GB2601285A

GB2601285A - DC-modulating modular electrical converter system and corresponding method for operation

Info

Publication number: GB2601285A
Application number: GB2012201.6A
Authority: GB
Inventors: Li Zhongxi; M Goetz Stefan
Original assignee: Individual
Current assignee: Individual
Priority date: 2020-08-05
Filing date: 2020-08-05
Publication date: 2022-06-01
Also published as: GB202012201D0

Abstract

An electric converter system 200 comprises at least one front-end circuit arrangement 500 and one back-end circuit arrangement 300. The front-end comprises at least one DC/AC inverter having a plurality of power switches and AC terminals 210 to connect to a load 211. The back-end comprises a plurality of modules M(1-N) having at least one storage element and at least two switches. The switches are arranged such that the modules can be connected in series or parallel with respect to each other, or bypassed. The energy storage modules may comprise a combination of batteries and capacitors. The modules may have different nominal voltages. The arrangement may be used to generate single- and multi-phase AC output with current or voltage control and may have applications in variable speed drives, grid-connected battery chargers, photovoltaic systems, or grid storage.

Description

Introduction and prior art

The invention relates to a system and method for a power electronic inverter as used in speed-variable electric drives or grid-connected battery chargers, photovoltaic systems, or grid storage.

In battery electric vehicles, motor drives convert battery power to specific electric form, such as three-phase alternating voltage and current with certain amplitude, frequency, and phase, in order to effectively drive the motor. Motor drives typically comprise at least two key parts, a battery bank (also called battery pack) as the power source and a main electric converter (also referred to as inverter, converter, or DC/AC converter). The battery bank comprises multiple batteries that are hard-wired in series and parallel to provide enough power. The electric power provided by the battery bank is in DC (direct-current) form, which is then converted to AC form (alternating-current) via the main converter to finally feed the motor. The main converter typically performs such DC/AC conversion via bridge circuits that alternately connect the converter's output terminals to the positive or the negative pole of the DC voltage source. In each switching state, the converter chooses the residence duration to approximate a certain AC waveform required by the motor. Prior art of said configuration is described, for instance, in US 8,441,224 B2, which is included here by reference.

Several drawbacks arise from such motor drive configuration: 1) the output AC voltage has a low quality and high distortion. The distorted voltage waveform stresses the winding insulation of the motors; 2) The battery bank-typically comprising numerous battery units-requires balancing circuits that are usually complicated and expensive; 3) the battery bank has to constantly output a high DC voltage to fully cover the operation profile. Since the DC/AC converter always switches at such higher voltage regardless of the real-time load, the energy loss stays high and stresses the cooling system. Furthermore, part of the energy loss is emitted electromagnetically and requires special measures to comply with EMI standards.

The problems mentioned above roots from the fixed battery pack whose output DC voltage cannot intentionally be varied, regardless of the actual operating condition. The DC/AC converter therefore mostly works under suboptimal conditions. Existing solutions can be categorized into the following.

US 8,760,122 B2, US 2019/0288617 Al, DE 10 2018 106 307 Al, JP 2019 165 623 A, and DE 10 2018 109 922 Al disclose the use of modular multilevel converters to disintegrate the battery bank and attach a micro-DC/AC converter to each battery unit. The modules, each comprising a battery unit and a micro-DC/AC converter, are wired into strings, and each string can effectively generate any AC waveform depending on the switching configuration of the modules' micro DC/AC converters. Importantly, the number of module strings equals to the number of phases of the motor. In power electronics, such configuration is called modular multilevel converter. There is no need for an independent converter because the DC/AC conversion is surrogated by the modules. The output AC voltage resembles a staircase, where each step corresponds to an inserted module. Clearly, such staircase AC waveform is less distorted than that generated by conventional motor drives. The micro-DC/AC converters grants additional controllability over the battery units and simplifies the battery management (e.g., cell balancing and/or battery module balancing). However, such fully modular configuration stresses the batteries with oscillatory load current, often referred to as ripple or circulating current, causing significant extra energy loss and thermal stress. The configuration also necessities significantly more circuit components. So far, modular multilevel motor drives remain as an academic interest and not yet practical.

Similarly, scientific reference Li et al. 2019 [Z. Li, R. Lizana, S. M. Lukic, A. V. Peterchev, and S. M. Goetz, "Current Injection Methods for Ripple-Current Suppression in Delta-Configured Split-Battery Energy Storage." IEEE Trans. Power Electron., vol. 34, no. 8, pp. 7411-7421, 2019, doi: 10.1109/TPEL.2018.2879613] discloses a method for battery-integrated modular multilevel converters to reduce the oscillatory load current.

US 9,502,960 B2 as well as Scientific references Goetz et al. 2016 [S. M. Goetz, Z. Li, X. Liang, C. Zhang, S. M. Lukic, and A. V. Peterchev, "Control of modular multilevel converter with parallel connectivity-Application to battery systems." IEEE Transactions on Power Electronics, vol. 32, no. 11, 8381-8392, 2016, doi: 10.1109/TPEL.2016.2645884] and Li et al. 2018 [Z. Li, R. Lizana, S. Sha, Z. Yu, A. V. Peterchev, and S. M. Goetz. "Module implementation and modulation strategy for sensorless balancing in modular multilevel converters." IEEE Transactions on Power Electronics, vol. 34, no. 9, pp. 8405-8416, 2018, doi: 10.1109/TPEL.2018.2886147] disclose various module circuit topologies and are included here by reference.

US 7,764,044 B2, US 7,279,855 B2, EP 1,881,596 Al and scientific reference Okamura, Masaki 2003 [Okamura, Masaki. "Development of hybrid electric drive system using a boost converter." EVS-20 November, 2003 (2003)] describe the use of a DC/DC stage between the battery bank and the DC/AC converter. The DC/DC stage in the middle takes as input the battery bank's DC voltage, and outputs regulated DC voltage for the DC/AC converter. The regulated DC voltage can be adjusted dynamically for different motor speeds-for instance, lowering the voltage at low speeds. Such configuration alleviates the electrical stress on the DC/AC converter and the motor. The DC/DC stage also decouples the design of the battery bank and the DC/AC converter.

Scientific reference Tekwani and Manila! 2017 [P. N. Tekwani and P. V. Manilal, "Novel approach employing buck-boost converter as DC-link modulator and inverter as AC-chopper for induction motor drive applications: An alternative to conventional AC-DC-AC scheme." in IEEE International Symposium on Industrial Electronics, vol. 26, pp. 793-800, 2017, doi: 10.1109/ISIE.2017.8001347] discloses and analyzes a relevant DC/DC stage, a method for control, and periphery for a motor drive.

Scientific reference Hsu et al. 2010 [J. S. Hsu, C. W. Ayers, and C. L. Coomer, "Report on Toyota/Prius motor design and manufacturing assessment." Oak Ridge Natl. Lab., p. 14, 2010, ORNL/TM-2004/137] describe an electrical vehicle drive train implementing a dc/dc stage. However, the primary side of the DC/DC stage constantly operates at the full voltage of the battery bank while conducting the entire current. In other words, the reduced switching stress on the DC/AC converter is transferred to the DC/DC stage at cost of additional components and hence higher conduction loss. Furthermore, large magnetic components are also required by the DC/DC stage, imposing unwanted trade-offs between weight and efficiency. Finally, a battery management system is still mandatory.

These solutions known to the person skilled in the art suggested a number of improvements, such as waveform quality and better battery management, over more prevailing simple technology at costs of more components and/or lower efficiency. The herein disclosed invention achieves the merits of those known solutions while mitigating the unwanted disadvantages.

Summary af the;wen den

A key aspect of the invention comprises at least a reconfigurable battery bank, advantageously 17C modularized, and one DC/AC converter. A method according to the invention operates both the reconfigurable battery bank and the DC/AC converter in their optimal conditions. A preferred embodiment of the invention comprises at least one string of modules and at least a DC/AC converter. Each module comprises at least one energy storage element and at least two power switches. The at least two power switches can form a micro DC/AC converter.

In contrast to canonical two-piece motor drive, i.e., a hard-wired battery bank and a DC/AC inverter, the invention retains the DC/AC converter but modularizes the battery bank. In contrast to modular multilevel motor drives, the invention employs similar electric modules but can implement only one module string, which is crucial to eliminate the large, low-frequency ripple current in the batteries. Preferably, the reconfigurable battery bank can only deliver 11c voltages of one polarity, e.g., positive voltages.

Backend The reconfigurable battery bank, henceforth called backend, comprises a plurality of modules. Each module comprises at least one energy storage unit, at least two power switches, and at least two electrical terminals. The at least two power switches can form a micro DC/AC converter. The energy storage can be a battery unit consisting of at least one battery cell, one or more capacitors, or combinations thereof. The micro DC/AC converter can dynamically wire the energy storage's terminals to the module's output electrical terminals, effectively presenting different voltages externally. Examples for switching elements are transistors, such as field-effect transistors (FET), bipolar transistors, and insulated gate bipolar transistors (IGBT). Further configurations can be gathered from the dependent claims and the description.

M2Cs (Modular Multilevel Converters), M2SPCs (Modular Multilevel Serial-Parallel Converters) and switched-capacitor converters generally utilize, as the basic structure, a concatenation of similar subcircuits that can be supplemented by further elements, for example serially adjacent circuits, circuits in parallel with the basic structure or circuit elements connecting different nodes of the basic structure. In this case, that part of the circuit which is repeated at least once, called module or individual module hereinafter without restricting the generality, need not be repeated structurally identically, but has functional similarity. Such topological modules can also form structural or physical modules improving manufacturability. Two modules generally already exhibit such a similarity if both can represent at least two so-called circuit functions or 131 circuit states of the same type. Furthermore, the implementation of the energy storages in the modules need not to be identical. For example, it is possible to have in the backend some modules containing only capacitors and, for instance, behaving as filters, while the other modules containing battery units and behaving as power sources.

All known module circuits (so-called microtopologies or module topologies) of modular converters, for example modular multilevel converters such as the M2C or the M2SPC and switched-capacitor converters, can be used as modules in the backend. Preferred module types are two-quadrant modules of the M2C (often also referred to as chopper modules, hereinafter called M2C-2q for short), four-quadrant modules of the M2C (hereinafter M2C-4q for short), short-circuit-protected M2C modules (hereinafter 4q-KGM2C for short), four-quadrant modules of the M2SPC (hereinafter M2SPC-4q for short), two-quadrant modules of the M2SPC (hereinafter M2SPC-2q for short), Marx converter modules (hereinafter MaM for short), and various switched-capacitor modules.

The backend modules are concatenated into a string. The two outmost terminals form a differential DC pair, whereas the terminals between the modules can be tapped to power any auxiliary electronics, e.g., at a lower voltage.

Frontend The main DC/AC converter (henceforth called frontend) comprises a plurality of switching elements, at least two DC electrical terminals of different polarity and a plurality of AC electrical terminals. The AC terminals are connected to the motor. The switching elements can be 151 controlled to dynamically wire the AC terminals to the DC terminals, effectively feeding any AC voltages to the motor.

The function of the frontend converter resembles that of canonical motor drives, i.e., to convert electric power from DC form to AC form. As such, all DC/AC converters known to the person skilled in the art can be used as the frontend. Possible implementations include two-level multi-phase converter, three-level neutral-point-clamped converter, and Marquardt's M2C macrotopology.

The backend and the frontend can either be directly connected, wherein the two outmost terminals of the backend are connected to the frontend's DC input terminals; or interfaced via an optional low-pass filter, wherein the filter comprises at least one passive circuit element 1:.]0 such as a capacitor or inductor. A preferred embodiment of the filter comprises an inductor and a capacitor (L-C filter).

Bret description of the drawings

Further advantages and refinements of the invention can be found in the description and the appended drawings.

The features summarized above and to be described below can be used not only in the respectively given combinations but also in other combinations or alone without departing from the scope of the present invention.

FIG. 1 shows schematic illustrations of three electric converter systems according to the prior art: FIG. la (per US 8,441,224 B2) displays a commonly used electric converter system for motor drives, wherein a hard-wired battery bank provides a fixed DC voltage that feeds a DC/AC inverter and eventually drives the load.

FIG.lb (per US 7,279,855 B2) shows an electric converter system that is similar to the setup in FIG. la but with a DC/DC stage inserted between the battery bank and the DC/AC inverter.

FIGs. lc-d (per US 8,760,122 B2 and US 20190288617 Al) show an electric converter system that primarily comprises electric modules. The modules form strings, which directly feed the electric load.

FIGs. 2a-b illustrate various embodiments of the invention.

FIGs. 3a-h illustrate several embodiments of the backend, using various modules. FIG. 4 shows several embodiments of the filter.

FIG. 5 shows several embodiments of the frontend according to the invention. FIGs. 6-7 illustrate control frameworks of the invention.

Detafled descn, on of the,:3'13'31..3'.," k § 3 Setup FIG. 2a shows an electric converter system 200 according to the invention. The electric load 211, e.g., a three-phase motor, is connected to at least two ac terminals 210 of at least one frontend 500. The energy supply of the electric converter system is made available by at least one backend 300, which is embodied in at least one, preferably exactly one, string comprising a plurality of modules 310. Modules 310 each comprises at least one energy storage element 312 and 313. The output voltages, measured across the ac terminals 210, are jointly synthesized by the backend 300 and the frontend 500. Between the backend and the frontend, a filter 400 can be optionally implemented to decouple the electrical switching transients. One or more auxiliary dc terminals 220 can be tapped between any two adjacent modules 310 in the backend 300. Optional auxiliary dc terminals 220 can be tapped in the backend 300, providing auxiliary electric power.

Modules 310 can cover a certain part of the current-voltage plane. If a module can cover two 220 quadrants of that plane, e.g., as it can generate only positive and zero voltage and currents with both polarities, it can absorb and deliver energy and is called two-quadrant module. If it can additionally generate voltages of both polarities and currents of both directions, it is called a four-quadrant module. Other combinations of quadrants or subareas of the current-voltage plane are likewise possible.

FIG. 213 shows an embodiment of the electric converter system 200, changed in respect of the filter 400 and the frontend 500.

FIG. 3a shows the backend 300, which serves as the energy supply of the electric converter system 200. Backend 300 contains a positive dc rail 230 and a negative dc rail 250, to which the rest of the electric converter system 200 is connected. The backend 300 is primarily based on a 210 string of multiple modules 310. The modules 310 are connected to one another via an interconnection 320, which is preferably a simple wire. The modules 310 can be embodied by different topologies, which, unmixed or in combinations with one another, form the backend 300. Typical module topologies featuring the single-conductor interconnection 320 are detailed in FIG. 313 and FIG. 3c.

FIG. 31,-c shows several alternatives for the modules 310a-c. Each module contains multiple power switches 316, a passive filter 312 and/or an energy storage 313. The energy storage 313 can be implemented by at least one battery, with which the module can provide active power. Without the energy storage, the module can serve as an active filter in series to the other modules. For the electric converter system ZOO, at least one module contains battery.

When analyzing DC/AC converters such as modules 310a-c, it is a common practice to combine every two power switches 316 in series, forming a so-called half-bridge 311 (FIG. 3b). The two extreme terminals of a half-bridge 311 connect to the at least one energy storage 312, 313. The middle terminal 314, 315 provides the output of the half-bridge 311. The two power switches 316 of half-bridges are typically switched ON and OFF in a complementary manner.

In a module 310, the combinations of ON and OFF states of the switches 316 effectively connect the at least one energy storage 312 and/or 313 to the middle terminals 314, 315 in different polarities. Each of such temporal switching combination is called a state of a module. Module embodiment 310a, featuring two half-bridges, can be configured in positive state 310a1, negative state 310a2, and at least one bypass state 310a3. Module 310a is therefore a four-quadrant module. Module embodiment 310b(310c), comprising only one half-bridge 311, can be configured in one polarized state 310b1(310c1), and one bypass state 310b2(310c2). Modules embodiments 310b and 310c are two-quadrant modules.

FIG. 3d shows a variation of FIG. 3a that features a two-conductor interconnection 330 between two adjacent modules 310. The two-conductor interconnection 330 allows the backend 300 to employ a wider range of modules, such as, advantageously, the series/parallel modules 310d-f to be introduced in FIG. 3e-g. Accordingly, the interconnection near the auxiliary terminal 220 takes different configurations, detailed in FIG. 3h.

FIG. 3e-g show various series/parallel modules 310d-f. Similar to the modules 310a-c, each series/parallel module comprises at least one energy storage 312, 313, as well as a plurality of power switches 316. Different from modules 310a-c in FIG. 3b-c, the series/parallel modules 310d-f comprise more half-bridges while offering the same operating quadrants when compared, respectively, to modules 310a-c. The additional half-bridges of series/parallel modules necessitate two-conductor interconnection 330 and allow parallel module states 310d1, 310e1, 310f1 where the at least one energy storage can be temporarily connected in parallel to the energy storages of the other modules. Modules 310d-f are connected to one another via terminals 314a-b, 315a-b, in the way illustrated in FIG. 3e-g.

The series/parallel module 310d comprises four half-bridges 311, or eight power switches 316, providing four-quadrant operations in which it is possible to switch between two polarities between terminal pairs 314a-b and 315b-c, shown as states 310d2 and 310d3. State 310d1 is _5C one of the parallel states offered by module 310d. Module 310e operates in two quadrants of the current-voltage plane, allowing a parallel state 310e1 and a polarized state 310e2. The other variations of the series/parallel circuits are also possible. One example is the module 310f, which is functionally identical to module 310e despite comprising only at least one energy storage 312, 313 and three independent power switches 316, i.e., fewer, transistors.

FIG. 3h shows alternative electrical configurations of the interconnection 340 of FIG. 3d.

FIG. 4 shows several embodiments for a filter 400. Embodiment 400a is an L-C filter that comprises an inductor and a capacitor, used to damp oscillations between the backend 300 and the frontend 500. Embodiment 400b only comprises a capacitor 403, placed physically near the frontend SOO to absorb the switching transients. Alternative filter 400c is merely a wire that directly connects the backend 300 and the frontend SOO.

FIG. 5 shows one embodiment of the frontend 500. Embodiment 500a comprises multiple half-bridges 510a-c, each comprising two power switches 316 in series. The two extreme terminals of each half-bridge connect to the positive dc rail 240 and the negative dc terminal rail 250. By changing the switching states, the three half-bridges 510a-c can produce AC voltages between terminals 210a-c, and thus driving the motor.

Operation FIG. 6 shows a control framework 600 for the electric converter system 200. The control diagram starts with the AC voltage references 611 calculated from an upper-level motor control 610. In one embodiment, the control framework comprises at least one backend controller 620 270 and one frontend controller 630. In this embodiment, at least one and typically at least three AC voltage references 611 are formed by at least one backend controller 620-which generates the switching signals 621 for the at least one backend 300-and by the at least one frontend controller 630, which generates the switching signals 631 for the at least one frontend 500. There are certain degrees of freedom in splitting the at least one AC voltage reference between the at least one backend controller 620 and the at least one frontend controller 630. Such control freedom can be used to optimize the switching loss and the output quality. In other embodiments, the at least one backend controller 620 and the at least one frontend controller 630 may be combined into one controller.

Alternatively, at least one rotating vector reference can be formed, e.g., by the at least one backend controller 620, according to the space-vector representation, which is subsequently split into at least two components, at least one first for the backend controller 620 and at least one second for the frontend controller 630.

For DC/AC converters such as frontend 500, the ratio between its peak output AC voltage-measured across ac terminals 210--and the at least one input DC-link voltage 622 is called the DC-link utilization ratio, which typically cannot exceed 100%. Most electric converter systems constrain the AC output voltages below the DC-link voltage to avoid output distortion (overmodulation). Such setting entails high-frequency switching actions on all power switches of the frontend converter and produces heavy losses. This invention, in contrast, variates the at least one DC-link voltage 622 in order to lock the DC-link utilization ratio at exactly 100% or 29C close to it. Working at close to 100% (e.g., >95%) DC-link utilization ratio spares a large portion of switching actions in the at least one frontend 500 and greatly reduces the switching loss without compromising the output quality. Such operation mode is embodied in FIG. 6a.

FIG. 6a shows an embodiment of the control framework 600 for a three-phase motor. In the embodiment 600a, the backend controller 620 comprises at least one DC-link voltage control 623 and at least one scheduler 624. An embodiment of the at least one DC-link voltage control 623a sets the at least one DC-link voltage 622 so that the DC-link utilization ratio is always 100% with respect to the AC voltage references 611. For M-phase systems with AC reference voltages vco, v,04, such DC-link voltage 622 can be calculated by DC = max { vo, , vom -min {V1, The voltage command vpc contains the 2M-th harmonic frequency and beyond, which is too high to be realized by single-stage DC/DC converters, but can be achieved effortlessly using the at least one backend 300 of the invention. Finally, the at least one DC-link voltage 622 is achieved by at least one scheduler 700, which assigns switching states to modules 310. Given vcc, there is a large degree of freedom of assigning the switching states to modules 310, which can be used to achieve various objectives including auxiliary dc voltage and battery management. The voltage command at the auxiliary dc terminal 220, vain° can be provided by sub-module string Ar-Aeo whereas the rest vx-vaux is provided by MM-M(k).By the same token, the voltage command we can also be fine-grained down to the module level, thus achieving the battery management. For instance, during the operation some modules (e.g., incorporated energy storage elements, such as batteries) might be discharged more than the rest. These outlier modules shall be commanded to generate less (more) voltage when the electric converter system 200 is discharging (charging). Once the distribution of the voltage commands is decided, there are numerous methods for translating the voltage commands to the module switching states (such as Li et al. (2019) [Z. Li, S. M. Goetz, et al., "Module Implementation and Modulation Strategy for Sensorless Balancing in Modular Multilevel Converters," in IEEE Transactions on Power Electronics, vol. 34, no. 9, pp. 8405-8416, Sept. 2019, doi: 10.1109/TPEL.2018.2886147.] and Goetz et al. (2017) [S. M. Goetz, Z. Li, et al., "Control of Modular Multilevel Converter With Parallel Connectivity-Application to Battery Systems," in IEEE Transactions on Power Electronics, vol. 32, no. 11, pp. 8381-8392, Nov. 2017, 32, doi: 10.1109/TPEL.2016.2645884.]), which are included by reference here and will not be detailed here further.

In conjunction to the DC-link voltage control defined above, the AC reference voltages 611 are processed by the at least one frontend voltage control 632 according to at least one so-called duty cycle of vok -min {vp v (1.2) o0_ VOM} k max (A.,1 v2 -nun tvo, 6'6 The resulting modulation references are sent to the at least one modulation stage 800 to be converted to transistor switching signals. Most modulation methods can be used here, including the pulse-width modulation with triangle carriers, e.g., after US 5,473,530 A. In short, the AC reference describes the temporal share of the ON time of the power switch(es) of the corresponding phase. When the reference is calculated by the above formula under the condition of (1.1), the reference spends 2/M share of time in either 0 (minimum) or 100% (maximum), wherein no switching actions take place for the corresponding switches of the frontend converter. For three phases, it is preferably at least one third of the time at its maximum level (e.g., >95%, preferably 100%). FIG. 6b visualizes the embodiment 600a with the help of several waveforms.

FIG. 7 illustrates the scheduler 700 for controlling the backend 300. In addition to producing the required DC-link voltage 622, the scheduler 700 may take advantage of the ample degrees of freedom in backend 300 to manage the batteries such as state-of-charge balancing, to pre-compensate the delay of the filter 400, and to provide power for auxiliary loads. Performing these additional tasks requires, respectively, the information of battery states 701, motor loads 702, and auxiliary loads 703. Various control methods could be implemented to convert the inputs 622, 701, 702, 703 into the switching signals 621. FIG. 7a shows an embodiment of the scheduler 700a, wherein the inputs 622, 701, 702, 703 are converted into modulation references 711 via reference calculation 710 before sent to the modulation 720 wherein the switching signals 621 are determined.

Claims

What Is cialmeth 1. An electric converter system comprising at least one frontend and at least one backend, wherein 350 each frontend comprises at least one DC/AC converter. Each of the at least one DC/AC converters comprises at least two electric terminals to which the backend is connected, a plurality of ac terminals to which the electric load is connected, and a plurality of power switches. Each of the power switches can be closed or opened in such a way that the each of the ac terminals can be connected to one of the at least two electric terminals of the input side of the frontend; each backend comprises at least two modules and each module comprises at least two middle terminals, at least one energy storage element, and at least two power switches, wherein said at least two power switches can be closed or opened in such a way that the at least two modules can be switched in series so 2.5C that the at least one energy storage elements of each of the modules are electrically in series, or in parallel so that the at least one energy storage elements of the each of the at least two modules are electrically in parallel, and bypass so that at least one of the energy storage elements is bypassed.
2. The electric converter system of claim 1, wherein the backend is composed of series/parallel modules, half-bridge modules, or full-bridge modules.
3. The electric converter system of claim 1, wherein the backend is composed of a mixture of series/parallel modules, half-bridge modules, and full-bridge modules.
4. The electric converter system of claim 1, wherein at least one of the modules in the backend employ capacitors as the energy storage elements, and the remaining modules comprise at least one battery as the energy storage element.
5. The electric converter system of claim 1, wherein the energy storage elements in the backend modules operate at different nominal voltages.
6. The electric converter system of claim 1, wherein at least one additional electrical terminal is tapped from the backend. The at least one additional electric terminal is 300 either connected to an energy storage element, or at an interconnection between two adjacent modules of the backend.
7. The electric converter system of claim 1, wherein an L-C filter is inserted between the backend and the frontend.
8. The electric converter system of claim 1, wherein a capacitor is inserted between the backend and the frontend.
9. The electric converter system of claim 1, wherein the frontend is implemented with three half-bridges, forming a two-level three-phase inverter.
10. A control method for an electric converter system with at least one frontend, which comprises at least one DC/AC converter, and at least one backend, which comprises at least two modules each with at least one energy storage elements and at least two power switches, wherein the control method comprises: at least one AC voltage reference is split into at least one first and at least one second contribution, of which the at least one first contribution is sent as a voltage reference to the at least one backend and the at least one second contribution is sent as a voltage reference to the at least one frontend.
11. The control method of claim 10, wherein the time average of the at least one second contribution is zero and the time average of the at least one first contribution is nonzero.
12. The control method of one of claims 10 and 11, wherein at least one of the AC voltage references is for an M-phase system and the at least one second contribution is controlled to contain a first frequency component and the at least one first contribution is controlled to contain a frequency that is at least twice of said first frequency.
13. The control method of claim 11, where the at least one first contribution is controlled to contain several frequencies, each a different even multiple of the first frequency and the voltage amplitude of each second frequency is controlled to effectively bring the DC-link voltage utilization ratio for the frontend to more than 95%.
14. The control method of one of claims 11 and 13, wherein the at least one first contribution is controlled to be the difference of the maximum voltage across the M phase AC reference voltages of the at least one M-phase system and the minimum voltage across the at M phase AC reference voltages of the at least one M-phase system at each point in time.
15. The control method of one of claims 12 through 14, wherein the at least one second contribution is formed by at least M differences of the respective M AC voltage references contained in the at least one AC voltage reference and the difference of the maximum voltage across the M phase AC reference voltages of the at least one M-phase system and the minimum voltage across the at M phase AC reference voltages of the at least one M-phase system at each point in time
16. The control method of claim 15, wherein at least one frontend modulator converts the at least one second contribution into at least one AC duty cycle and sends the at least 430 one AC duty cycle to at least one phase of the at least one frontend so that the at least one duty cycle describes the temporal share of the on time of the power switch(es) of the at least one phase and wherein the temporal maximum of the at least one duty cycle exceeds 95% and wherein the temporal minimum of the at least one duty cycle stays below 5%.
17. The control method of claim 16, wherein the at least one duty cycle exceeds 95% for at least one third of the time.
18. The control method of claim 16, wherein the temporal maximum of the at least one duty cycle equals 100% and wherein the temporal minimum of the at least one duty cycle equals 0%.
19. The control method of one of claims 16 through 19, wherein the method further comprises at least one backend modulator that converts the at least one first contribution into at least one DC duty cycle and sends the at least one DC duty cycle to at least one backend and wherein the at least one backend modulator receives as inputs the real-time states of at least one energy storage element, at least one electric load, and at least one auxiliary load to simultaneously perform at least battery management, feed-forward control of the DC-link voltage, and auxiliary power supply. 4 SC