HK40017832A - Efficient electric power conversion - Google Patents

Description

High efficiency electrical power conversion

Technical Field

The present invention relates to a power transfer unit for controlling the flow of electrical energy between two AC power units the present invention also relates to a power conversion unit for converting electrical power, said power conversion unit comprising a power transfer unit according to the present invention, an th AC power unit and a second AC power unit the present invention relates to a method for controlling the flow of electrical energy by using a power conversion unit according to the present invention, step .

Background

The power converter is typically implemented by using two AC-DC converters connected via a transformer, the transformer providing galvanic isolation and voltage level conversion to operate the power converter, the two AC-DC converters being operated at the same frequency the power flow is controlled by adjusting the phase and duty cycle of the voltage waveform the disadvantage of this method is that the phase shift necessary to establish the power flow generates significant reactive power flow which causes significant losses in the converter, the other disadvantage is that the power switches of the converter are switched at peak power (i.e. peak voltage and/or peak current), which generates significant switching losses.

For example, US5,027,264 (dedonacker et al) discloses a power conversion device for DC-DC conversion using dual active bridges with a transformer in between, controlling the active bridges to generate quasi-square wave voltages that are phase shifted with respect to each other to allow current to flow from bridges to another bridges, as long as certain conditions for current are met, the leakage inductance of the transformer and the buffer capacitance of the bridge switches form a resonant circuit for soft switching, otherwise, natural commutation (commutation) of the bridge devices will occur, causing switching losses, another disadvantages are that in order to increase the transmitted power of the converter, the phase shift between the voltages must be increased, and thus the phase shift between the voltages and currents must also be increased.

In order to reduce the switching losses of the electrical power converter, a resonant topology is proposed, in which capacitors are connected between in the AC-DC converter and the transformer.

To overcome these drawbacks, US 2015/0138841 (pahlevenneizhad et al) proposes a high efficiency DC-DC converter with a full bridge of current drive followed by a transformer and a diode rectifier.

Disclosure of Invention

It is an object of the present invention to provide a power transfer unit related to the initially mentioned technical field, whereby the drawbacks of the prior art are overcome or at least partly overcome, and it is an object of the present invention to provide a power transfer unit, which allows to increase the power transfer efficiency over its entire operating range, hi addition, it is an object of the present invention to provide a power conversion unit related to the initially mentioned technical field, whereby the drawbacks of the prior art are overcome or at least partly overcome, and it is an object of the present invention to provide a method for controlling the flow of electrical power by using a power conversion unit according to the present invention, whereby the drawbacks of the prior art are overcome or at least partly overcome.

According to the invention, a power transfer unit for controlling a flow of electrical energy between two AC power units comprises a main transformer having an th winding and a second winding, and a switchable auxiliary AC unit for applying a tuneable auxiliary AC voltage across an auxiliary AC side of the auxiliary AC unit, wherein the auxiliary AC side is connected in series with the th winding of the main transformer to form a series connection.

In the case of a power transfer unit according to the present invention, when connected to the power transfer unit, any desired power flow may be established between the two AC power units, preferably the and second windings of the main transformer are separated by galvanic isolation, thus the power transfer unit preferably provides galvanic isolation between the two power units due to its main transformer.

Preferably, the AC side of the AC power unit supplies or is capable of receiving a square wave AC voltage, said AC voltage preferably being between 200V and 4KV, and particularly preferably between 500V and 1KV, the power of the AC power unit is preferably between 200W and 3MW, and particularly preferably between 20KW and 400KW, the AC current of the AC power unit is preferably between 2A and 1000A, and particularly preferably between 10A and 100A, however, any other voltage, power and AC current is also possible with respect to the rating of the above mentioned AC power unit, the rating of the AC power unit is preferably increased with respect to the rating of the AC power unit, the rating of the AC power unit is increased with respect to the rating of the AC power module, or the rating of the AC power module is increased with respect to the rating of the AC power module, if the AC power unit is connected in parallel, or with more than the rating of the AC power module, or the rating of the AC power module is increased with respect to the rating of the AC power module, or the AC power module.

The main transformer may have a single phase or three phases. Such a main transformer is very common and advantageous. However, other numbers of phases are also possible, such as two phases or even more than three phases.

Preferably, the main transformer is an intermediate frequency transformer. Intermediate frequency means a much larger frequency compared to the 50Hz or 60Hz common line frequency. The intermediate frequency is preferably between 1KHz and 1MHz, and particularly preferably between 10KHz and 100 KHz. Alternatively, the main transformer may be a different transformer than the intermediate frequency transformer, such as for example a low frequency transformer or a high frequency transformer.

Advantageously, the main transformer is selected to have as small a leakage inductivity as possible. The smaller the leakage inductance, the faster the current through the transformer can rise and fall. This allows for a higher switching frequency, which in turn allows for a smaller transformer and thus a higher power density. However, the main transformer may have any leakage inductance if higher switching frequencies are not of interest.

The higher the main inductance of the main transformer, the smaller the phase shift between the th current through the th winding and the second current through the second winding of the main transformer.

The auxiliary AC unit may be any switchable electrical unit for applying a tunable auxiliary AC voltage across an auxiliary AC side of the auxiliary AC sides. Thus, the auxiliary AC unit is able to provide and receive an AC voltage. Examples of auxiliary AC units may include, but are not limited to: AC-AC converters, DC-AC converters, bidirectional AC-AC converters, and bidirectional AC-DC converters.

The second auxiliary AC unit may have or more of the features described for the switchable auxiliary AC unit as described in more detail below.

The auxiliary AC unit is a portion of the power transfer unit and is therefore separate from the two AC power units between which the flow of electrical energy may be controlled by the power transfer unit.

The th winding of the main transformer preferably has fewer turns or the same number of turns as compared to the second winding of the main transformer.

For example, the AC side of the th of the two AC power cells may be connected with the series connection formed by the auxiliary AC side and the th winding of the main transformer, while the second AC side of the second of the two power cells may be connected to the second winding of the main transformer.

In a preferred embodiment, the auxiliary AC unit of the power transfer unit further comprises an energy storage.

Using an energy storage is a simple way of providing an AC voltage across the auxiliary AC side of the auxiliary AC unit.

The energy storage may be available in various forms, such as, for example, a capacitor or a battery. As an alternative or in addition to the energy storage, an auxiliary AC unit may be connected to the grid. It is advantageous if the auxiliary AC unit is intended to transfer more electrical power than is needed to control only the power flow between the two AC power units.

In case the power transfer unit comprises a second auxiliary AC unit, the second auxiliary AC unit of the power transfer unit preferably comprises a second energy storage.

Preferably the auxiliary AC unit of the power transfer unit further comprises an auxiliary DC side this has the advantage that an energy storage for providing a DC voltage can be connected to the auxiliary DC side.

Many different energy storages, such as capacitors, supercapacitors and batteries, for providing DC batteries are available on the market. As an alternative to them, it is possible to use an energy storage that provides not a DC voltage but, for example, an AC voltage.

The auxiliary AC unit may further have no connection to a power supply or grid that delivers electrical power to be transmitted over the power transmission unit.

In case the power transfer unit comprises a second auxiliary AC unit, the second auxiliary AC unit of the power transfer unit preferably comprises a second auxiliary DC side. Advantageously, the second auxiliary AC unit comprises a second energy storage for providing a DC voltage, the second energy storage being connected to the second auxiliary DC side. However, the second auxiliary AC side may not have the second energy storage.

Preferably, the energy storage is a capacitor.

The capacitor may be designed such that there is substantially zero voltage change across the capacitor during operation of the power transfer unit. Typically, such capacitors are much larger than capacitors designed for resonant operation. The capacitor may maintain a predefined AC voltage during operation of the power transfer unit. Alternatively, the capacitor may be designed for resonant operation. In resonant operation, the capacitor may be exposed to an AC voltage.

In case the power transfer unit comprises a second auxiliary AC unit with a second energy storage, the second energy storage is preferably a capacitor.

In a preferred embodiment, the auxiliary AC unit further includes a converter.

The converter allows for a flexible provision of voltages, in particular AC voltages. Such a converter is easy to control. In particular when connected with an energy storage, the converter may provide any desired AC voltage across the auxiliary AC side of the auxiliary AC unit therefrom.

Advantageously, the converter is switchable. Alternatively, a non-switchable converter may be used, such as for example a lenard (Leonard) converter, also known as a lenard drive or control system.

The switches may be semiconductor devices, such as transistors, preferred transistors are IGBTs, BJTs and FETs, in particular MOSFETs , more than , or all switches may be without capacitors connected in parallel therewith.

Although the switches of the converter of the auxiliary unit may be switched "hard", i.e. at full current and at full voltage, the switching losses caused thereby are relatively small, since the voltage of the auxiliary AC unit, and thus the auxiliary AC voltage, is advantageously chosen to be significantly smaller compared to the voltage of the AC power unit.

Advantageously, the auxiliary AC unit may comprise an active half-bridge in parallel with the capacitor half-bridge. This topology has the following advantages: they require fewer semiconductor switches and can therefore be constructed more simply. However, their operating range is limited.

Where the power transfer unit comprises a second auxiliary AC unit, the second auxiliary AC unit of the power transfer unit preferably further comprises a second converter, the second converter preferably providing or more of the above mentioned features of the possible converters of the auxiliary units.

In a preferred embodiment of step , the auxiliary AC unit further comprises an auxiliary transformer.

The auxiliary transformer has the advantage that it enables an improved performance of the power transfer unit for applications.

The auxiliary transformer preferably has an th auxiliary winding and a second auxiliary winding the th auxiliary winding of the auxiliary transformer advantageously forms the AC side of the auxiliary AC unit.

Preferably, the auxiliary transformer is an intermediate frequency transformer, such as explained above with respect to the main transformer. However, the auxiliary transformer may also be any other transformer.

Further the auxiliary transformer is advantageously configured differently from the main transformer, preferably the th auxiliary winding of the auxiliary transformer has a much smaller voltage rating than the th winding of the main transformer.

In case the power transfer unit comprises a second auxiliary AC unit, in a preferred variant the second auxiliary AC unit of the power transfer unit comprises a second auxiliary transformer, wherein a second auxiliary winding of said second auxiliary transformer forms a second AC side of the second auxiliary AC unit, preferably a second auxiliary winding of the second auxiliary transformer has a smaller voltage rating than the second winding of the main transformer, i.e. the second winding of the main transformer is preferably adapted to handle a higher voltage than the second auxiliary winding of the second auxiliary transformer, in another preferred variants wherein the power transfer unit comprises a second auxiliary AC unit, the second auxiliary AC unit is free of such second auxiliary transformer.

Advantageously, the auxiliary AC unit is adapted to have a substantially zero energy balance.

In this case, the auxiliary AC units preferably do not provide continuous power flow, but only the auxiliary AC voltage needed to control the power flow between the AC power units.

The auxiliary AC unit advantageously acts as a provider of electrical power during the th part of the half-wave of the auxiliary AC voltage and as a receiver of electrical power during the second part of the half-wave of the auxiliary AC voltage accordingly, the th part of the half-wave of the auxiliary AC voltage is preferably different from the second part of the half-wave of the auxiliary AC voltage.

The term "substantially zero" means that the energy balance of the auxiliary AC unit is exactly zero when an ideal lossless auxiliary AC unit is used. However, real auxiliary AC units may have parasitic losses, such as ohmic losses. Thus, the energy balance of a real auxiliary AC unit is not exactly zero, but substantially zero. Preferably, the parasitic power is at least two orders of magnitude, at least three orders of magnitude or even at least four orders of magnitude smaller than the power transmitted by the power transmitting unit. Thus, the parasitic power is substantially zero compared to the transmitted power. This has the following advantages: the parasitic power only has to be supplemented to the auxiliary AC unit or, for example, to the energy storage of the auxiliary AC unit.

The peak auxiliary AC voltage of the auxiliary AC unit is preferably much smaller compared to the peak AC voltage of the th AC power unit the peak auxiliary AC voltage of the auxiliary AC unit is preferably between 1% and 20%, particularly preferably between 3% and 15%, and most preferably between 5% and 10% of the peak AC voltage of the th AC power unit however, the peak auxiliary AC voltage of the auxiliary AC unit may be 10% or more of the th AC voltage of the th AC power unit.

The frequency of the auxiliary AC voltage of the auxiliary AC unit may be equal to the frequency of the th AC voltage of the th AC power unit (and equal to the second AC voltage of the second AC power unit) — advantageously, the current flowing through the auxiliary AC unit during the th part of a half-wave of the auxiliary AC voltage is symmetrical to the current flowing through the auxiliary AC unit during the second part of the half-wave of the auxiliary AC voltage, with the axis of symmetry being in the middle of said half-wave.

In case the power transfer unit comprises a second auxiliary AC unit, the second auxiliary AC unit of the power transfer unit is preferably adapted to have a substantially zero energy balance. However, the second auxiliary AC unit may also be adapted to have a different energy balance.

In a further step preferred embodiment, the auxiliary AC unit further step includes a control unit for switching the auxiliary AC unit for applying a tunable auxiliary AC voltage across the auxiliary AC side of the auxiliary AC unit.

By using a control unit, any power flow between AC power units may be flexibly and/or automatically achieved.

In particular, the control unit may be adapted to switch the auxiliary AC unit to ensure that the energy balance of the auxiliary AC unit is substantially zero. The control unit may comprise, for example, a microcontroller.

In case the power transfer unit comprises a second auxiliary AC unit, the control unit may be further adapted to switch the second auxiliary AC unit of the power transfer unit for switching the second auxiliary AC unit for applying the tunable second auxiliary AC voltage across the second auxiliary AC side of the second auxiliary AC unit, or the second auxiliary AC unit may comprise a second control unit for switching the second auxiliary AC unit for applying the tunable second auxiliary AC voltage across the second auxiliary AC side of the second auxiliary AC unit.

According to another aspect of the invention, the power conversion unit for converting electrical power comprises a power transfer unit according to the invention the power conversion unit further step comprises a AC power unit connected to the series connection of power transfer units and a second AC power unit connected to the second winding of the main transformer.

The present power conversion unit enables to establish any desired power flow between the 84 th AC power unit and the second AC power unit while providing galvanic isolation between them, in particular the present power conversion unit enables to establish any DC-DC conversion, or AC-AC conversion, or DC-AC conversion, or AC-DC conversion 35in a very efficient way, additionally the advantages mentioned in the context of the power transfer unit apply also to the power conversion unit.

The power rating of the AC power unit, in this case, includes more than modular converters, for which the total voltage, current, and/or power rating of the AC power unit may be a multiple of the rating of the aforementioned modules.

Where the power conversion unit comprises a second auxiliary AC unit having a second auxiliary AC side, the second auxiliary AC side is preferably connected in series with the second winding of the main transformer to form a second series connection. The second series connection is preferably connected with a second AC power unit. However, the second auxiliary AC unit may also be connected differently.

In a preferred embodiment, the th AC power cell includes a converter having a th AC side connected with the series connection.

A power conversion unit with such a bidirectional DC-AC converter can convert a DC voltage to any other voltage with excellent efficiency, and vice versa, because the bidirectional DC-AC converter allows energy flow in both directions through the power transfer unit and through the AC power unit.

The bidirectional DC-AC converter may for example be a full bridge converter. However, as an alternative thereto, a unidirectional DC-AC converter may be employed.

In a preferred embodiment, the second AC power unit comprises a converter having a second AC side connected with a second winding of the main transformer.

With respect to the converter of the second AC power unit, the same applies as described above with respect to the converter of the th AC power unit.

In case the power transfer unit comprises a second auxiliary AC unit having a second auxiliary AC side, the second auxiliary AC side is preferably connected in series with the second winding of the main transformer to form a second series connection, and the second AC power unit is advantageously a DC-AC converter, preferably a bidirectional DC-AC converter, wherein the AC side thereof is preferably connected with the second series connection.

Advantageously, the power conversion unit further includes a control unit for controlling the auxiliary AC unit and/or the th AC power unit and/or the second AC power unit.

The control unit may advantageously be adapted to control or switch the power transfer unit or the auxiliary AC unit for controlling the flow of electrical energy between the th AC power unit and the second AC power unit such that a desired flow of energy passes through the power conversion unit.

The efficiency of the power conversion may also depend on the actual AC voltage of the st AC power unit and the second AC power unit if the control unit is not adapted to control the th AC power unit and/or the second AC power unit therefore the best results are achieved if the control unit is adapted to control the auxiliary AC unit, the th AC power unit and the second AC power unit, wherein the control unit is preferably adapted to switch the auxiliary AC unit.

The control unit may comprise, for example, a microcontroller and control software.

The control unit may be part of the power transfer unit as mentioned above.

In case the power transfer unit comprises a second auxiliary AC unit, the control unit may be further adapted to control or switch the second auxiliary AC unit of the power transfer unit.

In a preferred embodiment, the control unit is adapted for zero current switching of the th AC power unit and/or the second AC power unit.

The th AC power cell and/or the zero current switching of the second AC power cell allows for a reduction in switching losses of the th AC power cell and/or the power switch of the second AC power cell the reduction in switching losses results in an even further step increase in the efficiency of the power conversion cell.

The term "zero current" switching preferably comprises switching the current when the current is less than 20% of its peak value, particularly preferably when the current is less than 10% of its peak value and most preferably when the current is less than 5% of its peak value.

Preferably, the control unit is adapted to control the auxiliary AC unit such that the current through the th power unit is in phase with the th AC voltage of the th AC power unit and/or such that the current through the second AC power unit is in phase with the second AC voltage of the second AC power unit.

Thereby, the control unit implements a good power factor, which results in a further step increase in the efficiency of the power conversion unit.

If the auxiliary AC unit comprises a power switch, the control unit is advantageously adapted to control the power switch such that the current through the th AC power unit is in phase with the voltage of the th AC power unit and/or such that the current through the second AC power unit is in phase with the second AC voltage of the second AC power unit.

In case the power transfer unit comprises a second auxiliary AC unit, the control unit is preferably adapted to additionally control the second auxiliary AC unit such that the current through the th AC power unit is in phase with the th AC voltage of the th AC power unit and/or such that the current through the second AC power unit is in phase with the second AC voltage of the second AC power unit.

In a preferred embodiment, the control unit is adapted to switch the auxiliary AC unit such that a predetermined magnitude of the current through the th AC power unit is achieved, and/or such that a predetermined magnitude of the current through the second AC power unit is achieved.

Thereby, the control unit allows automation of the magnitude of the current through the power conversion unit.

According to an aspect of the invention of step , a method for controlling a flow of electrical energy by using a power conversion unit according to the invention comprises step a and step B step a comprises providing an auxiliary AC voltage across an auxiliary AC side of an auxiliary AC unit of the power conversion unit for shaping a th current through a th AC power unit of the power conversion unit and/or for shaping a second current through a second AC power unit of the power conversion unit, whereby the auxiliary AC voltage comprises pulses of different polarity during a half-wave of the auxiliary AC voltage step B comprises synchronizing a th AC voltage across a th AC side of the th AC power unit with the second AC voltage across the second AC side of the second AC power unit and/or synchronizing a th AC voltage across a th AC side of the th AC power unit with the auxiliary AC voltage.

By using the method according to the invention, any desired power flow can be established within the power conversion unit according to the invention. The power flow may be established at zero phase shift between the AC voltages of the AC power cells. The AC voltage may be in phase, or at least nearly in phase, with the current through the AC power unit. This results in a good power factor and a minimum reactive power within the power conversion unit. Therefore, the power conversion unit can be efficiently operated.

For example, if the th half-wave of the auxiliary AC voltage starts with a pulse having a positive voltage, followed by a pulse having a negative voltage, the second half-wave of the auxiliary AC voltage preferably starts with a pulse having a negative voltage, followed by a pulse having a positive voltage.

Other shapes than the rectangular shape of the pulses of the auxiliary AC voltage are also possible, such as for example a sawtooth shape, a triangular shape, or a sinusoidal shape. Such shapes may be advantageous in particular applications and particular situations. However, the rectangular shape of the pulse of the auxiliary AC voltage has the following advantages: they are provided with little effort.

The term "synchronizing the AC voltage X with the AC voltage Y" means herein ensuring that the AC voltage X has the same frequency as the AC voltage Y and the AC voltage X has the same phase as the AC voltage Y.

In case the auxiliary AC unit comprises an auxiliary converter and an energy storage, the method according to the invention may further comprise the initial step of charging the energy storage to reach a predetermined voltage of the energy storage.

In case the power transfer unit of the power conversion unit comprises a second auxiliary AC unit, step a preferably further step comprises providing a second auxiliary AC voltage across a second auxiliary AC side of the second auxiliary AC unit for shaping a th current through the th AC power unit and/or for shaping a second current through the second AC power unit, whereby the second auxiliary AC voltage preferably comprises pulses of different polarity during a half-wave of the second auxiliary AC voltage step B preferably further step comprises synchronizing a th AC voltage across a th AC side of the th AC power unit with the second auxiliary AC voltage.

Advantageously, generating the pulse of the auxiliary AC voltage includes a step A1 of switching the converter of the auxiliary AC unit such that the auxiliary AC unit has a th polarity, and a step A2 of switching the converter of the auxiliary AC unit such that the auxiliary AC voltage has a second polarity opposite to the th polarity further step synchronizing the th AC voltage across the th AC side of the th AC power unit with the second AC voltage across the second AC side of the second AC power unit preferably includes a step B1 of switching the converter of the th AC power unit such that the th AC voltage has a third polarity, and a step B2 of switching the converter of the th AC power unit such that the th AC voltage has a fourth polarity opposite to the third polarity.

If the th, second and/or auxiliary AC unit includes a converter, the method according to the invention can be easily implemented by switching the converter(s) in order to achieve the desired current through the AC power unit and thereby the desired power flow between the two AC power units.

The same applies to the third polarity and the fourth polarity, the fourth polarity is negative if the third polarity is positive, the fourth polarity is positive if the third polarity is negative.

For example, the AC voltage of the th AC power cell may have a square wave shape other shapes of the th AC voltage are possible, such as a sinusoidal shape although a sinusoidal shape has advantages in particular applications and particular situations, the th AC voltage having a square wave shape has the advantage that it may be provided with less effort.

Advantageously, step a1 is performed before step B1. Alternatively, step a1 may be performed simultaneously with step B1 or after step B1. Preferably, step a2 is performed before step B2. Alternatively, step a2 is performed concurrently with step B2 or after step B2.

In case the power transfer unit comprises a second auxiliary AC unit with a converter, generating the pulse of the second auxiliary AC voltage preferably comprises a step a11 of switching the converter of the auxiliary AC unit such that the second auxiliary AC voltage has a polarity and a step a21 of switching the converter of the second auxiliary AC unit such that the second auxiliary AC voltage has a second polarity opposite to the polarity .

Advantageously, the step of generating the pulse of the auxiliary AC voltage includes the step A3 of switching the converter of the auxiliary AC unit to provide a conductive path with zero voltage across the auxiliary AC side of the auxiliary AC unit more the step of stepping the AC voltage across the th AC side of the th AC power unit with the second AC voltage across the second AC side of the second AC power unit to preferably includes the step B3 of turning off all switches of the converter of the th AC power unit.

Performing step A3 allows maintaining a th current and/or maintaining a second current, thereby enabling even more different shapes for a th current and/or a second current, in particular, the waveform of the th current and/or the second current may be shaped, such as to achieve an optimal shape to minimize conduction losses.

If step B3 is performed when the th current is not zero, said current may flow through the anti-parallel diodes of the converter of the th AC power cell when all switches of the converter are off, if step B3 is performed when the th current is zero, the off-switches, in particular in the case of power semiconductor switches, may remain on until all charge is removed from said semiconductor switches.

During the th half-wave of the auxiliary AC voltage, step A3 is preferably performed after step a1 and before step a2 (i.e., between steps a1 and a2) during the second half-wave of the auxiliary AC voltage, step A3 is advantageously performed after step a2 and before step a1 (i.e., between step a2 and step a 1).

During the th half-wave of the auxiliary AC voltage, step B3 is preferably performed after step B1 and before step B2 (i.e. between steps B1 and B2) during the second half-wave of the auxiliary AC voltage, step B3 is preferably performed after step B2 and before step B1, however, steps a1, a2, A3, B1, B2 and B3 may also be performed in a different order.

In case the power transmission unit comprises a second auxiliary AC unit, step a3 preferably further comprises switching the converter of the second auxiliary AC unit to provide a conductive path with zero voltage across the second auxiliary AC side of the second auxiliary AC unit.

Preferably, stepping the th AC voltage across the th AC side of the th AC power cell with the second AC voltage across the second AC side of the second AC power cell includes step B4 switching the converter of the second AC power cell to provide the second AC voltage having the fifth polarity across the second AC side, and step B5 switching the converter of the second AC power cell to provide the second AC voltage having the sixth polarity across the second AC side opposite to the fifth polarity.

In this case, step B4 may initiate a rise or a fall of the second current through the second AC power unit depending on the polarity of the auxiliary AC voltage, while step B5 may initiate a fall or a rise of the second current depending on the polarity of the auxiliary AC voltage.

For example, the second AC voltage of the second AC power unit may have a square wave shape. Other shapes of the second AC voltage are also possible, for example sinusoidal. Even sinusoidal shapes may have advantages in particular applications and particular situations. However, the second AC voltage having a square wave shape may be supplied with less effort.

In an advantageous variant, step A1 is performed before step B4, further step A2 is advantageously performed before step B5, however, the order of step A1 and step B4, and the order of step A2 and step B5 may be different.

In a preferred embodiment, stepping the th AC voltage across the th AC side of the th AC power cell with the second AC voltage across the second AC side of the second AC power cell includes the step B6 of turning off all switches of the converter of the second AC power cell.

In case step B6 is performed, a second time period may be created during which the th current and/or the second current remains zero, whereby the switching losses may be reduced by a further step.

If step B6 is performed when the second current is not zero, the current may flow through the anti-parallel diodes of the converter of the second AC power cell when all switches of the converter are turned off. If step B6 is performed when the second current is zero, the switched-off switch, in particular in the case of a power semiconductor switch, may remain switched on until all charge is removed from the semiconductor switch.

During the th half-wave of the auxiliary AC voltage, step B6 is performed after step B4 and before step B5 (i.e., between step B4 and step B5.) during the second half-wave of the auxiliary AC voltage, step B6 may be performed after step B5 and before step B4 (i.e., between step B5 and step B4).

Advantageously, step B1 and/or step B2 and/or step B3 are performed when the th current is zero.

Therefore, performing step B1 and/or step B2 and/or step B3 when the th current is zero has the advantage that the efficiency of the power conversion unit, particularly the th AC power unit, can be improved.

Preferably, step B4 and/or step B5 and/or step B6 are performed when the th current is zero.

Preferably, step B1 and step B4 are performed simultaneously, and step B2 and step B5 are performed simultaneously. Step B3 and step B6 are also performed simultaneously.

By performing step B1 and step B4 simultaneously, and by performing step B2 and step B4 simultaneously, and by performing step B3 and step B6 simultaneously, the converter of the AC power unit and the converter of the second AC power unit operate synchronously to avoid losses caused by reactive power.

In a preferred variant, the second AC power unit is a diode rectifier. This has the following advantages: no active switching of the second AC power unit is required for synchronization, since synchronization occurs automatically.

Advantageously, the average value of the auxiliary AC voltage measured over a half-wave of the auxiliary AC voltage is zero.

This prevents the th current and/or the second current from having any DC component.

Preferably, the average value of the power flow through the auxiliary AC unit measured over a half-wave of the power flow through the auxiliary AC unit is substantially zero.

This has the advantage that the auxiliary AC units do not provide a continuous power flow, but only the auxiliary AC voltage needed to control the power flow between the AC power units, therefore, the auxiliary AC units can operate with minimal losses, therefore, the efficiency of the transmission unit is further improved . an explanation of this aspect of step has been given in the context of the discussion of the power transmission unit.

The method according to the invention and the power transmission unit and the power conversion unit according to the invention are particularly advantageous if used in data centers (in particular for providing DC power), in battery chargers, in railways and electric vehicles (also in particular for providing DC electrical power), in electrical networks as solid state transformers or for connecting AC electrical networks with DC electrical networks at all voltage levels, and in power conversion applications of renewable energy sources, such as wind energy plants and solar (photovoltaic) plants.

Other advantageous embodiments and combinations of features emerge from the detailed description below and from all the claims.

Drawings

The accompanying drawings, which are used to illustrate embodiments, illustrate:

fig. 1 is an th embodiment of a power transfer unit having an auxiliary AC unit in accordance with the present invention;

fig. 2A is an th embodiment of an auxiliary AC unit;

fig. 2B is a second embodiment of an auxiliary AC unit;

fig. 2C is a third embodiment of an auxiliary AC unit;

fig. 2D is a fourth embodiment of an auxiliary AC unit;

fig. 2E is a fifth embodiment of an auxiliary AC unit;

fig. 3 is a second embodiment of a power transfer unit according to the present invention;

fig. 4 is an th embodiment of a power conversion unit including a power transfer unit and two AC power units in accordance with the invention;

fig. 5A is an th embodiment of an AC power cell for use in a power conversion cell;

FIG. 5B is a second embodiment of an AC power unit for use in a power conversion unit;

FIG. 5C is a third embodiment of an AC power unit for use in a power conversion unit;

FIG. 6 is a waveform of voltage, current and power of a power conversion unit;

fig. 7 is a second embodiment of a power conversion unit comprising a power transfer unit and two AC power units according to the present invention;

fig. 8 is a third embodiment of a power conversion unit comprising a power transfer unit and two AC power units according to the present invention;

fig. 9 is a fourth embodiment of a power conversion unit comprising a power transfer unit and two AC power units according to the present invention;

fig. 10 is a fifth embodiment of a power conversion unit comprising a power transfer unit and two AC power units according to the present invention;

fig. 11 is a sixth embodiment of a power conversion unit comprising a power transfer unit and two AC power units according to the present invention; and

fig. 12 is a flow chart of a method according to the invention.

In the figures, like components are given like reference numerals.

Detailed Description

Fig. 1 shows an th embodiment of a power transfer unit 1.1 according to the invention the power transfer unit 1.1 comprises a main transformer 2 with a th winding 3 and a second winding 4 the power transfer unit 1.1 further comprises a switchable auxiliary AC unit 5 with an auxiliary AC side 6 the auxiliary AC unit 5 provides a tunable auxiliary AC voltage 7 across the auxiliary AC side 6 of the auxiliary AC unit 5 is connected in series with the th winding 3 of the main transformer 2 to form a series connection 8 the series connection 8 of the power transfer unit 1 may be connected to an AC power unit (not shown) the series connection 8 should not be short-circuited for normal operation of the power transfer unit 1.1.

Fig. 2A shows a th possible embodiment of a switchable auxiliary AC unit 5.1 in the present case the auxiliary AC unit 5.1 comprises a converter 9, which is a DC AC converter the auxiliary AC unit 5.1 has an auxiliary DC side 10 in addition to the auxiliary AC side 6.

Fig. 2B shows a second possible embodiment of a switchable auxiliary AC unit 5.2 comprising a full bridge converter 9.1. The auxiliary AC unit 5.2 likewise has an auxiliary AC side 6 and an auxiliary DC side 10. The full-bridge converter 9.1 has four Insulated Gate Bipolar Transistors (IGBTs) as switching devices with anti-parallel diodes.

Fig. 2C shows a third possible embodiment of a switchable auxiliary AC unit 5.3, the switchable auxiliary AC unit 5.3 having an auxiliary AC side 6, an auxiliary DC side 10 and a full bridge converter 9.2, the full bridge converter 9.2 having four Field Effect Transistors (FETs), in particular four metal oxide semiconductor FETs (mosfets) with integrated anti-parallel diodes, as switching devices.

Fig. 2D shows a fourth possible embodiment of a switchable auxiliary AC unit 5.4 comprising a converter 9.3, said converter 9.3 comprising a capacitive half-bridge with two capacitors and an active half-bridge with two IGBTs each with an anti-parallel diode.

Fig. 2E shows a fifth possible embodiment of a switchable auxiliary AC unit 5.5. The auxiliary AC unit 5.5 comprises a converter 9.4 with a capacitive half bridge and an active half bridge with six cascaded IGBTs. Each IGBT has an anti-parallel diode.

The auxiliary AC units 5.1-5.5 shown in fig. 2A-E may comprise an energy storage such as, for example, a capacitor or a battery. The energy storage may for example be connected to the auxiliary DC side of the respective auxiliary AC unit 5.1-5.5. However, the auxiliary AC units 5.1-5.5 may not comprise such an energy storage. For example, they may be connected to some energy supply with their auxiliary DC side.

Each of the embodiments of the auxiliary AC units 5.1-5.5 shown in fig. 2A-E may be used within the power transfer unit 1.1 as shown in fig. 1 or within the power transfer unit 1.2 as shown in fig. 3 to form an embodiment without the further step of the power transfer unit explicitly shown here.

Fig. 3 shows a second possible embodiment of a power transfer unit 1.2 according to the invention in which the auxiliary AC unit 5.2, 5.3 comprises a full bridge converter 9.1, 9.2 with four switches each having an anti-parallel diode as shown in fig. 2B and 2C the switches of such a full bridge converter may be IGBTs, FETs or mosfets the auxiliary DC side 10 of the auxiliary AC unit 5.2, 5.3 is connected to an energy storage 12 the energy storage 12 provides a DC voltage across the auxiliary DC side 10 the DC voltage is indicated by a polarity symbol adjacent to the energy storage 12 in this case the energy storage 12 is a capacitor the AC side of the full bridge converter 9.1, 9.2 is connected to an auxiliary transformer 11, said auxiliary transformer 11 is connected in series with the th winding 3 of the main transformer 2 to form a series connection 8 the auxiliary AC side 6 providing the auxiliary AC voltage 7 is formed by the windings of the auxiliary transformer 11.

Although shown in this second embodiment of the power transfer unit 1.2 as , the auxiliary AC unit 5.2, 5.3 does not require the inclusion of an auxiliary transformer 11 and an energy storage 12, therefore the auxiliary transformer 11 connecting the full bridge converter 9.1, 9.2 in series with the th winding 3 of the main transformer 2 may be omitted, similarly the energy storage 12 and/or the full bridge converter 9.1 may be omitted.

Fig. 4 shows a possible embodiment of a power conversion unit 20.1 comprising a power transfer unit 1.3 according to the invention, the power conversion unit 20.1 comprises a AC power unit 21, said 0AC power unit 21 provides a second current 22 and a AC voltage 23 across a AC side 24 of a second AC power unit 21, in this embodiment of the power conversion unit 20.1 a DC AC converter fed by a main voltage 25, a main voltage is a DC voltage, in this embodiment of the power conversion unit 20.7 AC power unit 21 a AC side 24 is connected to a power transfer unit 1.3 according to the invention, instead of the power transfer unit 1.3, any other power transfer unit according to the invention may be employed, for example, the power transfer unit 1.1 or the power transfer unit 1.2 shown in fig. 1 and fig. 3, respectively, or the power transfer unit 1.2, may be employed, the power transfer unit 1 or 363, the power transfer unit may be used for the purpose of energy transfer through a second AC voltage or energy transfer by a first main voltage or by a main voltage, the first switch may be used for the purpose of energy transfer between the first main voltage, the first 367 AC voltage transfer unit 72, the first 367 AC voltage, the first 363 and the main voltage may be used for the energy transfer unit 14, the energy transfer unit 14 is not be used for the purpose of the first polarity of the first main voltage transfer unit 21, the first polarity of the first main voltage, the first main voltage transfer unit 21, the first main voltage, the energy transfer unit 21 is used for the energy transfer unit 21, the energy transfer unit 3, the energy transfer unit 2 voltage, the energy transfer unit 3, the energy transfer unit 2 voltage is used for the energy transfer unit 21, the energy transfer unit 3, the energy transfer unit 2 voltage is used for the energy transfer unit 2 voltage, the energy transfer unit 3, the energy transfer unit 2 power transfer unit 3, the energy transfer unit is used for the energy transfer unit 21 is used for the energy transfer unit 3, the energy transfer unit 2 power transfer unit 3, the energy transfer unit 2 power transfer unit is used for the energy transfer unit 3, the energy transfer unit is used for the energy transfer unit 3 in the energy transfer unit 3 in the first power transfer unit 3 in the energy transfer unit 3, the first power transfer unit 3 in the energy transfer unit 3 in the first main voltage, the first power transfer unit 3 in the first main voltage, the first power transfer unit 3, the energy transfer unit 3, the first main voltage, the energy transfer unit 3.

Fig. 5A shows a th possible embodiment of the th AC power cell 21.1 and the second AC power cell 26.1, each comprising a full bridge converter with four Insulated Gate Bipolar Transistors (IGBTs) as switching devices with anti-parallel diodes.

Fig. 5B shows a second possible embodiment of th AC power cell 21.2 and second AC power cell 26.2, each comprising a converter with a capacitive half-bridge and an active half-bridge the capacitive half-bridge comprises two capacitors and the active half-bridge has two IGBTs, each IGBT with an anti-parallel diode.

Fig. 5C shows a third possible embodiment of th AC power cell 21 and second AC power cell 26, each comprising a converter with a capacitive half-bridge and an active half-bridge with cascaded IGBTs, i.e. six IGBTs, each with an anti-parallel diode.

Any of the in the embodiment of the th AC power cells 21.1-21.3 and any of the in the embodiment of the second AC power cells 26.1-26.3, or any combination thereof, may be used within the power conversion cell 21.1 as shown in fig. 4.

, as shown here for a single phase system, the topology of the auxiliary AC unit 5 and the topology of the AC power unit 21 and the second AC power unit 26 may be the same however, the power ratings may be very different, i.e. the power ratings of the AC power unit 21 and the second AC power unit 26 may be much higher than the power ratings of the auxiliary AC unit 5.

Fig. 6 shows waveforms of voltage, current and power when operating the power conversion unit 20.1 as shown in fig. 4 for simplicity of explanation it is assumed that the th winding 3 of the main transformer 2 and the second winding 4 of the main transformer 2 have the same number of turns, i.e. the ratio of the main transformer is 1, thus the th AC voltage 23 and the second AC voltage 28 have the same shape (uppermost in fig. 6 and thus the th curve) furthermore, the th current 22 and the second current 27 have the same shape (third curve in fig. 6).

At the beginning of the half wave of the waveform, it is assumed that all converters are switched off, as long as the converter of the th AC power unit 21 is switched off and as long as the converter of the second AC power unit 26 is switched off, the th AC voltage 23 and the second AC voltage 28 are zero ( th curve in fig. 6), simultaneously-i.e. at the same time time-both converters are switched on (step B1 and step B4) such that when there is no phase shift between the th AC voltage 23 and the second AC voltage 28-before switching on the converters of the AC power units 21, 26 or at the latest when switching on the converters of the AC power units 21, 26, the converter 9 of the auxiliary AC unit 5.1 is also switched on (step a1) to provide the auxiliary AC voltage 7 (second curve in fig. 6-thus initiating the rectangular shape of the th pulse winding of the main transformer 2 with the auxiliary AC voltage 7 of the third polarity pulse winding 463 is exposed to the second AC voltage 23 and the auxiliary AC voltage 28 is raised by the second AC voltage curve of the second AC power unit 21, 30-7-80-7-and the auxiliary power unit 23 is exposed to the linear transmission current transmission via the second voltage transfer curve of the second AC power unit 21, 30-27-7-18-23-7-18-2-so that the rectangular shape of the linear transmission current transmission curve of the auxiliary power unit 23-18-2-18-2-so that the linear transmission current transmission-2-transmission-has the transmission.

Next, the converter 9 of the auxiliary AC unit 5.1 is switched in order to reduce the auxiliary AC voltage 7 to zero (step a 3). thus, the th pulse of the auxiliary AC voltage 7 with polarity terminates in this state, the auxiliary AC unit 5.1 continues to conduct the th current 22, the th current 22 stops step up and remains constant.when the auxiliary AC voltage 7 is zero, the auxiliary power 31 of the auxiliary unit 5.1 drops to zero-contrary to this, the transmitted power 32 stops step up and remains constant.

To initiate a second pulse of the auxiliary AC voltage 7 having a second polarity, opposite to the th polarity, during the th half-wave of the waveform, the converter 9 of the auxiliary AC unit 5.1 is switched to provide the auxiliary AC voltage 7 having the second polarity (step a 2.) thus, the th current 22 and the second current 27 start to drop linearly, the auxiliary power 31 is now negative due to the inverted auxiliary AC voltage 7 and is delivered back to the auxiliary AC unit 5.1. as can be easily seen, the sum of the positive auxiliary power 31 during the th pulse of the auxiliary AC voltage 7 and the negative auxiliary power 31 during the second pulse of the auxiliary AC voltage 7 equals zero.

When the th and second currents 22, 27 become zero, the converter 9 of the auxiliary AC unit 5.1 is turned off (step A3), which terminates the second pulse of the th half wave of the auxiliary AC voltage 7. the converters of the th and second AC power units 21, 26 are also turned off (step B3 and step B6, respectively.) all voltages and currents are kept at zero for a short period of time to minimize switching losses if step A3 is omitted, there will be no period of time during which the current is zero, so that the second half wave starts without delay.

The second half-wave now starts in principle, the second half-wave is symmetrical to the th half-wave, but with opposite voltages and currents.

When the converters of the AC power unit 21 and the second AC power unit 26 are switched on simultaneously (i.e. at the same time) (steps B2 and B5, respectively) so that there is no phase shift between the th AC voltage 23 and the second AC voltage 28, said voltages jump to their respective main voltages 25, 30, but with a polarity opposite to the polarity during the th half-wave (curve of fig. 6) — before or at the latest when the converters of the AC power units 21, 26 are switched on, the converter 9 of the auxiliary AC unit 5.1 is also switched on (step a2) to provide the auxiliary AC voltage 7 (second curve in fig. 6) — thus, a third pulse is initiated which corresponds to the second pulse of the auxiliary AC voltage 7 with a second polarity opposite to the polarity of the th polarity-as a result of which the winding 3 of the main transformer 2 is exposed to the sum of the AC voltage 23 and the auxiliary AC voltage 7-while the second pulse of the main transformer 2 is exposed to the linear power unit 23 from the bottom of the auxiliary power unit 26, the voltage curve 9 is transmitted via the second AC voltage curve 636. the voltage curve 9 of the auxiliary power unit 21.7. as a voltage curve 9. the auxiliary power unit 23. the voltage curve 9 is transmitted by the linear power unit 30. the voltage curve 9. the auxiliary power unit 2. the voltage curve 9.

Next, the converter 9 of the auxiliary AC unit 5.1 is switched, such as to reduce the auxiliary AC voltage 7 to zero (step a 3.) thus, the third pulse of the auxiliary AC voltage 7 having the second polarity is terminated in this state, the auxiliary AC unit 5.1 continues to conduct the th current 22, the th current 22 stops further step down but remains constant, when the auxiliary AC voltage 7 is zero, the auxiliary power 31 of the auxiliary AC unit 5.1 falls to zero, contrary to which the transmitted power 32 stops further step up and remains constant.

To initiate a fourth pulse corresponding to the th pulse of the auxiliary voltage 7, also having the th polarity, which is opposite to the second polarity, the converter 9 of the auxiliary AC unit 5.1 is switched to provide the auxiliary AC voltage 7 having the th polarity (step a 1). therefore, the th current 22 starts to rise linearly, the auxiliary power 31 is negative due to the negative th current 22 and is delivered back to the auxiliary AC unit 5.1. as can be easily seen, the sum of the positive auxiliary power 31 during the third pulse of the auxiliary AC voltage 7 and the negative auxiliary power 31 during the fourth pulse of the auxiliary voltage 7 equals zero, therefore, on a half-wave, in particular the average value of the auxiliary power 31 measured on the second half-wave, is zero at step a1, the transmitted power 32 starts to fall linearly, but still provides a positive contribution to the power transmission during the entire second half-wave.

When the current 22 and the second current 27 become zero, the converter 9 of the auxiliary AC unit 5.1 is turned off (step A3), which terminates the fourth pulse of the auxiliary AC voltage 7 the converters of the AC power unit 21 and the second AC power unit 26 are also turned off (steps B3 and B6, respectively.) all voltages and currents remain zero for a short period of time to minimize switching losses if step A3 is omitted, there will be no period of time during which the current is zero, so that the th half wave is started again without delay.

Fig. 7 shows a second possible embodiment of a power conversion unit 20.2 comprising power units 21.1, 26.1 as shown in fig. 5A as the th AC power unit 21.1 and as the second AC power unit 26.1, and a power transfer unit 1.2 as shown in fig. 3 as the power transfer unit 1.2.

Fig. 8 shows a third possible embodiment of a power conversion unit 20.3, which again comprises power cells 21.1, 26.1 as shown in fig. 5A as the th AC power cell 21.1 and as the second AC power cell 26.1 contrary to the embodiment shown in fig. 7, the present power conversion unit 20.3 comprises a fourth possible embodiment of a power transfer unit 1.4 according to the invention with an auxiliary AC cell 5.2, 5.3 provided with a full bridge converter and a second auxiliary AC cell 55.2, 55.3 provided with a full bridge converter the second auxiliary AC cell 55.2, 55.3 is connected in series with the second winding 4 of the main transformer 2 to form a second series connection which is further connected to the second AC power cell 26.1 the auxiliary AC cells 5.2, 5.3 are connected in series with the th and th AC winding units 21.1 of the main transformer 2.

Fig. 9 shows a fourth possible embodiment of a power conversion unit 20.4, which again comprises AC power units 21.1, 26.1 as shown in fig. 5A as the th AC power unit 21.1 and as the second AC power unit 26.1 the power conversion unit 20.4 of fig. 9 comprises another power transfer units 1.5 according to the invention compared to the power conversion units 20.1, 20.2 and 20.3 shown in fig. 4, 7 and 8, respectively, the power transfer unit 1.5 has an auxiliary AC unit 5.6 with three phases and operates as an AC-AC converter and further comprises an auxiliary transformer 11.

Fig. 10 shows a fifth possible embodiment of a power conversion unit 20.5 comprising a th AC power unit 21.4 with a three-phase converter and a fourth embodiment of a second AC power unit 26.4 with another three-phase converters the main transformer 2 of this power conversion unit 20.5 is also configured as a three-phase transformer comprising a th winding 3 with three phases and a second winding 4 with three phases the three AC phases of the converter of the th AC power unit 21.4 are each connected in series to a different full bridge converter 5.2, 5.3 which is further connected to the respective phase of the th winding 3 of the main transformer 2 the second AC power unit 26.4 is connected to the second winding 4 of the main transformer 2.

Fig. 11 shows a sixth possible embodiment of a power conversion unit 20.6 which is in large part identical to the power conversion unit 20.1 shown in fig. 4, but comprises an additional control unit 33. this control unit 33 controls the th AC power unit 21, the second AC power unit 26 and the auxiliary AC unit 5.1. however, in a variant it is also possible that the control unit 33 controls only the auxiliary AC unit 5.1, only the th AC power unit 21, only the second AC power unit 26, only the auxiliary AC unit 5.1 and the AC power unit 21, only the auxiliary AC unit 5.1 and the second AC power unit 26 or only the th AC power unit 21 and the second AC power unit 26.

Fig. 12 shows a flow chart of the method according to the invention, the method comprising step a and step B in this example step a further comprises steps a1, a2 and A3, and step B comprises steps B1, B2, B3, B4, B5 and B6. which are explained in more detail in the context of fig. 6 above although these steps are explained for the power conversion unit 20.1 shown in fig. 4, the method with these steps can be applied to any power conversion unit comprising a power transfer unit according to the invention, a th power unit connected to the series connection of the power transfer unit and a second power unit connected to the second winding of the main transformer.

For example, various different types of auxiliary AC units with different converters 9, with and without an auxiliary transformer 11, with or without an energy storage 12, and different types of th AC power units 21 and different types of second AC power units 26 have been shown, all of these embodiments and variants can be combined resulting in various different advantageous power transfer units and various different advantageous power conversion units.

The power transfer unit as well as the power conversion unit and the method for controlling the power flow according to the invention provide interesting advantages over the prior art, such as improved efficiency over a practically unlimited operating range, the invention can therefore be used in -wide applications.

Claims (15)

1. Power transfer unit (1.1, 1.2, 1.3, 1.4, 1.5) for controlling the flow of electrical energy between two AC power units (21, 26), comprising:

a) a main transformer (2), the main transformer (2) having a th winding (3) and a second winding (4), an

b) A switchable auxiliary AC unit (5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6), the switchable auxiliary AC unit (5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6) for applying a tuneable auxiliary AC voltage (7) across an auxiliary AC side (6) of the auxiliary AC unit (5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6),

wherein the auxiliary AC side (6) is connected in series with the th winding (3) of the main transformer (2) to form a series connection (8).

2. The power transfer unit (1.1, 1.2, 1.3, 1.4, 1.5) of claim 1, wherein the auxiliary AC unit (5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6) further comprises an energy storage (12).

3. The power transfer unit (1.1, 1.2, 1.3, 1.4, 1.5) of any of the preceding claims, wherein the auxiliary AC unit (5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6) further steps comprise a converter (9).

4. The power transfer unit (1.1, 1.2, 1.3, 1.4, 1.5) of any of the preceding claims, wherein the auxiliary AC unit (5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6) further steps comprise an auxiliary transformer (11).

5. Power conversion unit (20.1, 20.2, 20.3, 20.4, 20.5) for converting electrical power, comprising:

a) the power transfer unit (1.1, 1.2, 1.3, 1.4, 1.5) of any of the preceding claims,

b) an th AC power unit (21) connected with the series connection (8) of power transfer units (1.1, 1.2, 1.3, 1.4, 1.5), and

c) a second AC power unit (26) connected to the second winding (4) of the main transformer (2).

6. The power conversion unit (20.1, 20.2, 20.3, 20.4, 20.5) of claim 5, wherein the th AC power unit (21) comprises a converter having a th AC side (24) connected with the series connection (8).

7. The power conversion unit (20.1, 20.2, 20.3, 20.4, 20.5) according to any of claims 5-6, wherein the second AC power unit (26) comprises a converter having a second AC side (29) connected with the second winding (4) of the main transformer (2).

8. The power conversion unit (20.1, 20.2, 20.3, 20.4, 20.5) according to any of claims 5-7, further steps comprising a control unit (33), the control unit (33) being adapted to control the auxiliary AC unit (5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6) and/or the AC power unit (21) and/or the second AC power unit (26).

9. The power conversion unit (20.1, 20.2, 20.3, 20.4, 20.5) according to claim 8 and claim 6 or 7, wherein the control unit (33) is adapted for zero current switching of the th AC power unit (21) and/or the second AC power unit (26).

10. Method for controlling the flow of electrical energy by using a power conversion unit (20.1, 20.2, 20.3, 20.4, 20.5) according to any of claims 5-9, comprising:

step A, providing an auxiliary AC voltage (7) across an auxiliary AC side (6) of an auxiliary AC unit (5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6) for shaping a th current (22) through an th AC power unit (21) and/or for shaping a second current (27) through a second AC power unit (26), wherein the auxiliary AC voltage (7) comprises pulses of different polarity during a half-wave of the auxiliary AC voltage (7), and

step B, synchronizing a th AC voltage (23) across a th AC side (24) of the th AC power cell (21) with a second AC voltage (28) across a second AC side (29) of the second AC power cell (26), and/or synchronizing a th AC voltage (23) across a th AC side (24) of the th AC power cell (21) with the auxiliary AC voltage (7).

11. The method of claim 10, wherein generating the pulse of the auxiliary AC voltage (7) comprises:

step A1, switching a converter (9) of the auxiliary AC unit (5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6) such that the auxiliary AC voltage (7) has a th polarity, and

step A2, switching a converter (9) of the auxiliary AC unit (5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6) such that the auxiliary AC voltage (7) has a second polarity opposite to the polarity,

and wherein synchronizing the AC voltage (23) across the AC side (24) of the AC power cell (21) with the second AC voltage (28) across the second AC side (29) of the second AC power cell (26) comprises:

step B1, switching the converter of the AC power cell (21) such that the AC voltage (23) has a third polarity, an

Step B2, switch the converter of the AC power cell (21) such that the AC voltage (23) has a fourth polarity opposite the third polarity.

12. The method of claim 11, wherein generating the pulse of the auxiliary AC voltage (7) further comprises:

step A3, switching a converter (9) of an auxiliary AC unit (5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6) to provide a conductive path with zero voltage across an auxiliary AC side (6) of the auxiliary AC unit (5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6),

and wherein synchronizing the AC voltage (23) across the AC side (24) of the AC power cell (21) with the second AC voltage (28) across the second AC side (29) of the second AC power cell (26) further comprises:

step B3, turn off all switches of the converter of the th AC power cell (21).

13. The method of any of claims 11-12, wherein step B1, and/or step B2, and/or step B3 are performed when the th current (22) is zero.

14. The method according to any of claims 10-13, wherein the average value of the auxiliary AC voltage (7) measured over half waves of the auxiliary AC voltage (7) is zero.

15. The method according to any of claims 10-14, wherein an average value of the power flow through the auxiliary AC unit (5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6) measured over half waves of the power flow through the auxiliary AC unit (5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6) is substantially zero.