METHOD FOR OPERATING AN ENERGY SUPPLY SYSTEM, DEVICE FOR EXCHANGING ELECTRICAL POWER IN AN ENERGY SUPPLY SYSTEM AND ENERGY SUPPLY SYSTEM

Description

Technical area

[0001] The application relates to the exchange of electrical power in an energy supply system with a DC network via which the electrical power is exchanged with DC consumers and which is at least partially supplied with electrical power from a DC source. State of the art

[0002] Direct current electrical networks, referred to as DC networks in the following, are increasingly being used to supply systems with a large number of consumers, for example so-called industrial DC networks. DC networks can be fed from an alternating current electrical network, referred to as the AC network in the following, e.g. from a higher-level energy supply network. As DC network participants, various DC consumers and DC sources can be connected to the DC network via suitable DC branches. DC consumers and DC sources are direct voltage units that consume (DC consumers) or supply (DC sources) electrical direct voltage. Examples of DC sources are DC energy generation units, e.g. PV systems, or batteries, which can also be operated either as DC sources or as DC consumers.Depending on which network device is to be connected to a DC feeder, a DC feeder can be protected by two-pole, fast-acting semiconductor switches to protect the line in the event of a short circuit or ground fault. DC fuses can also be provided additionally or alternatively.

[0003] In a DC grid, it may be necessary for safety reasons to connect certain DC units, i.e., certain DC sources such as power generation units and / or certain DC loads, to the DC grid in a galvanically isolated manner. This involves considerable effort when there are a large number of DC units that require individual galvanic isolation, for example, because galvanic isolation must be provided in each corresponding DC branch.

[0004] Photovoltaic systems (PV systems) as energy-generating DC sources or storage devices, such as batteries, as energy-absorbing and energy-emitting DC units can in principle also be connected to the DC grid via non-isolating DC / DC converters. For safety reasons, a suitable DC / DC converter should have leakage current detection and, if necessary, must determine the insulation resistance of the connected PV system.

[0005] To prevent the PV system and / or the storage unit or its cables from influencing the overall insulation resistance of the DC grid to ground potential, the PV generators or storage units can also be connected to the DC grid via isolating DC / DC converters. The isolating DC / DC converters provide galvanic isolation between the DC grid and the DC unit. In this case, each connected PV generator requires its own regular insulation monitoring.

[0006] In addition, galvanic isolation of a DC unit from the DC grid may be necessary for other reasons, e.g., if the maximum possible voltage of a connected PV generator against ground potential exceeds the permissible line-to-ground voltage of the DC grid. This may also be necessary if the PV system, if galvanically coupled to the DC grid, would create an impermissible asymmetry of the DC grid against ground potential, and / or if a leakage current limit of the DC grid would be regularly or sporadically exceeded due to a galvanically coupled PV system.

[0007] As a rule, the length of the DC cables and thus the distance of the DC units from the DC grid is also limited for technical reasons. However, a PV system as a DC unit, designed as a free-field or roof-mounted system, can be located correspondingly far away from a possible connection point to the DC grid, meaning that the PV system would have to be connected over long distances using suitable DC / PV cables. The same can apply to storage systems, particularly if they have a large capacity and, for example, have to be installed away from the DC grid of a production facility or similar for safety reasons. Connecting a DC unit in this way using DC cables can therefore be expensive and inefficient, particularly for DC units that have large and extensive PV generators and / or high-performance storage systems.

[0008] Document US 2017 / 353123 A1 discloses relevant prior art. Task

[0009] The application is based on the object of further improving the supply of a DC grid with electrical power from a DC source, and in particular of coupling PV systems or storage systems to a DC grid without influencing the DC grid's behavior with respect to earth. Solution

[0010] The object is achieved by a method having the features of independent claim 1. The object is achieved by a device having the features of independent claim 7. Advantageous embodiments of the method are claimed in the dependent claims. Description of the invention

[0011] A method for operating an energy supply system with at least one energy generation unit comprises the following steps: Exchanging electrical power between a first DC unit and an AC bus via a first power converter, wherein a first DC power of the first DC unit is converted into a first AC power via the first power converter and / or vice versa, Exchanging electrical power between a second DC unit and the AC bus via a second power converter, wherein a second DC power of the second DC unit is converted into a second AC power via the second power converter and / or vice versa, Combining the first and the second AC power to form an AC bus power of the AC bus, Transferring the AC bus power via a transformer, Exchanging electrical power between the AC bus and a DC grid via a third power converter, wherein the AC bus power transferred by the transformer is converted into a DC grid power via the third power converter and / or vice versa.

[0012] The first, the second and in particular the third power converters can be designed as bidirectional AC / DC converters that transfer electrical power from their AC side to their DC side and / or vice versa.

[0013] At least the first DC unit is designed as an energy generation unit, in particular as a PV system with at least one PV generator. A frequency and an amplitude of the AC voltage and / or the AC current on the AC bus are set by the method. In particular, the third power converter can stabilize the amplitude of the AC voltage by adjusting the DC grid power as a function of the AC bus power. The frequency on the AC bus can be adjusted by the third power converter in order to establish a power balance between the DC power of the DC units fed into the AC bus as AC power and the DC grid power discharged from the AC bus via the third power converter. In particular, the third power converter can use the set frequency on the AC bus to adjust the DC power of the DC units fed into the AC bus and thus the AC bus power according to the DC grid power.In addition, the third power converter can specifically change the DC grid power, creating a power imbalance in the AC bus, and modify the frequency in the AC bus. The DC units can adjust their respective DC power depending on the frequency in the AC bus. Through feedback between the frequency on the AC bus and the resulting DC grid power, a control loop can be created that offers largely automatic operation and the advantage of simple control engineering, for example, using appropriate P(f) characteristics, which specifically specify the dependence of the DC power of the DC units on the frequency in the AC bus.

[0014] Alternatively or additionally, f(P) control can be used in the third power converter. The third power converter reacts to any power imbalance with a frequency change, which in turn can cause a change in the DC power of individual DC units. For example, a change in irradiation on the PV generator can be compensated by increasing the power of another DC unit, such as a storage unit, before a response with a change in the DC grid power is required. This allows the power balance to be restored and the frequency in the AC bus to be adjusted to a setpoint.

[0015] In one embodiment, the first power converter operates the first DC unit at a maximum power operating point and converts the first DC power into a first AC current. The first power converter feeds the first AC current into the AC bus, with the first AC current following the AC voltage in the AC bus. Particularly for the operation of a PV system, it is useful to operate the PV generator or generators at the maximum power operating point, also known as the MPP, in order to maximize yield. The task of finding this maximum power operating point and operating the PV generators at this point can be performed by a so-called MPP tracker, which can be installed in the first power converter.

[0016] In one embodiment, the second DC power is generated, stored, and / or consumed by the second DC unit. The second DC power is converted into a second AC current by the second power converter and fed into the AC bus, with the second AC current following the AC voltage in the AC bus. The second DC unit can be, for example, another DC energy generation unit, e.g., another PV system, a DC consumer, such as an electrolyzer, or a DC storage device, e.g., a battery. The second power converter can be configured to operate the second DC unit at an adjustable operating point, for example, at the maximum power operating point of a PV generator or at an operating point with maximum efficiency of an electrolyzer; in the case of a storage device as the second DC unit, the power exchange with the storage device can be adjusted by the second power converter.

[0017] In one embodiment, the DC grid power is generated, stored, and / or consumed by DC grid participants connected to the DC grid. The DC grid power fed into the DC grid by the third power converter may differ from the sum of the powers of the DC grid participants. Any power imbalances in the DC grid can be compensated by connecting the DC grid to an AC grid, for example, to a higher-level energy supply network with which electrical power can be exchanged via a suitable grid exchange converter. This grid exchange converter can be responsible for the stable operation of the DC grid and have appropriate measurement and control technology as well as sufficient power reserves to operate the DC grid stably exclusively with electrical power from the AC grid, if necessary.

[0018] The safe operation of the DC grid, and in particular of the DC grid participants, is not compromised by the connection to the remote DC units. In particular, the galvanic isolation of the remote DC units decouples the potentials of the DC grid from any insulation and / or leakage current effects that may occur at the remote DC units. Overall, the present invention thus enables simple, cost-effective integration of storage units and local energy generation units into the power supply via a DC grid and proves particularly advantageous for use in industrial DC grids, which can, for example, supply a factory with electricity from renewable energy sources.

[0019] In one embodiment, the method comprises exchanging power between the AC bus and a further DC unit via a further power converter, wherein a further DC power of the further DC unit is converted into a further AC power via the further power converter and / or vice versa. The further DC power can be generated, stored and / or consumed by the further DC unit. The further DC unit can in particular comprise a further DC energy generation unit, e.g. a further PV system, a DC consumer, such as an electrolyzer, or a DC storage device, e.g. a battery. The further power converter can be configured to operate the further DC unit at an adjustable operating point.

[0020] The method offers the advantage that the frequency and / or amplitude of the AC voltage and / or the AC current of the AC bus can be selected relatively freely without being subject to the sometimes strict requirements of a public AC grid. Nevertheless, standard components such as contactors, fuses, monitoring, transformers, etc., which are also suitable for the conventional public AC supply grid, can be used. For example, it is possible to use a conventional transformer to galvanically isolate the AC bus from the DC grid. It is also possible to ground the transformer used on the AC bus side. In addition, particularly low-loss operation of the power converters is possible by adjusting the amplitude of the AC voltage on the AC bus, optionally on both sides of the transformer, in an optimal ratio to the amplitude of the DC voltage in the DC grid and optionally on the DC units.Furthermore, the adjustable parameters of the AC bus, in particular frequency, voltage, and / or reactive power, can be used to control and regulate electrical power transmission in the power supply system. In particular, by combining suitable characteristics that define the dependencies between the frequency on the AC bus and the exchanged power of the power converters involved, the power supply system can be operated stably and with reliably predictable performance.

[0021] A device for exchanging electrical power in an energy supply system having at least one energy generation unit has an AC bus to which a first power converter and a second power converter are connected, wherein the first power converter is designed to convert a first DC power from a first DC unit into first AC power and / or vice versa, and wherein the second power converter is designed to convert a second DC power from a second DC unit into second AC power and / or vice versa. The first and second power converters are connected on the AC side to an AC bus, such that the first and second AC power can be transmitted as AC bus power via the AC bus. The device has a transformer arranged between the AC bus and an AC side of a third power converter, wherein the transformer is designed to transmit the AC bus power and to effect galvanic isolation.The third power converter is configured to convert the AC bus power transmitted via the transformer into DC grid power and / or vice versa. A DC grid can be connected to a DC side of the third power converter, in which DC grid devices can be operated with the DC grid power. At least the first DC unit is designed as an energy generation unit, in particular as a PV system with at least one PV generator. The third power converter is configured to set a frequency and an amplitude of the AC voltage on the AC bus. In particular, a control unit of the third power converter can set the frequency on the AC bus as a function of the DC grid power and the DC grid power as a function of the AC bus power.

[0022] The device thus makes it possible to connect multiple DC units, in particular power generation units, storage units, and consumers, to an AC bus via respective power converters, e.g., DC / AC converters, and to exchange the electrical power collected by the DC units in the AC bus with the DC grid. The AC bus is galvanically isolated via the transformer and connected to the DC grid via the third power converter.

[0023] The frequency and / or amplitude of the AC voltage and / or current can be optimally configured on the AC bus. The optimal configuration can be based on the overall system design and, in particular, also depend on current conditions, e.g., the solar radiation on a PV system and the resulting currently available DC power. It is also conceivable to make the frequency and / or amplitude in the AC bus variably adjustable, at least within limits, and to operate the power converters depending on the current frequency and / or amplitude in the AC bus, particularly using f(P) and / or P(f) characteristics.

[0024] Thus, the DC grid, where the DC voltage can be essentially fixed, is connected to the AC bus via the third power converter and the transformer. Several DC sources and / or DC consumers are connected in parallel to the AC bus, which can be operated with an optimal – possibly also within limits variable – frequency and / or amplitude of the AC voltage and / or current, via respective power converters. They make their power available to the DC grid via the AC bus, largely avoiding any other impacts on the safe operation of the DC grid.

[0025] In one embodiment of the device, the first power converter is configured to operate the first DC unit at an operating point of maximum power and to convert the first DC power into a first AC current and feed it into the AC bus, wherein the first AC current follows the AC voltage in the AC bus.

[0026] In one embodiment of the device, a further DC unit can be connected to the AC bus via a further power converter, wherein the further power converter is configured to convert further DC power from the further DC unit into further AC power and / or vice versa. The second and / or the further DC unit can be configured to generate, store, and / or consume the respective DC power.

[0027] Optionally, the second DC unit can be configured as an additional power generation unit, in particular as an additional PV system with at least one additional PV generator. Optionally, the second power converter can then be configured to operate the second DC unit at a maximum power operating point and convert the second DC power into a second AC current and feed it into the AC bus, with the second AC current following the AC voltage in the AC bus.

[0028] In one embodiment of the device, the transformer is grounded on its side connected to the AC bus.

[0029] In one embodiment of the device, the control is at least partially implemented by the first, the second and / or the third power converter. The first, the second and / or the third power converter can in particular be implemented as a so-called active power converter. Active power converters in this case have a bridge circuit in which power semiconductor switches are actively controlled by a controller for electrical power conversion. The control of the power semiconductor switches and the control of the device can be implemented on the same hardware. Alternatively or additionally, the control of the device can be implemented at least partially outside the device. In such a case, a controller implemented outside the device preferably communicates with the device or the individual power converters via a communication interface.

[0030] An energy supply system can comprise such a device as described above. The energy supply system can further comprise a first DC unit, which is preferably designed as a PV system with at least one PV generator. A second DC unit of the device can also be designed as a PV generator or as an energy storage device. DC grid participants can be connected to a DC grid of the device for drawing DC grid power. A higher-level AC supply grid can be connected to the DC grid via a grid exchange converter for supplying the DC grid.

[0031] In one embodiment, the energy supply system comprises the controller configured to receive a setpoint for the DC grid power and to operate the first power converter, the second power converter, and / or the third power converter such that the DC grid power corresponds to the setpoint. A suitable setpoint may comprise an absolute value, for example, a power currently required to supply the DC grid, or a generic value, for example, retrieving the maximum available renewably generated power from the AC bus.

[0032] It is possible to use the controller to interact the DC units and the respective power converters, via which the DC units are connected to the AC bus, so that the AC bus as a whole can provide specific functions for the DC grid. For example, a so-called DC control power can be made available to the DC grid via the third power converter. This power is used to stabilize the voltage in the DC grid and is exchanged between the DC units and the DC grid via the respective power converters and the AC bus.

[0033] The device thus makes it possible to provide an AC bus for connecting a PV system. This bus can be operated separately from a public AC supply grid and can therefore be designed in such a way that, on the one hand, standard AC components such as fuses, inverters, transformers, residual current detection, load break switches, AC cables, etc. can be used. On the other hand, some requirements of conventional public AC supply grids do not necessarily have to be implemented, e.g., the normative specification of a target voltage or target frequency, but can be implemented if necessary. This can be used to achieve the simplest and most efficient design possible, whereby, in particular, losses along long DC lines and / or due to inefficient galvanically isolating DC / DC conversion, as well as any disruptive influences from potential shifts or leakage currents on the part of the energy generation units, can be avoided.

[0034] The device can save costs and minimize losses, especially in applications where several DC units, especially DC sources, are to be galvanically isolated and possibly connected to a DC grid over long distances, for example over several hundred meters.

[0035] The transformer for galvanic isolation can be arranged as close as possible to the third power converter in order, for example, to have as much of the cabling as possible galvanically isolated from the DC network. Figures

[0036] In the following, the invention is explained in more detail with the help of figures. Fig. 1 shows an example of an energy supply system, Fig. 2 shows an example of a method for operating an energy supply system; Fig. 3 shows schematically an embodiment of a power supply system and Fig. 4shows schematically another embodiment of an energy supply system.

[0037] In the figures, identical or similar elements are designated by the same reference numerals.

[0038] In Fig. 11 shows an exemplary energy supply system 10, which comprises a DC section 1, an AC supply network 2, and an energy generator 3. The energy supply system 10 serves in particular to supply DC network participants 40 in the DC section 1 with electrical power, which is provided in particular via a network exchange converter 36 from an AC supply network 2. The network exchange converter 36 can draw electrical power from the AC supply network 2 and feed it into a DC network 30 to which the DC network participants 40 are connected. For stable operation of the DC network 30, the network exchange converter 36 can have a suitable control system, which, for example, compensates for power imbalances in the DC network 30 by varying the network exchange power and thereby, in particular, keeps the DC voltage in the DC network 30 within a permissible range.

[0039] The energy generator 3 is located outside the DC section 1 and connected to the DC grid 30 via a connection 4. In order to feed electrical power from the energy generator 3 into the DC grid 30 via the connection 3, certain boundary conditions and guidelines must be observed depending on the specific design of the energy generator 3. In particular, galvanic isolation between the energy generator 3 and the DC grid 30 may be necessary for insulation and safety reasons. Depending on the performance of the energy generator 3, its output can provide part of the electrical power required in the DC grid 30 and / or supply the DC grid 30 completely with electrical power, at least temporarily; any excess power from the energy generator 3 can be fed into the AC supply grid 2 as surplus power via the grid exchange converter 36 and / or (temporarily) stored within the energy supply system 10.

[0040] In Fig. 2 A method for operating an energy supply system 10 is shown schematically. A concrete energy supply system 10 with two DC energy generation units as DC units, e.g. two PV systems 22, which form the energy generator 3 and whose power is to reach the DC section 1 via the connection 4, is shown by way of example in Fig. 2 or Fig. 3 The procedure comprises the following steps: Step S1: Exchanging power between a first PV system 22 and an AC bus 20 via a first power converter 12, wherein a first DC power of the first PV system 22 is converted into a first AC power via the first power converter 12 and / or vice versa. Step S2: Exchanging power between a second DC unit 22, 26, 28 and the AC bus 20 via a second power converter 14, 16, 18, wherein a second DC power of the second DC unit 22, 26, 28 is converted into a second AC power via the second power converter 14, 16, 18 and / or vice versa. The second DC unit can be Fig. 2 a second PV system 22. For the example of Fig. 3the second DC unit can be, for example, the second PV system 22, a battery 26 or a consumer 28, such as, for example, an electrolyzer. Further DC units can be, for example, the second or a further PV system 22, the battery 26 or a further battery 26 and / or a consumer 28 and / or a further consumer 28. Step S3: Combining the first and the second AC power to form an AC bus power of the AC bus 20. Step S4: Transferring the AC bus power via a transformer 32. Step S5: Exchanging power between the AC bus 20 and a DC grid 30 via a third power converter 34, wherein the AC bus power transferred by the transformer 32 is converted into a DC grid power via the third power converter 34 and / or vice versa.

[0041] At least the first DC unit 22 is designed as a power generation unit, in particular as a PV system 22 with at least one PV generator 24. A frequency and / or an amplitude of the AC voltage and / or the AC current is set on the AC bus 32, with the third power converter 34 adjusting the DC grid power depending on the AC bus power. The DC grid power can be predetermined, and the third power converter ensures a balanced power inflow and outflow in the AC bus. The method is executed, for example, by a controller that can be part of the energy supply system 10.

[0042] In Fig. 31 schematically shows an embodiment of an energy supply system 10 with two PV systems 22. Each of the PV systems 22 has one or more PV generators 24. The DC power generated by the PV systems 22 is converted into AC power for each of the PV systems 22 by a power converter 12, 14, which can be designed as an inverter. The AC power of the power converters 12, 14 is combined on the AC bus 20 as AC bus power. In addition to the two PV systems 22, further DC units can be connected to the AC bus 20 via further power converters. The AC power on the AC bus 20 is transmitted to the third power converter 34 via a transformer 32.The AC power transmitted via transformer 32 is converted by the third power converter 34, which can be configured as an active rectifier, into DC grid power, which is exchanged with the DC grid 30 and is thus available to DC grid participants 40. The DC grid 30 can be, for example, an industrial DC grid that can exchange power with a public AC supply grid 2 via a grid exchange converter 36. For this purpose, the grid exchange converter 36 can be connected on the AC side to the AC supply grid 2 via a transformer, in particular via a grounded medium-voltage transformer 38.

[0043] Such a design of the energy supply system 10 allows a

[0044] To connect energy generators 3 with multiple DC units 22 to the DC grid 30 without having to design the individual power converters 12, 14 with galvanic isolation. The galvanic isolation of the PV systems 22 from the DC grid 30 can be achieved, for example, via the transformer 32. This allows for a more cost-effective implementation of a standard-compliant and safe energy supply system 10, since the galvanic isolation of the transformer 32 allows multiple DC units 22 to be galvanically isolated from the DC grid 30 and to be operated and protected in a coordinated manner.

[0045] In Fig. 41 shows a further exemplary embodiment of an energy supply system 10. In addition to two PV systems 22, which are connected to an AC bus 20 via a first and a second power converter 12, 14, a storage device 26 for electrical energy is provided, which is designed, for example, as a battery and can store and release electrical energy. The storage device 26 is connected to the AC bus 20 via the power converter 16 and can absorb electrical DC power during charging and release it during discharging via the power converter 16. The power converter 16 is configured to convert the DC power of the storage device 26 into AC power and to exchange it with the AC bus 20. A DC consumer 28, e.g., an electrolyzer, is also provided in the energy supply system 10. The DC consumer 28 can be supplied with electrical power from the AC bus 20 via the power converter 28.

[0046] On the AC bus 20, the electrical power exchanged by the power converters 12, 14, 16, and 18 with the AC bus is combined into an AC bus power and transferred via the transformer 32 to the third power converter 34. Via the third power converter 34, electrical power is exchanged with the DC grid 30, to which DC grid participants 40 can be connected. The DC grid participants 40 can be, for example, DC consumers or DC generators. Power exchange between the DC grid 40 and the AC supply grid 2 is also possible via the grid exchange converter 36 and, if applicable, the medium-voltage transformer 38.

[0047] The transformer 32 galvanically isolates the DC units 22, 26, 28 from the DC network 30. The galvanic isolation of the transformer 32 is sufficient, in particular, for a large distance between the DC network 30 and the energy generator 3 as well as for a large number of DC units 22, 26, 28, so that the power converters 12, 14, 16, 18 individually assigned to the DC units 22, 26, 28 can be designed without galvanic isolation.

[0048] On the AC bus 20, the AC voltage, e.g., the amplitude and / or frequency of the AC voltage, can be specified within certain limits and adjusted accordingly. These values ​​can be selected largely independently of the values ​​required by a public grid, such as AC supply grid 2.

[0049] In one possible system design, the DC voltage in the DC grid 30 can be predetermined, e.g., by specifications for an industrial DC grid. The DC voltage output by the PV systems 22 is generally predetermined by the design of the PV generators 24 and also varies due to the fluctuating solar radiation. The operating voltages of the battery and / or electrolyzer can be predetermined by the units' design. The disclosed energy supply system 10 offers a number of degrees of freedom for coupling these generally different operating voltages, i.e., the voltages in the energy generator 2 and the voltage in the DC grid.Examples of parameters that can be optimized in the system design in order to set a desired power flow between energy generator 2 and DC grid 1 as well as AC supply grid 3 with as little loss and as much efficiency as possible are the amplitude and / or frequency of the AC voltage on the AC bus 20 and the transformation ratio of the transformer 32.

[0050] A control of an energy supply system 10 according to Fig. 3 or Fig. 4 is described below.

[0051] The DC voltage in the DC bus 30 is initially assumed to be given. The third power converter 34 is designed as a power electronic bridge circuit that operates in a clocked mode, whereby the specific clocking sets a frequency of the AC-side voltage as well as a transformation ratio of the AC-side voltage relative to the DC-side voltage. The third power converter 34 thus determines the frequency and amplitude of the voltage in the AC bus 20 according to Fig. 3 and Fig. 4 The frequency can be fixed, e.g., set at 50 Hz or 74.1 Hz, or variable within an adjustable range, e.g., within a permissible frequency band around a nominal frequency of e.g., 60 Hz or 41.3 Hz.

[0052] The voltage of the AC bus 20, which is thus predetermined in terms of frequency and amplitude, is detected by the power converters 12, 14, 16, 18 on the AC bus 20 and the power converters 12, 14, 16, 18 synchronize their feed with this AC voltage, ie they impress an AC current into the essentially given AC voltage in the AC bus 20.

[0053] In the embodiment according to Fig. 3the third power converter 34 can additionally be controlled such that the PV generators 24 are operated at an operating point of maximum power. The power converters 12, 14 can be operated with a fixed voltage transformation ratio, for example with a largely fixed clock speed with optimal efficiency. A change in the AC voltage in the AC bus 20 caused by the third power converter 34 is thus passed on by the power converters 12, 14 directly to the PV generators 24 and causes a change in the operating point of the PV generators 24, i.e. the DC voltage of the PV generators 24 rises and falls with the set AC voltage in the AC bus 20. The third power converter 34 can now detect the currently converted electrical power as a function of the AC voltage and increase or decrease the AC voltage in order to maximize the power.This establishes the voltage in the AC bus 20 at which the PV generators 24 deliver their maximum power, taking into account the transformation ratios of the power converters 12, 14. Should it be necessary or desired to reduce the power fed into the DC grid 30 via the third power converter 34, for example, if there is an excess of DC power in the DC grid 30, the AC voltage in the AC bus 20 can be increased or decreased by the third power converter 34 to shift the operating point of the PV generators 24 toward open-circuit or short-circuit, respectively, and thus reduce the power compared to the operating point with maximum power.

[0054] An embodiment of the control of the energy supply system 10, which is particularly applicable to the energy supply system 10 according to Fig. 4applicable, can efficiently utilize the energy storage device 26 on the AC bus 20. The amplitude and frequency of the AC voltage in the AC bus 20 are initially set by the third power converter 34. Setpoints for the amplitude and frequency of the AC voltage in the AC bus 20 are specified for all participating power converters 12, 16, 34. Each power converter 12, 16, 34 has a specific P(f) or f(P) characteristic (compare Fig. 5 ), which specifies the power P to be converted depending on the current frequency f in the AC bus 20 or the frequency to be set depending on the current DC grid power.

[0055] Specifically, characteristic curves P12, P14, P34 can be provided for the power converters 12, 14, 34 and, if necessary, characteristic curves P16, P18 for the power converters 16, 18. An embodiment of such characteristic curves is shown in Fig. 5 shown as an example. In Fig. 5the horizontal axis shows the frequency f in the AC bus 20 and the vertical axis the power flow P in the power supply system 10, where positive values ​​mean a power flow P towards the DC grid 30 and vice versa.

[0056] In this example, the characteristic curve P34 of the third power converter has a constant gradient and intersects the frequency axis at a frequency f1. At a frequency above a threshold f1, the third power converter 34 can supply power from the AC bus 20 to the DC grid 30 (rectifier operation) and at a frequency below the threshold f1, it can feed power from the DC grid 30 into the AC bus 20 (inverter operation). A specific specification for a power supply by the DC grid 30 or a withdrawal from the DC grid 30 can be made, in particular, at a higher level, for example by striving for maximum utilization of energy generated from renewable sources and / or by providing control power based on the stability requirements of the DC grid 30. For a specific amount of power exchanged via it, the power converter 34 uses the f(P) characteristic curve to determine the frequency for the AC bus and then adjusts it.

[0057] The power converters 12, 14 of the PV systems 22 can generally adjust the respective operating point of the PV generators 24 to their maximum power operating point (so-called maximum power point tracking, or MPP tracking for short). In the respective characteristic curves P12, P14, this occurs at frequencies below a threshold f2, so that the maximum possible PV power is fed into the AC bus 20 as long as the frequency is below the threshold f2. Above the frequency f2, the power output is reduced and reaches a value of zero at a threshold f3.

[0058] The power converter 16 of the storage 26 operates according to a P(f) characteristic curve P16(f) which has a threshold f4 at which the storage 26 does not exchange any power. Below the threshold f4 the storage 26 is discharged and above the threshold f4 the storage 26 is charged.

[0059] The frequencies f2, f3, f4 on the characteristic curves P12, P14, P16 are selected here such that the PV systems 22 can always deliver power - either via the AC bus 20 and the third power converter 34 into the DC grid 30 or via the AC bus 20 and the power converter 16 into the storage 26. In particular, at the frequency f4, the maximum possible PV power is transferred to the DC grid 30, whereby no power is exchanged with the storage 26.

[0060] Between the thresholds f2 and f3, the DC grid power according to characteristic curve P34 is lower than the maximum possible PV power, which is nevertheless retrieved and transferred partly via the third power converter 34 into the DC grid 30 and partly via the power converter 16 into the storage 26.

[0061] At threshold f1, no power transfer takes place via the third power converter 34, and the PV power flows almost entirely into the storage unit 30. Depending on the power capacity of the storage unit 26 and the current irradiation on the PV systems 22, it may be necessary to reduce the PV power compared to the MPP power according to characteristic curve P12, P14.

[0062] The characteristic curves can be selected so that the storage unit 26 can be charged even at a time when no PV power is available. In particular, at threshold f3, the third power converter 34 can specify a DC grid power based on the characteristic curve P34, which flows from the DC grid 30 via the third power converter 34 into the AC bus and from there is fed into the storage unit 26 via the power converter 16 according to the characteristic curve P16.

[0063] An optional additional load arranged in the energy supply system can be connected via a power converter 18, which has a characteristic curve P18 with the threshold f5. Below the threshold f5, the power converter 18 draws no power from the AC bus 20, i.e., the power flow according to characteristic curve P34 is supplied by the storage unit 26 and / or the PV systems 22. Above f5, the electrolyzer 28 is operated with a power according to characteristic curve P18 and draws a corresponding power from the AC bus 20, which, if necessary, in sum with the DC power according to characteristic curve P16 from the storage unit 26, results in the DC grid power according to characteristic curve P34.

[0064] Due to time-varying influences, the individual characteristic curves P12, P14, P34 and P16 and possibly P18 may change and / or specified power levels may not be implementable at certain frequencies, for example due to varying irradiation on the PV systems 22 or a storage system 26 that is too full or too empty. This can be taken into account by the method according to the application in that the third power converter 34 dynamically adapts the characteristic curve P34 if setting a specific frequency results in a power flow that deviates from the characteristic curve P34. For example, the gradient of the characteristic curve P34 may be greater the higher the instantaneous MPP power of the PV systems 22. Alternatively or additionally, the position of the thresholds f1-f5 can be adjusted, for example by having the thresholds f1 and f3 coincide when the storage system 26 is not available, e.g. because it is fully charged.

[0065] The frequencies f1, f2, f3, f4, and f5 can be coordinated to achieve stable and cost-efficient operation with a predictable power flow between the AC bus 20 and the DC grid 30, without requiring higher-level control of the AC bus 20 or the power converters connected to it. It is also possible to integrate additional DC units into the AC bus 20, whereby the number of thresholds P(fn) can be increased accordingly to operate the additional DC units efficiently and in a coordinated manner as needed. The frequency thresholds f1, f2, and f(n) can be selected to accommodate standardized protection technology and other components commonly found in AC grids.

[0066] The transformation ratio of transformer 32 can be selected so that all DC units 22, 26, 28 can exchange power with the AC bus 20. It must be taken into account that single-stage power converters can generally transfer power from a higher DC voltage to a lower AC voltage and vice versa, i.e., from a lower AC voltage to a higher DC voltage. For galvanic isolation, transformer 32 can be arranged as close as possible to the third power converter 34 or integrated into it, so that any extensive cabling of the AC bus 20 is galvanically isolated from the DC network 30. List of reference symbols

[0067] 10Energy supply system 12, 14, 16, 18Power converter 20AC bus 22PV system 24PV generator 26Battery 28Consumer 30DC grid 32Transformer 34Power converter 36Grid exchange converter 38Medium-voltage transformer 40DC grid participant PEPotential Earth S1, S2, S3, S4, S5Process steps

Claims

1. A method for operating an energy supply system (10) having at least one energy generation unit, comprising the steps of: • exchanging power between a first DC unit (22) and an AC bus (20) by means of a first power converter (12), wherein a first DC power of the first DC unit (22) is converted into a first AC power by means of the first power converter (12) and / or vice versa, • exchanging power between a second DC unit (22, 26, 28) and the AC bus (20) by means of a second power converter (14, 16, 18), wherein a second DC power of the second DC unit (22, 26, 28) is converted into a second AC power by means of the second power converter (14, 16, 18) and / or vice versa, • combining the first AC power and the second AC power into an AC bus power of the AC bus (20), • transmitting the AC bus power by means of a transformer (32), • exchanging power between the AC bus (20) and a DC grid (30) by means of a third power converter (34), wherein the AC bus power transmitted by the transformer (32) is converted by means of the third power converter (34) into a DC grid power and / or vice versa, • wherein at least the first DC unit (22) is formed as an energy generation unit, more particularly a PV system (22) comprising at least one PV generator (24), • wherein a frequency and an amplitude of the AC voltage and / or of the AC current on the AC bus (20) are set, characterised in that the third power converter (34) sets the frequency on the AC bus (20) as a function of the DC grid power and, for setting the AC voltage and / or the AC current, sets the DC grid power as a function of the AC bus power.

2. The method according to claim 1, wherein the first power converter (12) operates the first DC unit (22) at an operating point of maximum power and converts the first DC power into a first AC current and feeds it into the AC bus (20), wherein the first AC current follows the AC voltage in the AC bus (20).

3. The method according to claim 1 or 2, wherein the second DC power is generated, stored and / or consumed by the second DC unit (22, 26, 28), wherein the second DC power is converted by the second power converter (14, 16, 18) into a second AC current and fed into the AC bus (20), wherein the second AC current follows the AC voltage in the AC bus (20).

4. The method according to any one of the preceding claims, comprising the step of: exchanging power between the AC bus (20) and a further DC unit (26, 28) by means of a further power converter (16, 18), wherein a further DC power of the further DC unit (26, 28) is converted into a further AC power by means of the further power converter (16, 18) and / or vice versa.

5. The method according to any one of the preceding claims wherein the DC grid power is generated, stored and / or consumed by DC grid subscribers (40) connected to the DC grid (30), and / or wherein the further DC power is generated, stored and / or consumed by the further DC unit (26, 28).

6. The method according to any one of the preceding claims, wherein the frequency on the AC bus (20) is set by the third power converter (34) using an f(P) characteristic (P34) as a function of the DC grid power.

7. The method according to any one of the preceding claims, wherein the DC power of the DC units (22, 26, 28) is set by the respective power converter (12, 14, 16, 18) using a P(f) characteristic (P12, P14, P16, P18) as a function of the frequency on the AC bus (20).

8. A device for exchanging electrical power in an energy supply system having at least one energy generation unit, comprising: an AC bus (20) to which a first power converter (12) and a second power converter (14, 16, 18) are connected, wherein the first power converter (12) is designed to convert a first DC power of a first DC unit (22) into first AC power and / or vice versa, and wherein the second power converter (14, 16, 18) is designed to convert a second DC power of a second DC unit (22, 26, 28) into second AC power and / or vice versa, and wherein the first and second power converter are each connected on the AC side to an AC bus (20) so that the first and second AC power are transmittable as AC bus power via the AC bus (20), wherein the device has a transformer (32) arranged between the AC bus (20) and an AC side of a third power converter (34), wherein the transformer (32) is configured to transmit the AC bus power and to provide galvanic isolation, wherein the third power converter (34) is configured to convert the transmitted AC bus power into DC grid power and / or vice versa, and wherein, on a DC side of the third power converter (34), a DC grid (30) can be connected in which DC grid users (40) can be operated with the DC grid power, wherein at least the first DC unit (22) is formed as an energy generation unit, more particularly a PV system (22) having at least one PV generator (24), characterized that the third power converter (34) is designed to set a frequency and an amplitude of the AC voltage and / or the AC current on the AC bus (20), wherein in particular a control unit of the third power converter (34) sets the frequency on the AC bus (20) as a function of the DC grid power and sets the DC grid power as a function of the AC bus power.

9. The device according to claim 8, wherein the first power converter (12) is designed to operate the first DC unit (22) at an operating point of maximum power and to convert the first DC power into a first AC current and to feed it into the AC bus (20), wherein the first AC current follows the AC voltage in the AC bus (20).

10. The device according to claim 8 or 9, wherein a further DC unit (22, 26, 28) can be connected to the AC bus (20) via a further power converter (14, 16, 18), wherein the further power converter (14, 16, 18) is designed to convert a further DC power of the further DC unit (22, 26, 28) into a further AC power and / or vice versa.

11. The device according to any one of claims 8 to 10, wherein the second and / or the further DC unit (22, 26, 28) are configured to generate, store and / or consume the respective DC power.

12. The device according to any one of claims 8 to 11, wherein the transformer (32) is grounded on its side connected to the AC bus (20).

13. The device according to any one of claims 8 to 12, wherein the controller is at least partially implemented by the first, the second and / or the third power converter (12, 14, 16, 18, 34).

14. An energy supply system (10) comprising a device according to any one of claims 8 to 13, having: - a first DC unit (22) which is formed as a PV system (22) having at least one PV generator (24), - a second DC unit (22, 26) which is formed as a PV system (22) having at least one PV generator (24) or formed as an energy store (26), and - a DC grid (30) to which DC grid users (40) can be connected for drawing DC grid power and a superordinate AC supply grid for supplying the DC grid (30).

15. The energy supply system (10) according to claim 14, having the controller, which is configured to receive a setpoint for the DC grid power and to operate the first power converter (12), the second power converter (12, 14, 16, 18) and / or the third power converter (34) such that the DC grid power corresponds to the setpoint.