DE102012108363A1 - Method of changing sine voltage to rated voltage in single phase power supply, involves changing voltage between outer and neutral conductors and closing or opening a switch during successive periodic cycles to modulate induced voltage - Google Patents

Abstract

A switch corresponding to primary windings (11,12) of transformer (1) is switched in current path between first portion of outer conductor (L1) and neutral conductor (N) of power system, among the switches (2,3). A voltage in the secondary winding (13) of transformer is induced with the current flowing through one of the primary windings by the switching of a switch in current path. The voltage between second portion of outer conductor and neutral conductor is changed, and one of the switches is closed or opened during the successive periodic cycles, to modulate the induced voltage. An independent claim is included for a transformer circuit for implementing the method for changing the sine voltage into rated voltage in single phase power supply.

Description

• – mittels eines ersten Transformators, der wenigstens zwei Primärwicklungen und eine elektrisch zwischen einen ersten Abschnitt eines Außenleiters und einen zweiten Abschnitt des Außenleiters angeordnete Sekundärwicklung aufweist, und
• – mittels wenigstens zwei ersten steuerbaren Schaltern und wenigstens einem zweiten steuerbaren Schalter,
• – wobei wenigstens eine von den Primärwicklungen des ersten Transformators mit den ersten Schaltern in einen Strompfad zwischen dem ersten Abschnitt des Außenleiters und einem Neutralleiter des Stromnetzes geschaltet wird und
• – wobei durch das Schalten des Strompfades Strom durch wenigstens eine der Primärwicklungen des ersten Transformators fließt, was eine Spannung in der Sekundärwicklung des ersten Transformators induziert, wodurch sich die Spannung zwischen dem zweiten Abschnitt des Außenleiters und dem Neutralleiter ändert
• – wobei die Primärwicklung mittels des zweiten Schalters kurzgeschlossen wird, wenn die Spannung zwischen dem zweiten Abschnitt des Außenleiters und dem Neutralleiter gegenüber einer Nennspannung unverändert bleiben soll.

The present invention relates to a method for changing the voltage in a power grid relative to a nominal voltage

• - By means of a first transformer having at least two primary windings and an electrically disposed between a first portion of an outer conductor and a second portion of the outer conductor secondary winding, and
• By means of at least two first controllable switches and at least one second controllable switch,
• - Wherein at least one of the primary windings of the first transformer with the first switches is connected in a current path between the first portion of the outer conductor and a neutral conductor of the power network, and
• Wherein, by switching the current path, current flows through at least one of the primary windings of the first transformer, inducing a voltage in the secondary winding of the first transformer, thereby changing the voltage between the second portion of the outer conductor and the neutral conductor
• - Wherein the primary winding is short-circuited by means of the second switch, when the voltage between the second portion of the outer conductor and the neutral conductor is to remain unchanged from a nominal voltage.

The invention further relates to a transformer circuit for carrying out the method.

A method of the type mentioned is in the document DE 10 2009 014 243 A1 disclosed. With the method, it is possible to change the voltage in a power grid, without the power in the power grid must be interrupted. For this purpose, the voltage in an outer conductor is changed with the method. Since power grids are typically three-phase, the method is applied to each phase conductor to change the voltage in the outer conductor.

The change in the voltage is effected by the voltage induced in the secondary coil, the voltage between the second portion of the outer conductor and the neutral conductor is increased or decreased. To the voltage between the first section of the outer conductor and the neutral conductor, the voltage drop across the secondary winding is added to obtain the voltage in the second section opposite to the neutral conductor. The potential of the second section is thus shifted from the potential of the first section by the voltage drop across the secondary winding.

The change of the voltage is possible only by discrete values. Changing the current paths changes the gear ratio of the first transformer. When the second switches are closed, the primary side of the first transformer is shorted. Then no current is transformed. A change in the voltage in the second section of the outer conductor then does not take place. If at least one of the second switches is opened, the transmission ratio is determined by the position of the first switches.

A continuous change of the effective voltage is not possible. Although one can make the circuit, with which the process is carried out, so that a fine-step switching between discrete voltage levels is possible. For this purpose, however, a large number of primary coils and first controllable switches is necessary so that the transmission ratio of the first transformer can also be changed in small steps. This is very complicated.

However, many applications are conceivable in which a fine-level or even a continuous change of the voltage is necessary or desirable.

An application for which one wishes to change the voltage as continuously as possible is to adjust the voltage in a local network, as it did in the document DE 10 2009 014 243 A1 has been described.

Also in the Magazine etz, issue 4/2012 is under the heading "Intelligent local network stations as an alternative to network expansion" describes the need for changeability of the voltage in a local network.

The desire to change a voltage is not only known in relation to local networks.

The invention therefore an object of the invention to develop the method described above for (uninterruptible) changing the voltage in a power grid so that a fine-level or continuous adjustment of the effective voltage is possible without a large number of switches or coils is required.

This object is achieved according to the invention in that at least one of the first controllable switches is closed and / or opened during successive periodic clocks in order to modulate the induced voltage.

This object is also achieved in that the first controllable switch and the second controllable switch is closed and / or opened during successive periodic clocks to modulate the induced voltage.

If no modulation takes place, a sinusoidal voltage is induced in the secondary winding as in the prior art. This voltage is modulated by switching. Modulation in the sense of the application means that the amplitude, the angular frequency or the zero phase angle of the induced voltage is time-dependent.

According to the invention, the switching ratio of the first transformer is changed by firing and opening the switches during a periodic cycle, which results in the modulation of the voltage induced in the secondary coil. The modulation of the induced voltage results in a change in the reduction or increase in the voltage between the second section of the outer conductor and the neutral conductor. This manifests itself in a changed effective value of the voltage of the second section of the outer conductor relative to the neutral conductor.

According to the invention, during at least a first time interval within the clock in the secondary winding, a voltage having a first rms value and during at least a second time interval within the clock in the secondary winding a voltage having a second rms value can be induced, the first rms value and the second rms value being different from each other differ. The first time interval and the second time interval can last in total the whole cycle.

However, it is also conceivable that further time intervals are provided within the clock in which voltages with different rms values are induced in the secondary winding.

The modulation can be done during several periodic cycles. The calculation of the effective value of the voltage of the second section of the outer conductor relative to the neutral conductor takes place at least over one cycle. In contrast, only the first, second or one of the further time intervals is considered in the calculation of the effective value of the voltage induced in the secondary winding.

According to the invention, the cycle length may correspond to the length of one period of the voltage in the power network. Clock and network period thus correspond to each other. The first, the second and possibly another time interval then have in sum the length of the period.

The modulation then takes place within a period of the mains voltage. The modulation can be continued unchanged in the following periods of the mains voltage. The modulation can be changed from the period of the mains voltage to the period of the mains voltage. It is also possible that no modulation takes place.

The first controllable switch or the second controllable switch that is closed can be closed according to the invention only during a single first, a single second or possibly a single further time interval. It is possible for one of the switches to be closed during the entire period of the voltage in the single-phase power grid.

The switches can be controlled by a phase control or phase section control. Advantageously during a period a switch is controlled to the phase angle or phase section while one of the remaining switches remains closed during the entire period. In another period, at least one other switch may be controlled to the phase angle or phase portion while one of the remaining switches remains closed during the entire period.

The switches can be operated in primary-side voltage sequencing, as for the secondary side of a transformer in the book "Thyristorised Power Controllers" by GK Dubey, SR Doradla, A. Joshi and RMK Sinha in Section 5.1.4 is described.

According to the invention, it is also possible that the cycle length corresponds to the length of several periods of the voltage in the power grid. The periodic clock may correspond, for example, to 2, 5, 10, 25 or 50 periods of the mains voltage. The periodic clock is advantageously an integer multiple of the period of the mains voltage. The first, the second and possibly another time interval then have in the sum of the length of the clock.

The modulation then takes place within one cycle. The modulation can be continued unchanged in subsequent measures. The modulation can be changed from bar to bar. It is also possible that no modulation takes place.

The first controllable switch or the second controllable switch that is closed can be closed according to the invention only during a single first, a single second or a single third time interval. It is possible that one or more of the switches will be closed during the entire cycle.

The switches can be controlled by a vibration packet controller. The first, second or any further time interval then comprise an integer number of periods of the mains voltage.

Advantageously, a switch is closed during a first time interval, while one of the remaining switches remains closed during the entire periodic clock. In another periodic clock, at least one other switch may remain closed only in the first time interval while one of the remaining switches remains closed throughout the periodic clock.

Inventive embodiments are described below with reference to FIGS. Show in it

1 a simplified circuit diagram of a transformer circuit according to the invention,

2 a first course of a voltage induced in a secondary winding in an idealized representation,

3 a second course of a voltage induced in a secondary winding in an idealized representation,

4 a third course of induced in a secondary winding voltage in idealized representation and

5 a fourth course of a voltage induced in a secondary winding in an idealized representation.

In the 1 illustrated transformer circuit according to the invention comprises a first transformer 1 with a first primary winding 11 , a second primary winding 12 and a secondary winding 13 on.

The two primary windings each have two connections 111 . 112 . 121 . 122 ,

A first connection 111 the first primary winding 11 is electrically connected to a point P1.

A second connection 112 the first primary winding 11 is with a first connection 121 the second primary winding in a point P2 electrically connected.

A second connection 122 the second primary winding is connected to a neutral conductor N of a power network.

The primary windings have from the first connection 111 . 121 to the second connection 112 . 122 a same winding sense. The sense of winding could also be opposite.

The primary windings may be formed by a winding with a tap between the ends of the winding. The tap is then connected to point P2. The transformer 1 can also have more than two primary windings, which can then be realized by further taps of a winding.

The points P1 and P2 are each via a first controllable bidirectional switch 2 . 3 , in particular power controller, for example, anti-parallel connected thyristors, electrically conductively connected to a first portion L11 of an outer conductor L1 of the power grid.

Point P1 is also a bidirectional switch 4 , which may be constructed of IGBTs, for example, connected to the neutral conductor N.

In addition to the first section L11, the outer conductor L1 also has a second section L12. The two sections L11, L12 are connected to each other via the secondary winding of the first transformer.

Furthermore, the transformer circuit has a controller 5 on which the first controllable switch 2 . 3 and the second controllable switch 4 during operation of the circuit controls.

The transformer circuit is connected to a local power transformer 6 connected to a power supply network. The local power transformer 6 connects a grid of the medium voltage level and a grid of the low voltage level. The transformer circuit according to the invention and the local network transformer 6 are combined to form a transformer station according to the invention. The transformer circuit according to the invention is connected to the secondary side of the local network transformer 6 connected.

Power grids are usually three-phase. With the transformer circuit according to the invention, however, the voltage of an outer conductor is increased. In order to adjust the voltage of the three outer conductors of the power network, three transformer circuits according to the invention to the secondary side of the local power transformer 6 to change the voltage in each outer conductor. Above and below, a transformer circuit for changing the voltage in an outer conductor is described. The description may, however, refer to the change of tension in the others External conductors of the power network on the secondary side of the local power transformer 6 be transmitted.

By closing the first switch 2 with simultaneously opened first switch 3 is a current path from the first portion L11 of the outsider L1 to the neutral conductor N through both primary coils 11 . 12 connected. Is on the other hand the first switch 3 closed, is a current path from the first portion L11 of the outsider L1 via the second primary coil 12 switched to the neutral conductor N. In both cases, the sinusoidal current causes through the primary winding 12 or primary windings 11 . 12 the induction of a voltage in the secondary winding 13 of the first transformer 1 , Depending on which current path is connected, the transmission ratio of the first transformer changes 1 , Will be on the primary side of the first transformer 1 increases the number of current flowing through the windings, which decreases in the secondary winding 13 of the first transformer 1 induced voltage. Is the switch 3 closed, the primary side has fewer turns than when the switch 2 is closed and the switch 3 is open. Therefore, when the switch is closed 3 a larger voltage in the secondary winding 13 induced as if the switch 2 closed and the switch 3 is open.

Switching between current paths, ie between voltage stages, has hitherto been such that two discrete voltages are induced, depending on the winding sense of the windings 11 . 12 . 13 lead to an increase or decrease in the voltage between the second section L12 of the outer conductor L1 and the neutral conductor N, which is used to change the rms voltage in the second section L12 of the outer conductor.

If the voltage in the second section L12 remain unaffected, the primary coils 11 . 12 of the first transformer 1 by closing the second switch 4 shorted.

According to the invention is now by pressing the first switch 2 . 3 and the second switch 4 in the secondary coil 13 induced voltage of the first transformer 1 modulated. The modulation of the voltage takes place in a periodic clock Tp. The rms value of the induced voltage Ui can therefore also be changed between the known voltage levels.

The in the 2 to 5 illustrated examples of voltage waveforms of the induced voltage Ui may result in an operation of the transformer circuit according to the invention.

The in the 2 and 3 shown voltage curves in the secondary winding 13 of the first transformer 1 induced voltage Ui arise in a method according to the invention, in which the duration of the clock Tt the duration of a period Tp of the mains voltage in the first section L11 of the outer conductor L1 corresponds.

The in the 4 and 5 shown voltage curves in the secondary winding 13 of the first transformer 1 induced voltage Ui arise in a method according to the invention, in which the duration of the clock Tt the duration of three period Tp corresponds to the mains voltage in the first section L11 of the outer conductor.

In the figures, Ui denotes the voltages induced in the secondary coil, Tp the duration of a line period, Tt the duration of a clock, t2 the time of closing of the first switch 2 and at t3 the time of closing of the first switch 3 designated.

Further, U2 denotes the voltage in the secondary coil 13 would be induced when the first switch 2 constantly closed and the switch 3 constantly open. U3 is the voltage that would be induced in the secondary coil when the first switch 3 constantly closed.

With t2 the times are designated, at which one of the thyristors of the first switch 2 is ignited and with t3 the times are designated, at which one of the thyristors of the first switch 3 is ignited.

The voltage curve according to 2 results when, with the beginning of the clock, the first switch 2 is closed and at time t3, the first switch 3 is closed. At each zero crossing of the induced voltage Ui becomes the first switch 3 reopened and the first switch 2 closed again. The time t3 corresponds approximately to a firing angle of 110 ° of the thyristors of the first switch 3 ,

After a zero crossing, a current first of the first section L11 of the outer conductor via the first switch 2 , the point P1, the primary winding 11 , the primary winding 12 led to the neutral conductor N. This current induces in the secondary winding 13 the voltage Ui with an amount in the 2 With 1 is specified. By closing the first switch 3 the current gets from the first switch 2 and the primary winding 11 at the first switch 3 accepted. The first switch 2 and the primary winding 11 are then without electricity. The through the primary winding 12 guided current then induces due to the changed transmission ratio of the first transformer 1 the voltage Ui with an amount in the 2 With 2 is specified.

The voltage curve according to 3 results when at the beginning of the clock, the first switch 2 . 3 are open and the second switch 4 is closed and at time t2 the first switch 2 is closed. At each zero crossing of the induced voltage Ui becomes the first switch 2 opened again and the second switch 4 closed again. The time t2 corresponds approximately to a firing angle of 130 ° of the thyristors of the first switch 2 , This mode can be referred to as a primary-side voltage follower.

After a zero crossing, the series connection of the primary windings 11 . 12 shorted. Then no current flows through the primary windings. As a result, no voltage is induced in the secondary winding. By opening the second switch 4 and simultaneously closing the first switch 2 becomes a current from the first portion L11 of the outer conductor via the first switch 2 , the point P1, the primary winding 11 , the primary winding 12 led to the neutral conductor N. The through the primary winding 12 guided current then induces due to the changed transmission ratio of the first transformer 1 the voltage Ui with an amount in the 2 With 1 is specified. This mode can be referred to as a primary-side phase control.

The voltage curve according to 4 results when, with the beginning of the clock, the first switch 3 is closed and remains closed for two periods of length Tp, igniting the thyristors of the first switch 3 is reached. After two periods of length Tp, the first switch becomes the first time at time t2 2 closed. It remains closed for a period of duration Tp until the cycle of duration Tt ends. With the next cycle, the switching pattern repeats from the first cycle.

After the beginning of the clock, a current first of the first portion L11 of the outer conductor via the first switch 3 , the point P2, the primary winding 12 led to the neutral conductor N. This current induces in the secondary winding 13 the voltage Ui with an amount in the 4 With 2 is specified. By opening the first switch 3 after two periods of length Tp and the simultaneous closing of the first switch 2 the current gets from the first switch 3 and the primary winding 11 accepted. The current then flows from the first section L11 of the outer conductor via the first switch 2 , the point P1, the primary winding 11 , the point P2, the primary winding 12 to the neutral conductor N. The through the primary windings 11 . 12 guided current then induces due to the changed transmission ratio of the first transformer 1 the voltage Ui with an amount in the 2 With 1 is specified.

The voltage curve according to 5 results when, with the beginning of the clock, the first switch 2 is closed and remains closed for a period of length Tp, igniting the thyristors of the first switch 2 is reached. After a period of length Tp, the second switch is at time t4 4 closed. It remains closed for two periods of duration Tp until the clock of duration Tt ends. With the next cycle, the switching pattern repeats from the first cycle.

After the beginning of the clock, a current first of the first portion L11 of the outer conductor via the first switch 2 , the point P1, the primary winding 11 , the point P2, the primary winding 12 led to the neutral conductor N. This current induces in the secondary winding 13 the voltage Ui with an amount in the 5 With 1 is specified. By opening the first switch 2 after a period of the length Tp and the simultaneous closing of the second switch 4 becomes the series connection of the primary windings 11 . 12 shorted. Then no current flows through the primary windings. As a result, no voltage is induced in the secondary winding. The voltage of the second portion L12 of the outer conductor L1 is then unchanged from the nominal voltage.

The in the 2 and 4 Voltage curves shown can be changed by shifting the ignition timing of the thyristors to generate voltages with other RMS values. The change in the RMS values can take place continuously within limits set by the thyristors.

The in the 2 and 3 Voltage curves shown can be changed by shifting the ignition timing of the thyristors, ie by changing the firing angle of the thyristors used for the phase angle, to generate voltages with other RMS values. The change in the RMS values can take place continuously within limits set by the thyristors.

The in the 4 and 5 Voltage curves shown can be changed by changing the number of periods in which the first switch 2 . 3 and the second switch 4 are energized, to produce voltages with other RMS values. The change can not be made continuously, but has the basis of the 4 and 5 illustrated method that by switching the first switch 2 . 3 and the second switch 4 hardly any upper oscillations arise.

QUOTES INCLUDE IN THE DESCRIPTION

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

Cited patent literature

• DE 102009014243 A1 [0003, 0008]

Cited non-patent literature

• Magazine etz, issue 4/2012 is under the heading "Intelligent local network stations as an alternative to grid expansion" [0009]
• "Thyristorised Power Controllers" by GK Dubey, SR Doradla, A. Joshi and RMK Sinha in Section 5.1.4 [0023]

Claims (12)

Method for changing the sinusoidal voltage in a power grid compared to a nominal voltage - by means of a first transformer ( 1 ), the at least two primary windings ( 11 . 12 ) and a secondary winding arranged electrically between a first section (L11) of an outer conductor (L1) and a second section (L12) of the outer conductor (L1) ( 13 ), and - by means of at least two first controllable switches ( 2 . 3 ), - with the first switches ( 2 . 3 ) at least one of the primary windings ( 11 . 12 ) of the first transformer ( 1 ) is switched into a current path between the first section of the outer conductor (L1) and a neutral conductor (N) of the power network, and - wherein the current path through the switching of the current path through at least one of the primary windings ( 11 . 12 ) of the first transformer ( 1 ), which has a voltage in the secondary winding ( 13 ) of the first transformer ( 1 ), whereby the voltage between the second portion of the outer conductor and the neutral conductor changes, characterized in that at least one of the first controllable switch ( 2 . 3 ) is closed and / or opened during successive periodic clocks to modulate the induced voltage. Method for changing the sinusoidal voltage in a single-phase power network with respect to a rated voltage, in particular according to claim 1 - by means of a first transformer ( 1 ), which has at least one primary winding ( 11 . 12 ) and a secondary winding arranged electrically between a first section (L11) of an outer conductor (L1) and a second section (L12) of the outer conductor (L1) ( 13 ), and - by means of at least one first controllable switch ( 2 . 3 ) and at least one second controllable switch ( 4 ), - with the first switch ( 2 . 3 ) the primary winding ( 11 ) of the first transformer ( 1 ) is switched into a current path between the first section (L11) of the outer conductor (L1) and a neutral conductor (N) of the power network, and - wherein the current path through the switching of the current path through at least one of the primary windings ( 11 . 12 ) of the first transformer ( 1 ), which has a voltage in the secondary winding ( 13 ) of the first transformer, whereby the voltage between the second section (L12) of the outer conductor (L1) and the neutral conductor (N) changes with respect to a nominal voltage, - wherein the primary winding ( 11 . 12 ) by means of the second switch ( 4 ) is short-circuited if the voltage between the second section of the outer conductor and the neutral conductor is to remain unchanged from the nominal voltage, characterized in that - the first controllable switch ( 2 . 3 ) and the second controllable switch ( 4 ) are closed and / or opened during successive periodic clocks to modulate the induced voltage. A method according to claim 1 or 2, characterized in that during at least a first time interval within the clock in the secondary winding a voltage with a first rms value and during at least a second time interval within the clock in the secondary winding a second voltage with a second rms value is induced, wherein the first RMS value and the second RMS value differ from each other. Method according to one of claims 1 to 3, characterized in that the cycle length of the length corresponds to a period of the voltage in the power grid. Method according to Claim 4, characterized in that the first controllable switch ( 2 . 3 ) or the second controllable switch ( 4 ), which is closed, is closed only during a part of a period of the voltage in the single-phase power network. Method according to claim 5, characterized in that the switch closed during the part of the period of the voltage ( 2 . 3 . 4 ) is controlled by a phase angle or phase section control. Method according to one of Claims 1 to 3, characterized in that the cycle length (Tt) corresponds to the length (Tp) of several periods of the voltage in the power network. Method according to claim 7, characterized in that the first controllable switch ( 2 . 3 ) or the second controllable switch ( 4 ), which is closed, is closed during one or more periods during a bar. A method according to claim 8, characterized in that the switch closed for one or more periods of a cycle ( 2 . 3 . 4 ) is controlled by a swinging pact control. A transformer circuit for performing the method of changing the sinusoidal voltage in a single-phase power network between an outer conductor (L1) and a neutral conductor (N) to a rated voltage With a first transformer ( 1 ), the at least two primary windings ( 11 . 12 ) and a secondary winding arranged electrically between a first section (L11) of an outer conductor (L1) and a second section (L12) of the outer conductor (L1) ( 13 ), and - having at least two first controllable switches ( 2 . 3 ), Wherein at least one of the primary windings ( 11 . 12 ) of the first transformer ( 1 ) with the first switches ( 2 . 3 ) is switchable in a current path between the first section (L12) of the outer conductor (L1) and a neutral conductor (N) of the power network, - wherein the current path through the switching of the current path through at least one of the primary windings ( 11 . 12 ) of the first transformer ( 1 ), which causes a voltage in the secondary winding ( 12 ) of the first transformer ( 1 ), whereby the voltage between the second portion (L12) of the outer conductor (L1) and the neutral conductor (N) changes, characterized in that the transformer circuit according to one of claims 1 to 9 is operable and has a control which is suitable and is set up, during several periodic clocks the first controllable switch ( 2 . 3 ) to modulate the induced voltage. Transformer circuit for carrying out the method for changing the sinusoidal voltage in a single-phase power network between an outer conductor (L1) and a neutral conductor (N) with respect to a nominal voltage, in particular according to claim 1 - with a first transformer ( 1 ), the at least two primary windings ( 11 . 12 ) and a secondary winding arranged electrically between a first section (L11) of an outer conductor (L1) and a second section (L12) of the outer conductor (I1) ( 13 ), and - having at least two first controllable switches ( 2 . 3 ) and at least one second controllable switch ( 4 ), Wherein at least one of the primary windings ( 11 . 12 ) of the first transformer ( 1 ) with the first switches ( 2 . 3 ) is switchable in a current path between the first portion (L11) of the outer conductor (L1) and a neutral conductor (N) of the power network, - wherein the current path through the switching of the current path through at least one of the primary windings ( 11 . 12 ) of the first transformer ( 1 ), which causes a voltage in the secondary winding ( 13 ) of the first transformer ( 1 ), whereby the voltage between the second portion (L12) of the outer conductor (L1) and the neutral conductor (N) changes, - wherein the primary winding ( 11 . 12 ) by means of the second switch ( 4 ) is short-circuitable when the voltage between the second portion (L12) of the outer conductor (L1) and the neutral conductor (N) is to remain unchanged from a nominal voltage, characterized in that the transformer circuit according to one of claims 1 to 9 is operable and a controller which is suitable and arranged, during a plurality of periodic clocks the second controllable switch ( 4 ) to modulate the induced voltage. Transformer circuit according to claim 10 or 12, characterized in that the first and / or the second controllable switch ( 2 . 3 . 4 ) are bidirectional switches, in particular triacs, anti-parallel connected thyristors, anti-parallel connected IGBTs or the like.

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