Converter device and method for exchanging power

Abstract

A converter device (1) for exchanging power between a first electrical network (10) and a second electrical network (13) comprises: a first inverter (11) which can be connected to the first electrical network (10) on the AC side and to an intermediate circuit (14) of the converter device (1) on the DC side; a second inverter (12) which can be connected to the second electrical network (13) on the AC side and to the intermediate circuit (14) on the DC side; and a solar generator (15) directly connected to the intermediate circuit (14). A first controller (17) of the first inverter (11) is arranged to adjust the converter power of the first inverter (11), and a second controller (18) of the second inverter (13) is arranged to adjust the voltage of the intermediate circuit (14) such that the power of the solar generator (15) is maximized. A method of operating such a converter device is also described.

Description

Field of the Invention

[0001] The invention relates to a converter device for exchanging power between two electrical networks and a method for exchanging power. Background Art

[0002] It is known to exchange power between electrical networks which operate at different frequencies or asynchronously with respect to one another by means of frequency converters. For this purpose, the power to be transmitted is converted into a DC voltage by a first inverter or rectifier and fed into an intermediate circuit. A second inverter draws the power to be transmitted from the intermediate circuit and converts it into an AC voltage power which is compatible with the feed-in in the target electrical network. In order to ensure stable operating conditions, the power fed into the intermediate circuit must always correspond to the power drawn. Therefore, the inverters involved usually operate in a coordinated manner, for example by means of a common control device.

[0003] It is also known to connect a memory (such as a battery) to the intermediate circuit by means of a converter. In this case, the power fed into and drawn from the intermediate circuit must also be the same, wherein the difference between the power fed in by one inverter and the power drawn by the other inverter is compensated by the converter and used to charge / discharge the memory.

[0004] It is also known to generate DC voltage power by means of a solar generator and to feed this power into a connected AC voltage electrical network by means of an inverter. Here, the inverter can maximize the power generated by the solar generator by changing the DC voltage at the solar generator.

[0005] Document EP2 190 095A1 discloses an energy supply system which is connected to two different electrical networks, such as a railway electrical network and an AC current electrical network, by means of two inverters. The inverters are connected by means of a common DC intermediate circuit electrical network, and additionally, an energy generator is also connected to this DC intermediate circuit electrical network. It is not disclosed whether and how the output of the connected energy generator should be optimized. Summary of the Invention

[0006] The object of the invention is to expand a device for transmitting power between two AC voltage electrical networks which operate independently in principle with a solar generator and to upgrade the device to control the power generation of the solar generator and to selectively or also proportionally feed the generated power into these two electrical networks. In addition, the object of the invention is to clarify an operating method for such a device.

[0007] This task is solved by a converter device for exchanging power between a first electrical network and a second electrical network according to the invention and by a method for exchanging power between a first electrical network and a second electrical network by means of the converter device according to the invention. Preferred embodiments are described below.

[0008] In a first aspect of the invention, a converter device for exchanging power between a first electrical network and a second electrical network comprises: a first inverter which is connectable to the first electrical network on the AC side and to an intermediate circuit on the DC side; a second inverter which is connectable to the second electrical network on the AC side and to the intermediate circuit on the DC side. Thus, the intermediate circuit enables the transfer of power between the inverters and thus between the electrical networks. A solar generator is connected directly (i.e., without an intermediate connection of a converter) to the intermediate circuit. A first controller of the first inverter is provided for setting a pre-given converter power of the first inverter, and a second controller of the second inverter is provided for setting the voltage of the intermediate circuit such that the power of the solar generator is set as desired. Here, the set power can be the maximum power (MPP power, MPP = Maximal Power Point).

[0009] In this way, a converter device designed for exchanging power between electrical networks is upgraded without additional components or with a minimum number of additional components to flexibly supply the power that can be generated by a solar generator to one or both electrical networks. Here, the solar generator can be dimensioned with a rated power between 10% and 100% of the rated power of one of the inverters of the converter device. However, it is also conceivable that the solar generator is equipped with a rated power above 100% of the rated power of one of the inverters of the converter device, for example with 150% or in extreme cases up to 200%, so that the function of the power exchange between the electrical networks may then be impaired depending on the situation or the solar generator must be regulated down. Then, for example, it may no longer be possible to draw the desired power from one electrical network, but the PV power is fed into both electrical networks or the solar generator is regulated down by increasing the intermediate circuit voltage accordingly.

[0010] Since the solar generator is directly connected to the intermediate circuit, the second inverter can maximize the power generated by the solar generator by, for example, determining the power of the solar generator with the aid of a current sensor and varying the voltage such that it approximates the maximum power point MPP (Maximum PowerPoint) of the solar generator. Instead of determining the power of the solar generator, the second inverter can also merely determine the change in its own converter power that results from the change in the intermediate circuit voltage itself in order to approximate the MPP. In this way, no additional current sensor is required. For this purpose, the second inverter can use known MPP tracking algorithms. Depending on the power expectation value to which the first inverter is adjusted, different power flows and flow directions occur at the second inverter. Accordingly, the first power expectation value allows the power generated by the solar generator to be flexibly distributed to the two power grids. In parallel therewith, the desired power exchange between the two power grids can be effected within the framework of the converter capacity of the inverters involved.

[0011] Preferably, at least one of the power grids is implemented three-phase. Particularly preferably, both power grids are implemented three-phase. However, it is also conceivable that one or even both of the power grids are implemented single-phase.

[0012] In an advantageous embodiment, there is additionally a memory connected to the intermediate circuit via a converter. Thereby, the converter device according to the invention obtains further flexibility with respect to the power flow into or from the two power grids. The power generated by the solar generator (however, this power should not be fed into any of the power grids at the time of generation) or the power to be drawn from the power grid can be temporarily transferred to the memory to allow a later feed-in. It is also temporarily possible to increase the sum of the power fed into the two power grids to exceed the value of the power generated by the solar generator by discharging the memory.

[0013] In order to enable a flexible design of the power flow, the first inverter and the second inverter can be implemented bidirectionally.

[0014] In one configuration, the power expectation value of the inverter operating in a power regulation mode can be determined as a function of the power flowing through the inverter operating in a voltage regulation mode. For example, the power expectation value can be determined as a percentage of the power flowing through the inverter operating in a voltage regulation mode, for example in order to ensure a fixed percentage distribution of the power generated by the solar generator between the power grids. However, the adaptation of the power expectation value can also be effected only when the inverter operating in a voltage regulation mode is about to be overloaded.

[0015] Another aspect of the invention relates to a method for exchanging power between a first electrical network and a second electrical network by means of a converter device, the converter device comprising a first inverter connected to the first electrical network and a second inverter connected to the second electrical network, the first inverter and the second inverter being connected to each other via an intermediate circuit, wherein a solar generator is directly connected to the intermediate circuit. The method according to the invention comprises power regulation operation of the first inverter and intermediate circuit voltage regulation operation of the second inverter. Herein, the intermediate circuit voltage is regulated such that the power of the solar generator is maximized. Herein, the maximization of the solar generator power can also be carried out only temporarily, for example, operating in one of a plurality of operating modes. Herein, when the maximum converter power of the second inverter is reached, the second inverter will operate the solar generator in a reduced manner to avoid overload.

[0016] In an advantageous design of the invention, at least one of the inverters, preferably both inverters, provides reactive power, wherein the reactive power is preferably provided as a function of the voltage of the electrical network connected to the at least one inverter or the electrical network connected to the respective inverter. By this measure, the voltage of the electrical network connected to the at least one inverter or both electrical networks can be supported.

[0017] Correspondingly, it is also advantageous that the first inverter selects a power setpoint as a function of the frequency of the electrical network connected to the inverter, since the grid frequency represents an indicator of power surplus or power deficit in the electrical network, and the first inverter can compensate or at least reduce this grid frequency in this way.

[0018] It should not be left unmentioned at this point that if one or both electrical networks have excess energy that can be directly transferred to the solar generator by the inverter within a short period of time, the solar generator can also be used as a dump-load in another operating type, in particular, by raising the intermediate circuit voltage above the open-circuit voltage of the solar generator.

[0019] In one embodiment of the invention, for example within one operating mode, the roles of the power regulation operation and the voltage regulation operation can be interchanged between the inverters based on a switching criterion. Preferably, before the roles are interchanged, the solar generator can be placed at an operating point having a voltage that is higher than the MPP voltage of the solar generator. At this operating point, the power of the solar generator decreases as the voltage increases or increases as the voltage decreases. If now, during a transition phase in which both inverters operate with a fixed converter power (which corresponds to the converter power immediately before the start of the transition phase), then the change in the power of the solar generator caused by the irradiation change does not require adaptation of the expected values of the fixed converter powers of the two inverters, such that stable operation of the converter device is obtained within the transition phase. The start and end of the transition phase can be coordinated by communication between the inverters or the associated controllers. Preferably, the inverter operating in a voltage regulation manner initiates the role interchange. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In the following, the invention will be explained with the aid of the drawings, in which:

[0021] Figure 1 shows a first embodiment of a converter device according to the invention, and

[0022] Figure 2 shows a second embodiment of a converter device according to the invention. DETAILED DESCRIPTION

[0023] Figure 1 Shows an embodiment of a converter device 1 according to the invention, which converter device 1 has a first inverter 11 and a second inverter 12. The first inverter 11 is connected on the AC side via a transformer 19 to a first power grid 10, which is implemented here as single-phase. The second inverter 12 is connected on the AC side via another transformer 19 to a second (here three-phase) power grid 13. The transformer 19 serves to adapt the output voltages of the two inverters to the voltages of the respectively connected power grids. The two inverters are connected to each other on the DC side via an intermediate circuit 14. Additionally, a solar generator 15 is connected to the intermediate circuit 14. The solar generator 15 is connected directly (i.e., without intermediate connection of a converter), such that the intermediate circuit voltage corresponds to the solar generator voltage and the power generation of the solar generator 15 can be directly influenced by control of the intermediate circuit voltage. The solar generator 15 can preferably be separated from the intermediate circuit 14 by means of a disconnecting switch 16. If necessary, the solar generator 15 can be electrically separated from the intermediate circuit 14 by the disconnecting switch 16.

[0024] The first controller 17 controls the operation of the first inverter 11, while the second controller 18 controls the operation of the second inverter 12. However, it is also conceivable that both inverters are operated by a common controller.

[0025] The first controller 17 is configured to implement a power regulation operation of the first inverter 11, in which the first inverter 11 feeds a desired power into the first power grid 10 or draws the desired power from the first power grid 10. The desired power can be selected according to the frequency of the first power grid 10, for example, by storing a power-frequency characteristic curve in the first controller 17. Additionally, the first controller 17 can provide reactive power according to the voltage of the first power grid 10, for example, based on the stored reactive power-voltage characteristic curve. Both options are used to stabilize the first power grid 10.

[0026] The second controller 18 is configured to implement an intermediate circuit voltage regulation operation of the second inverter 12, in which the second inverter 12 regulates the power exchanged with the second power grid 13 such that the intermediate circuit voltage applied to the intermediate circuit 14 assumes a desired value. Within the framework of this regulation, both a power flow into the second power grid 13 and a power draw from the second power grid 13 can be generated. The level and direction of the power flow are influenced both by the currently used desired power value at the first inverter 11 and by the power currently generated by the solar generator 15.

[0027] The second controller 18 can also provide reactive power similarly to the first controller 17, the level and type of which are selected in particular as a function of the voltage of the second power grid 13, wherein the apparent power limits of both inverters must be observed separately to avoid inverter overload.

[0028] The second controller is also configured to select the desired value of the voltage of the intermediate circuit 14, at least in one operating mode, such that the power generated by the solar generator 15 is maximized. Known MPP tracking algorithms are suitable for achieving this goal, in which the solar generator voltage or the intermediate circuit voltage is changed stepwise or continuously, and the power change caused by the voltage change is analyzed to achieve a stable intermediate circuit voltage in order to derive a further voltage change therefrom and thus drive the solar generator voltage in the direction of the maximum power point or hold the maximum power point there.

[0029] Preferably, the two controllers are arranged to exchange control parameters with each other via the control line 22. These parameters may include the desired values and / or actual values of the electrical parameters of the respective inverters, such as power values, current values or voltage values, or may include specific control instructions. In this way, one of the controllers can influence the other controller, or both controllers can pursue a common regulation goal. However, the exchange of control parameters can also be carried out wirelessly, in particular via radio transmission.

[0030] The roles of the power regulation controller and the voltage regulation controller may not be permanently assigned to the first controller 17 or the second controller 18, but may be switched between the two controllers. This switching can be selected based on the operating conditions of the converter device 1, where switching criteria for the role switching are pre-given. The role switching is carried out when the switching criteria are met, for example, by communication between the two controllers, which is represented by the dashed line. This role switching can have a short transition phase, during which both controllers operate in a power regulation manner. At the start of this transition phase, the current converter power of the inverter operating in a voltage regulation manner is selected as the initial desired power for the power regulation operation. At the end of this transition phase, the original power regulation inverter is placed in voltage regulation operation.

[0031] Before the transition phase, the solar generator 15 can preferably be placed at an operating point where the solar generator voltage is higher than the current MPP voltage. Thereby, the power-voltage characteristic curve of the solar generator 15 cancels out the voltage variations caused by possible incomplete power balancing in the intermediate circuit during the transition phase. It is conceivable that the current MPP voltage of the solar generator 15 is exchanged together within the framework of the communication for the role switching. This allows the solar generator to immediately return to its MPP after the role switching is completed.

[0032] The assignment or switching criteria for the roles of the inverter operating in a power regulation manner or a voltage regulation manner can preferably be selected such that the maximum possible control reserve for the fluctuations of the power generated by the solar generator 15 and / or the maximum possible control reserve related to the grid stability response of the inverter are achieved. For example, the role of the inverter operating in a voltage regulation manner can preferably be assigned to the inverter with a larger rated power.

[0033] Both the first power grid 10 and the second power grid 13 can be implemented in single-phase or multi-phase, in particular three-phase. Between the two power grids, the number of phases can be implemented as the same or different. For example, the power grid 10 connected to the first inverter 11 can be implemented in three-phase, and the power grid 13 connected to the second inverter 12 can be implemented in single-phase.

[0034] InFigure 2 Figure 2 shows a second embodiment of the converter device 1 according to the invention. Relative to the Figure 1 embodiment in, the converter device 1 is supplemented with a memory 20, which is also connected to the intermediate circuit via a converter 21. The converter 21 is preferably implemented as a galvanic isolation converter in order to operate the memory 20 in a galvanically isolated manner from the intermediate circuit 14 for reasons of operational safety. Furthermore, in Figure 2 both power grids 10, 13 are implemented three-phase.

[0035] The converter 21 is preferably controlled such that a desired, at least temporarily constant exchange power is exchanged between the intermediate circuit 14 and the memory 20. In this way, the connection of the memory 20 to the intermediate circuit 14 does not affect or does not significantly affect the operation of the voltage-regulated inverter to find the maximum power operating point. However, it is also conceivable that the converter 21 is operated in a voltage-regulated manner temporarily, especially during the operating mode in which both inverters are operated in a power-regulated manner, for example within the framework of the above-mentioned transition phase. In this operating mode or phase, the converter 21 can keep the intermediate circuit voltage stable such that, for example, the solar generator remains in the MPP or its other operating point.

[0036] The operating methods described for the first embodiment of the converter device 1 according to the invention, in particular with regard to MPP tracking, role switching and reactive or active power provision for grid support, can also be used in the second embodiment of the converter device 1 according to the invention. Here, the memory 20 increases the possibilities for grid support in that it provides additional control reserves for the necessary short-term power requirements.

[0037] List of reference numerals

[0038] 1 Converter device

[0039] 10 Power grid

[0040] 11 Inverter

[0041] 12 Inverter

[0042] 13 Power grid

[0043] 14 Intermediate circuit

[0044] 15 Solar generator

[0045] 16 Disconnecting switch

[0046] 17 Controller

[0047] 18 Controller

[0048] 19 Transformer

[0049] 20 Memory

[0050] 21 Converter

[0051] 22 Control Pipeline

Claims

1. A converter device (1) for exchanging power between a first electrical grid (10) and a second electrical grid (13), the converter device comprising: - A bidirectional first inverter (11) which is connectable to the first electrical grid (10) on the AC side and to an intermediate circuit (14) of the converter device (1) on the DC side, - A bidirectional second inverter (12) which is connectable to the second electrical grid (13) on the AC side and to the intermediate circuit (14) on the DC side, - A solar generator (15) which is directly connected to the intermediate circuit (14), wherein the solar generator (15) is connected to the intermediate circuit (14) without an intermediate connection of a converter, wherein a first controller (17) of the first inverter (11) is provided for setting a predefined converter power of the first inverter (11), and a second controller (18) of the second inverter (12) is provided for setting the voltage of the intermediate circuit (14) such that the power of the solar generator (15) is maximized.

2. The converter device (1) according to claim 1, wherein, At least one of the electrical grids is implemented three-phase.

3. The converter device (1) according to claim 1 or 2, wherein, The first electrical grid (10) is implemented single-phase.

4. The converter device (1) according to claim 1 or 2, wherein, The first controller (17) and the second controller (18) are provided for exchanging control parameters with each other.

5. The converter device (1) according to claim 1 or 2, wherein, A memory (20) is connected to the intermediate circuit (14) via a converter (21).

6. The converter device (1) according to claim 1 or 2, wherein, The solar generator (15) has a rated power which is between 10% and 200% of the rated power of one of the first inverter (11) and the second inverter (12) of the converter device (1).

7. A method for exchanging power between a first electrical grid (10) and a second electrical grid (13) by means of a converter device (1), said converter device comprising a bidirectional first inverter (11) connected to the first electrical grid (10) and a bidirectional second inverter (12) connected to the second electrical grid (13), said first inverter and said second inverter being connected to one another via an intermediate circuit (14), wherein, The solar generator (15) is directly connected to the intermediate circuit (14), wherein the solar generator (15) is connected to the intermediate circuit (14) without an intermediate connection of a converter, wherein the method includes a power regulation operation of the first inverter (11) and an intermediate circuit voltage regulation operation of the second inverter (12), and the intermediate circuit voltage is regulated such that the power of the solar generator (15) is maximized.

8. The method according to claim 7, wherein When the maximum converter power of the second inverter (12) is reached, the second inverter (12) operates the solar generator (15) in a reduced manner.

9. The method according to claim 7 or 8, wherein At least one of the first inverter (11) and the second inverter (12) provides reactive power, wherein the reactive power is provided as a function of the voltage of the electrical grid connected to the at least one inverter.

10. The method according to claim 7 or 8, wherein The first inverter (11) selects the power as a function of the frequency of the first electrical grid (10) connected to the first inverter (11).

11. The method according to claim 7 or 8, wherein The roles of the power regulation operation and the voltage regulation operation are interchanged between the inverters based on a switching criterion.

12. The method according to claim 11, wherein, Before interchanging the roles, the solar generator (15) is placed in an operating point having a voltage higher than the MPP voltage of the solar generator (15).

13. The method according to claim 7 or 8, wherein, The power desired value of an inverter operating in a power regulation mode is pre-given as a function of the power flowing through an inverter operating in a voltage regulation mode.

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