Energy production plant with a power converter unit
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- SMA SOLAR TECH AG
- Filing Date
- 2024-06-14
- Publication Date
- 2026-04-22
AI Technical Summary
Large PV energy generation systems face challenges in detecting fault currents due to high parasitic capacitance, leading to potential electrical hazards and increased complexity and cost in monitoring, especially when the fault current is small compared to the capacitive leakage current present during normal operation.
A power generation system with a power converter unit featuring a DC intermediate circuit insulated from ground, equipped with monitoring units including differential current measuring devices, circuit breakers, and controllers, which utilize defined leakage capacitances to distinguish between normal leakage currents and fault currents, allowing for reliable detection and disconnection of faulty PV main strings.
This solution enables safe and cost-effective detection of fault currents, adhering to normative specifications, even in high-power systems, by providing a defined return current path for ground current measurement, thus preventing false tripping and ensuring system safety without the need for a fenced safety protection area.
Smart Images
Figure EP2024066608_19122024_PF_FP_ABST
Abstract
Description
[0001] POWER GENERATION PLANT WITH POWER CONVERTER UNIT
[0002] Technical field of the invention
[0003] The invention relates to energy generation systems with a power converter unit, which in a first aspect comprises a DC / AC converter that can both feed energy into an alternating current (AC) grid and extract energy from the AC grid. Furthermore, the invention can also relate to a DC-DC converter. In a first embodiment of the invention, a DC-side energy source is a photovoltaic energy generation system (PV system); in another embodiment, it is a battery-electric energy storage system (BESS).
[0004] State of the art
[0005] Large-scale PV power generation systems are opening up an ever-expanding range of possible applications. In addition to private, home-based energy generation—that is, the conversion of DC voltage provided by PV generators into AC grid voltage using an inverter and supplying a household grid or feeding it into a public utility grid—PV power plants with increasingly larger power outputs are assuming a significant share of the public electricity supply as large-scale power plants. In addition, there are other applications as part of the general transition of industrial processes to more ecologically sustainable methods. For example, large-scale PV power generation systems can be used as a DC source, without converting and providing AC power, to power industrial facilities such as battery parks, factories, electrolyzers, or seawater desalination plants, and to provide a DC grid.
[0006] A PV system can comprise a multitude of electrical components, particularly PV modules, distributed in a decentralized manner over a large area. A group of PV modules that is grouped in strings, i.e. in series, is also called a PV string. A PV generator of a PV system can have one or more PV sub-generators or main strings, consisting of several PV strings that are connected in parallel to one another by means of a connecting device, also called a combiner box, possibly each via a separate DC / DC converter, to a common DC link of a PV inverter or, depending on the application, another power converter unit, such as a DC / DC converter. Each of the PV sub-generators can have one or more PV strings connected in parallel.Due to their design, the PV modules in a PV system always exhibit an electrical capacitance relative to their surroundings, particularly relative to their usually grounded mounting. This capacitance is not essential for the functioning of the PV system, but inevitably arises from the mechanical structure of the PV modules. It is therefore often referred to as "parasitic capacitance" or "leakage capacitance." The parasitic capacitance of the PV system typically increases with the size of its associated PV generator, which is why a powerful PV generator also exhibits a correspondingly large parasitic capacitance. Furthermore, the parasitic capacitance depends on ambient conditions and increases further, for example, in the event of rain due to the associated damp surface of the PV modules and / or a change in the dielectric constant of the air caused by increased humidity.
[0007] Due to the parasitic capacitance of the PV modules with respect to the earth potential, a more or less strong leakage current of the PV generator with respect to the earth potential always occurs during normal operation of the PV system.
[0008] If a fault, e.g., defective cable insulation, causes a grounded person to come into contact with a live component of the PV generator, such as the defective cable, an additional fault current against the earth potential will occur - usually abruptly due to the direct contact. Since a fault current of approximately 30 mA or more can be dangerous to persons and is relevant for fire protection above approximately 300 mA, it is standardized to reliably detect such a fault current and to initiate further measures upon detection of such a fault current, such as switching off and / or short-circuiting the PV generator, in particular the affected PV sub-generator. As a rule, two criteria must be met. Firstly, the fault current must not be abrupt, i.e.not exhibit rapid increases above a comparatively low limit value of, for example, 30 mA in order to ensure maximum personal protection. Secondly, for fire protection and system protection reasons, any total residual current or capacitive leakage current measured across the connecting cables of a PV generator must not exceed a significantly higher limit value of several hundred mA. Due to the ever-increasing nominal power of PV systems, the parasitic capacitances of the associated PV generators or PV sub-generators are also increasing, and thus also the capacitive leakage currents that are always present during normal operation of the PV system. However, the threshold value assigned to the leakage current, for example, 300 mA, remains constant and can at most be reduced due to stricter normative restrictions.Therefore, any fault current that may be present can be significantly smaller compared to the capacitive leakage current that is always present in the PV system. Due to the low signal-to-noise ratio and the associated need for sensitive measurement systems, fault current detection is becoming increasingly complex and expensive. It is therefore desirable, especially in larger PV systems, to be able to reliably yet cost-effectively detect a potentially occurring fault current, 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.
[0009] Normative requirements, such as IEC 63112, stipulate with regard to earth faults that PV power generation systems above a certain power class must either be operated behind a fence in an electrical operating area or, if they are publicly accessible, must be equipped with a so-called ROD (“residual current detection”) that meets the aforementioned criteria.
[0010] The problem is that the parasitic capacitance of a PV field connected to a central DC link of a power conversion unit, such as a central inverter, is so large that if a person or animal touches a pole of the PV field due to an insulation fault, they can be damaged by the large discharge current that occurs when the entire capacitance is transferred via their body.
[0011] In the case of particularly large PV fields, a parallel connection is created by connecting individual PV generators together to form PV strings, which in turn are combined in connecting units to form sub-generators or "main strings". The currents from several of these connecting units are then combined in a DC collection unit, such as a DC busbar or a common DC intermediate circuit, before being fed to a power converter unit, such as an inverter or a DC / DC converter. RCDs that carry out fault monitoring and preferably also include a fault disconnection device are preferably arranged in the connecting units in order to specifically monitor a sub-generator and be able to disconnect it if necessary. Earth fault monitoring in the RCDs is generally carried out via a differential current measurement of the DC and DC+ supply lines of the individual PV sub-generators.For an RCD to be used, the unwanted ground current must be able to bypass the RCD, at least partially. This can only happen if there is an earth connection on the side of the RCD facing away from the fault, especially if the PV field, or more precisely the DC link, is earthed, i.e., not floating or insulated.
[0012] However, for many applications, an isolated structure of the DC busbar or the DC intermediate circuit of the power converter unit is a basic requirement, for example in the case of DC coupling with batteries in parallel with the PV voltage.
[0013] For many such applications, it is therefore currently not possible to implement a design with a central DC link or a central power converter unit, without a fenced safety protection area and only with RCD monitoring.
[0014] For particularly large PV systems, there is therefore increased difficulty in extracting a fault current in order to monitor it for compliance with a low limit value and short-term increases.
[0015] Object of the invention
[0016] The invention is based on the object of providing a PV energy generation system that provides improved fault current monitoring even with high electrical power and correspondingly large capacity of connected PV generators.
[0017] Solution
[0018] The object is achieved by a power generation system with a power converter unit having the features of independent patent claim 1 and by a battery-electric storage system with a power converter unit according to claim 16. Advantageous embodiments of the invention are recited in claims 2 to 15.
[0019] Description of the invention
[0020] The energy generation system according to the invention comprises a power converter unit for connection to a PV generator, wherein the PV generator comprises a plurality of parallel-connected PV main strings, which are each connected via two DC input lines to the power converter unit of the energy generation system on the input side via a DC intermediate circuit, wherein the DC intermediate circuit of the power converter unit is electrically insulated, wherein each pair of DC input lines assigned to a PV main string is assigned a monitoring unit comprising a differential current measuring device, a disconnector and a controller, wherein the controller of the monitoring unit is configured to switch the disconnector and disconnect the PV main string after detecting a fault due to a differential current threshold being exceeded,wherein at least one defined leakage capacitance to earth is arranged at at least one pole of the DC intermediate circuit as a return current path for an earth current measurement of the differential current measuring device.
[0021] This ensures that a monitoring unit is always capable of triggering via the reverse current path defined by the defined leakage capacitance, so that an unwanted fault current can be detected by the residual current measuring device. The leakage capacitances are defined and dimensioned in such a way that the isolated (or floating) design of the system is ensured and a naturally occurring leakage current occurring via the parasitic capacitances of the individual sub-generators can be distinguished from an unwanted fault current. The residual current measuring devices assigned to the sub-generators can thus detect a difference between an input current into a sub-generator and a return current that would otherwise flow to earth via a fault location on the affected sub-generator.The described monitoring unit, which includes a residual current measuring device, a circuit breaker, and a control unit, is also called an RCD (residual current detection and interruption). The term RCD is also used synonymously for the monitoring unit assembly below.
[0022] The parasitic capacitances of the non-faulty sub-generators are usually not large enough for a recharging current to flow through these parasitic capacitances and to last long enough for the RCD in the fault path to be triggered before all capacitances have assumed a new steady-state voltage to earth.
[0023] Furthermore, the parasitic capacitance is not clearly defined, but depends on the ambient conditions, contamination, and the general condition of the PV modules. In new, dry PV modules, these parasitic capacitances are so small that the required tripping of an RCD in a faulty sub-generator would not be successful. By sufficiently dimensioning the discharge capacitances on the side of the RCD facing away from the fault, a charge reversal current can flow through the defined discharge capacitances and continue until all capacitances involved have assumed a new steady-state voltage to ground. This is sufficient to trip the RCD in the fault path.
[0024] Disconnection via the monitoring device's disconnector after fault detection by the residual current measuring device is only triggered when a defined residual current threshold is exceeded. This allows for adaptation to normative specifications regarding a permissible residual current, for example, in terms of absolute value or the dynamics of a rapid change, while also preventing tripping even at small currents introduced via the parasitic capacitances of the non-faulty sub-generators.
[0025] An energy generation system according to the invention is preferably formed by a photovoltaic (PV) energy generation system that has a plurality of parallel-connected PV main strings. These are connected via DC input lines to a DC intermediate circuit, for example, a DC busbar or busbar of the power converter unit. Within the scope of the invention, input lines together with a busbar can also be considered part of the DC intermediate circuit. The power converter unit can be implemented differently depending on the application. For example, the power converter unit can be formed by a DC / AC central inverter that is configured to convert the energy provided by the DC energy source, for example, the PV generators, and feed it into an alternating voltage grid (AC grid) and / or also draw energy from the AC grid.The inverter can be single-stage or multi-stage, for example, including additional DC / AC or DC / DC converter stages. To provide a return current path, the defined leakage capacitances must be arranged on the DC side between the power converter and the monitoring units of the sub-generators on the DC intermediate circuit. At least one leakage capacitance must be present at at least one pole of the DC intermediate circuit for the inventive effect to occur.
[0026] 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.
[0027] In a preferred embodiment of the device according to the invention, more than one discharge capacitor is arranged at at least one pole of the DC link. This leads to improved reliability and redundancy.
[0028] In a further preferred embodiment of the device according to the invention, the at least one leakage capacitance is arranged exclusively or additionally at one or more intermediate potentials of the DC link. In particular, a leakage capacitance can be arranged at a DC link center point. This is particularly advantageous for power converter units with symmetrical topologies.
[0029] In a further preferred embodiment of the device according to the invention, the leakage capacitances are additionally connected to damping resistors connected in series. These advantageously ensure that charge-reversal currents can flow for a sufficiently long time, so that the RCD's tripping time is not exceeded in the event of a dangerous fault current. It has proven particularly advantageous for the damping resistors to be between 10 ohms and 2.5 kOhms. In this way, the defined leakage capacitances can be more precisely adapted to the specific conditions of the system. Furthermore, disruptive interactions with any EMC suppression capacitors that may also be present are avoided.
[0030] In an advantageous embodiment, the defined leakage capacitances are dimensioned significantly smaller than the maximum total capacitance of the PV generator to ground, preferably less than 10%, particularly preferably less than 3% of the total capacitance of the PV generator to ground. In particular, the leakage capacitances are dimensioned significantly smaller in relation to the maximum total capacitance of the PV generator to ground at the most unfavorable operating point of the PV generator. In this way, the leakage capacitances only increase the total capacitance of the system very slightly and therefore have only a negligible impact on the efficiency of the PV system.
[0031] In a further preferred embodiment of the device according to the invention, the differential current threshold of the monitoring unit is less than or equal to 300 mA, and sudden changes in the range of 30 to 150 mA are additionally monitored. A value of 300 mA as the limit value for the residual current meets common normative fire protection requirements, such as those required for photovoltaic systems in the agricultural sector. Sudden changes in the range of 30 to 150 mA must be monitored according to the IEC 62109-2 standard. This enables safe operation of the system in fields of application that were previously inaccessible to isolated PV power generation systems.
[0032] In a preferred embodiment of the device, the DC link has a DC disconnect switch, with the leakage capacitances being arranged on the side of the DC disconnect switch facing the PV input side of the power converter unit. In many large systems, it is provided or even required that the power converter unit has a DC disconnect switch that disconnects the DC link, or the power converter unit's supply line, from all of the PV generators. For example, for diagnostic or maintenance purposes, or in systems in which several components are connected in parallel to the same DC link. To enable continuous monitoring for fault locations, a leakage capacitance is preferably arranged between the PV input side and the DC disconnect switch. In this way, monitoring can continue even when the DC disconnect switch is open, for example in cases where the PV modules still carry voltage.In one embodiment of the device according to the invention, the power converter unit is formed by a central inverter unit, which is connected on the output side to an AC voltage grid for providing electrical power via an AC disconnector, a transformer, and a grid connection device. In this application, the PV power generation system is designed to feed energy into an AC voltage grid.
[0033] In a further embodiment of the device, an electrically isolated battery-electrical storage system (BESS) is additionally provided, which is connected to the DC intermediate circuit on the side facing away from the DC isolating switch. A further monitoring unit with a differential current measuring device, a isolating switch, and a controller is assigned to the DC input lines assigned to the battery-electrical storage system, or to sub-batteries of the storage system. The controller of the monitoring unit is configured to switch the isolating switch and disconnect the battery-electrical storage system upon detection of a fault caused by exceeding a differential current threshold. Further bypass capacitors can be arranged on the side of the DC intermediate circuit facing away from the DC isolating switch.The differential current measuring device can consist of a plurality p of individual differential current measuring devices, each assigned to a sub-battery, preferably a battery rack. Accordingly, up to p isolating switches and controls can preferably also be provided. Corresponding battery-electric storage systems are advantageously designed to be electrically insulated, which also gives rise to the same problem of leakage current monitoring. In this way, the power generated by the PV generator can be used to charge the sub-batteries of the BESS. Alternatively or additionally, in the event that the PV generator is separated from the power converter unit by the DC isolating switch, electrical energy can be provided exclusively via the BESS.
[0034] It is particularly advantageous that the discharge capacitances of the battery-electric storage system are also connected to damping resistors connected in series. In a preferred embodiment, the monitoring units are arranged on DC input lines located within a housing of the power converter unit and are thus part of the power converter unit.
[0035] In an alternative embodiment, the monitoring units are arranged on DC input lines that are part of a connecting device assigned to each main string and that connects a plurality of PV strings into a main string. The connecting device (also called a combiner box) is arranged outside a housing of the power converter unit. This decentralized arrangement is particularly advantageous for large systems with a large number of main strings or for simplified system expansion or interchangeability of individual components.
[0036] In a further preferred embodiment of the device according to the invention, multiple monitoring units are provided, and each individual PV string of the main PV string is assigned a monitoring unit. This advantageously allows for more detailed monitoring by not disconnecting the entire sub-generator in the event of a fault, but rather only individual strings of the sub-generator and the non-faulty strings remain available.
[0037] It is further preferred that, in the device according to the invention, failure monitoring of the bypass capacitors can be carried out by means of an impedance measurement or a voltage measurement. If several bypass capacitors are arranged on a DC intermediate circuit, for example, one at a positive pole, one at a negative pole, and others at intermediate potentials, the design can be such that if one bypass capacitor fails, its function is compensated for by the other bypass capacitors.
[0038] In a further aspect of the invention, the energy generation system is designed without a PV generator and only a battery-electric storage system is present as a direct current source.
[0039] The overall system comprising a battery-electric storage system and a power converter unit comprises a plurality of parallel-connected sub-batteries, each of which is connected to a DC intermediate circuit of the power converter unit, the DC intermediate circuit of the power converter unit being electrically insulated. Each sub-battery is assigned a monitoring unit comprising a differential current measuring device, a disconnector, and a controller. The controller of the monitoring unit is configured to switch the disconnector and disconnect the sub-battery upon detection of a fault caused by a differential current threshold being exceeded. At least one defined earth leakage capacitance is arranged at at least one pole of the DC intermediate circuit as a return current path for earth current measurement by the differential current measuring device.The functionality and further embodiments of the individual components correspond to those of the version with a PV generator, so that at this point reference is made to the corresponding previous explanations in this regard.
[0040] Advantageous developments of the invention emerge from the patent claims, the description, and the drawings. The advantages of features and combinations of several features mentioned in the description are merely examples and can be used alternatively or cumulatively, without the advantages necessarily having to be achieved by embodiments according to the invention. Without altering the subject matter of the appended patent claims, the following applies to the disclosure content of the original application documents and the patent: further features can be found in the drawings—in particular the relative arrangement and functional connection of several components. The combination of features of different embodiments of the invention or features of different patent claims is also possible, deviating from the selected references of the patent claims, and is hereby suggested.This also applies to features that are illustrated in separate drawings or mentioned in their description. These features can also be combined with features of different patent claims. Likewise, features listed in the patent claims can be omitted for further embodiments of the invention.
[0041] The number of features mentioned in the patent claims and the description is to be understood as meaning that exactly this number or a greater number than the stated number is present, without the need for the explicit use of the adverb "at least." Thus, for example, if reference is made to one element, this is to be understood as meaning that exactly one element, two elements, or more elements are present. These features may be supplemented by other features or may be the only features of which the respective product consists.
[0042] The reference signs contained in the patent claims do not represent a limitation of the scope of the subject-matter protected by the patent claims. They serve solely to make the patent claims easier to understand.
[0043] Short description of the characters
[0044] The invention is illustrated below with the aid of figures, of which
[0045] Fig. 1 shows an embodiment of a device according to the invention;
[0046] Fig. 2 shows a further embodiment of a device according to the invention with a battery-electric system.
[0047] Character description
[0048] Fig. 1 shows an embodiment of a PV energy generation system according to the invention. The PV energy generation system comprises, as an embodiment of a DC generator, a photovoltaic generator formed by a plurality of PV main strings PV1, PV2... PVn. Each PV main string PV1 to PVn has a plurality of PV modules connected in series or a plurality of PV strings, which in turn consist of a plurality of PV modules. The PV main strings PV1 to PVn are similar in terms of the number and type of PV modules, in particular they are of the same design. In addition, the PV main strings PVn are arranged so close to one another that they are subject to at least similar ambient conditions with regard to irradiation and temperature. A power converter unit 20 is exemplarily designed as a so-called multi-string inverter. For this purpose, it has at least as many DC inputs for DC lines as there are PV main strings PVn in the system.The DC inputs are preferably protected by pairs of fuses 36. The individual PV main strings PVn are connected in parallel, for example via DC busbars, to a common DC intermediate circuit 7. This DC intermediate circuit 7 can, for example, also be formed by a split center-point intermediate circuit. The common DC intermediate circuit 7 is in turn connected to a DC side of a DC / AC converter 5 of the power converter unit 20. A DC isolating switch 6 is preferably also provided, which can disconnect the entire PV generator from the DC / AC converter 5 if necessary. An alternating current (AC) network 1, which is also designed as a three-phase network, for example a medium-voltage network, is connected to the AC side of the DC / AC converter 5, which is exemplary in Fig. 1 and designed as a three-phase network, via an AC isolating switch 4, a transformer 3, in particular a medium-voltage transformer, and a grid connection device 2. A single-stage DC / AC converter 5 is shown as an example.Within the scope of the invention, this can also be designed as a multi-stage converter, for example, with additional DC / DC stages and both unidirectional and bidirectional. A control unit (not shown) of the power converter unit 20 controls the switches of the DC / AC converter 5 for the desired voltage conversion. Further components such as EMC filters and line filters are not shown for the sake of clarity. In particular, the PV system, in particular the DC intermediate circuit 7, is insulated or floating, without a fixed potential relative to ground, and is galvanically separated from the AC grid 1 via the transformer 3.
[0049] To monitor the power converter unit 20 for the occurrence of critical fault currents, which indicate earth faults in the area of the PV strings and their parallel connection, several monitoring units 21.1 to 21.n, so-called RCDs (residual current detection and interruption), are provided. These each comprise a residual current measuring device 8.1 to 8.n, a disconnector 9.1 to 9.n, and a controller 17.1 to 17.n. These are each assigned to the PV main strings PV1 to PVn. The residual current measuring units 8.1 to 8.n each record the residual current across a pair of input lines of a PV main string PV1 to PVn. The monitoring units 21.1 to 21.n can be formed as part of the power converter unit 20, for example within a container housing of the power converter unit 20, or externally, for example in a connection unit or combiner box in which individual PV modules or PV strings are interconnected to form a PV main string PVn and in which further monitoring and security components can also be arranged.
[0050] The individual PV main strings PV1 to PVn exhibit a parasitic capacitance 14 relative to ground potential, which can vary depending on the individual PV main strings. Leakage currents always flow toward ground potential via the parasitic capacitances 14. These are capacitive reactive currents. The leakage currents, together with the parasitic capacitances 14, depend on the ambient conditions of the PV strings, such as humidity, temperature, precipitation, or similar factors. They can vary significantly over time, although they also change rather slowly. However, they change in a similar way for similar PV main strings PV1 to PVn.
[0051] In the event of a fault, for example if a grounded person 23 makes contact between one of the PV modules, shown in Fig. 1 as an example for the PV main string PV1, and the earth potential, a fault current flows against the earth potential in addition to the leakage current on the PV string on which the fault was caused.
[0052] To protect people against electric shock, sudden changes in currents must be detected, such as those that can occur when life-threatening currents flow through the human body. Such fault currents are life-threatening even at currents that can be significantly lower than the usual currents of harmless capacitive leakage currents.
[0053] Such monitoring can only be reliably determined via the differential current measuring devices 8.1 to 8.n if a current difference occurs across the two monitored lines of a PV main string. This is illustrated in Fig. 1 using the example of the PV main string PV1. In order to detect the currents flowing via the leakage capacitances 14 and, in the event of a fault, via the grounded person 23, a return current path is required, which, however, is not present in an insulated DC link 7 design. For this reason, defined leakage capacitances 31, 32 are provided, which create a return current path 35. These are located at at least one pole of the DC link 7, for example, at the positive pole, the negative pole, or at a possible midpoint, if present. Defined leakage capacitances 31, 32 can also be provided at multiple locations, here at both the positive and negative poles, as illustrated in Fig. 1, to increase safety.The bypass capacitors are arranged on the side of the DC disconnect switch 6 facing the PV generator. This also enables monitoring when the DC disconnect switch 6 is open. Only in this way is a defined return current path 35 provided for an isolated or floating structure that is not dependent on ambient conditions, as is the case with the parasitic capacitances 14. The differential current measuring device 8.1 can now detect a sudden change in the usual leakage currents and / or an exceedance of a specified limit. This signal is sent to the controller 17.1, which then triggers the disconnect switch 9.1 and disconnects the faulty PV main string PV1.
[0054] The differential current threshold of the monitoring unit 21.1 is preferably less than or equal to 300 mA, and sudden changes in the range from 30 to 150 mA are additionally monitored. The defined leakage capacitances 31, 32 are dimensioned significantly smaller than the maximum total capacitance of the PV generator to ground, preferably less than 10%, particularly preferably less than 3% of the total capacitance of the PV generator to ground. Thus, the leakage capacitances only very slightly increase the total capacitance of the system and therefore have only a negligible impact on the efficiency of the PV system.
[0055] In addition, the defined leakage capacitances are connected to series-connected damping resistors 33 and 34 to ensure that charge-reversing currents can flow for a sufficiently long time, ensuring that the RCD's tripping time is not exceeded in the event of a dangerous fault current. These range, for example, from 10 ohms to 2.5 kOhms.
[0056] Fig. 2 shows a further embodiment of a PV energy generation system according to the invention, which essentially corresponds to the embodiment shown in Fig. 1. Therefore, for reasons of clarity, not all details of the embodiment are explained and are provided with reference numbers that have already been explained in the embodiment shown in Fig. 1 and are clearly identical to this. In addition to the embodiment shown in Fig. 1, an electrically isolated battery-electrical storage system (BESS) 37 is provided, which is connected between the DC isolating switch 6 and the DC / AC converter 5 to the DC intermediate circuit 7 via a DC / DC converter 38. A further monitoring unit 2T with a differential current measuring device 8', a isolating switch 9', and a controller 17' is assigned to the DC input lines assigned to the battery-electrical storage system 37 or to sub-batteries of the storage system 37.The controller 17' of the monitoring unit 2T is configured, in a manner analogous to the monitoring units 21.1 to 21.n of the PV generator, to switch the disconnector 9' and disconnect the battery-electrical storage system 37 after a fault is detected due to a differential current threshold being exceeded, wherein further defined bypass capacitors 3T, 32' and damping resistors 33', 34' are arranged on the side of the DC intermediate circuit 7 facing away from the DC disconnector 6. The differential current measuring device 8' can consist of a plurality p of individual differential current measuring devices 8', each of which is assigned to a partial battery, preferably a battery rack. Accordingly, preferably up to p disconnectors 9' and controllers 17' can also be provided.Corresponding battery-electric storage systems are usually also electrically isolated, which also results in the same problem of leakage current monitoring. In this way, the power generated by the PV generator can be used to charge the BESS's sub-batteries. Alternatively or additionally, if the PV generator is disconnected from the power converter unit 20 by the DC disconnect switch 6, electrical energy is provided exclusively via the BESS 37.
[0057] List of reference symbols
[0058] 1 AC network
[0059] 2 mains connection device
[0060] 3 transformer
[0061] 4 AC disconnect switches
[0062] 5 DC / AC converters
[0063] 6 DC disconnect switches
[0064] 7 DC intermediate circuit / busbar
[0065] 8.1 - 8.n, 8' differential current measuring device
[0066] 9.1 - 9.n, 9' string disconnector
[0067] 13 Grounding
[0068] 14 Parasitic capacity
[0069] 17.1 - 17. n, 17' RCD control
[0070] 20 Power converter unit
[0071] 21 Monitoring unit
[0072] 23 people
[0073] 31 , 32, 31 ',32' Defined leakage capacity
[0074] 33, 34,33', 34' Damping resistors
[0075] 35 Return current path
[0076] 36 fuses
[0077] 37 Battery-electric storage system (BESS)
[0078] 38 DC / DC converters
[0079] PV1 - PVn sub-generators / PV main string
Claims
Patent claims 1. An energy generation plant with a power converter unit (20) for connection to a PV generator, wherein the PV generator comprises a plurality of parallel-connected PV main strings (PVn), which are each connected on the input side to the power converter unit (20) of the PV energy generation plant via two DC input lines, the DC intermediate circuit (7) of the power converter unit (20) being electrically insulated, and a monitoring unit (21.n) comprising a differential current measuring device (8.n), a disconnector (9.n), and a controller (17.n) being assigned to each pair of DC input lines assigned to a PV main string (PVn), the controller (17.n) of the monitoring unit (21.n) being configured to switch the disconnector (9.n) off after a fault has been detected due to a differential current threshold being exceeded.n) and to disconnect the PV main string (PVn), wherein at least one defined leakage capacitance (31, 32) to earth is arranged on at least one pole of the DC intermediate circuit (7) as a return current path (35) for an earth current measurement of the differential current measuring device (8.n).
2. Device according to claim 1, wherein more than one discharge capacitance (31, 32) is arranged on at least one pole of the DC intermediate circuit (7).
3. Device according to claim 1 or 2, wherein the at least one discharge capacitance (31, 32) is arranged exclusively or additionally at one or more intermediate potentials of the DC intermediate circuit (7).
4. Device according to one of claims 1 to 3, wherein the discharge capacitances (31, 32) are additionally connected to damping resistors (33, 34) connected in series.
5. Device according to claim 4, wherein the damping resistors (33, 34) are between 10 ohms and 2.5 kOhm.
6. Device according to claim 1 or 2, wherein the discharge capacitances (31, 32) are significantly smaller than the maximum total capacitance of the PV generator to earth, preferably less than 10%, particularly preferably less than 3% of the total capacitance of the PV generator to earth.
7. Device according to one of the preceding claims, wherein the differential current threshold value of the monitoring unit (21.n) is less than or equal to 300 mA and sudden changes in the range of 30 to 150 mA can be detected.
8. Device according to one of the preceding claims, wherein the DC intermediate circuit (7) has a DC isolating switch (6), wherein the discharge capacitances (31, 32) are arranged on the side of the DC isolating switch (6) which faces the PV input side of the power converter unit 20.
9. Device according to one of the preceding claims, wherein the power converter unit (20) is formed by a central inverter unit which is connected on the output side to an AC voltage network (1) for providing electrical power via an AC isolating switch (4), a transformer (3) and a network connection device (2).
10. Device according to claim 9, wherein an electrically isolated battery-electrical storage system (37) is connected to the DC intermediate circuit (7) via a DC / DC converter unit (38) on the side facing away from the DC isolating switch (6), wherein at least one further monitoring unit (2T), comprising a differential current measuring device (8'), a isolating switch (9') and a controller (17') is assigned to the DC input lines assigned to the battery-electrical storage system (37) or to partial batteries of the storage system (37), wherein the controller (17') of the monitoring unit (2T) is configured to switch the isolating switch (9') and to disconnect the battery-electrical storage system (37) after a fault is detected due to a differential current threshold being exceeded, wherein further bypass capacitors (3T, 32') are arranged on the DC intermediate circuit (7).
11. Device according to claim 10, wherein the discharge capacitances (31', 32') of the battery-electric storage system (37) are additionally connected to damping resistors (33', 34') connected in series.
12. Device according to one of claims 1 to 11, wherein the monitoring units (21. n, 21 ') are arranged on DC input lines which are arranged within a housing of the power converter unit (20) and are thus part of the power converter unit (20).
13. Device according to one of claims 1 to 11, wherein the monitoring units (21. n, 2T) are part of a connecting device which is assigned to each PV main string (PVn) and which connects a plurality of PV strings to a PV main string (PVn), wherein the connecting devices are arranged outside a housing of the power converter unit (20).
14. Device according to claim 13, wherein a plurality of monitoring units (21. n, 2T) are provided and each individual PV string of the PV main string (PVn) is assigned a monitoring unit (21. n, 2T).
15. Device according to one of the preceding claims, wherein failure monitoring of the bypass capacitors (31, 32, 3T, 32') can be carried out by an impedance measurement or a voltage measurement.
16. Battery-electric storage system (37) with a power converter unit (20) comprising a plurality of parallel-connected sub-batteries which are electrically insulated and connected to a DC intermediate circuit (7) of the power converter unit (20), wherein the DC intermediate circuit (7) of the power converter unit (20) is designed to be electrically insulated, wherein each sub-battery is assigned a monitoring unit (21.n) comprising a differential current measuring device (8.n), a disconnector (9.n) and a controller (17.n), wherein the controller (17.n) of the monitoring unit (21.n) is designed to switch the disconnector (9.n) and disconnect the sub-battery after a fault has been detected by exceeding a differential current threshold value, wherein at least one defined leakage capacitance (31, 32) to earth is provided at at least one pole of the DC intermediate circuit (7) as a return current path for a ground current measurement of the differential current measuring device (8.n) is arranged.