Patent Publication Number: US-2022231513-A1

Title: Method for operating an energy generating system, and energy generating system comprising said method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Patent Application number PCT/EP2020/075742, filed on Sep. 15, 2020, which claims priority to German Patent Application number 10 2019 127 198.3, filed on Oct. 9, 2019, and is herby incorporated by reference in its entirety. 
    
    
     FIELD 
     The disclosure relates to a method for operating an energy generating system (EEA). The disclosure relates in particular to the operation of an EEA, in which a case of a fault in the EEA is detected and its damaging effect on components of the EEA is avoided or at least reduced. 
     BACKGROUND 
     In the so-called power-to-gas method, electrical energy is converted by an electrolyzer to a gaseous energy carrier, in particular to hydrogen. The electrical energy can advantageously be renewable electrical energy. In such methods, the electrolyzer and the renewable EEA supplying the electrolyzer can be connected to one another in the form of an island network, which is not connected at all or only temporarily connected to a public power grid (EVN) operating with an AC voltage. 
     Many of the renewable energy sources are often inherently designed as DC sources. Since the electrolyzer operates as a DC consumer, it is advantageous with regard to avoiding conversion losses if the island network is also designed as a DC network. For example, the EEA within the island network can have a plurality of DC sub-generators designed as photovoltaic (PV) sub-generators, which are connected in each case via a DC/DC converter and in parallel with one another to an input of the electrolyzer or another DC load. A voltage of the DC sub-generators can be decoupled via the DC/DC converters from a voltage at the input of the electrolyzer. Each of the DC sub-generators can thus be operated at its respective maximum power point (MPP) and can nevertheless supply the electrolyzer with an output voltage that is the same for all DC/DC converters. An operating point of the electrolyzer, in particular its power consumption, can be set via the output voltage of the DC/DC converters. Specifically, a higher-level control unit can control the output voltage of the DC/DC converters in such a way that, on the one hand, there is always an equilibrium between the total generated and consumed power in the island network and, on the other hand, the maximum possible power of renewable power of the PV sub-generators can always be harvested. 
     Now, if one or possibly even several of the PV sub-generators operating as DC sub-generators have a fault, for example a short-circuit fault or a double insulation fault, the DC sub-generators operating without faults feed their power into the one faulty DC sub-generator (or, where applicable, into the plurality of faulty DC sub-generators). Usually, a DC sub-generator comprises one or more DC sources. The DC sources of a DC sub-generator, depending on the size of the EEA, are connected via one or more fuses to the DC/DC converter assigned to the DC sub-generator. However, in the case of a questionable fault within the EEA, it may happen that a resulting current from the fault-free DC sources of the EEA into the faulty DC source is not sufficiently high to trip at least one of the fuses via which the faulty DC source is connected to its assigned DC/DC converter. Thus, there is the risk that, in the case of a fault, the generated fault current is not sufficient without further measures to separate the faulty DC source from the remaining DC sources, which operate in a fault-free manner, by tripping a fuse downstream of the faulty DC source. The resulting current representing a fault current is not reliably interrupted and, depending on the current strength and duration of the fault current, can damage or even destroy further components of the EEA, in particular the DC/DC converter concerned. 
     Document DE 10 2015 007 443 A1 discloses a method and a device for supplementarily feeding current from at least one power source into a final power network having several consumption points. The final power network is connected to a higher-level power supply, wherein the consumption points are protected against the higher-level power supply via at least one overcurrent fuse. A control unit detects the current flowing from the higher-level power supply and the current flowing from the power source into the final power network. The instantaneous total value formed from the currents is compared with a predetermined maximum current in order to reduce the current coming from the current source when the maximum current is exceeded. The method can prevent an overload situation in the final power network. 
     Publication DE 10 2013 111 869 A1 discloses a PV system with an inverter, which is connected to a power grid via an AC disconnecting means, and at least one PV sub-generator, each of which has at least one PV string and is connected via DC lines to a DC connection area of the inverter. A DC separator close to the generator, a DC short-circuit switch downstream of the energy flow direction during feeding for short-circuiting the at least one PV string, and a reverse current protection downstream of the DC short-circuit switch in the energy flow direction are assigned to the PV sub-generator. Furthermore, an AC short-circuit switch is arranged in the energy flow direction upstream of the AC disconnecting means. 
     A photovoltaic (PV) system with an arc detection device is known from publication US 2018/210022. In the PV system, a PV string is connected to a power converter via first power lines, a DC-DC converter, and further power lines. The arc detection device comprises capacitors, which form bypass current paths for the DC-DC converter, and a current sensor arranged on one of the further power lines between the DC/DC converter and the power converter. 
     Publication DE 11 2012 007 202 T5 describes a method for operating a controller for maximum power point tracking (MPPT controller), which is designed to transmit power between an input terminal and an output terminal, comprising the following steps:
         In a first operating mode of the MPPT controller: operating a first switching device of the MPPT controller at a fixed duty cycle; and   In a second operating mode of the MPPT controller: causing a regulating switching device of the MPPT controller to repeatedly switch between its conductive and non-conductive states in order to maximize a power taken from a PV device electrically connected to the input terminal.       

     Publication US 2016 181781 A1 discloses a PV string having a plurality of PV modules which are electrically connected in series and have a first end and a second end. A power line with a first string protection unit is electrically connected to the first end, and a return line having a second string protection unit is connected to the second end. One of the string protection units comprises a plurality of protective devices selected from overcurrent protection, arc fault protection, reverse current protection, and ground fault protection. The other of the string protection units comprises a plurality of protective devices selected from overcurrent protection, arc fault protection, reverse current protection, ground fault protection, and a remote-controlled switch in series with the power line or the return line. 
     A PV system with a plurality of PV modules and their assigned DC/DC converters, which are connected in parallel with one another to a central inverter on the output side, is known from publication DE 101 36 147 A1. The inverter converts a DC link voltage generated by the DC/DC converters into a sinusoidal AC voltage. The PV modules are electrically decoupled by their individual DC/DC converters. 
     SUMMARY 
     The disclosure is directed to a method for operating an EEA with a plurality of DC sub-generators connected in parallel with one another, with which damage to the EEA is reliably avoided in the case of a fault. In one embodiment, the method operates to separate a faulty DC source of a DC sub-generator from the remaining DC sources, which operate in a fault-free manner, by tripping a fuse, if possible. However, the resulting fault current is to be controlled such that damage to components of the EEA, which are sensitive to overcurrent, is ruled out in any case, even if tripping the fuse is not possible. Also disclosed is an EEA suitable for performing the method. 
     The method according to the disclosure is aimed at the operation of an energy generating system (EEA) with a plurality of DC sub-generators, which are connected in parallel with one another and in each case via a DC/DC converter to a shared DC load. Each of the DC sub-generators has at least one DC source, which is connected, via at least one fuse that is connected in series to the DC source, to the DC/DC converter that is assigned to the respective DC sub-generator. The method comprises: monitoring each of the DC sub-generators for a fault, in particular a short-circuit fault, and if the monitoring of the DC sub-generators indicates a faulty DC sub-generator, the DC/DC converters that are not assigned to the faulty DC sub-generator but rather to a fault-free DC sub-generator are operated at a common total current I Rest  which corresponds to a default value. 
     Each of the DC sub-generators may have a DC source or a plurality of DC sources connected in parallel with one another to the corresponding DC/DC converter. The fuse can be arranged in each case, in relation to a power flow direction during normal operation of the EEA, between the DC source and the DC/DC converter. The fuse can be merely one fuse, or may be a plurality of fuses, which are arranged in a series connection between the DC source and the DC/DC converter. In one embodiment, one pole of the DC source is connected to the corresponding DC/DC converter via the one fuse or the plurality of fuses connected in series. However, it is also within the scope of the disclosure that each of the two poles of the DC source is connected to the assigned DC/DC converter via a fuse or a plurality of fuses connected in series. In one embodiment, each of the DC/DC converters can accordingly be connected to one DC source on the input side. Alternatively, however, it is also possible for one, several, or even each of the DC/DC converters to be connected on the input side to a plurality of DC sources connected in parallel with one another. In one embodiment, when at least one of the DC/DC converters is connected to a plurality of DC sources on the input side, a power flow of the DC sources is combined in a cascading manner within the corresponding DC sub-generator in a plurality of stages. Each individual stage can have a separate fuse in one embodiment. For example, a so-called main string fuse can secure a generator box which in each case contains a plurality of so-called string fuses. In this way, a plurality of fuses of different types can be connected in series between a DC source and the DC/DC converter. In one embodiment, a tripping threshold of the fuses can increase with increasing distance from the DC source and with decreasing distance to the DC/DC converter. 
     Monitoring each of the DC sub-generators for faults may comprise, in one embodiment, monitoring an electrical parameter of the DC sub-generators. In one embodiment, a current and/or a voltage is detected in a temporally successive manner. In one embodiment, it is not necessary for each of the DC sources within a DC sub-generator to be monitored separately. In particular when a plurality of DC sources connected in parallel with one another is connected as one DC sub-generator to a shared DC/DC converter, it is sufficient to combine the DC sources in groups or, in other words, to monitor the corresponding DC sub-generator. During monitoring, it can be detected, for example, whether a return current flows into a faulty DC sub-generator which is affected by a short-circuit fault. Although the monitoring does not directly yield the faulty DC source, it still yields the DC sub-generator which contains the faulty DC source. 
     The disclosure makes use of the effect that, in the event of a fault, in particular in the event of a short-circuit fault of one of the DC sources, the voltage of the faulty DC source, as well as the voltage of the DC sub-generator that contains the faulty DC source, drops and causes a return current from non-faulty DC sources. The dropped voltage is applied, on the input side, to the DC-DC converter assigned to the faulty DC source, which DC-DC converter is therefore no longer able to provide a voltage on the output side that corresponds to the output-side voltage of the remaining DC/DC converters, which are not connected on their respective input sides to a faulty DC source or a faulty DC sub-generator. This results in a total current I Rest  of those DC/DC converters that are not assigned to the faulty DC sub-generator, i.e., are not connected on the input side to the faulty DC sub-generator, into the faulty DC sub-generator and there, in particular, into the faulty DC source of the faulty DC sub-generator. The total current I Rest  thus flows backward, i.e., counter to the current direction during normal operation of the EEA, via freewheeling diodes of the DC/DC converter that is assigned to the faulty DC sub-generator. 
     A fault of a DC source can also affect the EEA such that a voltage drop does not only take place at an output of the DC/DC converter that is assigned to the faulty DC source. Instead, a voltage drop can also be present more or less strongly at the outputs of all other DC/DC converters. In this case, the voltage drops to a small voltage value in an entire DC bus of the EEA. In this case, however, the voltage drop is usually more pronounced at the DC/DC converter that is assigned to the faulty DC source than the voltage drops at the remaining DC/DC converters that are not assigned to the faulty DC source, so that the already described total current I Rest  from the fault-free DC sub-generators into the faulty DC sub-generator also results here. 
     According to one embodiment of the disclosure, only the DC/DC converters that are not assigned to a faulty DC sub-generator are operated with the aim that their common total current I Rest  corresponds to a default value. This can take place by controlling, in a coordinated manner, the DC/DC converters that are not assigned to the faulty DC sub-generator. The coordinated control can take place using a control unit acting on all the DC/DC converters. In case of a coordinated control, the default value to be adjusted must be taken into account by the control unit. The control unit may be furthermore provided with the individual currents of all DC/DC converters participating in the coordinated control. However, this is not absolutely necessary. Rather, it is sufficient if the control unit, in addition to the default value to be adjusted, has available the common total current I Rest  of the DC/DC converters participating in the coordinated control. The common total current I Rest  may optionally also include a portion which flows from the DC load in the direction of the DC/DC converter that is assigned to the faulty DC sub-generator. Since fewer values need to be measured and/or communicated during the coordinated control, the coordinated control can be simplified overall and the corresponding control unit can be less sophisticated. A fault current corresponding to the total current I Rest  thus does not flow in an uncontrolled manner but is controlled in particular in terms of its current strength in such a way that the total current I Rest  corresponds to the default value. The default value can be selected such that damage to an overcurrent-sensitive component of the EEA, in particular of the DC/DC converter that is assigned to the faulty DC sub-generator or, in other words, that is assigned to the faulty DC source is ruled out. The overcurrent-sensitive component can in particular be a freewheeling diode of the corresponding DC/DC converter. On the one hand, the total current I Rest  can be limited upward (i.e., not to exceed a predetermined amount) by means of the default value. On the other hand, however, by controlling the relevant DC/DC converters with the aim that the common total current I Rest  corresponds to the default value, the total current I Rest  is at the same time also limited downward (i.e., not to fall below a predetermined amount). Specifically, the default value can be selected such that the fuse connected in series to the DC/DC converter and the faulty DC source or, in the case of a plurality of fuses, one or more of the fuses connected in series to the DC/DC converter and the faulty DC source trip reliably. In the case of a DC sub-generator having a plurality of DC sources connected in parallel with one another, the fuse closest to the faulty DC source advantageously trips first for circuit reasons, since it usually has, on the one hand, relative to the other fuses within the DC sub-generator, a relatively low tripping threshold and is, on the other hand, passed through not only by the total current I Rest  generated outside the faulty DC sub-generator but additionally also by a fault current generated within the faulty DC sub-generator. However, the fault current generated within the faulty DC sub-generator is generally significantly lower than the total current I Rest  generated outside the faulty DC sub-generator. Both currents add up and thus support the desired tripping of the fuse in the case of a fault. 
     The current strength of the total current I Rest  can be regulated or set to the default value by suitable operation of the DC/DC converters that contribute to the total current I Rest . For this purpose, in one embodiment, a current flowing via the DC/DC converter can be detected for all DC/DC converters and transmitted to the higher-level control unit. The higher-level control unit can add up the detected currents of the DC/DC converters that are not assigned to a faulty DC sub-generator, and can calculate therefrom the currently present total current I Rest . By comparing the currently present total current I Rest  with the default value, the control unit can control the relevant DC/DC converters with the aim that the total current I Rest  corresponds to the default value. If the total value I Rest  exceeds or threatens to exceed the default value, at least one current (I 2 -I n ) can be reduced by a respective one of the DC/DC converters that are not assigned to the faulty DC sub-generator. Alternatively, however, several or all currents (I 2 -I n ) can be reduced by the respective ones of the DC/DC converters in order to set the total current I Rest  to the default value. Conversely, if the total value I Rest  falls below or threatens to fall below the default value, at least one current (I 2 -I n ) can be increased by a respective one of the DC/DC converters, optionally even several or all of the currents can be increased by the respective ones of the DC/DC converters that are not assigned to the faulty DC sub-generator. 
     In one embodiment of the method, when monitoring the DC sub-generators for faults, a current flowing via the DC/DC converter can be detected for all DC/DC converters in each case. A faulty DC sub-generator can be indicated when the current flowing in the direction of the DC load from the DC/DC converter assigned to the DC sub-generator falls below a current threshold value I TH  or, in particular, when the current flowing in the direction of the DC load from the DC/DC converter assigned to the DC sub-generator changes its current direction. In this case, a current flowing from the DC/DC converter in the direction of the DC load is evaluated as a positive current and a current flowing from the direction of the DC load to the DC/DC converter is evaluated as a negative current. According to this definition, a negative current flowing via the DC/DC converter in the direction of the DC load also indicates that a faulty DC sub-generator is connected on the input side to the relevant DC/DC converter. When the current flowing via the DC/DC converter is detected, an input current or an output current of the DC/DC converter can be detected. 
     Alternatively or cumulatively to detecting a current flowing via the DC/DC converter, it is likewise possible to detect a voltage of the DC sub-generator in each case for monitoring the DC sub-generators for faults. In this case, a faulty DC sub-generator is indicated when the voltage detected at the DC sub-generator falls below a voltage threshold value U TH . The voltages can advantageously be detected on the input side at the DC/DC converters assigned to the respective DC sub-generators. Alternatively, however, the output voltages of the DC/DC converters can also be detected. Advantageously, measuring units which are already available in the DC/DC converters can be used during the detection of the voltages as well as during the detection of the currents. 
     In one embodiment of the method, the default value can be selected in such a way that a time integral formed from the total current I Rest  exceeds an i2t value of an overcurrent-sensitive component of the DC/DC converter assigned to the faulty DC source or, in other words, of the DC/DC converter assigned to the faulty DC sub-generator. This prevents the overcurrent-sensitive component of the DC/DC converter(s) from being damaged as an overcurrent-sensitive component. Furthermore, in one embodiment, the default value can be selected in such a way that a time integral formed from the total current I Rest  exceeds an i2t value of the fuse that is connected in series between the faulty DC source and the DC/DC converter assigned to the faulty DC source. Such selection of the default value has the aim of tripping the fuse and thus of separating the faulty DC source from the remaining non-faulty DC sources within the faulty DC sub-generator as well as from the remaining non-faulty DC sources outside the faulty DC sub-generator. 
     In the case of a fault occurring within the EEA, situations can occur in which tripping of the fuse that is connected in series to the faulty DC source does not take place even in the case of such a selection of the default value. This can be facilitated, for example, by component tolerances in conjunction with i2t values of the overcurrent-sensitive component and the fuse that are close to one another. In such a case too, however, the faulty DC sub-generator can be separated in a simple manner from the shared DC load and thus from the remaining non-faulty DC sub-generators. Specifically, when tripping the fuse at the selected default value does not take place, the DC/DC converters that are not assigned to a faulty DC sub-generator can be deactivated in order to set the total current I Rest  to a current value of 0 A. In this way, a zero-current state is effected between the DC/DC converters, in which state the faulty DC source or the faulty DC sub-generator can be galvanically separated from the shared DC load via an electromechanical switching element. Such switching elements are usually already present in the EEA in question. However, since the switching action takes place in a zero-current state, it is not absolutely necessary for the switching elements to have arc-extinguishing means. They can therefore be designed significantly more cost-effectively. 
     In one embodiment of the method, a fault indicated during the monitoring of the DC sub-generators, optionally also a successful or an unsuccessful tripping of the fuse, can be signaled by the EEA. This can take place, for example, by a communication unit of the EEA. In this way, an operator of the EEA can accordingly be informed of the fault and correspondingly required repair measures in a timely manner. The operator can thus already take precautions in advance in order to carry out a repair of the faulty DC source or of the EEA as efficiently as possible. 
     An energy generating system according to the disclosure comprises a plurality of DC sub-generators which are connected in parallel with one another and in each case via a DC/DC converter to a shared DC load. Each of the DC sub-generators has one DC source, which is connected, via at least one fuse that is connected in series to the DC source, to the DC/DC converter that is assigned to the respective DC sub-generator. As a characteristic feature, the energy generating system furthermore comprises a control unit which is designed and configured to carry out the method according to the disclosure. The control unit can be a separately designed control unit of the EEA. Alternatively or cumulatively, however, it is also possible for the control unit to be contained within a control unit of a DC/DC converter or within a plurality of control units of the DC/DC converters. In other words, the control unit can thus also be distributed to the control units of a plurality of DC/DC converters of the EEA. The advantages already mentioned in connection with the method result. 
     In one embodiment of the EEA, the DC sub-generators each have a plurality of DC sources which are connected in parallel with one another via at least one fuse to the DC/DC converter assigned to the respective DC sub-generator. The DC sources can also each be connected via a series connection of a plurality of fuses to the DC/DC converter assigned to the respective DC sub-generator. In this case, a tripping threshold or the i2t value of the fuses can increase, the further the fuse is removed from the DC source in the series connection or, in other words, the closer the fuse is arranged to the corresponding DC/DC converter in the series connection. In a further embodiment of the EEA, the DC sources of one or more DC sub-generators can comprise a photovoltaic (PV) string and/or a battery. Furthermore, the shared DC load can in particular comprise an electrolyzer. 
     The EEA can comprise a step-down converter as DC/DC converter. Specifically, a DC/DC converter, a plurality of or possibly also all DC/DC converters of the EEA can each comprise a step-down converter and/or a DC/DC converter that has a step-down function, i.e., is designed and configured to convert an input voltage into a smaller output voltage. The overcurrent-sensitive component of the DC/DC converter assigned to the faulty DC source or, in other words, of the DC/DC converter assigned to the faulty DC sub-generator, can comprise a separate freewheeling diode connected in parallel with a semiconductor switch of the DC/DC converter. Alternatively, the overcurrent-sensitive component of the DC/DC converter can also comprise a semiconductor switch of the DC/DC converter, in particular a body diode of the semiconductor switch. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The disclosure is illustrated below with the aid of figures. The following is shown: 
         FIG. 1  shows an EEA according to the disclosure in a first embodiment; 
         FIG. 2  shows an EEA according to the disclosure in a second embodiment; 
         FIG. 3  shows a flowchart of the method according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure relates to a method for operating an energy generating system (EEA). The disclosure relates in particular to the operation of an EEA, in which a case of a fault in the EEA is detected and its damaging effect on components of the EEA is avoided or at least reduced. The EEA can in particular be a renewable EEA with a plurality of DC sub-generators which are connected in parallel with one another and in each case via a DC/DC converter to a shared DC load in order to supply the same. A case of a fault can exist in particular if one, possibly even several of the DC sub-generators, have a fault, for example a short-circuit fault. The disclosure furthermore relates to an EEA which is designed and configured for such a method. 
       FIG. 1  shows an EEA  1  according to the disclosure in a first embodiment. The EEA  1  comprises a plurality of DC sub-generators  5 . 1 - 5 . n , which are connected in parallel with one another and in each case to a shared DC load  20 , here illustrated for example as electrolyzer  21 , via a DC/DC converter  4 . 1 - 4 . n . The EEA  1  furthermore comprises a control unit  15  for controlling the DC/DC converters  4 . 1 - 4 . n . The control unit  15  is configured to receive and analyze measured values for current and or voltage, which are detected by measuring units of the DC/DC converters  4 . 1 - 4 . n . The control unit  15  is furthermore configured to control, alone or in conjunction with control units present in the DC/DC converters  4 . 1 - 4 . n , the individual ones of the DC/DC converters  4 . 1 - 4 . n  with the aim that the common total current I Rest  generated by the individual ones of the DC/DC converters  4 . 1 - 4 . n  corresponds to a default value. The DC/DC converters  4 . 1 - 4 . n  are shown by way of example in  FIG. 1  as step-down converters with an input  10 . 1 - 10 . n  in each case, an input capacitance  8 . 1 - 8 . n , that is connected to the input  10 . 1 - 10 . n , two semiconductor switches  6 . 1 - 6 . n  and an inductance  7 . 1 - 7 . n  that is connected to an output  11 . 1 - 11 . n  of the DC/DC converter  4 . 1 - 4 . n . However, according to the disclosure, it is also possible to use as DC/DC converter  4 . 1 - 4 . n  any type of DC/DC converter with a step-down function, i.e., which can convert an input voltage U 1 -U n  applied to the input  10 . 1 - 10 . n  into a smaller output voltage U a  applied to the output  11 . 1 - 11 . n . In the embodiment according to  FIG. 1 , each of the DC sub-generators  5 . 1 - 5 . n  includes only one DC source  2 . 1 - 2 . n , which is connected via a fuse  9 . 1 - 9 . n  to the input  10 . 1 - 10 . n  of the DC/DC converter  4 . 1 - 4 . n  assigned thereto. Each of the DC sources  2 . 1 - 2 . n  is shown by way of example as a photovoltaic (PV) string  3 . 1 - 3 . n . However, it is also possible for one or more of the DC sources  2 . 1 - 2 . n  to be a battery. 
     During normal operation of the EEA  1 , the DC sources  2 . 1 - 2 . n  configured as a PV string  3 . 1 - 3 . n  are operated at their respective maximum power point (MPP). Here, a voltage U 1 -U n  of the respective DC source  2 . 1 - 2 . n , which voltage is assigned to the respective MPP and applied to a respective input  10 . 1 - 10 . n  of the DC/DC converter  4 . 1 - 4 . n  assigned thereto, is converted into a shared output voltage U a . The electrolyzer  21  as the shared DC load  20  is supplied with the output voltage U a . This ensures that the maximum possible renewable power is always converted by the EEA  1  and supplied to the DC load  20 . 
     During normal operation of the EEA  1 , the DC sub-generators  5 . 1 - 5 . n  are each monitored for a possible fault  30 , for example a possible short-circuit fault. Monitoring is carried out according to one embodiment of  FIG. 1  such that any measuring devices (not shown in  FIG. 1 ) already present in the DC/DC converters  4 . 1 - 4 . n  are used. The measuring devices can be used in each case to detect a voltage U 1 -U n  applied to the respective input  10 . 1 - 10 . n  of the respective DC/DC converters  4 . 1 - 4 . n  and/or a current I 1 -I n  flowing via the respective output  11 . 1 - 11 . n  of the respective DC/DC converters  4 . 1 - 4 . n  in the direction of the DC load  20 . According to one embodiment, if one of the voltages U 1 -U n  applied to the inputs  10 . 1 - 10 . n  falls below a voltage threshold value U TH  and/or if one of the currents I 1 -I n  flowing in the direction of the DC load  20  via the output of the DC/DC converters  4 . 1 - 4 . n  falls below a current threshold value I TH , this is signaled to the control unit  15  by the corresponding DC/DC converter  4 . 1 - 4 . n . Cumulatively or alternatively, it is also possible to transmit the values of the detected currents I 1 -I n  and voltages U 1 -U n  of the higher-level control unit  15 , which then itself evaluates said values and detects an indication of a fault  30 . 
       FIG. 1  illustrates by way of example a fault  30  in the form of a short-circuit fault at the DC source  2 . 1  of one of the DC sub-generators  5 . 1 - 5 . n , here of the DC sub-generator  5 . 1 . Due to the short-circuit fault, the voltage U 1  of the faulty DC source  2 . 1  drops to a very small absolute value. In this case, unlike the remaining DC/DC converters  4 . 2 - 4 . n , the DC/DC converter  4 . 1  assigned to the faulty DC sub-generator  5 . 1  or, in other words, the DC/DC converter  4 . 1  assigned to the faulty DC source  2 . 1  is no longer capable of converting the voltage U 1  applied on the input side to the shared value U a . Instead, an output voltage that is significantly smaller than the shared value U a  is now also applied to the output of the DC/DC converter  4 . 1  assigned to the faulty DC source  2 . 1 . Sometimes, there may be a drop not only in the output voltage of the DC/DC converter  4 . 1  assigned to the faulty DC source  2 . 1 ; it may also be possible that in response to the one faulty DC source  2 . 1 , a voltage drop forms on an entire DC bus of the EEA  1 , i.e., is present also at the DC/DC converters  4 . 2 - 4 . n  that are not assigned to the faulty DC source  2 . 1 . However, the drop in output voltage at the DC/DC converter  4 . 1  assigned to the faulty DC source  2 . 1  is somewhat more pronounced than the drops in voltage at the other DC/DC converters  4 . 2 - 4 . n . The at least slightly more pronounced drop in voltage at the output  11 . 1  of the DC/DC converter  2 . 1  results in a total current I Rest  according to I Rest =I 2 +I 3 + . . . +I n  of the DC/DC converters  4 . 2 - 4 . n  assigned in each case to a fault-free DC sub-generator  5 . 2 - 5 . n , in the direction of the DC/DC converter  4 . 1  assigned to the faulty DC source  2 . 1  (wherein I Rest =I 1  in  FIG. 1 ). Without further measures, the total current I Rest  can assume values that may irreversibly damage overcurrent-sensitive components of the DC/DC converter  4 . 1  concerned. This is the case, in particular, when a fuse  9 . 1  connected in series to the faulty DC source  2 . 1  does not trip, or does not trip on time, in order to interrupt the fault current. 
     According to the disclosure, the fault  30  is detected by the control unit  15  based on the detected currents I 1 -I n  and/or voltages U 1 -U n . In response, in the above example with a fault  30  associated with DC source  2 . 1 , the DC/DC converters  4 . 2 - 4 . n  assigned to respective fault-free DC sub-generators  5 . 2 - 5 . n  are operated by the control unit  15  with the aim that the common total current I Rest  generated by them assumes a default value according to I Rest =I 2 +I 3 + . . . +I n . In this case, not only the default value itself but also the respective currents I 2 -I n  contributed by the individual DC/DC converters  4 . 2 - 4 . n  to achieve the shared default value can be predetermined from the outset. The default value is selected such that, on the one hand, damage to an overcurrent-sensitive component of the DC/DC converter  4 . 1  connected to the faulty DC sub-generator  5 . 1  is ruled out. Specifically, the default value can be selected such that it is smaller than a value of the overcurrent-sensitive component of the corresponding DC/DC converter  4 . 1 . On the other hand, the default value is selected to be such (high), if possible, that a fuse  9 . 1  downstream of the faulty DC source  2 . 1  trips reliably and thus separates the faulty DC source  2 . 1  from the remaining fault-free DC sources  2 . 2 - 2 . n . Since the DC/DC converters  4 . 1 - 4 . n  of the EEA  1  are DC/DC converters designed and configured for a step-down function, the DC/DC converters  4 . 2 - 4 . n  are capable of providing a sufficiently high total current I Rest  at their output, even if irradiation on the DC sources  2 . 2 - 2 . n  configured as PV strings  3 . 1 - 3 . n  is relatively low. However, should the fuse  9 . 1  downstream of the faulty DC source  2 . 1  not trip before the i2t value of the overcurrent-sensitive component of the relevant DC/DC converter  4 . 1  is exceeded, damage to the overcurrent-sensitive component is avoided in that the total current I Rest , under such conditions, is set to a value of 0 A by suitable operation of the remaining DC/DC converters  4 . 2 - 4 . n . In particular, the remaining DC/DC converters  4 . 2 - 4 . n  can be deactivated in this case. The faulty DC sub-generator  5 . 1  can then be permanently and galvanically separated in the zero-current state, via a DC separator not shown in  FIG. 1  (similar to the DC separators  13 . 1 - 13 . n  shown in  FIG. 2 ), from the shared DC load  20  and the remaining fault-free DC sub-generators  5 . 2 - 5 . n . Thereafter, the EEA  1  can be further operated without the defective DC sub-generator  5 . 1  but with the remaining fault-free DC sub-generators  5 . 2 - 5 . n . The disconnected DC sub-generator  5 . 1  or its faulty DC source  2 . 1  can be repaired without hazard and can then be reconnected to the EEA  1 . Thus the control unit  15  operates to set a first default value for the current upon detecting a fault that is non-zero to trip a fuse associated with the faulty DC source, and if the fuse does not trip in response to the first default value, a second default value for the current is set to about 0 A in order to for the faulty DC sub-generator to be galvanically separated via a DC separator (not explicitly shown in  FIG. 1 ) arranged between the faulty DC sub-generator and the DC/DC-converter assigned thereto. 
     Although  FIG. 1  shows only one faulty DC sub-generator  5 . 1 , the method can also be applied to a case with a plurality of simultaneously present faulty DC sub-generators. This applies at least as long as a number of fault-free DC sub-generators outweighs a number of faulty DC sub-generators. 
       FIG. 2  shows a second embodiment of an EEA  1  according to the disclosure, which in many features resembles the EEA  1  shown in  FIG. 1 . For the sake of clarity, reference is therefore made only to the differences to the first embodiment of the EEA  1 , while reference is made to the descriptions in  FIG. 1  with regard to identical features. 
     The EEA  1  shown in  FIG. 2  also has a plurality of DC sub-generators  5 . 1 - 5 . n , which are connected in each case in parallel with one another and to a shared DC load  20  via a DC/DC converter  4 . 1 - 4 . n . However, in contrast to  FIG. 1 , each of the DC sub-generators  5 . 1 - 5 . n  in  FIG. 2  comprises a plurality of DC sources  2 . 1 - 2 . n , which are each connected in parallel with one another and in each case via a series connection of a plurality of fuses  9 . 1 - 9 . n  to the DC/DC converter  4 . 1 - 4 . n  that is assigned to the respective DC sub-generator  5 . 1 - 5 . n . Here, an i2t value of the fuses  9 . 1 - 9 . n  within each of the series connections increases with increasing distance of the fuse  9 . 1 - 9 . n  from the DC source  2 . 1 - 2 . n  assigned thereto. A DC separator  13 . 1 - 13 . n  is furthermore arranged between each of the DC sub-generators  5 . 1 - 5 . n  and the DC/DC converter  4 . 1 - 4 . n  assigned thereto, with which DC separator the corresponding DC sub-generator  5 . 1 - 5 . n  can be galvanically separated from the DC/DC converter  4 . 1 - 4 . n  assigned thereto. The DC separators  13 . 1 - 13 . n  can advantageously be free of means for extinguishing an arc and can therefore be designed relatively cost-effectively. 
     Also in  FIG. 2 , currents I 1 -I n  and/or voltages U 1 -U n  of the DC sub-generators  5 . 1 - 5 . n  are detected by measuring units of the DC/DC converters  4 . 1 - 4 . n  (not shown in  FIG. 2 ) and transmitted to the control unit  15 . Should one of the currents I 1 -I n  and/or one of the voltages U 1 -U n  fall below the corresponding one of the threshold values I TH , U TH , this indicates a fault  30  of the corresponding DC sub-generator  5 . 1 - 5 . n .  FIG. 2  illustrates, by way of example, a fault  30  in the form of a short-circuit fault in the upper DC sub-generator  5 . 1 , and there in the lower one of the DC sources  2 . 1  shown associated with the upper DC sub-generator  5 . 1 . Here, too, the input voltage of the DC/DC converter  4 . 1  that is assigned to the faulty DC source  2 . 1  and thus also to the faulty DC sub-generator  5 . 1  or, in other words, connected thereto on the input side, drops. The relevant DC/DC converter  4 . 1  is no longer capable of providing, at its output  11 . 1 , an output voltage U a  identical to the other DC/DC converters  4 . 2 - 4 . n ; instead, the voltage drops there as well. As a result, here, the non-faulty DC/DC converters  5 . 2 - 5 . n  also feed a total current I Rest  into the one faulty DC/DC converter  5 . 1 . 
     Similarly to the method already described in connection with  FIG. 1 , the fault  30  is detected by the control unit  15  monitoring the DC sub-generators  5 . 1 - 5 . n . In response thereto, the remaining DC/DC converters  4 . 2 - 4 . n  assigned in each case to a fault-free DC sub-generator  5 . 2 - 5 . n  are operated via the control unit  15  with the aim that the total current I Rest  generated by the remaining DC/DC converters  4 . 2 - 4 . n  assumes a default value. In  FIG. 2 , as well, the default value is selected such (low) that, on the one hand, an overcurrent-sensitive component of the DC/DC converter  4 . 1  assigned to the faulty DC sub-generator  5 . 1  is protected and the damage thereof is avoided. On the other hand, however, it is also selected such (high) that a fuse  9 . 1  arranged within the series connection of fuses  9 . 1  between the faulty DC source  2 . 1  and the downstream DC/DC converter  4 . 1  trips. Since the i2t value of the fuses  9 . 1  within the series connection decreases with decreasing distance from the faulty DC source  2 . 1 , tripping precisely the fuse connected directly downstream of the faulty DC source  2 . 1  is most likely. Advantageously, this fuse is also usually the most cost-effective one of the fuses  9 . 1  within the series connection. 
       FIG. 3  shows a variant of the method according to the disclosure in the form of a flowchart, as can be carried out with the EEA  1  according to  FIG. 1  or the EEA  1  according to  FIG. 2 . In a first act S 1 , the method starts with an MPP operation of all fault-free DC sub-generators  5 . 1 - 5 . n . Since, at the beginning of the method, it is assumed by way of example that there are no faulty DC sources  2 . 1  and hence no faulty DC sub-generators  5 . 1 , the number of all fault-free DC sub-generators corresponds at the beginning to the number of all available DC sub-generators  5 . 1 - 5 . n  of the EEA  1 . In a second act S 2 , all DC sub-generators  5 . 1 - 5 . n  of the EEA  1  that are currently in operation are monitored for a fault  30 . Monitoring takes place in that for each of the DC sub-generators  5 . 1 - 5 . n  a voltage U 1 -U n  of the respective DC sub-generator  5 . 1 - 5 . n  and/or for each of the DC/DC converters  4 . 1 - 4 . n  a current I 1 -I n  flowing via the output  11 . 1 - 11 . n  of the respective DC/DC converter  4 . 1 - 4 . n  in the direction of the DC load  20  is detected and transmitted to the control unit  15 . In a third act S 3 , the control unit  15  checks whether one of the detected voltages U 1 -U n  falls below a voltage threshold value U TH  and/or whether one of the detected currents I 1 -I n  falls below a current threshold value I TH . If this is not the case (NO at S 3 ), the method jumps back to the first act S 1 . However, if one of the detected voltages U 1 -U n  falls below the voltage threshold value U TH  and/or if one of the detected currents I 1 -I n  falls below the current threshold value I TH  (YES at S 3 ), it is indicated according to the method that the DC sub-generator  5 . 1 - 5 . n  with the undershooting voltage U k  and/or the undershooting current I k  (with k=1 . . . n) is a faulty DC sub-generator  5 . k  which has a faulty DC source  2 . k . In this case, the method jumps to a fourth act S 4 , in which all remaining DC/DC converters  4 . 1 - 4 . n  in operation that are not assigned to the faulty DC sub-generator  5 . k  are operated via the control unit  15  with the aim that their total current I Rest  assumes a default value. In one embodiment, the total current I Rest  is hereby limited both upward and downward. A fifth act S 5  checks whether the total current I Rest  was capable of tripping a fuse  9 . k  connected downstream of the faulty DC source  2 . k . This can be checked, for example, by the detection of current I k  and/or voltage U k  of the DC/DC converter  4 . k  assigned to the faulty DC sub-generator  5 . k . It can be assumed that the fuse  9 . k  did not trip (NO at S 5 ) if a current I k  flowing via the output  11 . k  of the DC/DC converter  4 . k  in the direction of the DC load continues to fall below the current threshold value I TH  and/or if a voltage U k  applied to the input  11 . k  of the DC/DC converter  4 . k  continues to fall below the voltage threshold value U TH . If this is not the case, however, the method assumes that the fuse  9 . k  connected downstream of the faulty DC source  2 . k  has tripped (YES at S 5 ) and as a result, the faulty DC source  2 . k  has been separated from the shared DC load  20 . Once the fuse  9 . k  has tripped (YES at S 5 ), the DC sub-generator previously indicated as a faulty DC sub-generator  5 . k  shows exclusively fault-free DC sources  2 . k , i.e., it can be referred to again as a fault-free DC sub-generator and can continue to be operated. Accordingly, when fuse  9 . k  is tripped, the method jumps back to the first act S 1 , in which all fault-free DC sub-generators are again operated at their MPP. The number of DC sub-generators continues to correspond to the total number of DC sub-generators of the EEA  1 . However, the DC sub-generator  5 . k  previously identified as faulty now has at least one DC source  2 . k  less than operating previously. If it is however concluded in the fifth act S 5  that a fuse  9 . k  connected downstream of the faulty DC source  2 . k  did not trip (NO at S 5 ), all remaining DC/DC converters  4 . 1 - 4 . n  that are not assigned to the faulty DC sub-generator  5 . k  are deactivated in a sixth act S 6 , as a result of which their total current I Rest  assumes the value 0 A. In a seventh act S 7 , the faulty DC sub-generator  5 . k  is then galvanically separated, via the DC separator  13 . k  assigned to it, from the shared DC load  20  and from the remaining fault-free DC sub-generators. The method then jumps to the first act S 1  in which all fault-free DC sub-generators are again operated at their respective MPP. However, the EEA  1  now continues to be operated without the DC sub-generator  5 . k  previously identified as faulty; the number of fault-free DC sub-generators has decreased by 1 in this case.