Method for increasing the service life of converter switches, and system
Patent Information
- Application Number
- EP2023721908
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-05-23
- Filing Date
- 2023-04-24
- Publication Date
- 2025-04-02
AI Technical Summary
Temperature fluctuations reduce the service life of semiconductor switches in power converters used in energy generation systems, such as PV systems, due to inefficient heat management during periods of available energy with no intended transmission.
A method where the DC/DC converter operates to generate power loss without transferring energy, maintaining the switches in a heated state through internal current flow, by controlling switches to create circulating currents and power dissipation, especially when the battery is fully charged or no energy is needed for grid feed-in, utilizing the available energy for heating.
This method extends the service life of converter switches by maintaining them in a heated state during inactive periods, utilizing excess energy for self-heating and preventing thermal stress, thus enhancing reliability and longevity.
Smart Images
Figure 1.1
Abstract
Description
[0001] Method for increasing the service life of transformer switches and system
[0002] Technical area
[0003] The application relates to a method for increasing the service life of converter switches and to a system comprising a generator and a DC / DC converter having the converter switches.
[0004] background
[0005] Power converters are used in energy generation systems, especially photovoltaic (PV) systems. These converters may incorporate pulsed semiconductor switches, known as converter switches. Temperature fluctuations lead to a reduced service life for such semiconductor switches.
[0006] EP2600510A1 describes a method in which an electrical power converter is controlled in such a way that, in addition to an output current, a current is generated which does not influence the output current and at the same time heats converter switches to a predetermined target temperature.
[0007] The application is based on the task of increasing the service life of converter switches.
[0008] The object is achieved by a method having the features of patent claim 1 and by a system having the features of independent patent claim 8, as well as by a power generation plant according to claim 14. Preferred embodiments are specified in the dependent patent claims.
[0009] Overview
[0010] In a system, an energy source, in particular a generator, is connected to a DC bus together with a DC / DC converter. The energy source is, for example, a PV generator. The DC / DC converter has converter switches in the form of semiconductor switches. The DC / DC converter is intended, for example, to transfer electrical power from the DC bus to a DC energy storage device, for example, a battery, or in the opposite direction. The DC storage device can, for example, be intended to absorb and store electrical energy generated by the energy source. A method for increasing the service life of converter switches of the DC / DC converter comprises the following steps:
[0011] • Determining a system state in which electrical power is available from the energy source for which no transmission via the DC / DC converter and no output to other participants of the DC bus is intended.
[0012] • Operating the DC / DC converter during the detected system condition such that power dissipation is generated in the DC / DC converter without power being transferred through the DC / DC converter.
[0013] The detected system state therefore relates to a system state in which electrical energy is not intended to be transferred to the DC storage device via the DC / DC converter. This can be the case, for example, if the battery is already fully charged. Another device on the DC bus can be, for example, an inverter that is intended to transfer electrical energy from the DC bus to an AC grid. The AC grid can be, for example, an AC supply grid. Other devices on the DC bus can be, for example, other DC loads. A DC bus usually has two conductors that are at different electrical potentials and between which the DC bus voltage is applied.
[0014] The method thus makes it possible to operate the DC / DC converter during the determined system state in such a way that power loss is generated in the DC / DC converter, for example, by a circulating current flowing within the DC / DC converter, without power being transferred via the DC / DC converter, so that the converter switches are kept in a warm state compared to an inactive state. DC (direct current) refers to direct current in this application. AC (alternating current) refers to alternating current in this application.
[0015] The system state in which electrical power from the energy source is available and for which no transmission via the DC / DC converter and no output to other DC bus devices is provided can, for example, be a system state in which the battery is already fully charged, no or at least no complete feed-in of the currently available power into an AC grid via an inverter connected to the DC bus is provided, and at the same time no other DC bus devices, for example, other DC loads, are provided to absorb the electrical energy. In such a state, electrical energy from the energy source is available, possibly unused, and can be used for the method according to the invention without disadvantages.
[0016] During a period of good weather in a PV system, this could, for example, be the afternoon until early evening on a nice day, when the battery has already been charged by the PV generator earlier in the day and the PV generator can produce more electrical energy than is required to supply the DC bus due to the ongoing solar radiation. The method makes it possible to detect such an operating state and, at the same time, to operate the DC / DC converter in which no energy is transferred via the DC / DC converter (e.g. towards the DC storage device), but rather the DC / DC converter only absorbs energy to keep the converter switches warm through an internal current flow in the DC / DC converter. The heating is achieved through the current flow in the DC / DC converter and the resulting power loss in the form of heat.
[0017] In one embodiment, the DC / DC converter has a first half-bridge with a series connection of a first switch and a second switch. The first and second switches are preferably embodied as semiconductor switches. The first and second switches are converter switches of the DC / DC converter. The first half-bridge is arranged between output-side terminals of the DC / DC converter. The output-side terminals are intended, for example, to be connected to a DC storage device. The DC / DC converter further has a second half-bridge with a series connection of a third switch and a fourth switch. The third and fourth switches are converter switches of the DC / DC converter and are preferably embodied as semiconductor switches. The second half-bridge is arranged between input-side terminals of the DC / DC converter connected to the DC bus.A center connection of the first half-bridge and the second half-bridge are connected to each other via a choke. The center connections of the first and second half-bridge are each arranged between the two switches. A choke is to be regarded as an electrical choke, i.e. it is designed as an inductor. The first and the fourth switch have a common connection which can be connected, for example, to one of the conductors of the DC bus. For such a DC / DC converter, in one embodiment of the method, the second switch is permanently closed and the first switch is permanently opened during the determined system state. This effectively prevents power flow via the DC / DC converter. The third switch is clocked during the determined system state. Clocking means that the switch is controlled in such a way that it is temporarily opened and temporarily closed.
[0018] Controlling the switches in this way allows electrical energy to be taken from the DC grid and flowed as current, which generates power loss, within the DC / DC converter. The flowing current generates power loss, and this power loss heats the switches. When the third switch is switched on, a current flows through the third and second switches and through the choke. When the third switch is open, an electrical current flows through the second switch and the fourth switch. Depending on the switching state of the fourth switch, the current flows through a freewheeling diode arranged in parallel with the fourth switch or through the closed fourth switch.
[0019] In one embodiment, the third and fourth switches are clocked to operate the DC / DC converter in the detected operating state. In another embodiment, the third and fourth switches are clocked complementarily. Complementary clocking means that during periods in which the third switch is closed, the fourth switch is open, and vice versa. It is also possible to keep the fourth switch open during the detected system state, since the desired current flow for heating the converter switches can also occur via the freewheeling diode of the fourth switch.
[0020] In one embodiment of the method, the third switch is clocked at a duty cycle such that a permissible nominal current of the DC / DC converter is not exceeded. For example, the third switch is operated in a current-controlled manner and, for example, opens when a switch-off threshold below the nominal current is exceeded and closes again when a switch-on threshold is undershot. In the method, the current flowing through the DC / DC converter is determined by the duty cycle of the third switch, since electrical power is only drawn from the DC grid when the third switch is closed. With complementary clocking of the fourth switch, however, the fourth switch is clocked with a corresponding duty cycle complementary to the third switch.
[0021] In one embodiment of the method, the third switch is clocked with a duty cycle that depends on a temperature measured in the DC / DC converter. The duty cycle can, for example, be selected to achieve a predefined target temperature in the DC / DC converter. It is also possible, at low temperatures in the DC / DC converter, to increase the current flow in the detected system state accordingly in order to continue to achieve the desired heating of the converter switches. For example, the aforementioned switch-off threshold and / or the switch-on threshold can be determined as a function of the measured temperature. The duty cycle can, of course, also depend on other influencing factors, such as the DC bus voltage.
[0022] Instead of or in addition to the temperature measured in the DC / DC converter, the clocking of the third switch or the power loss generated by the method according to the invention can be determined as a function of a temperature measured externally to the DC / DC converter. For this purpose, determining the system state comprises detecting the temperature of a component with which the DC / DC converter is in thermal contact, but which is not part of the DC / DC converter and whose temperature should not fall below a predetermined value. In one embodiment, this can be, for example, a battery that is charged and discharged via the DC / DC converter. Batteries of frequently used types have the property that they cannot be charged below a critical charging temperature, or should be charged, even if discharging at such a low temperature is possible.Here, the generated power loss can be used to raise the temperature of the battery in thermal contact with the DC / DC converter or to maintain it above the specified temperature. The power loss can also be drawn entirely or partially from the battery, and the fourth switch can be clocked with a suitable duty cycle for this purpose.
[0023] In an advantageous embodiment of the method, the system state is determined depending on the voltage of the DC bus. The system state to be determined, in which no consumption by other DC bus devices is planned and, at the same time, no power transmission via the DC / DC converter is planned, can be determined depending on the voltage on the DC bus. The voltage on the DC bus is determined, for example, by the inverter on the DC bus, the other loads on the DC bus, and / or the energy source.
[0024] In a further embodiment of the method, the determination of the system state is indicated to the DC / DC converter by an input signal. This embodiment can be used, for example, if the system state is determined by a control device and then transmitted to the DC / DC converter, where a controller of the DC / DC converter controls the converter switches of the DC / DC converter according to the described method. The control device of the DC / DC converter can be understood as part of a control device of the system. This embodiment can also be used if the system state is determined, for example, based on a voltage measurement on the DC bus and transmitted to the DC / DC converter, where a controller of the DC / DC converter then controls the converter switches according to the method. Control devices can be computing units that have at least one processor, memory, and a communication interface.
[0025] A system comprises an energy source and a DC / DC converter. The energy source and the DC / DC converter are each connected to a DC bus. The DC / DC converter has converter switches which can be controlled to open and close by the DC / DC converter for power conversion. A control device is also provided which is designed and intended to carry out one of the methods described above. The control device can, for example, be arranged in the DC / DC converter and / or on another computing unit of the system. The control device can also, for example, be distributed between the DC / DC converter and one or more other computing units of the system or be arranged remotely from the system. The control device can also be designed to control the DC / DC converter and / or other participants of the DC bus.
[0026] In one embodiment of the system, the first switch is thermally coupled to at least one of the second, third, and / or fourth switches. In the method, the current generated to generate the power loss flows through the second, third, and fourth switches. These switches can thus be heated by the power loss. By coupling the first switch to the second, third, and / or fourth switches, the first switch can also be heated to increase its service life.
[0027] In one embodiment, the first switch is thermally coupled to the inductor as a heat accumulator. During the execution of the method, the current for generating the power loss flows through the inductor of the DC / DC converter. To heat the first switch, it is thermally coupled to the inductor, which serves as a heat accumulator. This can also heat the first switch.
[0028] In one embodiment, the converter switches and the choke are thermally coupled with a common heat sink. The common heat sink ensures uniform heat exchange between the converter switches and the choke, thus achieving uniform heating of the converter switches.
[0029] In one embodiment of the system, the energy source is a PV generator. The method is particularly advantageous for implementing a system with a PV generator, since solar energy generated by the PV generator can be available when the system state is determined in which no, or at least no complete, grid feed-in of the power generated by the energy source is possible or desired, and at the same time, a battery connected to the system is already fully charged or further charging is not planned. This solar energy can then be used to heat the converter switches, thus extending the service life of the converter switches.
[0030] It is also conceivable that, during the execution of the method according to the invention, a check is made continuously or occasionally to determine whether the condition that no other use of the power used to heat the DC / DC converter is intended is still met. If this is not the case, the heating can be stopped or the power used for heating can be reduced.
[0031] In one embodiment of the system, the DC / DC converter has a first and a second sub-converter. The two sub-converters are each connected to the DC bus on the input side. When the system status is determined, the two sub-converters are connected to one another on the output side via an optional coupling switch such that power flows of equal but opposite magnitudes are transmitted through the two sub-converters. However, the sub-converters can also be permanently connected to one another on the output side without a coupling switch. Due to the equal but opposite power flows, power loss is generated in the sub-converters by the converter switches, thus keeping the converter switches in a heated state. This occurs without any overall power transfer from the DC / DC converter, i.e. without electrical power being output via the output terminals of the DC / DC converter.Advantageously, the two sub-converters are operated in parallel when transmitting power via the DC / DC converter. This means that, to transmit total power via the DC / DC converter, both sub-converters are connected to the DC bus on the input side, and both sub-converters are connected to the output terminals of the DC / DC converter on the output side. When the system state is determined, the output sides of the sub-converters are then connected via the coupling switch so that they transmit equal but opposite power flows to each other. Instead of a coupling switch, the sub-converters can also be permanently connected in parallel on the output side.
[0032] The application further relates to a power generation system comprising one of the systems described above and an inverter connected to the DC bus for feeding the power generated by the energy source into an alternating current grid. The alternating current grid is preferably an AC supply grid.
[0033] Fiourenliste
[0034] Examples of the application are explained in more detail below with reference to the accompanying figures. They show:
[0035] Fig. 1 is a schematic representation of an embodiment of a system with a power source and a DC / DC converter;
[0036] Fig. 2 schematically shows a method for increasing the service life of converter switches; Fig. 3 is a schematic representation of an embodiment of a system with a power source and a DC / DC converter.
[0037] The same reference numerals are used in the figures for the same or similar elements.
[0038] Flower description
[0039] Figure 1 schematically shows an embodiment of a system 10 which comprises the DC bus DC+ DC-, to which the energy source PV with a voltage U_PV and the DC / DC converter 12 are connected. The DC bus DC+ DC- has two conductors DC+ and DC-, which are at different electrical potentials. A DC bus voltage U_Bus is applied between the two conductors of the DC bus DC+ DC-. Additional DC bus devices, which are not shown in Figure 1, can be connected to the DC bus DC+ DC-. The additional DC bus devices can be, for example, an inverter, which can be provided to feed electrical power generated by the energy source PV into an AC supply grid.
[0040] The DC / DC converter 12 is provided to draw electrical power from the DC bus DC+ DC-, convert it, and transfer it, for example, to a DC storage device Bat connected to its output terminals 12A+ 12A-, in order to be able to make this power available to the DC bus again at a later time. The DC storage device Bat has a voltage U_Bat between its terminals. The DC / DC converter has a series connection of a first and a second semiconductor switch Q1, Q2 between its output terminals 12A+ 12A-. A capacitor is arranged in parallel with this series connection of the first and the second switches Q1 and Q2. Between its input terminals 12E+ 12E-, the DC / DC converter 12 has a series connection of a third and a fourth semiconductor switch Q3, Q4. Between its input terminals 12E+ 12E-, a capacitor is arranged in parallel with the series connection of the third and the fourth switch Q3, Q4.The semiconductor switches Q1, Q2, Q3, and Q4 correspond to converter switches of the DC / DC converter 12 and can have intrinsic freewheeling diodes or freewheeling diodes connected in antiparallel. The two series circuits of switches Q1, Q2, and Q3, Q4 correspond to two half-bridges of the DC / DC converter 12. The two half-bridges are connected midway between the two switches by a choke D. An ammeter A is arranged in series with the choke D. The second switch Q2 and the fourth switch Q4 are connected to each other, for example, to a DC conductor of the DC bus.
[0041] Figure 2 schematically shows a method for increasing the service life of converter switches Q1, Q2, Q3, Q4. In a system 10 shown by way of example in Figure 1 or Figure 3 with a DC / DC converter 12 and an energy source PV, both of which are connected to a DC bus DC+ DC-, electrical energy generated by the energy source PV can be fed into the DC bus DC+ DC-. The energy source PV has a PV voltage U_PV prevailing between its terminals. The DC / DC converter 12 can draw electrical power from the DC bus DC+ DC- and output it via its output terminals 12A+ 12A-, for example to a DC storage device Bat.
[0042] In the method, a system state is determined in a step S1 in which electrical power from the energy source PV is available on the DC bus DC+ DC-. The system state is determined when no transmission of electrical power via the DC / DC converter 12 is provided and, at the same time, no output of the electrical power to other participants in the DC bus DC+ DC- is provided. Other participants in the DC bus DC+ DC- can be, for example, other DC loads (not shown) and / or an inverter (not shown). The inverter can be provided, for example, to feed electrical power into an AC supply network. A system state can now arise in which the energy source PV can generate electrical power, but no or at least no complete transmission of the generateable power via the inverter is provided. The reasons for this can be, for example, regulatory in nature.At the same time, it is possible that no transfer to the DC storage Bat is planned, for example, because the DC storage Bat is already full. This is an example of a system state determined in step S1.
[0043] In a step S2, after determining the system state, the DC / DC converter 12 is operated in such a way that power loss is generated in the DC / DC converter 12 without power being transferred via the DC / DC converter 12. This causes the converter switches Q1, Q2, Q3, Q4 of the DC / DC converter 12 to be kept in a heated state compared to an inactive state. The heated state is caused, on the one hand, by the switching operations and by the current flowing in the DC / DC converter 12 and the resulting power loss.
[0044] The system state to be determined in method step S1 can, for example, be determined by measuring the voltage U_Bus of the DC bus DC+ DC-. To operate the DC / DC converter 12 in step S2, the first switch Q1 can now be permanently open. The second switch Q2 can remain permanently closed. The third switch Q3 is clocked. The fourth switch Q4 can, for example, be permanently open. It is also possible to close the fourth switch Q4 when the third switch Q3 is in a state in which it is open. Complementary clocking of the third and fourth switches Q3, Q4 would therefore be possible. Incompletely complementary clocking of the third and fourth switches Q3, Q4 is also possible, provided the fourth switch Q4 is open at the times when the third switch Q3 is closed. One possible clocking of the third switch Q3 is a duty cycle of 30 ps at a clock frequency of 100 Hz.
[0045] During operation in step S2, when the third switch Q3 is closed, a current flows via the third switch Q3, via the inductor D, and the second switch Q2. When the third switch Q3 is then opened, the current can flow further via switch Q4, via the inductor D, and back to the second switch Q2. Depending on the switch type, the current can flow via the fourth switch Q4 either via a freewheeling diode arranged anti-parallel to the fourth switch Q4, an intrinsic freewheeling diode, or, when the switch Q4 is closed, directly via the closed switch Q4. This generates a current flow via the third, second, and fourth switches Q3, Q2, Q4 without any current flowing via the output terminals 12A+ 12A- of the DC / DC converter 12. This current flow generates power loss, which heats the converter switches Q1, Q2, Q3, Q4.Since no direct current flows through the first switch Q1, it can be heated, for example, via the inductor D as a shared heat storage device or a shared heat sink with the inductor D and the switches Q2, Q4, Q3. However, it is also conceivable that the converter switch Q1 is installed together with the converter switch Q2 in a module, and is thus already thermally coupled to it and heated.
[0046] In the system 10 shown in Figure 3, the DC / DC converter 12 has two sub-converters 14, 16. The first sub-converter 14 and the second sub-converter 16 are each connected on the input side to the DC bus DC+ DC-. The input terminal 14E+ of the first sub-converter 14 and the input terminal 16E+ of the second sub-converter 16 are connected to the input terminal 12E+ of the DC / DC converter 12. The input terminal 14E- of the first sub-converter 14 and the input terminal 16E- of the second sub-converter 16 are connected to the input terminal 12E- of the DC / DC converter 12. The two sub-converters 14, 16 also correspond to DC / DC converters. In normal operation, i.e. when the DC / DC converter 12 draws electrical power from the DC bus DC+ DC- and transfers it to the DC storage Bat, the sub-converters 14, 16 are connected in parallel on the input side, as shown in Figure 3.On the output side, the DC / DC converter 12 has separate output terminals 12A+, 12A- for each sub-converter 14, 16, to which a battery can be connected. A coupling switch 18 can optionally be provided between the output terminals 14A+ and 16A+ and / or between the output terminals 14A- and 16A- to enable a temporary parallel connection of the sub-converters 14, 16 on the output side or to separate them from one another on the output side in a single-pole or all-pole manner. However, it is also conceivable that the output terminals 12A+, 12A- assigned to the sub-converters are connected to the same battery, thus permanently connecting the sub-converters 14, 16 in parallel on the output side.
[0047] During operation of step S2 of the method, the sub-converters 14, 16 are now connected in parallel on the output side via the optional coupling switch, unless they are already permanently connected in parallel. The electrical power output by one of the sub-converters 14, 16 is equal in magnitude but is absorbed by the other sub-converter, so that no electrical power is exchanged via the output terminals 12A+ 12A- of the DC / DC converter 12. In the state of S2, electrical power is absorbed via the input terminals 12E+ 12E-, but further power flows run entirely within the DC / DC converter 12, and no electrical power is exchanged via the output terminals 12A+ 12A-.
[0048] 10 systems
[0049] 12 DC / DC converters
[0050] 12A+, 12A- output terminals DC / DC converter
[0051] 12E+, 12E- Input terminals DC / DC converter 14 first sub-converter
[0052] 14A+, 14A- Output terminals of the first partial transformer
[0053] 14E+, 14E- Input terminals first sub-converter 16 second sub-converter
[0054] 16A+, 16A- output terminals of the second partial converter
[0055] 16E+, 16E- Input connections of the second partial converter 18 coupling switches
[0056] A ammeter
[0057] DC+, DC- DC bus
[0058] U_Bus DC bus voltage
[0059] Bat Battery
[0060] U_Bat battery voltage
[0061] PV energy source
[0062] U_PV PV voltage
[0063] D throttle
[0064] Ql, Q2, Q3, Q4 converter switches
Claims
Claims: Method for increasing the service life of converter switches (Q1, Q2, Q3, Q4) in a system (10) comprising a power source (PV) which is connected to a DC bus (DC+, DC-) together with a DC / DC converter (12) comprising the converter switches (Q1, Q2, Q3, Q4), wherein the method comprises the steps: • Determining a system state in which electrical power from the energy source (PV) is available for which no transmission via the DC / DC converter (12) and no output to other participants of the DC bus (DC+, DC-) is intended, and • Operating the DC / DC converter (12) during the detected system state such that power loss is generated in the DC / DC converter (12) without power being transferred through the DC / DC converter (12). Method according to claim 1, wherein the DC / DC converter (12) is operated during the detected operating state such that a circulating current flowing through the converter switches (Q1, Q2, Q3, Q4) is generated within the DC / DC converter (12).A method according to claim 1 or 2, wherein the DC / DC converter (12) comprises a first half-bridge with a series connection of a first switch (Q1) and a second switch (Q2) arranged between output terminals (12A+, 12A-) of the DC / DC converter (12), and a second half-bridge with a series connection of a third switch (Q3) and a fourth switch (Q4) arranged between input terminals (12E+, 12E-) of the DC / DC converter (12) connected to the DC bus (DC+, DC-), wherein the center terminals of the first half-bridge and the second half-bridge are connected to each other via an inductor (D), and the second switch (Q2) has a common terminal with the fourth switch (Q4), wherein the DC / DC converter (12) permanently closes the second switch (Q2) and permanently opens the first switch (Q1) during the detected system state, while the third switch (Q3) is clocked. A method according to claim 3, wherein the third switch (Q3) and the fourth switch (Q4) are clocked, in particular complementarily clocked. A method according to claim 3 or 4, wherein the clocking of the third switch (Q3), in particular the complementary clocking of the third switch (Q3) and the fourth switch (Q4), is performed with a duty cycle such that a permissible rated current of the DC / DC converter (12) is not exceeded. A method according to claim 3, 4, or 5, wherein the clocking of the third switch (Q3), in particular the complementary clocking of the third switch (Q3) and the fourth switch (Q4), is performed with a duty cycle selected such that a temperature measured in the DC / DC converter (12) is regulated to a target temperature. A method according to any one of the preceding claims, wherein the system state is determined as a function of the voltage of the DC bus (DC+, DC-).A method according to any one of the preceding claims, wherein determining the system state comprises sensing the temperature of a component with which the DC / DC converter (12) is in thermal contact. A method according to any one of the preceding claims, wherein the determination of the system state is indicated to the DC / DC converter (12) by an input signal. A system (10) comprising a power source (PV) and a converter switch (Q1, Q2, Q3, Q4), a DC / DC converter (12) comprising a DC / DC converter (12) which are jointly connected to a DC bus (DC+, DC-), and comprising a control device configured and provided to execute the method according to any one of the preceding claims. A system according to claim 10, wherein the first switch (Q1) is thermally coupled to at least one of the second, third, and / or fourth switches (Q2, Q3, Q4). A system according to claim 10 or 11, wherein the first switch (Q1) is thermally coupled to the inductor (D) as a heat storage element. System according to any one of claims 10 to 12, wherein the converter switches (Q1, Q2, Q3, Q4) and the choke (D) are thermally coupled to a common heat sink. System according to any one of claims 10 to 13, wherein the energy source (PV) comprises a PV generator. System according to one of claims 10 to 14, wherein the DC / DC converter (12) comprises a first sub-converter (14) and a second sub-converter (16), which are each connected to the DC bus (DC+, DC-) on the input side (14E+, 14E-, 16E+, 16E-) and are configured to transmit power flows of equal magnitude but opposite to each other when the system state is detected in a parallel-connected output-side state, so that the converter switches (Q1, Q2, Q3, Q4) of the first and second sub-converters (14, 16) are kept in a heated state without a total power transfer of the DC / DC converter (12).System according to claim 15, wherein the system is configured to bring about the parallel-connected state on the output side via a coupling switch (18) arranged between the output terminals (14A+, 14A-, 16A+, 16A-). Energy generation plant comprising a system (10) according to any one of claims 10 to 16 and an inverter connected to the DC bus (DC+, DC-) for feeding the power generated by the energy source (PV) into an AC grid.