Method for characterising and / or optimising at least one energy storage module

Abstract

The invention relates to a method for characterising and / or optimising at least one energy storage module with a multi-level converter system, in which a plurality of energy storage modules and transistors are provided, wherein each energy storage module can be connected in series and / or in parallel to a respective neighbouring energy storage module, and the energy storage modules, preferably the transistors, are connected in such a way that at least one energy storage module is characterised and / or optimised on the basis of a, preferably current-independent, frequency behaviour.

Description

[0001] Method for characterizing and / or optimizing at least one energy storage module

[0002] The invention relates to a method for characterizing and / or optimizing at least one energy storage module, in which a plurality of energy storage modules and transistors are provided.

[0003] Current energy storage systems are usually loaded with direct current (DC). This is due to the design of conventional converter systems. The aim is to keep the alternating current components, i.e., harmonic oscillations, away from the energy storage devices.

[0004] Since many energy storage devices must be connected in series or parallel, a battery management system (BMS) is necessary. A DC link capacitor, for example, can be connected downstream of the energy storage devices. This serves to further smooth the three-phase currents of the converter and to keep high-frequency oscillations away from the energy storage devices. It also absorbs switching overshoots, as the inductance of the energy storage devices would otherwise drive the current further. The goal of this approach is to apply DC load to the energy storage devices, as this is assumed to contribute to the durability of the battery cell and reduce losses.

[0005] For example, in a conventional electric vehicle, the DC bus can house converters that transfer energy to the electric motor or, during braking, return it to the battery. This bus can also be used to connect chargers that operate with alternating current (AC) or direct current (DC).

[0006] These converters are usually designed as two-point converters, e.g. as a B6 bridge in a three-phase version, or - especially in the area of ​​solar systems - as three-point converters.

[0007] As an alternative to bridge circuits as converters, so-called multilevel converter systems (MMC systems) are known.

[0008] Batteries, such as accumulators, capacitors, fuel cells, and / or solar panels, can be used as energy storage devices or energy sources. The energy storage devices are not hard-wired together, but rather combined as individual submodules. This structure is required for each phase. Therefore, the energy storage devices are distributed across these phases and can be connected in series or parallel, for example. Previous methods for characterizing energy storage devices or energy sources are often inaccurate. Furthermore, losses often occur with energy storage devices or energy sources, for example, frequency-dependent ones.

[0009] It is therefore an object of the invention to provide a method which enables an accurate characterization of at least one energy storage module and / or reduces losses.

[0010] This problem is solved by the method having the features of claim 1 .

[0011] According to the invention, the method for characterizing and / or optimizing at least one energy storage module with a multilevel converter system, preferably a modular multilevel battery system (B2M), is designed or can be used for this purpose.

[0012] Optimization is preferably carried out with a view to low losses.

[0013] The losses P depend according to P = I 2 * Z depends on the real part of the alternating current resistance Z.

[0014] The energy storage module can be a storage device for a preferably frequency-dependent electrical source, for example a battery, e.g. an accumulator, a fuel cell, a solar cell and / or a (super)capacitor.

[0015] The process can be used, for example, in electric vehicles, such as electric cars, electric trucks, and / or electric buses. Its use in hydrogen vehicles is also conceivable. Furthermore, it can be applied to stationary energy storage systems and / or other converter systems that are connected to the power grid and / or powered by an AC motor.

[0016] A variety of energy storage modules and transistors are provided.

[0017] A preferably modular multilevel converter system describes a type of arrangement or circuit of multiple energy storage modules or transistors. Each energy storage module can have at least one or exactly one battery, e.g., an accumulator, and / or at least one or exactly one capacitor.

[0018] The transistors serve, for example, as switches that can be used to select current and / or voltage paths. This allows the energy storage modules to be integrated into or excluded from a desired configuration.

[0019] Preferably, each energy storage module is assigned at least or exactly two, three, four, five, six, seven, eight, nine, ten or more transistors.

[0020] The transistor can, for example, be designed for a voltage of less than 500 V, 400 V, 300 V, 200 V, 100 V, 50 V, 40 V, 30 V, 20 V, or 10 V. Preferably, the transistor can be designed for a voltage between 2 V and 8 V, e.g., 3 V, 4 V, 5 V, 6 V, or 7 V.

[0021] Each energy storage module can be connected in parallel and / or in series with its adjacent energy storage module. Preferably, each energy storage module can be connected in series with its adjacent energy storage module. The option of parallel connection is advantageous but not necessary.

[0022] Preferably, the adjacent energy storage modules are each connected to each other via two current and / or voltage paths. A transistor can be assigned to each path.

[0023] For example, three transistors are provided between two adjacent energy storage modules. This allows the energy storage modules to be connected in parallel or in series, for example.

[0024] Multilevel converter systems are significantly more versatile than bridge circuits. This allows for virtually any configuration to be created. For example, the energy storage modules can be connected to each other in any way, e.g., parallel or series. Individual energy storage modules can also be included in or excluded from a desired configuration.

[0025] The energy storage modules, preferably the transistors, are connected in such a way that at least one energy storage module is characterized and / or optimized based on a frequency response, preferably one that is independent of the current. For characterization purposes, for example, a current pulse can be applied or impressed to the energy storage module, and the resulting voltage response can be measured.

[0026] Alternatively or additionally, electrochemical impedance spectroscopy (EIS) investigates the behavior of a system under alternating current load. Every system under investigation can essentially be understood as a combination of resistances, capacitances, and / or inductances, which correspond to specific real-world components or processes. A precise understanding of the system enables the creation of an equivalent circuit. This allows changes in electrical behavior to be interpreted as changes in individual system components.

[0027] In most cases, however, the electrical resistance is frequency-dependent. The current and voltage are not in phase, meaning the voltage and current curves are offset in time. This behavior is common to all energy storage systems, such as (super)capacitors, batteries, and fuel cells.

[0028] An (ideal) capacitor is a typical example of this. At direct current and very low alternating current frequencies, its resistance is high, so no or only a very small current flows. At higher frequencies, the resistance becomes increasingly smaller. However, in an ideal capacitor, the flowing current and the voltage are 90° out of phase.

[0029] Capacitors can be found in real systems, for example, in the form of non-conductive covering layers and / or air gaps. An electrochemical double layer that forms at the interface between a metal and an electrolyte also represents a capacitor.

[0030] An inductor, such as a coil, behaves in exactly the opposite way to a capacitor. The phase shift between current and voltage is also 90°, but with a different sign. An inductor has an extremely high resistance at high frequencies, while at low frequencies or with direct current this resistance is almost zero. In the technical systems considered here, inductors are therefore only significant at high frequencies. For example, the cables or (unintentional) cable loops can act as inductors. This should preferably be taken into account in the measurement setup for impedance spectroscopy. Other frequency-dependent phenomena include diffusion processes in a battery. These are represented by a so-called Warburg impedance.Non-ideal capacitive behavior, caused for example by a porous and / or rough surface, can be described by a constant-phase element instead of a capacitor.

[0031] If the frequency response of the energy storage module has been characterized, the aging and / or temperature of the energy storage module can be estimated, for example, if the state of charge is known and / or estimated.

[0032] The frequency response of the energy storage module serves as a basis for developing further characteristics of the energy storage module.

[0033] It was surprising that the frequency response can be determined by switching the energy storage modules, preferably the transistors. The frequency, e.g., the switching frequency, is independent of the current. Only the voltage can influence it—for example, through the different switching patterns.

[0034] Furthermore, it was found that the losses of an energy storage module depend on the frequency, e.g. the switching frequency, at which the energy storage module is operated.

[0035] An energy storage module can thus be operated at a specific switching frequency to reduce losses. The multilevel converter system can preferably be operated in a frequency range that is ideal for the energy storage module. This frequency range can change over its lifetime or the state of charge.

[0036] For example, an energy storage module can be switched on and / or off at a predetermined frequency.

[0037] The optimization preferably depends only on the frequency and not on the current.

[0038] The frequency, e.g., the switching frequency (typically above the fundamental harmonic of, for example, 50 Hz), can preferably be shifted to a frequency range where the energy storage module's losses are lowest. This can, for example, reduce the overall battery losses of a vehicle. An optimized frequency can preferably be between 1 kHz and 4 kHz. In this range, the Joule losses of the energy storage modules are low.

[0039] In order to optimize an energy storage module with regard to losses, it can be connected accordingly, e.g. in parallel or in series with one or more energy storage modules.

[0040] Switching can preferably be done using pulse width modulation (PWM).

[0041] At least one transistor, preferably all transistors, has / have a switching frequency of at least 1 Hz, 2 Hz, 3 Hz, 4 Hz, 5 Hz, 6 Hz, 7 Hz, 8 Hz, 9 Hz, 10 Hz, 15 Hz, 20 Hz, 30 Hz, 40 Hz, 50 Hz or 100 Hz.

[0042] For example, a converter string can have 100 stages. The entire converter string can therefore have 100 times the frequency.

[0043] For example, the maximum switching frequency of a transistor and / or the entire converter train can be at least 50 kHz, 60 kHz, 70 kHz, 80 kHz, 90 kHz, 100 kHz, 150 kHz, 200 kHz, 250 kHz, 300 kHz, 400 kHz, 500 kHz, 600 kHz, 700 kHz, 800 kHz, 900 kHz, 1 MHz, 1.1 MHz, 1.2 MHz, 1.3 MHz, 1.4 MHz, 1.5 MHz, 2 MHz, 5 MHz, 10 MHz, 20 MHz, 30 MHz, 40 MHz, 50 MHz, 80 MHz, 100 MHz or 120 MHz.

[0044] For example, the transistor may be designed as a metal-oxide-semiconductor field-effect transistor (MOSFET) or may comprise a MOSFET.

[0045] MOSFETs can be switched at high frequencies.

[0046] The semiconductor material could be silicon, gallium nitride, gallium arsenide, and / or silicon carbide, for example. Organic semiconductors are also conceivable.

[0047] For example, preconditioning and / or optimum operating settings for MOSFETs are possible.

[0048] Further developments of the invention can also be found in the dependent claims, the description and the accompanying drawings.

[0049] According to one embodiment, the energy storage module is designed as a battery, preferably an accumulator. The energy storage module is preferably rechargeable. However, one-time energy storage modules are also conceivable.

[0050] Preferably, the energy storage module enables longer-term storage of energy than, for example, a capacitor.

[0051] According to a further embodiment, the energy storage module to be characterized is integrated into a configuration.

[0052] For example, an energy storage module may initially not be integrated into a configuration for characterization. It can then be integrated into a configuration and, for example, connected in series with at least one other energy storage module. This leads from a current flow of 0 A to one with > 0 A, during which the voltage response is measured. The relationship between voltage and current results in the complex resistance. Based on knowledge of the frequencies, the equivalent circuit can be provided with data, for example.

[0053] The ability to connect the energy storage modules in series is sufficient for characterization. Parallel connection is therefore not necessary, but is conceivable in principle if the energy storage modules have corresponding paths. Furthermore, this increases the degree of freedom in data collection. For example, the points can also be recorded with different current strengths.

[0054] In this way, the frequency response can be determined.

[0055] A single integration into a configuration is sufficient to obtain an initial measurement point. For more precise measurements, however, it is advantageous to perform multiple measurements. The configuration (or reconfiguration) can remain the same, for example. For example, you can first select 0 A and then—if necessary, multiple times—select a value (or different values) > 0 A.

[0056] According to a further embodiment, the configuration is varied.

[0057] By changing the configuration, different measurement points can be determined. For example, an energy storage module can be connected first in series and then in parallel. This allows the frequency response to be determined for different configurations.

[0058] Preferably, the configuration can change at any stage. Alternatively or additionally, the configuration can be changed within a stage.

[0059] The energy storage modules can be integrated into or excluded from a desired configuration.

[0060] Exclusion can be achieved, for example, by closing a switch, e.g. the upper one, so that no current flows through the corresponding energy storage module.

[0061] According to a further embodiment, a current pulse is switched or impressed on the energy storage module and the resulting voltage response is measured.

[0062] Meanwhile, the multilevel converter system can generate a sine wave at the load, e.g. a motor or a power source, e.g. a power grid.

[0063] Pulse characterization can be achieved by measuring the voltage response.

[0064] A portion of the sinusoidal waveform of the current or voltage curve can be extracted, for example, by changing the configuration, e.g., by integrating it into a configuration, or by connecting it in series and / or parallel. Depending on the switching frequency and the sinusoidal frequency, this can result in a constant current, for example, in this journal. This method also works ideally when the pulse is not constant.

[0065] The real part of the alternating current resistance Z depends on the voltage and the current according to Z = u / i.

[0066] The real part of the AC resistance varies depending on the frequency range. Therefore, the frequency response can be determined based on the voltage response.

[0067] The frequency response can be determined without the need for parallel connection. In a vehicle, it is also possible to characterize one, several, or all energy storage modules while driving, meaning that the vehicle does not need to be stationary.

[0068] According to another embodiment, the duration of the current pulse is varied. The frequencies to be examined, for example, determine how long the current pulse must be.

[0069] By varying the duration of the current pulse, additional measurement points can be determined to determine the frequency response.

[0070] According to a further embodiment, an energy storage module to be optimized is integrated into a configuration such that the real part of the alternating current resistance is low and / or the frequency is high.

[0071] In high-frequency ranges, the real part of the AC resistance is high, while in medium-frequency ranges, it is smaller. An optimized energy storage module is therefore preferably operated in a high-frequency range with low AC resistance.

[0072] For example, an energy storage module can be connected in parallel or in series with at least one other energy storage module for optimization.

[0073] The configuration can remain the same at each stage and / or within a stage.

[0074] Alternatively, the configuration may change at each stage and / or within a stage.

[0075] A configuration may contain only the energy storage module to be optimized. Alternatively, a configuration may also include at least one additional energy storage module. Preferably, each configuration includes a plurality of energy storage modules connected in parallel and / or in series.

[0076] For example, the frequency can be adjusted by changing the configuration.

[0077] If it is known that an energy storage module has the lowest losses at 20 Hz, for example, a (re)configuration with the frequency 20 Hz can be selected.

[0078] According to a further embodiment, a characterization according to the invention is carried out prior to optimization. In this way, the frequency response of the energy storage module can be determined first.

[0079] Alternatively, existing data can be used. This data can include, for example, the frequency response of comparable energy storage modules, simulations, and / or historical data of the energy storage module.

[0080] If the frequency response is known, the energy storage module can be operated in a range where losses are low. The energy storage module is therefore switched into a specific configuration.

[0081] According to a further embodiment, the optimization is carried out with regard to a single energy storage module.

[0082] The energy storage module to be optimized is therefore integrated into a configuration in such a way that the losses of the energy storage module to be optimized are as low as possible.

[0083] This is technically relatively easy to implement, as no consideration needs to be given to the other energy storage modules. However, this can lead to higher losses in the other energy storage modules.

[0084] According to a further embodiment, the optimization takes place with regard to several, preferably all, energy storage modules.

[0085] For example, two, three, four, five or more energy storage modules can be optimized.

[0086] Preferably, optimization is carried out with regard to all energy storage modules.

[0087] This leads to a reduction in overall losses.

[0088] Software can be used to find one or more configurations that optimize the total loss of several or all energy storage modules. For example, the software can work with neural networks, Kalman filters, and / or artificial intelligence. This allows the process to be continuously improved using recorded data.

[0089] The invention also relates to a multilevel converter system, preferably a modular multilevel battery system, for carrying out the method according to the invention with a plurality of energy storage modules and transistors, wherein each energy storage module is connected in parallel and / or in series to the respectively adjacent energy storage module.

[0090] The system comprises a control device which is designed to switch the energy storage modules, preferably the transistors, in such a way that at least one energy storage module is characterized and / or optimized.

[0091] At least one transistor has a switching frequency of at least 1 Hz.

[0092] An optimized frequency can preferably be between 1 kHz and 4 kHz.

[0093] All embodiments and components of the multilevel converter system described here are preferably designed to be operated according to the method described here, e.g., by means of a control device. Furthermore, all embodiments of the device described here, as well as all embodiments of the method described here, can be combined with one another, preferably independently of the specific configuration in whose context they are mentioned.

[0094] The invention is described below by way of example with reference to the drawings.

[0095] Fig. 1 shows an embodiment of an inventive

[0096] MMC system,

[0097] Fig. 2 shows a curve of the output voltage of a PWM system according to the state of the art,

[0098] Fig. 3 shows a curve of the output voltage of an embodiment of an MMC system according to the invention, Fig. 4 shows a (loss) power-optimised configuration of an MMC-

[0099] State-of-the-art systems,

[0100] Fig. 5 shows a configuration of an embodiment of an MMC system according to the invention for characterizing and / or optimizing energy storage modules,

[0101] Fig. 6 shows a configuration of another embodiment of an MMC system according to the invention for characterizing and / or optimizing energy storage modules,

[0102] Fig. 7 a Nyquist diagram of an electrochemical

[0103] Impedance spectroscopy,

[0104] Fig. 8 shows a diagram of the current load over time for energy storage modules connected in parallel, and

[0105] Fig. 9 a diagram of the frequency components.

[0106] First, it should be noted that the illustrated embodiments are purely exemplary in nature. Individual features can be implemented not only in the combination shown, but also individually or in other technically feasible combinations. For example, the features of one embodiment can be combined arbitrarily with features of another embodiment. The configuration and / or number of energy storage modules, paths, and transistors shown are purely exemplary and fundamentally arbitrary.

[0107] If a figure contains a reference symbol that is not explained in the immediately corresponding description, reference is made to the corresponding preceding or following explanations in the figure description. Thus, the same reference symbols are used for identical or comparable components in the figures and these are not explained again.

[0108] Fig. 1 shows a multilevel converter system for characterizing and / or optimizing at least one energy storage module 10, 12, 14, 16.

[0109] Adjacent energy storage modules 10, 12, 14, and 16 are each connected to each other via several paths. A switch embodied as a transistor 18 is provided in each path.

[0110] The adjacent energy storage modules 10, 12, 14, 16 can thus be connected in series or parallel. Individual energy storage modules 10, 12, 14, 16 can also be bridged if necessary, e.g., by closing the upper switch 18, and thus excluded from a configuration.

[0111] Fig. 2 shows the voltage curve U over time t of a PWM modulation.

[0112] For a three-phase DC / AC system coupling, six switches are required.

[0113] In a B6 bridge or a two-level converter, the DC voltage is switched on synchronously via several or one switch, so that an AC voltage is only generated on average over time.

[0114] The sinusoidal target voltage 20 is therefore only rudimentarily simulated by the output voltage 22 of the PWM system.

[0115] Fig. 3 shows the voltage curve U in volts over time t in seconds of an MMC system.

[0116] The sinusoidal target voltage 20 is simulated by the construction of individual stages 24. The output voltage 24 therefore simulates the sinusoidal target voltage 20 much better.

[0117] Fig. 4 shows a power-loss-optimized configuration of an MMC system. The voltage U is plotted in volts over time t in seconds.

[0118] An example shows how the first three voltage levels can be formed by connecting the energy storage modules 10, 12, 14, 16 in parallel.

[0119] To optimize performance and achieve the best efficiency, all energy storage modules 10, 12, 14, 16 are always integrated into the configuration in each stage.

[0120] Fig. 5, however, shows a configuration in which all energy storage modules 10, 12, 14, 16 are connected in series. Such a configuration can be used to characterize the energy storage module 10.

[0121] In this example, the energy storage module 10 is always integrated so that a measured value can be determined for different levels.

[0122] Because the energy storage module 10 is integrated into the configuration alone in the first stage and is connected in series with another energy storage module 12 in the second stage, with two further energy storage modules 12, 14 in the third stage and with three further energy storage modules 12, 14, 16 in the fourth stage, the behavior in different frequency ranges can be determined.

[0123] The configuration shown in Fig. 5 can thus be used to characterize the energy storage module 10.

[0124] Alternatively or additionally, the configuration shown in Fig. 5 may also represent an optimized configuration in which the losses for one or more energy storage modules 10, 12, 14, 16 are low.

[0125] Another exemplary configuration is shown in Fig. 6.

[0126] In the first stage, the energy storage module 10 is connected alone and in the further stages in series with at least one of the other energy storage modules 12, 14, 16.

[0127] The configuration shown in Fig. 6 may represent an optimized configuration in which the losses for one or more energy storage modules 10, 12, 14, 16 are low.

[0128] The configurations shown in Figs. 5 and 6 are purely exemplary. Depending on which energy storage module(s) 10, 12, 14, 16 are to be characterized and / or optimized with regard to losses, other configurations are also conceivable.

[0129] Fig. 7 shows a Nyquist diagram of electrochemical impedance spectroscopy (EIS) using the example of a lithium-ion battery as energy storage module 10, 12, 14, 16.

[0130] The procedure is similarly applicable to other storage technologies. The EIS of battery cells varies greatly depending on the manufacturer, but even cells from one manufacturer differ significantly due to different protective circuits, cathode and anode materials, and / or electrolytes. What's particularly interesting here is that the EIS differs even among cells of the same type and manufacturer. However, the basic curve is the same for all storage types.

[0131] The x-axis represents the real part of the impedance, i.e. the complex alternating current resistance in Q. This corresponds to the actual losses of the battery at the respective operating points.

[0132] In addition, the y-axis shows the imaginary part of the impedance in Q at the respective operating points.

[0133] The different operating points are caused by the different voltage frequencies applied to the battery. The frequencies in the upper right area are low frequencies (LF) and are strongly influenced by diffusion. The losses in the lower left area correspond to the losses at high frequencies (HF) and are strongly influenced by the inductive behavior.

[0134] The Nyquist diagram does not provide the ability to directly read exact frequency values. These must be displayed separately.

[0135] However, the Nyquist diagram shows that the point of smallest losses (P = I 2 * Re(Z)) depends on the x-axis and is the point furthest to the left, since only the ohmic losses occur here.

[0136] A conventional DC load is located in the Nyquist diagram at the low frequencies NF and thus at the maximum losses.

[0137] Measurements were also conducted with different charge levels (not shown), resulting in different curves.

[0138] Battery losses therefore depend not only on frequencies and current, but also on the battery's state of charge. Adjusting the losses for different battery states would therefore be advantageous.

[0139] Temperature and / or aging also influence the frequency response and / or losses of the battery. For the battery shown, it would be desirable to load it with a frequency of approximately 2 kHz, for example, in order to operate it at the operating point with low losses. This point corresponds to the intersection with the x-axis.

[0140] The Nyquist diagram of this battery shows very homogeneous behavior at the point of lowest resistance. However, this is not the case for all batteries.

[0141] From these measurements, a battery model, a so-called equivalent circuit diagram (ESB), can now be calculated.

[0142] The battery can be modeled using three RC elements, with the filter capacitance chosen to be negligibly small to avoid distorting the simulation. This results in parameters that reflect the battery's frequency response.

[0143] Models without or with one RC element or two, three or more RC elements are also possible.

[0144] High frequency ranges above 4 kHz as well as ranges with very low frequencies can be neglected.

[0145] The present invention challenges the conventional approach, especially since DC loading of battery storage systems is detrimental. It has been found that high-frequency sinusoidal loads have a positive effect on the aging of battery cells and can positively influence the so-called state of health (SOH) of the batteries.

[0146] It is therefore advantageous to operate the battery at suitable operating points in order to minimize aging effects.

[0147] However, this is difficult to achieve with a conventional converter. This would require additional converters to model these harmonic oscillations on the battery, since the three-phase design of conventional converter systems neutralizes the currents of the three phases on average. This results in one advantage of the system: reactive currents, i.e., currents that do not perform any work, do not burden the entire battery system.

[0148] Batteries in MMC systems can experience DC stress at low frequencies. While filtering this with capacitors would be conceivable, these would be disproportionately large, as they are unlikely to lose any charge over an entire fundamental cycle. In battery-operated MMC systems, the batteries are exposed to the fundamental frequency of the output current.

[0149] In the specific case of battery-powered MMC systems with parallel connection, the current follows the main current. However, this current is chopped by changing the configurations.

[0150] In addition, when connected in parallel, the current is distributed among several energy storage modules (10, 12, 14, 16). This results in a smaller current flowing in each of the energy storage modules (10, 12, 14, 16) than the total current flowing through the entire system. One converter type that makes this possible is the Modular Multilevel Battery Converter (M2B), essentially a subtype of an MMC system.

[0151] Fig. 8 shows the waveform of the current I applied to an M2B module during a sine wave (measurement with eight M2B modules and an additional capacitor module equipped with 860 pF) over time t.

[0152] The waveform resembles a rectified sine wave. The differences to the envelope of a rectified sine signal arise from the parallel connection of the energy storage modules 10, 12, 14, and 16. Jumps in the current load occur when previously parallel-connected energy storage modules 10, 12, 14, and 16 transition to series mode.

[0153] Fig. 9 shows the frequency components of the current curve shown in Fig. 8 in percent. As expected for a rectified sine wave, the frequencies f = 0 Hz and f = 100 Hz dominate. Other frequency components such as 50 Hz, 150 Hz, etc. are significantly lower and have an amplitude of less than 15% of the DC component.

[0154] Especially low frequencies up to about 300 Hz have a high relative proportion.

[0155] The use of an MMC system, or in particular an M2B, results in a 100 Hz load on the batteries on the European power grid (50 Hz). This corresponds to twice the value, since both the positive and negative half-waves of a sine wave flow through the batteries.

[0156] When the converter is used on a three-phase motor, the frequency load depends on the motor's electrical speed and corresponds to twice this speed in the converter modules. If the losses in this example battery are lowest at 2 kHz, then the losses can be reduced by an MMC system operating at an output frequency of 1 kHz.

[0157] However, the variable battery interconnection of MMC systems allows the frequency range in which the energy storage modules 10, 12, 14, 16 are loaded to be freely influenced. This is also necessary because the optimal operating point depends on various parameters and changes, for example, over the lifetime of an energy storage module 10, 12, 14, 16 (and possibly the other energy storage modules 10, 12, 14, 16).

[0158] This can be achieved, for example, via software and / or software updates.

[0159] For example, dependencies on the temperature of the energy storage modules 10, 12, 14, 16 can also be compensated.

[0160] Accordingly, a dependence on the number of cycles (service life) of the energy storage modules 10, 12, 14, 16 can be compensated.

[0161] The method according to the invention can preferably be repeated at intervals to account for aging effects. To save on temperature sensors, the method can be repeated continuously, for example.

[0162] The load frequency of the converter can preferably be set to different charging and / or discharging curves.

[0163] Due to the offset component of the load current, 100% of the expected optimization cannot be achieved, since a sine wave that has been shifted by its amplitude also has a DC component. This corresponds to 50%, and for this 50%, no improvement can be achieved with the method presented here.

[0164] In electrical engineering, the root mean square (RMS) is used to calculate losses. This allows AC quantities to be converted into their DC losses or effective values. For a sine wave offset by an offset, as applied to the energy storage module 10, 12, 14, 16 during operation of an MMC system, the following results:

[0165] So, if a sine wave is modeled on a DC current, the effective value (RMS) increases. This is the same across all frequencies. However, the energy storage module 10, 12, 14, 16 still has different losses. For a sine wave with the same amplitude as the offset, the following results:

[0166] I en = J(12 + (A) 1.2247.

[0167] The losses can be calculated directly using Ohm's law (P = I 2 * R). With increasing frequency, the ohmic losses can be reduced.

[0168] As a reference system at 0 Hz, a current can be selected that corresponds to the effective value of a sine wave with offset, namely 1.23 A. Since the load current is specified by the consumer, it must now be modeled in such a way that it is possible to operate the energy storage module 10, 12, 14, 16 at the optimal operating point. This, for example, exploits a property of MMC systems with parallel connection capability.

[0169] The stages in these are generated by parallel and / or serial connection, as shown by way of example in Fig. 4 to 6.

[0170] To generate a four-stage sine wave, configurations can be formed as shown in Fig. 5, wherein in the first stage one energy storage module 10 is connected in series, in the second stage two energy storage modules 10, 12 are connected in series, in the third stage three energy storage modules 10, 12, 14 and in the fourth stage four energy storage modules 10, 12, 14, 16.

[0171] The remaining energy storage modules 10, 12, 14, 16 serve to reduce the internal resistance by connecting them in parallel.

[0172] This configuration can be maintained across all stages. However, the possibility of parallel connection now offers alternative representations of the stages. These can be used to model higher frequencies on the battery current.

[0173] For example, Fig. 6 shows that no alternative vector exists for the fourth stage, since all energy storage modules 10, 12, 14, 16 are required to generate the output voltage and there are no remaining energy storage modules 10, 12, 14, 16. However, there are a wide variety of possible configurations for the first, second, and third stages, since the energy storage modules 10, 12, 14, 16 can be integrated into a parallel or series configuration in almost any way, or can be excluded entirely.

[0174] However, the battery losses are suboptimal for vectors that do not use all energy storage modules 10, 12, 14, 16, since the effect of parallel connection of energy storage modules 10, 12, 14, 16 and the resulting reduced battery losses is not utilized.

[0175] For example, in the first stage, all energy storage modules 10, 12, 14, 16 can be connected in parallel so that the internal resistance - neglecting the complex components - is 1 / 4.

[0176] Even with the four energy storage modules 10, 12, 14, 16 shown, a multitude of different configurations are conceivable, which enables modeling of the battery current.

[0177] A simulation has shown that a reduction in battery losses can be achieved by blunt switching.

[0178] Here, for example, it is possible to halve the current through an energy storage module 10, 12, 14, 16 by connecting another energy storage module 10, 12, 14, 16 in parallel or to pass the full load current through the energy storage module 10, 12, 14, 16.

[0179] This is always possible if not all energy storage modules 10, 12, 14, 16 are needed to output the maximum voltage.

[0180] In the example of an electric car, this is always the case when the driver is not traveling at maximum speed or is not currently charging from the power grid. However, this limitation is irrelevant, as it only applies to the highest level and can be integrated into the modulation scheme, since the sine wave only has its maximum amplitude briefly, even at maximum speed.

[0181] Switching back and forth between two configurations results in a high-frequency load on the energy storage module 10, 12, 14, 16. Switching back and forth between parallel and series changes the losses over the frequency.

[0182] 10, 12, 14, 16 energy storage module

[0183] 18 Transistor 20 Target voltage

[0184] 22 Output voltage PWM system

[0185] 24 stage, output voltage MMC system

[0186] U voltage t time

[0187] Re(Z) Real part of the complex resistance lm(Z) Imaginary part of the complex resistance NF low frequencies

[0188] HF high frequencies I current f frequency

Claims

Claims 1. Method for characterizing and / or optimizing at least one energy storage module (10, 12, 14, 16) with a multilevel converter system in which a plurality of energy storage modules (10, 12, 14, 16) and transistors (18) are provided, wherein each energy storage module (10, 12, 14, 16) can be connected in parallel and / or in series with the respective adjacent energy storage module (10, 12, 14, 16), and the energy storage modules (10, 12, 14, 16), preferably the transistors (18), are connected in such a way that at least one energy storage module (10, 12, 14, 16) is characterized and / or optimized based on a frequency response that is preferably independent of the current.

2. Method according to claim 1, characterized in that the energy storage module (10, 12, 14, 16) is designed as a battery, preferably an accumulator.

3. Method according to claim 1 or 2, characterized in that the energy storage module (10, 12, 14, 16) to be characterized is integrated into a configuration.

4. Method according to claim 3, characterized in that the configuration is varied.

5. Method according to one of the preceding claims, characterized in that a current pulse is applied to the energy storage module (10, 12, 14, 16) and the resulting voltage response is measured.

6. Method according to claim 5, characterized in that the duration of the current pulse is varied. A method according to claim 1 or 2, characterized in that an energy storage module (10, 12, 14, 16) to be optimized is integrated into a configuration such that the real part of the AC resistance is low and / or the frequency is high. A method according to claim 7, characterized in that a characterization according to one of claims 1 to 6 is carried out before the optimization. A method according to claim 7 or 8, characterized in that the optimization is performed with respect to a single energy storage module (10, 12, 14, 16). A method according to one of claims 7 to 9, characterized in that the optimization is performed with respect to several, preferably all, energy storage modules (10, 12, 14, 16).

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