DE102014219788A1 - Modular multilevel direct converter with single-phase variable frequency output - Google Patents

Abstract

The invention relates to a multi-level energy converter (22, 24) for converting electrical energy, having a terminal (34, 36) for supplying electrical energy to be converted and another terminal (34, 36) for outputting the converted electrical energy, wherein the multi-level energy converter (22, 24) comprises a first converter circuit (38) connected to a first of the two of the terminals (36) of the multi-level energy converter (22, 24) and comprising a plurality of series connected converter units (26) with a converter unit capacitor (48) and providing a center port coupled to the second of the ports (34), the converter circuit (38) being connected to a controller for controlling to convert the electrical energy. According to the invention, the control unit controls the converter units (26) for operating both the first and second terminals (34, 36) in AC operation.

Description

The present invention relates to a multi-level energy converter for converting electrical energy, having a terminal for supplying electrical energy to be converted and another terminal for outputting the converted electrical energy, the multi-level energy converter having a first converter circuit connected to a first of the two of the terminals of Multi-level energy converter is connected and which comprises a plurality of series-connected converter units with a converter unit capacitor and which provides a center terminal coupled to the second of the terminals, wherein the converter circuit is connected to a control unit for controlling to convert the electrical energy. Furthermore, the invention relates to a power coupling device for electrically isolated coupling of a first power supply network with a second power grid, with an isolating transformer with two magnetically coupled and galvanically isolated windings, wherein a first of the windings of the isolation transformer is electrically coupled to a first power supply network and the second of the windings of Isolation transformer is electrically coupled to the second power supply network. Moreover, the invention also relates to a method for converting electrical energy by means of a multi-level energy converter in which the electrical energy supplied to one terminal is converted and delivered to another terminal, the electrical energy being supplied to at least one of the first and second terminals For which purpose the converter circuit comprises a plurality of series-connected converter units with a converter unit capacitor and provides a center terminal coupled to the second terminal, the converter circuit being controlled by a control unit to convert the electrical energy.

Multi-level energy converters and energy coupling devices of the generic type are basically known, so that it does not require a separate documentary evidence for this. Multi-level energy converters serve to convert electrical energy between a DC voltage port and an AC voltage port. Energy coupling devices serve to enable an energy exchange between galvanically separated electrical power supply networks. In general, the power supply networks are three-phase power grids that are operated as a public power grid at 50 Hz or 60 Hz with AC voltage. In this area, an effective voltage between two phases of 400 V is often used. For various reasons, for example for reasons of security of supply or the like, it is expedient to allow an energy exchange between different energy supply networks. However, it makes sense at the same time to provide a galvanic separation, due to which undesired interactions of the coupled electrical power supply networks can be avoided or at least reduced with each other. In the simplest case, a three-phase isolation transformer is used in this regard, which provides a corresponding three-phase connection for each of the connected power supply networks and allows direct coupling. Such transformers are designed for the line frequency of the power supply networks to be coupled, that is, for operation at 50 Hz or at 60 Hz.

Although such isolation transformers have proven themselves in practical operation, there are still disadvantages. Although it is possible to achieve a galvanic isolation with such isolating transformers and at the same time make a voltage adjustment, on the one hand the energy flow can not be controlled and on the other hand the use of the isolating transformer requires that the two power supply networks to be coupled be operated synchronously with the same frequency , This may result in practical operation problems.

In order to be able to bring about an improvement here, it is also known to couple the power supply networks to be coupled by means of inverters coupled via common DC voltage intermediate circuits and a single-phase high-frequency isolation transformer. For this purpose, each of the energy supply networks to be coupled is connected to a bidirectional three-phase inverter, to whose intermediate circuit in each case a likewise bidirectional inverter for single-phase high-frequency operation is connected. On the output side, that is to say on the alternating voltage side, the single-phase inverters are connected to galvanically separated windings of the high-frequency isolation transformer assigned to them. With such an energy coupling device, it is possible, independently of electrical parameters of the energy supply networks to be coupled to enable energy flow in a desired predeterminable direction.

Although it is possible with such a power coupling device compared to a conventional isolating transformer smaller size in relation to the mains frequencies of the power grids can be achieved, however, the overall efficiency of such an energy coupling device turns out over the aforementioned isolation transformer as disadvantageous. For this reason, such energy coupling devices are used only for special requirements. The efficiency is particularly affected by switching elements of the inverter.

It is also known to use a multi-level energy converter as an inverter. Although this can be improved efficiency, however, the total circuit complexity proves to be high by the required four inverters for two to be coupled power grids still high. Generic multi-level energy converters are called in the English literature also Modular Multi Level Converter or MMC or M2C.

The invention is therefore an object of the invention to provide a multi-level energy converter a Vrefahren to its operation and an energy coupling device, which require improved efficiency and at the same time a little effort.

As a solution, the invention proposes a multi-level energy converter and an energy coupling device according to the independent claims 1 and 8. In addition, as a solution and a method according to the further independent claim 9 is proposed. Further advantageous embodiments will become apparent from the features of the dependent claims.

With regard to the multi-level energy converter, it is in particular proposed that the control unit controls the converter units for operating both the first and the second connection in AC operation.

With respect to the energy coupling device, the invention particularly proposes that the energy coupling device has two multi-level energy converters according to the invention, the multi-level energy converters each having a first AC voltage connection for connection to the respective winding of the isolation transformer and a second AC voltage connection for connection to the respective power supply network, wherein a first the multi-level energy converter electrically couples the first winding to the first power grid and the second of the multi-level power converters electrically couples the second winding to the second power grid.

In terms of the method, the invention particularly proposes that the control unit controls the converter units in such a way that both the first and the second connection are operated in AC operation.

The invention is based on the recognition that the multi-level energy converter can not only be operated as an inverter on a DC voltage intermediate circuit, but basically does not require the DC voltage intermediate circuit. Unlike inverters, the multi-level energy converter can therefore be operated with direct current DC voltage. This makes it possible to save effort compared to the energy coupling device of the prior art, which is based on the use of four inverters in the coupling of two power supply networks. In order to couple the isolating transformer or its two windings to the respective power supply networks, only one single converter device in the form of the multi-level energy converter is thus required for each energy supply network. As a result, the effort in terms of conversion can be significantly reduced, which not only increases the efficiency, but also costs and space can be reduced.

Multi-level energy converters and methods for their operation are often used in the field of high-voltage direct current (HVDC) transmission, with DC voltages in the range of several 100 kV and powers in the range of 1 GW are provided. Preferably, such multi-level energy converters are used bidirectionally, wherein the electrical energy can each be converted into a desired, preferably predeterminable direction. The conversion of the electrical energy preferably takes place without a significant change in voltage levels, that is to say that a voltage level of a maximum amplitude of the AC voltage of the energy supply network essentially corresponds to a voltage level of the transformer-side AC voltage.

Each of the converter units preferably has two parallel-connected series circuits each having two switching elements. To the series circuits of the switching elements of the converter unit capacitor is further connected in parallel. This makes it possible to form the circuit structure of a full bridge with respect to each converter unit. Central terminals of the respective series circuits of the switching elements provide the converter unit terminals by means of which the connections to adjacent converter units can be made. As a result, a high degree of flexibility with regard to the control of the multi-level energy converter can be achieved.

The switching elements are connected to the control unit, which controls the switching elements in a suitable manner to realize the desired conversion process. Basic control methods relating to the conversion of energy by means of a multi-level energy converter between a DC voltage and a changeover voltage are known to the person skilled in the art, so that a detailed description of the general conversion principle is omitted here. For the rest, a publication of Lesnicar, A. and Marquardt, R., entitled "An innovative modular multi-level converter topology for wide power range" published by the IEEE Power Tech Conference, Bologna, Italy, June 2003 ,

The converter unit capacitor may be formed by a film capacitor, a ceramic capacitor, but also by an electrolytic capacitor suitable for frequency applications or the like. Of course, the converter unit capacitor can also be formed by a combination of a plurality of individual capacitors, in particular of different types, as mentioned above.

For the purposes of this disclosure, a switching element is preferably a controllable electronic switching element, for example a controllable electronic semiconductor switch, for example a transistor, a thyristor, combination circuits thereof, preferably with freewheeling diodes connected in parallel, a gate turn-of-thyristor (GTO) an isolated-gate bipolar Transistor (IGBT), combinations thereof or the like. Basically, the semiconductor switch may also be formed by a metal oxide semiconductor field effect transistor (MOSFET). Preferably, the switching element is controllable by the control unit.

Particularly advantageous, the invention can be used in the field of medium voltage. Of course, the invention can be used both in the field of low voltage and in the range of medium or high voltage. As a low voltage according to the invention is in particular a definition according to the Directive 2006/95 / EC of the European Parliament and of the Council of 12.12.2006 on the approximation of the laws of the member states of electrical equipment for use within certain voltage limits. However, the invention is not limited to this voltage range, but can be used particularly advantageously benso in the range of medium voltage, which may preferably include a voltage range greater than 1 kV up to and including 52 kV.

The multi-level energy converter of the invention has the first AC voltage terminal for connection to the respective winding of the isolation transformer, to which the plurality of series connected converter units for converting electrical energy are connected, each with a converter unit capacitor as a converter circuit. Compared to the multi-level energy converter of the prior art, the invention thus also applies AC voltage to the terminal, which is a DC voltage terminal in the prior art.

Overall, the invention uses a property of the multi-level energy converter, namely on the one hand to be able to be operated substantially without DC link and on the other hand to be able to directly convert an AC voltage to another AC voltage without the generation of a DC voltage must be interposed.

The invention thus requires for the realization of an energy coupling device in addition to a single-phase isolation transformer only two more energy level converter according to the invention, by means of which an energy conversion between two AC voltages, one of which may be multiphase, can be realized immediately. A DC voltage intermediate circuit can be completely saved. This can be designed as a high-frequency transformer, as a medium-frequency transformer or as a low-frequency transformer, depending on the application. Accordingly, the multi-level energy converters and their operation are designed adapted. As a result, the efficiency can be significantly increased and / or the volume of construction can be significantly reduced.

According to a development, it is proposed that the multi-level energy converter has a circulation capacitor connected to the first connection, and the control unit is set up to flow a circulation current through the series-connected converter units of the multi-level energy converter and optionally the circulation capacitor. With this configuration, it is possible to provide the converter unit capacitors in a predeterminable manner with electrical charge, so that they are less stressed during normal operation. This allows the converter unit capacitors to be smaller in their capacitance value. In addition, in a three-phase operation on a three-phase power supply network can be achieved that interactions between the individual phases can be reduced during the conversion by means of the multi-level energy converter. Overall, the effort for the multi-level energy converter and its size can be reduced. This proves to be advantageous because in particular in the range of medium voltage and high voltage suitable capacitors are difficult to obtain.

The optional circulation condenser is preferably selected so that the circulation flow can flow in the intended manner. He is not with a DC link capacitor comparable, especially since the connection to which the circulation capacitor is connected, to be acted upon by an AC voltage. An intermediate circuit capacitor would result in a malfunction at this point. As a result, a capacitance value of the circulating capacitor is, of course, considerably smaller than that of a suitable DC intermediate capacitor. Its capacitance value is preferably chosen such that its influence on the AC voltage at the first terminal is low, particularly preferably negligible.

It proves to be particularly advantageous if the second AC voltage connection of the multilevel energy converter is designed as a three-phase AC voltage connection. This can be achieved, for example, by providing, for each phase of the second AC voltage connection, in each case series-connected converter units as described above, which are each connected in parallel to one another. The converter units are controlled by the control unit taking into account a phase shift between the three AC voltages at the second AC terminal. In this way, it is easy to provide a connection possibility for a three-phase power supply network.

According to a development, it is proposed that the multi-level energy converter has, for each phase, its own converter circuit in the manner of a series connection of series-connected converter units. These converter circuits or series circuits are preferably connected in parallel and provide respective phase connections for the alternating voltages of the three-phase power supply network.

The series-connected converter units of a respective series circuit are preferably connected to the first AC voltage terminal of the multi-level energy converter. Thus, an immediate conversion of energy from one AC side to the other AC side can be achieved. A DC intermediate circuit, as required when using inverters, can be completely eliminated. Due to the immediate conversion beyond the effort for the power coupling device and thus on the overall volume can be further reduced.

Preferably, a center terminal of the series-connected converter units of a respective series circuit is coupled to the second AC voltage terminal. This embodiment provides in particular that an even number of converter units are each connected in series and the center connection is connected to two directly connected converter units. Preferably, the center port is provided after half of the number of converter units in the series circuit.

According to a further embodiment, it is proposed that the series-connected converter units in the region of the coupling of the second AC voltage connection are supplemented by a series connection of two series circuit inductances whose connection is connected to the second AC voltage connection. The series circuit inductances are thus preferably interposed between the converter units and the second AC voltage terminal in this embodiment, on the one hand provide the second terminal at its common connection point and on the other hand are connected in series to the other converter units of the series connection. Preferably, the interposition of the series circuit inductances is likewise provided in the region of the middle terminal of the series connection of the converter units.

Moreover, the method proposes that the electrical energy is converted by means of a second and a third converter circuit parallel to the first converter circuit, wherein the series circuits of the converter circuits are identical to the first converter circuit and each provide a third and a fourth terminal, wherein the converter units of Converter circuit in AC operation by means of the control unit are controlled such that with the second, third and fourth terminals, a three-phase AC operation is achieved. This makes it possible to upgrade the multi-level energy converter with little effort for a three-phase operation. This shows the particular advantage of the invention that namely for the three-phase operation a complex DC voltage intermediate circuit can be completely saved basically.

Preferably, the control unit controls the converter units of the converter circuit such that a circulation current is generated by the converter units. This makes it possible to enable an energy balance in the individual converter units, and in particular in the converter unit capacitors, so that their capacity and thus the overall volume can be reduced. For given capacities, therefore, a higher power can be converted.

In particular, in at least two converter circuits for a multi-level energy converter, the circulation current can be controlled by appropriately controlling the switching elements of the converter units the two converter circuits can be realized. Of course, this is also possible with more than two converter circuits.

In addition, the circulation capacitor may additionally be provided, in particular if only a single converter circuit is provided. By means of the circulation condenser, the circulation flow can then be realized. Of course, the circulation capacitor may also be provided in a plurality of converter circuits to improve the flexibility of the realization of the circulation current. It is chosen in terms of capacity such that its influence on the voltage applied to the first terminal AC voltage can be substantially neglected.

Further advantages and features can be taken from the following description of exemplary embodiments with reference to the figures. In the figures, like reference numerals designate like components and functions.

Show it:

1 a schematic block diagram for a power coupling device of the prior art,

2 a schematic block diagram for a power coupling device according to the invention,

3 a schematic block diagram of an energy coupling device according to 2 with a multi-level energy converter,

4 a schematic diagram representation of a transducer unit for the multi-level energy converter according to 3 .

5 2 shows diagrammatically two diagrams, of which the upper diagram represents a time section of a current at the second connection of the multilevel energy converter and the lower diagram shows a schematic representation of a Fourier transform of the signal represented in the upper diagram,

6 schematically two superimposed diagrams as in 5 However, for the first AC voltage terminal of the multi-level energy converter according to 3 .

7 three superimposed diagrams in a schematic representation, wherein the uppermost diagram of three phases at the second terminal of the multi-level energy converter according to 3 , the middle diagram according to a first graph, the corresponding AC voltage of a single phase from the upper diagram and superimposed thereto according to a second graph, the corresponding associated current, and the lower diagram represents voltages to the respective converter unit capacitors,

8th a schematic diagram of the second terminal of the multi-level energy converter according to 3 generated AC voltages for a three-phase power grid,

9 temporally assigned a diagram in a schematic representation, in which corresponding phase currents are shown, the generated AC voltages according to 8th are opposite,

10 a schematic diagram of a voltage waveform at the first terminal supplied electrical voltage from the isolation transformer,

11 in a schematic representation of a superimposed representation accordingly 10 in which the supplied alternating voltage is shown superimposed on the first connection of the multilevel energy converter with a corresponding current,

12 1 is a schematic representation of an overall representation of an energy coupling device according to the invention for a simulation setup;

13 a schematic diagram of a voltage and a current waveform of a phase at one of three phase terminals of the second terminal of the multi-level energy converter according to 12 , where there is an equal power factor for all three phases,

14 in a schematic representation of two superimposed and temporally associated diagrams in which converter unit capacitor voltages of a first and a second multi-level energy converter according to 12 are shown

15 and 16 schematic representation of energy supply network side three-phase voltage waveforms of the two in 12 illustrated multi-level energy converters, and

17 in a schematic representation of a voltage curve of the transformer-side AC voltage of the multi-level energy converter according to 12 ,

1 shows an energy coupling device 10 according to the prior art, the two power grids 12 . 14 coupled with each other. The energy supply networks 12 . 14 are by means of Energy coupling apparatus 10 coupled galvanically isolated. The energy coupling device 10 allows a bidirectional flow of energy.

The energy supply networks 12 . 14 are each designed in three-phase with a phase voltage, that is, an effective voltage of 400 V at 50 Hz. To the power grids 12 . 14 is each an inverter 40 connected to a DC voltage intermediate circuit 44 connected. To the respective DC voltage circuits 44 are single-phase medium frequency inverters 42 connected, the AC voltage side in each case to windings 18 . 20 a medium-frequency isolation transformer 16 are connected. This energy coupling device 10 The prior art requires four inverters 40 . 42 to the energy technology coupling of the energy coupling device 10 to be able to represent in the intended manner. It turns out that in addition to the high cost and the efficiency compared to a power engineering coupling using a standard isolation transformer is significantly reduced at mains frequency.

2 shows an energy coupling device 10 according to the invention, in place of the four inverters 40 . 42 two multi-level energy converters 22 . 24 be used. Opposite inverters 40 . 42 have multi-level energy converters 22 . 24 a higher efficiency. The invention also makes it possible, in that the multi-level energy converter 22 . 24 be operated at their two terminals in AC operation that a DC voltage intermediate circuit, such as the DC intermediate circuit 44 according to 1 , can be saved. The isolation transformer 16 according to 2 corresponds in this case to the isolating transformer 16 according to 1 ,

3 shows a schematic block diagram for the multi-level energy converter 22 . 24 according to 2 , In the present case it is provided that the multi-level energy converters 22 . 24 are formed identically, but may be provided in alternative embodiments that it is different multi-level energy converters, for example when using the isolation transformer 16 at the same time a voltage transformation is provided.

The multi-level energy converter 22 . 24 according to 3 has connections 34 . 36 on, where electrical energy can be supplied or removed. The electrical energy is transmitted by means of three at the first of the two connections 36 of the multi-level energy converter 22 . 24 connected converter circuits 38 changed. For this purpose, the converter circuits 38 several series-connected converter units 26 with a converter unit capacitor 48 on ( 4 ). The converter circuits 38 Provide center connections with the second of the ports 34 are coupled. The converter circuits 38 be by means of a control unit 62 . 64 ( 12 ) is controlled so that electrical energy is converted. In the present case, it is provided that by means of the three converter circuits 38 at the second connection 34 a three-phase alternating voltage network is provided.

The control unit controls the converter units 26 further such that both the first and second terminals 34 . 36 be operated in AC operation.

The multi-level energy converters 22 . 24 each have a first AC voltage connection 36 for connection to the respective winding 18 . 20 of the isolation transformer 16 and a second AC voltage terminal 34 for connection to the respective power supply network 12 . 14 on. A first of the multi-level energy converters 22 electrically couples the first winding 18 with the first power supply network 12 , The second of the multi-level energy converters 24 electrically couples the second winding 20 with the second power grid 14 , The second AC voltage connection 34 of the multi-level energy converter 22 . 24 is designed in each case as a three-phase AC voltage connection. For this purpose, three corresponding unsigned ports are provided ( 4 ).

In the present case it is provided that the converter circuits 38 six converter units each 26 have, each connected in a series circuit. The series circuits of the converter circuits 38 are connected in parallel and to the first connection 36 the multi-level energy converter 22 . 24 connected.

The series connected converter units 26 the converter circuits 38 are each in the range of coupling the second AC terminal 34 by a series connection of two series circuit inductances 28 added. The connection between the two series connection inductances 28 is to the second AC voltage connection 34 or connected to the corresponding phase connections. In the present case it is provided that the respective phase connections via a connection inductance 30 to the respective connections of the series connection inductances 28 are connected.

4 shows a schematic diagram representation of one of the converter units 26 , In the present case are the converter units 26 formed identical to each other. The converter unit 26 according to 4 comprises two parallel series circuits of IGBTs 50 , parallel to which Conversion unit capacitor 48 connected. Centers of the series connections of the IGBTs 50 form connections 52 . 54 the transducer unit 26 , By means of the connections 52 . 54 can the converter units 26 be connected in series connection. In the present case it is provided that the converter units 26 are all identical. In alternative embodiments, however, deviating converter units may also be provided, for example with regard to the converter unit capacitor and / or the like.

Out 3 It can also be seen that the first connection 36 a connection inductance 32 having. This serves for better adaptation of the first connection 36 to the isolation transformer 16 ,

12 schematically shows a simulation structure for a power coupling device 10 according to the invention. The energy coupling device 10 then includes the two multi-level energy converters 22 . 24 whose first connections 26 to the transformer 16 are connected. In addition, the multi-level energy converters 22 . 24 via a connecting cable 66 connected to a common clock signal. Finally, for each of the multi-level energy converters 22 . 24 a control unit associated with it 62 . 64 provided, which likewise over the connection line 66 to the respective multi-level energy converter 22 . 24 connected.

At the first connection 36 of the multi-level energy converter 22 is also a measuring device 68 connected, by means of an electrical voltage of the first terminal 36 can be detected.

The second connections 34 the multi-level energy converter 22 . 24 are via decoupling networks 76 . 78 as well as measuring blocks 70 . 72 connected in parallel and via another measuring block 60 to a three-phase source of energy 74 connected.

In the present embodiment, it is provided that the inductance 32 has a value of 4.5 mH. The connection inductance 30 in the present case has an inductance of 10 μH. The capacity of the converter unit capacitors 48 in the present case is 1.5 mF. The series circuit inductances 28 in the present case have an inductance of 0.8 μH. For each of the three phases, a voltage in the peak-to-peak value of about 700V is provided. The frequencies of the AC voltage at the second terminal 34 is 50 Hz. The peak-to-peak voltage of the AC voltage at the first AC terminal 36 the multi-level energy converter 22 . 24 in the present case is about 400 V. The frequency of the AC voltage at the first AC voltage terminal 36 is about 1000 Hz.

The above values are given only for the illustrative embodiment and can be adjusted as desired according to the need or application. Basically, of course, the frequency at the first port 36 of the multi-level energy converter 22 . 24 also lower than the frequency of the AC voltage at the second terminal 34 be.

5 schematically shows two superimposed diagrams, wherein in the upper diagram by means of a graph, a current waveform of one of the phases at the second AC voltage terminal 34 is shown. The abscissa is a time axis in which the time is indicated in s. On the ordinate the current in A is plotted. It can be seen that the graph shown in the upper diagram shows an alternating current with an amplitude of about 40 A at a frequency of 50 Hz.

The lower diagram of the 5 represents a corresponding Fourier transform of the signal detail shown in the upper diagram. Accordingly, the abscissa is assigned to the frequency in Hz. It can be seen that at 50 Hz a large value is shown, whereas harmonics and subharmonics are very small. The course of the current shown in the upper diagram is thus sinusoidal in high quality.

6 shows two diagrams arranged one above the other as in 5 , In the upper diagram, a graph shows the current flow at the first connection 36 of the multi-level energy converter 22 . 24 shown. Accordingly, the ordinate is assigned to the current measured in A, whereas the abscissa is a time axis on which the time is indicated in s. It can be seen from the upper diagram that the current profile shown there has an amplitude of approximately 100 A at a frequency of 1 kHz.

In the lower diagram of the 6 again the Fourier transform of the signal section of the upper diagram is shown. Accordingly, the abscissa is assigned to the frequency in Hz. To recognize is from the lower representation of the 6 in that the Fourier transform takes place at the operating frequency of 1 kHz, a very large deflection. A negligible rash is recognizable only at 3000 Hz and even lower at 500 Hz. This results in that by means of the multi-level energy converter 22 . 24 an AC voltage can be generated with high quality, with which the isolation transformer 16 is charged. The high quality of the AC voltage allows losses in the isolation transformer 16 keep as low as possible.

7 shows three superimposed diagrams showing the second port 34 of the multi-level energy converter 22 according to 12 affect. at 12 is provided in the present case that electrical energy from the multi-level energy converter 22 over the isolating transformer 16 to the multi-level energy converter 24 is encouraged. The upper illustration of the 7 shows the electrical voltages of the three phases of the second AC terminal 34 of the multi-level energy converter 22 supplied electrical energy. The ordinate is therefore assigned to the electrical voltage in V, whereas the abscissa is a time axis on which the time is shown in s. It can be seen that it is a three-phase AC voltage, in which each phase has a voltage amplitude of about 230 V at a frequency of 50 Hz. The three AC voltages are each shifted by 120 °.

In the middle illustration of the 7 is by means of a graph 80 the electrical voltage of a single phase of the above diagram is separated with an associated phase current according to a graph 82 shown. It can be seen that the amplitude of the phase current is about 40 A. The alternating voltage and the alternating current are equal in phase. The time axis corresponds to the time axis of the upper representation.

In the lower illustration according to 7 is another diagram with a same time axis as the two upper diagrams shown. In this diagram, the voltage waveform on one of the converter unit capacitors 26 shown. Accordingly, the ordinate of the electrical voltage in V is assigned. In the present case, one of the converter unit capacitors is in each case for each of the phases 48 in terms of its voltage exemplified. It can be seen that the voltage across the converter unit capacitors 48 in the rhythm of the AC voltage at the phase terminals of the second AC voltage terminal 34 fluctuates. At the same time a ripple is superimposed, in the present case by the AC voltage at the first AC voltage connection 36 is superimposed due to the action of the corresponding alternating current. Accordingly, the ripple has a frequency of 1 kHz.

8th schematically shows a diagram with the second AC voltage terminal 34 of the multi-level energy converter 24 provided electrical AC voltage, which is also three-phase here. In the diagram of 8th Therefore, the abscissa is a time axis in which the time is shown in s and the ordinate a voltage axis in which the voltage is indicated in V. It can be seen that the AC voltage provided has a mean voltage amplitude of about 500 V at a frequency of 50 Hz. Again, three phases are generated, which are shifted from each other by about 120 °. From the illustration according to 8th It can also be seen that each of the voltages at the phase terminals is superimposed by a high frequency ripple. This ripple has the frequency of 1 kHz due to the energy transfer from the multi-level energy converter 22 over the isolating transformer 16 to the multi-level energy converter 24 results. If necessary, additional filtering measures can be provided to dampen the ripples.

9 shows in timely representation to 8th Current curves of the respective in 8th illustrated electrical phase voltages. The diagram according to 9 Therefore, the abscissa indicates the time axis as 8th whereas the ordinate is assigned to the electric current of the respective phase in A. It can be seen that the current profile of each of the three phases substantially corresponds to the sinusoidal shape and there is substantially no phase shift between the alternating voltages at the phase terminals and the associated alternating currents.

In the 10 is a voltage curve of the first connection 36 of the multi-level energy converter 24 wherein the abscissa is again assigned a time axis with time in s and the ordinate a voltage axis with an electrical voltage in V. It can be seen that there is an AC voltage of about 450 V in amplitude and 1 kHz in frequency.

11 shows another diagram like 10 in which, however, a graph 84 in terms of electrical voltage, as in 10 a corresponding associated electrical current is superimposed with a second graph 86 is shown. It can be seen that the electrical current is in opposite phase to the electrical voltage and has an amplitude of about 80 A.

13 shows in an upper representation by means of a graph 88 a phase voltage and by means of a graph 90 a phase current of one of the phases at the second electrical connection 34 of the multi-level energy converter 22 . 24 when changing the direction of energy flow and at a uniform power factor. A correspondingly assigned representation of the voltages on the converter unit capacitors 48 can be found in 14 , In 14 Two diagrams are arranged one above the other with identical time axes as abscissa and stress axes as ordinate. The time is given in s and the voltage in V. The upper of the two representations is the converter unit capacitors 48 of the multi-level energy converter 22 and the lower of the two representations of the converter unit capacitors 48 of the multi-level energy converter 24 assigned. It can be seen that at about 0.27 s a change of direction of the energy flow takes place. In the range of 0.2 s to 0.27 s, the Energy flow from the multi-level energy converter 22 to the multi-level energy converter 27 , After that, that is, from about 0.27 s, the direction of energy flow reverses.

In the 15 and 16 are the voltage curves of the three phases of the second AC voltage connection 34 the two multi-level energy converters 24 . 26 each shown in a diagram. In the 15 and 16 in each case the abscissa is a time axis in which the time is indicated in s. The ordinate is assigned in the two figures in each case to the voltage which is indicated there in V. 15 is the multi-level energy converter 22 assigned, whereas 16 the multi-level energy converter 24 assigned. The time axes are also the time axes of 13 and 14 assigned. It can be seen that an energy flow change takes place in the range of 0.27 s.

17 shows a voltage curve at the first AC voltage connection 36 as he does in both the multi-level energy converter 22 as well as the multi-level energy converter 24 occurs. The abscissa is again a time axis on which the time is shown in s, whereas the ordinate is a voltage axis on which the voltage in V is shown. It can be seen that the alternating voltage has a frequency of 1 kHz and an amplitude of about 600 V.

The embodiments are only illustrative of the invention and are not limiting for these. Of course, functions in particular embodiments in relation to the multi-level energy converter and the converter units can be designed arbitrarily, without departing from the spirit of the invention.

Finally, it should be noted that the advantages and features and embodiments described for the device according to the invention apply equally to the corresponding method and vice versa. Consequently, corresponding device features and vice versa can be provided for device features.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• Lesnicar, A. and Marquardt, R., entitled "An Innovative Modular Multi-level Converter Topology for Wide Power Range" published by the IEEE Power Tech Conference, Bologna, Italy, June 2003 [0015]
• Directive 2006/95 / EC [0018]

Claims (11)

Multi-level energy converters ( 22 . 24 ) for converting electrical energy, with a connection ( 34 . 36 ) for supplying electrical energy to be converted and another connection ( 34 . 36 ) for delivering the converted electrical energy, wherein the multi-level energy converter ( 22 . 24 ) a first converter circuit ( 38 ) at a first of the two of the terminals ( 36 ) of the multi-level energy converter ( 22 . 24 ) and the several series-connected converter units ( 26 ) with a converter unit capacitor ( 48 ) and the one with the second of the terminals ( 34 ) provides a coupled center connection, wherein the converter circuit ( 38 ) is connected for control to a control unit in order to convert the electrical energy, characterized in that the control unit converts the converter units ( 26 ) for operating both the first and second terminals ( 34 . 36 ) in AC operation. Multi-level energy converter according to claim 1, characterized in that the multi-level energy converter ( 22 . 24 ) one at the first AC voltage terminal ( 36 ) and the control unit is set up, a circulation current through the series-connected converter units ( 26 ) of the multi-level energy converter ( 22 . 24 ) and to circulate the circulation condenser. Multi-level energy converter according to claim 1 or 2, characterized in that the second connection ( 34 ) of the multi-level energy converter ( 22 . 24 ) is designed as a three-phase AC voltage connection. Multi-level energy converter according to claim 3, characterized in that the multi-level energy converter ( 22 . 24 ) for each phase its own converter circuit, each having a series connection of series-connected converter units ( 26 ) having. Multi-level energy converter according to one of claims 1 to 4, characterized in that the series-connected converter units ( 26 ) at the first connection ( 36 ) of the multi-level energy converter ( 22 . 24 ) are connected. Multi-level energy converter according to one of claims 1 to 5, characterized in that a middle terminal of the series-connected converter units ( 26 ) with the second connection ( 34 ) is coupled. Multi-level energy converter according to claim 6, characterized in that the series-connected converter units ( 26 ) in the region of the coupling of the second terminal ( 34 ) by a series connection of two series circuit inductances ( 28 ), whose connection to the second port ( 34 ) connected. Energy coupling device ( 10 ) for the galvanically isolated coupling of a first power supply network ( 12 ) with a second energy supply network ( 14 ), with an isolating transformer ( 16 ) with two magnetically coupled and galvanically isolated windings ( 18 . 20 ), wherein a first of the windings ( 18 ) of the isolation transformer ( 16 ) with the first power grid ( 12 ) is electrically coupled and the second of the windings ( 20 ) of the isolation transformer ( 16 ) to the second power grid ( 14 ) is electrically coupled, characterized by two multi-level energy converters ( 22 . 24 ) according to one of the preceding claims, wherein the multi-level energy converters ( 22 . 24 ) each have a first AC voltage connection ( 36 ) for connection to the respective winding ( 18 . 20 ) of the isolation transformer ( 16 ) and a second AC voltage terminal ( 34 ) for connection to the respective energy supply network ( 12 . 14 ), wherein a first of the multi-level energy converters ( 22 ) the first winding ( 18 ) with the first power grid ( 12 ) and the second of the multi-level energy converters ( 24 ) the second winding ( 20 ) to the second power grid ( 14 ) is electrically coupled. Method for converting electrical energy by means of a multi-level energy converter ( 22 . 24 ), in which the at a connection ( 34 . 36 ) converted electrical energy and at another terminal ( 34 . 36 ) is discharged, wherein the electrical energy by means of at least a first at a first of the two of the terminals ( 36 ) of the multi-level energy converter ( 22 . 24 ) connected converter circuit ( 38 ), for what purpose the converter circuit ( 38 ) a plurality of series converter units ( 26 ) with a converter unit capacitor ( 48 ) and one with the second of the terminals ( 34 ) provides a coupled center connection, wherein the converter circuit ( 38 ) is controlled by means of a control unit in order to convert the electrical energy, characterized in that the control unit converts the converter units ( 26 ) such that both the first and second terminals ( 34 . 36 ) are operated in AC operation. Method according to Claim 9, characterized in that parallel to the first converter circuit ( 38 ) the electrical energy by means of a second and a third converter circuit ( 38 ), wherein series circuits of the converter circuits ( 38 ) identical to the first converter circuit ( 38 ) are formed and one each provide third and fourth ports, with the transducer units ( 26 ) of the converter circuits ( 38 ) are controlled in AC operation by means of the control unit such that a three-phase AC operation is achieved with the second, third and fourth terminals. Method according to claim 9 or 10, characterized in that the control unit converts the converter units ( 26 ) of the converter circuit ( 38 ) controls such that a circulation current through the converter units ( 26 ) is produced.

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