Continuously and uninterruptedally adjustable power transformer

Abstract

A transformer arrangement with adjustable output voltage is disclosed. The transformer arrangement comprises a primary winding (2), a secondary winding (3), and a first core (4). The primary winding (2) is configured to input an alternating voltage (Uin). The secondary winding (3) is configured to output a voltage (Uout) to a load. The first core (4) passes through the primary winding (2) and the secondary winding (3) and directs a magnetic flux generated by the primary winding (2) to the secondary winding (3).Furthermore, the transformer arrangement (1) comprises a control winding (8), a second core (7) and a control unit (9), wherein the second core (7) is guided through the secondary winding (3) or the primary winding (2), wherein the second core (7) is additionally guided through the control winding (8), wherein the control unit (9) is connected to the control winding (8) and wherein the control unit (9) is configured to input a control current into the control winding (8) such that the magnetic flux acting on the secondary winding (3) and thus the output voltage (Uout) is increased or reduced.

Description

[0001] The invention relates to a transformer arrangement with adjustable output voltage.

[0002] Energy supply networks must maintain a certain level of supply quality to keep the network stable and prevent damage to connected consumers, generators, and equipment. A key aspect of supply quality is the network voltage, which must be kept within a relatively narrow tolerance range. Ensuring adequate supply quality is no trivial task in networks with a high proportion of distributed energy generation, such as photovoltaic or wind power plants.

[0003] Every generator and every consumer can influence the quality of supply in the areas surrounding them within the power supply network. A consumer's draw of power from the network can lead to a reduction in the grid voltage, for example, if more power is drawn than the network can supply. Conversely, every generator requires a slightly higher output voltage than the grid voltage to feed energy into the power supply network. If too much power is fed in relative to the power drawn, this can lead to an increase in the grid voltage. The behavior of equipment within the power supply network also depends on the operating conditions. For example, a high current flow through a conductor increases the voltage drop across that conductor.

[0004] In practice, this means that the energy supply network must be able to react flexibly to changing requirements. For generators with inverters featuring active inverter bridges, this is usually not too challenging, as necessary adjustments can be made quickly via software. However, the performance and application areas of such inverters are limited.

[0005] One effective way to adjust the output voltage would be directly at the transformer. However, in conventional transformers, the output voltage is determined by the input voltage and the turns ratio, i.e., the ratio of the number of turns in the primary and secondary windings. Therefore, flexible adjustment of the output voltage is not possible with such transformers.

[0006] A simple solution is to change the number of effective secondary windings. This can be achieved by manually switching a predefined number of turns of the secondary winding on or off. The higher the number of effective turns, the higher the output voltage. However, this only allows for abrupt changes in the output voltage. Furthermore, manually adjusting the voltage always requires disconnecting the transformer, which inevitably leads to power outages. This makes it unsuitable for flexibly responding to changing operating conditions.

[0007] Alternatively, the windings could be switched on or off via switching devices, which could also be done during operation. However, the problem here is that such switching devices would incur considerable additional costs and also increase maintenance requirements.

[0008] Another transformer with adjustable output voltage is known, for example, from DE 18 73 209 U1. In this transformer, the output voltage can be adjusted via sliding contacts that are guided over a sliding section of a toroidal transformer. However, these transformers are maintenance-intensive, complicated to manufacture, and can only be used for lower power transmission capacities.

[0009] Another controllable transformer is known from DE 76 17 26 A. This design utilizes flux displacement in one leg of a three-leg core. However, this results in cores requiring considerable material. Furthermore, the transmission power is also limited in this case.

[0010] Another method of voltage adjustment is known from EP 3 101 796 A1. The primary winding of a transformer is coupled into a phase conductor of the supply network. Various current paths between the primary and secondary windings can be selected using a number of switches, thus influencing the output voltage. An additional winding, arranged within the magnetic circuit of the primary and secondary windings, can be used to maintain the magnetic flux within the circuit. This prevents abrupt changes in the magnetic flux. However, the input and output of this arrangement are galvanically connected, and the adjustment options are limited.

[0011] For further prior art, reference is made to DE 28 11 205 A1, DE 10 2004 018 613 B3, GB 1 053 297 A, US 2 644 109 A and DE 607 127 C, each comprising a transformer arrangement according to the preamble of claim 1.

[0012] The invention is based on the objective of providing a transformer arrangement in which the output voltage can be flexibly adjusted. It is desirable for the transformer arrangement to be relatively simple in design, to be usable in principle even at higher transmission power levels, and to allow continuous or quasi-continuous adjustment.

[0013] This problem is solved by the combination of features of claim 1. Further, particularly advantageous embodiments are disclosed in the dependent claims.

[0014] It should be noted that the features listed individually in the claims can be combined with one another in any technically meaningful way and demonstrate further embodiments of the invention. The description further characterizes and specifies embodiments of the invention.

[0015] It should also be noted that the conjunction "and / or" used herein, which stands between two characteristics and links them together, is always to be interpreted in such a way that in a first embodiment only the first characteristic can be present, in a second embodiment only the second characteristic can be present, and in a third embodiment both the first and the second characteristic can be present.

[0016] The present disclosure utilizes the effect that, in a conductor loop, the induced voltage U depends on the rate of change of the magnetic flux Φ through the conductor loop (induction law). This means that by influencing the magnetic flux through the secondary coil of a transformer, the output voltage of the transformer can be influenced. Since magnetic fluxes superimpose, the effect of a first magnetic flux can be amplified or attenuated by a second magnetic flux, depending on the flux direction. Thus, if the primary coil of the transformer generates a first magnetic flux, the magnetic flux acting on the secondary coil, and therefore the output voltage of the transformer, can be changed by a second magnetic flux.

[0017] To modify the magnetic flux acting on the secondary winding, a second magnetic flux can be introduced into the primary and / or secondary winding. According to the present disclosure, this is achieved using a second core and a control winding, wherein the second core is guided through the primary or secondary winding and the control winding is arranged around the second core. If a second flux is to be coupled to both the primary and secondary sides, two second cores and two control windings can be provided. The control winding generates a second magnetic flux in the second core, and the second core conducts this second magnetic flux to the primary or secondary winding.

[0018] When the second core passes through the primary winding, the first magnetic flux generated by the primary winding is conducted through both the first and second cores. The second magnetic flux, generated by the control winding in the second core, superimposes itself on the first magnetic flux within the second core. This second magnetic flux then passes through the second core to the primary winding and acts upon it. If the first and second magnetic fluxes are aligned in the region of the primary winding, the second magnetic flux increases the magnetic flux in the first core. If the first and second magnetic fluxes are opposed in the region of the primary winding, the second magnetic flux reduces the magnetic flux in the first core.In both cases, the magnetic flux effective at the secondary winding, and thus the output voltage, can be influenced by the control winding and the magnetic flux it generates.

[0019] When the second core is routed through the secondary winding, both the first magnetic flux generated by the primary winding and the second magnetic flux generated by the control winding are directed through the secondary winding, both contributing to an induced voltage according to Faraday's law of induction. If the first and second magnetic fluxes through the secondary winding are aligned, the induced voltage, and thus the output voltage, is increased. If the first and second magnetic fluxes through the secondary winding are oppositely aligned, the induced voltage, and thus the output voltage, is decreased. In this way, the control winding can influence the magnetic flux acting on the secondary winding and thus the output voltage.

[0020] In all cases, the task of a control unit connected to the control winding is to appropriately control the control winding with a control current. The specific design of the control unit and how it fulfills this task is not crucial for the present disclosure. Typically, the control unit is designed to generate a current flow through the control winding that leads to the desired change in the output voltage of the transformer assembly. The direction and magnitude of the current flow can be particularly important in this regard. The specific settings of the current flow are likely to depend on the particular design of the transformer assembly, for example, the winding direction and number of turns of the control coil, the power transmitted by the transformer, the arrangement of the second core (on the primary or secondary winding), etc.It can be advantageous if the control current and the current through the primary winding are approximately in phase. This makes adjusting the output voltage particularly effective.

[0021] Overall, the output voltage of the secondary winding, and thus the output voltage of the transformer assembly, can be adjusted by means of the control winding, the second core, and the control unit. The transformer assembly itself can be of a simple design. If the control unit is designed such that the current flow through the control winding can be continuously or at least quasi-continuously adjusted, this also results in continuous or quasi-continuous adjustability of the output voltage of the transformer assembly. Furthermore, the technology disclosed here is not limited to low transmission power levels but can be scaled almost arbitrarily. This allows the transformer assembly disclosed here to be used in power supply networks, for example, to stabilize the voltage level or to control the feed-in capability of distributed energy generators by raising or lowering the grid voltage.

[0022] A "continuous adjustment" means that the output voltage can, in principle, be changed continuously. A "quasi-continuous adjustment" means that while there are steps in the output voltage, these steps are very small compared to the output voltage and therefore practically imperceptible. This can mean that a step is less than or equal to 1% of the output voltage, preferably less than or equal to 0.1%.

[0023] The degree of "adjustable output voltage" is, in principle, arbitrary. With a suitably dimensioned control coil, drive unit, and second core, the output voltage can, in principle, be reduced to zero volts or below. It is also conceivable to significantly increase the output voltage compared to a nominal voltage (i.e., an output voltage without influence from the control winding), for example, by doubling the output voltage. In practical applications, however, a transformer configuration that allows the output voltage to be increased and / or decreased in the single- or double-digit percentage range is likely to be more advantageous. The smaller the desired adjustability, the smaller the control winding, the second core, and the drive unit can be.

[0024] In general, a "transformer arrangement" is understood to be an arrangement that converts an alternating voltage into an alternating voltage. For this purpose, an input alternating voltage is applied to a primary winding, converted by the primary winding into a magnetic flux, guided through a first core to a secondary winding, and the magnetic flux in the secondary winding is converted into an output alternating voltage. This results in galvanic isolation between the input and output. In principle, it is irrelevant whether the input voltage is higher than the output voltage or vice versa. That is, the transformer arrangement can be used to step up or step down a voltage. The transformer arrangement can also, in principle, be designed for bidirectional operation.It is also conceivable in principle that the nominal input and nominal output AC voltages have the same amplitude, so that the transformer arrangement for adjusting the output voltage is designed at a comparable voltage level.

[0025] The term "primary winding" refers to a coil connected to an input of the transformer assembly. The term "secondary winding" refers to a coil connected to an output of the transformer assembly. In a common application scenario, this can mean that the primary winding is connected to a power supply network and the secondary winding is connected to a load. Although the term "primary winding" is often used to describe a winding connected to the higher voltage, this is not a mandatory requirement for the purposes of this disclosure. The same applies to the "secondary winding." In bidirectional operation, the functions of the primary and secondary windings can even be reversed.

[0026] The specific construction of the windings (especially primary, secondary, and control windings) is not crucial, as long as they fulfill the functions disclosed herein. In particular, no special winding types are required. Rather, the windings can be wound in the usual way onto a hollow core and then placed onto the core. This allows the windings to be produced using conventional winding machines.

[0027] The windings can be constructed as hollow cylinders, with the individual turns of the winding running along the circumference. Possible shapes for the base of the hollow cylinder include circles, ellipses, rectangles, squares, hexagons, rectangles with rounded corners, etc. Particularly to avoid field potentiometers, it can be advantageous if the turns are wound with minimal bending, which is why base shapes such as circles or ellipses can be beneficial.

[0028] The control winding can be constructed and dimensioned in various ways. It is also conceivable that the control winding is divided into two or more sub-windings, for example, to make optimal use of available space. Crucially, the control winding must be able to conduct a control current that enables the required change in the output voltage. This typically means that a larger conductor cross-section is required for high maximum control currents than for lower maximum control currents. Accordingly, increasing the number of turns may be advantageous in order to reduce both the control current and the conductor cross-section.

[0029] When it is stated that a "core is guided through a winding," this means that the winding is arranged around the core. Typically, one axis of the winding and the core are arranged parallel to each other during the winding process.

[0030] The "first core" and the "second core" can be constructed and dimensioned in a conventional manner. This means that the core can be formed by a stack of transformer laminations, which can be insulated from each other. The core can be composed of several sub-cores, which can particularly simplify the assembly of the transformer. For dimensioning purposes, it is advantageous if the core can conduct the expected maximum magnetic flux without any, or at least no significant, saturation effects. However, such dimensioning is well-established in practice.

[0031] In one embodiment, an air gap is arranged between the first and second cores, and / or a portion of the first core and a portion of the second core are arranged parallel to each other. An air gap allows for a stronger magnetic separation of the magnetic circuits of the first and second cores. This can simplify the core dimensioning. The air gap forms a region with increased magnetic resistance. This does not necessarily mean that the air gap actually contains air. Rather, it can also contain another material with low or no magnetic conductivity, such as a plastic or a potting compound. In one embodiment, the air gap is greater than or equal to one or a few millimeters. In another embodiment, the air gap is less than or equal to 10 centimeters. This allows for a high magnetic resistance.In another embodiment, the air gap is less than or equal to 5 centimeters. This reduces the space requirement. In yet another embodiment, the air gap is less than or equal to one centimeter. This provides good magnetic resistance in many transformer configurations while maintaining a manageable space requirement. Parallel arrangement of parts of the first and second cores can facilitate the conduction of magnetic flux through the secondary or primary winding (depending on the arrangement of the second core).

[0032] In one embodiment, the control unit is designed as a controller, where the controller regulates a secondary-side output voltage to a target output voltage or a secondary-side output power to a target output power. Using a controller allows for a rapid response to changes in an operating situation. This enables the actual state to be detected and compared with a target state. If a deviation between the actual and target states exceeds a predetermined threshold, the controller can take appropriate countermeasures, for example, by increasing or decreasing the output voltage. Regulating to a target output voltage allows for stable output voltage in varying operating conditions. Regulating to a target output power can prevent overload.

[0033] In one embodiment, the cross-section of the secondary winding or the primary coil is egg-shaped or elliptical, perpendicular to a coil axis, to accommodate the second core. The cross-section of the winding in which the second core is not located can be shaped in the usual way. An elliptical cross-section allows for the accommodation of a first and second core with similar dimensions. An egg-shaped cross-section allows for the accommodation of a larger first core and a second core with a smaller cross-section than the first. The second core can be positioned at the apex of the "egg."

[0034] According to the invention, a pickup winding is additionally arranged around the first core such that a magnetic flux generated by the primary winding induces a pickup voltage in the pickup winding, wherein the pickup winding is connected to the control unit and wherein the control unit is configured to generate the control current from the pickup voltage. In this way, the control winding can be supplied from the primary side, thus eliminating the need for an additional power supply. Furthermore, this design simplifies maintaining a defined phase angle of the control current relative to the current in the primary winding.

[0035] In one embodiment, the control unit for generating the control current comprises a current adjustment circuit, wherein the current adjustment circuit is configured for stepless or quasi-stepless adjustment of the control current and preferably includes a potentiometer or an inverter with a DC link. The current adjustment circuit can include a current regulator. In this context, an adjustable voltage divider is considered the potentiometer. This allows for a particularly simple implementation of a current adjustment circuit. The use of an inverter with a DC link enables particularly flexible adjustment of the control current.

[0036] In one embodiment, the control winding for reducing the control current by a factor of x comprises x times the number of turns. Here, x is a real number, preferably a natural number, with a value of at least 2. The number of turns should not be too large, as this would significantly increase the winding complexity. Therefore, in one embodiment, x is less than or equal to 100, in another embodiment less than or equal to 50, and in yet another embodiment less than or equal to 25. The number of turns in the respective main winding (i.e., the winding through which the associated second core is routed) can serve as the basis for this x-fold increase in the number of turns. For example, if the second core, containing the control winding to be dimensioned, is routed through the secondary winding and the output voltage is to be changed by up to 10%, the base number of turns in the control winding can be dimensioned to be 10% of the number of turns in the secondary winding.In this configuration, the number of turns of the control winding would be set to x times the base number of turns.

[0037] In one embodiment, the transformer assembly is designed for connection to a single-phase or multi-phase power supply, wherein the transformer assembly comprises a primary winding, a secondary winding, a control winding, and preferably a second core for each phase. The use of a single-phase system is particularly suitable for lower transmission power levels. Multi-phase systems, especially a three-phase AC system, can transmit significantly higher power levels. In particular, the three-phase configuration allows the transformer assembly to be used in conventional power supply networks. With a multi-phase power supply, a separate single-phase transformer assembly can be used for each phase. Specifically, with a three-phase power supply, a common first core can be used for all three primary windings and all three secondary windings.

[0038] In a further development, the control unit, when designed for connection to a multi-phase power supply, is configured for identical control of each control winding or for individual control of each phase. Identical control of the individual control windings ensures that the transformer arrangement allows for reliable adjustment of the output voltages even with unbalanced loads across the individual phases. "Identical control" means that the control currents through the individual control windings are fundamentally identical, but shifted relative to each other by a network-specific phase angle. This phase angle can be, for example, 360° divided by the number of phases, such as 120° in a three-phase network. Individual control of each phase allows for the implementation of different control objectives for each phase.

[0039] In one embodiment, the transformer assembly comprises a transformer housing in which the primary winding, the secondary winding, the first core, the second core, and the control winding are arranged. A transformer housing provides good protection against contact with live parts. Furthermore, the transformer can be cooled in this way, for example, by an oil filling. Connections can be provided on the transformer housing, in particular a voltage input, a voltage output, and a control input. If the transformer assembly has a pickup winding, this can also be arranged in the transformer housing. The control unit can also be arranged in the transformer housing or in an additional housing, which can be attached to the outside of the transformer housing.

[0040] In one embodiment, the control winding and the control unit are dimensioned such that an increase and / or reduction of the output voltage of up to 50% of the nominal output voltage, preferably up to 25%, and particularly preferably up to 10%, is possible. The nominal output voltage is defined here as the output voltage that the transformer assembly outputs without a control current through the control winding. It is assumed that a nominal input voltage is applied to the transformer assembly. A reduction or increase of the output voltage by 50% allows for a large adjustment range. A reduction or increase of the output voltage by 25% allows for a relatively large output voltage range with lower requirements for the second core, the control winding, and the control unit. A reduction or increase of the output voltage by 10% allows for useful adaptability in practical power supply networks.At the same time, the additional effort for the second core, the control winding and the control unit is manageable.

[0041] In one embodiment, the transformer arrangement is designed to transmit a power of at least one kilovolt-ampere, preferably at least 30 kilovolt-amperes, more preferably at least 150 kilovolt-amperes, particularly preferably at least 3 megavolt-amperes, and most preferably at least 500 megavolt-amperes. In principle, the transformer arrangement can be used for a wide variety of power requirements. With a power rating of at least 30 kilovolt-amperes, the transformer arrangement is well-suited for use as an industrial transformer or in comparable applications. With a power rating of at least 150 kilovolt-amperes, smaller transformer substations in a power supply network can be equipped. With a power rating of at least 3 megavolt-amperes, larger transformer substations can also be equipped. A power rating of at least 500 megavolt-amperes is also suitable for machine transformers.

[0042] In one embodiment, the transformer assembly is designed for connection on the input side to a low-voltage network, a medium-voltage network, or a high-voltage network. This allows the transformer assembly to be used at various points within a power supply network.

[0043] Further optional features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention, which are not to be understood as limiting.

[0044] According to a first embodiment of a transformer arrangement according to the present invention, a single-phase transformer arrangement comprises a primary winding, a secondary winding, and a first core, wherein the first core extends through the primary winding and the secondary winding. The primary winding is connected to a voltage input configured for inputting an input voltage Uin. The secondary winding is connected to a voltage output configured for outputting an output voltage Uout.

[0045] A second core is routed through the secondary coil. A control winding is arranged around this second core and can be driven by a control unit with a control current. The control unit is powered by a pickup winding, which is arranged around the first core and thus also carries a magnetic flux generated by the primary coil. The control unit is designed as a controller. For this purpose, the control unit includes a controller input into which, in particular, a setpoint, but also measured values ​​from the transformer assembly, can be input.

[0046] An air gap may be formed between the first core and the second core. Furthermore, in the area of ​​the secondary coil, parts of the first core and parts of the second core are arranged parallel to each other.

[0047] The transformer assembly further includes a transformer housing. The primary winding, the secondary winding, the first core, the second core, the control winding, and the pickup winding are arranged within the transformer housing.

[0048] During operation of this transformer arrangement, a current flowing through the primary winding generates a magnetic flux that is directed through the first core to the secondary winding and the pickup winding. This induces a pickup voltage in the pickup winding, which supplies the control unit. The control unit generates a control current from the pickup voltage, which is fed into the control winding. This, in turn, generates a magnetic flux in the second core of the control winding, which is also directed to the secondary winding. Thus, the secondary winding is traversed by a magnetic flux generated by the primary winding and by a magnetic flux generated by the control winding, with the two fluxes practically not influencing each other due to the air gap. The sum of these magnetic fluxes then induces a voltage in the secondary coil, which is output as the output voltage Uout to the voltage output.Depending on whether the magnetic fluxes are aligned (i.e., generally have the same sign in equal time units) or opposite (i.e., generally have different signs in equal time units), the output voltage U out can be raised or lowered by the control winding or the control current flowing through it.

[0049] The input voltage Uin is typically a sinusoidal alternating voltage. Accordingly, the output voltage Uout is also a sinusoidal alternating voltage. It is obvious that the control current should then also have at least an approximately sinusoidal waveform. According to a second embodiment of a transformer arrangement according to the present invention, the transformer arrangement is three-phase. A first core is three-armed, with a primary winding and a secondary winding arranged around each arm. A second core is routed through each secondary winding. A control winding is arranged around each of the second cores. Furthermore, a pickup winding, as specified in claim 1, is provided.

[0050] In this transformer arrangement, the second cores can be arranged parallel to the first core, with an air gap between each of the first and second cores. Furthermore, the secondary windings can be egg-shaped.

[0051] Regarding further advantageous embodiments, reference is made to the general part of the description and to the attached claims to avoid repetition.

[0052] Finally, it should be expressly pointed out that the exemplary embodiments described above serve only to illustrate the claimed teaching, but do not limit it to these exemplary embodiments.

Claims

1. A transformer arrangement having an adjustable output voltage, comprising a primary winding which is designed for inputting an AC voltage, a secondary winding which is designed for outputting an output voltage to a load, a first core which is guided through the primary winding and the secondary winding and guides a magnetic flux generated by the primary winding to the secondary winding, a control winding, a second core and an activation unit, wherein the second core is guided through the secondary winding or the primary winding, wherein the second core is additionally guided through the control winding, wherein the activation unit is connected to the control winding and wherein the activation unit is designed in such a manner for inputting a control current into the control winding that the magnetic flux that is effective at the secondary winding and thus the output voltage is increased or reduced characterized in that a pick-up winding is additionally arranged around the first core in such a manner that a magnetic flux generated by the primary winding induces a pick-up voltage in the pick-up winding, and in that the pick-up winding is connected to the activation unit, wherein the activation unit is designed to generate the control current from the pick-up voltage.

2. The transformer arrangement according to Claim 1, characterized in that an air gap is arranged between the first core and the second core.

3. The transformer arrangement according to Claim 1 or 2, characterized in that the activation unit is designed as a regulator, wherein the regulator regulates a secondary-side output voltage to a set-point output voltage or a secondary-side output power to a set-point output power.

4. The transformer arrangement according to one of Claims 1 to 3, characterized in that, to accommodate the second core, the cross section of the secondary winding or the primary coil is designed to be ovate or elliptic perpendicular to a coil axis.

5. The transformer arrangement according to one of Claims 1 to 4, characterized in that the activation unit comprises a current adjustment circuit for generating the control current, wherein the current adjustment circuit is designed for infinitely variable or virtually infinitely variable adjustment of the control current and preferably comprises a potentiometer or a power inverter having a constant-voltage / constant-current DC link.

6. The transformer arrangement according to one of Claims 1 to 5, designed for connection to a single-phase or multiphase power supply, wherein the transformer arrangement comprises a primary winding, a secondary winding, a control winding and preferably a second core for each phase.

7. The transformer arrangement according to Claim 6, characterized in that, in one embodiment for connection to a multiphase power supply, the activation unit is designed for an identical activation of the individual control windings or for individual regulation of the individual phases.

8. The transformer arrangement according to one of Claims 1 to 7, characterized by a transformer housing in which the primary winding, the secondary winding, the first core, the second core and the control winding are arranged.

9. The transformer arrangement according to one of Claims 1 to 8, characterized in that the control winding and the activation unit are dimensioned in such a manner that an increase and / or reduction of the output voltage of up to 50 % of the nominal output voltage, preferably of up to 25 %, particularly preferably of up to 10 %, is possible.

10. The transformer arrangement according to one of Claims 1 to 9, designed for transmitting a power of at least one kilovolt-ampere, preferably at least 30 kilovolt-amperes, further preferably at least 150 kilovolt-amperes, particularly preferably at least 3 megavolt-amperes, very particularly preferably at least 500 megavolt-amperes.

11. The transformer arrangement according to one of Claims 1 to 10, designed for input-side connection to a low-voltage network, a medium-voltage network or a high-voltage network.