RU2453010C2 - Electric transformer with constant flow compensation - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: electric transformer (20) with constant flow compensation has soft magnetic core (4) on which compensating winding (3) is arranged additionally to primary and secondary windings (1, 2). That winding is connected to current control device (12, 13) which according to control signal (14, 15) which represents measuring device (30) of magnetic field from measurement of magnetic flow connected to current in primary or secondary windings leads compensating current (16, 17) to compensation winding (3) so that its action in core (4) is directed opposite to magnetic constant flow (5).

EFFECT: easier reduction of core heating and emission of interference determined with constant magnetic flow in core.

9 cl, 8 dwg

Description

Technical field

The invention relates to an electric transformer with constant flow compensation.

State of the art

It is known that in an electric transformer that is operated in conjunction with a current rectifier, due to inaccuracies in the control of powerful semiconductor switches, a current component may occur that is superimposed on the operating current of the transformer. This current component, which with respect to the network can be regarded as direct current, is hereinafter referred to as the “DC component” or the “DC component”. It often amounts to only a few ppm of the rated current of the transformer, however, it causes a magnetic constant flux in the transformer core, which is superimposed on the primary or secondary alternating flux, and determines the asymmetric control of the BH characteristic of the ferromagnetic core material. Already an insignificant component of the constant flow can, due to the high permeability of the ferromagnetic material of the core, cause saturation of the core and severe distortion of the magnetization current. Also, a geostationary magnetic field can contribute to the constant flux component. The consequences of this asymmetric control are increased magnetic losses and, thereby, increased core heating, as well as peaks in the magnetization current, which cause increased emission of industrial noise.

The undesirable effect of saturation could, in principle, be counteracted by the fact that the cross section of the magnetic circuit increases and, therefore, the magnetic flux density B is kept lower, or an (equivalent) air gap is introduced into the magnetic circuit, as proposed, for example, in DE 19854902 A1. However, the former leads to an increased structural volume, and the latter to a higher magnetization current, both of which are a drawback.

In order to reduce the interference emission of an electric transformer, US 5726617 and DE 69901596 T2 offer actuators that excite the oil in the transformer housing in such a way that the pressure waves of the fluid that come from the transformer core sheet and transformer windings are attenuated. However, these actuators consume a significant amount of energy during operation; they are also subject to interference and are costly.

Representation of the invention

The objective of the invention is the creation of a transformer in which, as simple as possible, the heating of the core and the emission of interference due to a constant magnetic flux in the core are reduced.

The solution to this problem is carried out by the features of paragraph 1 of the claims. Preferred embodiments of the invention are defined in the dependent claims.

The invention proceeds from the idea that it is not necessary to combat the undesirable manifestations of preliminary magnetization, but to eliminate their cause. A transformer according to the invention is characterized as follows:

- The transformer has a magnetically soft core, on which, in addition to the primary and secondary winding, a compensation winding is placed.

- The compensation winding is connected to a current control device, which according to the control parameter, which provides a magnetic field measuring device from measuring the magnetic flux associated with the current in the primary or secondary windings, introduces a compensation current in the compensation winding so that its action in the core is directed opposite magnetic constant flux.

Due to this, it is achieved that the magnetic component of the constant flow in the core of the transformer can be determined in a simple way by technical means of measurement and compensated by the regulation process to eliminate the mismatch. If the constant flow component is excluded, then the control of the HV characteristic is symmetrical. The ferromagnetic core material no longer works in the saturation region. Thus, the magnetostriction of the material is reduced, as a result of which the emission of industrial noise is also reduced. The transformer windings are less stressed thermally, since magnetic losses and, thus, the operating temperature in the core are reduced.

In accordance with the invention, the compensation current in the compensation winding is set according to the measured magnetic field value that the magnetic field measuring device provides. To determine the measured value of the magnetic field, self-known magnetic field sensors are suitable which measure either the field in the core of the transformer or the scattered magnetic field, which closes outside the core by an air gap. The operating principle underlying these sensors can be, for example, induction in a measuring coil, Hall effect or magnetoresistive effect. The measured magnitude of the magnetic field can also be determined by using a magnetometer (flux probe or Ferster probe). Compared with the accurate measurement of the DC component (which, in particular, in large transformers is much less than the rated current, and therefore more difficult to determine), the measuring and technical costs for determining the measured magnetic field are less.

A preferred embodiment of the invention may be characterized in that the magnetic field measuring device is formed by a signal processing unit, which is connected to at least two magnetic field detectors by a signal conductor. With a three-phase transformer of a conventional structure, the determination of the two components of the constant flow may be sufficient, since the total flow should give zero.

Preferably, the signal processing unit is configured to determine higher harmonics from the corresponding measured signal provided by the magnetic field detector, and to generate a control signal on this basis. Thus, at a relatively low circuit costs, it is possible to obtain a control value suitable for compensating the constant flow component. Harmonic analysis can be carried out electronically or with computer support.

Particularly suitable for this are even harmonics, in particular the first higher harmonic (second harmonic), the amplitude of which is functionally related to the constant magnetic flux, which should be compensated.

In particular, a preferred embodiment is one in which two magnetic field detectors outside the core are arranged such that they determine the diffuse flux of the transformer. In the case of magnetic saturation of the core, the scattered flux of the transformer increases very strongly, which is favorable for determining the control signal.

The magnetic field detector can be simply implemented as an induction probe, which determines the change in the scattered flux and converts it into an electrical measurement signal, from which even harmonics, in particular the second harmonic, can then be filtered.

In a particularly preferred embodiment, the induction probe may be configured as an air core coil. Compared with a semiconductor-based transmitter, the electrical measurement signal of this air-core coil is independent of long-term and temperature drift and is also economical.

In order to keep the influence of the network on the compensation winding as low as possible, it can be favorable if a locking circuit (for example, reactive two-terminal device) is included in the current path to the current control device. Thus, the voltage load of the controlled current source, which is introduced into the compensation current in the compensation winding, can be kept negligible. A bipolar circuit is suitable for this, for example, formed from an L-C-parallel circuit, which blocks the mains frequency, however, with respect to the compensation direct current, it practically does not present any resistance.

The favorable spatial arrangement of the magnetic field detector is carried out most simply by experimental or numerical simulation. In particular, the measurement site is favorable in which the magnetic fields caused by the primary and secondary load currents are largely compensated. Preferred is the arrangement in which the air-core coil is placed in the gap formed by the outer circumferential surface of the transformer core and the concentrically surrounding compensation winding or secondary winding, at approximately the average height of the core core.

The preferred location of the compensation winding in the case of a three-rod transformer is yoke, and in the case of a five-rod transformer, a rod that closes the return flow; Thus, the existing transformer can be easily upgraded using a compensation winding.

Brief Description of the Drawings

For further explanation of the invention, in the subsequent part of the description, references are made to the drawings, from which other preferred embodiments, features and improvements of the invention follow, and which show the following:

figure 1 - corresponding to the invention, a three-phase current transformer (three-rod transformer) with constant current compensation, in which the compensation winding is placed on the main rods;

figure 2 - corresponding to the invention, a three-phase current transformer (three-rod transformer) with constant current compensation, in which the compensation winding is placed on the yoke;

figure 3 - corresponding to the invention, a three-phase current transformer with constant current compensation, in which the compensation winding is placed on a yoke that closes the return flow;

figure 4 - corresponding to the invention, a three-phase current transformer (five-core transformer) with constant current compensation, in which the compensation winding is placed on the main rods;

5 is a block diagram of a signal processing according to the invention for practicing deviation (compensation) of a constant flow component;

6 is a flowchart of a measurement method for measuring a constant flow component on a 4 MVA high-power transformer, wherein the signal processing of FIG. 5 is applied;

Fig. 7 is a diagram which, as a result of the measurement method according to Fig. 6, shows a linear relationship between the DC component and the second harmonic at a primary voltage of 6 kV;

Fig. 8 is a diagram which, as a result of the measurement method according to Fig. 6, shows a linear relationship between the DC component and the second harmonic at a primary voltage of 30 kV.

The implementation of the invention

Figure 1 shows an electric transformer 20 with a housing 7, which has a transformer core 4. The structural form of the core 4 corresponds to the known three-rod structural form with three rods 21, 22, 23 and a transverse yoke 32. On each of the rods 21, 22, 23 located, as usual, the primary winding 1 and the secondary winding 2.

According to the invention, a compensation winding 3 is additionally provided on the outer rods 21 and 23. In FIG. 1, in the region of the first rod 21, the arrow 5 denotes the magnetic “constant flow”. For this magnetic "constant flow" 5, it is accepted that it is caused by a DC component (DC component), which flows from the primary side or secondary side. A “constant flux” can also be introduced by the Earth’s magnetic field. By “constant current” or “direct current” here we will mean a physical quantity, which in time, in comparison with variable frequency values of 50 Hz, fluctuates only very slowly, if at all. This magnetic constant flux, which is superimposed on the alternating flux in the rod 21, causes the pre-magnetization, which causes an asymmetric regulation of the magnetic material and, thereby, increased emission of noise. For the compensation of this component of the direct flow corresponding to the invention, in FIG. 1, two controlled current sources 12 and 13 are provided. These current sources 12 and 13 are introduced, respectively, in the sense of deviation (compensation), into the corresponding compensation winding 3, the compensation current 16 or 17, the value and the direction of which is chosen so that the magnetic constant flux 5 in the core 4 is compensated. (In Fig. 1, this is shown by arrow 6, equal in height to arrow 5 and directed opposite to it.) This compensation is carried out by means of control signals 14, 15, which, as regulation parameters, are supplied to current sources 12 and 13 via conductors 9, 10.

The control values 14, 15 are provided by the signal processing unit 11, which is described in more detail below. As shown in figure 1, between the compensation winding 3 and the outer rod 21 or 23 of the core 4, respectively, approximately in the middle of the placed detector 8 of the magnetic field. Each of these magnetic field detectors 8 is located outside the magnetic circuit and measures the scattering field of the transformer 20. The half-wave of the magnetizing current, which is controlled in saturation, is particularly noticeable in the scattering field, so that the constant flux component in the core is well defined. The measurement signal of the detectors 8 by means of conductors 9, 10 is supplied to the signal processing unit 11.

In the presented example, both magnetic field detectors 8 consist respectively of a measuring coil (several hundred turns, diameter about 25 mm). Already two detectors 8 can, as shown in the presented example of a three-rod transformer, be sufficient, since the sum of the components of the constant flow over all the rods should give zero.

As already mentioned above, in principle, many sensory principles can be applied to measure the magnetic field. What is decisive is that the magnetic field parameter of the transformer is measured, from which the DC component or component of the constant flow can be determined by signal-technical means and then compensated.

Figure 2 differs from Figure 1 only in that here the compensation winding 3 is placed not on the main shaft 21, 22, 23, but on the yoke 32 of the core 4. On each main shaft 21, 22, 23 again in the gap between the core 4 and the secondary winding 2 is a magnetic field detector 8 (here, for reservation reasons, only three).

Figure 3 shows a five-core transformer in which a corresponding compensation winding 3 is placed on each terminal 31 closing the return flow. In this structure, the core stream at the entrance to the yoke is not bisected in two; based on the law of continuity, the backward flow component from the rod 31 that closes the return flow must correspond to a constant flow in the main rods 21, 22, 23, so that each rod 31 that closes the return flow conducts a 1.5-fold constant flow component. Each rod 21, 22, 23 again corresponds to a magnetic field detector 8 located outside the core 4. Each measured signal of these three magnetic field detectors 8 is again supplied to the signal processing unit 11, which at the output provides control values 14, 15 for controlled current sources 12 and 13, so that the compensation current 16 or 17 of the constant current component can be compensated in the terminals 31, trailing return flow.

FIG. 4 shows an embodiment of the embodiment of FIG. 3. Here, the compensation windings 3 are located on the main rods 21, 22, 23. Each of these compensation windings 3 again corresponds to one of the current control devices. The compensation current is set, as described above, by the signal processing unit 11.

FIG. 5 shows a schematic block diagram of a possible embodiment of a signal processing unit 11, which acts as a DC compensation regulator. As already described above, the signal processing unit 11 determines the second harmonic from the harmonic spectrum, which is a direct display of the constant flux component (DC components).

The following functional blocks are explained in more detail: the sensor coil 8 determines the scattering flux of the transformer 20. The measured signal of the sensor coil 8 is supplied to the differential amplifier 19. Following the shown signal path, the output signal of the differential amplifier 19 is supplied to the notch filter 24, which filters out the main oscillation ( component at a frequency of 50 Hz). Through the low-pass filter 25 and the band-pass filter 26, the measured signal is supplied to the integrator 27. Due to the integration, a voltage signal is proportional to the magnetic change in the flux in the measuring coil 8, which is supplied to the highly selective band-pass filter 26 to filter out the second harmonic, which displays the component of the constant flow . This voltage signal gets after the circuit 28 of the selection and storage and the low-pass filter 25 through the conductor 16 to a controlled current source 12 with an integrated control device. This current source 12 with a control device in a closed current loop 33 is connected to the compensation winding 3. It supplies a constant current to the compensation winding 3, which acts opposite to the constant current component in the core 4. Since the direction of the compensated DC component is not known a priori, it is used bipolar current regulator, in the presented experiment with IGBT transistors in a full bridge. The integrator 27 causes a 99 degree phase delay with respect to the second harmonic. Reactive bipolar 18, consisting of a parallel oscillatory circuit, blocks the inverse effect on the network of network frequency components.

In Fig. 5, you can still see the auxiliary winding 29, the signal of which is supplied through the filter and rectifier to the sampling and storage circuit 28. It serves in the circuit shown for generating sample signals, so that a phase-dependent sampling of the second harmonic of the measured signal is possible. It should be noted here that this sampling and storage scheme only serves for a phase-dependent measured signal provided by an induction probe 8 (second harmonic at a frequency of 100 Hz).

The signal generation shown in FIG. 5 shows, by way of example only, a possible method for measuring the second harmonic. A person skilled in the art is provided with a number of analog and digital functional components. Thus, the current control values 14, 15 can also be obtained, for example, by means of a suitable digital calculation method in a microcomputer or in a freely programmable logic component (FPGA), which determines the second harmonic from the Fourier transform.

FIG. 6 shows an experimental device in which the signal processing unit 11 shown in FIG. 5 and explained above was used in a powerful 4 MVA transformer in order to determine, using measurement techniques, the relationship between the constant flux component and the first highest harmonic (second harmonic ) under real conditions. A powerful 4 MVA transformer was in this experiment in idle mode with a primary voltage of 6 kV to 30 kV. At the zero points (neutrals) of the primary or secondary windings (Fig. 6), a DC component in the range of 0.2 and 2 A was introduced by means of a current source. A detector coil with 200 turns, which was placed outside the core, served as a magnetic field detector 8. transformer and determined the scattering flux.

In Fig.7 and Fig.8 in the diagram recorded the measurement result of the experimental device according to Fig.6. In the diagrams in FIGS. 7 and 8, the constant flow component (IDC) introduced at the zero point is plotted along the ordinate; and the abscissa shows the effective value of the first higher harmonic (U100 Hz). The diagram in Fig. 7 shows the dependence at a primary voltage of 6 kV, and the diagram in Fig. 8 shows a dependence at a primary voltage of 30 kV. Both diagrams in FIGS. 7 and 8 show that the relationship between the constant flow component (IDC) and the resulting distortion (second harmonic of U100 Hz) can be considered linear with sufficient accuracy.

As a result, this means that the value determined from the measurement of the magnetic field of a powerful transformer is very well suited to form a control quantity that is used to ensure that the component of the constant flow is without prejudice to its cause, that is, when it is involved and Earth’s magnetic field - to determine and compensate by means of measuring equipment, so that industrial noise and heating of the transformer can be kept low.

List of applicable reference positions

1 primary winding

2 secondary winding

3 compensation winding

4 soft core

5 magnetic constant flux

6 magnetic flow compensation

7 transformer housing

8 magnetic field detector

9 measuring conductor, signal

10 measuring conductor, signal

11 signal processing unit

12 current control device

13 current control device

14 control signal

15 control signal

16 compensation current

17 compensation current

18 reactive bipolar

19 differential amplifier

20 transformer

21 first transformer rod

22 second transformer rod

23 third terminal of the transformer

24 notch filter

25 low pass filter

26 bandpass filter

27 integrator

28 sampling and storage scheme

29 auxiliary winding

30 magnetic field measuring device

31 rod, closing the return flow

32 yoke

33 current path.

Claims (9)

1. An electric transformer with constant flow compensation, characterized by the following features:
a) the transformer (20) has a soft magnetic core (4) on which, in addition to the primary and secondary windings (1, 2), a compensation winding (3) is placed,
b) the magnetic field measuring device (30) measures the scattered magnetic field, which closes outside the core (4) through the air gap, and provides a control signal (14, 15), while the device (30) has a signal processing unit (11) that connected by signal conductors to at least two magnetic field detectors (8), each of which is designed as an induction probe,
c) a control signal (14, 15) is supplied to the current control device (12, 13),
d) a current control device (12, 13) through a current path (33) that contains a reactive two-terminal device (18) is connected to the compensation winding (3) and injects a compensation current (16, 17) into it according to the control signal (14, 15) ) so that its action in the core (4) is directed opposite to the constant magnetic flux (5). 2. The transformer according to claim 1, characterized in that the signal processing unit (11) is made in such a way that higher harmonics are determined from the corresponding measurement signal provided by the magnetic field detector (8), so that a control signal is determined from them (14, 15 ) to compensate for the constant flow (5). 3. The transformer according to claim 2, characterized in that the control signal (14, 15) is formed from the first higher harmonic (second harmonic). 4. A transformer according to any one of claims 1 to 3, characterized in that each of at least two magnetic field detectors (8) is located outside the core (4) to determine the scattering flux of the transformer (20). 5. A transformer according to claim 1, characterized in that each induction probe (8) is an air core coil. 6. The transformer according to claim 5, characterized in that the reactive bipolar (18) contains a parallel oscillatory circuit. 7. The transformer according to claim 5, characterized in that the core (4) has three rods (21, 22, 23), of which at least two rods (21, 23) are equipped with a compensation winding (3), and each a coil (8) with an air core is placed in the gap formed by the outer circumferential surface and the surrounding compensation winding (3) or winding (2) at approximately the average height of the rod. 8. The transformer according to claim 5, characterized in that the core (4) has three rods (21, 22, 23) and two rods (31) that close the return flow, on which a compensation winding (3) is respectively placed. 9. The transformer according to claim 5, characterized in that the compensation winding (3) is placed on the yoke (32) of the transformer.

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