Energy storage device, system with energy storage device and method for controlling an energy storage device

Abstract

Energy storage device (1) for generating a three-phase supply voltage, comprising: three parallel connected power supply branches, each of which is coupled between an output terminal (1a, 1b, 1c) and a reference potential rail (4), wherein each of the power supply branches has a plurality of series connected energy storage modules (3), each comprising: an energy storage cell module (5) comprising at least one energy storage cell (5a, 5n), and a coupling device (9) with coupling elements (7, 8) designed to selectively connect the energy storage cell module (5) to the respective power supply branch or to bypass it in the respective power supply branch; and a control device (6) which is designed to: to determine the power supply branch with the lowest possible maximum output voltage; to calculate the amount of the asymmetric voltage components from the difference between the second lowest possible maximum output voltage and the lowest possible maximum output voltage, to generate the asymmetrical voltage components in such a way that they are distributed with a phase shift of 60° between the two power supply branches that do not form the power supply branch with the lowest possible maximum output voltage, to generate the symmetrical voltage components between the three power supply branches with a phase shift of 120°, and to control the coupling devices (9) of the power supply branches so that the output voltage is formed from the sum of the symmetrical voltage components and the asymmetrical voltage components.

Description

[0001] The invention relates to an energy storage device, a system with an energy storage device and a method for controlling an energy storage device, in particular in a battery direct converter circuit for supplying power to electrical machines. State of the art

[0002] It is becoming apparent that in the future, electronic systems that combine new energy storage technologies with electric drive technology will increasingly be used both in stationary applications, such as wind turbines or solar systems, and in vehicles, such as hybrid or electric vehicles.

[0003] Fig. Figure 1, for example, shows the supply of three-phase current to a three-phase electrical machine 101. A DC voltage provided by a DC link 103 is converted into a three-phase AC voltage via a converter in the form of a pulse-controlled inverter 102. The DC link 103 is supplied by a string 104 of serially connected battery modules 105. In order to meet the power and energy requirements of a particular application, several battery modules 105 are often connected in series in a traction battery 104.

[0004] Connecting multiple battery modules in series poses the problem that the entire string fails if a single battery module fails. Such a failure of the power supply string can lead to a failure of the entire system. Furthermore, temporary or permanent performance reductions in a single battery module can lead to performance reductions in the entire power supply string.

[0005] US 5,986,909 A describes an energy storage device in which asymmetrical voltage components are generated and distributed between power supply branches with a fixed phase offset. Control is performed taking into account the output voltages of the individual power supply branches.

[0006] US Pat. No. 5,642,275 A describes a battery system with an integrated inverter function. Systems of this type are known as multilevel cascaded inverters or battery direct inverters (BDIs). Such systems comprise DC voltage sources in multiple energy storage module strings, which can be directly connected to an electrical machine or an electrical grid. Single-phase or multi-phase supply voltages can be generated.The energy storage module strings comprise a plurality of energy storage modules connected in series, wherein each energy storage module has at least one battery cell and an associated controllable coupling unit which, depending on control signals, allows the respective energy storage module string to be interrupted or the respective associated at least one battery cell to be bridged or the respective associated at least one battery cell to be connected into the respective energy storage module string. By appropriately controlling the coupling units, e.g., using pulse width modulation, suitable phase signals for controlling the phase output voltage can also be provided, so that a separate pulse-width inverter is no longer required. The pulse-width inverter required to control the phase output voltage is thus, in a sense, integrated into the BDI.

[0007] BDIs typically have higher efficiency and higher reliability than conventional systems, as in Fig. 1. Failure safety is ensured, among other things, by the fact that defective, failed, or not fully efficient battery cells can be switched off from the power supply lines by appropriate bridging control of the coupling units.

[0008] However, if defective, failed, or not fully capable battery cells can no longer or should no longer contribute to the voltage supply of the respective power supply string, the available power in conventional operation is determined by the lowest of the remaining maximum voltages of the power supply strings. To continue generating a symmetrical rotating field at the BDI outputs, the output voltages of the individual power supply strings must currently be reduced to the lowest maximum voltage of all power supply strings. However, this severely impairs the performance and available voltage range of the BDI. Disclosure of the invention

[0009] The present invention provides an energy storage device for generating a three-phase supply voltage, a system, and a method for controlling such an energy storage device with the features of the independent claims. Further advantageous embodiments are the subject of the dependent claims.

[0010] According to one aspect, an energy storage device for generating a three-phase supply voltage is provided, comprising three parallel-connected power supply branches, each coupled between an output terminal and a reference potential rail, wherein each of the power supply branches comprises a plurality of series-connected energy storage modules. Each of the energy storage modules comprises an energy storage cell module having at least one energy storage cell, and a coupling device with coupling elements configured to selectively connect the energy storage cell module to the respective power supply branch or to bypass it in the respective power supply branch.The energy storage device further comprises a control device which is designed to determine the energy supply branch with the lowest possible maximum output voltage and to control the coupling devices of the two remaining energy supply branches such that their output voltage is formed from the sum of a symmetrical voltage component, the amount of which corresponds to the lowest possible maximum output voltage, and an asymmetrical voltage component, the amount of which corresponds to a maximum of the difference between the amounts of the second lowest possible maximum output voltage of the energy supply branches and the lowest possible maximum output voltage, wherein the phase offset between the symmetrical voltage components of the two remaining energy supply branches is 120° and the phase offset between the asymmetrical voltage components of the two remaining energy supply branches is 60°.

[0011] According to a further aspect, a system is provided with a three-phase electrical machine, three phase lines, each of which is coupled to one of three phase terminals of the electrical machine, and an energy storage device according to the invention, wherein the phase lines are each coupled to one of the output terminals of the energy storage device.

[0012] According to a further aspect, a method is provided for controlling an energy storage device for generating a three-phase supply voltage, which has three parallel-connected energy supply branches, each coupled between an output terminal and a reference potential rail, for generating a respective output voltage at respective ones of the output terminals.The method comprises the steps of determining the power supply branch with the lowest possible maximum output voltage, and controlling the remaining two power supply branches so that their output voltage is formed from the sum of a symmetrical voltage component, the amount of which corresponds to the lowest possible maximum output voltage, and an asymmetrical voltage component, the amount of which corresponds at most to the difference between the amounts of the second lowest possible maximum output voltage of the power supply branches and the lowest possible maximum output voltage, wherein the phase offset between the symmetrical voltage components of the two remaining power supply branches is 120° and the phase offset between the asymmetrical voltage components of the two remaining power supply branches is 60°. Advantages of the invention

[0013] One idea of ​​the present invention is to use a battery direct converter for the voltage supply of a three-phase electrical machine, in which the generation of the phase voltages is balanced in such a way that differences in the maximum output voltages in the individual power supply branches are compensated by cleverly controlling the individual power supply branches. This can be achieved by dividing the output voltages of the individual power supply branches into symmetrical and asymmetrical voltage components in the space vector diagram, and by adjusting the phase offset of the asymmetrical voltage components of the more powerful power supply branches to at least partially compensate for the lack of performance of the remaining power supply branch.

[0014] Due to the interdependence of the differential voltages between the output voltages of the power supply branches, a supply voltage with a corresponding phase position can be generated by appropriately controlling the two power supply branches with higher remaining power, which causes a star point shift at the star point of a connected electrical machine. This (virtual) star point shift allows a symmetrical rotating field to be maintained over a wider voltage range than would actually be possible with the power supply branch with the lowest possible maximum output voltage.

[0015] A significant advantage of this approach is that, when the maximum output voltages of the power supply branches differ, the required power reduction of the overall system is lower than when controlling all power supply branches, which depends on the respective performance of the other power supply branches. This increases the performance of the overall system even in the event of uneven discharge of the energy storage modules, different aging effects of the energy storage modules, and defective or failed energy storage modules.

[0016] According to one embodiment, the energy storage device can have semiconductor switches, for example MOSFET switches, as coupling elements. According to a further embodiment, the coupling elements can be configured as a full-bridge circuit. In an alternative embodiment, the coupling elements can be configured as a half-bridge circuit.

[0017] Further features and advantages of embodiments of the invention will become apparent from the following description with reference to the accompanying drawings. Short description of the drawings

[0018] They show: Fig. 1 is a schematic diagram of a voltage supply system for a three-phase electrical machine; Fig. 2 is a schematic representation of a system with an energy storage device according to an embodiment of the present invention; Fig. 3 a schematic representation of an energy storage module in full bridge circuit of an energy storage device according to Fig. 2 according to another embodiment of the invention; Fig. 4 a schematic representation of an energy storage module in a half-bridge circuit of an energy storage device according to Fig. 2 according to another embodiment of the invention; Fig. 5 is a schematic illustration of a space vector diagram for voltage vectors of the output voltages of an energy storage device according to another embodiment of the invention; Fig. 6 is a schematic illustration of a space vector diagram for modified voltage vectors of the output voltages of an energy storage device according to another embodiment of the invention; and Fig. 7 a schematic representation of a method for controlling an energy storage device according to a further embodiment of the present invention.

[0019] Fig. Figure 2 shows a system 200 for converting DC voltage, which is provided by m energy storage modules 3, into an n-phase AC voltage. The system 200 comprises an energy storage device 1 with energy storage modules 3, which are connected in series in power supply branches. Examples are shown in Fig. 9 shows three power supply branches suitable for generating a three-phase alternating voltage, for example, for a three-phase machine 2. However, it is clear that any other number of power supply branches may also be possible. The energy storage device 1 has an output terminal 1a, 1b, 1c on each power supply branch, which are each connected to phase lines 2a, 2b, and 2c, respectively. The system 200 in Fig. 2 for supplying an electrical machine 2. However, it can also be provided that the energy storage device 1 is used to provide electrical power for an energy supply network 2.

[0020] The system 200 may further comprise a control device 6 which is connected to the energy storage device 1 and by means of which the energy storage device 1 can be controlled in order to provide the desired output voltages at the respective output terminals 1a, 1b, 1c.

[0021] The power supply branches can be connected at their end to a reference potential 4 (reference rail), which in the illustrated embodiment carries a medium potential with respect to the phase lines 2a, 2b, 2c of the electric machine 2. The reference potential 4 can, for example, be a ground potential. Each of the power supply branches has at least two energy storage modules 3 connected in series. For example, the number of energy storage modules 3 per power supply branch is Fig. 2 three, although any other number of energy storage modules 3 is also possible. Preferably, each of the energy supply branches comprises the same number of energy storage modules 3, although it is also possible to provide a different number of energy storage modules 3 for each energy supply branch.

[0022] The energy storage modules 3 each have two output terminals 3a and 3b, via which an output voltage of the energy storage modules 3 can be provided.

[0023] Examples of designs of the energy storage modules 3 are shown in the Fig. 3 and Fig. 4 in greater detail. The energy storage modules 3 each comprise a coupling device 9 with a plurality of coupling elements 7 and 8. The energy storage modules 3 further each comprise an energy storage cell module 5 with one or more series-connected energy storage cells 5a, 5n.

[0024] The energy storage cell module 5 can, for example, comprise series-connected batteries 5a to 5n, for example lithium-ion batteries. The number of energy storage cells 5a to 5n in the Fig. 3, two energy storage cells 5a to 5n are shown as an example, although any other number of energy storage cells 5a to 5n is also possible.

[0025] The energy storage cell modules 5 are connected via connecting lines to input terminals of the associated coupling device 9. The coupling device 9 is in Fig. 3 is embodied, for example, as a full-bridge circuit with two coupling elements 7 and two coupling elements 8 each. The coupling elements 7 can each have an active switching element 7a, for example a semiconductor switch 7a, and a freewheeling diode 7b connected in parallel thereto. Similarly, the coupling elements 8 can each have an active switching element 8a, for example a semiconductor switch 8a, and a freewheeling diode 8b connected in parallel thereto. The semiconductor switches 7a and 8a can, for example, have field-effect transistors (FETs). In this case, the freewheeling diodes 7b and 8b can also be integrated into the semiconductor switches 7a and 8a.

[0026] The coupling elements 7 and 8 in Fig. 10 can be controlled in this way, for example by means of the control device 6 in Fig. 2, that the energy storage cell module 5 is selectively connected between the output terminals 3a and 3b or that the energy storage cell module 5 is bridged or bypassed in the power supply branch. For example, the energy storage cell module 5 can be connected in the forward direction between the output terminals 3a and 3b by placing the active switching element 8a at the bottom right and the active switching element 7a at the top left into a closed state, while placing the two remaining active switching elements into an open state. A bridging state or bypassing state can be set, for example, by placing the two active switching elements 8a in the closed state while keeping the two active switching elements 7a in the open state.

[0027] By appropriately controlling the coupling devices 9, individual energy storage cell modules 5 of the energy storage modules 3 can therefore be specifically integrated into the series circuit of a power supply branch.

[0028] Fig. 4 shows a further exemplary embodiment of an energy storage module 3. The Fig. The energy storage module 3 shown in Figure 4 differs from the one shown in Fig. 3 only in that the coupling device 9 has two instead of four coupling elements 7, 8, which are connected in a half-bridge circuit instead of a full-bridge circuit.

[0029] In the illustrated embodiments, the active switching elements 7a and 8a or the coupling elements 7 and 8 can be designed as power semiconductor switches, for example in the form of IGBTs (Insulated Gate Bipolar Transistors), JFETs (Junction Field-Effect Transistors) or as MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors).

[0030] Using the active switching elements 7a and 8a or the coupling elements 7 and 8, the output voltage of each of the energy supply branches can be varied in steps from a negative maximum value to a positive maximum value via a suitable control. The gradation of the voltage levels depends on the gradation of the individual energy storage cell modules 5. For example, in order to obtain an average voltage value between two voltage levels predetermined by the gradation of the energy storage cell modules 5, the coupling elements 7, 8 of an energy storage module 3 can be controlled in a clocked manner, for example in pulse width modulation (PWM), so that the respective energy storage module 3 delivers a module voltage on average over time, which can have a value between zero and the maximum possible module voltage determined by the energy storage cells 5a to 5n.The coupling elements 7, 8 can be controlled, for example, by a control device 6 which is designed, for example, to carry out current control with a subordinate voltage control, so that individual energy storage modules 3 can be switched on or off in stages.

[0031] By using such an energy storage device 1, it is possible to provide an n-phase supply voltage, for example, for an electrical machine 2. For this purpose, phase lines 2a, 2b, 2c can be connected to respective output terminals 1a, 1b, 1c, wherein the phase lines 2a, 2b, 2c can in turn be connected to phase terminals of the electrical machine 2. For example, the electrical machine 2 can be a three-phase electrical machine, for example, a three-phase rotating field machine.

[0032] For a three-phase electrical machine 2, Fig. 5 schematically shows the space vector diagram of the three phase voltages u, v and w, which are generated by an energy storage device 1 at the respective output terminals 1a, 1b, 1c. In this case, the phase voltage u, for example, can be generated at a first output terminal 1a, the phase voltage v, for example, at a second output terminal 1b, and the phase voltage w, for example, at a third output terminal 1c. The phase voltages u, v and w, which have a relative phase offset of 120° to one another, can be fed into the phase terminals of the three-phase electrical machine 2, for example via phase lines 2a, 2b, 2c, in order to ensure a three-phase voltage supply to the electrical machine 2.

[0033] In the case of differing maximum amplitudes of the phase voltages u, v and w, for example due to uneven discharge of the energy storage modules 3 or due to the failure of individual energy storage modules 3, the space vectors have different lengths. As shown in the example in Fig. As shown in Figure 5, the space vector u is that of the lowest possible maximum output voltage |u|, the space vector w is that of the second lowest possible maximum output voltage |w|, and the space vector v is that of the highest possible maximum output voltage of all power supply branches. For a conventional control system, in this case, only the voltage level |u| of the lowest possible maximum output voltage would be available, meaning that all power supply branches would have to have their phase voltage throttled to the level of the power supply branch with the lowest power capacity in order to ensure a symmetrical rotating field for supplying an electrical machine 2. This would result in a significant loss of power.

[0034] However, for three-phase power supply systems in which the star point is not contacted, the sum of the three phase currents is zero at any time, i.e., the currents flowing into the electrical machine 2 via the phase lines 2a, 2b, 2c cancel out those flowing out via the phase lines 2a, 2b, 2c at any time during operation. This means that in three-phase systems, a phase current can be expressed as the relationship between the other two phase currents. The same applies to the pairwise differential voltages between two of the three phase connections of the electrical machine 2. It is therefore possible to provide a three-phase supply voltage for an electrical machine 2 using only two energy supply branches in an energy storage device.

[0035] If one now carries out a virtual star point shift of the output voltages of all power supply branches in the direction of the voltage level of the power supply branch with the lowest possible maximum output voltage, that is, in the example of the Fig. 5 in the direction of the phase voltage u, one can obtain a shifted space vector system which can be operated like a two-phase system with respect to the phase voltage u.

[0036] This can be done as follows: First, the power supply branch with the lowest possible maximum output voltage is determined, in the case of Fig. 5 of the power supply branch with the phase voltage u. Finally, the coupling devices 9 of the two remaining power supply branches are controlled such that their output voltage is formed from the sum of a symmetrical voltage component us and an asymmetrical voltage component ua. The symmetrical voltage component us is defined by the magnitude |u| of the phase voltage u of the lowest possible maximum output voltage. The asymmetrical voltage component ua, on the other hand, corresponds to the magnitude of the difference between the magnitudes of the second lowest possible maximum output voltage |w| and the lowest possible maximum output voltage |u|.

[0037] The symmetrical voltage components us of the individual power supply branches follow the conventional control method and therefore exhibit a phase shift of 120° from each other in pairs. However, the phase shift between the asymmetrical voltage components of the two power supply branches with the higher maximum possible output voltages is 60°, in order to build up a corresponding symmetrical counter-voltage through the differential voltage of the asymmetrical voltage components in the phase line of the power supply branch with the lowest possible maximum output voltage.By selecting a phase offset of 60° for the asymmetrical voltage components of the two power supply branches with the higher maximum possible output voltages, the phase voltage of the power supply branch with the lowest possible maximum output voltage can be increased beyond the actually maximum possible voltage level |u| at the electrical machine 2 in . Fig. 2. In other words, the more efficient energy supply branches can at least partially compensate for the lack of efficiency of the currently weakest energy supply branch.

[0038] Fig. Figure 6 shows a schematic illustration of a space vector diagram with space vectors u, v', and w' subjected to a virtual star point shift sv (length 58% of the asymmetric voltage component). As a result, the space vectors u, v', and w' form a three-phase power supply system in which a three-phase symmetrical rotating field with an improved voltage level |u'| compared to the voltage level |u| at the load can be generated.

[0039] The space vectors u, v' and w' are formed according to the above explanations (for a three-phase system) according to the following mathematical relationships (exemplary for the Fig. 5 shown case): u=|u|*[1 0]T v'=|u|*[−0.5 q]T+(|w|−|u|)*[−q 0.5]T w'=|u|*[−0.5 −q]T+(|w|−|u|)*[−q −0.5]T, where q = 0.5*3 0,5 As in Fig. As can be seen in Figure 6, the gain in performance compared to a conventional control system is 3 -0,5 ≈ 0.58 of the difference |w|-|u| (asymmetric stress component). Other mathematical solutions are, of course, also conceivable.

[0040] Fig. 7 shows a schematic representation of a method 20 for controlling an energy storage device, for example an energy storage device 1 as in Fig.2, which is designed to generate a three-phase supply voltage. The method 10 comprises, in a first step 21, determining the power supply branch that currently has the lowest possible maximum output voltage. For example, this can be a power supply branch of the energy storage device 1 in which most of the energy storage modules 3 are deactivated due to defects or failures, or in which the energy storage modules 3 have, on average, reached the highest degree of aging. This power supply branch with the lowest possible maximum output voltage is controlled to generate the phase voltage during normal operation.

[0041] The remaining two power supply branches are then controlled in step 22 such that their output voltage is formed from the sum of a symmetrical voltage component, the magnitude of which corresponds to the lowest possible maximum output voltage, and an asymmetrical voltage component, the magnitude of which corresponds to the difference between the magnitudes of the second lowest possible maximum output voltage of the power supply branches and the lowest possible maximum output voltage. The phase offset between the symmetrical voltage components of the two remaining power supply branches is 120°, and the phase offset between the asymmetrical voltage components of the two remaining power supply branches is 60°.

Claims

[1] Energy storage device (1) for generating a three-phase supply voltage, comprising: three parallel-connected power supply branches, each coupled between an output terminal (1a, 1b, 1c) and a reference potential rail (4), each of the power supply branches having a plurality of series-connected energy storage modules (3), each comprising: an energy storage cell module (5) which has at least one energy storage cell (5a, 5n), and a coupling device (9) with coupling elements (7, 8) which are designed to selectively connect the energy storage cell module (5) to the respective energy supply branch or to bypass it in the respective energy supply branch; and a control device (6) which is designed to: to determine the power supply branch with the lowest possible maximum output voltage; to calculate the amount of the asymmetrical voltage components from the difference between the second lowest possible maximum output voltage and the lowest possible maximum output voltage, to generate the asymmetrical voltage components in such a way that they are distributed to the two power supply branches that do not form the power supply branch with the lowest possible maximum output voltage, with a phase shift of 60°, to generate the symmetrical voltage components between the three power supply branches with a phase shift of 120°, and to control the coupling devices (9) of the power supply branches in such a way that the output voltage is formed from the sum of the symmetrical voltage components and the asymmetrical voltage components. [2] Energy storage device (1) according to claim 1, wherein the coupling devices (9) comprise coupling elements (7, 8) in a full bridge circuit. [3] Energy storage device (1) according to claim 1, wherein the coupling devices (9) comprise coupling elements (7, 8) in a half-bridge circuit. [4] System (20), with: a three-phase electrical machine (2); three phase lines (2a, 2b, 2c), each coupled to one of three phase terminals of the electrical machine (2); and an energy storage device (1) according to one of the preceding claims, wherein the phase lines (2a, 2b, 2c) are each coupled to one of the output terminals (1a, 1b, 1c) of the energy storage device (1). [5] Method (20) for controlling an energy storage device (1) for generating a three-phase supply voltage, which has three parallel-connected energy supply branches, each coupled between an output terminal (1a, 1b, 1c) and a reference potential rail (4), for generating a respective output voltage at respective ones of the output terminals (1a, 1b, 1c), comprising the steps: Determining (21) the power supply branch with the lowest possible maximum output voltage; Calculating the amount of the asymmetrical voltage components from the difference between the second lowest possible maximum output voltage and the lowest possible maximum output voltage; Generation of the asymmetrical voltage components and distribution to the two power supply branches that do not form the power supply branch with the lowest possible maximum output voltage, with a phase shift of 60°; Generating the symmetrical voltage components with a phase shift of 120° between the three power supply branches; Controlling (22) the power supply branches so that their output voltage is formed from the sum of a symmetrical and asymmetrical voltage component.

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