LU93202B1 - Multi-strand photovoltaic system, method for operating such and reverse current protection circuit for such - Google Patents

Abstract

The invention relates to a multi-strand photovoltaic system (1), comprising: two or more photovoltaic strings (10, 10 ') which are each formed by series-connected photovoltaic modules (12, 12'), a central collecting point (22a , b) to which the two or more photovoltaic strings (10, 10 ') are connected in parallel, at least one of the photovoltaic strings (10, 10') comprising a reverse current protection circuit (40). The invention further relates to a method for interrupting the connection of a photovoltaic string to a central collection point with other photovoltaic strings and / or Wiederanschaiten the photovoltaic string to the central collection point and the reverse current protection circuit. 93202

Description

Multi-strand photovoltaic system, method for operating such and reverse current protection circuit for such

description

Field of the invention

The invention relates to a multi-strand photovoltaic system with a reverse current protection circuit, a method for disconnecting and / or reactivation of a photovoltaic string from or to a parallel connection with other photovoltaic strings to a common inverter and a reverse current protection circuit for this purpose.

Background of the invention

A photovoltaic system typically includes a plurality of photovoltaic modules connected in series to form a nominal photovoltaic generator DC voltage, typically currently up to 1000 volts or even 1500 volts. Further, depending on the number of interconnected photovoltaic modules and their individual voltage in turn, if necessary, several of the photovoltaic strings connected in parallel, of which each photovoltaic string can produce, for example, a nominal current of 10A. Due to the high voltage and the high current in the DC part of the photovoltaic system, there is a need for maintenance or incidents such. in the event of a fire, the risk of people being exposed to life-threatening tensions.

Although a photovoltaic system typically has a main isolator in the so-called generator junction box, it can be used, e.g. In the event of damage caused by fire, water, hail etc. on the solar panels or on the connection lines, do not disconnect the area in front of the generator connection box. Furthermore, a selective activation of individual photovoltaic modules is also not possible.

Therefore, photovoltaic module protection circuits have been developed, with which the photovoltaic modules can be switched off individually (see WO2013 / 026539 A1 as well as the product SCK-RSD-100 of the Applicant). Furthermore, starting boxes have been developed, with which such "intelligent" photovoltaic modules can be reactivated (see WO2014 / 122325 A1 and the products SCK-RSD-400 and SCK-RSD-600 of the applicant).

If multiple strings, sometimes referred to as "strings", are connected in parallel, this is referred to as multistream or multi-string connection. In a multistream interconnection, it is generally possible to use return currents, sometimes also as crossflows or

Compensation flows referred to arise. Reverse currents in multi-strand photovoltaic systems are particularly virulent, in which the photovoltaic modules can be individually switched off and bridged with a short-circuit switch on the photovoltaic modules. In a multistream photovoltaic system, the return currents can cause a photovoltaic string to act as a source and a second photovoltaic string to sink, and the current flows via the parallel connection in front of the inverter or inverter. Thus, a disconnection would be at the inverter, which normally leads to the shutdown of such a secure photovoltaic system, although possible, but would not necessarily lead to the desired shutdown of the entire multi-strand photovoltaic system. The current could flow in this interconnection through the second strand as a drain and would be short-circuited there with line impedances. In doing so, a self-sustaining condition could be achieved which could persist without disconnecting or waiting until sunset.

Although it would be possible to insert stranded diodes before the parallel switching point, they disadvantageously have a relatively high power loss, which is typically about 7 W at 10 A. This power loss is particularly undesirable when used in conventional plastic housings in the strand cabling. Providing such a stranded or blocking diode in an additional housing may also be undesirable, as it adds further expense, maintenance, and susceptibility to malfunction, e.g. can go along with the additional plug connections. In general, such stranded diodes can simply break down over time.

Currently constructed photovoltaic systems accept negative return currents and secure the photovoltaic system only by means of string fuses against overloading by these unwanted back currents between the parallel-connected strands. However, such string fuses are not optimal, e.g. if one or more strands are shaded by a large number of parallel strands or if strands are to be selectively separated from the inverter. In principle, such string fuses only protect against overloading, but not in principle against backflowing cross currents, which can impair the safety and longevity of the photovoltaic system.

General description of the invention

The invention has therefore set itself the task of providing a multi-strand photovoltaic system that meets high security requirements and their wiring and control has a low power loss and heat.

Another object of the invention is to provide a multi-strand photovoltaic system which meets high safety requirements and can reconcile the yield and longevity of the photovoltaic system.

Another object of the invention is to provide a multi-strand photovoltaic system that can avoid unwanted back currents between parallel photovoltaic strings.

A further object of the invention is to provide a multi-strand photovoltaic system which meets high safety requirements and makes it possible to disconnect photovoltaic strings individually, e.g. to wait for this.

Another object of the invention is to provide a method for operating a multi-line photovoltaic system, which offers a high level of safety and flexibility in operation, maintenance and accidents.

Yet another object of the invention is to provide a safe and low-loss reverse current protection circuit for a multi-line photovoltaic system.

The object of the invention is solved by the subject of the independent claims. Advantageous developments of the invention are defined in the subclaims.

The multi-strand photovoltaic system according to the invention comprises two or more photovoltaic strands. The photovoltaic strings are each formed by serially connected photovoltaic modules, wherein the photovoltaic modules are connected in series by means of a two-pole string line.

The multi-strand photovoltaic system also has a central collection point at which the photovoltaic strings are connected in parallel, so that the central collection point forms a parallel switching point of the photovoltaic strings.

The multi-prong photovoltaic system further preferably includes an inverter, sometimes referred to as a solar inverter, which converts the DC voltage generated by the photovoltaic modules into AC voltage. The PMT photovoltaic system may also be used without an inverter, e.g. be connected to a charging unit.

The inverter includes a DC input to which the photovoltaic strings are connected and electrically connected during production. In the

Multi-strand photovoltaic system so several photovoltaic strings are connected in parallel to the same DC input of the inverter. For this purpose, the plurality of photovoltaic strings are connected in parallel via the central collection point, which is upstream of the DC input of the inverter, and then connected to the same DC input of the inverter. Preferably, the plurality of parallel photovoltaic strings are controlled by the same power optimizer, typically a multi-power point tracker (MPPT), to provide their performance in accordance with the

To optimize the irradiation situation.

At least one of the parallel photovoltaic strings, preferably all of the parallel photovoltaic strings, at the central collection point or at the inverter now have a reverse current protection circuit between the series-connected first photovoltaic modules and the central collection point. In particular, the reverse current protection circuit does not consist of a stranded or blocking diode subjected to a power loss, but is an actively switchable and / or electronically controlled reverse current protection circuit.

In other words, it is possible to dispense with additional stranded or blocking diodes for the reverse current protection. Preferably, the reverse current protection circuit has a power loss in the production operation of the associated photovoltaic string of less than 1 W, for example at 1000 volts and 10 amps per photovoltaic string.

The reverse current protection circuit has at least one switch, and a control device for controlling the switch or switches. Although the reverse current protection circuit can also comprise a semiconductor switch, the reverse current protection circuit preferably does not exclusively comprise semiconductor components, but at least one non-semiconductor-based switch, e.g. a relay on. Preferably, at least at times, in particular in the permanent production operation of the photovoltaic string, the photovoltaically generated current flows in the part of the photovoltaic string in which the voile nominal voltage of the several serial photovoltaic modules or the entire photovoltaic string (typ.> 500 volts) ) is applied, preferably exclusively, by metallic conductors, eg Metal cables, metal connectors and / or relays and at least not permanently by semiconductor devices. As a result, the power loss can be kept low in an advantageous manner, so that the reverse current protection circuit can optionally be installed in existing housing, without these overheat. Depending on the requirements, it may also be possible to use a switching device which comprises only semiconductor switches.

Preferably, each parallèle photovoltaic strand in front of the central collection point on such a reverse current protection circuit. The Rückstromschutzschaitungen have the input side (ie strand or photovoltaic module-side) a Piuspoleingang and a Minuspoleingang for connecting the Piuspols or Minuspols the strand line and the output side (ie collection point side) a positive terminal and a negative pole input for connecting the Piuspols or Minuspols a continuation of Catenary up to the central collection point of the photovoltaic strings or up to the DC input of the inverter. In particular, all of the string voltage is applied to the reverse current protection circuit and / or the entire string current flows through the reverse current protection circuit to be parallelized downstream in the direction of the inverter at the central collection point with the currents of the other parallel photovoltaic strings. Thus, the plus pole outputs and the minus pole outputs of the parallel photovoltaic strings are connected via the central collection point with the positive pole or the negative pole of the DC input of the inverter. The reverse current protection circuit now provides protection against a reverse current, which can be caused by the parallel circuit, from the other parallel photovoltaic strings in the associated photovoltaic string, so the photovoltaic string in the string line the respective reverse current protection circuit is turned on before the central collection point. In particular, the reverse current protection circuit prevents cross current from becoming so large that the direction of current flow in the associated photovoltaic string is reversed, i. a negative current flows back into the associated photovoltaic string. Advantageously, therefore, unwanted back currents can be prevented, which could otherwise result if the associated photovoltaic string were to be more low-impedance than the inverter or corresponding current collector downstream of the central bus. This can have a positive effect on the life of the associated photovoltaic modules. Depending on the safety equipment of the photovoltaic system, the reverse current protection circuit can also increase the safety of the system, in particular when switching off individual photovoltaic strings or the photovoltaic system, e.g. due to inadequate irradiation, maintenance or emergency shutdown, in particular in that the reverse current protection circuit prevents the shutdown of the associated photovoltaic string is prevented due to a return current.

The reverse current protection circuit is preferably installed in an electrically insulating housing which has a positive pole connection for the plus pole of the branch line and a negative pole connection for the negative pole of the phase line of the associated photovoltaic line on the line-side input. Further preferably, the backflow protection circuit at the collection point side output has a plus pole terminal for the plus pole and a minus pole terminal for the negative pole of the extension of the branch line leading to the central collection point. The terminals on the housing are preferably as photovoltaic connectors, e.g. trained according to the Applicant's SUNCLIX® system. The housing with the reverse current protection circuit, which can be inserted with the connectors in the two lines of the strand line of the associated photovoltaic module, can therefore form a separate plug-in unit in the form of a Rückstromschutzbox, which also as a retrofit solution in existing strand lines between the photovoltaic modules each of a photovoltaic string and the central collection point or the common inverter can be inserted.

Preferably, the reverse current protection circuit includes a control circuit and a sensor connected to the control circuit for measuring an electrical characteristic on the associated photovoltaic string. In response to the measured with the sensor electrical characteristic, the control circuit switches the at least one switch. In particular, the at least one switch, controlled by the control circuit, interrupts the current carrying connection of the associated photovoltaic string to the central collection point, e.g., in response to the electrical characteristic measured by the sensor. if the measured electrical parameter is outside of a definitive safety condition for the first electrical parameter for the return current.

The sensor is preferably a current sensor or a voltage sensor which measures at least one of the following electrical characteristics: input voltage at the upstream input of the reverse current protection circuit, output voltage at the collector side or inverter side output of the reverse current protection circuit, phase current of the associated photovoltaic string or through the reverse current protection circuit or box, - negative current flow.

A negative current flow means that the current in the photovoltaic string is flowing through the reverse current protection circuit contrary to the direction of flow of the photovoltaically generated current in operation. the current sensor is in particular designed to also be able to measure a negative current flow.

In response to one or more of the electrical characteristics measured by the sensor or sensors, the reverse current protection circuit switches the at least one switch and breaks the connection of the associated photovoltaic string to the parallel circuit with the other photovoltaic strings and to the DC input of the inverter. The reverse current protection circuit therefore preferably includes a current sensor which can also measure negative currents and / or an input voltage sensor and / or an output voltage sensor.

Advantageously, this can be determined in a simple manner separately on each photovoltaic string continuously, whether a state of the associated photovoltaic string is present, which is outside the desired operating and safety parameters.

According to a preferred embodiment of the invention, the sensor is accordingly a current sensor which can measure a negative current flow through the reverse current protection circuit. The reverse current protection circuit may connect the associated photovoltaic string to the central collection point, i. to the parallel connection with the other photovoltaic strings and to the DC input of the inverter, at least if the current in the associated photovoltaic string is negative and / or exceeds a predefined threshold value for the magnitude of the negative current. However, a negative current is not a necessary condition for disconnection. Depending on which operating and safety parameters are specified, the reverse current protection circuit can already perform the interruption even if the current is still positive, but is below a predefined safety threshold. This can e.g. occur when the photovoltaic modules of the associated photovoltaic string are significantly more shaded than the photovoltaic modules of the other photovoltaic strings. In this case, the cross-currents may not yet provide for a total negative current flow in the associated photovoltaic string, but may already reduce the photovoltaic-generated current from the associated photovoltaic string to such an extent that the production operation of that associated photovoltaic string is no longer economically viable, so it is better to connect the associated Photovoltaik strand to the

Parallel connection with the other photovoltaic strings and to interrupt the common inverter. This may be useful, in particular, if the current measured by the reverse current protection circuit is still positive, but considerably below the maximum current of the photovoltaic string, e.g. is less than 10% of the maximum current of the photovoltaic string. In other words, the control circuit is preferably designed to switch the reverse current protection circuit automatically or to interrupt the connection of the associated photovoltaic string to the parallel circuit and to the inverter when the condition occurs that the phase current IS becomes smaller than a predefined floating value I0. The predefined threshold value I0 can be selected between positive and significantly smaller than the maximum current of the associated photovoltaic string and a negative safety value for the current flow. In mathematical terms, the switching condition can be defined as IS <IO, where I0 is selected from an interval [11,12], where 11 is a negative confidence value at which the associated photovoltaic string could be damaged and I2 is a positive value, below which An economic operation no longer seems sensible.

In particular, the control circuit of the reverse current protection circuit also electrically switches the associated photovoltaic string electrically to the parallel circuit with the other photovoltaic strings and to the common inverter if the operating and safety parameters permit. For this purpose, the control circuit is connected to a sensor device for measuring at least one electrical parameter. This electrical characteristic may be the same electrical characteristic used to break the connection, but preferably it is another electrical characteristic on the associated photovoltaic string. In response to this measured with the sensor device electrical characteristic, the control circuit automatically switches the photovoltaic string whose connection has been interrupted again electrically to the other parallel photovoltaic strings and to the DC input of the inverter when this measured electrical characteristic within one for the after the electrical start-up expected return current relevant operating and safety condition is.

In other words, the reverse current protection circuit is preferably configured to switch off the associated photovoltaic string in response to a measured electrical parameter if the measured electrical parameter is outside a safety condition which governs the return current and / or the associated photovoltaic cell.

Strand to turn on electrically, if the measured electrical parameter is within a decisive for the return current safety condition.

With this, advantageously, the operating and safety parameters of the associated photovoltaic string can be determined not only during the production operation, but also during the period of the separation from the parallel circuit and from the inverter, e.g. be monitored permanently in case of shading or malfunction, as soon as possible to turn on the associated photovoltaic string again electrically, sobaid the operating and safety parameters allow this again. There is thus a permanent reverse current monitoring of the associated photovoltaic string during the production operation and / or during the period of interruption of the connection to the parallel circuit and to the inverter. This can reconcile the yield of the photovoltaic system with operational and safety parameters.

According to a preferred embodiment, the photovoltaic string electrical switching-on sensor device comprises an input voltage sensor which measures the input side voltage to the reverse current protection circuit and / or an output voltage sensor which measures the collector side or inverter side output voltage at the backflow protection circuit. In response to the measured line-side input voltage and / or the measured collector-side or inverter-side output voltage, the controller automatically switches the associated photovoltaic line whose connection to the central collection point is interrupted to the other parallel photovoltaic lines and to the DC input of the photovoltaic line Inverter electrically.

According to a further preferred embodiment of the invention, the input voltage is compared with the output voltage and the controller switches the at least one switch in response to the comparison of the measured line side input voltage and the measured collector side or inverter side output voltage, in particular that the associated photovoltaic line, whose connection to the central collection point is interrupted is automatically switched back to the other parallel photovoltaic strings and to the DC input of the inverter. The voltage comparison may ensure that after the electrical restart of the associated photovoltaic string the desired operating and safety parameters are met, e.g. that no negative flow back into the associated

Photovoltaic strand flows or the positive production flow exceeds a minimum value by the associated photovoltaic string is only electrically switched on again when the difference between output voltage U2 minus input voltage U1 is smaller than a predefined Schwelienwert UO. If U0 = 0, this means that the output voltage U2 is smaller than the input voltage U1. In this case, a certain tolerance may be present, in particular the effect can be exploited that the input voltage of the associated photovoltaic string whose connection to the central collection point is interrupted, the open-circuit voltage, whereas the output voltage, that of the other parallel photovoltaic modules generated voltage already represents a load voltage, which is typically about 20% lower than the open circuit voltage, because the inverter is already working. Thus, in response to the comparison of the open circuit voltage of the associated photovoltaic string with the load voltage of the other photovoltaic strings, the controller may automatically redirect the associated photovoltaic string whose connection to the central collection point is interrupted to the other parallel photovoltaic strings switch on the DC input of the inverter. In addition to the voltage match, it can still be checked whether the voltage of the associated photovoltaic string exceeds a predefined threshold value Ujnin, e.g. to ensure that the associated photovoltaic string can generate sufficient power at this time.

The reverse current protection circuit preferably comprises a switching device which in the closed state electrically connects the associated photovoltaic string in parallel with the other photovoltaic strings and in the open state interrupts the current-carrying connection of the associated photovoltaic string to the central collection point or to the inverter within the string line ,

In this case, the switching device should be designed for a separation voltage of at least 500 volts, better at least 1000 volts and / or for a through current of at least 5 amps, more preferably at least 10 amps. Namely, in particular, the total voltage of the associated photovoltaic string is interrupted or switched by the switching device and / or the entire production flow of the associated photovoltaic strand in the production operation of the photovoltaic string through the reverse current protection circuit, more precisely by the switching device, flow.

Preferably, the switching device includes a relay, in particular a parallel circuit of a semiconductor switch, in particular a MOS-FET, and a relay. Preferably, the control device closes when electrical Wiederanschaiten the associated photovoltaic strand to the other parallel photovoltaic strings and to the DC input of the inverter first, the semiconductor switch and only after a time delay, the parallel relay. This has the advantage that the semiconductor switch is relieved in the production operation of the photovoltaic string by the relay from the flow of current, so that on the one hand reduces the loss of line and on the other hand, the relay can be spared. The thermal and spatial conditions of the reverse current protection circuit are suitable for placing them in existing housings, e.g. Initially, only the semiconductor switch executes the required switching operations, which can also be repeated several times, and only when it is ensured that the photovoltaic string is permanently in production operation, the relay is closed. This can on the one hand keep the number of switching operations of the relay low and on the other hand, the power loss occurs on the semiconductor switch only for a short period of time, namely only as long as the relay is not yet closed.

Preferably, the semiconductor switch and / or the relay are designed as normally open (normally open). While this may seem disadvantageous at first glance, electrical power is required to hold the on-state. However, this is accepted in order to achieve the associated safety, namely that the connection of the associated photovoltaic string in the normal state of the switching device is interrupted to the parallel connection with the other photovoltaic strings and to the DC input of the inverter.

Advantageously, a semiconductor switch, e.g. a MOS-FET may be used, which has an RDS-on (Resistance-Drain-Source-On, also referred to as on-resistance) of e.g. at least 50 mOhms, possibly even several 100 mOhms or more. Although this generates a relatively large power loss of possibly one watt, a few watts or more, but this is acceptable in particular by the discharge through the parallel relay.

The presently disclosed invention or reverse current protection circuit is particularly advantageous to use in photovoltaic systems in which the series-connected photovoltaic

Modules in at least one of the photovoltaic strands, preferably in all photovoltaic strings, each have protective circuits by means of which the photovoltaic modules can be switched off individually from the associated strand, and wherein the protective circuits of the individual photovoltaic modules control the output of the respective photovoltaic Short-circuit the module at the connection points for the line to create a low-impedance bypass for the respective photovoltaic module. Such protective circuits for the photovoltaic modules are described in more detail in WO2013 / 026539 A1, to which reference is hereby made and which in this regard is hereby incorporated by reference. Furthermore, the presently disclosed invention or reverse current protection circuit is particularly advantageous to use in photovoltaic systems in which the associated photovoltaic string has a start circuit, which is designed to re-activate such or other protection circuits of the individual photovoltaic modules of the associated photovoltaic string , Such starting circuits for the photovoltaic strings are described in more detail in WO2014 / 122325 A1, to which reference is hereby made and which in this regard is hereby incorporated by reference.

Preferably, such a start-up circuit for starting the associated photovoltaic string and the reverse current protection circuit for the associated photovoltaic string can be accommodated in a common housing, so that a combined start-and-Rückstromschutzbox for the associated photovoltaic string is formed. The combined start and reverse current protection box is provided with plus and minus pole terminals at the input and plus and minus pole terminals at the output of the box between the series of photovoltaic modules of the associated photovoltaic string and the central collection point with the other photovoltaic strings connected in the associated photovoltaic string, where it can perform both functions - starting the associated photovoltaic string and reverse current protection for the same photovoltaic string. As a result, superfluous components, in particular connectors can be saved, which can have a positive effect on the cost, life and loss of the photovoltaic system. The power loss of the reverse current protection circuit is so low in the production mode, in particular when the relay is closed, that the thermal load is acceptable.

Preferably, each photovoltaic string of the parallel circuit at the central collection point or the DC input of the same inverter with the described reverse current protection circuit, Rückstromschutzbox or combined start and Rückstromschutzbox is equipped.

The invention also relates to a method for interrupting the live connection to the central collection point or to the inverter for protection against Rückstrôme and to turn on the photovoltaic strings of the multi-strand photovoltaic system described above to the central collection point or to the DC input of the inverter , The method comprises the following steps: During the entire operation of the multi-strand photovoltaic system is permanently permanently an electrical characteristic in the photovoltaic strings by means of each of the respective photovoltaic string zugehôrigen reverse current protection circuit for protection against the other photovoltaic strands in the respective a photovoltaic strand is measured back flowing cross-flow or return current.

If the measured electrical characteristic is outside a safety condition for the electrical characteristic of the associated photovoltaic string, which is relevant for the return current, e.g. if a negative total current flow occurs, the connection of that photovoltaic string to the other parallel photovoltaic strings and to the DC input of the inverter is automatically interrupted.

If in the associated photovoltaic string one or another measured electrical characteristic is within a safety condition relevant to the return current, e.g. when the input line side voltage is greater than the collector point side or inverter side output voltage, the associated photovoltaic string is automatically electrically reconnected to the parallel other photovoltaic strings and to the DC input of the inverter.

Depending on the complexity of the security architecture, a longer period, e.g. one night and / or further testing processes can be carried out and / or the individual photovoltaic modules can be activated from a protection state.

The invention also provides the backflow protection circuit for a photovoltaic string of a multi-strand photovoltaic system for protection against backflow into the associated photovoltaic string from the other parallel photovoltaic strings, as disclosed herein. For this purpose, the reverse current protection circuit comprises in particular: a switching device which, in the closed state, electrically connects the associated photovoltaic string in parallel with the other photovoltaic strings and in the open state the connection of the associated photovoltaic string to the parallel connection with the other photovoltaic strings and to the Inverter interrupts, a control circuit at least for controlling the switching device and one or more sensors connected to the control circuit for measuring an electrical characteristic at the associated photovoltaic strand.

In response to a first electrical characteristic measured with one of the sensors, the reverse current protection circuit interrupts the connection of the associated photovoltaic string to the parallel connection with the other photovoltaic strings and to the DC input of the inverter in which the control circuit opens the switching means when the measured one electrical characteristic is outside a decisive for the return current safety condition for the first electrical characteristic, eg a negative current in the reverse current protection circuit.

In response to the first and / or another second electrical characteristic measured with the first and another second sensors, the reverse current protection circuit automatically switches the associated photovoltaic string whose connection to the central collection point is interrupted back to the other parallel photovoltaic strings and to the DC input of the inverter electrically, when the first and second electrical characteristic are within a relevant for the return current safety condition, eg when the input voltage is greater than the output voltage.

It will be apparent to those skilled in the art that although the invention can be used particularly advantageously for photovoltaic systems, it can also be used in principle for other DC generators, in particular if they have a parallel connection to a common inverter. These should not be excluded.

In the following the invention will be explained in more detail on the basis of exemplary embodiments and with reference to the figures, wherein identical and similar elements are in part provided with the same reference symbols and the features of the various embodiments

Embodiments can be combined with each other.

Brief description of the figures

Show it:

1 is a block diagram of a photovoltaic system with parallel photovoltaic strings,

2 is a block diagram of a protection circuit for a photovoltaic module,

3 is a block diagram of a start box with reverse current protection circuit,

Fig. 4 is a more detailed block diagram of the switching device S1, and

5 is a flowchart for starting and monitoring the associated photovoltaic

Train.

Detailed description of the invention

Referring to Fig. 1, the main photovoltaic array 1 comprises a plurality of photovoltaic strings connected in parallel, of which only two photovoltaic strings 10, 10 'are shown for the sake of simplicity.

Each photovoltaic string comprises a plurality of photovoltaic modules or photovoltaic panels 12, 12 ', each equipped with a protection circuit 14, 14', e.g. as described in WO 2013/026539 A1. The protective circuits 14, 14 'are each assigned to a photovoltaic module 12, 12', and the respective branch line 16, 16 'leads through the photovoltaic string 10, 10' to the protective circuit 14, 14 'through two poles.

Referring to FIG. 2, each protection circuit 14, 14 'includes a short-circuiting switch S3 for strand-side shorting of the associated photovoltaic module 12, 12' and a serial disconnecting switch S4 with which the associated photovoltaic module 12, 12 'of the photovoltaic string 10, 10 'can be separated. In case of shading or failure of a photovoltaic module 12,12 'closes the short-circuit switch S3 and the serial disconnect switch S4, so that the respective photovoltaic module 12,12' is separated from the strand, leerlâuft and the photovoltaic string 10,10 'still closed remains, so that further the photovoltaic generated electricity of the other photovoltaic modules of this photovoltaic string can flow through the closed even in this protective condition line 16,16 '. For further details of the exemplary protection circuit 14, 14 ', reference is made to WO 2013/026539, which in this regard is hereby made the subject of the present disclosure by reference.

Referring again to Figure 1, in each strand 10, 10 'is inserted a starter box 20, 20' through which both strand lines 16, 16 'pass. Inverter side of the

Starter boxes 20, 20 'are continuations 17,17' of the strand lines 16,16 'at the central collection point 22a, b or its plus and minus connected in parallel to the photovoltaic power of the multiple photovoltaic strings connected in parallel in the DC input 24a, b of the common inverter 26 feed. At the AC output 28 of the inverter 26, the AC power is provided for feeding into a utility grid.

The start boxes 20, 20 'are powered in this example by an external 24 volt power supply 32 to perform the desired switching operations. However, the supply can also be done by a start photovoltaic module, which has no protection circuit 14,14 'and thus automatically supplied with light incidence the respective strand 10,10' and thus the zugehôrige starter box 20,20 'with electrical power. For further details, reference is made to WO 2014/122325 A1, which is hereby incorporated by reference into the subject of the present disclosure.

Referring now to FIG. 3, a strand 10 of the plurality of parallel strands is shown. Aile other parallel connected photovoltaic strings are the same structure, so that in this respect can be dispensed with a corresponding repetition.

The photovoltaic modules 12 with connected protection circuit 14, which is not shown separately in FIG. 3 for the sake of simplicity, form the photovoltaic string 10 in a serial circuit. The positive pole 16a and the negative pole 16b of the strand line 16 are connected to a positive terminal 34a, respectively ., a Minuspoleingang 34 b of the start box 20 connected to be fed into the start box 20. Corresponding continuations 17 of the stranded line 16 are connected to a positive pole output 36a or a negative pole output 36b to the positive pole 24a or to the negative pole 24b of the DC input 24 of the inverter 26. The photovoltaic power generated by the photovoltaic modules 12 of the strand 10 accordingly flows through the starting box 20 during production operation. At the inputs and outputs 34a, b; 36a, b are the strand lines 16,17 preferably with connectors (not shown) connected. The parallel-connected second photovoltaic string 10 'or further parallel photovoltaic strings are symbolized by the dashed lines which lead to the two poles of the central collecting point 22a, b.

The start box 20 now contains a reverse current protection circuit 40, which is integrated in the housing 21 of the start box. The reverse current protection circuit 40 comprises a switching device S1, which is connected in series in the photovoltaic string 10, in this example in a branch of the strand line 16. With the serial switching device S1, the connection of the associated photovoltaic string 10 to the inverter 26 can be interrupted electronically controlled. A controller 42 in the form of a microcontroller controls the serial disconnect switching device S1 and monitors a current sensor 44, an input voltage sensor 46 and an output voltage sensor 48 to obtain electrical characteristics, the switching condition for the switching device S1 in response to these electrical characteristics from the controller 42 is taken automatically.

The input voltage sensor 46 is connected in parallel with the input terminals 34a, 34b of the start box 20 to measure the input voltage U1, which is the phase voltage of the associated photovoltaic string 10. The output voltage sensor 48 is connected in parallel with the output terminals 36a, 36b of the start box 20 to measure the output voltage U2, which is the voltage applied to the inverter 26, i. which is the voltage applied to the inverter 26 from the parallel connection of all other photovoltaic strings. The current sensor 44 measures, in the production operation, the phase current flowing through the strand line 16, the start box 20, the continuation of the strand line 17, and the inverter 26 when the photovoltaic string 10 is produced in the normal direction, which is referred to herein as positive to be fed correctly pole into the inverter. However, the current sensor 44 is also preferably designed to allow current to flow in the reverse direction, i. to measure a negative current relative to the normal current direction of the production of the photovoltaic string 10 (ie reverse pole to the direct current inputs 24a, 24b of the inverter). Alternatively, however, two separate current sensors may be provided, one for the positive current flow and one for the negative current flow (not shown). The reverse current protection circuit 40 is therefore designed to measure positive and / or negative current flow.

The start box 20 further includes a shorting switch S2, which is also controlled by the microcontroller 42 and can inject a starting current into the associated photovoltaic string 10 to protect the protection circuits 14, e.g. to start after sunrise. At night, during maintenance or in the event of a fault, the protective circuits 14 are in the safety state, whereby each photovoltaic module 12 is separated individually from the associated strand 10 and the stranded line 16 is short-circuited. For this purpose, the switch S3 is closed and the switch S4 is opened. In the associated basic position of the start box 20 and the reverse current protection circuit 40, the serial switching device S1 and the short-circuit switch S2 are both open and are in their normal state (Normal Open). A major function of the serial switching device S1 is to interrupt the live connection of the associated photovoltaic string 10 to the inverter 26 and to the other parallel photovoltaic strings. The short-circuit switch S2 is used to trigger string-side short circuits in the associated photovoltaic string 10 and the photovoltaic modules 12 of the associated photovoltaic string 10 via an impressed starting current, which in this example is generated by the external power supply 32 and the internal starting current transformer 33 , to activate. After transmitting the start pulse, the line voltage U1 and the inverter voltage U2 are measured and compared to electrically reconnect the associated photovoltaic line to the parallel circuit and the inverter 26 in response to the voltage comparison if the corresponding switching condition is met. The serial switching device S1 is closed in response to a comparison of the strand voltage U1 and the inverter voltage U2, i. the switching condition for the connection of the serial switching device S1 depends on a comparison of the phase voltage U1 and the inverter voltage U2. The serial switching device S1 is closed in particular only when the strand voltage U1 of the associated photovoltaic string 10 is either greater than the inverter voltage U2 or only smaller by a predetermined threshold U0 than the inverter voltage U2. In other words, a switching condition for the serial switching device S1 for electrically connecting the photovoltaic string 10 to the parallel circuit and to the inverter 26 is U1> = U2-U0, where U0 is a predefined value for the maximum allowable voltage difference for securely turning on the associated photovoltaic String is 10 to the inverter. The predefined and stored in the microcontroller switching value U0 is therefore (significantly) smaller than the maximum possible voltage of the associated photovoltaic string 10. Only if such a switching condition, which electrical characteristics of the associated photovoltaic string 10 and the other Photovoitaiksträngen depends is met , the microcontroller 42 controls the serial switching device S1 so that the switching device S1 is closed, thereby turning on the associated photovoltaic string 10 to the inverter 26. Thus, system start-up of the associated photovoltaic string 10 is possible without the risk of the occurrence of undesirable back currents.

In order to detect unwanted return currents from the other photovoltaic strings into the associated photovoltaic string 10 in the production operation, in which the serial switching device S1 is closed, the current flow through the backflow protection circuit is monitored during the production operation. For this purpose, a current measurement with the current sensor 44 is performed permanently. By means of the current sensor 44, therefore, a permanent monitoring of the phase current is carried out, including the sign of the phase current, ie whether the phase current is negative. Palis should the strand current become negative, or a condition approaching that is reached, the microcontroller 42 controls the serial switching device S1 to open and the current carrying connection of the photovoltaic string 10 to the central collection point 22a, b and to the inverter 26 interrupts.

Thus, all start boxes cleanly disconnect the respective photovoltaic string from the inverter 26, e.g. When the entire photovoltaic system 1 is shut down, the serial switching devices S1 of all reverse current protection circuits 40 of all photovoltaic circuits are also opened as soon as the inverter voltage U2 is below a predefined minimum voltage U_min, Ujnin being e.g. can be in the range of about 30 Voit.

Referring to FIG. 4, the exemplary switching device S1 includes a parallel circuit of a semiconductor switch, in this example a MOSFET 52, and a relay 54. Both are designed for the full nominal strand voltage and full nominal strand current of the associated photovoltaic string 10 , which typically up to 1000 volts or even 1500 volts and, for example 10A can be. However, MOSFETs 52 of this type have a typical on-resistance, the so-called resistance-drain-source-on or RDS-on, of several 100 mOhms. This results in a typical photovoltaic string 10 on the MOS-FET 52, a power loss in the range of a few watts. Since such a power dissipation is primarily for installation in existing housings, e.g. in which the present start box 20, undesirable, relieves the relay 54, the MOS-FET 52 in continuous operation. That the switching operations are initially performed by the MOSFET 52, and when the associated photovoltaic string 10 is in production for a certain minimum time, the parallel relay 54 is closed to relieve the MOS FET 52. As a result, on the one hand the permanent generation of high power loss is avoided on the MOS-FET 52 and on the other hand, the relay 54 is spared, since the number of Schaltvorgänge can be kept low on the relay 54.

Referring to FIG. 5, the start and monitor sequence of the reverse current protection circuit 40 proceeds as follows, for example.

At system startup 102, the switching device S1 is open at 104. In a query in step 106, it is checked whether the input voltage U1 falls below a minimum voltage U_min. If the condition U1 <U_min is satisfied, the start pulses are triggered in the step 108 with the short-circuit switch S2 in order to start the associated photovoltaic string 10. The serial switching device S1 is still open and the loop goes back to step 104.

Now, if the associated photovoltaic string 10 is started, in which the associated protection circuits 14 have electrically connected the associated photovoltaic modules 12 to the strand 16, the strand voltage U1 should be well above the predefined minimum value U_min, provided no fault exists and the associated one Photovoltaic strand 10 is not totally shaded. Thus, after the condition U1 <U_min is no longer met in normal Einstrahiung and without interference in step 106, there is a further query 110 as to whether the input voltage U1 of the associated photovoltaic string 10 is greater than or equal to the output voltage U2 of the associated photovoltaic string 10th minus a predefined threshold value U0, where U0 can also be zero (U1> = U2-U0). This condition should preferably be fulfilled for a predefined minimum time to. Furthermore, it is again checked whether the voltage U1 is greater than or equal to the minimum value U_min to ensure that the associated photovoltaic string 10 is still electrically connected. If these conditions are not met in query step 110, the loop goes back to step 104 and to query 106. However, if all conditions of query 110 are met, microcontroller 1er 42 controls serial switch S1 in step 112 to close it , As a result, the associated photovoltaic string 10 is connected to the inverter 26 and the associated photovoltaic string 10 thus goes into production. During production operation, in a test step 114, it is then permanently or regularly checked whether a negative current is greater than or equal to a predefined minimum value I0 over a predefined minimum time t1 and / or if the input voltage U1 falls below a predefined minimum value U_min. The minimum value I0 for the negative current is typically in the milli-ampere range. As long as these test results are negative, the switching device S1 is kept closed. Accordingly, the two steps 112, 114 form a permanent or continuous monitoring circuit for the associated photovoltaic string 10, so that even in the production mode no unwanted return currents from the other photovoltaic strands 10 'occur in the associated photovoltaic string 10, in particular thus no undesirably high negative Total current can flow.

If the query in step 114 should result in an undesirably high negative return current, e.g. the associated photovoltaic string 10 is significantly more shaded than the other photovoltaic strings and / or because the associated photovoltaic string 10 has even lost strand voltage to such an extent that the threshold condition U1 <U_min is satisfied, the switching means S1, i. the loop goes back to step 104. Preferably, the switching device S1 is designed as a normally open so that it automatically closes in the open state if it is not held in the closed state by the control device 42.

It is also not mandatory that the interruption of the connection takes place only when the total current actually becomes negative, i. E. that the cross-flow from the other photovoltaic strings is greater than the current which can be called up from the associated photovoltaic string 10 in the case of a single connection. It is also possible that the connection of the associated photovoltaic string 10 is already interrupted when the total current I measured with the sensor 44 is still positive but below a predefined threshold for the economic operation of the associated photovoltaic string 10.

In summary, with the measurement technology and the switches, it is possible to simulate the functionality of a stranded diode via a software and / or hardware interface, but without generating their additional power loss. For this purpose, the switching device is designed to 1000 V or higher in an existing start box of the MOS-FET S1 and added a parallel relay 54, and adapted measurement technology and the device firmware. By inserting the relay 54, the conduction losses can be minimized. All switching operations that are important for the function of the start box 20 are initially taken over by the semiconductor switch 52 and the relay 54 finally relieves the semiconductor switch 52. In measurement technology, the current measurement for negative currents is upgraded in order to detect reverse currents during operation.

Both voltages applied to the start box, i. the input voltage / phase voltage of the associated strand U1 and the output voltage / voltage at the inverter U2, measured. The starter box 20 starts the associated photovoltaic string 10 and compares the voltage U1 with the voltage on the inverter side U2. By measuring both voltages U1 and U2, voltage differences are detected on the basis of which transverse or equalizing currents can flow. If the voltage U2 on the collection point side or inverter side is greater than the voltage U1 on the side of the strand, a transient current could flow from the other parallel photovoltaic strings into the associated strand. In this case, the connection of the associated photovoltaic string 10 to the inverter 26 is not closed to prevent the flow of negative cross or Ausgleichsstrôme and an undesirable state of the photovoltaic system. If the voltage U2.inverter is greater than the voltage U1 of the associated photovoltaic string 10, after a predefined time, e.g. After 2 minutes, the associated photovoltaic string 10 is restarted. The inverter / inverter 26, which is interconnected with multi-string, will already start operation with a connected photovoltaic string and reduce the voltage of the producing photovoltaic string by about 20% for power optimization (MPPT). The newly started photovoltaic string which could not start at a first start attempt now has this 20% as a range around e.g. compensate for one or two defective or shaded photovoltaic modules 12. The associated photovoltaic string 10 is restarted and now switches to production mode due to the acceptable voltage difference. This will be done gradually on all photovoltaic strings connected in parallel until all are productive without cross or equalizing currents flowing. In operation, if transverse or equalizing currents were to flow through shadow strike, module faults or the like, this can be detected by the permanent current measurement, which involves measuring a negative current. In response to this, the connection of the associated strand to the multi-strand interconnection can then be interrupted so that a permanent reverse current monitoring of all photovoltaic strands is also provided. The system disclosed herein can thus permanently monitor a photovoltaic multi-circuit interconnection and actively prevent crossover or equalizing currents.

It will be apparent to those skilled in the art that the above-described embodiments are to be understood as illustrative and that the invention is not limited to them, but rather may be varied in many ways without departing from the scope of the claims. It can also be seen that the characteristics, irrespective of whether they exist in the

Even if they are described together with other features, they will also individually define essential components of the invention.

Claims (15)

claims: A multi-strand photovoltaic system (1) comprising: two or more photovoltaic strings (10, 10 ') formed respectively by serially connected photovoltaic modules (12, 12'), a central collection point (22a, b ) at which the two or more photovoltaic strings (10, 10 ') are connected in parallel, wherein at least one of the photovoltaic strings (10, 10j) comprises a reverse current protection circuit (40). 2. multi-strand photovoltaic system (1) according to claim 1, wherein an inverter (26) for converting the photovoltaic modules (12,12 ') generated DC voltage is included in AC voltage and the inverter (26) has a DC input (24) to which the photovoltaic strings (10, 10 ') are connected in parallel. 3. multi-strand photovoltaic system (1) according to any one of the preceding claims, wherein the reverse current protection circuit (40) comprises a control circuit (42) and connected to the control circuit (42) sensor (44,46,48) for measuring an electrical characteristic ( U1, U2,1) on the associated photovoltaic strand (10, 10j), wherein the control circuit (42) is adapted to connect the associated photovoltaic in response to the electrical characteristic measured with the sensor (44, 46, 48) -Strang (10,10 ') to the central collection point (22a, b) automatically interrupt. 4. Muitistrang photovoltaic system (1) according to any one of the preceding claims, wherein the sensor measures at least one of the following electrical characteristics: - input voltage (U1) at the string-side input (34a, b) of the reverse current protection circuit (40), - output voltage (U2 ) at the collection point side output (36a, b) of the reverse current protection circuit (40), - strand current (I) of the associated photovoltaic string (10, 10 '), - negative current flow through the reverse current protection circuit (40), and wherein the control circuit (42) thereto is formed, in response to one or more of these with the sensor (44,46, 48) measured electrical parameters (U1, U2,1) the connection of the associated photovoltaic string (10,10j to the central collection point (22a, b) automatically interrupt. A multi-strand photovoltaic device (1) according to claim 3 or 4, wherein the sensor (44) is a current sensor adapted to measure a negative current flow, the control circuit (42) being particularly adapted to connect of the associated photovoltaic string (10, 10j to the central collection point (22a, b)), if the current in the associated photovoltaic string (10, 10j, either i) is positive, but below a predefined safety threshold ( IO) or ii) is even negative. 6. M u Itistrang photo of Itaik-An position (1) according to any one of the preceding claims, wherein the reverse current protection circuit (40) comprises a control circuit (42) and connected to the control circuit (42) sensor means (46,48) for measuring at least an electrical parameter (U1, U2) on the associated photovoltaic string (10, 10j), wherein the control circuit (42) is designed to generate at least one electrical parameter (U1, U1, U1, U2) in response to the sensor device (46, 48). U2) the associated photovoltaic string (10,10 j automatically to the central collection point (22a, b) to turn on electrically. 7. A microphotograph of an audio equipment (1) according to claim 6, wherein the sensor device comprises: an input voltage sensor (46) which measures the input voltage (U1) at the reverse current protection circuit (40) and / or an output voltage sensor ( 48) which measures the collector point side output voltage (U2) at the reverse current protection circuit (40), and wherein the controller (42) is arranged to respond in response to the measured line side input voltage (U1) and / or the measured collector point side output voltage (U2). automatically connect the associated photovoltaic string (10, 10 ') to the central collection point (22a, b). 8. The multi-strand photovoltaic system (1) according to claim 6 or 7, wherein the sensor device comprises: an input voltage sensor (46) which measures the input voltage (U1) at the reverse current protection circuit (40) and an output voltage sensor (48) which measures the input voltage collector point side output voltage (U2) across the reverse current protection circuit (40), and wherein the controller (42) is configured to responsive to a comparison of the measured line side input voltage (U1) and the measured collector side output voltage (U2) the associated photovoltaic line (10, 10j automatically to the central collection point (22a, b) to turn on. 9. multi-strand photovoltaic system (1) according to one of the preceding claims, wherein the reverse current protection circuit (40) comprises a switching device (S1) which in the closed state, the associated photovoltaic string (10,10 j via the central collection point (22a, b) connects electrically in parallel with the other photovoltaic strands and in the open state, the connection of the associated photovoltaic string (10,10 j to the central collection point (22a, b) interrupts. 10. MuItistrang photovoltaic system (1) according to claim 9, wherein the switching device (S1) is designed for a separation voltage of at least 500 V and / or for a through-current of at least 5 A. 11. M u Itistrang Photo voltai k-An I age (1) according to claim 9 or 10, wherein the switching device (S1) includes a parallel circuit of a semiconductor switch, in particular a MOS-FET (52), and a Reiais (54) and the control device (42) is designed to first close the semiconductor switch (52) when the associated photovoltaic line (10, 10 ') is electrically connected to the central collecting point (22a, b), and only after a time delay has the relay ( 54) to relieve the semiconductor switch (52) in the production operation of the photovoltaic string (10, 10j from current flow. 12. multi-strand photovoltaic system (1) according to claim 11, wherein the semiconductor switch (52) and / or the relay (54) are designed as normally open. 13. multi-strand photovoltaic system (1) according to any one of the preceding claims, wherein the series-connected photovoltaic modules (12,12 ') in at least one of the photovoltaic strands each protection circuits (14,14'), by means of which the photovoltaic Modules (12, 12 ') can be individually switched off from the associated photovoltaic string (10, 10') and / or a connection box of the respective photovoltaic module (12, 12 ') can be short-circuited on the string side and wherein the associated photovoltaic string ( 10, 10 ') has a starting circuit (32, 33, S2) which is designed to activate the protective circuits of the photovoltaic modules of the associated photovoltaic string (10, 10'), the starting circuit and the reverse current protection circuit (40) are housed in a common housing (21) of a start-and-Rückstromschutzbox (20) and the start-and-Rückstromschutzbox (20) between the series of photovoltaic modules (12,12 ') of the associated photovoltaic Strand (10,10j and the central collection point (22a, b) with the other photovoltaic strings in the associated photovoltaic string (10,10 ') is connected. 14. A method for disconnecting and reconnecting a photovoltaic string (10,10 ') of a multi-strand photovoltaic system (1), in particular according to one of the preceding claims, of other parallel to a central collection point (22a, b) connected photovoltaic strings the multi-strand photovoltaic system (1) for protection against reverse currents, comprising the following steps: during the entire operation of the multi-strand photovoltaic system (1) permanent measurement of an electrical characteristic (U1, U2,1) in the photovoltaic strings ( 10,10 ') by means of in each case one of the respective photovoltaic string (10,10') Zugehôrigen reverse current protection circuit (40) for protection against flowing from the other photovoltaic strands in each of a photovoltaic string return current, automatic interruption of the connection to the central collection point (22a, b) of those photovoltaic strings (10,10j, in which the measured electrical characteristic (U1, U2,1) outside a f for the reverse current relevant safety condition for the electrical characteristic of the associated photovoltaic string, automatic electrical Wiederanschaiten those photovoltaic strands (10,10j, in which one or another measured electrical characteristic is within a prevailing safety condition for the return current, to the central Collection point (22a, b). 15. reverse current protection circuit (40) for a photovoltaic string of a multi-strand photovoltaic system (1) for protection against in the associated photovoltaic string (10, 10 ') from other parallel photovoltaic strings flowing return current, in particular according to one of the preceding claims comprising: a switching device (S1) which in the closed state electrically connects the associated photovoltaic string (10, 10j) in parallel with the other photovoltaic strands via a central collecting point (22a, b) and in the open state connects the associated photovoltaic line Strand (10, 10j to the central collection point (22a, b) interrupts a control circuit (42) at least for controlling the switching device (S1), one or more sensors (44, 46, 48) connected to the control circuit (42) for measuring each of an electrical characteristic (U1, U2,1) to the associated photovoltaic string (10,10j, wherein the control circuit (42) is adapted to in response on a first with one of the sensors (44, 46,48) measured electrical characteristic (U1, U2,1), the connection of the associated photovoltaic string (10,10 j to the central collection point (22a, b) to interrupt by the Control circuit (42) opens the switching device (S1) when the measured first electrical characteristic is outside a safety condition for the first electrical characteristic relevant to the return current, and wherein the control circuit (42) is adapted to respond in response to the first and / or another second with the first or another second sensor (44, 46, 48) measured electrical characteristic automatically connect the associated photovoltaic string (10,10 ') again to the central collection point, if the first or the second electrical characteristic lie within a relevant for the return flow safety condition.