figures. Show it

• 1 a PV system with a PV inverter according to the invention in one embodiment;
• 2 a detection device according to the invention in a first embodiment with two PV strings and a shared flow converter;
• 3 a detection device according to the invention in a second embodiment with three PV strings and three flow converters;
• 4th a flowchart of the method according to the invention for detecting a fault current in one embodiment.

Figure description

In 1 is a photovoltaic (PV) system 1 with an embodiment of a PV inverter according to the invention 10 shown. The PV system 1 contains a first PV string 2.1 and a second PV string 2.2 . Any PV string 2.1 , 2.2 has several PV modules connected in series 3 on. Both PV strings 2.1 , 2.2 form a PV generator 8th . The two PV strings 2.1 , 2.2 are with regard to the number and type of PV modules 3 similar, in particular designed the same. In addition, the two PV strings are 2.1 , 2.2 Arranged so close to one another that they are subject to at least similar environmental conditions in terms of irradiation and temperature. The photovoltaic inverter 10 is designed as a so-called multi-string inverter. For this 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 on. Each of the DC inputs 11.1 , 11.2 is via a separate DC / DC converter 13th with a common DC link 14th connected. The common DC link 14th is in turn with a DC side of a DC / AC converter 15th connected. The AC side of the DC / AC converter 15th is with an AC output 12th of the PV inverter 10 connected. At the in 1 three-phase AC output as an example 12th of the PV inverter 10 an alternating voltage (AC) network 20, which is also designed to be three-phase, is connected. The DC inputs 11.1 , 11.2 are via a current transformer 31 with the DC / DC converters 13th connected, a control unit 16 of the PV inverter 10 controls the switches of the DC / DC converter 13th and the DC / AC converter 15th for the desired voltage conversion. The PV inverter 10 further comprises a detection device 30th with a current transformer 31 and one with the current transformer 31 connected evaluation unit 32 on. The current transformer 31 is between the entrances 11.1 , 11.2 and the DC / DC converters 13th of the PV inverter 10 arranged and can be designed as a flow converter. A measurement signal from the current transformer represents 31 a difference of one from the first PV string 2.1 against the earth potential PE flowing first total current I _{Sum, 1} and one from the second PV string 2.2 against the earth potential PE flowing second total current I _{Sum, 2} . The evaluation unit 32 is designed to further process the measurement signal measured by the current transformer, e.g. B. evaluate. It is also control technology and for communication with the control unit 16 of the PV inverter 10 connected.

The first PV string 2.1 has a first parasitic capacitance 6.1 compared to the earth potential PE on. The second PV string 2.2 has a second parasitic capacitance 6.2 compared to the earth potential PE on. Parasitic capacities 6.1 , 6.2 occur with PV modules 3 and are dependent on the environmental conditions, such as B. humidity, temperature, precipitation or the like dependent.

About the first parasitic capacitance 6.1 a first leakage current flows I _{oj, 1} from the first PV string 2.1 towards earth potential PE . About the second parasitic capacitance 6.2 a second leakage current flows I _{oj, 2} from the second PV string 2.2 towards earth potential PE . With the leakage currents I _{oj, 1} I _{oj, 2} it concerns capacitive reactive currents that result from it result that a DC voltage potential of the PV modules 3 an AC potential is superimposed on the earth potential. The leakage currents I _{oj, 1} , I _{oj, 2} are are together with the parasitic capacitances 6.1 , 6.2 the environmental conditions of the PV strings 2.1 , 2.2 such as humidity, temperature, precipitation, etc. dependent. They can change significantly over time, even if rather slowly over time. However, they change for the similar PV strings 2.1 , 2.2 in a similar way.

In the event of an error, e.g. B. if a grounded person 7th a contact between one of the PV strings 2.1 , 2.2 and the earth potential PE produces flows in addition to the leakage current I _{oj, 1} I _{oj, 2} on that PV string 2.1 , 2.2 at which the fault was caused is a fault current I _miss against the earth potential PE . This is in 1 using the example of the first PV string 2.1 illustrated.

The measuring signal of the current transformer 31 now represents the difference of the first total current I _{Sum, 1} and the second sum stream I _{Sum, 2} . The first includes total current I _{Sum, 1} the sum of the first leakage current I _{oj, 1} and - as in 1 shown in the event of a fault - the fault current I _miss . The second sum stream I _{Sum, 2} includes the sum of the second leakage current I _{oj, 2} and in the event of a fault on the second PV string 2.2 fault current also present there I _miss . For the in 1 illustrated case that on the second sting 2.2 If there is no fault, the second total current corresponds I _{Sum, 2} the second leakage current I _{oj, 2} .

Especially with powerful PV inverters 10 Without galvanic separation between the DC and AC sides, the vast majority of the total currents correspond I _{Sum, 1} I _{Sum, 2} the leakage currents I _{oj, 1} , I _{oj, 2} caused by the parasitic capacitance 6.1 , 6.2 caused. For protection against electric shock to people 7th must now have sudden changes in the total currents I _{Sum, 1} , I _{Sum, 2} can be detected as they are caused by the flow of life-threatening currents through the human body of persons 7th can arise. Such resistive fault currents I _miss are already life-threatening at currents that are well below the usual currents of non-dangerous capacitive leakage currents I _{oj, 1} , I _{oj, 2} can lie. To now a potentially occurring resistive fault current I _miss To be able to measure reliably and with little effort with such low current intensities, it is proposed to use a separate measurement of a total current in each case I _{Sum, 1} I _{Sum, 2} with a possibly high one for both PV strings 2.1 , 2.2 but similar leakage current I _{oj, 1} , I _{oj, 2} to renounce. Instead, the current transformer gives 31 as the measurement signal, the difference, i.e. the difference in the total currents I _{Sum, 1} I _{Sum, 2} and thus also the leakage currents I _{oj, 1} , I _{oj, 2} between the two PV strings 2.1 , 2.2 out. This difference is, according to the assumption mentioned above, the similarity of the PV strings 2.1 , 2.2 , small amount. This means that there is also a measurement signal that is usually permanently present, ie the measurement signal that is generated during normal operation of the PV system 1 without fault, i.e. without fault current I _miss , is present, due to the compensation of the total currents that occurs I _{Sum, 1} , I _{Sum, 2} also low.

A change in the total current I _{Sum, 1} , I _{Sum, 2} in one of the PV strings 2.1 , 2.2 For example, if a person touches a live conductor, it only affects those of the PV strings 2.1 , 2.2 on which the error occurred, i.e. which is assigned to the error. In the event of a fault, the remaining difference between the total currents therefore increases I _{Sum, 1} , I _{Sum, 2} of similar PV strings 2.1 , 2.2 significant relative to the otherwise compensated and usually permanently present measurement signal. This effect is in the measuring signal of the current transformer 31 good to see. As a result, a correspondingly sensitive, inexpensive current transformer 31 can be selected and the fault current I _miss can be detected with reliable and inexpensive means.

In 2 is a first embodiment of a detection device according to the invention 30th shown, for example, in a PV inverter according to the invention 10 of the PV system 1 out 1 can be arranged. The detection device 30th includes a current transformer 31 , and one with the current transformer 31 connected evaluation unit 32 . Here is the current transformer 31 in 2 as a flow converter 33 formed by the two PV strings 2.1 , 2.2 of the PV system 1 is enforced. The enforcement is designed in such a way that there is one in the positive DC line 4.1 flowing DC current I _{Str, 1st} of the first PV string 2.1 with one in the negative DC line 5.1 in the opposite direction through the flow converter 33 flowing and usually equally large DC current I _{Str, 1st} compensated. The same applies to the DC currents I _{Str, 2} , the second PV string 2.2 that is in its positive DC line 4.2 and in its negative DC line 5.2 flow. The DC currents I _{Str, 1st} , I _{Str, 2} are therefore usually push-pull signals that flow within the DC lines. These DC currents appearing as push-pull signals I _{Str, 1st} , I _{Str, 2} in particular do not flow, at least not in normal operation of the PV system 1 where there is no fault current I _miss is present, from the DC lines to earth potential.

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 superimposed on the earth potential. The AC signal is present due to the topology, because the PV inverter 10 to which the PV strings 2.1 , 2.2 connected on the input side is connected to the AC grid on the output side. With the AC signal, which is in 2 is illustrated by means of the wavy lines, it is usually a common-mode signal with respect to the ground potential. This results in connection with the parasitic capacitances 6.1 , 6.2 of the two PV strings 2.1 , 2.2 one leakage current each I _{oj, 1} I _{oj, 2} , and thus a total current in each case I _{Sum, 1} I _{Sum, 2} that of each of the PV strings 2.1 , 2.2 against the earth potential PE flows. Enforcement of the forward converter 33 with regard to the total currents I _{Sum, 1} I _{Sum, 2} , is now such that the first total current I _{Sum, 1} of the first PV string 2.1 the forward converter in the opposite direction to the second total current I _{Sum, 2} of the second PV string 2.2 interspersed. Because the parasitic capacitance 6.1 , 6.2 the PV strings 2.1 , 2.2 are similar, results in normal operation of the PV strings 2.1 , 2.2 (ie in the absence of a fault current) the greatest possible compensation of the measuring signal of the flow converter 33 If a fault current occurs in one of the two PV strings 2.1 , 2.2 on, so adds to the leakage current of the faulty PV string and a significant measurement signal of the current transformer results 33 .

The principle described is also applicable to PV inverters 10 Can be used with more than two PV strings by always placing the DC cables of two similar PV strings in pairs in the in 2 way shown by a current transformer 31 , especially a forward converter 33 be guided. With a number n of PV strings, there are thus a number n / 2 of current transformers 31 , especially forward converters 31 needed. Alternatively, however, the DC cables can also be used 4.1 , 4.2 , 5.1 , 5.2 any other even number of similar PV strings 2.1 , 2.2 can be combined in this way, namely by the total current of one half of the PV strings in one direction and the total current of the other half of the PV strings in the opposite direction through exactly one common flow converter 33 as a current transformer 31 to be led.

For applications in which an even number of PV strings 2.1 , 2.2 with a common current transformer 31 , as in 1 and 2 represented, monitored, is a localization of a sudden change in a total current I _{Sum, 1} , I _{Sum, 2} on one of the PV strings 2.1 , 2.2 only possible indirectly. There can be an abrupt change in the entirety of the two PV strings 2.1 , 2.2 can be recognized, however, due to the structure, it cannot yet be recognized from which of the two PV strings 2.1 , 2.2 the sudden increase and thus the fault current I _miss is caused. One way of determining which of the PV strings is faulty 2.1 , 2.2 will be used in conjunction with 3 explained in more detail.

3 shows a second embodiment of a detection device according to the invention 30th . The second embodiment is similar in some respects to the first embodiment of the detection device 30th according to 2 executed, which is why with regard to the similarities on the description of the figures 2 is referred. The following mainly shows the differences between the second and the first embodiment.

In contrast to the first embodiment, the second embodiment is the detection device 30th for a total of three PV strings 2.1 , 2.2 , 2.3 designed and includes three flow converters 33.1 , 33.2 , 33.3 with the evaluation unit 32 are connected. The first DC current I _{Str, 1st} of the first PV string 2.1 flows through the first forward converter 33.1 twice, each time in the opposite direction. In addition, the first DC current flows through it I _{Str, 1st} of the first PV string 2.1 the third forward converter 33.3 also twice, each time in the opposite direction. The second DC current I _{Str, 2} of the second PV string 2.2 flows through the second forward converter 33.2 twice, each time in the opposite direction. In addition, the second DC current flows through it I _{Str, 2} of the second PV string 2.2 the first forward converter 33.1 also twice, each time in the opposite direction. The third DC current I _{Str, 3} of the third PV string 2.3 flows through the second forward converter 33.2 twice, each time in the opposite direction. In addition, the third DC current flows through it I _{Str, 3} of the third PV string 2.3 the third forward converter 33.3 twice, each time in the opposite direction. Each forward converter is therefore always powered by the DC currents from a pair of two of the PV strings 2.1 - 2.3 flowed through. The enforcement is designed in such a way that the leakage currents I _{oj, 1} , I _{oj, 2} IAbl, 3, as well as the total currents I _{Sum, 1} , I _{Sum, 2} I _{Sum, 3rd} for each pair of PV strings 2.1 - 2.3 with a common flow converter 33.1 - 33.3 , the common forward converter 33.1 - 33.3 prevail in the opposite direction. In this way, there is also a compensation of the total currents that acts in pairs for each of the flow converters I _{Sum, 1} , I _{Sum, 2} I _{Sum, 3rd} .

With an in 3 The embodiment shown is a localization of that PV string 2.1 , 2.2 , 2.3 , which actually triggered a sudden change in the total current, is possible. Thereby the flow converters 33.1 , 33.2 , 33.3 so used that through it the DC lines 4.1 , 4.2 , 4.3 , 5.1 , 5.2 , 5.3 of two PV strings each 2.1 , 2.2 , 2.3 be guided in a compensating manner in the manner of a "daisy chain".

Each of the PV strings 2.1 , 2.2 , 2.3 in 3 is each with two specific for him characteristic of the three forward converters 33.1 , 33.2 , 33.3 connected. A sudden increase in the total current I _{Sum, 1} , I _{Sum, 2} I _{Sum, 3rd} on one of the PV strings 2.1 , 2.2 , 2.3 therefore generates a jump in the corresponding measurement signal in the two flow converters that are characteristic of it 33.1 - 33.3 . By comparing which two of the three flow converters 33.1 - 33.3 now show a sudden increase in the measurement signal, conclusions can be drawn about the PV string that caused the error. The comparison of the measurement signals of the individual flow converters and the determination of the faulty PV string 2.1 - 2.3 via the evaluation unit 32 respectively.

The procedure described here can also be applied to a configuration of the detection device 30th with more than three PV strings 2.1 , 2.2 , 2.3 transfer. In general, it follows that for a number n of PV strings 2.1 - 2.n also a number n of forward converters are required to avoid just one fault in one of the PV strings 2.1 - 2.n, but also the faulty PV strings 2.1 - 2.n to be determined.

An inequality of the PV strings considered in pairs and each penetrating a flow converter 2.1 , 2.2 , 2.3 can override the respective flow converter 33.1 , 33.2 , 33.3 cause. Conversely, from too great an inequality of the across a flow converter 33.1 , 33.2 , 33.3 on the PV inverter 10 connected PV strings 2.1 , 2.2 , 2.3 can be assumed if the measurement signal of a shared flow converter 33.1 , 33.2 , 33.3 even during normal operation of the PV system 1 permanently a difference in the total currents I _{Sum, 1} , I _{Sum, 2} I _{Sum, 3rd} detected with a high amperage, so if this amperage has a certain rated value as the second threshold value S _{TH, 2} permanently exceeds. In such a case, however, appropriate measures can be taken, e.g. B. a different grouping in relation to the shared forward converters 33.1 - 33.3 and / or, if necessary, a change in individual PV strings 2.1 - 2.3 in terms of type and number of the PV string 2.1 - 2.3 assigned PV modules 3 .

In 4th is a flowchart of a method according to the invention for detecting a fault current according to one embodiment.

In step S1 becomes a measurement signal of the at least one current transformer 31 detected. The measurement signal represents z. B. a difference of one from the first PV string 2.1 against the earth potential PE flowing first total current I _{Sum, 1} and one from the second PV string 2.2 against the earth potential PE flowing second total current I _{Sum, 2} . When using a detection device 30th that according to 3 for more than two PV strings 2.1 , 2.2 is designed, the measurement signal can also be a difference between one of the first PV string 2.1 against the earth potential PE flowing first total current I _{Sum, 1} and one of the third PV string 2.3 against the earth potential PE flowing third sum stream I _{Sum, 3rd} and / or a difference in one of the second PV string 2.2 against the earth potential PE flowing second total current I _{Sum, 2} and one from the third PV string 2.3 against the earth potential PE flowing third sum stream I _{Sum, 3rd} include.

In step S2 it is checked whether the detected measurement signal has a first threshold value S _{TH, 1} exceeds. If not, with step S1 proceeded. If so, the optional step follows S3 , which checks whether the measurement signal changes suddenly. If this is the case, then in the optional step S4 checked whether the resistive component of the measurement signal changes abruptly. If this is the case, step S5 proceeded. If checking the optional steps S3 or S4 is negative, will step with S1 proceeded. If the optional steps S3 and S4 are not carried out, after the check in step S2 when the detected measurement signal has reached the first threshold value STH, 1 exceeds, with the step S5 proceeded.

In step S5 becomes a fault current I _miss signals when the detected measurement signal according to the test in step S2 the first threshold S _{TH, 1} exceeds. Signaling can, for example, occur within the evaluation device 32 be generated alarm signal that can trigger further measures. A signaling of the fault current I _miss can be initiated by the evaluation unit 32 also via a radio transmitted signal to an operator of the PV system 1 respectively.

At step S5 closes the optional step S6 on, in particular for a design of the detection device 30th according to 3 can be carried out. In step S6 becomes the affected PV string 2.1 , 2.2 , 2.3 determines the change in the measurement signal and thus the fault current I _miss triggered. After determining the affected PV string 2.1 , 2.2 , 2.3 can do this in step S7 unlocked to avoid possible dangers to people 7th avert. The PV system can 1 together with the rest not from the fault current I _miss affected PV strings continue to be operated.

List of reference symbols

1

Photovoltaic (PV) system

2.1, 2.2, 2.3

PV string

3

PV module

4.1, 4.2, 4.3

DC line

5.1, 5.2, 5.3

DC line

6.1, 6.2

parasitic capacitance

7th

person

8th

PV generator

10

Inverter

11.1, 11.2

DC input

12th

AC output

13th

DC / DC converter

14th

DC link

15th

DC / AC converter

16

Control unit

20th

Alternating voltage network (AC network)

30th

Detection device

31

Power converter

32

Evaluation unit

33, 33.1, 33.2, 33.3

Flow converter

S1, S2, S3, S4, S5, S6, S7

Procedural steps

ISum, 1, ISum, 2, ISum, 3

Total current

I mistake

Fault current

IAbl, 1, IAbi, 2

Leakage current

IStr, 1, Istr, 2, IStr, 3

DC power

PE

Earth potential

STH, 1

first threshold

STH, 2

second threshold

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• EP 2372857 A1 [0006]

Claims (16)

Detection device (30) for detecting a fault current (I _fault ) on a PV generator (8) and / or on the DC lines (4.1 - 4.3, 5.1 - 5.3) assigned to the PV generator (8) of a PV system (1 ), the PV generator (8) having at least a first PV string (2.1) and a second PV string (2.2), which are connected to a PV via two DC lines (4.1, 5.1, 4.2, 5.2) Inverters (10) of the PV system (1) are connected, the detection device (30) having at least one current transformer (31) and an evaluation unit (32) connected to the at least one current transformer (31), characterized in that - the current transformer (31) is designed as a current transformer (31) that can be used jointly by the first PV string (2.1) and the second PV string (2.2), a measurement signal from the jointly usable current transformer (31) being a difference between one of the String (2.1) against the ground potential (PE) flowing first total current (I _{Sum, 1} ) and one from the second PV string (2 .2) represents the second total current (I _{sum, 2} ) flowing against the earth potential (PE), and where the fault current (I _Fehl ), if it occurs, is a component of the first total current (I _{Sum, 1} ) and / or the second total current (I _{sum, 2} ) is. Detection device (30) after Claim 1 , characterized in that the jointly usable current transformer (31) is designed as a jointly usable flow converter (33) which is used to detect the measurement signal from the two DC lines (4.1, 5.1) of the first PV string (2.1) and the two DC lines (4.2, 5.2) of the second PV string (2.2) is penetrated. Detection device (30) after Claim 2 , the two DC lines (4.1, 5.1) of the first PV string (2.1) being arranged in relation to one another in such a way that, during normal operation of the PV system (1), a DC current (I _{Str, 1} ) flowing in the flow converter (33) penetrated in two mutually opposite directions, and wherein the two DC lines (4.2, 5.2) of the second PV string (2.2) are arranged in such a way that, during normal operation of the PV system (1), a DC flowing therein -Current (I _{Str, 2} ) passes through the flow converter (33) in two mutually opposite directions, and - wherein a DC line (4.1) connected to a positive pole of the first PV string (2.1) is relative to one with a positive pole of the second PV string (2.2) connected DC line (4.2) is arranged so that during normal operation of the PV system (1) the DC currents (I _{Str, 1} , I _{Str, 2} ) in the positive poles assigned DC lines (4.1, 4.2) pass through the flow converter (33) in opposite directions. Detection device (30) according to one of the preceding claims, wherein the current transformer (31) is designed for a maximum value which is a value of a maximum of 50%, preferably a maximum of 25%, particularly preferably a maximum of 15% of the larger of the two values from a first Leakage current (I _{Abl, 1} ) and a second leakage current I _{Abl, 2} corresponds, with the first leakage current (I _{Abl, 1} ) being one of the first PV string (2.1) against earth potential (2.1) during normal operation of the PV system (1) PE) characterizes the flowing current and the second leakage current (IA _{bl, 2} ) characterizes a current flowing from the second PV string (2.2) against the earth potential (PE) during normal operation of the PV system (1). Detection device (30) according to one of the Claims 1 until 4th , the PV system (1) having a number of PV strings (2.1-2.3), the number being greater than two, the detection device (30) having an equal number of flow converters (33) which are connected to the evaluation unit (32 ) of the detection device (30) are connected, each of the flow converters (33) being penetrated by the DC lines (4.1-4.3, 5.1-5.3) of two different PV strings (2.1-2.3) for the detection of the measurement signal, and the DC lines (4.1-4.3, 5.1-5.3) of each PV string (2.1-2.3) pass through two different flow converters (33). Method for detecting a fault current (I _Fehl ) in a PV system (1), wherein a PV generator (8) of the PV system (1) has at least a first PV string (2.1) and a second PV string (2.2 ), which are each connected to a PV inverter (10) of the PV system (1) via two DC lines (4.1, 5.1, 4.2, 5.2), with a detection device (30) according to one of the preceding claims comprising the Steps: - Detection of a measurement signal from the at least one current transformer (31), the measurement signal being a difference between a first total current (I _{Sum, 1} ) flowing from the first PV string (2.1) to the ground potential (PE) and one from the second PV -String (2.2) against the ground potential (PE) represents the second total current (I _{sum, 2} ) flowing; - Signaling of a fault current (I _Fehl ) when the detected measurement signal _{exceeds a first threshold value (S TH, 1} ). Procedure according to Claim 6 , the fault current (I _{Fehl, 1} ) is only signaled when the measurement signal exceeds the first threshold value (S _{TH, 1} ) with a sudden increase that results in a change in the difference between the first total current (I _{Sum, 1} ) and the second total current (I _{sum, 2} ) of at least 10mA, advantageously represented by at least 20mA. Method according to one of the Claims 6 or 7th , where a resistive current component of the at least one current transformer (31) detected measurement signal is determined, optionally the fault current (I _Fehl ) is only signaled if the measurement signal detected by the current transformer (31) shows a sudden change in a resistive _{value when the first threshold value (S TH, 1) is exceeded} Current component by at least 10mA, advantageously by at least 20mA. Method according to one of the Claims 6 until 8th , the PV generator (8) of the PV system (1) having a number of PV strings (2.1-2.3), the number being greater than two, and the detection device (30) having an equal number of current transformers (31 ), wherein a PV string (2.1-2.3) causing the fault current is determined by the fact that two of the PV strings (2.1-2.3) each share one of the current transformers (31) to detect the measurement signal, and each of the PV -Strings (2.1-2.3) each uses two of the current transformers (31) .. Method according to one of the Claims 6 until 9 , whereby the PV generator (8) or only the PV string (2.1-2.3) of the PV generator (8) causing _{the fault current (I Fehl} ) is activated in response to the signaling of the fault current (I _fault). Method according to one of the Claims 6 until 10 , whereby DC lines (4.1-4.3, 5.1 - 5.3) of a first group of PV strings (2.1-2.3) and a second group of the same size of PV strings enforce a common forward converter (31) so that each of the PV strings Strings (2.1-2.3) of the first group has a first total current (I _{Sum, 1} ) flowing against the ground potential (PE), which is derived from a second total current (I _{sum, 2} ) of a corresponding PV string flowing against the ground potential (PE) of the second group is at least approximately compensated. Method according to one of the Claims 6 until 11th , whereby the two PV strings (2.1, 2.2) of a jointly used current transformer (31) are so similar that during normal operation of the PV system (1) there is a difference between one of the first PV strings (2.1) and the earth potential ( PE) flowing first leakage current (I _{Abl, 1 ) and a second leakage current (IA bl, 2} ) flowing from the second PV string (2.2) against the ground potential (PE) falls below a second threshold value (S _{TH, 2} ). Procedure according to Claim 12 , the second threshold value (S _TH , 2) corresponding to a value of 25%, advantageously a value of 15% based on a maximum of the first leakage current (I _{Abl, 1} ) and the second leakage current (IA _{bl, 2).} PV inverter (10) with an AC output (12) for connection to an AC voltage network (20) and at least two DC inputs (11.1, 11.2) for connection of at least two PV strings (2.1, 2.2, 2.3) of a PV -Generator (8), characterized in that the PV inverter (10) has a detection device (30) according to one of the Claims 1 until 5 has, and / or for performing a method according to one of the Claims 6 until 13th is designed and furnished. PV inverter (10) Claim 14 , characterized in that the PV inverter (10) has a nominal power of at least 10 kW. PV inverter (10) Claim 14 or 15th , characterized in that the PV inverter (10) is designed as a transformerless PV inverter.

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