POWER CONVERTER FOR ENERGY TRANSMISSION

Description

[0001] The invention relates to an improved power converter and a corresponding method and particularly relates to power converters for bidirectional energy transmission and with the possibility of changing the output voltage in steps.

[0002] Power converters are used to convert one type of electrical current (direct current, alternating current) into the other, or to change characteristic parameters such as voltage or frequency. Both principles are often used in combination.

[0003] Power converters are used in a wide variety of areas. In particular, they are used to control electrical machines, especially electric motors in vehicles. Another area of ​​application is the energy supply / power supply in power grids, on-board electrical systems, or in energy supply networks in general. Accordingly, there are different types of power converters. On the one hand, there are DC-to-DC converters (DC-DC converters), which convert a first direct voltage into a second direct voltage, which can be higher, lower, or inverted. On the other hand, there are so-called AC-DC converters, which convert alternating current into direct current (rectifiers) or direct current into alternating current (inverters). By connecting two AC-DC converters via the DC side, AC-AC converters can also be constructed.

[0004] Modern power converters use semiconductor-based electronic components such as transistors, diodes, and thyristors. Two-quadrant and four-quadrant converters represent an important basic circuit. The latter allow both negative and positive voltages to be processed at a single terminal pair, regardless of the current direction. Applications include the controlled supply of electrical energy to an electric motor and the conversion of mechanical energy into electrical energy during the braking phase (regeneration).

[0005] The ongoing development of semiconductor technology in power electronics, introduced about 35 years ago, enables and supports the realization of such power converters for ever higher power and voltage ranges. Today, for example, IGBTs and IGCTs are predominantly used in the medium-voltage range to switch electrical currents and voltages. Due to the constant advancement of semiconductors and their manufacturing processes, the reliability and power density of these components has increased significantly.

[0006] Advances in semiconductor technology are also driving the development of innovative converter topologies. HVDC (high-voltage direct current) systems in the high-voltage range (up to several GVA) are still largely implemented using thyristor circuits. These operate according to the phase-angle control principle and can generate or process different voltage levels depending on the firing angle. However, complex filter circuits are required to filter out the high harmonic content. Recently, multi-point converters with significantly improved properties for power transmission, voltage and frequency conversion, and reactive power compensation have become the focus of attention.

[0007] Particularly in high-power applications in the power supply sector, the voltages to be switched by the power converter are considerably higher than the blocking voltages of available power semiconductors. Therefore, circuit topologies are used in such applications that allow the high input voltage to be evenly distributed across multiple switching elements / power semiconductors, for example, through series connection.

[0008] Many disadvantages of conventional high-power converters can be solved with so-called modular multilevel converters, see, for example, DE 102 17 889 A1. This system is capable of converting virtually any voltage waveform between the terminal pairs on one side into equally any waveform between the terminal pairs on the other side. Due to its bidirectional nature, there is no need to distinguish between input and output. The converter's electronics responsible for each phase are constructed from a multitude of identical individual modules connected in series.

[0009] Figure 1shows a schematic view of a three-phase two-point converter 110 for an electric motor 120, such as is used in electric vehicles. The two-point converter is formed by three parallel-connected phase modules or half-bridges 130, 132, 134. Each phase module consists of two series-connected switches 140, 142. At the connection points of these switches, there is a connection for each phase 150, 152, 154. The input terminal pair of the two-point converter is fed by a schematically indicated DC power source 160.

[0010] Figure 2shows corresponding series connections of identical modules, where the three upper modules 201, 102, and 203 together form a so-called bridge arm 211. A combination of two bridge arms 211, 212 (consisting of any number of modules) is referred to as a phase module 213. This phase module is typically connected to the terminals of a DC system or a DC intermediate circuit. The number of modules in each bridge arm defines the voltage and harmonic characteristics of the power converter. The connection points of the two bridge arms of each phase module each form an AC terminal of a power converter.

[0011] Each individual module can be electrically considered a two-terminal network and contains an energy storage element and several switching elements that can selectively absorb or release current in one or both voltage directions. The modules can either switch only one voltage polarity in two possible current directions, and are then referred to as two-quadrant modules; or they can reach all four quadrants of the current-voltage graph, and are then referred to as four-quadrant modules.

[0012] Figures 3a - 3c show a basic form of a two-terminal module. The three four-quadrant modules shown can also be used in a similar way for the present invention. A four-quadrant module 330 according to Figure 3arequires at least four switching transistors 301-304. Such a circuit is also referred to as a full or H-bridge. Two of the series-connected transistors 301 and 302, or 303 and 304, form a so-called half-bridge. A diode 305-308 is connected in parallel to each of these transistors, which can also be the integrated body diode of the respective transistor. In principle, any type of controllable switch can be used for the switching elements. Accordingly, the full bridge 340 can generally be represented by four switching devices 341-344, as shown in Figure 3b shown. The transistors or switches can electrically connect the output terminals 310 and 311 to the capacitor 309.

[0013] Explained using the Figure 3a Basically, four states can be switched: Specification of a positive terminal voltage with respect to the two terminals 310 and 311 in any current direction; achievable by simultaneously switching the transistor switches 301 and 304; Specification of a negative terminal voltage with respect to the two terminals 310 and 311 in any current direction; achievable by simultaneously switching the transistor switches 302 and 303; Bypass state (i.e. no energy absorption or absorption by the individual module), free current flow in any direction; achievable by simultaneously switching the transistor switches 301 and 303 or the transistor switches 302 and 304; Forcing energy absorption by the individual module by forcing the voltage level; for example, when all transistor switches are blocked and the amount of the externally applied terminal voltage exceeds the voltage of the capacitor 309.

[0014] Alternatively, the modules can be designed as two-quadrant modules 350, as in Figure 3c shown. In this case, only a half-bridge consisting of two series-connected switches 323 and 324 is used. This allows the two output terminals 320 and 321 to be switched so that either a positive voltage is applied to the output terminals or they are switched to the bypass state. A chain of modules can then represent almost any voltage waveform, for example, a sine wave, depending on the individually selected switching states of the modules and their change over time according to a step function.

[0015] Depending on the application, the modules of the modular multilevel converter can be connected for n sources and m outputs (e.g. m=3 for a three-phase low-voltage system).

[0016] The phase modules form the basic building blocks of a single-phase or multi-phase converter. For example, three phase modules 401, 402 and 403 connected together according to Figure 4a system for converting a direct voltage into a 3-phase alternating voltage can be used, whereby the connection points of two bridge branches each form a connection 410, 411 and 412 for the three phases of the three-phase system.

[0017] In Figure 5 shows an example of the interconnection of five phase modules 501 to 505 to couple a three-phase AC network with a two-phase network. Such a system can be used, for example, to convert a 50 Hz AC power grid into a 60 Hz AC power grid. For this purpose, the three phases of the 50 Hz system would be connected via the three terminals to the left part of the Figure 5 shown circuit, which creates a direct voltage, which is then fed via the common terminals to the right part of the circuit, which creates a two-phase alternating voltage at 50 Hz. In the phase module according to Figure 2, consisting of 2 bridge arms of 3 modules each, the operating voltage for each individual module can be, for example, 1 kV. In the case of two-quadrant modules, each module can switch a voltage of 0 V or 1 kV at its terminals. Therefore, a voltage of 3 kV should be present in the DC circuit. This allows all module configurations of all 6 modules involved, whose total series connection results in a voltage of 3 kV. Compared to a fictitious center tap of the DC circuit, the output of the phase module would be Figure 2so that the states -1.5 kV, -0.5 kV, +0.5 kV, 1.5 kV can be assumed. When using four-quadrant modules, according to the above example, each module would be able to switch a voltage of -1 kV, 0 kV or 1 kV at its terminals. This means that such a phase module can also output voltages that are greater than the positive voltage of the DC circuit or whose negative voltage can extend below the negative voltage of the DC circuit. Accordingly, a lower voltage can be used in the DC circuit. Generally speaking, with a number z of modules per bridge branch, 2z+1 different stable voltage states can be generated across each phase module.

[0018] The described modular multilevel converters are generally used for high power applications, for example in energy networks similar to the one described in relation to Figure 5 described example.

[0019] In contrast, smaller electrical power outputs (up to several 100 kVA) are converted using pulse-width modulated (PWM) converters, in which the average voltages and currents are achieved by a suitable duty cycle (instead of the step function of the multilevel converter) of the switched variables. The voltage is switched either between "+" and "-" or between "+" and "0" or "-" and "0" of the DC voltage circuit. They are referred to as two-point or three-point converters, as they switch back and forth between two or three voltage levels. A sine wave, for example, is formed by the duty cycle steadily increasing from 0 to reach the maximum duty cycle near the maximum of the sine wave.From there, the duty cycle decreases again, reaching the value 0 at the zero crossing and the maximum duty cycle with negative voltage at the maximum negative value of the sine wave (three-point converter), or reaching the value 0 at the maximum negative value of the sine wave (two-point converter). However, similar to thyristor-based converters, this type of converter generates a very high level of harmonics due to the high-frequency switching processes and consistently high pulse amplitude, which necessitates complex filtering of the inputs and outputs. For example, a switching frequency of 50 kHz is typically required to simulate a sine wave of 50 Hz. At very high voltages (and thus also high power levels), the necessary dielectric strength of the components is not available, and switching losses and filtering complexity also increase significantly.

[0020] Conventional modular multilevel converters (MMC) offer great advantages in this respect, but have the following disadvantages for smaller and medium power outputs (e.g. 100 VA to 1 MVA): In order to generate a sufficiently finely stepped voltage curve, a large number of modules are necessary for MMCs (for example, 100 modules per phase are necessary for a total of 50 voltage steps). This large number of modules is independent of the switched power and therefore high costs are incurred even where only low power levels are switched. For many applications, the costs are therefore too high. Since each module in the MMC has to be controlled and supplied individually, the electronics expenditure required for this is disproportionately high, particularly for low power levels, which makes such systems very expensive. At low frequencies of the associated AC system, high capacitances are required for the module capacitors in order to be able to maintain the voltage levels for a sufficiently long time.While this isn't a particular disadvantage for grid-connected converters, it is particularly detrimental in motor converters, such as those used in electric vehicles. The dimensioning would have to take into account the low frequency of regularly recurring motor start-ups.

[0021] From WO2007 / 028350 A1, a power converter is known which has phase elements, each of which has at least one series connection of switching elements, each of which comprises at least two turn-off power semiconductors and at least two freewheeling diodes and energy storage means connected in parallel thereto.

[0022] US6005788 A discloses a multi-level power converter that includes multiple DC voltage sources that provide different DC voltage levels. The multi-level electrical power converter also includes inverter switching devices connected to the DC voltage sources and control means for controlling the inverter switching devices.

[0023] Multilevel converter circuits are known from WO02 / 063758 A1, in which each module has a voltage source and these voltage sources are graded relative to one another according to powers of two.

[0024] In Rodriguez J et al: "Multilevel Voltage-Source-Converter Topologies for Industrial Medium-Voltage Drives" an overview of available multilevel converter types is given, with particular emphasis on redundant module configurations and voltage grading by powers of two.

[0025] Du Z et al., "A Cascade Multilevel Inverter Using a Single DC Power Source," proposes a multilevel converter in which a main module is supplied with a constant voltage and additional modules are supplied via energy storage devices. The voltage gradation of the individual modules relative to each other is achieved according to powers of two, and the modules can contribute their voltage directly or inverted to the voltage path, or they can be bridged. This allows specific output voltage levels to be achieved with different module configurations. OVERVIEW OF THE INVENTION

[0026] It is an object of the present invention to provide an improved modular power converter for outputting different voltage levels and a method for operating such a power converter.

[0027] The problem is solved by the subject matter of the independent patent claims.

[0028] Preferred embodiments of the present invention are the subject of the subclaims.

[0029] According to a first aspect, the invention relates to a modular power converter which is constructed from at least one main module to be actively supplied and any number of N-1 additional modules. All modules are connected to one another in series (e.g. via module terminals, whereby the term module terminal is intended to include any other type of electrical connection and in particular plug connections). Each module has switching devices and at least one energy storage device, whereby the individual module is capable of being charged with an adjustable voltage. The switching devices, which are preferably implemented by transistors, allow the module to be connected according to active operation with respect to the series connection or according to bypass operation, in which the module in question is essentially bridged and thus cannot contribute to the voltage path of the series connection.The output voltage of the power converter is tapped at the end points of the series connection. Tapping therefore preferably occurs at the corresponding output terminals, whereby the term output terminals encompasses any type of electrical connection option. The main module is powered by a DC voltage source, so that the main module is maintained at a substantially constant voltage level by this DC voltage source, which voltage level can be supplied by the main module to the voltage path during active operation. The main module preferably has a storage device, such as a capacitor, but this is not absolutely necessary due to the supply from a DC voltage source. As a result, each module can contribute to the total voltage of the voltage path with the voltage prevailing at its storage device (active operation) or not (bypass operation).The corresponding operation can be adjusted via the respective switching devices.

[0030] The voltage contributions to be delivered by the individual modules differ from one another. The voltage contributions are preferably graded in powers of two, but other gradations such as powers of three or non-exponential ratios are also possible. Since only the main module (or modules), which do not necessarily have to be the first module in the series, are supplied with DC voltage, the other modules are charged via the serial connection to the main module. Since a large number of different voltage levels are therefore available, such as U 0 / 0, U 0 / 2, U 0 / 4, U 0 / 8, U 0 / 16, etc., these partial voltages can be used to represent any achievable output voltage at any time, corresponding to a desired output voltage curve.The modules whose partial voltages or voltage contributions are required to generate the desired output voltage are switched to active mode, while the unused modules are switched to bypass mode, so that the desired output voltage is achieved overall. Thus, starting, for example, from a gradation based on powers of two, a large number of resulting voltage levels can be achieved with a relatively small number of modules, in this example according to the binary principle.

[0031] According to a preferred aspect of the present invention, the switching devices of the modules are designed such that the module can be connected not only in series with the voltage path but also in anti-serial, thus additionally enabling an inverted operating state. This allows a specific module to act on the voltage path in an inverted manner with its terminal voltage, for example with -Uo / 4. This inverted operation enables an increase in the possible combinations of the individual voltage contributions and, in particular, makes it possible to represent a specific desired output voltage using at least two or more different module configurations or module operating mode connections. The inverted operating mode of a module has the advantage that the relevant storage device, preferably a capacitor, can be recharged during operation in this way.If a module contributes its voltage component to the voltage path, the corresponding storage device is discharged due to the load current. In inverted operation, however, the load current charges the module in question, so that the resulting voltage range of this module can be kept within predetermined limits. If the charge level of a storage device in a module threatens to fall below a limit value W 1 , this module is switched to inverted operation in a subsequent phase, as a result of which the storage device recharges (it may well have been switched to bypass mode before this subsequent phase). This charging then takes place up to the limit value W 2 , at which point the module must either be switched back to discharge mode, i.e. active operation, or to bypass mode to prevent overcharging.In this way, each storage device in each module can be maintained within a predetermined voltage range. From an electrical perspective, this can be independent of the module's actual storage capacity. The use of smaller capacities will therefore require more frequent switching operations than when using larger capacities, as recharging must occur at an earlier time. The capacities of the individual modules can be designed identically, even though the modules are intended to provide different voltage contributions. However, they can also be matched to their assigned voltage level. All of this must be taken into account when controlling the operating mode switching. The voltage range at which a specific module is maintained, for example the voltage Uo / 4 + / - Δ, is essentially achieved by specifically switching the module via the switching devices into either active or bypass mode or, if available, inverted mode.If more than one module configuration is available to represent a specific output voltage, this redundancy allows modules that are to be recharged to be set to inverted operation in one of the next cycles if necessary, while still achieving an essentially constant output voltage.

[0032] According to a further aspect of the invention, the switching devices of the modules are controlled by a controller device, which preferably switches all modules simultaneously and is responsible for detecting the current charge states of the modules and selecting the appropriate operating settings for each module. Due to the interaction of the controller device with the power converter, it is thus possible to map a desired output voltage curve over time.Due to the possibility of maintaining a specific output voltage level at a constant level in this aspect of the invention despite switching from one module configuration to another (thereby enabling modules that reach a lower charging limit to be recharged in subsequent inverted operation without the overall output voltage having to change as a result), this not only enables the representation of changing output voltage waveforms, for example periodic waveforms, but also the representation of DC voltage in the output circuit. This behavior is particularly interesting for applications in which very slow step changes from one output level to the next output level are to take place over time and thus the output voltage is to remain at one voltage level (e.g. binary level) for a certain period of time.

[0033] Preferably, so-called four-quadrant modules are used for the modules, although other circuit types, such as two-quadrant modules, are also possible, particularly for the main module.

[0034] According to a further aspect, the power converter according to the invention also enables energy reversal, in which energy is introduced from the output side into the DC voltage source that feeds the main module. In this operating mode, the load current temporarily flows in the opposite direction, which results in the sign of the charging behavior of the modules' storage devices being reversed. For example, a module in inverted operation is then no longer charged but discharged. These relationships must be taken into account by the controller device that switches the operating states of the modules.

[0035] The controller device can be centralized or decentralized. In particular, it is possible for there to be a central controller device, but each module can also have a local controller device that switches the switching device in the module and records the current charge level of the storage device and reports it to the central controller device.

[0036] According to a further aspect, the power converter according to the invention is provided without a controller device and thus essentially comprises the aforementioned module chain or modules. It can be coupled to an external controller device for subsequent operation.

[0037] According to a further aspect, the power converter according to the invention is designed for multi-phase systems and then has multiple module chains. For example, the power converter can have three module chains if it is intended to control a three-phase electric motor. For such multi-phase systems, it can be advantageous to provide adapted switching devices in the modules, in particular multi-phase modules.

[0038] According to a preferred embodiment, the main module is supplied with a direct voltage Uo, and in the case of single-phase systems, this results in output voltages in the range between +Uo and -Uo. However, it will be clear to those skilled in the art that, at least briefly, output voltages in the range of almost +2Uo and -2Uo can also be achieved, whereby the generation of these voltages above Uo or below -Uo does not fall under the protection of the independent patent claims. In particular, these maximum voltages are achieved by switching all modules to active mode (Uo + ½ Uo + ¼ Uo + 1 / 8 Uo, etc.) or all to inverted mode (-Uo - ½ Uo - ¼ Uo - 1 / 8 Uo, etc.).

[0039] According to a further aspect, the present invention relates to a method for operating a power converter according to which N modules are connected in series, a DC voltage Uo is applied to the input terminals of the main module, a desired temporal profile of the output voltage in relation to the amplitude over time is specified, an output voltage is tapped at the two end points of the series connection of the modules and predetermined charge states are maintained for the individual modules by each module in its active operating state making essentially a predetermined voltage contribution to the series connection, wherein the voltage contributions of the modules are different from one another and are preferably graded according to powers of two, and by the circuit devices of the modules are controlled in order to switch each module to an operating state in each case, such thatthat the sum of the individual module voltages as provided by the energy storage device of each module according to its active, inverted or bypass operation corresponds at any time to the specified range of the output voltage to be supplied.

[0040] According to a further aspect of the invention, the power converters according to the invention are used for AC / AC operation by coupling two such power converters via the DC circuit. In this way, for example, a power converter can be constructed that connects two AC systems with different voltages and / or different frequencies and / or different phase numbers, thus enabling a bidirectional energy exchange between these systems. According to a further preferred aspect, a power converter according to the invention is used to power an electric motor of an electric vehicle. According to a further preferred aspect, the power converter according to the invention is used to connect typical three-phase mains voltages to a DC network, for example to connect house connections to a local DC network. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: Fig. 1 describes the principle of a three-phase two-level converter; Fig. 2 shows a phase module of a conventional modular multi-level converter, which is constructed from two bridge arms, each with three identical two-poles; Fig. 3a - c shows the basic structure of a four-quadrant module or a two-quadrant module, as can also be used for the present invention; Fig. 4 shows a system for converting a 3-phase alternating voltage into a 1-phase alternating voltage or a direct voltage, which consists of three interconnected phase modules; Fig. 5 shows a system for coupling a 3-phase three-phase network with a two-phase network, which consists of three and two interconnected phase modules; Fig. 6a shows a basic circuit diagram to explain the basic principle of the present invention using a preferred embodiment; Fig.6b - d shows the basic structure of a single-phase converter circuit according to one aspect of the invention, consisting of, for example, four modules; Fig. 7 shows an illustration of possible voltage combinations by activated modules for a single-phase converter circuit, consisting of, for example, four modules similar to . Figure 6shows; Fig. 8a - b shows illustrations of possible stepped curves of an alternating voltage, as they can be generated according to aspects of the invention; Fig. 9 shows the principle of supplying a three-phase load with separate, isolated connections via three single-phase devices according to a further aspect of the present invention; Fig. 10 shows a first embodiment of an actively supplied module for three phases of a multi-phase power converter according to a further aspect of the invention; Fig. 11 shows a second embodiment of an actively supplied module for one phase of a multi-phase power converter according to a further aspect of the invention; Fig. 12 shows possible switching states and the generated output voltages of the second embodiment for one phase within a multi-phase power converter; Fig.13 shows a third embodiment of an actively supplied module for one phase within a three-phase power converter according to a further aspect of the invention; Fig. 14 shows an illustration of the functional principle using a circuit with two modules for generating a set DC voltage and the time characteristic; Fig. 15 shows an illustration of the functional principle using a circuit with three modules for generating a set DC voltage and the time characteristic; Fig. 16 shows an illustration of a step function with an enlarged section for generating individual voltage steps; Fig. 17 shows sequences of switching steps as required to generate a step voltage curve.

[0042] In the drawings, like reference symbols designate like parts, components and arrangements. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Basic aspects:

[0043] According to one aspect, the invention is based on the discovery of realizing power converters for each phase by connecting modular two-terminal networks in series. The two-terminal networks are preferably staggered with respect to their terminal voltage according to powers of two, such that, with n modules, 2 (n-1)< voltage levels can be realized at the ends of the series connection. Preferably, the module with the highest voltage level is connected to the DC voltage circuit.

[0044] Preferably, the individual modules are constructed as four-quadrant modules, although the use of two-quadrant modules is also possible. Preferably, the terminal voltage of each module is adjusted by directly or indirectly measuring it and charging or discharging it accordingly, which in turn is achieved by appropriately switching the respective transistors (or other suitable switching devices).

[0045] Although a preferred embodiment assumes a strict staggering of the modules in series according to their maximum terminal voltage in order to optimize component complexity, particularly for the module capacitor (the module with the highest terminal voltage uses the capacitor with the highest dielectric strength), according to one aspect of the invention, the sequence can be chosen arbitrarily (as long as the dielectric strengths of the individual capacitors are not exceeded), and this sequence can be changed during operation by a controller or control device designed for this purpose. For example, if a module fails, its value can be set by the controller to the smallest possible binary value, whereby the failure of this module would only be noticeable in a slight deterioration in the staggering.

[0046] Preferably, the design of an individual module does not necessarily define the maximum adjustable terminal voltage at it; instead, this is set by a higher-level control or controller device that can detect the current potential through appropriate charging and discharging. The current terminal voltage at a module can be detected directly at the module (direct measurement) or at a higher level (indirect measurement) by measuring only the total terminal voltage of the series circuit, taking into account at least the current switching states and preferably previous switching states. Preferably, the voltage changes of the module capacitors can be taken into account via the current flowing through the series circuit and their capacitance.

[0047] According to a further aspect, the invention is based on the finding that a power converter constructed from a series circuit of several individual modules functioning as two-terminals must be designed such that the voltages of these individual modules have different values, preferably staggered by powers of two. The individual modules should contain energy storage elements and be designed such that they output the voltage of the energy storage element at their respective two terminals, output its inverted voltage, or be switched to a bypass mode (voltage drop 0), in which a conductive connection is established between the terminals without charging or discharging the energy storage element.In this way, the series connection of the individual modules via the corresponding activation, deactivation / bypass mode and reversing / inverting of the energy storage elements results in a very finely graded adjustment of the output voltage and thus of the desired voltage curve and at the same time enables the energy storage elements of the modules to be optionally charged or discharged.

[0048] According to a further aspect of the invention, the gradation of the selected voltages of the modules enables there to be at least two module configurations for each voltage level to be output at the terminals of the series connection, so that the energy storage elements of the respective modules can be selectively charged or discharged. In other words, and expressed for the operating state of a power output by the series connection to a load, this should enable the recharging of a module in the inverted connection, even though the same module was actively connected in a previous switching state and was discharged in the process. This is important because recharging is not possible in bypass mode or deactivated mode.

[0049] According to a further aspect of the invention, a DC power supply is coupled to a special module such that this special module optionally supplies the voltage of the DC power supply, a bypass state, or optionally the inverted voltage of the DC power supply at its terminals. The further individual modules connected in series with this special switching module, together with this special module, form a phase of a single- or multi-phase power converter, with the ends of this series connection representing the outer connections for each phase. This aspect enables the construction of power converters that are fundamentally different from conventional power converters with two bridge arms. Figure 2In other words, the dedicated module itself already forms one phase of a two- or three-level converter, while the additional modules in the series are passively connected and—unless switched to standby mode—are supplied by the current through the series circuit, further increasing the number of displayable voltage levels.

[0050] Figure 6ashows the basic structure of a power converter according to this aspect of the invention. The power converter is formed by a chain of n modules 602 connected in series. From an electrical perspective, the modules represent two-pole networks. Preferably, at least with the exception of the top main module, all n-1 modules are constructed identically. The main module has terminals for applying a DC voltage Uo. The modules have switching devices inside, preferably formed by transistors 620. These switching devices are controlled by a controller device 604. This control can be carried out centrally, as shown. Alternatively or additionally, each module can have its own control device. Each module also has a storage device, such as a capacitor 606.Through interaction with the controller device, the charge state of at least each of the n-1 additional module capacitors is detected or measured and thus monitored. By switching the switching devices of the modules at specific times as a function of this, the module voltage that can be delivered to the voltage path in the active state for each module can be kept within a desired range. According to the invention, the module voltages of the individual modules are different. Preferably, starting from the main module with the voltage U 0, each additional n-1 module has a gradually reduced voltage. Preferably, the module voltages are staggered according to powers of two with respect to one another, as stated. The output is the voltage at the end points of the series circuit.

[0051] Each module can be switched to active mode, and its module voltage can be introduced into the voltage path. In bypass mode, the module is switched through, and its module voltage is not introduced into the serial voltage path. Switched to a preferably provided inverted mode, each module can introduce its inverted voltage into the voltage path. It will be clear to those skilled in the art that this allows for a large number of displayable voltage levels at the output, achievable with a small number of modules. Switching to inverted mode enables the storage device of the corresponding module to be recharged.

[0052] Further details are described in the following figures, although for the sake of simplicity the controller device is partially not shown.

[0053] The special module or main module powered by the DC power supply preferably represents the module with the highest dielectric strength and can be considered as the actively supplied module (or main module). The other individual modules connected in series can be designed as four-quadrant modules 330, 340 of a modular multilevel converter, as described in the Figures 3a and 3b MOSFETs or IGBTs, for example, can be used as transistors here. Instead of the transistors 301 to 304 shown, other electrically switching components can also be used as transistors, such as GTOs or IGCTs. Furthermore, diodes can be connected in parallel with the switching components in such a way that they block the corresponding forward direction of the switching component.

[0054] The special wiring according to Figures 3a and 3ballows each individual energy storage element in the series circuit to be connected either positively or negatively in series with the voltage source, or to be left out of the circuit in the form of a bypass circuit. Thus, selected energy storage elements (such as capacitors) in the series circuit are connected in series or anti-serial with the voltage source at different voltages. The two terminals at the ends of such a chain are considered the system's output. However, due to the bidirectional power flow, they can also function as the circuit's inputs.

[0055] The energy storage elements are preferably charge storage devices, such as capacitors or batteries, which do not require an additional external power supply. However, other energy storage elements, such as double-layer capacitors or lithium-ion capacitors, can also be used. By connecting these energy storage elements in series with the actively supplied modules and a load, the system can transport charge from the actively supplied modules into or out of the energy storage elements—depending on the currently selected polarity in the module.In particular, with appropriate selection of the voltages of the storage elements and their interconnection, voltages Uo ≤ U ≤ Uo can be generated at the output terminals of the series connection, where Uo represents the voltage of the actively supplied module (this range applies at least if recharging of individual modules is to be possible; without this requirement, almost twice the voltage swing would be possible for a short time). The voltage at the output terminals is therefore the sum of the partial voltages of the actively switched modules, i.e. the modules that are not in bypass operation. Since the non-supplied modules in particular are designed as four-quadrant modules, their voltage contributions can also be negative. The properties of the actively supplied module result in two basic options for connecting the modules in series: . the actively supplied module is designed as a two-quadrant module; in this case it can only output a positive voltage Uo or 0V. Accordingly, the output terminals of the series connection only produce a stepped voltage between 0V and U0, since the sum of the voltage contributions from the other modules is negative as long as the voltage at the actively supplied module is Uo. In this variant, the other modules can be designed as four-quadrant or two-quadrant modules. the actively supplied module is designed as a four-quadrant module; in this case it can output a positive voltage Uo, a negative voltage -Uo or 0V. Accordingly, a stepped voltage between -U0 and Uo - and thus a true AC voltage waveform can be output at the output terminals. In this variant, the other modules should also be designed as four-quadrant modules.

[0056] The current at the system's output terminals (which can be a load current or a source current) creates a charge transport through all modules, which in turn changes the voltages of the individual active energy storage elements over time. The voltage of a capacitor (which serves as the energy storage element of a module) decreases over time if this capacitor is connected in series with the voltage source—according to the currently selected polarity—or its voltage increases if it is connected in reverse series with the source.

[0057] By continuously switching the energy storage elements of all participating modules (except for the module with the highest dielectric strength) during operation, the average charge and thus also the voltage of each energy storage element can be maintained. In particular, for such switching processes, the modules to be activated can be selected such that reversing the polarity of the energy storage elements, on the one hand, balances the charge of each energy storage element in the desired manner (so that its voltage value is roughly maintained) and, on the other hand, also maintains the desired total voltage of the series connection of the modules. In this way, through constant periodic switching, almost constant DC voltages can be generated at the system output or changed in small steps.

[0058] Changing the output voltage by changing the configuration of actively switched modules occurs with very low losses, since only the forward losses of the transistors of the modules, the ohmic losses of the energy storage elements and possibly switching losses of the transistors occur.

[0059] For grading the voltages of the individual energy storage elements, grading in powers of two is particularly advantageous. This means that the energy storage element with the highest voltage is designed for a voltage of Uo; for the other storage elements, the voltage is halved. This design has the advantage that a large number of equidistant voltage levels can be generated using the method described above with relatively few energy storage devices. If the voltage source inputs are considered a DC intermediate circuit, the actively supplied module is designed as a four-quadrant module, and the output terminals are considered an AC circuit (although the system can also optionally generate DC voltages), the resulting system forms a single-phase power converter. Single-phase system:

[0060] The above principles apply analogously to the further explanations, which initially focus on a single-phase system.

[0061] Single-phase systems, as a preferred embodiment, are required, for example, to generate single-phase alternating current, such as 230V / 50Hz, from a direct current source. One possible application is in the field of photovoltaics. An energy storage unit is charged over time using photovoltaic panels and is intended to supply a system, such as a house. The house system typically requires 230V / 50Hz. The power converter is capable of managing this conversion between the energy storage unit and the house's electrical system.

[0062] Figure 6bshows, by way of example and schematically, such a system consisting of four modules, each designed as a four-quadrant module. It is clear to the person skilled in the art, and in particular from the above explanations, that any number of modules can be present and that these can be connected via any type of switching device. They can preferably be designed as two-quadrant or four-quadrant modules. A single-phase power converter 610 consists of four modules 640, 650, 660, 670, with each module symbolically represented by four switches. Module 640 represents the actively supplied module. The serially connected modules 650, 660, 670 following below are all designed as passive four-quadrant modules, each with a storage capacitor 652, 662, 672, whereby the voltages of the modules 650, 660, 670 are each halved again, as shown in Figure 6aspecified. For the four modules shown (one actively supplied module and three passive capacitor modules), the individual voltages are divided according to the values ​​U 0 , U 0 / 2, U 0 / 4, U 0 / 8. The output voltage can therefore be shown in steps of one eighth relative to the maximum voltage Uo. The order of the modules with their respective voltages is not fixed to the order shown. For example, the module with Uo / 2 could be the last in the series. Only the actively supplied module, which provides the voltage Uo, is fixed. This is preferably the first module in the series, although this is not mandatory. The terminal pair 680, 681 represent the connections for the DC voltage system (e.g. battery). The output terminals 690, 691 form the AC circuit. With this system, a variable positive or negative voltage in the range -U 0 and U 0 can be generated at the output terminals.Typically, the output voltage is an AC voltage with the desired frequency and amplitude (e.g., 230V / 50Hz). However, due to the ability to recharge the module capacitors, a DC voltage can also be generated on the output side. The system can be used to output energy from the DC source and feed it into an AC grid via the output terminals as AC voltage. However, the reverse operation is also possible, with the AC grid charging the DC voltage source.

[0063] Figure 6cshows a further embodiment with additional details. All modules are designed as four-quadrant modules. The module chain 620 represents the power converter with transistors and the associated body diodes. Furthermore, the module chain 620 also shows the associated controller circuit 622. This controller circuit serves to determine the required switching states of the modules and pass them on to the modules. For example, the voltages of the energy storage elements of all modules are recorded, as is the current flowing through the module chain (this corresponds to the output current of the power converter, which can be positive or negative due to the bidirectionality of the power converter). Based on the individual voltages of the energy storage elements and a comparison with a respective target voltage, the controller circuit decides for each individual module whether it should be charged or discharged, provided it should not be switched to the bypass state.By specifying a desired instantaneous target voltage, which in turn is derived from a desired voltage waveform, and the polarity of the output current, the controller circuit can then calculate the switching states of each module required to achieve the desired voltage waveform at the current time. It can then output the required transistor control signals (or switching device control signals in general) to all modules accordingly.

[0064] The modular design of the power converter in combination with the controller circuit allows for a very high degree of flexibility of the power converter, as a large part of the power converter's operating strategy can be stored in the controller circuit in the form of replaceable software. In this way, the controller circuit can be easily adapted to different operating strategies of the power converter through appropriate reprogramming or parameter adjustment. For example, the voltage tolerance can be determined, according to which the controller circuit decides whether an energy storage element of a module should be recharged or discharged. Such a control program can also be used to variably define whether pulse width modulation at the level of a voltage step should be used in addition to the step functions.This allows for a smoother average output voltage curve; on the other hand, it increases switching losses. The converter can also be adapted to a changed voltage in the DC circuit by reconfiguring all of the modules' target voltage values ​​accordingly. Furthermore, the controller circuit can be adjusted via its software so that the converter generates fewer voltage steps than would be technically possible. Such an operating strategy can, for example, reduce semiconductor switching losses and capacitor losses at high converter power levels if the converter's operation allows for a lower number of steps.The corresponding programming and reconfiguration of the controller circuit can also be carried out optically or wirelessly, so that no galvanic connection to the power converter or its control system is required.

[0065] Some of the functionality required to control the power converter can also be transferred to individual control units, each of which is directly connected to the individual modules. These control units can, for example, check the charge levels of the energy storage elements, so that only the charge level is reported to the controller circuit, not the charging voltage. Furthermore, the control units can perform safety functions and only allow certain switching combinations of the switching elements, or, in the event of a fault, put a particular module into bypass mode. Communication between these control units and the controller circuit can, for example, take place via a potential-free bidirectional data channel. Here, too, data can be transmitted wirelessly or optically.

[0066] The module chain 630 in Figure 6dsymbolically shows the same modules with a length corresponding to their respective voltage in terms of powers of two (if so chosen).

[0067] A corresponding load current (or current from a source) discharges or charges the module capacitors depending on the switched polarity, which leads to a corresponding increase or decrease in the capacitor voltage. Therefore, a module whose capacitor is, for example, too deeply discharged (i.e., whose voltage is below a respective target value by a small tolerance limit) is switched in a next step so that its capacitor is in the opposite direction in the current path. In this way, the corresponding capacitor is recharged by the load current. However, since this measure changes the overall voltage, a different combination of modules must then be switched according to the desired target voltage, but which delivers the same output voltage. Details on this principle of recharging with a constant output voltage are described in connection with the Figures 14 and 15 explained.

[0068] The operating principle can also be reversed; this means that electrical power is supplied to the output terminals and delivered via the DC circuit. The voltage supplied via the output terminals must be at least equal to the voltage of the energy storage element with the lowest voltage (lowest power of two) (i.e., typically the lowest module in a strictly staggered arrangement).

[0069] Instead of a single actively supplied module or voltage source, several such actively supplied modules can be connected in series.

[0070] Figure 7 shows examples of possible different switching combinations of the four modules according to Figure 6busing voltage arrows. The arrow length in this representation is proportional to the respective voltage. The black arrows symbolize the voltages of the currently active modules; the grey arrow corresponds to the resulting output voltage of the system, ie the output voltage of the series connection of modules. Arrow 730 of the switching combination 710 represents the voltage Uo of the actively supplied module. Figure 7 The modules 650, 660, 670 are made of Figure 6b All are active, but have inverse polarity. Accordingly, arrows 732, 734, and 736 are shown with the opposite direction.

[0071] The arrow directions also indicate the direction of current flow. If a load is connected to the output terminals, a downward arrow direction corresponds to current output, and an upward arrow direction corresponds to current consumption. In this case, the actively supplied module 640 is being discharged, while modules 650, 660, and 670 are being charged. The resulting voltage, which can be tapped at terminal pair 690, 691, is represented by arrow 738. Accordingly, the voltage at the output terminals is positive and has the value U 0 / 8.

[0072] With the switching combination 712 according to Figure 7The same output voltage of Uo / 8 is generated as in the first configuration. However, the actively supplied module is now operated in bypass mode, so it does not output any voltage. Module 650 is now discharged, while modules 660 and 670 continue to be charged. Accordingly, there are switching combinations not shown here in which modules 660 and 670 can be discharged.

[0073] This aspect is discussed in connection with Figure 17 explained in detail. Switching combinations 714 and 716 each generate voltages of U 0 / 4, while switching combinations 718 and 720 generate voltages of 3 / 8 Uo. Switching combination 722 can generate a voltage of Uo / 2 at the output terminals. By reversing all arrow directions, corresponding negative voltages can be generated.

[0074] For a module chain consisting of an actively supplied module with voltage Uo and a further n-1 four-quadrant modules (i.e. there are n modules in total), with module voltages graded in powers of two, the following relationship can be shown: At the output terminals of such a module chain, any voltage from -Uo to Uo can be represented in steps of U 0 / (2 (n-1)< ), regardless of the current state of charge of the individual energy storage elements (e.g. capacitors) in the modules. State of charge means that for each module the respective energy storage device is either in a state that it should be charged (current voltage is below the target voltage for this module) or in a state that it should be discharged (current voltage is above or equal to the target voltage for this module). Since, for example, when capacitors are used as energy storage elements, the voltage changes during charging or discharging.Since the voltage changes continuously during discharging, in order to maintain a certain output voltage under load for an extended period - depending on the charge level of the module capacitors - it is necessary to constantly switch back and forth between certain module configurations. These specific module configurations together supply a constant output voltage, but allow the necessary recharging or discharging of certain module capacitors without this leading to a noticeable change in the output voltage. Therefore, for each representable output voltage - except for 0V (all modules in bypass state), the maximum voltage Uo (only the actively supplied module supplies voltage) and the minimum voltage -Uo (here the actively supplied module supplies its inverted voltage) - there are always at least two module configurations to represent this output voltage. Further details on this principle are provided in connection with the . Figures 14 and 15 explained.

[0075] Although grading module voltages in powers of two is preferred and allows for maximum flexibility and ease of control, other gradations are also conceivable, for example, in powers of three. However, this may require certain module voltages to be present twice or more than in the case of powers of two.

[0076] Furthermore, in case several DC circuits are available, configurations can also be used in which several actively supplied modules are used, each connected to a DC circuit.

[0077] The frequency at which module configurations must be switched is determined, on the one hand, by the permissible tolerance of the module capacitor voltages and their influence on the output voltage, the load current, and the capacitance of the capacitors. In particular, very low switching frequencies can be achieved by using large capacitances for the module capacitors, since the voltage changes occur more slowly. However, when outputting alternating voltage, the frequency of this alternating voltage determines the minimum permissible switching frequency (i.e., the switching instant must be significantly earlier than would actually be electrically required by the module capacitor due to the frequency to be represented and thus the rise of the step function).Conversely, the capacitance of the capacitors and thus the required capacitor volume - which, in addition to the filter costs, represents an important cost factor for power converters - can be greatly reduced by deliberately carrying out the above process at a relatively high switching frequency.

[0078] Furthermore, and especially in connection with the above, the modules can be used to apply pulse width modulation to the generated step function. This is particularly useful where a relatively coarse stepping occurs due to a small number of modules. The superimposed pulse width modulation can be used to further optimize the average value. In connection with Figure 8b Further details about this variant are explained.

[0079] High switching frequencies also have the advantage that a filter circuit connected between the module chain and the load to smooth the output current and voltage only needs to store a small amount of energy (depending on the voltage to be absorbed and the period between switching states) and can therefore be designed smaller. However, increasing the switching frequency increases the switching losses of the transistors.

[0080] The above considerations for generating variable, step-adjustable output voltages can also be reversed, so that the circuit allows a power flow from the right to the left terminals. The voltage applied to the right terminals only needs to be slightly higher, i.e., the voltage of the module with the lowest voltage U 0 / 2 n-1<. In this case, each passive module can be equivalently recharged first and transfer its energy to the actively supplied module (which is connected to the fixed DC voltage), thus feeding it into the DC voltage system via the left terminals. In this case, a module whose capacitor voltage exceeds a tolerance threshold due to the energy feed-in - with a corresponding change in the entire module configuration, according to the applied voltage - is then reversed into the current path and thus discharged again. Figure 6In the single-phase system shown, the output terminals have a fluctuating potential relative to the input terminals. Therefore, for example, the negative pole on the left side must not be connected to one of the two output terminals on the right side. Solutions to this problem are discussed below in connection with multi-phase systems and the Figures 9 to 13 explained.

[0081] According to a preferred embodiment, the main module is supplied with a DC voltage Uo, and in the case of single-phase systems, this results in output voltages in the range between +Uo and -Uo. However, it is clear to the person skilled in the art that under certain conditions, at least for a short time, output voltages in the range of almost +2Uo and -2Uo can be generated, whereby the generation of these voltages above Uo or below -Uo does not fall under the protection of the independent patent claims. In particular, these

[0082] Maximum voltages are achieved by switching all modules to active operation (U 0 + ½ U 0 + ¼ U 0 + 1 / 8 U 0 etc.) or all to inverted operation (-U 0 - ½ U 0 - ¼ U 0 - 1 / 8 U 0 etc.). Rules for setting voltage levels:

[0083] The following explanations explain further preferred details and can be applied analogously to the above basics and single-phase systems, as well as to the other systems described below.

[0084] According to one aspect of the present invention, a stepped voltage can be generated at the output terminals of a single-phase or multi-phase power converter by setting respective switching states for the corresponding modules of the power converter according to the following rules.

[0085] The system consists of an actively supplied module with a freely switchable voltage and additional four-quadrant modules that can be operated with either positive or negative voltage depending on the charge level or alternatively switched to bypass mode.

[0086] The combination of these modules is intended to represent total voltage values ​​up to the largest module voltage (i.e. the largest voltage that the actively supplied module can deliver), which correspond to a multiple of the smallest module voltage (i.e. the voltage of the energy storage of a four-quadrant module with the smallest terminal voltage).

[0087] Any positive output voltage generated from a combination of exclusively positive module voltages (so-called ground states) can still be represented if, for any four-quadrant module, the only option is to operate with a negative voltage value in addition to bypass mode (this statement does not apply to the maximum voltage Uo, which can only be supplied by the actively supplied module, and to the voltage 0, at which all modules are operated in the bypass state). The resulting multiple representability of an output voltage enables a sequence of switching states in which discharged modules can be charged and charged modules can be discharged in one of the subsequent switching states. This principle can also be used to change the switching state within a voltage level. The same applies to negative output voltages.

[0088] The above considerations refer to the case of energy being delivered to a load. When energy is being absorbed from a source (e.g., braking in an electric vehicle), negatively charged modules are discharged and positively charged modules are charged. This must be taken into account when connecting the modules to achieve the desired charge level.

[0089] The flexibility mentioned is based on the fundamental possibility of creating a desired positive voltage value for a specific module (except for the actively supplied module), which is currently unavailable due to the state of charge of the module in question, by combining the positive voltage of the module with the next higher voltage in combination with the negative voltage (due to the state of charge) of the module in question. Example: if a voltage of 4V is desired in a chain of modules with voltages of 1V, 2V, 4V, 8V, etc., but this module is currently being charged and therefore supplies -4V, a voltage of 4V can be achieved by combining an 8V module with the -4V of the module in question. This principle applies recursively, i.e. if the next higher module (e.g. 8V) is also only available in negative voltage (i.e. -8V), it can be compensated for with the next higher module (here 16V): 16V - 8V - 4V = 4V.Since the highest module is actively supplied, this module is always available for balancing. These considerations also apply to all negative voltages. Example: -4V = -16V + 8V + 4V.

[0090] For a specific state in which the four-quadrant modules are available with any positive or negative voltage, the following method can be used to determine the switching state of the individual modules and thus for the balancing process. Expressed as a rule, the following cases can be distinguished for these balancing processes (gradation of the voltages of the energy storage devices in powers of two is assumed as an example): The four-quadrant modules are connected, starting with the lowest voltage value, in bypass mode or with a positive voltage value depending on the base state. If a four-quadrant module is required in the base state with a positive voltage value, but can only be connected with a negative voltage value due to the state of charge, it is connected with a negative voltage value. The next four-quadrant modules are then connected according to the following rule: ▪ If the four-quadrant module with the next higher voltage is not required for a base state, it is always connected. With a positive voltage state, voltage compensation is thus completed. With a negative voltage state (inverted to the charge), the level of the voltage to be compensated is doubled and must then be compensated with the module with the next higher voltage.▪ If the voltage of the four-quadrant module with the next higher voltage was required for the ground state, it is always operated in bypass mode. The requirement for the voltage to be balanced is thus doubled again.

[0091] Since the requirement for the voltage to be balanced is either balanced or doubled by the next four-quadrant module, a final balancing by the actively supplied module is always possible at the latest.

[0092] The same logic applies with the opposite sign for the generation of negative total voltages.

[0093] Example: If an output voltage of 6V is desired in the previously mentioned chain of modules with voltages of 1V, 2V, 4V, 8V, etc., the base state would be 2V + 4V = 6V; i.e., only the two modules for 2V and 4V would need to be switched to positive, while all other modules would need to be switched to bypass. If—deviating from the base state—the 2V module with positive voltage is not available, it is still needed (according to the rules above); therefore, it is switched on inverted. Furthermore, the next higher module, the 4V module, is switched to bypass (since it was needed for the base state according to the rules above). The next module in terms of voltage, the 8V module, is switched back on (since it is not needed for the base state according to the rules above). In this case, the result would be: -2V + 8V = 6V.However, if the 8V module is also not positively available, it is still used – albeit inverted – and the next higher module is activated: -2V - 8V + 16V = 6V. This can be continued if necessary until the actively supplied module, which is always available, is reached. Number of adjustable voltage levels:

[0094] In the following, the number of possible adjustable voltage levels is considered, based on a gradation of the voltages of the individual energy storage elements in powers of two. The highest voltage level is not the sum voltage of all n modules, but only the level of the actively supplied module. Expressed in binary notation, where a 1 describes a positively connected module and a 0 a module in bypass operation, this would be the vector 10000 for an assumed 5 modules. The highest theoretically possible voltage in this case would be 11111, i.e. all n modules in use with the same voltage direction - this case, however, is usually not practical, since the modules can no longer be recharged at will when operating as power converters. This vector representation (e.g. state 1; 0; -1) is a good description of the switching behavior of all modules required for a specific output voltage.The switching of the modules thus corresponds to the change of the vector to the next vector in the clock cycle. The clocking, and thus the switching to the next module configuration, preferably occurs regularly and at a frequency significantly higher than the frequency of the output signal to be displayed. Depending on the output function to be displayed and the storage capacity of the transistors, it may also be lower.

[0095] This gives the number S + positive levels as: S + = 2 n − 1 + 1

[0096] This is the number of possibilities without the highest level and "+1" through the highest level. According to the binary system, zero is already included. If you want to include the number of possible negative levels S - , you have to omit level 0, resulting in a total number S of representable voltage levels: S = S + + S − = 2 n − 1 + 1 + 2 n − 1 = 2 ⋅ 2 n − 1 + 1 = 2 n + 1

[0097] Figure 8shows the stepped waveform of a sinusoidal voltage, as it can be output via a power converter in a design consisting of five modules. The output voltage can be output in a total of 33 voltage steps. The output voltage can then be further smoothed optionally by filter circuits or, optionally, by additional pulse-width modulation of the modules; see the explanations above. Multiphase systems:

[0098] All of the above statements, particularly regarding fundamentals and single-phase systems, apply accordingly to the multi-phase systems described below.

[0099] In a multi-phase system, the fact that the actively powered module of the single-phase system is constantly reversed in polarity means that the output terminals do not have a defined ground point compared to the DC voltage connections of the input.

[0100] When using multi-phase consumers or sources where the individual phases can be operated in isolation from each other, i.e. which are not connected as a star (e.g. transformers or motors / generators with insulated, unconnected windings), multi-phase converter systems can be constructed by connecting several such single-phase converters according to Figure 6 be connected via the DC circuit. Such a system is Figure 9for controlling a three-phase motor 910. The three individual windings 920, 930 and 940 of the motor are fed by the three single-phase converters 950, 960 and 970 in such a way that a rotating field is generated in the three windings. The advantage here is that the potential of the DC voltage can be easily reversed for each phase to generate the output voltage. Accordingly, for a voltage of U 0 in the DC voltage circuit, a possible output voltage between +Uo and -Uo results for each phase (in contrast, the voltage at the terminals of a conventional converter would be Figure 1 only cover a range of +U 0 / 2 and -Uo / 2). The controller device required to control the switching devices and to record the charge states, the load current, the rotating field, etc., is not shown in this illustration for the sake of simplicity.

[0101] If the output terminals are to be referenced to a common potential or a common reference point between input and output is required, modified circuit topologies are proposed according to further aspects of the invention. Regarding the passive four-quadrant modules with their capacitors as energy storage elements, the multi-phase system is constructed similarly to the single-phase system, see above. Since the output terminals of all phases must be referenced to a common potential, an adapted, actively supplied module must be used, which is described below.

[0102] In the figures of the systems described below, the voltage in the DC circuit is formally represented as 2·U 0 , since they generate output voltages for the corresponding multi-phase systems that are in the range of +U 0 and -U 0 .

[0103] In a first embodiment of this kind, the DC power supply is designed to provide a center tap. This center tap represents "0V" on both the input and output sides and can thus simultaneously serve as the star point of the output system.

[0104] The actively supplied module, hereinafter referred to as actively supplied multiphase module, must be configured so that the potential jumps back and forth between +Uo and -Uo depending on the voltage to be delivered.

[0105] Figure 10 shows a first embodiment of the circuit of such a module 1010. The actively supplied multiphase module shown (the other modules can be designed as described above) is required for each phase of the multiphase system to be constructed. The principle can be implemented for any number of phases. The actively supplied multiphase module according to Figure 10is supplied by a DC voltage supply with voltages + / -Uo via terminals 1050 and 1070 and the center tap 1060. If this actively supplied multiphase module is to pass a positive or negative voltage via its first output terminal 1090 to the subsequent four-quadrant module, the second terminal 1080 is connected to "0V". This voltage potential corresponds, on the one hand, to the optional star point of the output and, on the other hand, to the center tap of the DC voltage supply. Depending on the desired state, one of the three switches 1020, 1030, and 1040 must be closed. However, this schematic diagram places two special requirements on the switches. The top switch 1020 and the bottom switch 1040 must be designed for a dielectric strength of 2 Uo. Furthermore, the middle switch 1030 must be able to block in both voltage directions.One such actively supplied multi-phase module is required for each phase of the power converter.

[0106] A second embodiment with similar properties is the circuit 1110 according to Figure 11 , where an actively supplied multi-phase module according to the Figure 11 shown circuit is required per phase. Here, too, voltages can be passed via the DC voltage terminals 1150, 1160, 1170 via corresponding switches 1120, 1122, 1124, 1126, 1128, 1130 to the output terminals 1180, 1190. Although it requires a total of 6 switches per phase, these switches only need to be designed for half the voltage of the DC voltage circuit, i.e., only for Uo. Furthermore, the switches used only need to be able to block in one voltage direction. Therefore, IGBTs or MOSFETs, for example, can be used here.

[0107] Figure 12shows different switching states of the transistors of this second embodiment. The bars placed over the transistors represent the currently conductive transistors. Accordingly, the first switching configuration 1210 generates a positive voltage at the output terminals, configurations 1220 and 1240 generate 0V, and configuration 1230 generates a negative voltage. The circuit according to Figure 12 can therefore also be considered as a phase of a three-level converter that can supply the three voltage levels +U 0 , 0 and -U 0 .

[0108] If no star point is required in the multi-phase output circuit of the converter, a third embodiment of the actively supplied multi-phase modules can be used, as shown in Figure 13This embodiment uses, as an actively supplied multiphase module 1310, 1312, 1314, a simple half-bridge for each phase, consisting of two switches 1316, 1318, which can only switch between positive voltage 2·U 0 and 0V. Figure 13 shows an example of such a converter with three phases and three additional four-quadrant modules per phase. However, the principle shown can be applied to any number of phases as well as any number of four-quadrant modules per phase. The positive terminal of the DC voltage supply is connected to the upper input terminal 1320, the negative terminal to the lower input terminal 1322. A single-phase converter equipped in this way can only output positive voltages between 0V and 2·U 0 at each output terminal of the converter. Accordingly, the three chains 1330, 1340 and 1350 of the Figure 13The three-phase converter shown also only generates positive voltages against the negative terminal 1322 of the DC voltage supply. However, the three outputs 1360, 1370, and 1380 can be controlled among themselves to create a symmetrical three-phase voltage system. The advantage of this third embodiment is that only two transistor switches (or switching devices in general) are required per phase for the actively supplied multi-phase module, which also only need to be able to block in one current direction. Accordingly, the upper three modules 1310, 1320, 1330 of the circuit according to Figure 13 a two-point converter. If the DC power source connected to the input terminals 1320, 1322 also has a center tap, this can also serve as the star point for the multi-phase output system. This is the essential difference from the embodiment according to Figure 12 and Figure 13in that the circuit according to Figure 12 forms a three-point converter, while the circuit according to Figure 13 Accordingly, for the embodiment according to Figure 13 another four-quadrant module is required to achieve the same number of stages as in the embodiment according to Figure 12 .

[0109] Generally speaking, according to a further aspect of the invention, a chain of four-quadrant modules can be connected to each AC terminal of a single- or multi-phase two- or multi-level power converter such that this respective chain is located between the AC terminal and the single- or multi-phase load (or source). This chain (or these chains) thus functions as a type of filter circuit, which correspondingly increases the number of output voltage levels of the power converter. Thus, the power converter system can be split into a two- or multi-level power converter and the associated chains of four-quadrant modules. Such a division can be advantageous, for example, for electric vehicles, since the associated power converter can be used both to control the motor (or generator for recuperation operation) and to charge the battery (with high power).In such a case, the power converter, which is connected to the traction battery, can be connected to the motor and to the single- or three-phase AC mains via a changeover switch. To control the motor, a simple voltage output waveform (e.g., block commutation or PWM control) with a two- or three-level converter is often sufficient, whereas charging the battery via a single- or three-phase AC mains must meet more stringent requirements regarding the voltage waveform (due to possible grid interference). In such a case, the chains of four-quadrant modules only need to be located in the current path of the single- or three-phase AC mains and only be designed for the corresponding charging power. For motor operation, however, the simpler waveform of the two- or multi-level converter can be used, which further reduces the corresponding losses in the transistors of the module chains.

[0110] Accordingly, additional converter chains can also be used to generate networks with more than three phases. By coupling two such single- or multi-phase converter systems, a converter can be constructed – similar to a back-to-back converter – that can process variable DC and AC voltages on both sides, with bidirectional power flow. The connection between two such converter systems is achieved – similar to conventional converter systems – via the DC voltage circuit, which thus becomes the so-called DC intermediate circuit. In this way, any DC or AC voltage (with any number of phases and frequency) can be converted into any other DC or AC voltage.

[0111] In principle, all of the multiphase converter systems presented in the first, second, and third embodiments are suitable for such a coupling. If the potential of the DC link does not need to have a fixed relationship to the AC systems, the third embodiment according to Figure 13 the cheapest option. Switching losses of the transistor switches.

[0112] Switching losses in the semiconductor switches used arise from the finite switching time during which a corresponding switch is neither insulated nor conducts well, thus generating high ohmic power losses. These losses are therefore roughly proportional to the switched voltage, the switched current, the switching time, and the switching frequency. The highest voltage to be switched is applied to the module actively supplied with the DC voltage. However, compared to a two-point converter, the transistors of this module only have to switch half the voltage, namely the voltage of the supply circuit. Furthermore, in contrast to PWM converters, the switching frequency can be reduced by increasing the size of the module capacitors. When outputting alternating current, the required switching frequency of the individual modules can be reduced to approximately the frequency of the output AC voltage.If, as explained below, a switching threshold voltage is defined for the charge / discharge transition, the resulting switching frequency is inversely proportional to the load current. Thus, both switching and conduction losses decrease disproportionately with decreasing load current. This, in turn, leads to extremely high partial-load efficiency.

[0113] All switching elements can be implemented as transistors (e.g. MOSFET or IGBT) or other semiconductor switches and their current carrying capacity can be improved by parallelization. Simple examples to explain in detail the principle of charging and discharging processes:

[0114] The following explanations are merely examples and serve to improve understanding. It will be clear to those skilled in the art that these explanations also apply to any number n of modules, any type of storage elements, switching devices, any input voltages, and in particular significantly higher ones, and any output voltages that may be provided.

[0115] The following exemplary explanations based on the Figures 14 and 15 and 8b on the one hand depict specific details, and on the other hand explain basic principles of the invention which also apply to all other figures and embodiments. Example 1, generating a DC voltage with 2 modules:

[0116] The Figure 14The converter shown should initially consist of only two modules: an actively powered module 1410 (here designed as a four-quadrant module) with 12V (which is supplied from the DC circuit via terminals 1430, 1440) and a capacitor-based module 1420, which is designed for a voltage of 6V. Furthermore, the converter should initially deliver a DC voltage of 6V (i.e., the voltage of the "smallest stage") at its output terminals 1450, 1460, with a certain load current also flowing. The initial configuration of the converter should be selected so that the 6V module is precharged with 6V. The 12V module can always deliver 12V due to its power supply. Time span 0 to t 1 , 1470:

[0117] First, the 12V module is switched to 0 (bypass) (by switching on only the two right or the two left transistors of this module as shown in the figure).

[0118] The 6V module delivers the required 6V, but the module capacitor discharges over time until a lower tolerance threshold is reached. If the voltage falls below 5.9V, for example, the system must then switch over. Time period t 1 to t 2 , 1472:

[0119] Now the 12V module is actively switched to +12V, while the 6V module, with its reduced voltage of 5.9V, is connected in series, for example, by switching on the upper right and lower left switches of this module. This results in a new total voltage of 12V - 5.9V = 6.1V, which – this time due to the charging of the capacitor (caused by the transfer of charge from the negative terminal of the capacitor) – slowly decreases again to the tolerance threshold.Since the total effective capacitor capacitance in the circuit remains unchanged compared to the first time period (since only one capacitor continues to act electrically; the additional capacitor connected in the first stage is electrically neutral, as it is always maintained at full voltage due to the DC power supply), the rate of voltage drop also remains the same (however, the time period doubles compared to the start, as the output voltage changes by 0.2 V from 6.1 V to 5.9 V). Due to the inverted operation, the 6 V module is charged to a voltage of 6.1 V. Time period t 2 to t 3 , 1474:

[0120] Here again only the 6V module is positively active (the 12V module is again in bypass state), only it starts with 6.1V, so that the third time period is as long as the second.

[0121] In other words, the storage device of an active module is discharged due to the load current. In inverted operation, however, the load current leads to a charge of the module in question, so that the resulting voltage range of this module can be kept within predetermined limits. If the charge level of a storage device of a module threatens to fall below a limit value W 1 , this module is switched to inverted operation in a subsequent phase, as a result of which the storage device is recharged. This charging then takes place up to the limit value W 2 , at which time the module must then either be switched back to discharge mode, i.e. active mode, or to bypass mode to prevent overcharging. In this way, each storage device of each module can be kept within a predetermined voltage range.Electrically speaking, this can be independent of the module's actual storage capacity. Using smaller capacities will therefore require more frequent switching operations than using larger capacities, since recharging must occur at an earlier point in time. The capacities of the individual modules can be designed identically, even though the modules are intended to deliver different voltage contributions. However, they can also be matched to their assigned voltage level. All of this must be taken into account when controlling the operating mode switching. Example 2, generating a DC voltage with 3 modules:

[0122] The Figure 15The converter shown should initially consist of three modules: an actively supplied module 1510 with 12V (which is supplied from the DC circuit via terminals 1540, 1550) and two capacitor-based modules 1520, 1530, one for 6V, and one for 3V. The converter should now deliver a DC voltage of 3V at its output terminals 1560, 1570, with a load current flowing. The initial configuration of the converter should again be selected so that the 6V module is precharged with 6V and the 3V module with 3V. Time span 0 to t 1 , 1580:

[0123] First, the 12V module and the 6V module are switched to 0V and bypass, respectively. The 3V module delivers the required 3V, with the module capacitor discharging over time until a lower tolerance threshold of, for example, 2.9V is reached. Time period t 1 to t 2 , 1582:

[0124] The 6V module is now activated, and the 3V module is connected in series with it in reverse. The starting voltage is now 6V - 2.9V = 3.1V. This process continues until the total voltage drops back to 2.9V. At this point (assuming the same capacitance of the module capacitors), the voltage of the 3V module has risen back to 3.0V; the voltage of the 6V module is now 5.9V. The voltage difference is twice as high as in the first period; however, two module capacitors (e.g., of the same capacitance) are now connected in series, so the period is the same length as the first.

[0125] In other words, the capacitance of two module capacitors is effective in this phase compared to the previous phase, in which only one module capacitor was active. This results in a doubled voltage gradient. However, the entire charging phase is the same length as the first phase, since the doubled voltage swing must be compensated. These considerations assume the use of identical capacitor capacitances, which is a preferred embodiment due to the simplified switching processes. Of course, the capacitances could be matched to the module voltages, which are graduated, for example, in powers of two. Period t 2 to t 3 , 1584:

[0126] Since the 3V capacitor is now charged back to 3V, a cycle similar to the first period can be used again. At the end of this period, the 3V module capacitor is discharged to 2.9V. Period t 3 to t 4 , 1586:

[0127] Now both the capacitors of the 3V module and the 6V module are discharged. Therefore, during this time, these two module capacitors are connected anti-serially, i.e., inverted, with the 12V module. This results in a starting voltage of 12V - 5.9V - 2.9V = 3.2V. Because of the two series-connected capacitors, the voltage drops just as quickly as in the second phase (the capacitor of the actively supplied module, which is also connected in series, is electrically neutral due to the DC voltage supply; see the explanations for the first example above); however, the voltage swing is somewhat larger, as the voltage now drops from 3.2V to 2.9V. This leads to an extended switching phase. The controller device (not shown here), which monitors the charge states of the capacitors, must take this into account when switching the modules' switching devices.At the end, the capacitor voltage of the 6V module according to the example is 6.05V and that of the 3V module is 3.05V. Period t 4 to t 5 , 1588:

[0128] Now the 3V module can be activated again on its own; the voltage drops from 3.05V to 2.9V.

[0129] According to these examples, the Figure 8a The stages shown can be considered as a short-term DC voltage output. Thus, such a voltage stage can have a substructure, so to speak, if several charging and discharging cycles of modules are required during the period in which a voltage stage is to be maintained. This is Figure 16 explained in more detail.

[0130] Furthermore, in addition to the equalization of the voltage curve, a pulse width modulation with the voltage swing of a voltage stage can also be controlled via the power converter, as in Figure 8b shown.

[0131] For each switching period, the Figures 14 and 15 The state vector is given above, which indicates which module in this module configuration must be in active mode ("1"), inverted mode ("-1"), and standby ("0"). Ultimately, the switching behavior can be expressed via the sequence of these state vectors, and the controller device must be able to perform this switching behavior. For example, a sine wave at the output can be described by the sequence of suitable state or switching vectors. If, in addition, as in Figure 8b If pulse width modulation is used as shown, the controller device must also be able to perform this by switching on and off at appropriate times.

[0132] Figure 16illustrates that, particularly for a slowly rising output voltage, it may be necessary to maintain the same output voltage for at least two consecutive switching periods. The right-hand side shows an enlarged view of the circled section of the curve on the left-hand side. During this section, switching occurs 8 times over time, thus changing the module configuration. The output voltage level remains unchanged for three periods, and then remains unchanged at a higher level for 5 switching periods. While the output voltage is kept essentially constant for a short time in this way, the module configurations nevertheless change in order to enable recharging of heavily discharged switching devices, as already explained in detail above.

[0133] The information related to the Figure 15The state vectors explained above, which determine the switching behavior of the modules and lead to different module configurations that can deliver the same output voltage and thus enable the recharging of individual modules, are explained below in connection with Figure 17 explained again in a more systematic way. Example of generating a step function for arbitrary initial states:

[0134] Figure 17shows, as an example, the necessary switching states of a supplied main module (HM) in combination with three four-quadrant modules (M2, M3, M4) for the case of a load in order to gradually map a positive output voltage UA from 0 V to the main basic module voltage of 8 V and back to 0 V with the step height of the smallest four-quadrant module. For example, a positive sine half-wave could be generated in this way, as symbolically represented with 1790. The main module voltage is 8 V and the target voltages U target of the energy storage elements of the other four-quadrant modules (M1-M4) are 4 V, 2 V and 1 V respectively. Figure 17 represents sequences 1710 - 1780 of the switching states S, up to S 17 for all possible combinations of the initial charge states LZ Soll and thus for all possible combinations of the possible initial operating voltages U operation of the four-quadrant modules.

[0135] The term "state of charge" refers to the fact that the energy storage element of the respective module is either charged to such an extent that it may be discharged during the next use, or that the voltage of this energy storage element is slightly below the target value and should therefore be recharged in one of the following cycles. Accordingly, two possible charge states exist for each module: LZ Soll = Y ("Yes") means the energy storage element may be discharged; for LZ Soll = N ("No"), the energy storage element must be recharged in one of the following cycles.

[0136] For the case illustrated here, with a positive voltage and a load at the converter output, this means that a four-quadrant module can be connected with a positive voltage, provided the charge state LZ Soll = Y ("Yes") allows it. For LZ Soll = N ("No"), the module can only be connected with a negative voltage—i.e., in the inverted state. It can be seen that a different switching sequence occurs for each possible combination of U operation.

[0137] In the example shown, the charge level of the corresponding energy storage element changes after each module is used, so that each module is discharged again after charging in one of the subsequent switching states, and vice versa. This is not necessarily the case, of course, but is determined by the capacitor capacitance and the load current. Accordingly, depending on the capacitor capacitance, a single or multiple change in the switching sequence may be necessary during a single stage, or it may also be the case that the charge level does not change from one stage to the next.

[0138] For example, sequence 1710 starts with energy storage elements all having a charge level of "J", while sequence 1780 starts with energy storage elements all having a charge level of "N".

[0139] In any case, the actively supplied module can, in combination with one or more four-quadrant modules, compensate for negative voltages from connected four-quadrant modules.

[0140] According to the example shown, any step voltage can be represented with any combination of energy storage element states of charge. This, of course, also applies to any number of modules.

[0141] Figure 17 This simple example illustrates the great flexibility that can be achieved and that a specific output voltage can be achieved with completely different module configurations. In particular, this example illustrates the controller's task of sequentially switching different module configurations over time, starting from initial states, to achieve a desired output voltage profile. Control of the individual modules

[0142] Each individual module can, for example, be controlled by its own control unit, which is connected to a common controller device via a potential-free bidirectional data channel in order to control the switching elements on the one hand and to transmit the voltage values ​​of the energy storage element to the controller device on the other. Alternatively, other data, such as currents or temperatures of the components, can also be transmitted to the controller device. It is particularly useful if the control units in the individual modules are electrically supplied directly from the controller device via the voltage at the terminals of the individual modules or via DC-DC converters. Data can be transmitted to the common control unit, for example, optically or wirelessly to ensure galvanic isolation.Alternatively, it is possible to work with only one common controller device, or only with several individual control units related to a module. An example control algorithm for the process of generating a step voltage curve for one phase of the power converter is as follows:

[0143]

[0144] Here, three loops are nested within each other; the outermost loop runs from k = 1 to z and describes the temporal sequence of the voltage steps to be generated, which are required to convert a desired voltage curve into a step function that can be represented by the power converter. The lengths Δt of the time periods may vary from step to step according to the control algorithm used. The second loop runs from j = 1 to m and refers to the switching of module configurations within a voltage step. Such a temporal subdivision of the step function is particularly necessary if the module capacitors must be recharged within the short time period of a single step, as is the case, for example, in the Figure 16is shown. If the capacitor capacitances are sufficiently large, this temporal division is not necessary, so that the activities of the outer loop and the middle loop can coincide. The third, innermost loop runs from i = 1 to n (where n describes the number of modules) and determines the current charge state for each module capacitor in order to decide whether the corresponding module should be switched for a subsequent state so that the capacitor is charged or discharged (whereby there may be several bypass states for this module before this subsequent state). Details on the energy flow direction:

[0145] The above description is generally based on the assumption that energy is delivered to a load via the terminals of the series connection of modules, for example, when operating an electric motor. However, it has already been established in various places that the present invention is also suitable for supplying a direct current source with energy, so that the energy flow occurs in the opposite direction, namely from the terminals of the series connection of modules into the direct current circuit. This is the case, for example, during the braking process of an electric vehicle in so-called recuperation mode. In this case, the current direction is reversed compared to the load case. Thus, if in the case of a load, from the perspective of the power converter - when a positive voltage is delivered - the current direction previously came from the power converter (positive current direction), it is reversed when the energy flow direction is reversed. This means:If the voltage remains positive, the current direction becomes negative. The same applies if the voltage at the output terminals is negative. In this case, when a load is applied, the current direction is negative, whereas when the direction of energy flow is reversed, it becomes positive. Since the capacitors of the individual modules - unless switched to bypass mode - are located directly in this current path of the output terminals, the corresponding charging and discharging behavior of the switched capacitors (energy storage elements) is also reversed if the direction of energy flow is reversed. This means that a module capacitor that has been switched inverted will be charged when current is delivered to a load, whereas it will be discharged when the load flow direction is reversed. This must be taken into account by the controller device that switches the switching devices (and accordingly ensures that the voltage of each module capacitor remains within the specified voltage range).These considerations also apply to capacitive or inductive loads (or sources) where the current and voltage are not exactly in phase. Due to such a phase shift, the direction of energy flow constantly reverses depending on the frequency of the alternating current. Therefore, knowledge of the instantaneous current and voltage values ​​is helpful for the controller of the power converter in order to determine the corresponding control signals for the individual modules.

[0146] Whether a system connected to the converter (either via the DC circuit terminals or via the output terminals) acts as a load or as a source can also be determined by the corresponding instantaneous values ​​of the voltages between this system and the instantaneous values ​​of the converter voltages (whereby it is initially assumed that the system is not yet connected to the converter). For example, if the instantaneous value of the system voltage is slightly higher than the converter voltage, a current flow from the system into the converter can be expected. This - typically very small - voltage difference thus determines the direction and magnitude of the expected current. Accordingly, the converter (or alternatively, a similarly controllable source) can determine the current or load flow direction.For example, by slightly increasing the voltage in the DC circuit, the converter can ensure that energy is fed into the DC circuit (e.g. to charge a battery in the DC circuit). Another example is a DC motor / generator that is supplied with a constant voltage from the converter in order to generate a specific speed. Typically, the voltage induced in the motor / generator is directly proportional to the speed. Therefore, at speeds slightly below the target speed, the machine will operate as a motor and deliver torque (or mechanical energy). If the target speed is slightly exceeded, the machine switches to generator mode; the torque changes sign and mechanical energy is absorbed. This also applies analogously to rotating field machines that are operated with three-phase current.However, in addition to the voltage, the frequency of the three-phase current must also be adapted to the speed of the machine.

[0147] Since the converter outputs a stepped voltage curve, it must be considered – especially when using coarser voltage steps (i.e., converters with fewer modules) – that these voltage steps also cause corresponding current changes, as discussed above. This naturally applies to all types of converters, although PWM-based converters in particular generate very strong voltage fluctuations. If the connected source or load does not exhibit sufficiently flexible response to such voltage fluctuations (as, for example, a motor / generator does via the inductance of its windings), the use of additional filter circuits to smooth the current (or voltage) curve is helpful. Advantage of preferred embodiments of the converter according to the invention compared to conventional modular multilevel converters:

[0148] In a conventional MMC-based power converter, 2n modules are required to generate a total of n voltage levels (of one polarity) per phase module. For example, a three-phase power converter is to be supplied with a DC voltage of + / - 320V (thus, it could generate output voltages of 230V- against ground). The voltage levels should have a height of 20V, whereby the output voltage is divided into 16 positive voltage levels (or a total of 33 levels, see also Figure 11 ) can be displayed.

[0149] In a conventional modular multilevel converter consisting of 2-quadrant modules, each bridge arm would need 32 modules with a dielectric strength of 20V each: When a voltage of 0V to ground is output, 16 modules in both the upper and lower bridge arms are switched to active (16 x 20V = 320V) and 16 modules are switched to bypass. If the maximum voltage of 320V is to be output, all 32 modules in the upper bridge arm are switched to bypass mode, while all 32 modules in the lower bridge arm must be switched to active (32 x 20V = 640V). To output the maximum negative voltage of -320V, the situation is exactly the opposite; here, all modules in the positive bridge arm must be switched to active, and all modules in the negative arm must be switched to bypass mode. Therefore, a three-phase converter with this number of stages requires a total of 2 x 3 x 32 = 192 modules.This would require 384 transistors, 192 module capacitors and (at least) 192 voltage levels (at least one per 2-quadrant module, since neighboring modules may always share a voltage level) to supply the transistor drivers.

[0150] A three-phase power converter according to the present invention, which is intended to deliver this output voltage, requires only 5 modules per phase, including 3 actively supplied modules (e.g., with 6 transistors each) and 4 capacitor modules (each with 4 transistors). Therefore, 15 modules are required for the three-phase system. This would require 66 transistors, 12 to 15 module capacitors (the supply modules do not necessarily require a capacitor), and 36 voltage levels (two for each 4-quadrant module) for supply.

[0151] The total dielectric strength of all transistors combined is the same for both compared circuit types, at twice the DC link voltage (per phase). However, the new circuit allows for a significant reduction in component count. This reduces the cost of the power converter; in particular, the effort required to provide the potential-free supplies for the individual modules (e.g., via DC-DC converters) is reduced. The above example therefore shows a case in which – when using the circuit according to the invention – conventional components (including MOSFETs) can also be used for the dielectric strength of the module with the highest voltage. This could be a three-phase power converter that converts a power of just under 10 kW per phase at 10 A DC. Summary of further advantages of preferred embodiments:

[0152] The circuit topology of the individual modules provides overvoltage protection for the inputs / outputs (low-pass filter effect) through the energy storage elements and the diodes that are connected in parallel to the transistor switches; failure safety through the possible use of redundancies via the control system (installation of additional replacement modules or modification of the operating mode so that modules can take over the functionality of other failed modules); the system is also DC-capable at the output, meaning it can also supply an adjustable direct voltage; when using suitable actively supplied modules, converters for single- or multi-phase networks with any number of phases can be constructed; two individual converters can be connected via the DC circuit in the manner of a back-to-back converter and thus transfer electrical energy of any voltage, frequency and number of phases bidirectionally from one converter to the other.At the output terminals, a large number of equidistant voltage steps can be output with just a few modules - if the module voltages are gradated in powers of two - in order to generate a finely stepped output voltage curve; the system is bidirectional, i.e. energy can be both supplied and supplied via the output terminals. In particular, a significantly higher voltage can be generated in the DC circuit from a small input voltage (this property is very advantageous, for example, for the recuperation mode of electric vehicles at low speeds); the capacitance of the capacitors (general storage devices) is determined by the tolerated voltage drops during operation and by the switching frequency of the power converter; however, it is not dependent on the frequency of an output AC voltage.Therefore, with a sufficiently high switching frequency, the capacitances of the capacitors can be very small, which in turn enables an extremely compact design of the power converter. The capacitors are only used in one voltage direction, so in certain applications, electrolytic capacitors or accumulators can also be used. Due to the finely stepped voltage curve, almost no filtering is often necessary at the power converter terminals. Furthermore, a filter circuit can take advantage of the fact that the power converter, based on the principle of PWM control, constantly switches back and forth (and at high frequency) between a slightly too large and a slightly too small voltage step (corresponding to the voltage swing of the module with the lowest voltage).Therefore, compared to a conventional PWM converter, pulse width modulation is not carried out with the full voltage swing, but preferably only with the amplitude of the smallest step. Due to a potentially high switching frequency and a small voltage difference to be smoothed, the filter circuit only needs to store a small amount of energy and can therefore be designed very small. By adding an additional (e.g., longitudinally regulated) analog module, continuous output voltages can be generated. The voltage swing of such an analog module only needs to be slightly larger than the voltage value of the module with the lowest voltage. Such voltages can be relatively easily longitudinally regulated using power transistors, even at high currents. Alternatively, only the module with the lowest voltage can be operated as a PWM module to keep switching losses low.Another alternative is to add a dedicated additional PWM module instead of the analog module. The voltage swing to be achieved by this module must again only be within the voltage range of the module with the lowest voltage level. For control purposes, the voltages of the individual modules only need to be checked to see whether they are above or below a respective tolerance threshold. This means that simple comparators can be used to measure the module voltage instead of AD converters. Alternatively, algorithms can be used for control purposes that infer the voltages of the individual energy storage elements of the modules from the measured voltage and current at the output terminals of the power converter.To do this, it is necessary to take into account which modules are currently actively involved and to extrapolate the change in voltage of the energy storage elements using the measured current. The voltages of the individual energy storage elements can then be calculated with sufficient accuracy across a larger number of switched stages. If the requirements for the accuracy of the output voltage allow it, a corresponding power converter can optionally be operated in such a way that - starting with the module with the lowest voltage - one or more modules are switched to pure forward operation (i.e. bypass). In this case, the switchable voltage stages are larger (since only a smaller number of modules participate in stage formation), but in return the number of switching operations is significantly reduced and the capacitor losses of the bridged modules are also eliminated.Such an approach can be advantageous, for example, with a vehicle power converter if it is also used as a charger. When used as a drive power converter, larger voltage steps may be permissible than when used as a charger, as strict power quality standards must be adhered to here. In extreme cases, the electric machine could even be controlled in pure PWM mode (possibly by only one module per phase) (with the remaining modules possibly only in the charging circuit). Furthermore, such an approach can also be advantageous if the power converter is to output alternating voltages with very different amplitudes. If the amplitude is large, the smallest steps can then be omitted, while they are activated when small voltage amplitudes are output.If the design of the module's dielectric strength allows it, the system presented here also allows for dynamic adjustment of the individual module voltages to meet specific requirements. This allows for non-equidistant voltage gradations to be achieved if necessary. Selected areas of application

[0153] Due to properties such as easy scalability, this system offers an extremely broad field of application, ranging from low-voltage conversion and vehicle applications to the high-voltage level (railway traffic, DC systems, grid coupling, power compensation, grid coupling of different voltages and frequencies, etc.).

[0154] MOSFETs can be used as switching elements, particularly for small and medium-power converters for the low-voltage range with DC circuit voltages up to approximately 800V (for the module with the highest voltage level); for voltages up to approximately 2000V, IGBTs can be used. With a DC circuit voltage of UDC = + / -800V and a DC current requirement of 50A per phase, such a converter can control a power output of approximately 3 x 800V x 50A = 120kW at variable frequency and variable output voltage (in the range of 0 to 566V phase voltage or 980V line-to-line voltage). This makes such converters suitable for low-voltage applications in which three-phase or alternating current is to be exchanged with a low-loss local DC voltage system, as will be increasingly used in future distribution grids, for example.For example, house connections with typical three-phase voltages can be connected to a local direct current network.

[0155] The presented power converter is also suitable as a solar power converter. Such a converter must convert the DC voltage of a photovoltaic system into an AC voltage synchronized with the grid. At the same time, the DC voltage should be directly adjusted to the current output of the solar cells (maximum power point tracking). These functions can be achieved with the present system without the need for additional DC-DC converters or transformers.

[0156] A broad field of application also arises in the field of electromobility. The power converter is compact, causes less EMC interference, and can be used both to drive the electric motor of electric cars and to charge the battery with very high power. Furthermore, it can recuperate very well even at low rpm or speeds without the need for an external DC / DC converter. It is also possible to combine two functionalities: During driving, the motor can be controlled via PWM; charging takes place in multilevel mode. Since many rail-based vehicles, such as trams, are supplied with direct current of just a few kV, power converters based on the system presented here can also be advantageously used to control three-phase motors for propulsion.

[0157] Furthermore, the power converters presented here can also be used in wind turbines. Here, the generator connected to the wind turbine generates a three-phase voltage with a variable frequency. The electrical energy is fed either into a local direct current grid (for example, in offshore wind farms) or directly into the local multi-phase alternating current grid. In the first case, a suitable multi-phase power converter can be used to adapt the output direct voltage to the direct current grid. In the second case, a back-to-back converter can be used, which converts the multi-phase alternating voltage of the wind turbine into a multi-phase alternating voltage of suitable voltage and frequency and feeds it into the alternating current grid. Alternatively, a simple rectifier can be combined with a power converter connected as an inverter.

[0158] Another possible application for power converters based on the principle presented here is high-performance audio amplifier systems: For good sound quality, a 16- or even 20-bit system is not realistic, as charging and discharging the individual modules cannot be accomplished with sufficient accuracy. However, the required accuracy could easily be achieved with a 6- to 8-bit system in conjunction with an analog module by feeding the output back via a corresponding controller, thus controlling the analog module. The control of the power converter modules can then be derived directly from the signal values ​​of the analog module, as it occurs quickly enough that the analog module does not exceed its voltage limit. The power consumption of the analog module is then significantly lower than that of the entire system.In addition, at such low voltages, the corresponding power transistors can be subjected to significantly higher currents without becoming unstable.

Claims

1. Modular power converter for outputting different voltage levels, with at least one main module (602) to be actively supplied having input terminals for the application of an input voltage U0 for said power converter, n-1 further modules (602), where n is an integer and n ≥ 2 is true, all at least n modules (602) are connected to each other in series, output terminals at both end points of the series connection, to output an output voltage, where at least each of said n-1 further modules (602) comprises an energy storage device (606), preferably a capacitor for storing charge, and each module (602) comprises switching devices, preferably formed by transistors, for optional and controlled setting of at least one active normal operation, in which said respective module (602) introduces the voltage at its energy storage device (606) or its input into the voltage path of the series connection, and a bypass operation, in which said respective module (602) only connects through the voltage path of the series connection, and an inverted operation, in which said respective module (602) introduces the voltage at its energy storage device (606) or at its input inverted, into the serial voltage path, and where said voltage contributions of said modules (602) differ and are stepped down relative to each other according to the powers of two, accordingly U0, U0 / 2, U0 / 4, U0 / 8 etc., wherein U0 corresponds to the value of the input voltage, in a controller device (604) for directly or indirectly detecting the states of charge of said energy storage devices (606) of all modules (602) and for controlling the switching states of said switching devices of all modules (602) in order to be able to selectively set a specific operating state for each of said modules (602) for a certain point in time, where said controller device (604) is configured such that the state of charge of each energy storage element (606) of a respective module (602) in the operation of said power converter is held substantially between predetermined limit values and in the active operation of said respective module (602) thus leads to a predetermined voltage contribution into the voltage path, and where the controller device (604) is further adapted to select alternative module configurations, said alternative module configurations each resulting in substantially the same output voltage, except for the module configuration for the maximum output voltage, the minimum output voltage and the zero output voltage, wherein switching to this alternative module configuration causes said modules (602), which are to be recharged for maintaining their desired state of charge, to be switched to the inverted operating state as part of said alternative module configuration and without changing the resulting output voltage level, in order to thus effect the recharge of said respective module (602) while maintaining the same output voltage level, wherein a direct voltage U0 is applied to the input terminals and an alternating voltage in the range [-U0, +U0] is provided at the output terminals.

2. Power converter according to claim 1, characterized in that said energy storage devices (606) are formed by capacitors and the charging constant of said capacitors used is small in comparison to the cycle time of said output voltage and excessive discharge of the capacities is avoided by repeated module configuration changes for every output voltage level, in that individual capacitors are discharged as well as recharged during a constant output voltage level.

3. Power converter according to one of the claims 1 to 2, where said modules (602) are connected to each other in a staggered manner such that a connection in the sequence of said respective realizable voltage levels is obtained, and / or where said modules (602) are sized according to their assigned voltage contribution, in particular with regard to their storage device (606).

4. Power converter according to one of the preceding claims, where an alternating voltage provided at the output terminals controlled by the switching behavior of said modules (602) and the switching steps achievable with them, can assume almost any frequency, preferably including the possibility of realizing a direct voltage at said output.

5. Power converter according to claim 4 with a main module (602) and n-1 further modules (602), where the number of representable steps in the output signal is 2n + 1.

6. Power converter according to one of the claims 1 to 56, where a certain output voltage at the output can be held for any length of time, almost independent of the actual electrical properties of said storage devices (606) of said modules (602), in that the module configuration suitable for this specific output voltage range is switched back and forth with a switching frequency high enough to ensure that each module (602) is recharged early enough by way of its inverted operating state before it is possible for said output voltage to leave the desired range due to the excessive discharge of individual modules (602).

7. Power converter according to one of the preceding claims, where changing the switching states of said modules (602) takes place by way of a common predetermined clock and the predetermined clock is tuned to a desired output frequency of the voltage and the charging characteristics of said energy storage devices (606) of said modules (602).

8. Power converter according to one of the preceding claims, where the detection of the states of charge of each module (602) is carried out by said controller device, and this detection is effected either directly at said respective module (602) or on the basis of the output voltage and the instantaneous switching state, preferably also the previous switching state of said modules (602), as well as the load current.

9. Power converter according to one of the preceding claims, comprising several phases formed from several corresponding module chains, where each phase of said power converter is connected to a phase of a load and a source.

10. Power converter according to one of the claims 1-9, characterized in that the module configuration is changed for each voltage level, other than the maximum voltage level, the minimum voltage level and for zero voltage, at least twice, preferably multiple times and preferably between 5 and 15 times, particularly preferably about 10 times for each voltage level.

11. Power converter according to claim 10, where said capacitors used can have a relatively small capacity in that the switching frequency with which said module configurations are changed and thus recharging is carried out, is high in comparison to the frequency of the output voltage frequency to be realized, and / or where the charging time of said capacitors is shorter by a factor of 100, preferably be a factor of 1000 than the period duration of the output voltage to be realized.

12. Power converter for AC / AC conversion, with two power converters according to one of the claims 1-11, which are connected to each other via their direct voltage circuits.

13. Electric motor with a power converter according to claim 9.

14. Electric vehicle, in particular an electric car, with a battery device, an electric motor and a power converter according to one of the claims 1- 11.

15. System for converting an alternating current supplied via a power line to direct current for the local supply of a building cluster with direct current by way of a power converter according to one of the claims 1-14.

16. Method for operating a power converter, where said power converter comprises at least one main module (602) to be actively supplied having input terminals for the application of an input voltage U0 to said power converter and n-1 further modules (602), where n is an integer and n ≥ 2 is true, where said the main module (602) and the n-1 further modules (602) are preferably four-quadrant modules and each of said n-1 further modules (602) comprises an energy storage device (662), preferably a capacitor for storing charge, and the modules (602) comprise switching devices, preferably formed by transistors, for optional and controlled setting of at least one active normal operation, in which said respective module (602) introduces the voltage at its energy storage device (606) or at its input into the voltage path, a bypass operation, in which said respective module (602) only connects through, and an inverted operation, in which said respective module introduces the voltage at its energy storage device (606) or at its input inverted into the voltage path, and where the method comprises the following steps: connecting said n modules (602) in series, applying a direct voltage U0 at said input terminals of said main module (602), predetermining a desired temporal course of the output voltage in relation to the amplitude over time, tapping the output voltage at the two endpoints of the series connection of said modules (602), maintaining predetermined states of charge for said individual modules (602) in such a way that each module (602) in its active operating state substantially makes a predetermined voltage contribution to the series connection, where said voltage contributions of said modules (602) differ from each other and are stepped down relative to each other by the powers of two, actuating said switching devices of said modules (602) to switch each module (602) to an operating state such that the sum of the individual module voltages as they are provided by said energy storage device (606) of each module (602) according to its active, inverted or bypass operation at all times corresponds to the predetermined range of the output voltage, where at least one alternative module configuration exists for each module configuration, that is currently able to be set by switching said individual switching devices of said modules (602) and the resulting output voltage, with the exception of the module configurations for the maximum output voltage, U0, the minimum output voltage, -U0, and the zero output voltage, O, and that can be set and substantially leads to the same output voltage, where switching to this alternative module configuration causes the modules (602), that are to be recharged for maintaining their desired state of charge, to be switched to the inverted operating state as part of said alternative module configuration and without changing the resulting output voltage level, in order to thus effect the recharge of said respective module (602) while maintaining the same output voltage level, where a direct voltage U0 is applied to the input terminals and an alternating voltage in the range [-U0, +U0] is provided at the output terminals.