PV system with collection units, method for operating a PV system with collection units and collection unit for PV strings

Abstract

PV system (10) with an inverter (14) and at least one collection unit (20) for PV strings (12), wherein the collection unit (20) has several inputs for PV strings (12) and one output, wherein the inputs in the collection unit (20) are connected in parallel and are switchably connected to the output of the collection unit (20), wherein the output of the at least one collection unit (20) is switchably connected to a two-pole DC bus (16) and the DC bus (16) is switchably connected to a bridge circuit (18) of the inverter (14), wherein at least one collection unit (20) is assigned a residual current protection circuit (21) which is arranged between the parallel connection of the PV strings (12) and the DC bus (16) and in particular comprises a common-mode current sensor (22) and a circuit breaker (24), wherein the residual current protection circuit (21) is configured to cause a disconnection of the connection between the PV strings (12) and the DC bus (16) when a common-mode current on the connection between the PV strings (12) and the DC bus (16) exceeds a predefinable limit value, characterized by the fact that the residual current protection circuit (21) is designed to maintain the connection between the PV strings (12) and the DC bus (16) even in the event of an impermissible common-mode current and / or to prevent the disconnection of the connection between the PV strings (12) and the DC bus (16) when a passivation signal (30) is present at the residual current protection circuit (21).

Description

TECHNICAL AREA OF INVENTION

[0001] The invention relates to a photovoltaic system with collection units for photovoltaic strings, a method for operating such a photovoltaic system, and collection units for photovoltaic units. STATE OF THE ART

[0002] A photovoltaic system (PV system) can comprise a PV generator with a plurality of photovoltaic strings (PV strings), which in turn comprise a plurality of photovoltaic modules (PV modules), preferably connected in series. The PV strings of such a PV system can, in particular, be connected in parallel with one another, and a parallel connection of PV strings can be connected to a centrally located inverter.

[0003] Such a PV system, and in particular its generator, must be protected against earth faults for regulatory reasons. For this purpose, a large-scale PV system often includes an earth fault protection device (GFDI), which may be centrally located at the inverter. Furthermore, it may be mandatory to regularly measure the insulation resistance of the generator in such a PV system.

[0004] An inverter of a PV system can therefore have measuring and / or protective circuits, in particular a central earth fault protection circuit and an insulation resistance measuring device.

[0005] The parallel connection of multiple PV strings can be implemented using a manifold unit that includes inputs for several PV strings and an output for connection to an inverter. A PV system with such manifold units can include distributed protection circuits, in particular residual current or DI sensors and (automated) circuit breakers, which can be assigned to individual sub-generators and, for example, located within the manifold units. Such distributed protection circuits can operate autonomously by disconnecting the relevant PV strings from the inverter if a detected residual current exceeds a limit value.

[0006] From EP 2 386 870 A2 a method for checking the insulation of a PV system against earth by means of a test pulse on the connection lines of the PV system is known.

[0007] From DE 20 2009 015 987 U1, a connection cabinet is known as a connection device between PV modules and an inverter, wherein the connection cabinet has current-supplying and current-dissipating components, fuses and a DC disconnect switch.

[0008] DE 10 2012 218 504 A1 discloses a differential current monitoring device for monitoring differential currents in a PV system.

[0009] From EP 3 210 243 A1 a combiner box is known which includes an earth fault detection sensor, wherein a main switch of the combiner box is opened when an earth fault is detected.

[0010] US Patent 2012 0 050 924 A1 discloses a current collection box for collecting electrical current from a multitude of PV strings, comprising a detector that detects a ground fault in each of the PV strings, wherein a control unit can disconnect a PV string affected by a ground fault via an associated switch. The control unit in such a collection box is connected to a central control unit via a communication path.

[0011] DE 10 2013 111 869 A1 discloses a photovoltaic system with at least one inverter and several parallel-connected PV sub-generators, which are connected to a DC connection area of ​​the inverter via DC lines. It describes in particular protective measures such as generator-adjacent disconnect devices and short-circuit switches to prevent damage in the event of faults within the PV system.

[0012] DE 199 30 089 A1 relates to an arrangement for the selective tripping of circuit breakers connected in series, in which a circuit breaker directly affected by a short circuit transmits a blocking signal to an upstream circuit breaker. This blocking signal prevents unnecessary tripping of the upstream circuit breaker and thus contributes to operational continuity. TASK OF INVENTION

[0013] The invention is based on the objective of providing a PV system with improved protection against fault currents and increased operational reliability. SOLUTION

[0014] The problem is solved by a PV system according to claim 1, by a method for operating a PV system according to claim 15, and by a collection unit for a PV system according to claim 32. DESCRIPTION OF THE INVENTION

[0015] The present application relates to a photovoltaic system (PV system) designed for converting solar energy into electrical energy. The system comprises an inverter and at least one power collection unit for photovoltaic strings (PV strings). The power collection unit is equipped with several inputs for PV strings, which are connected in parallel and switchably linked to an output of the power collection unit. This output of the power collection unit is in turn connected to a two-pole direct current bus (DC bus), which is switchably connected to a bridge circuit of the inverter.

[0016] A feature of the invention is the assignment of a residual current protection circuit to the at least one power distribution unit. This residual current protection circuit is arranged between the parallel connection of the PV strings and the DC bus and can, in particular, comprise a common-mode current sensor and a circuit breaker. The residual current protection circuit is configured to disconnect the connection between the PV strings and the DC bus when a common-mode current on the connection between the PV strings and the DC bus exceeds a predefinable limit.

[0017] The registered PV system is characterized by the fact that the residual current device (RCD) is configured to maintain the connection between the PV strings and the DC bus even in the event of an otherwise impermissible common-mode current. This is achieved by preventing the connection from being interrupted when a passivation signal is present at the RCD. In other words, the RCD is configured to prevent the connection between the PV strings and the DC bus from being interrupted when a passivation signal is present at the RCD.

[0018] The invention is based on the understanding that, under certain circumstances, significant balancing and transfer currents can flow in a photovoltaic (PV) system. These currents can be detected by the residual current protection (RCP) and interpreted as (earth fault) faults. Due to the (supposed) fault current, the distributed protection circuits may trip and disconnect the affected sub-generators. This is undesirable because, firstly, there is no actual reason for the disconnection, and secondly, manual intervention may be required to reset the protection circuit. By using the passivation signal, such a disconnection can be reliably prevented.

[0019] In one embodiment, the residual current protection circuit includes an evaluation unit configured to evaluate the measured values ​​of the common-mode current sensor. The circuit breaker opens when the measured common-mode current exceeds the predefined limit and no passivation signal is present.

[0020] In another embodiment, several collection units are connected to the DC bus via their outputs and connected in parallel in or on the inverter. This allows for a flexible and efficient design of the PV system.

[0021] The registered PV system offers effective residual current detection and treatment at the level of the collector units, thereby increasing the safety and reliability of the system. The ability to prevent the PV strings from being disconnected from the DC bus by the residual current protection circuit under certain conditions significantly improves operational continuity, which is particularly beneficial for the efficiency and economic viability of the PV system.

[0022] In one embodiment of the PV system, a DC disconnect switch is arranged in the DC bus between the inverter's bridge circuit and the at least one power collection unit. Switching on the DC disconnect switch can generate equalizing currents, which can be detected as common-mode currents on the connection between the PV strings and the DC bus by the residual current device (RCD). By applying the passivation signal to the RCD, the DC disconnect switch can be switched on without any equalizing currents causing an unwanted disconnection of the connection between the PV strings and the DC bus by the RCD.

[0023] The residual current device (RCD) can be located within the power distribution unit in a connection between the parallel connection of the PV strings and the output of the power distribution unit. Alternatively, the RCD can be implemented separately and located outside the power distribution unit in a connection between the output of the power distribution unit and the DC bus.

[0024] The PV system can include a signaling device for generating the passivation signal, which is preferably located on or in the inverter. The signaling device can be configured, in particular, to inject a periodic differential-mode signal or a periodic common-mode signal into the DC bus. The differential-mode or common-mode signal can have a frequency in the range of several kilohertz, particularly between 10 kHz and 50 kHz. Alternatively or additionally, the passivation signal can have a variable frequency and / or an encoding, for example, a predefined clock pattern.

[0025] In one embodiment, the signaling device can comprise a signal switch and an inductor, wherein the signal switch and the inductor are arranged in series between the two poles of the two-pole DC bus. By periodically switching the signal switch, a push-pull signal can thus be generated particularly efficiently by periodically short-circuiting the DC bus across the inductor, so that, in particular, no separate power source is required to generate the passivation signal. The signaling device can also include a diode and, optionally, a resistor, which are arranged electrically in parallel with the inductor of the signaling device to prevent any reverse currents and, for example, to minimize resonance effects due to the periodic short-circuiting.

[0026] In further embodiments, the PV system includes detection means for detecting the passivation signal. Such a detection means can be assigned to a collection unit and arranged between the parallel connection of the PV strings and the DC bus. The detection means is specifically designed to detect the passivation signal on the connection between the PV strings and the DC bus and to apply it to the residual current protection circuit.

[0027] The detection device can, in particular, comprise a differential-mode current sensor, which can be implemented as a shunt resistor and arranged in at least one of the DC lines between the parallel connection of the PV strings and the DC bus. The detection device can further comprise signal processing, which is configured to evaluate the measured values ​​of the differential-mode current sensor and to forward the passivation signal.

[0028] In an alternative embodiment, the signaling device can be arranged between one pole of the two-pole DC bus and ground potential and configured to inject the passivation signal into the DC bus as a periodic common-mode signal. This passivation signal, generated on demand, can be used by the residual current protection circuit by having the circuit detect the periodic common-mode signal using the common-mode current sensor and recognize it as a passivation signal. This embodiment has the advantage that no additional detection device is required for detecting or recognizing the passivation signal.

[0029] The PV system can be configured to feed the electrical energy from the PV strings into an alternating current (AC) grid. For this purpose, the inverter can be connected to the AC grid, particularly via an AC disconnect switch. Operating, and especially switching on, the AC disconnect switch can also lead to equalizing currents, which could be perceived by the residual current device (RCD) as impermissible differential-mode currents. However, these currents should under no circumstances lead to the PV strings being disconnected from the DC bus, which is effectively prevented by applying the passivation signal, as per the application.

[0030] The present application further relates to a method for operating a photovoltaic (PV) system, which in particular improves the safety and efficiency of the PV system. The PV system comprises an inverter, at least one power collection unit, and several PV strings connected in parallel within the power collection unit. The electrical power of the PV strings is fed via an output of the power collection unit into a direct current (DC) bus, which is connected to a bridge circuit of the inverter.

[0031] A feature of the invention is the integration of a residual current protection circuit with the at least one power distribution unit. This residual current protection circuit is positioned between the parallel connection of the PV strings and the DC bus and can, in particular, comprise a common-mode current sensor and a circuit breaker. The residual current protection circuit is designed to disconnect the connection between the PV strings and the DC bus if the common-mode current exceeds a predefined limit.

[0032] The method is characterized by maintaining the connection between the PV strings and the DC bus, even when a normally impermissible common-mode current flows, provided a passivation signal is present at the residual current protection circuit. This enables a continued power supply under certain conditions that would otherwise result in an unintended shutdown of the system. The passivation signal can be permanently applied or initialized for a period during which the connection between the PV strings and the DC bus is to be maintained.

[0033] In one embodiment, an evaluation unit of the residual current protection circuit analyzes the measured values ​​of the common-mode current sensor and opens the circuit breaker if the common-mode current exceeds the specified limit value and no passivation signal is present.

[0034] In another embodiment, several collection units feed electrical power from connected PV strings into the DC bus of the inverter in parallel, with the same passivation signal being applied to the residual current protection circuits of all collection units.

[0035] The patented method offers an improved approach to managing fault currents in PV systems at the unit level, thereby increasing safety without unnecessarily interrupting energy production due to false tripping of the residual current device (RCD). Integrating a passivation signal for the RCD in the PV system allows for a flexible response to unavoidable current flows, thus contributing to maintaining operational continuity.

[0036] In one embodiment, the passivation signal is applied to the DC bus by a signaling device. This enables centralized control of the passivation signal, simplifying the handling and maintenance of the PV system. The passivation signal can have a predefined signal shape, whereby a signal frequency and / or an encoding, for example, a unique clock pattern in the signaling device, can be fixed or variable and is preferably in the range of a few kilohertz.

[0037] In another embodiment, a detection device is assigned to the at least one collection unit and arranged between the parallel connection of the PV strings and the DC bus. The detection device detects the passivation signal on the connection between the PV strings and the DC bus and applies it to the residual current protection circuit.

[0038] The passivation signal can be applied to the DC bus as a differential-mode signal. The detection device can detect this differential-mode signal using a differential-mode current sensor, in particular a shunt sensor, in at least one of the DC lines. Signal processing can evaluate the measured values ​​of the differential-mode current sensor and apply the passivation signal, if present, to the residual current protection circuit.

[0039] In one embodiment of the method, the passivation signal is induced by periodically short-circuiting the DC bus lines using the signaling device. The signaling device can comprise a signal switch and an inductor arranged in series between the two poles of the two-pole DC bus. Periodically switching the signal switch thus periodically short-circuits the DC bus via the inductor, thereby generating a differential-mode signal in the DC bus with particular efficiency.

[0040] The control of the signaling device, in particular the timing of the signal switch, can be carried out by a control unit of the inverter, whereby no separate energy source is necessary to generate the passivation signal, but the energy for the passivation signal is obtained from the DC bus and thus from the PV strings.

[0041] In an alternative embodiment, the passivation signal can be injected into the DC bus as a periodic common-mode signal. For this purpose, the signaling device can be arranged, in particular, between one pole of the two-pole DC bus and ground potential. The periodic common-mode signal can be detected by the residual current protection circuit itself using the common-mode current sensor and recognized as a passivation signal. This embodiment is particularly easy to implement, since no additional detection device is required to recognize the presence of a passivation signal.

[0042] During operation of a PV system, various situations can arise in which, for example, equalizing currents occur. These are unavoidable, but are perceived by the residual current device (RCD) as impermissible common-mode currents. This can lead to unwanted disconnection of PV strings from the DC bus. In embodiments of the method, the passivation signal is therefore generated, in particular, when an AC disconnect switch located between the inverter's bridge circuit and an AC network is closed, and / or when a DC disconnect switch in the DC bus is closed, and / or when an earth fault protection device is connected to the DC bus. This prevents the RCD from disconnecting the connection between the PV strings and the DC bus due to any common-mode currents caused by the aforementioned switching operations.In other words, the connection between the PV strings and the DC bus is maintained even if, in certain situations, impermissible common-mode currents occur and are detected by the residual current protection circuit. The passivation signal can be permanently active as long as the disconnection of the connection between the PV strings and the DC bus needs to be prevented. Alternatively, the passivation signal can be used to initialize a period during which the disconnection of the connection between the PV strings and the DC bus is prevented, without requiring the passivation signal to be permanently active.

[0043] In one specific embodiment of the method, electrical power from the PV strings is fed into the DC bus and processed by the inverter. During this feed-in, an earth fault protection circuit, which is switchably connected to the DC bus at or within the inverter, can be disconnected. This may be particularly necessary if the insulation resistance of the PV generator, i.e., the entirety of the PV strings connected to the DC bus, is to be measured using an insulation resistance measuring device. After the insulation resistance measurement is complete, the earth fault protection circuit is reconnected to the DC bus.To prevent unnecessary and undesirable disconnection of PV strings by the residual current device (RCD) due to one or more switching operations, a passivation signal is generated. This passivation signal can be initiated before disconnecting the RCD from the DC bus, or during or after the insulation resistance measurement, but at the latest before reconnecting the RCD to the DC bus after completion of the insulation resistance measurement. The passivation signal can be terminated after reconnecting the RCD to the DC bus; alternatively, a time period can be defined that includes the reconnection of the RCD or the entire duration of the insulation resistance measurement and is initiated by the passivation signal.

[0044] In one embodiment of the method, the insulation resistance of the PV system is checked regularly, in particular once a day. The earth fault protection circuit is initially connected to the DC bus in normal operation and is then disconnected and reconnected to check the insulation resistance of the PV system with the earth fault protection circuit disconnected. During this process, the passivation signal is generated to prevent the PV strings from being disconnected due to potentially impermissible common-mode currents.

[0045] In one specific embodiment, the passivation signal is received by the evaluation unit of the residual current protection circuit and used to prevent the circuit breaker from opening. The evaluation unit only opens the circuit breaker if the common-mode current measured by the common-mode current sensor exhibits impermissible values ​​and no passivation signal is present at the collection unit.

[0046] In an alternative embodiment, the passivation signal in the residual current protection circuit is used to prevent the circuit breaker from opening by interrupting the switch-off signal from the evaluation unit that would normally open the circuit breaker. For example, a switch in a signal line between the evaluation unit and the circuit breaker can be actuated based on the passivation signal, so that a switch-off signal does not reach the circuit breaker and a switch-off is prevented.

[0047] In another alternative embodiment, the passivation signal in the residual current protection circuit is used to prevent the circuit breaker from opening by controlling the circuit breaker with an OR gate consisting of a switch-on signal and the passivation signal. The circuit breaker is and remains closed when either a switch-on signal from the evaluation unit or the passivation signal is present at the circuit breaker. In other words, the circuit breaker only opens when the switch-on signal is absent, particularly due to an excessively high common-mode current, while simultaneously no passivation signal is present at the OR gate.

[0048] The present application relates to a power distribution unit for PV systems, designed to safely and efficiently connect multiple PV strings to an inverter. The power distribution unit has several inputs for connecting PV strings and one output that can be connected to the DC bus of an inverter. The PV strings are connected in parallel within the power distribution unit and are switchably connected to the output of the power distribution unit.

[0049] A key feature of this power distribution unit is an integrated residual current protection circuit. This circuit is positioned between the parallel connection of the PV strings and the output and can include, in particular, a common-mode current sensor and a circuit breaker. The protection circuit is designed to disconnect the connection between the PV strings and the output if the common-mode current on this connection exceeds a certain threshold.

[0050] The collection unit is characterized by the fact that the residual current protection circuit has an input for a passivation signal. This makes it possible to maintain the connection between the PV strings and the DC bus even when a normally impermissible common-mode current flows under certain conditions, by applying a passivation signal to the residual current protection circuit.

[0051] The residual current protection circuit can include an evaluation unit that processes the readings from the common-mode current sensor. This evaluation unit is responsible for opening the circuit breaker if the measured common-mode current exceeds the defined limit and no passivation signal is present.

[0052] In one embodiment of the power collection unit, a detection device is installed between the parallel connection of the PV strings and the output. This detection device is capable of detecting a passivation signal on the connection between the PV strings and the output of the power collection unit and applying it to the residual current protection circuit. Specifically, the detection device can comprise a differential-mode current sensor, in particular a shunt resistor, which can be arranged in one of the DC lines between the parallel connection of the PV strings and the output of the power collection unit. Signal processing by the detection device can then detect the passivation signal in the measured values ​​of the differential-mode current sensor and forward it to the residual current protection circuit.

[0053] In an alternative embodiment of the collection unit, the evaluation unit of the residual current protection circuit is further configured to detect a periodic common-mode signal by means of the common-mode current sensor and to recognize it as a passivation signal. This embodiment does not require an additional detection means for detecting the passivation signal, since the common-mode current sensor of the residual current protection circuit itself, in combination with the evaluation unit, forms the input for the passivation signal.

[0054] The registered collection unit offers a robust and flexible solution for the parallel connection of PV strings by achieving increased safety through the residual current protection circuit in the collection unit, and improving the continuity of operation of the PV system through the possibility of temporarily passivating the residual current protection circuit, so that supposedly impermissible common-mode currents occurring in certain situations do not lead to the disconnection of PV strings. BRIEF DESCRIPTION OF THE FIGURES

[0055] The invention will now be further explained and described with reference to exemplary embodiments shown in the figures. Fig. Figure 1 shows a PV system in a first embodiment as per the application, Fig. Figure 2 shows another embodiment of a PV system according to the application, and Fig. Figure 3 shows an embodiment of a method according to the application. FIGURE DESCRIPTION

[0056] Fig. Figure 1 shows a PV system 10 with three PV strings 12 and an inverter 14. The PV strings 12 consist in particular of a large number of PV modules connected in series and, if necessary, in parallel. The individual PV strings 12 are connected in parallel in a power distribution unit 20a. The power distribution unit can have several inputs for connecting the PV strings 12. Alternatively, the PV strings 12 can be connected directly to a common DC line, for example, a busbar in the power distribution unit 20a.

[0057] The parallel connection of the PV strings 12 is connected to a DC bus 16, for example via an output of the collection unit or directly. Multiple parallel connections of PV strings 12 can be connected to the DC bus 16 via a plurality of collection units 20a, 20b, 20c. For example, between ten and twenty PV modules can form a PV string 12, and between ten and twenty PV strings 12 can be connected in parallel in a collection unit 20. In turn, ten to twenty collection units 20 can be connected in parallel to the DC bus 16. This results in a total nominal power output of the PV system 10, which can be at least a few hundred kilowatts and up to several megawatts.

[0058] The DC bus 16 is connected to a bridge circuit 18 of the inverter 14 via a DC disconnect switch. The inverter 14 is dimensioned according to the total rated power of all connected PV strings 12 and can convert the electrical DC power received from the PV strings 12 via the power distribution units 20 and the DC bus 16 into AC power and feed it into an AC grid 39. The AC grid 39 is switchable to the bridge circuit 18 of the inverter 14 via an AC disconnect switch 38. The AC grid 39 can be a public distribution grid, a transmission grid, or an island grid.

[0059] The collection unit 20 is assigned a residual current protection circuit 21, which includes a common-mode current sensor 22 and a circuit breaker 24. Fig. In Figure 1, the residual current protection circuit 21 is arranged in the assembly unit 20a; however, it can alternatively also be implemented as a separate component. The residual current protection circuit 21 can, in particular, include an evaluation unit 23, which receives the measured values ​​from the common-mode current sensor 22 and checks the measured common-mode current according to predefined criteria, for example, by comparing it with a limit value. If the common-mode current exhibits impermissible, in particular excessively high, values, for example, due to an earth fault 44 on one of the PV strings 12, the evaluation unit 23 causes the circuit breaker 24 to open.

[0060] Any (suddenly occurring) differential current between the DC bus 16 and the PV strings 12 at the collector unit 20a is thus detected by the residual current device 21 and interrupted by opening the circuit breaker 24. This also applies analogously to the other identically constructed collector units 20b, 20c, and any further collector units 20 not shown here, which are connected in parallel to each other via the DC bus 16. As a result, the PV system 10 may include a multitude of residual current devices 21, each responsible for a subset of the total PV strings 12 and thus offering better protection, for example, through higher sensitivity of the individual measurements compared to a central residual current device and through the spatial localization of any earth fault to the subset of PV strings 12 connected to the affected collector unit 20.

[0061] In a registered PV system 10, a passivation signal 30 can be applied to the residual current protection circuits 21 of the collector units 20a, 20b, 20c. The residual current protection circuits 21 are configured to open the circuit breaker 24 as described when an impermissible common-mode current is detected, but only if no passivation signal is present at the same time. This ensures that the PV strings 12 remain connected to the DC bus 16, even if an impermissible common-mode current is detected by the residual current protection circuit 21, and prevents the connection between the PV strings 12 and the DC bus 16 from being disconnected when a passivation signal 30 is present at the residual current protection circuit 21.The disconnection of the connection between the PV strings 12 and the DC bus 16 is prevented as long as the passivation signal 30 is present at the residual current protection circuit 21, or until a predetermined period has elapsed after the application of an initializing passivation signal 30, i.e., triggering the predetermined period.

[0062] The passivation signal 30 can be generated, for example, by a controller 19, which can be centrally located in the PV system 10, particularly in or on the inverter 14, and is generally applied to the residual current protection circuits 21 in the collector units 20a, 20b, 20c via a wireless or wired signal connection. The controller 19 can generate the passivation signal 30, in particular, when a common-mode current between the DC bus 16 and the PV strings 12 is expected or may occur, caused by a known factor and which should not be interpreted as a fault current by the residual current protection circuits 21. For example, switching on the DC disconnect switch 32 or the AC disconnect switch 38 can lead to equalizing currents between the inverter 14, particularly the capacitances of the inverter bridge 18, and the PV strings 12 or 20c, respectively.whose leakage capacitances flow to ground, which are common-mode currents in form and are therefore detected by the common-mode current sensor 22. However, the cause of these common-mode currents, namely the switching of the respective disconnect switch 32, 38, is known and determined, so that there is no need to disconnect the connections between the PV strings 12 and the DC bus 16. To prevent this for a limited period, the passivation signal 30 can be generated, in particular, when the DC disconnect switch 32 is actuated (see Figure 3). Fig. 3), so that the connection between the PV strings 12 and the DC bus 16 is temporarily maintained even in the event of an impermissible common-mode current.

[0063] Fig. Figure 2 shows an embodiment of a PV system 10 according to the application, which, in addition to the components of the PV system 10 according to Fig. The inverter 14 has one further feature. In this example, the inverter 14 includes the DC disconnect switch 32 and a signaling device 40, which is configured to generate a passivation signal 30. Furthermore, the inverter 14 includes an earth fault protection circuit 36, which is connected to the DC bus 16 and, for example, during normal operation of the PV system 10, establishes a switchable resistive connection between the DC bus 16 and an earth potential, at which, in particular, an earth fault current can be measured and compared with a limit value. An insulation measuring device 34 can also be connected to the DC bus 16, which is configured to measure the insulation resistance of all the PV strings 12. However, as a rule, an insulation resistance measurement can only be performed when the earth fault protection circuit 36 ​​is disconnected from the DC bus 16, for example, by the earthing switch 37 of the earth fault protection circuit 36.

[0064] The DC disconnect switch 32, the insulation measuring device 34, the earth fault protection circuit 36 ​​and the signaling device 40 are shown in the example according to Fig. 2 are arranged in the inverter 14, but can alternatively be arranged individually or completely separately from the inverter 14, as long as they are each connected to the DC bus 16.

[0065] The passivation signal is in the embodiment according to Fig. 2. First, a periodic push-pull signal is injected into the DC bus 16 by the signaling device 40. For this purpose, the signaling device 40 comprises, in particular, a series circuit consisting of a signal switch and an inductor, which is arranged between the two poles of the two-pole DC bus 16. To generate a periodic push-pull signal, the signal switch of the signaling device 40 can be switched on and off periodically, whereby a switching frequency of a few kilohertz to a few tens of kilohertz can be used. The signal switch of the signaling device can be controlled by the controller 19 of the inverter 14 as required with a suitable periodic control signal. Alternatively, the signaling device 40 itself can perform a clocked control of the signal switch if a signal from the controller 19 or another control unit of the PV system 10 is present at the signaling device 40.

[0066] By periodically short-circuiting the DC bus 16 with the signaling device 40, a very robust push-pull signal is impressed into the DC bus 16. This signal propagates via the DC lines of the PV system 10 towards the PV strings 12 and is particularly visible in all parallel-connected collection units 20a, 20b, 20c during normal operation of the PV system 10. The signaling device 40 can include a diode and optionally a resistor, which are arranged electrically in parallel with the inductance of the signaling device 40 and, in particular, minimize reverse currents and resonances resulting from the periodic short-circuiting of the DC bus 16.

[0067] A differential-mode signal impressed into the DC bus 16 by the signaling device 40 can be detected by detection devices 25, which are assigned to the collection units 20a, 20b, 20c and are, in particular, arranged in the collection units 20a, 20b, 20c. The detection device 25 can, in particular, comprise a differential-mode current sensor 26, for example, a shunt resistor in at least one of the DC lines between the DC bus 16 and the parallel connection of the PV strings 12. Furthermore, the detection device 25 can have a signal processing circuit 27 that evaluates the measured values ​​of the differential-mode current sensor 26 with regard to an impressed passivation signal and, if necessary, applies the passivation signal 30 to the residual current protection circuit 21.

[0068] In the embodiment according to Fig. 2. The passivation signal 30 is used to influence the control of the circuit breaker 24, in particular to interrupt a switch-off signal 28. This prevents the circuit breaker 24, which is normally closed, from being opened when the passivation signal 30 is activated. This would otherwise occur due to the control signal 23 if an impermissible common-mode current were present in the collector unit 20.

[0069] Alternatively, the passivation signal 30 from the control unit 23 of the residual current protection circuit 21 can be used to prevent the circuit breaker 24 from opening. This is achieved by the evaluation unit 23 opening the circuit breaker 24 only if the measured common-mode current has impermissible values ​​and no passivation signal 30 is present at the collection unit 12. Furthermore, the passivation signal 30 can be used in the residual current protection circuit 21 to prevent the circuit breaker 24 from opening. This is achieved by controlling the circuit breaker 24 with an OR gate consisting of an on-signal and the passivation signal 30, so that the circuit breaker 24 is closed when either an on-signal from the evaluation unit 23 or the passivation signal 30 is present at the circuit breaker 24.

[0070] In an alternative embodiment not shown, the passivation signal 30 can be impressed into the DC bus 16 as a periodic common-mode signal, detected by the fault current protection circuit 21 by means of the common-mode current sensor 22 and recognized by the evaluation unit 23 as passivation signal 30.

[0071] Fig. Figure 3 shows an example of a method for operating a PV system 10. The method begins with a start-up phase in which the DC bus 16 is interrupted by the open DC disconnect switch 32, while the circuit breakers 24 in the distribution units 20 are closed, since no common-mode signal and therefore no fault current is detectable in the DC lines. During the start-up phase, for example, the control unit 19 of the inverter 14 can be initialized or other preparations for operating the PV system 10 can be carried out.

[0072] After completion of the preparatory start-up phase, the passivation signal 30 is activated (PS→on). This passivates the residual current protection circuits 21 in the collection units 20. With the passivation signal 30 activated, or within a specified period after activation of the passivation signal 30, the DC disconnect switch 32 can now be closed (DC→on) to connect the DC bus 16, including the connected collection units 20a, 20b, and 20c, to the bridge circuit 18 of the inverter 14 and to convert the electrical power of the PV strings 12 in the inverter.Any common-mode currents that may arise, which are normally considered impermissible by the residual current protection circuit 21 and lead to the activation of the circuit breaker 24 (e.g., charging currents between the capacitances of the PV strings and the capacitances of the bridge circuit), are assumed to be harmless and / or temporarily tolerable within the scope of this procedure and should not lead to the disconnection of any PV string 12. This is ensured by the applied passivation signal 30, which maintains the connection between the PV strings 12 and the DC bus 16 even in the case of an otherwise impermissible common-mode current, provided the passivation signal 30 is present at the residual current protection circuit 21.

[0073] After the DC bus 16 is connected to the bridge circuit 18 by closing the DC disconnect switch 32, the passivation signal can be deactivated (PS→off). Simultaneously, the PV system can switch to normal operation (NOM = normal operating mode), in which electrical power flows from the PV strings 12 to the inverter bridge 18 and then into the AC grid 39. In normal operation NOM, the residual current protection circuits 21 are active as usual; that is, the connections between the DC bus 16 and the PV strings 12 are monitored for any fault currents and are disconnected if necessary, if common-mode currents on the connections between the DC bus 16 and the PV strings 12 exceed predefined limits.

[0074] As part of normal operation (NOM), it may be necessary to regularly check the insulation resistance of the PV system 10 to earth, e.g., once a day. The insulation resistance measuring device 34 can be used for this purpose; however, it only operates reliably if the DC bus 16 has no other earth reference. Therefore, the earth fault protection circuit 36 ​​must be disconnected from the DC bus 16 for the duration of the insulation resistance measurement.

[0075] In Fig.Figure 3 shows a subroutine called Iso-Test within the context of normal operation (NOM). The Iso-Test subroutine begins with an initialization (Start) that, for example, initiates the measurement of the insulation resistance of the PV system 10 at specific times. After the initialization (Start), the ground fault protection circuit 36 ​​is first disconnected from the DC bus 16 (GFDI=off; where GFDI stands for Ground Fault Detection Interruption) to enable the measurement of the insulation resistance (R_Iso) in the next step.After the insulation resistance (R_Iso) measurement is complete, the earth fault protection circuit 36 ​​can be reconnected to the DC bus 16 (GFDI=on). However, since equalizing and / or transfer currents can occur during this step, which are perceived by the residual current protection circuits 21 as impermissible common-mode currents, the passivation signal 30 is activated (PS=on) before the earth fault protection circuit 36 ​​is (re)connected, and only then, for example, is the earthing switch 37 of the earth fault protection circuit 36 ​​closed (GFDI=on). This reliably prevents the PV strings 12 from being unnecessarily disconnected from the DC bus 16, which is particularly advantageous for the continuous operation of the PV system 10.

[0076] Alternatively or additionally, the passivation signal 30 can be activated during the measurement of the insulation resistance (R_Iso) or even before the earth fault protection circuit 36 ​​is disconnected from the DC bus 16. This further minimizes the probability of unnecessary and / or undesired disconnection of the PV strings 12 from the DC bus 16.

[0077] The subroutine Iso-Test is terminated after the passivation signal 30 is deactivated again or a specified period of time has elapsed after the passivation signal 30 was applied (PS→off), and the PV system 10 continues to operate in normal operation NOM with residual current protection circuits 21 now active again.

[0078] In summary, as per the application, the distributed residual current protection circuits 21 of the collective units 20a, 20b, 20c of the PV system 10 are temporarily passivated as needed by generating a passivation signal 30 and applying it to the residual current protection circuits 21 as required, particularly during switching operations on the DC bus 16, in order to prevent unwanted and / or unnecessary disconnection. The passivation signal 30 is generated, in particular, for periods during which a (central) switching operation is scheduled, for example, switching on or off a central ground fault protection circuit 36 ​​(GFDI), a DC disconnect switch 32, or an AC network 39, and / or during other active measurements, in particular a measurement of the insulation resistance of the PV system 10. The passivation signal 30 can, in particular, be imprinted in the DC lines between the inverter 14 and the PV generator and, for example,The signaling device 40 is generated in or on the inverter 14. The signaling device 40 can, for example, be configured such that the DC lines at the inverter 14 are pulsed and short-circuited via a large inductor, thus generating a robust differential-mode signal in the DC lines to the PV generator. Differential-mode current sensors 26, such as simple shunts already installed in the DC lines in the distribution units 20, can be used to detect such a differential-mode signal as a passivation signal 30. The receivers of the passivation signal 30 are therefore located at the same installation location as the distributed residual current protection circuits 21.In the case of shunts as differential-mode current sensors 26, a signal processing unit 27 can be provided that detects a passivation signal 30 in the measured values ​​of the respective differential-mode measurement signal, for example, by the amplitude of a differential-mode current exceeding a threshold value at a predefinable frequency. Upon detecting a passivation signal 30, the signal processing unit 27 can deactivate the respective residual current protection circuits 21, i.e., in particular, prevent the disconnection of the respective PV strings 12 from the DC bus 16 even if an otherwise impermissible common-mode current occurs, at least as long as the passivation signal 30 is detected or for a predefinable period after its detection. For this purpose, the signal processing unit can, for example, intervene directly in the respective residual current protection circuit 21 or be connected to a control circuit for the respective residual current protection circuit 21.As an alternative to a push-pull signal on the DC lines, other signal forms and / or transmission paths are also conceivable, e.g., other powerline communication signals from a broadcast-capable transmitter on inverter 14, broadcast transmission using a robust AM or FM signal, or other wired or wireless signal transmission. However, actual communication is not required, neither unidirectional nor bidirectional, as only a binary signal with a unique function is necessary.

[0079] In principle, the larger the PV generator as a whole, i.e., the more collection units 20 with their associated, essentially self-contained residual current protection circuits 21 and, if applicable, detection devices 25, there are, the more difficult it becomes to generate reliable signaling in the DC lines that produces sufficient signal levels at all recipients of the passivation signal 30. The proposed signaling device 40 for generating a push-pull signal represents an optimal compromise by generating a clear and robust passivation signal 30 with comparatively simple means, which is equally perceptible in all parallel-connected collection units 20.In order to decouple any large intermediate circuit capacitance in the inverter from the generation of the passivation signal, an inductor can additionally be arranged in one of the DC lines between the transmitting circuit and the intermediate circuit of the inverter, which also increases the signal level of the passivation signal 30 in the direction of the PV strings 12. REFERENCE MARK LIST 10 Photovoltaic system (PV system) 12 photovoltaic strings (PV strings) 14 inverters 16 Direct Current Bus (DC Bus) 18 Bridge circuit 19 Control 20, 20a, 20b, 20c Collection unit 21 Residual current protection circuit 22 Common-mode current sensor 23 Evaluation unit 24 circuit breakers 25 detection agents 26 Push-pull current sensor 27 Signal processing 28 Switch-off signal 30 Passivation signal 32 DC disconnect switches 34 Insulation resistance measuring device 36 Earth fault protection circuit 37 Earthing switches 38 AC disconnect switches 39 Alternating current network (AC network) 40 signaling devices 44 Earth faults

Claims

[1] PV system (10) with an inverter (14) and at least one collection unit (20) for PV strings (12), wherein the collection unit (20) has several inputs for PV strings (12) and one output, wherein the inputs in the collection unit (20) are connected in parallel and are switchably connected to the output of the collection unit (20), wherein the output of the at least one collection unit (20) is switchably connected to a two-pole DC bus (16) and the DC bus (16) is switchably connected to a bridge circuit (18) of the inverter (14), wherein at least one collection unit (20) is assigned a residual current protection circuit (21) which is arranged between the parallel connection of the PV strings (12) and the DC bus (16) and in particular comprises a common-mode current sensor (22) and a circuit breaker (24), wherein the residual current protection circuit (21) is configured to cause a disconnection of the connection between the PV strings (12) and the DC bus (16) when a common-mode current on the connection between the PV strings (12) and the DC bus (16) exceeds a predefinable limit value, characterized by , that the residual current protection circuit (21) is designed to maintain the connection between the PV strings (12) and the DC bus (16) even in the event of an impermissible common-mode current and / or to prevent the disconnection of the connection between the PV strings (12) and the DC bus (16) when a passivation signal (30) is present at the residual current protection circuit (21). [2] PV system (10) according to claim 1, wherein the residual current protection circuit (21) comprises an evaluation unit (23) which is configured to evaluate the measured values ​​of the common-mode current sensor (22) and to open the protective switch (24) when the measured common-mode current exceeds the specified limit value and no passivation signal (30) is present. [3] PV system (10) according to claim 1 or 2, wherein several collection units (20a, 20b, 20c) are connected to the DC bus (16) via their outputs and are connected in parallel in or on the inverter (14). [4] PV system (10) according to one of claims 1 to 3, wherein a DC disconnect switch (32) is arranged in the DC bus (16) between the bridge circuit (18) of the inverter (14) and the at least one collection unit (20). [5] PV system (10) according to one of the preceding claims, wherein an earth fault detection unit (36) and optionally an insulation measuring device (34) are arranged between the bridge circuit (18) of the inverter (14) and the at least one collecting unit (20) and are connected to one pole of the two-pole DC bus (16). [6] PV system (10) according to one of the preceding claims, wherein the residual current protection circuit (21) is arranged in the collection unit (20). [7] PV system (10) according to one of the preceding claims, wherein a signaling means (40) for generating the passivation signal (30) is arranged at or in the inverter (14), wherein the signaling means (40) is in particular configured to impress a periodic differential-mode signal or a periodic common-mode signal into the DC bus (16). [8] PV system (10) according to claim 7, wherein the signaling means (40) comprises a signal switch and an inductor, wherein the signal switch and the inductor are arranged in series between the two poles of the two-pole DC bus (16). [9] PV system (10) according to claim 8, wherein the signaling means (40) comprises a diode and optionally a resistor, which is / are arranged electrically in parallel to the inductance of the signaling means (40). [10] PV system (10) according to one of the preceding claims, wherein a detection means (25) is arranged between the parallel connection of the PV strings (12) and the DC bus (16), which is assigned to the respective collection unit (20) and is configured to detect the passivation signal (30) on the connection between the PV strings (12) and the DC bus (16) and to apply it to the residual current protection circuit (21). [11] PV system (10) according to claim 10, wherein the detection means (25) comprises a differential-mode current sensor (26), in particular a shunt resistor, and a signal processing unit (27). [12] PV system (10) according to claim 7, wherein the signaling means is arranged between one pole of the two-pole DC bus (16) and earth potential and is configured to imprint the passivation signal (30) as a periodic common-mode signal into the DC bus (16). [13] PV system (10) according to claim 12, wherein the residual current protection circuit (21) is configured to detect the periodic common-mode signal by means of the common-mode current sensor (22) and to recognize it as a passivation signal (30). [14] PV system (10) according to one of the preceding claims, wherein the inverter (14) is switchably connected to an AC network (39) via an AC disconnect switch (38). [15] Method for operating a PV system (10) with an inverter (14), at least one collection unit (20) and a plurality of PV strings (12), wherein the PV strings (12) are connected in parallel in the collection unit (20) and feed their electrical power into a DC bus (16) via an output of the collection unit (20), which is connected to a bridge circuit (18) of the inverter (14), wherein the at least one collection unit (20) is assigned a residual current protection circuit (21) which is arranged between the parallel connection of the PV strings (12) and the DC bus (16) and in particular comprises a common-mode current sensor (22) and a circuit breaker (24), wherein the residual current protection circuit (21) causes a disconnection of the connection between the PV strings (12) and the DC bus (16) when a common-mode current on the connection between the PV strings (12) and the DC bus (16) exceeds a predefinable limit value, characterized by, that the connection between the PV strings (12) and the DC bus (16) is maintained even in the case of an impermissible common-mode current when a passivation signal (30) is present at the residual current protection circuit (21). [16] Method according to claim 15, wherein an evaluation unit (23) of the residual current protection circuit (21) evaluates the measured values ​​of the common-mode current sensor (22) and opens the protective switch (24) when the measured common-mode current exceeds the predetermined limit value and no passivation signal (30) is present. [17] Method according to claim 15 or 16, wherein several collection units (20a, 20b, 20c) feed electrical power from each connected PV strings (12) in parallel into the DC bus (16) of the inverter (14), wherein the same passivation signal (30) is applied to the residual current protection circuits (21) of all collection units (20a, 20b, 20c). [18] Method according to one of claims 15 to 17, wherein the passivation signal (30) is impressed onto the DC bus (16) by a signaling means (40). [19] Method according to one of claims 15 to 18, wherein the at least one collection unit (20) is associated with a detection means (25) which is arranged between the parallel connection of the PV strings (12) and the DC bus (16), wherein the passivation signal (30) on the connection between the PV strings (12) and the DC bus (16) is detected by the detection means (25) and applied to the residual current protection circuit. [20] Method according to claim 19, wherein the passivation signal (30) is impressed as a differential-mode signal onto the DC bus (16), wherein the detection means (25) detects the differential-mode signal by means of a differential-mode current sensor (26), in particular by means of a shunt sensor, and in particular applies it as a passivation signal (30) to the residual current protection circuit (21) via a signal processing (27). [21] Method of one of claims 18 to 20, wherein the passivation signal (30) is impressed by periodically short-circuiting the lines of the DC bus (16) by means of the signaling means (40), wherein the signaling means (40) comprises a signal switch and an inductor arranged in series with each other between the two poles of the two-pole DC bus (16). [22] Method according to claim 18, wherein the signaling means is arranged between one pole of the two-pole DC bus (16) and the earth potential and the passivation signal (30) is impressed into the DC bus (16) as a periodic common-mode signal. [23] Method according to claim 22, wherein the periodic common-mode signal is detected by the fault current protection circuit (21) by means of the common-mode current sensor (22) and is recognized by the evaluation unit (23) as a passivation signal. [24] Method according to claims 18 to 23, wherein the signaling means (40), in particular the signal switch of the signaling means (40), is controlled by a control (19) of the inverter (14). [25] Method according to any one of claims 15 to 24, wherein the passivation signal (30) is generated when an AC disconnect switch (38) arranged between the bridge circuit (18) of the inverter (14) and an AC network (39) is closed, and / or when a DC disconnect switch (32) in the DC bus (16) is closed, and / or when an earth fault protection circuit (36) is connected to the DC bus (16). [26] The method of claim 25, wherein the method comprises the following steps: - Feeding electrical power from the PV strings (12) into the DC bus - Disconnecting the earth fault protection circuit (36) from the DC bus (16), - Measuring the insulation resistance of the PV strings (12) using an insulation resistance measuring device (34), - Generating the passivation signal (30), - Connecting the earth fault protection circuit (36) to the DC bus (16), especially after completion of the insulation resistance measurement. [27] Method according to claim 26, wherein the generation of the passivation signal (30) is started before disconnecting the earth fault protection circuit (36) from the DC bus (16) or during or after the insulation resistance measurement and is terminated after connecting the earth fault protection circuit (36) to the DC bus (16). [28] Method according to one of claims 25 to 27, wherein the earth fault protection circuit (36) is connected to the DC bus (16) in normal operation (NOM) and is regularly, in particular once daily, disconnected and reconnected in order to check an insulation resistance of the PV system (10) when the earth fault protection circuit (36) is disconnected. [29] Method according to any one of claims 16 to 28, wherein the passivation signal (30) is received by the evaluation unit (23) of the residual current protection circuit (21) and is used to prevent the opening of the circuit breaker (24), so that the evaluation unit (23) opens the circuit breaker (24) only when the measured common-mode current has impermissible values ​​and no passivation signal (30) is present at the collection unit (20). [30] Method according to any one of claims 16 to 28, wherein the passivation signal (30) in the residual current protection circuit (21) is used to prevent the opening of the circuit breaker (24) by interrupting a switch-off signal (28) of the evaluation unit (23) to open the circuit breaker (24). [31] Method according to any one of claims 16 to 28, wherein the passivation signal (30) in the residual current protection circuit (21) is used to prevent the circuit breaker (24) from opening by controlling the circuit breaker (24) with an OR gate consisting of a switch-on signal and the passivation signal (30), such that the circuit breaker (24) is closed when either a switch-on signal from the evaluation unit (23) or the passivation signal (30) is present at the circuit breaker (24). [32] Collecting unit (20) for a PV system (10) with a plurality of inputs for connecting PV strings (12) and an output for connection to a DC bus (16) of an inverter (14), wherein the PV strings (12) are connected in parallel in the collecting unit (20) and are switchably connected to the output of the collecting unit (20), wherein the collecting unit (20) comprises a residual current protection circuit (21) which is arranged between the parallel connection of the PV strings (12) and the output and in particular comprises a common-mode current sensor (22) and a circuit breaker (24), wherein the residual current protection circuit (21) is configured to cause a disconnection of the connection between the PV strings (12) and the output when a common-mode current on the connection between the PV strings (12) and the output exceeds a predefinable limit value, characterized by, that the residual current protection circuit (21) has an input for a passivation signal (30) and is designed to maintain the connection between the PV strings (12) and the DC bus (16) even in the case of an impermissible common-mode current, when a passivation signal (30) is applied to the residual current protection circuit (21). [33] Collecting unit (20) according to claim 32, wherein the residual current protection circuit (21) comprises an evaluation unit (23) which is configured to evaluate the measured values ​​of the common-mode current sensor (22) and to open the protective switch (24) when the measured common-mode current exceeds the predetermined limit value and no passivation signal (30) is present. [34] Collecting unit (20) according to claim 32 or 33, wherein a detection means (25) is arranged between the parallel connection of the PV strings (12) and the output of the collecting unit (20), which is configured to detect a passivation signal (30) on the connection between the PV strings (12) and the output and to apply it to the residual current protection circuit (21). [35] Collection unit (20) according to claim 34, wherein the detection means (25) comprises a differential-mode current sensor (26), in particular a shunt resistor, and a signal processing unit (27), wherein the signal processing unit (27) is configured to detect the passivation signal (30) in the measured values ​​of the differential-mode current sensor (26) and forward it to the residual current protection circuit (21). [36] Collecting unit (20) according to claim 33, wherein the evaluation unit (23) is further configured to detect a periodic common-mode signal by means of the common-mode current sensor (22) and to recognize it as a passivation signal (30).

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