EP3783630B1 - 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 for suppressing a magnetic direct current component in the magnetizable core of an electrical device, with a compensation winding for generating a magnetic flux in the core, the effect of which is opposite to the direct flux component, a circuit in which the compensation winding is arranged, at least one in which Electric circuit and a power converter unit arranged in series with the compensation winding, which enables current to flow therethrough in only one direction, a number of switching branches which are arranged in parallel to one another in the circuit and each in series with the compensation winding and the power converter unit, with a switching unit and in each switching branch a current-limiting choke are connected in series, and a control unit connected to each switching unit, which is set up to actuate each switching unit, so that a current flow is enabled over so many switching branches that an input signal of the control unit is minimized, the current-limiting chokes being designed identically or differently to one another are.

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

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

In electrical transformers, such as those used in energy transmission and distribution networks, an undesirable direct current can be fed into the windings, for example. Also power electronics Components in the network, such as the control of electric drives, converters for flexible AC transmission systems, stray currents from railway systems powered by direct current or high-voltage direct current transmission can cause direct currents in electrical devices. Another cause of direct currents can be so-called “Geomagnetically Induced Currents” (hereinafter also referred to as GIC for short).

A direct current component results in a magnetic direct flux component in the core of the transformer, which is superimposed on the alternating flux. This results in an asymmetrical control of the magnetic material in the core, 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 (e.g.: 20-30%). Heating problems can occur, especially with large GIC. There is also increased noise emission during operation, which is perceived as particularly disturbing when the transformer is operated near a living area.

Various active and passive devices are known for direct current compensation or reduction of operating noise of a transformer as an electrical device.

The device mentioned at the beginning and the method mentioned at the beginning are in the EP 3 080 821 B1 described. The device disclosed there has a compensation winding that is part of a 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. Phase control is implemented with the help of a control unit. In other words, the current flowing via the switching branch in the circuit is regulated by varying the phase shift between the ignition point of the thyristor and the voltage in the compensation circuit. The adjustable current range can be used by a second switching branch, which is the first

Switching branch is connected in parallel can be increased. The second switching branch can be used to increase the achievable flow in the core, so that larger direct current components can be compensated for. However, the voltage-synchronous ignition of the thyristor or thyristors requires complex electronics that are costly and maintenance-prone.

Further state of the art is in the WO 2012/041368 A and the DE 27 23 767 A1 disclosed.

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

Based on the device mentioned at the beginning, the invention solves this problem in that each current-limiting choke is composed of partial coils, with two partial coils being arranged next to each other between two metal plates to form a partial coil pair and with each partial coil being equipped with connection terminals.

The invention further solves this problem by an electrical device with 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 solves the problem by a method in which an output signal from a sensor, which detects the direct current component in one or the winding of an electrical device, is fed to a control unit, the control unit actuates so many switching units that a current flow occurs in a circuit , which has a compensation winding that is inductively coupled to a core of an electrical device, so that a direct flux component in the electrical device is minimized.

According to the invention, a direct current flowing through a compensation winding is adjusted by switching current limiting chokes arranged in parallel on and off. All current-limiting chokes are arranged in series with the compensation winding in a circuit in which a converter unit ensures the rectification of the current flowing in the circuit. Within the scope of the invention, the more switching branches are connected, the greater the compensating direct current in the compensation choke. A complex phase control has become unnecessary within the scope of the invention. A simple digital control is sufficient. Within the scope of the invention, this switches on so many switching branches that the direct current component measured by a 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 switching branches in the direction of the current permitted by the converter unit in the circuit.

Within the scope of the invention, the inductance acting in the circuit is changed gradually by switching the current limiting chokes on and off. The compensation winding in the circuit acts as an ideal voltage source. When operating 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 magnitude of which depends on the inductance set in the circuit. According to the invention, the circuit in which the compensation winding and the switching branches are arranged is a closed circuit that is grounded at one point.

If you switch a current direction unit in a closed circuit that has an ideal voltage source, For example, a diode and a current-limiting choke in series, the following current i(t) flows in the circuit: $$i\left(t\right)={\mathrm{<figure-callout\; id="1"\; label="I\; DC"\; filenames="imgf0001.png"\; state="\{\{state\}\}">I</figure-callout>}}_{\mathit{DC}}\left(1-\cos \left(\mathit{\omega t}\right)\right)\mathit{with}{I}_{\mathit{DC}}=\frac{{\mathrm{<figure-callout\; id="2"\; label="U\; eff"\; filenames="imgf0001.png"\; state="\{\{state\}\}">U</figure-callout>}}_{\mathit{eff}}\sqrt{2}}{\mathit{\omega L}}$$

A direct current I _DC therefore flows, which is superimposed by an alternating current, with the amplitude of the alternating current approximately 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. U _eff is the effective value of the alternating voltage induced in the compensation winding.

By successively connecting N identical partial inductances, the direct current flowing in the circuit is changed in equal jumps, i.e. H. Each addition of a partial inductance changes the current by one increment.

When operating the electrical device, it is connected to a high-voltage network within the scope of the invention. The electrical device is therefore designed for high voltages and is, for 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 and has a magnetizable core and at least one winding. During operation, at least one winding is connected to the high-voltage network carrying alternating voltage. An ester liquid or a mineral oil, for example, can be considered as an insulating fluid. In addition to electrically insulating the active part from the tank at ground potential, it also serves to dissipate the heat generated in the components.

Within the scope of the invention, any sensor that detects direct currents and provides an electrical signal on the output side can be considered as a sensor. The electrical signal can be an analog electrical one Signal, for example an electrical current or a voltage, the strength or intensity of which corresponds to the size 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 were 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 a 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 alternating voltage network. But a combination of alternating and direct 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, which is transferred by a control or ignition signal from a blocking position, in which a current flow via the power semiconductor is interrupted, into a through position, in which a current flow via the power semiconductor switch is possible. A controllable power semiconductor is, for example, a thyristor, GTO, IGBT, IGCT or the like.

According to an expedient embodiment of the invention in this regard, 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 transferred from a blocking position to the through position become. In other words, they can only be switched on or off. 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 alternating voltage in the circuit, which, in the event of a polarization change, ensures a current in the circuit that is below the holding current of the thyristor. If a circuit is to be switched on, the thyristor is permanently ignited.

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

In a preferred embodiment of the invention, the thyristors serve not only as switches, but also as a current limiting unit or, in other words, as a current valve. If 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.

The current limiting chokes are expediently designed to be identical or different from one another. If all current-limiting chokes are identical, the current in the circuit can only be changed in uniform steps if the current-limiting chokes are connected in the same way. According to the invention, each current-limiting choke is composed of partial coils, with two partial coils being arranged side by side between two metal plates to form a partial coil pair and with each partial coil being equipped with connection terminals. This design of the current limiting chokes has proven to be particularly simple and robust. In addition, this design allows the introduction of an intermediate inductance, which will be discussed in more detail later.

Further costs can be saved within the scope of the invention if partial coil pairs are arranged one above the other, so that a coil stack is formed. According to this variant of the invention, the partial coil pairs share a metal plate. In other words, the thickness of the metal plate between two pairs of partial coils, which is arranged one above the other, is exactly the same as the thickness of the lower or upper metal plate of the stack. If two adjacent pairs of partial coils are active, almost no magnetic flux can be measured in this metal plate, so that the flux of the lower and upper pairs of partial coils cancel each other out. The metal plates with their constant thickness are necessary if an adjacent pair of partial coils is not active, i.e. does not generate a magnetic field.

Each metal plate advantageously consists of a ferromagnetic material. In this way, stray field losses are minimized.

The partial coils of a partial coil pair are advantageously connected in series or parallel to one another. This advantageous further development allows the number of partial coil pairs required to be reduced, so that material and construction volume are saved. If the partial coils of a partial coil pair are not connected in series, but parallel to one another, without otherwise changing the geometry, the inductance of the partial coil pair is quartered.

According to one variant, the partial coils of a partial coil pair are designed identically. According to a further development in this regard, partial coils of different partial coil pairs have different numbers of turns and conductor cross sections. Based on the stack explained above, this means that the partial coils arranged between two metal plates are identical to one another. However, the partial coils of a pair of partial coils arranged further up or down in the stack can have a smaller number of turns, but a larger conductor cross section to be able to carry the higher currents that then occur without errors. Of course, it is also possible to connect the partial coils of a partial coil pair in parallel or in series.

Figur 1

ein Ausführungsbeispiel der erfindungsgemäßen Vorrichtung,

Figur 2

eine Ausführungsbeispiel der in der Vorrichtung gemäß Figur 1 verwendeten Spulen,

Figur 3

ein weiteres Ausführungsbeispiel der in der Vorrichtung gemäß Figur 1 verwendeten Spulen,

Figuren 4,5

Ausführungsbeispiele der Verschaltung der Spulen gemäß der Figuren 2 und 3 schematisch verdeutlichen.

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 in the drawing, whereby the same reference numerals refer to components with the same effect and where the

Figure 1

an embodiment of the device according to the invention,

Figure 2

an embodiment of the device according to Figure 1 coils used,

Figure 3

a further embodiment of the device according to Figure 1 coils used,

Figures 4.5

Embodiments of the connection of the coils according to Figures 2 and 3 illustrate schematically.

Figure 1 shows an exemplary embodiment of the device 1 according to the invention, which shows a circuit 2 in which a compensation winding 3 is arranged. The compensation winding 3 is inductively coupled to a high-voltage winding of a power transformer, the power transformer being connected to an alternating high-voltage network with a nominal voltage of 325 kV. In the circuit 2, a power converter unit 4 can be seen in series with the compensation winding 3, which is designed, for example, as a diode. A diode can also be referred to as a non-controllable power semiconductor.

The power converter unit 4 allows current to flow via it in only one direction, which is through the tip of the triangle (i.e. in Figure 1 from left to right). The grounded circuit 2 also has switching branches 5 ₁ , 5 ₂ , 5 ₃ ... 5 _n-1 , 5 _n connected in parallel. In every switching branch a switching unit 6 ₁ , 6 ₂ , 6 ₃ ... 6 _n-1 , 6 _n and a current-limiting choke 7 ₁ , 7 ₂ , 7 ₃ ... 7 _n-1 , 7 _n are connected in series. The switching units 6 ₁ , ... 6 _n are in the in Figure 1 The exemplary embodiment shown is realized as tyristors, which take on the function of the diode 4 shown in the figure. The diode 4 is therefore part of the switching unit and integrated into it. In other words, the rectifying effect of a power converter unit is taken over by the electronic switching unit. Once fired, a thyristor allows current to flow through it in only one direction. Tyristor 6 ₁ , ... 6 _n is connected via signal lines shown in dashed lines to a control unit 8, which is set up to ignite the respective thyristor. The control unit 8 is also connected to the output of a sensor 9, with the help of which a direct current component is detected in one of the windings of the transformer, which is otherwise not shown. On the output side of the sensor 9 there is a signal which corresponds to the direct flow component and which is transferred to the control unit 8.

The compensation winding 3, which is inductively coupled to the high-voltage winding, generates a voltage U in the circuit 2. In other words, the compensation winding 3 provides an ideal voltage source for the circuit 2. The induced voltage now drives a direct current with an alternating current component across circuit 2. The size of this current flowing in the circuit depends on the number of active switching branches connected in parallel. The more switching branches are actively switched by igniting the thyristor assigned to it, the greater the current flowing in the circuit 2, which also occurs in the compensation winding 3, so that the inductive coupling creates a direct flow component in the core, which is caused by a direct current flowing through the high-voltage circuit. and/or undervoltage winding is compensated for.

The control unit 8 is a particularly simple control unit, since these only switch branches through continuous ignition the tyristors or by omitting continuous ignition, a number of switching branches are connected in parallel, so that the direct current component detected by the sensor 9 is minimized. A complex phase control is avoided within the scope of the invention. According to the invention, a simple, almost error-free and robust device for direct current compensation is provided.

Figure 2 illustrates the structure of an inductor 7 ₁ , ... 7 _n as an example. It can be seen that the current-limiting choke 7 ₁ is composed of a pair of partial coils 10 and 11, which are arranged between two iron plates 12, the partial coils 10 and 11 being provided with taps or connection terminals (not shown in the figure).

The two partial coils 10 and 11 form a partial coil pair 13. An inductor constructed in this way can be assembled together with other inductors in a particularly simple and compact manner to form a stack 14, with adjacent coil pairs 13 sharing a metal or iron plate 13 arranged between them. In other words, all metal plates 13 of the stack 14 have the same thickness. Only a metal plate is necessary between two pairs of partial coils 13 arranged one above the other. The metal plates are preferably designed identically to one another. The metal plates are preferably plates made of thin electrical sheets.

The number of coil pairs required can be limited by appropriate interconnection. In Figure 4 Two partial coils 10 and 11 are connected in series, for example. Figure 5 shows the two partial coils 10 and 11 of a partial coil pair 13 in parallel connection. Since the partial coil pairs are according to Figure 4 and Figure 5 Otherwise no further distinction is made, the partial coil pair 13 according to Figure 5 only a quarter of the inductance of the partial coil pair Figure 4 on. If partial coil pairs with full inductance are referred to as L1, see Figure 4 , and partial coil pairs according to Figure 5 , which are connected in parallel and thus have a quarter of the inductance of L1, with L2, can be used with eight partial choke pairs 12 switching stages can be implemented, as shown in Table 1. Table 1: n L1 2 L2 3L1 4 L2 5 L1 6 L2 7L1 8L2 1 1 0 0 0 0 0 0 0 2 0 1 0 0 0 0 0 0 3 1 1 0 0 0 0 0 0 4 1 1 1 0 0 0 0 0 5 1 1 0 1 0 0 0 0 6 1 1 1 1 0 0 0 0 7 1 1 1 1 1 0 0 0 8th 1 1 1 1 0 1 0 0 9 1 1 1 1 1 1 0 0 10 1 1 1 1 1 1 1 0 11 1 1 1 1 1 1 0 1 12 1 1 1 1 1 1 1 1

In addition, it is possible to design the partial chokes differently. If n is the number of partial coil pairs with different inductances, the number of possible switching stages is 2 ⁿ -1. Ideally, the partial coil pairs are graded so that the following partial coil pair only has half the inductance. This means that 2 ⁿ -1 switching stages with the same increments can be achieved with n pairs of partial coils.

If, for example, the partial chokes 10 and 11 of a partial choke pair 13 are designed with only half the number of turns but with a doubling of the conductor cross section compared to an adjacent partial choke pair, the size of the said partial choke pair remains approximately the same. By connecting in parallel, the inductances of the partial choke pairs can now be quartered again, so that, according to this further development of the invention, there are four partial choke pairs with different inductances L1, L2, L4 and L8.

You can see that you only need four pairs of partial coils to implement 15 switching stages, 2 ⁴ -1 = 15. This corresponds to a saving of up to 60%. The switching options are illustrated in Table 2: Table 2: n 1 L1 2 L2 3L4 4 L8 1 1 0 0 0 2 0 1 0 0 3 1 1 0 0 4 0 0 1 0 5 1 0 1 0 6 0 1 1 0 7 1 1 1 0 8th 0 0 0 1 9 1 0 0 1 10 0 1 0 1 11 1 1 0 1 12 0 0 1 1 13 1 0 1 1 14 0 1 1 1 15 1 1 1 1

Claims (12)

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 number of switching branches (5₁, 5₂, ... 5_n), which are arranged in the circuit in parallel with one another and in each case in series with the compensation winding (3) and with the power converter unit (4), wherein a switching unit (6₁, 6₂, ... 6_n) and a current-limiting reactor (7₁, 7₂, ... 7_n) are connected in series in each switching branch (5₁, 5₂, ... 5_n), and

   - a control unit (8) that is connected to each switching unit (6₁, 6₂, ... 6_n) and is configured to actuate each switching unit (6₁, 6₂, ... 6_n), such that a flow of current is enabled through so many switching branches (5₁, 5₂, ... 5_n) that an input signal, which corresponds to a detected DC component, of the control unit (8) is minimized,

   - wherein the current-limiting reactors (7₁, 7₂, ... 7_n) are designed so as to be identical to one another or different from one another,

   - characterized in that

   - each current-limiting reactor (7₁, 7₂, ... 7_n) is composed of partial coils (10, 11), wherein in each case two partial coils (10, 11) are arranged next to one another between two metal plates (12) so as to form a partial coil pair (13), and wherein each partial coil (10, 11) is equipped with connection terminals.
2. Apparatus (1) according to Claim 1,
   characterized in that
   each switching unit is an electronic switching unit (6₁, 6₂, ... 6n) .
3. Apparatus (1) according to Claim 2,
   characterized in that
   each electronic switching unit is a thyristor (6₁, 6₂, ... 6_n).
4. Apparatus (1) according to Claim 2 or 3,
   characterized in that
   each switching unit (6₁, 6₂, ... 6_n) has at least two thyristors that are connected in parallel with one another in opposite directions.
5. Apparatus (1) according to Claim 3 or 4,
   characterized in that
   the thyristors (6₁, 6₂, ... 6_n) are at least partially used as a power converter unit (4).
6. Apparatus (1) according to Claim 1,
   characterized in that
   the partial coil pairs (13) are arranged one on top of the other, such that a coil stack (14) is formed.
7. Apparatus (1) according to Claim 1 or 6,
   characterized in that
   the partial coils (10, 11) of a partial coil pair (13) are connected in series or in parallel with one another.
8. Apparatus (1) according to one of Claims 1, 6 and 7,
   characterized in that
   the partial coils (10, 11) of a partial coil pair (13) are designed so as to be identical to one another.
9. Apparatus (1) according to Claim 8,
   characterized in that
   partial coils (10, 11) of different partial coil pairs (13) have numbers of turns and conductor cross sections that differ from one another.
10. 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 7 that is inductively coupled to the core.
11. Electrical device according to Claim 10,
    characterized in that
    a sensor (9) is provided to ascertain the DC component in a or the winding, wherein the sensor (9) is connected to the control unit (8) on the output side.
12. 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 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 has a number of switching branches (5₁, 5₂, ... 5_n), which are connected in the circuit (2) in parallel with one another and in each case in series with the compensation winding (3) and with the power converter unit (4), wherein a switching unit (6₁, 6₂, ... 6_n) and a current-limiting reactor (7₁, 7₂, ... 7_n) are connected in series in each switching branch (5₁, 5₂, ... 5_n), in which

    - the output signal of a sensor (9) that detects the DC component in a or the winding is supplied to a control unit (8),

    - the control unit (8) switches on so many switching units (6₁, 6₂, ... 6_n) that the number of switching branches that are active and connected in parallel results in a flow of current in the circuit (2) that is adjustable in switching stages and minimizes the DC component.

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