EP3786987B1 - Device for suppressing a direct current component during the operation of an electrical appliance connected to a high-voltage network - Google Patents

Description

The invention relates to a device and a method for suppressing a direct current component when operating an electrical device connected to a high-voltage network.

The invention also relates to an electrical device with such a device.

In the case of electrical transformers, such as those used in energy transmission and distribution networks, an undesired direct current can be fed into the windings. Power electronic components in the network, for example the control of electrical drives, converters for flexible AC transmission systems, stray currents from railway systems operated with direct current or high-voltage direct current transmission can cause direct currents in electrical devices. Another cause of direct currents can be what are known as “Geomagnetically Induced Currents” (hereinafter also referred to as GIC for short).

A DC component results in a magnetic DC component in the core of the transformer, which is superimposed on the AC flux. An asymmetrical modulation of the magnetic material in the core occurs, which has a number of disadvantages. Even a direct current of a few amperes leads to saturation of the core with magnetic flux. This is associated with a significant increase in losses in the core (eg: 20-30%). Heating problems can occur, especially with large GICs. There is also increased noise emission during operation, which is felt to be particularly annoying when the transformer is operated in the vicinity of a living area. Various active and passive devices are known for DC compensation or reduction of operating noise of a transformer as an electrical device.

So the device mentioned and the method mentioned in the EP 3 080 821 B1 described. The device disclosed there has a compensation winding which is part of an electric circuit. In said circuit, a switching branch is provided in series with the compensation winding, with a choke and a thyristor being connected in series in the switching branch. A phase angle control is implemented with the aid of a control unit. In other words, the current flowing in the circuit via the switching branch is regulated by varying the phase shift between the triggering point of the thyristor and the voltage in the compensation circuit. The controllable current range can be increased by a second switching branch, which is connected in parallel to the first switching branch. The achievable throughput in the core can be increased by the second switching branch, so that larger DC components can be compensated. However, the voltage-synchronous firing of the thyristor or thyristors requires complex electronics that are expensive and susceptible to maintenance.

Further prior art is in DE 27 23 767 A1 described.

The object of the invention is therefore to create a device, an electrical device and a method of the type mentioned at the outset that are inexpensive, reliable and low-maintenance.

Based on the device mentioned above, the invention solves this problem in that the device with a compensation winding for generating a magnetic flux in the core, the effect of which is opposed to the direct flux component, a circuit in which the compensation winding is arranged, at least one in the circuit and converter unit arranged in series with the compensation winding, which enables a current flow via this in only one direction, a coarse stage branch arranged in the circuit and in series with the compensation winding with at least two main choke sections connected in series, with a low-impedance bridging path being connected in parallel with at least one main choke section, in which a switching unit is arranged, which can be converted from a blocking position, in which a current flow via the switching unit is enabled, into an interrupting position, in which a current flow via the switching unit is prevented, or vice versa, a sensor for detecting the direct current component and one with each switching unit and the control unit connected to the sensor, which is set up to actuate each switching unit so that a current flow can be set over so many main throttle sections that the direct current component detected by the sensor is minimized.

The invention also achieves this object with an electrical device having a core and at least one winding which is set up to generate a magnetic flux in the core, the electrical device having a device according to the invention which is inductively coupled to the core. Finally, the invention achieves the object through a method for suppressing a direct current component when operating an electrical device connected to a high-voltage network, which has a core, at least one winding for generating a magnetic flux in the core and one arranged in a circuit and inductively coupled to the core Compensation winding, wherein the circuit has at least one power converter unit arranged in series with the compensation winding, which allows current to flow via it in only one direction, and a coarse stage branch with at least two series-connected main choke sections, with at least one Main throttle section connected in parallel with a low-impedance bypass path is arranged, in which a switching unit is arranged, which can be transferred from a blocking position to an open position or vice versa, and has a control unit, which is connected to a sensor for detecting the direct current component and each switching unit, the method having the following steps: detecting the Direct current component through the sensor while providing an output signal, transferring the output signal to the control unit, driving the circuit units by the control unit, so that a current path over so many main throttle sections is made possible that a current flow is established in the circuit, which minimizes the DC component.

According to the invention, a direct current flowing through a compensation winding is set by connecting and disconnecting main choke sections. All the main choke sections are arranged in series with the compensation winding in a circuit in which a power converter unit rectifies the current flowing in the circuit. Within the scope of the invention, the compensating direct current in the compensating winding becomes greater the more main choke sections are bypassed. A complex phase control has become superfluous within the scope of the invention. A simple digital control is sufficient. Within the scope of the invention, this bridges so many main throttle sections that the direct current component measured by the sensor in an electrical device is minimized. The circuit of the device according to the invention is expediently grounded at a potential point. This potential point is connected downstream of the main choke sections in the direction of the current in the circuit permitted by the converter unit.

The main part of the generation of the direct current in the compensation winding at the core of the electrical device is taken over by the main choke, which has main choke sections with the step inductance L. The converter unit is used to rectify the current in the circuit. The effective in the circuit The inductance of the main choke can be realized by connecting and disconnecting partial turns as main choke sections of a tapped main choke or by connecting and disconnecting partial chokes connected in series as main choke sections. The partial throttles are, for example, designed as independent throttles and are arranged in series in the coarse stage branch. Each partial inductor can be bypassed by a bypass branch when the switching unit arranged in the bypass path is switched on.

The direct current I _DC generated according to the invention is inversely proportional to the inductance effective in the circuit according to I _DC ∝1/L.

According to an advantageous further development of the invention, the main choke sections all have the same inductance L. In other words, by bridging main throttle sections, the adjustment range (0 . . . I _dc,max ) can be divided into N equal sections. Deviating from this, however, the main choke sections can also be designed differently within the scope of the invention, that is to say have an inductance L that differs from one another.

According to a preferred further development of the invention, a fine-stage branch is provided which is connected in parallel to the coarse-stage branch and has at least two secondary choke sections connected in series, with at least one secondary choke section being connected in parallel with a low-impedance bridging path, in which a switching unit is arranged which switches from a blocking position to an open position or can be reversed.

Each switching unit is preferably an electronic switching unit or, in other words, an electronic switch. Electronic switches have a faster switching time compared to mechanical switches. The term switching time is understood to mean the period of time from the triggering time to the time at which the disconnected position is reached. The switches in the fine-stage branch have to carry less current than the switches in the main-stage branch and can therefore be designed to be more compact than these. The secondary throttle sections are preferably designed as independent partial throttles or in the form of taps on a large partial throttle. The sub-throttle sections divide a control range of the main-stage branch into sub-sections, so that more precise fine control is enabled. If the control range is (0 . . . I _dc,max ) and the coarse stage branch divides this control range into N equal coarse sections I _dc,max /N, then the fine stage branch with its M partial inductances divides the coarse sections into M fine stage sections.

If the precision choke sections are also divided into N equal partial inductances and each partial inductance has the value N*L, the direct current I _DC can be set with an accuracy of 1/(2*N) of the maximum direct current I _dc,max

A further advantage of the invention can be seen in the fact that the current pulses generated by a choke and converter unit only have an AC component of the fundamental harmonics and a DC mean value at the level of the amplitude of the AC current. Therefore, there are no high harmonic current components, so that eddy current losses in the windings are reduced.

Within the framework of the invention, no converter is necessary, instead only relatively simple switching elements are required. A complex and error-prone phase control is also avoided within the scope of the invention. According to the invention, the control unit increases or decreases the current flowing in the circuit in steps until the direct current component detected by the sensor is minimized.

This is done with a simple digital control of the switches. Problems with what is known as a tendency of a PI controller to oscillate are avoided within the scope of the invention. Also no tedious controller settings (Pl) have to be made. According to the invention, a simple zero-point control (minimum sensor signal) by switching the respective throttle sections on and off is sufficient. Furthermore, the time-consuming calibration of the sensor is no longer necessary.

Within the scope of the invention, the inductance acting in the circuit is changed in stages. The compensation winding in the circuit acts as an ideal voltage source. During the operation of an electrical device that has a device according to the invention, a voltage is induced in the compensation winding, which is inductively coupled to the winding or windings of the electrical device. This voltage drives a current in the circuit that is rectified by the converter unit, the size of which depends on the inductance set in the circuit. According to the invention, the circuit in which the compensation winding and the main and, if necessary, secondary choke sections are arranged is a closed circuit that is grounded at one point.

If you connect a power converter unit, for example a diode, and main and, if necessary, secondary choke sections in series in a closed circuit that has an ideal voltage source, the following current flows in the circuit: $$i\left(t\right)={\mathrm{<figure-callout\; id="1"\; label="I\; DC"\; filenames="imgf0001.png"\; state="\{\{state\}\}">I</figure-callout>}}_{\mathit{DC}}\left(\mathrm{1}-\cos \left(\mathit{\omega t}\right)\right)\mathit{With}{I}_{\mathit{DC}}=\frac{{\mathrm{<figure-callout\; id="2"\; label="u\; eff"\; filenames="imgf0001.png"\; state="\{\{state\}\}">u</figure-callout>}}_{\mathit{eff}}\sqrt{2}}{\mathit{\omega L}}$$

A direct current I _DC thus flows, which is superimposed by an alternating current, with the amplitude of the alternating current roughly corresponding to the value of the direct current. ω corresponds to the angular frequency of the alternating current. L stands for the inductance of the circuit. Ueff is the effective value of the AC voltage that is induced in the compensation winding.

When the electrical device is in operation, it is connected to a high-voltage network within the scope of the invention. The electrical device is therefore designed for high voltages and example a transformer, in particular a power transformer or a choke. Such a transformer or such a choke preferably has a tank filled with an insulating fluid. An active part is arranged in the tank, which has a magnetizable core and at least one winding. At least one winding is connected to the high-voltage network carrying AC voltage during operation. For example, an ester liquid or a mineral oil can be considered as the insulating fluid. In addition to the electrical insulation of the active part from the tank, which is at ground potential, it is also used to cool the heat-generating components.

Within the scope of the invention, any sensor that detects direct currents and provides an electrical signal on the output side as a function of the magnitude of the direct current can be considered as a sensor. The electrical signal can be an analog electrical signal, for example an electrical current or a voltage, the strength or intensity of which corresponds to the magnitude of the detected direct current. However, within the scope of the invention, the output signal of the sensor can also be a digital signal, such as a sequence of digital values, which have been generated, for example, by sampling an analog signal to obtain sample values and digitizing the sample values.

Within the scope of the invention, the electrical device is designed for operation in the voltage or high-voltage network, i.e. for an operating voltage between 1 kV and 1200 kV, in particular 50 kV and 800 kV. The high-voltage network is preferably an AC voltage network. However, a combination of AC and DC voltage networks is also possible within the scope of the invention.

According to the invention, an electrical device, for example a transformer, in particular a power transformer, a choke or the like.

According to a further variant of the invention, each switching unit is an electronic switching unit. An electronic switching unit or, in other words, an electronic switch is, for example, a controllable power semiconductor that is switched by a control or ignition signal from a blocked position, in which a current flow through the power semiconductor is interrupted, into a through position, in which a current flow through the power semiconductor switch is enabled. A controllable power semiconductor is, for example, a thyristor, GTO, IGBT, IGCT or the like.

According to an embodiment of the invention that is expedient in this respect, each electronic switching unit is a thyristor. Thyristors are particularly robust power semiconductors and are available on the market at low cost. Thyristors can only be actively switched from a blocking position to the open position. In other words, they can only be switched on or switched on. In order to move from the through position to the blocking position, the current flowing through the thyristor must fall below a holding current. However, this is guaranteed within the scope of the invention, since the compensation winding generates an AC voltage in the circuit, which ensures a current in the circuit in the event of a polarization change that falls below the holding current of the thyristor. If a circuit is to be switched on, the thyristor is continuously fired.

According to one variant of the invention, each switching unit has at least two thyristors connected in parallel in opposite directions to one another.

In a preferred embodiment of the invention, the thyristors serve not only as switches, but also as a power converter unit. When a thyristor is in its open position, current can only flow through it in one direction. In other words, the thyristor directs the current at the same time.

Figur 1

ein Ausführungsbeispiel der erfindungsgemäßen Vorrichtung, und

Figur 2

die Stellbereiche der Vorrichtung gemäß Figur 1 schematisch verdeutlicht.

Further expedient refinements and advantages of the invention are the subject of the following description of exemplary embodiments of the invention with reference to the figure of the drawing, in which the same reference symbols refer to components which have the same effect and in which

figure 1

an embodiment of the device according to the invention, and

figure 2

the adjustment ranges of the device according to figure 1 schematically illustrated.

figure 1 shows an exemplary embodiment of the device 1 according to the invention, which comprises a circuit 2 in which a compensation winding 3 is arranged. During operation of the device 1, the compensation winding 3 is inductively coupled to a high-voltage winding of a power transformer (not shown in the figure), the power transformer being connected to a high-voltage network carrying AC voltage with a nominal voltage of 325 kV.

In circuit 2, in series with compensation winding 3, a power converter unit 4 can be seen, which is designed as a diode 4 in the example shown. The diode 4 is a non-driven power semiconductor.

The power converter unit, or in other words the diode 4, enables current to flow in the circuit 2 in only one direction, which is indicated by the apex of the triangle, i.e. in figure 1 is indicated from left to right.

The grounded circuit 2 also has a coarse branch 5 and a fine branch 6. In the coarse branch 5, main throttle sections 7 ₁ , 7 ₂ , 7 ₃ . . . 7 _n are connected in series. Except for the first main throttle section 7 ₁ , all other main throttle sections 7 ₂ , 7 ₃ . . . 7 _n can be bridged by a bridging path 8 ₂ , 8 ₃ . . . 8 _n . There is a switching unit in each of the listed bridging paths 8 ₂ , 8 ₃ . . . 8 _n 9 arranged. Each switching unit 9 is designed as a thyristor, which is connected to a control unit 10 via a signal line shown in dashed lines. In order to switch a main choke section or a secondary choke section into circuit 2, the thyristor must be in its interrupting position. In other words, the thyristor, which can only be actively transferred from its blocking position to its open position, must not be fired. In order to turn off the thyristor again, i.e. to transfer it from its open position to its blocking position, the current flowing through the thyristor must fall below a holding current. The holding current is in the range of a zero current. Due to the AC voltage coming from the compensation winding, the holding current is periodically undershot. The thyristor is continuously fired for bridging.

The control unit 10 is connected on the input side to a sensor 11 which is set up to detect a direct current component in an electrical device such as a transformer. N auxiliary throttle sections 12 ₁ , 12 ₂ , 12 ₃ . . . 12 _n are connected in series in fine-stage branch 6 . The fine stage sections 12 ₂ , 12 ₃ . . . 12 _n following the first fine stage section can again be bridged by bridging _branches 13 ₂ , 13 ₃ .

The switching units 9 of the fine-stage branch 6 are also connected to the control unit 10 via signal lines shown in dashed lines. The main throttle sections 7 ₁ , 7 ₂ , 7 ₃ have the same inductance L. The inductance of the precision choke sections 12 ₁ , 12 ₂ , 12 ₃ is greater. In the exemplary embodiment shown, it is N×L. However, since the precision choke sections carry a lower current, they can be designed to be comparatively more compact. figure 2 shows the control range of the main throttle sections 7 ₁ , 7 ₂ , 7 ₃ . . . 7 _n and the control range of the fine throttle sections 12 ₁ , 12 ₂ , 12 ₃ ... 12 _n . In figure 2 only two main control areas 14 ₁ , 14 ₂ are shown. The main control range 14 ₂ is in turn divided into fine control ranges 15 ₁ to 15 _n .

The control unit 10 can now bridge as many main throttle sections 7 ₁ , 7 ₂ , 7 ₃ ... 7 _n and secondary throttle sections 12 ₁ , 12 ₂ , 12 ₃ ... 12 _n by opening the thyristors 9 that there is a current in circuit 2 sets, which minimizes the DC component in the transformer in which the device 1 is installed.

Claims (10)

1. Apparatus (1) for suppressing a magnetic DC component in the magnetizable core of an electrical device, comprising

   - a compensation winding (3) for generating a magnetic flux in the core, the effect of which is counter to the DC flux component,

   - a circuit (2) in which the compensation winding (3) is arranged,

   - at least one power converter unit (4) that is arranged in the circuit (2) and in series with the compensation winding (3), and enables a flow of current therethrough in just one direction,

   - a coarse stage branch (5) that is arranged in the circuit (2) and in series with the compensation winding (3) and comprises at least two main inductor sections (7₁, 7₂, ... 7_n) that are connected in series, wherein a low-resistance bypass path (8₂, ... 8_n) is connected in parallel with at least one main inductor section (7₂, ... 7_n), in which bypass path a switching unit (9) is arranged that can be changed over from an on-state position, in which a flow of current through the switching unit (9) is enabled, into an interrupter position, in which a flow of current through the switching unit (9) is prevented, or vice versa,

   - a sensor (11) for detecting the DC component, and

   - a control unit (10) that is connected to each switching unit (9) and the sensor (11) and is configured to actuate each switching unit (9) so that a flow of current can be set through so many main inductor sections (7₁, 7₂, ... 7_n) that the DC component detected by the sensor (11) is minimized.
2. Apparatus (1) according to Claim 1,
   characterized by
   a fine stage branch (6) that is connected in parallel with the coarse stage branch and comprises at least two auxiliary inductor sections (12₁, 12₂, ... 12_n) that are connected in series, wherein a low-resistance bypass path (13₂, ... 13_n) is connected in parallel with at least one auxiliary inductor section (12₁, 12₂, ... 12_n), in which bypass path a switching unit (9) is arranged that can be changed over from an off-state position into an on-state position, or vice versa.
3. Apparatus (1) according to Claim 1 or 2,
   characterized in that
   each switching unit is an electronic switching unit (9).
4. Apparatus (1) according to Claim 3,
   characterized in that
   each electronic switching unit is a thyristor (9).
5. Apparatus (1) according to Claim 3 or 4,
   characterized in that
   each switching unit has at least two thyristors (9) that are connected in parallel with one another in opposite directions.
6. Apparatus (1) according to one of the preceding claims, characterized in that
   at least some of the switching units (9) are used as the power converter unit (4).
7. Apparatus (1) according to one of the preceding claims, characterized in that
   main inductor sections (7₁, 7₂, ... 7_n) are of identical or different design.
8. Apparatus (1) according to one of Claims 2 to 7, characterized in that
   auxiliary inductor sections i (12₁, 12₂, ... 12_n) are of identical or different design.
9. Electrical device having a core and at least one winding that is configured to generate a magnetic flux in the core, and an apparatus (1) according to one of Claims 1 to 8 that is inductively coupled to the core.
10. Method for suppressing a DC component during operation of an electrical device that is connected to a high-voltage network and has a core, at least one winding for generating a magnetic flux in the core and a compensation winding (3) that is arranged in a circuit (2) and is inductively coupled to the core, wherein the circuit (2) has at least one power converter unit (4) that is arranged in series with the compensation winding (3) and enables a flow of current therethrough in just one direction, and a coarse stage branch (5) that is arranged in the circuit and in series with the compensation winding (3) and comprises at least two main inductor sections (7₁, 7₂, ... 7_n) that are connected in series, wherein a low-resistance bypass path (8₂, ... 8_n) is connected in parallel with at least one main inductor section (7₂, ... 7_n), in which bypass path a switching unit (9) is arranged that can be changed over from an off-state position into an on-state position, or vice versa, and a control unit (10) that is connected to a sensor (11) for detecting the DC component and each switching unit (9), wherein the method includes the following steps:

    - detecting the DC component by way of the sensor (9), with provision of an output signal,

    - transferring the output signal to the control unit (10),

    - activating the switching units (9) by way of the control unit (10) so that a flow of current is enabled through so many main inductor sections (7₁, 7₂, ... 7_n) that a flow of current is established in the circuit (2) that minimizes the DC flux component.

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