TRANSFER UNIT, SYSTEM AND METHOD FOR PERFORMING AN INTRA-BATTERY EQUALIZATION PROCESS

Abstract

The application describes a transfer circuit or system with a first DC bus for connecting a plurality of battery racks to an inverter bridge for a first power exchange with an AC grid. The transfer circuit or system has a second DC bus which is connected to the first DC bus via a DC/DC converter. The transfer circuit or system is arranged to disconnect at least one battery rack of the plurality of battery racks from the first DC bus and to connect it to the second DC bus for performing an intra-battery equalization process. The application also describes a system with a transfer unit and a method for performing an intra-battery equalization process.

Claims

1. A transfer circuit or system comprising: a first DC bus configured to connect a plurality of battery racks to an inverter bridge for a first power exchange with an AC network; a second DC bus connected to the first DC bus via a DC/DC converter, the DC/DC converter having a rated power at least a factor of 5 lower than a rated power of the inverter bridge; and a control circuit configured to disconnect at least one battery rack of the plurality of battery racks from the first DC bus via a switching circuit in order to carry out an intra-battery equalization process, and to connect the at least one battery rack to the second DC bus, wherein the control circuit, after connecting the at least one battery rack to the second DC bus, is configured to initiate the intra-battery equalization process for the at least one battery rack connected to the second DC bus, wherein during or after, or both during and after, the intra-battery equalization process a second power exchange takes place between the first DC bus and the second DC bus via the DC/DC converter in response to a command from the control circuit, in order to achieve a balancing of a state of charge of the at least one battery rack connected to the second DC bus with a state of charge of the battery racks connected to the first DC bus, and wherein the control circuit, after the balancing of the state of charge, is configured to disconnect the at least one battery rack from the second DC bus via a switching circuit and connect the at least one battery rack to the first DC bus.

2. The transfer circuit or system according to claim 1, wherein the control circuit is configured to determine or receive a notification of a need of the at least one battery rack to perform the intra-battery equalization process.

3. A system comprising a plurality of battery racks and a transfer circuit or system according to claim 1, wherein the plurality of battery racks are converter-free battery racks and are connectable to the inverter bridge via the first DC bus for the first power exchange with the AC grid.

4. A method for performing an intra-battery equalization process in which a state of charge of cell sections of at least one battery rack in a system according to claim 3 is balanced, comprising: disconnecting the at least one battery rack from the first DC bus and connecting the at least one battery rack to the second DC bus, initiating the intra-battery equalization process with the at least one battery rack after connecting the at least one battery rack to the second DC bus, during or after the intra-battery equalization process, initiating a second power exchange between the first DC bus and the second DC bus via the DC/DC converter, in order to achieve a balancing of a state of charge of the at least one battery rack with a state of charge of the battery racks connected to the first DC bus, after the balancing of the state of charge, disconnecting the at least one battery rack from the second DC bus and connecting the at least one battery rack to the first DC bus.

5. The method according to claim 4, wherein the acts thereof are performed successively for each of the plurality of battery racks.

6. The method according to claim 4, wherein the second power exchange occurs at least temporarily during the intra-battery equalization process.

7. The method according to claim 4, wherein the second power exchange occurs at least intermittently such that the first power exchange of the system with an AC grid is supported.

8. The method according to claim 6, wherein the second power exchange is initiated based on the first power exchange via the inverter bridge, a completion of the balancing of the state of charge at a time of the completion of the intra-battery equalization process, or as soon as possible thereafter is enabled.

9. The method of claim 6, wherein the second power exchange is initiated at a time with regard to a completion time of the intra-battery equalization process such that a completion of a balancing of a state of charge due to the intra-battery equalization process is synchronized with a completion of the second power exchange.

10. The method according to claim 4, wherein the at least one battery rack independently detects a need to perform the intra-battery equalization process and communicates the detected need to a transfer circuit.

11. The method according to claim 10, wherein the communicated detected need comprises information about an urgency of performing the intra-battery equalization process.

12. The method according to claim 4, wherein the control unit of the transfer unit is configured to detect a suitable time for carrying out the intra-battery equalization process and initiates the intra-battery equalization process at this time.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0031] In the following, the disclosure is illustrated with the aid of figures, of which

[0032] FIG. 1 shows an example of a system with a transfer circuit or system, and

[0033] FIG. 2 shows a method for performing an intra-battery equalization process, for example, using the transfer circuit or system of FIG. 1.

DETAILED DESCRIPTION

[0034] FIG. 1 shows an embodiment of a battery inverter system with a transfer circuit or system 12. The transfer circuit or system 12 has a first DC bus 7 and a second DC bus 8. FIG. 1 shows one conductor of each of the two conductors of each DC bus. It is possible that the first DC bus 7 and the second DC bus 8 share a common second conductor (not shown). The transfer circuit or system 12 comprises a DC/DC converter 6 connected between and arranged to transfer power between the first DC bus 7 and the second DC bus 8. The power transfer via the DC/DC converter 6 can be bidirectional. A plurality of battery racks 1 can be connected to the transfer circuit or system 12 via a switching circuit 5 comprising a plurality of switches. The battery racks 1 are connected in parallel to each other via the first DC bus 7. Each battery rack 1 has a plurality of cell sections 2 and optionally a battery management circuit or system 3. The cell sections 2 of a battery rack 1 are connected in series. The battery management circuit or system 3 manages the cell sections 2 and controls, for example, charging and/or discharging processes. Cell sections 2 may comprise several cells, which may be connected in series and/or parallel within the cell section 2.

[0035] An inverter bridge circuit 9 can be connected to the first DC bus 7 of the transfer circuit or system 12, via which electrical power can be exchanged with an AC network 10 such as an AC grid. The inverter bridge 9 can be integrated in an inverter which can be operated in grid-forming and/or grid-operated mode. The transfer circuit or system 12 may be disconnected from the inverter bridge 9 by a disconnect switch 13. The power exchange of the transfer circuit or system 12 with the AC grid 10 via the inverter bridge 9 can be bidirectional, so that the battery racks 1 connected to the transfer circuit or switch 12 can be charged with electrical power from the AC grid 10 via the inverter bridge 9 and/or can feed electrical power into the AC grid 10 via the inverter bridge 9.

[0036] There is a communication link between the battery management circuits or systems 3, the switches of the switching circuit 5, the DC/DC converter 6, an optional control circuit 11, the disconnector switch 13 and the inverter bridge. The communication link can, for example, be a communication bus 4 as shown in FIG. 1. Other communication connections, wired and/or wireless, are possible.

[0037] During operation of battery racks 1, differences in the state of charge between the cells or between the cell sections 2 of a battery rack 1 may occur, for example, due to cyclical charging and discharging of the cells. This may require an equalization process in which the states of charge of the cells are balanced again. Such equalization processes can take place approximately once a week, for example. Otherwise, the estimation of the currently available energy in the form of the battery state of charge could become increasingly inaccurate over time and make further operation difficult or only allow it with uncertainties about availability. In addition, there could be a risk of premature ageing of cells or cell sections 2.

[0038] In one embodiment, the battery management circuits or systems 3 may be configured to detect and/or perform an equalizing demand of the associated battery rack 1. Based on the detected need, an affected battery rack 1 can be disconnected from the first DC bus 7 by the switching circuit 5 and connected to the second DC bus 8. The battery management circuit or system 3 can start and perform the intra-battery equalization process within the affected battery rack 1. In this process, the states of charge of the individual cell sections 2 are balanced with each other. During the equalization process and/or afterwards, a power exchange with the first DC bus 7 can take place via the DC/DC converter 6 in order to balance the state of charge of the affected battery rack 1 connected to the second DC bus 8 with the state of charge of those battery racks 1 connected to the first DC bus 7 as well as to supply any energy required for the balancing process. In one embodiment, the nominal power of the DC/DC converter 6 can be considerably lower than the nominal power of the inverter bridge 9 and, for example, only up to 20% of the same. After the balancing process and the power exchange process have been carried out, the affected battery rack 1 can be disconnected again from the second DC bus 8 via the switching circuit 5 and connected to the first DC bus 7.

[0039] The optional control circuit 11 can be provided to receive an equalization demand signal in response to such demand being detected by the battery management systems 3. The equalization demand signal is received via the communication bus 4 and in turn disconnects the affected battery rack 1 from the first DC bus 7 via the switching circuit 5 and connects the affected battery rack 1 to the second DC bus 8. The equalization process and the balancing process can also be coordinated with each other via the control circuit 11. For this purpose, the control circuit 11 can be connected to the switching circuit 5 and the DC/DC converter 6 via the communication bus 4 and control them. After the equalization process and the balancing process have been carried out, the control circuit 11 can cause affected the battery rack 1 to be disconnected again from the second DC bus 8 via the switching circuit 5 and to be re-connected to the first DC bus 7. Optionally, the inverter bridge 9 and/or the disconnector switch 13 can also be connected to the communication bus 4.

[0040] At the end of the equalization process, the affected battery rack 1 may have a state of charge that differs from the state of charge of the other battery racks 1. A direct connection of this battery rack 1 to the first DC bus 7 with the other battery racks 1 should therefore be avoided because of the equalization currents that then occur. It is therefore advantageous in one embodiment to balance the state of charge of the affected battery rack 1 to the state of charge of the other battery racks 1 in a balancing process via a power exchange via the DC/DC converter 6 before it is disconnected again from the second DC bus 8 and re-connected to the first DC bus 7 in parallel with the other battery racks 1. Such a balancing process concerns the balancing of the state of charge between different battery racks 1, i.e. the DC voltages.

[0041] FIG. 2 shows an example of a method for carrying out an intra-battery equalization process in which the state of charge of cell sections 2 of at least one battery rack 1 can be balanced, for example, in a system shown in FIG. 1. In 51, the at least one battery rack 1 is disconnected from the first DC bus 7 and is connected to the second DC bus 8. After connecting the at least one battery rack 1 to the second DC bus 8, the intra-battery equalization process is initiated in S2. In S3, during or after the intra-battery equalization process of S2, a power exchange takes place between the first DC bus 7 and the second DC bus 8 via the DC/DC converter 6 in order to achieve a balancing of the state of charge of the at least one battery rack 1 with the state of charge of the battery racks 1 connected to the first DC bus 7. After the state of charge has been balanced, the at least one battery rack 1 is disconnected from the second DC bus 8 in S4 and the at least one battery rack 1 is connected to the first DC bus 7.

[0042] The control circuit 11 shown in FIG. 1 can be designed and configured to carry out the method. For this purpose, it can, for example, communicate with the battery management systems 3, the DC/DC converter 6, the inverter bridge 9 and/or the switching circuit 5 via the communication bus 4 and control the aforementioned elements accordingly. To disconnect the transfer circuit or system 12 from the inverter bridge 9, the disconnecting switch 13 can be controlled by the control circuit 11. Alternatively, the switching state of the disconnector switch 13 can be transmitted to the control circuit 11.