EP2622614B1 - Device and method for reducing a magnetic unidirectional flux fraction in the core of a transformer - Google Patents

Description

Technical area

The invention relates to an apparatus and a method for reducing a magnetic DC component in the core of a transformer, with a measuring device which provides a sensor signal corresponding to the magnetic DC component, with a compensation winding which is magnetically coupled to the core of the transformer with a switching unit which is electrically arranged in a current path in series with the compensation winding for feeding into the compensation winding a current whose effect is opposite to the DC component, the switching unit being controllable by means of a manipulated variable provided by a control device; Furthermore, the present invention relates to a method for converting a transformer.

State of the art

In electrical transformers, as used in energy distribution networks, there may be an undesirable feed of a direct current into the primary winding or secondary winding. Such DC supply, hereinafter also referred to as DC component, may for example come from electronic components, such as those used today in the control of electrical drives or in the reactive power compensation. Another cause may be so-called "Geomagnetically Induced Currents" (GIC).

A DC component in the core of the transformer results in a DC component, which is superimposed on the AC flux. This leads to an asymmetric modulation of the magnetic material in the core and brings a number of disadvantages. Even a DC of a few amps can cause a local heating in the transformer, which can affect the life of the winding insulation. Another undesirable effect is an increased noise emission during operation of the transformer. This is especially troublesome when the transformer is installed near a living area.

To reduce the operating noise of a transformer, various active and passive devices are known. In the DE 40 21 860 C2 For example, it is proposed to counteract the noise emission already in its origin, namely to directly combat the magnetic effect of the injected DC component. For this purpose, an additional winding is attached to the transformer, a so-called compensation winding. In this compensation winding, which usually has only a few turns, a compensation current is fed, which is directed in its magnetic action so that it is directed in the core of the transformer to the magnetic flux of the interfering DC component. The adjustment of the injected direct current takes place in accordance with an adjuster or a control device in conjunction with an associated measuring sensor, eg a microphone. However, such a measuring device does not meet the requirements for reliability and the lowest possible maintenance required, as they are made for transformers in an energy distribution network today.

In order to capture the DC component in the core of a transformer as reliable as possible, is in the unpublished PCT / EP2010 / 054857 proposed a sensor device which in the manner of a "magnetic bypass" works: by means of a ferromagnetic shunt part of a part of the main magnetic flux is branched off at the transformer core and fed back downstream. From this branched and guided in the shunt to the core flux fraction is determined either directly, or indirectly from a derived physical variable, the magnetic field strength in the bridged by the shunt branch core section. This detection of the magnetic field strength, or the magnetic excitation, is more reliable and more suitable for long-term use.

Also from the WO 2004/013951 A2 For example, a semiconductor switching unit is known, by means of which in a compensation winding of a transformer for the purpose of DC minimization, a compensation current is fed. From a control device with independent power source, a controllable frequency for the current flow duration of the semiconductor switch (MOSFET) is specified. The electrical energy for generating the compensation current is taken from a capacitor which is charged cyclically via the freewheeling circuit of the MOSFET. For transformers, as used in an energy distribution network, but a capacitor is not desirable as energy storage for reasons of reliability and because of the desired low-maintenance long-term operation.

Presentation of the invention

It is an object of the present invention to provide an apparatus and a method for reducing a direct flux component of a magnetic flux in a core of a transformer, which are more suitable in practical use for transformers in an energy distribution network. Furthermore, the invention relates to a method for converting a transformer.

This object is achieved with respect to a device having the features of patent claim 1 and with respect to a method having the features of patent claim 9. Furthermore, the object is achieved by a method for converting a transformer having the features of claim 16. Advantageous embodiments, aspects and details of the invention will become apparent from the dependent claims, the description and the accompanying drawings.

The invention is based on the idea to use the voltage induced in the compensation winding voltage and to use for the compensation of the disturbing magnetic DC component. According to the invention, a compensation current is generated by means of an electronic switching unit, wherein the switching-on of the switching unit is synchronized with the network and according to a predetermined switching strategy. According to the invention, the switch-on time is triggered by the phase of the voltage induced in the compensation winding, and the switch-on time depends on a sensor signal provided by a measuring device. In this way, a sinusoidally pulsating direct current is fed into the compensation winding whose size is limited by a current-limiting device. An energy source, that is a battery or a capacitor, is not required for the generation of this pulsating direct current. The current flow duration of this pulsating direct current can be set in a simple manner and very precisely in accordance with the supplied sensor signal, which predetermines the direction and magnitude of the DC component to be compensated. The mean value of this pulsed direct current generated in the soft magnetic core of the transformer, a reduction of the DC component, or completely eliminates its effect in the core. As a result, it no longer comes to undesirable asymmetric modulation of the soft magnetic core. As a result, the thermal load of the winding of the transformer is lower. When operating the transformer are losses and Noise lower. The device can be realized with comparatively simple means. Both discrete and or programmable devices can be used and are commercially available. Of great advantage is that no energy storage, such as a battery or a capacitor, is required for the generation of the compensation current. The energy for generating the compensation current is taken directly from the compensation winding. Due to its simplicity, the reliability of the circuitry is high. It is well suited for the low-maintenance long-term operation of a transformer in an energy distribution network. The application area includes both transformers in the low or medium voltage range, as well as transformers of very high power. Neither the size nor safety-relevant devices or other design criteria of the transformer are adversely affected by the use of the invention.
The control device consists essentially of two function blocks, a phase detector and a timer. The phase detector detects the zero crossing of the voltage induced in the compensation winding and supplies the trigger signal for the switch-on time of the time interval whose duration is predetermined in accordance with the sensor signal.

Of particular advantage may be, if, for the purpose of current limiting in the current path in series with the switching unit and the compensation winding, an inductance is arranged. The advantage of using an inductance in the current path is already due to the fact that the coil current of the compensation winding corresponds to the time integral of the coil voltage and thus by a suitable control strategy over a period in a simple manner equal proportions of this voltage integral and thus the coil current can be achieved. With appropriate choice of inductance, the load when switching on can be very low are held, since the temporal change of the current is limited at startup by the inductance. In principle, another two-pole could be used instead of the inductance. Circuit technology would also be an ohmic resistance conceivable, but its losses would be a disadvantage.

Another protective measure for protecting the switching device against inductive voltage peaks may be that an overvoltage protection is provided in parallel with the series connection of inductance and switching unit in a parallel circuit branch.

In a very particularly preferred embodiment, the switching unit is formed from at least one thyristor. The advantage of using a thyristor is first that a thyristor with a current pulse "ignited", that can be brought into the conductive state. During the positive half-wave of the mains voltage, the thyristor has the property of a diode until the next current zero crossing. The end of the current flow time is effected by the thyristor itself by the holding current is exceeded and the thyristor automatically "clears", that is, goes into the non-conductive state. Of course, other semiconductor switches, such as GTO, IGBT transistors or other switching elements are conceivable.

In order to be able to feed a DC current in both directions of current in the compensation winding, different circuit variants are possible. It could be used in combination with a unipolar semiconductor switch, or a winding with bipolar semiconductor switches, two oppositely wound compensating windings. Basically, a Umposchaltung could be used. However, a particularly simple implementation can be achieved by an anti-parallel connection of two switching units, in particular two antiparallel thyristors. It can be advantageous if a switch for switching on and off as well as a fuse limiting the current flow are provided in series in the current path. As a result, the compensation device can be activated or deactivated. In the event of an error, the fuse ensures that an excessively high current is limited.

It may be favorable if the switching unit and the control device are arranged outside the boiler of a transformer. The entire electronic circuit is thus accessible from the outside for inspection and maintenance.

A very particularly preferred embodiment of the invention can consist in that the measuring device for detecting the magnetic DC component comprises a magnetic shunt part with a sensor coil. The shunt part is connected to the core of the transformer e.g. is disposed adjacent to a leg or yoke to bypass a portion of the magnetic flux. From this, guided in the shunt magnetic flux can be obtained by means of a sensor coil very easily a long-term stable sensor signal, which if necessary after a signal processing the DC component shares very well. The measurement result is largely free of drift and for long-term stability. Since this detector consists essentially of the shunt part and the sensor coil arranged thereon, it has a high reliability.

The object stated in the introduction is also achieved by a method which is characterized in that the switch-on time of the switching unit is synchronous with the voltage induced in the compensation winding and in accordance with a sensor signal, wherein the sensor signal from a measuring device for detecting the magnetic DC component of the Control device is supplied. Such a thing In terms of circuitry, the method can be implemented very simply with a few components.

The switching unit is controlled with a manipulated variable which is predetermined by a timer present in the control device, the timer being triggered by a phase detector which detects the phase of the voltage induced in the compensation winding. The timer may be a discrete device, or part of a digital circuit. It may be advantageous if the manipulated variable is the result of an arithmetic operation of a microprocessor. The microprocessor can be used at the same time for signal processing of the sensor signal.

In a particularly preferred embodiment, the switching unit is driven so that in the compensation winding, a pulsating direct current is fed. This has the advantage that the arithmetic mean of this pulsating direct current can be specified very simply in accordance with the DC component to be compensated. For the purpose of reducing the magnetic energy stored in the inductance, the electronic switching unit remains meaningfully switched on until the pulsating direct current has decayed. Thus, overvoltage protection after turning off the electrical switching unit does not in fact absorb any residual magnetic energy stored in the coil.

Furthermore, a method for converting a transformer is given to achieve the above object. The device according to the invention, or the method according to the invention, can also be used advantageously in already operating transformers. The effort is very low. A conversion is particularly easy if a already arranged in the transformer tank compensation winding according to the present invention can be used. In this case, the transformer tank does not need to be opened, but the device according to the invention can only be connected to the already led out terminals of the compensation winding.

Brief description of the drawing

Figur 1

ein Ausführungsbeispiel der erfindungsgemäßen Vorrichtung, dargestellt in einer vereinfachten Skizze;

Figur 2

eine Darstellung des zeitlichen Verlaufs der in der Kompensationswicklung induzierten elektrischen Spannung des Kompensationsstroms;

Figur 3

eine Darstellung des Kompensationsstroms als Funktion der Stellgröße;

To further explain the invention, reference is made in the following part of the description to the drawings, from which further advantageous embodiments, details and further developments of the invention can be found by way of non-limiting embodiment.
Show it:

FIG. 1

an embodiment of the device according to the invention, shown in a simplified sketch;

FIG. 2

a representation of the time course of the induced voltage in the compensation winding of the compensation current;

FIG. 3

a representation of the compensation current as a function of the manipulated variable;

Embodiment of the invention

The FIG. 1 shows a device 1 according to an embodiment of the invention in a simplified representation. The device 1 consists essentially of a circuit arrangement which is connected via the terminals K1 and K2 to a compensation winding arrangement K. The Compensation winding assembly K is housed in the transformer tank 12 and magnetically coupled to the core 4 of the transformer. It usually consists only of a winding with few turns, which is wound for example around a leg or a yoke part of the transformer. From the compensation winding K in the transformer tank 12, the terminals at the terminals K1 and K2 are led out into the outer space 13.

During operation of the transformer an electrical voltage is induced in the compensation winding K, which is used according to the invention to combat the disturbing DC component of the magnetic flux in the core 4. This is done by network-controlled switching a switching unit T.

The following explains in more detail how the in FIG. 2 illustrated course of the compensation current is generated:

As from the representation of FIG. 1 can be seen, the terminals K1 and K2 of the compensation winding K are connected to a control device 2. The control device 2 consists essentially of a phase detector P and a timer TS. The phase detector P, for example a zero-crossing detector, derives from the induced voltage a trigger signal 8, which is fed to a timer TS. Together with a likewise supplied to the control device 2 control signal 6, the control device 2 on the output side a manipulated variable 9 ready, which is fed to an electronic switching unit T. The switching unit T is in a current path 3 in series with the compensation winding K and in series with an inductance L. The inductance L is so dimensioned that when switching the switching unit T flowing in a current direction, sinusoidally pulsating current waveform in the compensation winding K is fed.

In the current path 3, a fuse Si is provided for the purpose of current limitation. This fuse Si is in FIG. 1 placed between the terminal K1 and a switch S. The switch S serves to close or disconnect the current path 3.

According to the invention, the switching of the electronic switching unit T is carried out in phase synchronism with the voltage in the compensation winding K and according to a predetermined switching strategy. That is, depending on the size and direction of the compensation current to be introduced, the switch-on time is controlled by means of the controlled by the phase detector P timer TS according to a functional relationship explained in more detail below such that the resulting arithmetic mean of the pulsating current in the compensation winding K by its effect the disturbing DC Reduced share or this fully compensated.

The information regarding size and direction of the DC field component to be compensated in the core 4 is obtained by the control device 2 from a measuring device 7 for measuring the DC component. This provides the sensor signal 6, which is supplied to the control device 2. With particular advantage, the measuring device 7 operates according to the above-quoted measuring principle of the magnetic bypass ( PCT / EP2010 / 054857 ). That is, it consists essentially of a magnetic shunt portion disposed on the core to conduct a portion of the magnetic flux in a bypass, from which then, for example, a sensor coil arranged at the shunt portion in connection with signal conditioning the DC component can be determined.

Turning off the electronic switching unit T takes place at zero crossing of the current (see FIG. 2 ). This point in time can be determined very simply, since the current flow duration 16 is twice the manipulated variable x (signal 9 in FIG FIG. 2 ) corresponds. It is thereby achieved that the overvoltage protection V provided in the parallel branch 5 must absorb only a small residual magnetic energy when switched off. The switching losses of the electronic switching unit are minimal, since when switching, due to the inductance L in the current path 3, the inrush current is low; even when switching off the switching losses are low, since the switch-off is set so that it takes place at zero crossing or at least near zero current in the current path 3.

The arithmetic mean of the compensation current I _GL is thus determined solely by the predetermined by the manipulated variable switch-on. Thyristors are particularly suitable as a switch for the switching unit T, as they inherently go on reaching the de-energized state, more precisely, when falling below the so-called holding current, by itself again in the non-conductive state.

By the switching-on time is predetermined by the signal 9 and takes place synchronously to the network, and by turning off the switching unit T is performed at zero crossing of the current, the arithmetic mean of the compensation current I _{GL is} very precisely adjusted by the manipulated variable x and the manipulated variable signal 9.

The FIG. 2 shows the time course of the voltage induced in the compensation winding K 10 and the predetermined by the switching strategy according to the invention pulsating DC 11 (compensation current I _GL ). The compensation current I _GL has the form of juxtaposed sinusoidal half-waves 18 which are interrupted by current gaps 17, each half-wave 18 being symmetrical to half the period T / 2 of the induced voltage 10. The switch-on 14 is set as described above in synchronism with the network and in accordance with the manipulated variable 9. The synchronization point for switching on is in FIG. 2 By appropriate choice of the inductance L follows after switching through the switching unit T, the current in current path 3 the integral of the electrical voltage 10, that is, it has at the zero crossing of the electrical voltage 10 its maximum value and then stops again , When the compensation current 11 is almost zero, the switching unit T, eg a thyristor, transitions to the non-conductive state. The current flow time 16 is determined by the manipulated variable 9 or by the deletion of the thyristor. Each half-waves 18 follows a current gaps 17th

In order to predetermine in the winding K a compensation current I _GL in both directions, is in FIG. 1 in a broken line, a second switching unit T 'indicated. The two switching units T and T 'can be, for example, two anti-parallel connected thyristors.

There is a non-linear relationship between the generated compensation current I _GL and the manipulated variable x FIG. 3 is shown graphically and explained in more detail below:

In the following discussion it is assumed that the ohmic resistance of the coil can be neglected.

Thus, for the functional relationship between coil current I _L (t) and coil voltage U _L (t) approximately: $$\left[{\mathrm{I}}_{\mathrm{L}}\left(\mathrm{t}\right)-{\mathrm{I}}_{\mathrm{L}}\left(\mathrm{t}=\mathrm{0}\right)\right]=\left[\mathrm{1}/\mathrm{L}\right]\mathrm{,}\left[\int {\mathrm{I}}_{\mathrm{L}}\left(\mathrm{t}\right)\mathrm{,}\mathrm{dt}\right]$$

• T := Periodendauer der Spannung an der Kompensationswicklung [s]
• Û := Scheitelwert der Spannung an der Kompensationswicklung [V]
• L := Induktivität der Spule [H]
• x := Stellgröße in Prozenten [%]

und sei ferner die Zeit t definiert durch: $$t=x\mathrm{.}\frac{\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">T</figure-callout>}}{2}\cdot \frac{1}{100}$$

dann folgt für den maximal erreichbaren arithmetischen Mittelwert (Gleichanteil) des Spulenstromes bzw. des Kompensationsstromes I_MAX bei einer Stellgröße von 100 Prozent $$I_{\mathit{MAX}}=\frac{\hat{U}}{L}\mathrm{.}\frac{\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">T</figure-callout>}}{2\pi }\mathrm{.}$$

Now be:

• T: = period of the voltage at the compensation winding [s]
• Û: = peak value of the voltage at the compensation winding [V]
• L: = inductance of the coil [H]
• x: = manipulated variable in percent [%]

and let the time t be defined by: $$t=x\mathrm{,}\frac{\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">T</figure-callout>}}{2}\cdot \frac{1}{100}$$

then follows for the maximum achievable arithmetic mean (DC component) of the coil current or the compensation current I _MAX at a control value of 100 percent $$I_{\mathit{MAX}}=\frac{\hat{U}}{L}\mathrm{,}\frac{\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">T</figure-callout>}}{2\pi }\mathrm{,}$$

For the arithmetic mean (DC component) of the coil current or compensation current I _GL [A] as a function of the manipulated variable x [%], after some intermediate calculation: $${\mathit{I}}_{\mathit{GL}}={\mathit{I}}_{\mathit{MAX}}\frac{T\mathrm{<figure-callout\; id="2"\; label="sin"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">sin</figure-callout>}\left(\frac{2\pi t}{T}\right)-2\pi t\mathrm{<figure-callout\; id="2"\; label="cos"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">cos</figure-callout>}\left(\frac{2\pi t}{T}\right)}{\pi \mathrm{,}T}$$

For the effective value of the fundamental component I _GW [A _EFF ] contained in the compensation current signal as a function of the manipulated variable x [%] follows: $${\mathit{I}}_{\mathit{GW}}={\mathit{I}}_{\mathit{MAX}}\frac{T\sin \left(\frac{4\pi t}{T}\right)-4\pi \mathrm{<figure-callout\; id="2"\; label="t"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">t</figure-callout>}}{2\mathit{\pi T}\sqrt{2}}$$

Mit : k ∈ N und k≥2In addition, for the rms value of the spectral component I _0W [A _EFF ] of the (k) th harmonic contained in the compensation _{current signal} , the following applies as a function of the manipulated variable x [%]: $${\mathit{I}}_{\mathit{OW}}={\mathit{I}}_{\mathit{MAX}}\frac{\cos \left(k\pi \right)\left[\left(1+k\right)\mathrm{<figure-callout\; id="2"\; label="sin"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">sin</figure-callout>}\left(\frac{2\pi t\left(k-1\right)}{T}\right)-\left(k-1\right)\mathrm{<figure-callout\; id="2"\; label="sin"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">sin</figure-callout>}\left(\frac{2\pi t\left(k+1\right)}{T}\right)\right]}{k\left(k^2-1\right)\pi \sqrt{2}}$$

With: k ∈ N and k ≥2

FIG. 3 shows the functional relationship between the compensation current I _GL (based on the maximum achievable compensation current I _MAX at 100 percent) as a function of the manipulated variable according to equation (4).

If the size and direction of the DC component to be compensated is known (sensor signal 6), the control device determines according to the above illustration or the in FIG FIG. 3 shown relationship, the manipulated variable x required for compensation (signal 9). As a result, in a simple manner in a transformer, the thermal load of the winding and the disturbing emission be reduced by noises. The above-described electronic circuit can be constructed floating. As a result, no insulation problems occur even in the application of high mains voltages.

Compilation of AI used correct reference numerals

1

circuit device

2

control device

3

current path

4

magnetic core of the transformer

5

parallel branch

6

sensor signal

7

Measuring device for detecting the DC component

8th

trigger signal

9

Signal manipulated variable x

10

time course of the electrical voltage at the compensation winding

11

time course of the compensation current I _GL in the current path 3

12

Boiler room transformer

13

outer space

14

switch-on

15

off time

16

Conducting period

17

current gaps

18

half-wave

L

Kitchen sink

T

Switching unit, thyristor

V

Overvoltage protection

TS

timer

P

phase detector

K

compensation winding

S

switch

Si

fuse

I _GL

compensating current

K1

terminal

K2

terminal

X

manipulated variable

Claims (16)

1. Device for reducing a magnetic unidirectional flux fraction in the core of a transformer, said device comprising:

   - a measuring device (7), providing a sensor signal (6) corresponding to the magnetic unidirectional flux fraction,

   - a compensation winding (K) which is coupled magnetically to the core (4) of the transformer,

   - a switching unit (T) which is arranged electrically in a path (3) of the current in series with the compensation winding (K), in order to feed a current into the compensation winding (K), the action of said current being in an opposite direction to the unidirectional flux fraction, wherein the switching unit (T) can be controlled by means of a control variable (9) provided by a control device (2),

   - characterised in that

   - the switching unit (T) can be switched into a conducting state during a predetermined time interval (16), the turn-on time (14) of said time interval (16) taking place in a manner synchronous to the mains, i.e. phase-synchronously to the voltage in the compensation winding (K) and in accordance with the control variable (9),

   - a device for current limitation (L) is provided in the path (3) of the current,

   - the control device (2) comprises a device (P) for detecting the voltage phase in the compensation winding (K) and a timer device (TS) triggered by said compensation winding (K) for specifying the time interval (16),

   - and that the sensor signal (6) is supplied to the control device (2).
2. Device according to claim 1, characterised in that the current limiting device is formed by an inductance (L) connected in series in the path (3) of the current to the compensation winding (K) and to the switching unit (T).
3. Device according to one of claims 1 to 2, characterised in that the switching unit (T) is controlled in such a way that the current (I_GL) flowing in the path (3) of the current is a pulsating d.c. current and switches off the switching unit (T) when the current (I_GL) in the path (3) of the current is zero or almost zero.
4. Device according to claim 3, characterised in that the pulsating d.c. current (I_GL) is formed of periodically recurring half-waves (18) and of current gaps (17) connecting adjacent half-waves (18).
5. Device according to one of claims 1 to 4, characterised in that the switching unit (T) is formed of at least one semiconductor switch, preferably of at least one thyristor, GTO or IGBT.
6. Device according to claim 5, characterised in that the switching unit (T) is formed of two thyristors connected in anti-parallel arrangement.
7. Device according to one of claims 1 to 5, characterised in that a fuse (Si) and a switch (S) are arranged in the path (3) of the current.
8. Device according to one of claims 1 to 7, characterised in that for detecting the magnetic unidirectional flux fraction, the measuring device (7) comprises a magnetic shunt part with a sensor coil, said shunt part being arranged at the core of the transformer, in order to guide part of the magnetic flux as a by-pass and the sensor signal is derived or formed from the voltage induced in the sensor coil.
9. Method for reducing a magnetic unidirectional flux fraction in the core of a transformer, wherein by means of a switching unit (T), which is controlled by a control device (2), a compensation current (I_GL) is fed into a compensation winding (K) coupled to the core (4), the action of said compensation current in the core being in a direction opposite to the unidirectional flux fraction, wherein the switching unit (T) is arranged in a path (3) of the current in series with the compensation winding (K), wherein the current flowing in the path (3) of the current is restricted by means of a current limiting device (L),
   wherein

   - the switching unit (T) is connected synchronously to the voltage induced in the compensation winding (K) and is switched on at a turn-on time (14) according to a sensor signal (6),

   - the switching unit (T) is controlled by a control variable (9), which is predetermined by a timer (TS) present in the control device (2), the timer (TS) being triggered by a phase detector (P), which phase detector (P) detects the phase of the voltage induced in the compensation winding (K), and

   - the sensor signal (6) is provided by a measuring device (7) for detecting the magnetic unidirectional flux fraction and supplying it to the control device (2).
10. Method according to claim 9, characterised in that the current limiting device is formed by an inductance (L) in the path (3) of the current connected in series to the compensation winding (K) and to the switching unit (T).
11. Method according to claim 9 or 10, characterised in that the switching unit (T) is controlled by a control variable (9), which control variable is predetermined by a timer (TS) present in the control device (2), wherein the timer (TS) is triggered by a phase detector (P), which phase detector (P) detects the phase of the voltage induced in the compensation winding (K).
12. Method according to claim 9, 10 or 11, characterised in that the switching unit (T) is controlled, such that a pulsating direct current (11) is fed into the compensation winding (K).
13. Method according to claim 12, characterised in that the pulsating direct current (11) is formed by periodically recurring sinusoidal half-waves (18) and current gaps (17) lying therebetween.
14. Method according to claim 13, characterised in that the switching unit (T) is switched off at the end of a half-wave in a currentless or almost currentless state.
15. Method according to one of claims 9 to 14, characterised in that the switching unit (T) comprises at least one thyristor and the switching off is predetermined by the current falling below the holding current of the at least one thyristor.
16. Method for equipping a transformer, wherein a compensation winding (K) coupled magnetically to the core (4) of the transformer is connected to a device according to one of claims 1 to 8 or the method defined in claims 9 to 15 is used in connection with the compensation winding (K).

Images