EP2156448B1 - Electrical transformer with unidirectional flux compensation - Google Patents

Description

Technical area

The invention relates to an electrical transformer with DC compensation.

State of the art

It is known that in an electrical transformer operated in conjunction with a power converter, due to inaccuracies in the driving of the power semiconductor switches, a current component may arise which is superimposed on the operating current of the transformer. This current component, which can be regarded as DC with respect to the network, is also referred to below as "DC component" or "DC component". It is usually only a few parts per thousand of the rated transformer current, but causes in the core of the transformer a magnetic direct flux, which is superimposed on the primary or secondary alternating flux and causes an asymmetrical modulation of the BH characteristic of the ferromagnetic core material. Even a small proportion of direct current can cause a saturation of the core due to the high permeability of the ferromagnetic core material and result in strong distortions of the magnetizing current. The geostationary magnetic field can also contribute to a DC component in the nucleus. The consequence of this asymmetrical modulation are increased magnetic Losses and thus increased heating of the core, as well as magnetizing current peaks, which cause an increased emission of operating noise.

The undesirable saturation effect could basically be counteracted by increasing the cross section of the magnetic circuit and thus keeping the magnetic flux density B lower, or by inserting a (replacement) air gap into the magnetic circuit, as in the US Pat DE 198 54 902 A1 proposed. But the former leads to an increased construction volume of the transformer, the latter to a larger magnetizing current; both are disadvantageous.

In order to reduce the noise emission of an electric transformer, in US 5,726,617 and in the DE 699 01 596 T2 each proposed actuators, which excite the oil in a transformer housing so that the fluid pressure waves that emanate from the laminated core of the core and the transformer windings during operation of the transformer, are attenuated. However, these actuators consume a not inconsiderable amount of energy during operation; They are also prone to failure and consuming.

From the JP 59 013313A is an electrical transformer with Gleichflusskompensation according to the preamble of claim 1, in which the magnetic field in the core of the transformer is measured and from a compensation current is derived.

Presentation of the invention

It is an object of the present invention to further develop the prior art.

The solution of this object is achieved by the features of claim 1. Advantageous embodiments of the invention are defined in the dependent claims.

• Der Transformator weist einen weichmagnetischen Kern auf, auf dem zusätzlich zu einer primären und einer sekundären Wicklungsanordnung eine Kompensations-Wicklungsanordnung angeordnet ist.
• Die Kompensations-Wicklungsanordnung ist mit einer Strom-Steuereinrichtung verbunden, welche nach Maßgabe einer Steuergröße, die eine Magnetfeld-Messeinrichtung aus einer Messung eines mit einem Strom in der primären oder sekundären Wicklungsanordnung verketteten magnetischen Flusses bereit stellt, in die Kompensations-Wicklungsanordnung einen Kompensationsstrom so einspeist, dass dessen Wirkung im Kern einem magnetischen Gleichfluss entgegen gerichtet ist.

The invention is based on the idea not to combat the unwanted effects of the bias, but to eliminate their cause. The transformer according to the invention is characterized as follows:

• The transformer has a soft magnetic core on which a compensation winding arrangement is arranged in addition to a primary and a secondary winding arrangement.
• The compensation winding arrangement is connected to a current control device which, in accordance with a control variable, which provides a magnetic field measuring device from a measurement of a magnetic flux linked to a current in the primary or secondary winding arrangement, into the compensation winding arrangement a compensation current feeds in that its effect in the core is directed against a magnetic direct flux.

This ensures that a magnetic DC component in the core of a transformer, can be detected in a simple manner by measurement and compensated by a Ausregelungsvorgang. When the DC component is eliminated, the modulation of the BH characteristic is symmetrical. The ferromagnetic material of the core is no longer driven into saturation. As a result, the magnetostriction of the material is lower, as a result of which the emission of operating noise also decreases. The transformer windings are less thermally stressed, as the magnetic losses and thus the operating temperature in the core are lower.

According to the invention, the specification of the compensation current in the compensation winding takes place in accordance with a magnetic field measured variable which supplies a magnetic field measuring device. For determining the magnetic field measured quantity, known magnetic field sensors are suitable, which either measure the field in the core of the transformer, or the stray magnetic field, which closes outside the core via the air path. The basic operating principle of these sensors can be, for example, the induction in a measuring coil, the Hall effect or the magneto-resistive effect. The magnetic field measured variable can also be determined by using a magnetometer (fluxgate or Förster probe). In comparison to an accurate measurement of the DC component (which is much smaller than the rated current, in particular in the case of a large transformer, and is therefore difficult to detect), the metrological outlay for determining the magnetic field measured variable is lower. According to the invention, the magnetic field measuring device is formed from a signal processing unit which is signal-conducting with at least two magnetic field detectors. In a three-phase transformer of conventional design, the determination of two DC components may be sufficient, since the total flux must be zero. In this case, the signal processing unit is set up to determine harmonics from a respective measurement signal provided by the magnetic field detector and to form the control signal therefrom. As a result, with a comparatively low circuit complexity, one suitable for compensating the DC component can be used Control variable to be won. The harmonic analysis can be done electronically or computer-aided.

In this case, even-numbered harmonics are particularly suitable, according to the invention the first harmonic (2nd harmonic) whose amplitude is functionally related to the magnetic direct flux which it is to be compensated for. According to the invention, two magnetic field detectors are arranged outside the core so that they detect a leakage flux of the transformer. The stray flux increases very strongly in the case of the magnetic saturation of the core, which is favorable for the determination of the control signal.

The magnetic field detector is simply designed as an induction probe, which detects the leakage flux change and converts it into an electrical measurement signal, from which the even harmonics, according to the invention the second harmonic, can be filtered out. According to the invention, the induction probe is designed as an air-core coil. Compared to a semiconductor-based transmitter, the electrical measurement signal from this air-core coil is independent of long-term and temperature drift and is also cost-effective.

In order to minimize the effects of the network on the compensation winding, it may be favorable if a blocking circuit (according to the invention a reactance dipole) is connected in the current path to the current control device. This allows the voltage burden of the controlled current source, which feeds the compensation current into the compensation winding, be kept low. Suitable for this purpose, for example, a two-pole network, for example, formed from an LC parallel circuit, which blocks the mains frequency, but hardly represents a resistance with respect to the compensation DC.

A favorable spatial arrangement of the magnetic field detector is most easily done by trial or numerical field simulation. Particularly favorable is a measuring location at which the magnetic fields caused by the primary and secondary load currents largely compensate each other. According to the invention this is an arrangement in which an air coil in a gap formed of an outer peripheral surface of a transformer leg and the concentrically enclosing compensating winding or secondary winding, approximately in the middle leg height, is arranged.

A preferred arrangement of the compensation winding may be the yoke in a three-arm transformer or the yoke in a five-arm transformer; As a result, a compensation winding can be retrofitted to an existing transformer in a simple manner.

Brief description of the drawings

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.

Figur 1

einen erfindungsgemäßen Drehstromtransformator (Dreischenkel-Transformator) mit Gleichfluss-Kompensation, bei dem die Kompensations-Wicklungsanordnung auf den Hauptschenkeln angeordnet ist;

Figur 2

einen erfindungsgemäßen Drehstromtransformator (Dreischenkel-Transformator) mit Gleichfluss-Kompensation, bei dem die Kompensations-Wicklungsanordnung auf dem Joch angeordnet ist;

Figur 3

einen erfindungsgemäßen Drehstromtransformator mit Gleichfluss-Kompensation, bei dem die Kompensations-Wicklungsanordnung auf einem Rückschlussjoch angeordnet ist;

Figur 4

einen erfindungsgemäßen Drehstromtransformator (Fünfschenkel-Transformator) mit Gleichfluss-Kompensation, bei dem die Kompensations-Wicklungsanordnung auf den Hauptschenkeln angeordnet ist;

Figur 5

ein Blockschaltbild der erfindungsgemäßen Signalaufbereitung zur Ausregelung der Gleichfluss-Komponente;

Figur 6

ein Blockschaltbild eines Messversuchs, zur Messung des Gleichfluss-Anteils an einem 4-MVA Leistungstransformator, wobei die Signalaufbereitung gemäß Figur 5 verwendet wird;

Figur 7

ein Diagramm, das als Ergebnis des Messversuchs gemäß Figur 6 den linearen Zusammenhang zwischen DC-Anteil und 2. Harmonischer bei einer Primärspannung von 6 kV zeigt;

Figur 8

ein Diagramm, das als Ergebnis des Messversuchs gemäß Figur 6 den linearen Zusammenhang zwischen DC-Anteil und 2. Harmonischer bei einer Primärspannung von 30 kV zeigt.

Show it:

FIG. 1

a three-phase transformer (three-arm transformer) according to the invention with DC compensation, wherein the compensation winding assembly is arranged on the main legs;

FIG. 2

a three-phase transformer (three-arm transformer) according to the invention with DC compensation, wherein the compensation winding assembly is disposed on the yoke;

FIG. 3

a three-phase transformer according to the invention with DC compensation, wherein the compensation winding assembly is disposed on a yoke yoke;

FIG. 4

a three-phase transformer (five-limb transformer) according to the invention with DC compensation, wherein the compensation winding assembly is disposed on the main legs;

FIG. 5

a block diagram of the signal processing according to the invention for regulating the DC component;

FIG. 6

a block diagram of a measurement experiment, for measuring the DC component of a 4-MVA power transformer, the signal processing according to FIG. 5 is used;

FIG. 7

a diagram as a result of the measurement experiment according to FIG. 6 the linear relationship between DC component and 2nd harmonic at a primary voltage of 6 kV;

FIG. 8

a diagram as a result of the measurement experiment according to FIG. 6 shows the linear relationship between the DC component and the second harmonic at a primary voltage of 30 kV.

Embodiment of the invention

In the FIG. 1 an electrical transformer 20 with a housing 7 can be seen, which has a transformer core 4. The design of the core 4 corresponds to the known three-limb design with three legs 21, 22, 23 and a transverse yoke 32. On each of the legs 21, 22, 23 is as usual a primary winding 1 and a secondary winding. 2 ,

According to the invention, a compensation winding 3 is additionally provided on the outer legs 21 and 23. In the drawing of the FIG. 1 is in the region of the first leg 21 with an arrow 5, a magnetic "DC" indicated. This magnetic "direct current" 5 is assumed to be caused by a "direct current component" (DC component) flowing on the primary side or the secondary side. The "direct flow" can also be interspersed by the earth's magnetic field. By "direct current" or "direct current" is here to be understood a physical quantity, which, seen in time compared to 5o Hz cycles, varies only very slowly, if this is the case at all. This magnetic flux 5, which is superimposed on the alternating flux in the leg 21, causes a bias, which is an asymmetrical modulation of the magnetic Material and thus causes an increased noise emission. For compensation according to the invention this DC component are in FIG. 1 two controlled current sources 12 and 13 are provided. These current sources 12, 13 respectively feed a compensating current 16 or 17 in the sense of a compensation into an associated compensation winding 3, whose magnitude and direction are dimensioned such that the magnetic direct flux 5 in the core 4 is compensated. (In the FIG. 1 This is indicated by means of the control signals 14, 15, which are supplied as control variable to the current sources 12 and 13 by means of the lines 9, 10.

The control variables 14, 15 provide a signal processing unit 11, which will be explained in more detail below.

Like in the FIG. 1 can be seen, between the compensation winding 3 and an outer leg 21 and 23 of the core 4 each approximately centrally a magnetic field detector 8 is arranged. Each of these magnetic field detectors 8 is located outside the magnetic circuit and measures a stray field of the transformer 20. In the stray field, in particular, that half-wave of the magnetizing current occurs, which is controlled to saturation, so that the DC component in the core can be determined well. The measuring signal of the detectors 8 is fed to the signal processing unit 11 by means of the lines 9, 10.

In the present example, the two magnetic field detectors 8 each consist of a measuring coil (several hundred turns, diameter about 25 mm). Already two detectors 8, as shown in the present example of a three-arm transformer, may be sufficient, since the sum of the DC components over all legs must be zero.

As already mentioned above, a multiplicity of sensor principles is fundamentally possible for the magnetic field measurement. It is only decisive that a magnetic field characteristic of the transformer is measured, from which the DC component or the DC component can be determined by means of signaling technology and subsequently corrected.

The FIG. 2 differs from FIG. 1 merely in that here the compensation winding arrangement 3 is not arranged on a main leg 21, 22, 23 but on the yoke 32 of the core 4. At each main leg 21, 22, 23 is again in a gap between the core 4 and the secondary winding 2, a magnetic field detector 8 is arranged (here for redundancy reasons a total of three).

The FIG. 3 shows a five-limb transformer, in which at each conclusion legs 31 each have a compensation winding 3 is arranged. In this structure, the core flux does not split in half when entering the yoke to two sides; on the basis of the law of continuity, the respective direct flow component flowing back from the return leg 31 must correspond to the direct flow in the main legs 21, 22, 23, so that each return leg 31 carries 1.5 times the direct flow component. Each leg 21, 22, 23 is again associated with a magnetic field detector 8 arranged outside the core 4. Each measurement signal of these three magnetic field detectors 8 is again supplied to the signal processing unit 11, which provides the output side, the control variables 14, 15 for the controlled current sources 12 and 13, so that the compensation current 16 and 17 can compensate for the DC component in the yoke legs 31.

In the FIG. 4 is a variant of the embodiment according to FIG. 3 shown. Here are the compensation windings 3 on the main legs 21, 22 and 23. Each of these compensation windings 3 is again assigned to one of three current control device. The specification of the compensation current takes place as described above by the signal processing unit 11.

The FIG. 5 shows in a schematic block diagram a possible embodiment of the signal processing unit 11, which acts as a DC compensation controller. As already stated above, the signal processing unit 11 determines the second harmonic from the spectrum of the harmonics, which is a direct image of the DC component.

This is explained in more detail below on the basis of the illustrated functional blocks: A sensor coil 8 detects leakage flux of the transformer 20. The measuring signal of the sensor coil 8 is supplied to a differential amplifier 19. In the illustrated signal path following the output signal of the differential amplifier 19 reaches a notch filter 24, which filters out the fundamental (50 Hz component). Through a low pass 25 and a bandpass 26, the measurement signal is applied to an integrator 27. By integrating a, the magnetic flux change in the measuring coil 8 proportional voltage signal, which is a very selective bandpass filter 26 is supplied to the second harmonic, the DC Share figures, filter out. This voltage signal passes after a sample-and-hold circuit 28 and a low-pass filter 25 via line 16 to the controlled current source 12 with integrated control device. This current source 12 and control device is connected in a closed circuit 33 with a compensation winding 3. She gives in the Compensation winding 3 before a DC, which counteracts the DC component in the core 4. Since the direction of the DC component to be compensated is not known a priori, a bipolar current regulator, in the present experiment with IGBT transistors in a full bridge, is used. An integrator 27 causes a phase lag of 99 degrees with respect to the 2nd harmonic. The Reaktanzzweipol 18, consisting of a parallel resonant circuit, blocks the network feedback of the power-frequency components.

In the FIG. 5 is still an auxiliary winding to see 29, the signal is fed to the sample-hold circuit 28 via filters and rectification. It serves in the illustrated circuit for conditioning the scanning signal, so that a phase-related sampling of the second harmonic of the measuring signal is possible. At this point it should be noted that this sample and hold circuit ultimately only for the phase-related sampling of the provided by the induction probe 8 measuring signal (second harmonic 100 Hz) is used.

In the FIG. 5 illustrated signal processing shows only an example of a possible measurement method of the second harmonic. The expert expert has a number of analog and digital function blocks available for this purpose. For example, the current control variable 14, 15 could also be obtained by a suitable digital calculation method in a microcomputer or a freely programmable logic device (FPGA), which determines the second harmonic (100 Hz) from the Fourier transform.

In the FIG. 6 an experimental arrangement is shown in which the in FIG. 5 illustrated and discussed above signal conditioning unit 11 was used in a 4-MVA power transformer to determine the relationship between the DC component and the first harmonic (2nd harmonic) under real conditions by measurement. The 4 MVA power transformer in this experiment was idle at a primary voltage of 6 KV and 30 KV, respectively. In the star points of the primary or secondary winding arrangement ( FIG. 6 ) was fed by means of a current source, a DC component between 0.2 and 2 A. As a magnetic field detector 8 was a sensor coil with 200 turns, which was located outside the core of the transformer and detects the leakage flux.

In FIG. 7 and in FIG. 8 is in each case in a diagram, the measurement result of the experimental arrangement according to FIG. 6 logged. In the diagrams of FIGS. 7 and 8 the DC component (IDC) fed in the star point is plotted on the ordinate; on the abscissa the rms value of the first harmonic (U100Hz) is plotted. The diagram in FIG. 7 shows the connection at a primary voltage of 6 kV, that diagram in FIG. 8 effective at a primary voltage of 30 KV. The two diagrams in FIGS. 7 and 8 show that the relationship between the DC component (IDC) and the associated distortion (second harmonic U100Hz) can be considered with sufficient accuracy as linear.

As a result, this means that the characteristic value determined from a magnetic field measurement of a power transformer is very well suited to form a control variable that has a direct-current component, regardless of its cause, ie even if the earth's magnetic field is involved. metrologically detect and compensate, so that operating noise and heating of the transformer can be kept low.

Compilation of the reference numbers used

1

primary

2

secondary winding

3

compensation winding

4

Soft magnetic core

5

magnetic direct flow

6

magnetic compensation flux

7

transformer housing

8th

Magnetic field detector

9

Measuring line, signal

10

Measuring line, signal

11

Signal processing unit

12

Current control means

13

Current control means

14

control signal

15

control signal

16

compensating current

17

compensating current

18

Reaktanzzweipol

19

differential amplifier

20

transformer

21

first leg of the transformer

22

second leg of the transformer

23

third leg of the transformer

24

notch filter

25

lowpass

26

bandpass

27

integrator

28

Sample and hold circuit

29

auxiliary winding

30

Magnetic field measuring device

31

Inference leg

32

yoke

33

current path

Claims (3)

1. Electrical transformer with unidirectional flux compensation, with the following features:

   a) the transformer (20) has a soft-magnetic core (4) on which, in addition to a primary and secondary winding arrangement (1, 2), a compensation winding arrangement (3) is arranged,

   b) a magnetic field measuring device (30) measures a flux interlinked with a current in the primary or secondary winding arrangement and provides a control signal (14, 15),

   c) the control signal (14, 15) is fed to a current control device (12, 13),

   d) the current control device (12, 13) is connected to the compensation winding arrangement (3) via a current path (33) that contains a reactance dipole (18) and the current control device (12, 13) in accordance with a control signal (14, 15) feeds into said compensation winding arrangement (3) a compensation current (16, 17) in such a way that the effect of said compensation current in the core (4) is in a direction opposite to a magnetic unidirectional flux (5).

   e) the magnetic field measuring device (30) is formed from a signal processing unit (11) that is connected in a signal-conducting manner to at least two magnetic field detectors (8),

   f) the signal processing unit (11) is set up for ascertaining overtones from in each case one measurement signal provided by the magnetic field detector (8) in order to ascertain from said overtones the control signal (14, 15) for correctively adjusting the unidirectional flux (5),

   g) the control signal (14, 15) is formed from the first overtone (second harmonic),

   h) the core (4) has three limbs (21, 22, 23), at least two of which (21, 23) in particular are fitted with a compensation winding (3),
   characterised in that

   i) the magnetic field measuring device (30) measures the stray magnetic field that closes outside the core (4) via the air path in order to provide the control signal,

   j) each of the at least two magnetic field detectors (8) is arranged outside the core (4) for registering a stray flux of the transformer (20),

   k) each magnetic field detector (8) is embodied as an induction probe.

   l) each induction probe (8) is an air-cored coil,

   m) each air-cored coil (8) is arranged in a gap, formed from an outer circumferential surface and an enclosing compensation winding (3) or a winding (2), approximately at centre limb height.
2. Transformer according to claim 1, characterised in that the core (4) has three limbs (21, 22, 23) and two return limbs (31), on each of which a compensation winding (3) is arranged.
3. Transformer according to claim 1, characterised in that the compensation winding (3) is arranged on the yoke (32) of the transformer.

Images