Abstract
A power transmission unit for controlling a flow of electric energy between two AC power units is provided. The power transmission unit comprises a main transformer having a first winding and a second winding as well as a switchable auxiliary AC unit for applying a tunable auxiliary AC voltage across an auxiliary AC side of the auxiliary AC unit. The auxiliary AC side is connected in series with the first winding of the main transformer to form a series connection. Further, a power conversion unit comprising the power transmission unit and two AC power units as well as a method for controlling a flow of electric energy by using such a power conversion unit are provided.
Claims
1. A power conversion unit for converting electric power, comprising: a) power transmission unit for controlling a flow of electric energy between two AC power units, comprising a main transformer having a first winding and a second winding and a switchable auxiliary AC unit for applying a tunable auxiliary AC voltage across an auxiliary AC side of the auxiliary AC unit, wherein the auxiliary AC side is connected in series with the first winding of the main transformer to form a series connection, b) a first AC power unit connected with the series connection of the power transmission unit, and c) a second AC power unit connected with the second winding of the main transformer, wherein the first AC power unit comprises a converter having a first AC side being connected with the series connection and/or the second AC power unit comprises a converter having a second AC side being connected with the second winding of the main transformer, d) a control unit for controlling the auxiliary AC unit and/or the first AC power unit and/or the second AC power unit, wherein the control unit is adapted for zero-current switching of the first AC power unit and/or the second AC power unit, and/or wherein the control unit is adapted for controlling the auxiliary AC unit such that a current through the first AC power unit is in phase with a first AC voltage of the first AC power unit, and/or such that a current through the second AC power unit is in phase with a second AC voltage of the second AC power unity and wherein the control unit is adapted for controlling the auxiliary AC unit and/or the first AC power unit and/or the second AC power unit by using a step A of providing the auxiliary AC voltage across the auxiliary AC side of the auxiliary AC unit for shaping a first current through the first AC power unit and/or for shaping a second current through the second AC power unit, wherein the auxiliary AC voltage comprises pulses of different polarities during a half wave of the auxiliary AC voltage, and a step B of synchronizing a first AC voltage across the first AC side of the first AC power unit with a second AC voltage across the second AC side of the second AC power unit, and/or synchronizing the first AC voltage across the first AC side of the first AC power unit with the auxiliary AC voltage.
2. The power conversion unit according to claim 1, wherein the auxiliary AC unit further comprises an energy storage.
3. The power conversion unit according to claim 1, wherein the auxiliary AC unit further comprises a converter.
4. The power conversion unit according to claim 1, wherein the auxiliary AC unit further comprises an auxiliary transformer.
5. A method for controlling a flow of electric energy by using the power conversion unit according to claim 1, comprising: a step A of providing the auxiliary AC voltage across the auxiliary AC side of the auxiliary AC unit for shaping the first current through the first AC power unit and/or for shaping the second current through the second AC power unit, wherein the auxiliary AC voltage comprises pulses of different polarities during the half wave of the auxiliary AC voltage, and a step B of synchronizing the first AC voltage across the first AC side of the first AC power unit with the second AC voltage across the second AC side of the second AC power unit, and/or synchronizing the first AC voltage across the first AC side of the first AC power unit with the auxiliary AC voltage.
6. The method according to claim 5, wherein generating the pulses of the auxiliary AC voltage comprises: a step A1 of switching a converter of the auxiliary AC unit such that the auxiliary AC voltage has a first 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 first polarity, and wherein synchronizing the first AC voltage across the first AC side of the first AC power unit with the second AC, voltage across the second AC side of the second AC power unit comprises: a step B1 of switching the converter of the first AC power unit such that the first AC voltage has a third polarity, a step B2 of switching the converter of the first AC power unit such that the first AC voltage has a fourth polarity opposite to the third polarity.
7. The method according to claim 6, wherein generating the pulses of the auxiliary AC voltage further comprises a step A3 of switching the converter of the auxiliary AC unit to provide a conducting path with zero voltage across the auxiliary AC side of the auxiliary AC unit, and wherein synchronizing the first AC voltage across the first AC side of the first AC power unit with the second AC voltage across the second AC side of the second AC power unit further comprises a step B3 of switching off all switches of the converter of the first AC power unit.
8. The method according to claim 6, wherein steps B1, and/or B2, and/or B3 are performed when the first current is zero.
9. The method according to claim 5, wherein a mean value of the auxiliary AC voltage measured over the half wave of the auxiliary AC voltage is zero.
10. The method according to claim 5, wherein a mean value of a power flow through the auxiliary AC unit measured over a half wave of the power flow through the auxiliary AC unit is essentially zero.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The drawings used to explain the embodiments show:
(2) FIG. 1 a first embodiment of a power transmission unit according to the invention with an auxiliary AC unit;
(3) FIG. 2A a first embodiment of the auxiliary AC unit;
(4) FIG. 2B a second embodiment of the auxiliary AC unit;
(5) FIG. 2C a third embodiment of the auxiliary AC unit;
(6) FIG. 2D a fourth embodiment of the auxiliary AC unit;
(7) FIG. 2E a fifth embodiment of the auxiliary AC unit;
(8) FIG. 3 a second embodiment of the power transmission unit according to the invention;
(9) FIG. 4 a first embodiment of a power conversion unit comprising a power transmission unit according to the invention and two AC power units;
(10) FIG. 5A a first embodiment of a AC power unit for use in the power conversion unit;
(11) FIG. 5B a second embodiment of the AC power unit for use in the power conversion unit;
(12) FIG. 5C a third embodiment of the AC power unit for use in the power conversion unit;
(13) FIG. 6 waveforms of the voltages, currents and power of the power conversion unit;
(14) FIG. 7 a second embodiment of the power conversion unit comprising a power transmission unit according to the invention and two AC power units;
(15) FIG. 8 a third embodiment of the power conversion unit comprising a power transmission unit according to the invention and two AC power units;
(16) FIG. 9 a fourth embodiment of the power conversion unit comprising a power transmission unit according to the invention and two AC power units,
(17) FIG. 10 a fifth embodiment of the power conversion unit comprising a power transmission unit according to the invention and two AC power units;
(18) FIG. 11 a sixth embodiment of the power conversion unit comprising a power transmission unit according to the invention and two AC power units; and
(19) FIG. 12 a flow chart of the method according to the invention.
(20) In the figures, the same components are given the same reference symbols.
PREFERRED EMBODIMENTS
(21) FIG. 1 shows a first embodiment of a power transmission unit 1.1 according to the invention. The power transmission unit 1.1 comprises a main transformer 2 having a first winding 3 and a second winding 4. The power transmission 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. The auxiliary AC side 6 of the auxiliary AC unit 5 is connected in series with the first winding 3 of the main transformer 2 to form a series connection 8. The series connection 8 of the power transmission unit 1 can be connected to an AC power unit (not shown). For proper operation of the power transmission unit 1.1, the series connection 8 should not be shorted.
(22) FIG. 2A shows a first possible embodiment of the 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 in addition to the auxiliary AC side 6 also an auxiliary DC side 10.
(23) FIG. 2B shows a second possible embodiment of the switchable auxiliary AC unit 5.2 comprising a full bridge converter 9.1. The auxiliary AC unit 5.2 has again an auxiliary AC side 6 and an auxiliary DC side 10. The full bridge converter 9.1 has four insulated gate bipolar transistors (IGBT) as switching devices with antiparallel diodes.
(24) FIG. 2C shows a third possible embodiment of the switchable auxiliary AC unit 5.3 with an auxiliary AC side 6, an auxiliary DC side 10 and a full bridge converter 9.2 having four field effect transistors (FET), in particular four metal oxide semiconductor FET (MOSFET) with integrated antiparallel diodes as switching devices.
(25) FIG. 2D shows a fourth possible embodiment of the switchable auxiliary AC unit 5.4 comprising a converter 9.3 with a capacitive half bridge having two capacitors and an active half bridge having two IGBTs with each having an antiparallel diode.
(26) FIG. 2E shows a fifth possible embodiment of the 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 having six cascaded IGBTs. Each IGBT has an antiparallel diode.
(27) The auxiliary AC units 5.1-5.5 shown in FIG. 2A-E may comprise an energy storage like for example a capacitor or a battery. This energy storage may for example be connected with the auxiliary DC side of the respective auxiliary AC unit 5.1-5.5. The auxiliary AC units 5.1-5.5 may however not comprise such an energy storage. For example, they can be connected with their auxiliary DC side to some energy supply.
(28) Each of the embodiments of the auxiliary AC unit 5.1-5.5 shown in FIG. 2A-E can be used within the power transmission unit 1.1 as shown in FIG. 1 or within the power transmission unit 1.2 as shown in FIG. 3 to form further embodiments of the power transmission unit without being explicitly shown here.
(29) FIG. 3 shows a second possible embodiment of the power transmission unit 1.2 according to the invention. In this embodiment, the auxiliary AC unit 5.2, 5.3 comprises a full bridge converter 9.1, 9.2 having four switches with each having an antiparallel diode as the ones shown in FIGS. 2B and 2C. The switches of such full bridges converters can 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 the polarity sign next to the energy storage 12. In the present 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, which is further connected in series with the first winding 3 of the main transformer 2 to form the series connection 8.
(30) The auxiliary AC side 6 providing the auxiliary AC voltage 7 is formed by a winding of the auxiliary transformer 11.
(31) Although shown together in this second embodiment of the power transmission unit 1.2, the auxiliary AC unit 5.2, 5.3 is not required to comprise the auxiliary transformer 11 and the energy storage 12. Thus, the auxiliary transformer 11 connecting the AC side of the full bridge converter 9.1, 9.2 in series with the first winding 3 of the main transformer 2 can be omitted. Similarly, the energy storage 12 and/or the full bridge converter 9.1 can be omitted.
(32) FIG. 4 shows a first possible embodiment of a power conversion unit 20.1 comprising a power transmission unit 1.3 according to the invention. The power conversion unit 20.1 comprises a first AC power unit 21 providing a first current 22 and a first AC voltage 23 across a first AC side 24 of the first AC power unit 21. The first AC power unit 21 is a DC AC converter fed by a first main voltage 25 which is in this embodiment of the power conversion unit 20.1 a DC voltage. The first AC side 24 of the first AC power unit 21 is connected to a power transmission unit 1.3 according to the invention. Instead of the power transmission unit 1.3, any other power transmission unit according to the invention could be employed. For example, one of the power transmission units 1.1 or 1.2 shown in FIGS. 1 and 3, respectively, could be employed. The power transmission unit 1.3 shown here in FIG. 4 comprises the auxiliary AC unit 5.1 as shown in FIG. 2A with the converter 9 and the energy storage 12 being a capacitor. The power transmission unit 1.3 further comprises the main transformer 2 with a first winding 3 and a second winding 4. The second winding 4 of the main transformer 2 is connected to a second AC power unit 26. The second AC power unit 26 provides a second current 27 and a second AC voltage 28 across a second AC side 29 of the second AC power unit 26. The second AC power unit 26 is a DC AC converter fed by the second main voltage 30 which is in this embodiment of the power conversion unit 20.1 a DC voltage. The direction of the arrows indicating the first current 22, the first AC voltage 23, the first main voltage 25, the second current 27, the second AC voltage 28 and the second main voltage 30 in FIG. 4 is for counting purpose only and shall not indicate an actual current direction or voltage polarity. In fact, several of these currents or voltages are alternating, i.e. have two directions or polarities, respectively, during a period of the alternating voltage or current. In this embodiment of the power conversion unit 20.1, a flow of energy from the first AC power unit 21 via the power transmission unit 1.3 to the second AC power unit 26 is possible, as well as vice versa. The flow of energy is controlled by the tunable auxiliary AC voltage 7 across the auxiliary AC side 6. The voltage across the first winding 3 of the main transformer 2 corresponds to the sum of the first AC voltage 23 and the auxiliary AC voltage 7. The voltage across the second winding 4 of the main transformer 2 corresponds to the second AC voltage 28. Assuming, the main transformer 2 has a winding ratio of one, i.e. the first winding 3 and the second winding 4 have a same number of turns, and further assuming that the first main voltage 25 and the second main voltage 30 are equal and, therefore, the first AC voltage 23 and the second AC voltage 30 are equal as well, then the polarity and the magnitude of the auxiliary AC voltage 7 determine the direction and the magnitude of the flow of current and energy through the power transmission unit 1.3. It is remarked, that a mean power flow of the auxiliary AC unit can be zero. It is further remarked that the series connection formed by the auxiliary AC side 6 and the first winding 3 of the main transformer 2 is devoid of being short circuited for proper operation of the power transmission unit 1.3. Rather, as mentioned before, the series connection formed by the auxiliary AC side 6 and the first winding 3 of the main transformer 2 is connected to the first power unit 21. Detailed curves of the currents and voltages of the power transmission unit 20.1 will be discussed in FIG. 6.
(33) FIG. 5A shows a first possible embodiment of the first AC power unit 21.1 and the second AC power unit 26.1, each comprising a full bridge converter having four insulated gate bipolar transistors (IGBT) as switching devices with antiparallel diodes.
(34) FIG. 5B shows a second possible embodiment of the first AC power unit 21.2 and the second AC power unit 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 with each having an antiparallel diode.
(35) FIG. 5C shows a third possible embodiment of the first AC power unit 21 and the second AC power unit 26 comprising a converter with a capacitive half bridge and an active half bridge with cascaded IGBTs, i.e. six IGBTs. Each IGBT has an antiparallel diode.
(36) Any of the embodiments of first AC power unit 21.1-21.3 and any of the embodiments of the second AC power unit 26.1-26.3, or any combination thereof, can be used within the power conversion unit 21.1 as shown in FIG. 4.
(37) In general, as shown here for a single phase system, the topologies of the auxiliary AC unit 5 and the topologies of the first AC power unit 21 and the second AC power unit 26 can be the same. However, the power ratings can be very different, i.e. the power rating of the first and the second AC power unit 21, 26 can be much higher than the power rating of the auxiliary AC unit 5. For example, they can differ by a factor of more than 20.
(38) FIG. 6 shows waveforms of the voltages, currents and power when operating the power conversion unit 20.1 shown in FIG. 4. To simplify the explanation, it is assumed that the first 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 one. Therefore, the first AC voltage 23 and the second AC voltage 28 have the same shape (most upper and thus first curve in FIG. 6). Also, the first current 22 and the second current 27 have the same shape (third curve in FIG. 6).
(39) At the beginning of the first half wave of the waveforms, all converters are assumed to be switched off. As long as a converter of the first AC power unit 21 is switched off and as long as a converter of the second AC power unit 26 is switched of, the first AC voltage 23 and the second AC voltage 28 are zero (first curve in FIG. 6). Upon switching on both converters simultaneously (steps B1 and B4), i.e. at the same time such that there is no phase shift between the first AC voltage 23 and the second AC voltage 28, said voltages jump to their respective main voltages. Before or latest upon switching on the converters of the AC power units 21, 26, the converter 9 of the auxiliary AC unit 5.1 is switched on as well (step A1) to provide the auxiliary AC voltage 7 (second curve in FIG. 6). Thus, a first pulse of the auxiliary AC voltage 7 having a first polarity is initiated. The first winding 3 of the main transformer 2 is exposed to the sum of the first AC voltage 23 and the auxiliary AC voltage 7 while the second winding 4 of the main transformer 2 is exposed to the second AC voltage 28, only. The voltage difference (corresponding to the auxiliary voltage 7) causes the first current 22 and the second current 27 (third curve in FIG. 6) to rise. Due to the voltages 7, 23, 28 having rectangular waveforms, the first current 22 and the second current 27 rise linearly. Also, the auxiliary power 31 (fourth curve of FIG. 6) delivered by the auxiliary AC unit 5 as well as the transmitted power 32 (bottom curve of FIG. 6) from the first AC power unit 21 via the power transmission unit 1.3 to the second AC power unit 26 rise linearly.
(40) 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 A3). Therefore, the first pulse of the auxiliary AC voltage 7 having a first polarity is terminated. In this state, the auxiliary AC unit 5.1 continues to conduct the first current 22 which stops rising further and remains constant. The auxiliary power 31 of the auxiliary unit 5.1 falls to zero as the auxiliary AC voltage 7 is zero. In contrast thereto, the transmitted power 32 stops to rise further and remains constant.
(41) To initiate a second pulse of the auxiliary AC voltage 7 having a second polarity opposite to the first polarity during the first half wave of the waveforms, the converter 9 of the auxiliary AC unit 5.1 is switched to provide the auxiliary AC voltage 7 having the second polarity (step A2). Thus, the first current 22 and the second current 27 start to fall linearly. Due to the reversed auxiliary AC voltage 7, the auxiliary power 31 is now negative and delivered back to the auxiliary AC unit 5.1. As can be seen easily, the sum of the of the positive auxiliary power 31 during the first pulse of the auxiliary AC voltage 7 and the negative auxiliary power 31 during the second pulse of the auxiliary AC voltage 7 is equal to zero. Therefore, the mean value of the auxiliary power 31 measured over a half wave, in particular over the first half wave, is zero. At step A2, the transmitted power 32 starts to fall linearly but still provides a positive contribution to the power transmission during the entire first half wave.
(42) When the first current 22 and the second current 27 become zero, the converter 9 of the auxiliary AC unit 5.1 is switched off (step A3) which terminates the second pulse of the first have wave of the auxiliary AC voltage 7. The converters of the first AC power unit 21 and the second AC power unit 26 are switched off as well (step B3 and step B6, respectively). All voltages and currents remain zero for a short period of time to minimize switching losses. If step A3 were omitted, there would be no period of time during which the currents are zero such that the second half wave starts without delay.
(43) Now, the second half wave begins. In principal, the second half wave is symmetrical with the first have wave, but with reversed voltages and currents.
(44) Upon switching on the converters of the first AC power unit 21 and the second AC power unit 26 simultaneously (steps B2 and B5, respectively), i.e. at the same time such that there is no phase shift between the first 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 first half wave (first curve of FIG. 6). Before or latest upon switching on the converters of the AC power units 21, 26, the converter 9 of the auxiliary AC unit 5.1 is switched on as well (step A2) to provide the auxiliary AC voltage 7 (second curve in FIG. 6). A third pulse corresponding to the second pulse of the auxiliary AC voltage 7 having a second polarity opposite to the first polarity is thus initiated. As a result, the first winding 3 of the main transformer 2 is exposed to the sum of the first AC voltage 23 and the auxiliary AC voltage 7 while the second winding 4 of the main transformer 2 is exposed to the second AC voltage 28, only. The voltage difference (corresponding to the auxiliary voltage 7) causes the first current 22 and the second current 27 (third curve in FIG. 6) to fall below zero. Due to the voltages 7, 23, 28 having rectangular waveforms, the first current 22 and the second current 27 fall linearly. The auxiliary power 31 (fourth curve of FIG. 6) delivered by the auxiliary AC unit 5.1 and the transmitted power 32 (bottom curve of FIG. 6) from the first AC power unit 21 via the power transmission unit 1.3 to the second AC power unit 26 rise linearly.
(45) 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 A3). Therefore, the third pulse of the auxiliary AC voltage 7 having a second polarity is terminated. In this state, the auxiliary AC unit 5.1 continues to conduct the first current 22 which stops falling further but remains constant. The auxiliary power 31 of the auxiliary AC unit 5.1 falls to zero as the auxiliary AC voltage 7 is zero. In contrast thereto, the transmitted power 32 stops to rise further and remains constant.
(46) To initiate a fourth pulse corresponding to the first pulse of the auxiliary voltage 7 having again the first polarity opposite to the second polarity, the converter 9 of the auxiliary AC unit 5.1 is switched as to provide the auxiliary AC voltage 7 having the first polarity (step A1). Thus, the first current 22 starts to rise linearly. Due to the negative first current 22, the auxiliary power 31 is negative and delivered back to the auxiliary AC unit 5.1. As can be seen easily, the sum of the 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 is equal to zero. Therefore, the mean value of the auxiliary power 31 measured over a half wave, in particular over 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.
(47) When the first current 22 and the second current 27 become zero, the converter 9 of the auxiliary AC unit 5.1 is switched off (step A3) which terminates the fourth pulse of the auxiliary AC voltage 7. The converters of the first AC power unit 21 and the second AC power unit 26 are switched off as well (steps B3 and B6, respectively). All voltages and current remain zero for a short period of time to minimize switching losses. If step A3 were omitted, there would be no period of time during which the currents are zero such that the first half wave starts again without delay.
(48) FIG. 7 shows a second possible embodiment of the power conversion unit 20.2 comprising as first AC power unit 21.1 and as second AC power unit 26.1 a power unit 21.1, 26.1 as shown in FIG. 5A and as power transmission unit 1.2 the power transmission unit 1.2 shown in FIG. 3.
(49) FIG. 8 shows a third possible embodiment of the power conversion unit 20.3 again comprising as first AC power unit 21.1 and as second AC power unit 26.1 a power unit 21.1, 26.1 shown in FIG. 5A. In contrast to the embodiment shown in FIG. 7, the present power conversion unit 20.3 comprises a fourth possible embodiment of the power transmission unit 1.4 according to the invention which has an auxiliary AC unit 5.2, 5.3 with full bridge converter and a second auxiliary AC unit 55.2, 55.3 with full bridge converter. The second auxiliary AC unit 55.2, 55.3 is connected in series with second winding 4 of main transformer 2 to form a second series connection which is further connected to the second AC power unit 26.1. Auxiliary AC unit 5.2, 5.3 is connected in series with the first winding 3 of the main transformer 2 and to the first AC unit 21.1.
(50) FIG. 9 shows a fourth possible embodiment of a power conversion unit 20.4 again comprising as first AC power unit 21.1 and as second AC power unit 26.1 the AC power unit 21.1, 26.1 shown in FIG. 5A. In contrast to the power conversion units 20.1, 20.2 and 20.3 shown in FIGS. 4, 7 and 8, respectively, the power conversion unit 20.4 of FIG. 9 comprises another power transmission unit 1.5 according to the invention. This power transmission unit 1.5 has an auxiliary AC unit 5.6 with three phases and works as an AC-AC converter and further comprises an auxiliary transformer 11.
(51) FIG. 10 shows a fifth possible embodiment of a power conversion unit 20.5 which comprises a first AC power unit 21.4 with a three phase converter and the fourth embodiment of the second AC power unit 26.4 with another three phase converter. The main transformer 2 of this power conversion unit 20.5 is as well configured as three phase transformer with a first winding 3 having three phases and a second winding 4 having three phases. The three AC phases of the converter of the first 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 a respective phase of the first 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.
(52) FIG. 11 shows a sixth possible embodiment of a power conversion unit 20.6. This embodiment is in most parts 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 first AC power unit 21, the second AC power unit 26 and the auxiliary AC unit 5.1. In variants, it is however as well possible that the control unit 33 only controls the auxiliary AC unit 5.1, only the first AC power unit 21, only the second AC power unit 26, only the auxiliary AC unit 5.1 and the first AC power unit 21, only the auxiliary AC unit 5.1 and the second AC power unit 26 or only the first AC power unit 21 and the second AC power unit 26.
(53) FIG. 12 shows a flow chart of the method according to the invention. The method comprises steps A and B. In the present example, step A further comprises steps A1, A2 and A3, and step B comprises steps B1, B2, B3, B4, B5 and B6. These steps are explained in more detail above in the context of FIG. 6. Even though the steps are explained there in view of 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 transmission unit according to the invention, a first power unit connected with the series connection of the power transmission unit, and a second power unit connected with the second winding of the main transformer.
(54) In summary, it is to be noted that the invention is not limited to the above mentioned embodiments. For example, a variety of different types of the auxiliary AC unit with different converters 9, with and without auxiliary transformer 11, with or without energy storage 12, as well as different types the first AC power unit 21 and different types of the second AC power unit 26 have been shown. All of these embodiments and variants can be combined resulting in a variety of different advantageous power transmission units and a variety of different advantageous power conversion units.
(55) The power transmission unit according to the invention, as well as the power conversion unit and the method for controlling a flow of electric energy provide interesting advantages over the prior art as for example improved efficiency over a virtually unlimited operating range. Therefore, the invention can be used for a wide range of applications.
