EP3783630A1 - Device for suppressing a direct current component during the operation of an electrical appliance connected to a high-voltage network - Google Patents

Abstract

The invention relates to a device (1) for suppressing a magnetic direct current component in the magnetizable core of an electrical device, with - a compensation winding (3) for generating a magnetic flux in the core, the effect of which is opposite to the direct current component, - a circuit (2), in which the compensation winding (3) is arranged, - at least one converter unit (4) arranged in the circuit (2) and in series with the compensation winding (3), which enables a current to flow through it in only one direction, - a number of switching branches ( 5 ₁, 5 ₂, ... 5 _n), which are parallel to each other in the circuit and each in series with the compensation winding (3) and to the converter unit (4) are arranged, with a switching unit (6th) in each branch (5 ₁, 5 ₂, ... 5 _n) ₁, 6 ₂, ... 6 _n) and a current limiting choke (7 ₁, 7 ₂,. .. 7 _n) are connected in series, and - one with each switching unit (6 ₁, 6 ₂, ... 6 <sub > n </sub>) connected control unit (8), which is used to actuate each switching unit (6 ₁, 6 ₂, ... 6 <sub> n </ sub >) is set up so that a current flow over as many switching branches (5 ₁, 5 ₂, ... 5 _n) is possible, that an input signal of the control unit (8) is minimized.

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 feed of a direct current can occur, for example in the windings. Power electronic components in the network, for example the control of electrical drives, converters for flexible AC transmission systems, stray currents from rail systems operated with direct current or high-voltage direct current transmission can also ensure direct currents in the electrical device. 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. An asymmetrical modulation of the magnetic material in the core occurs, which has a number of disadvantages. A direct current of just a few amperes leads to saturation of the core with magnetic flux. This is associated with a significant increase in core losses (e.g. 20-30%). Heating problems can occur, especially with a large GIC. In addition, there is an increased noise emission during operation, which is perceived as particularly annoying in particular when the transformer is operated in the vicinity of a living area.

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

The device and the method mentioned in the introduction are thus in FIG EP 3 080 821 B1 described. The device disclosed there has a compensation winding which is part of a circuit. In said circuit, a switching branch is provided in series with the compensation winding, a choke and a thyristor being connected in series in the switching branch. Phase 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 ignition time of the thyristor and the voltage in the compensation circuit. The controllable current range can be increased by a second switching branch that is connected in parallel to the first switching branch. The achievable throughflow in the core can be increased by the second switching branch, so that larger direct current components can be compensated. The voltage-synchronous ignition of the thyristor or thyristors, however, requires complex electronics, which are cost-intensive and require maintenance.

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

Based on the device mentioned at the beginning, the invention solves this problem in that the device has 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 the circuit and converter unit arranged in series with the compensation winding, which allows current to flow through it in only one Direction enables a number of switching branches which are arranged in the circuit parallel to one another and each in series with the compensation winding and with the converter unit, with a switching unit and a current limiting inductor being connected in series in each switching branch. The device according to the invention further comprises a control unit connected to each switching unit, which is set up to actuate each switching unit, so that a current flow is made possible over so many switching branches that an input signal of the control unit is minimized.

The invention also achieves this object by means of 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 an inventive device inductively coupled to the core.

Finally, the invention achieves the object by a method in which an output signal of a sensor that 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 is established in a circuit , which has a compensation winding which 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 set by connecting and disconnecting current-limiting chokes arranged parallel to one another. 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. In the context of the invention, the compensating direct current in the compensation throttle becomes greater the more switching branches are connected. 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 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.

In the context of the invention, the inductance acting in the circuit is changed in stages 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 rectified by the converter unit in the circuit, 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 switching branches are arranged is a closed circuit that is earthed at one point.

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

A direct current I _DC thus flows on which an alternating current is superimposed, 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 rms value of the alternating voltage that is induced in the compensation winding. By successively switching in N identical partial inductances, the direct current flowing in the circuit is changed in equal steps, ie each switching in 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, 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, which has a magnetizable core and at least one winding. At least one winding is connected to the high-voltage network during operation. An ester liquid or a mineral oil, for example, can be used as the insulating fluid. In addition to electrically isolating the active part from the tank at ground potential, it also serves to dissipate the heat generated in the components.

In the context of the invention, any sensor can be considered as a sensor which detects direct currents and provides an electrical signal on the output side as a function of the magnitude of the direct current. 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, for example, a sequence of digital values that have been generated, for example, by sampling an analog signal, obtaining sampling values and digitizing the sampling values.

The electrical device is designed within the scope of the invention for operation in the voltage or high voltage network, ie 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. However, a combination of AC and DC voltage network 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 transferred by a control or ignition signal from a blocking position, in which a current flow through the power semiconductor is interrupted, to a through position, in which a current flow through the power semiconductor switch is made possible. A controllable power semiconductor is, for example, a thyristor, GTO, IGBT, IGCT or the like.

According to an advantageous embodiment of the invention in this regard, each electronic switching unit is a thyristor. Thyristors are particularly robust power semiconductors and are inexpensive on the market. Thyristors can only be actively transferred from a blocked position to the open position. In other words, they can only be switched on or on. In order to move from the pass-through position to the blocked 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, when the polarization changes, ensures a current in the circuit which falls below the holding current of the thyristor. If a circuit is to be switched on, the thyristor is continuously 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 current limiting units or, in other words, as flow valves. If a thyristor is in its through position, the current can flow through it in only one direction. In other words, the thyristor directs the current at the same time.

The current limiting chokes are expediently designed to be identical to or different from one another. If all current limiting reactors are identical, the current in the circuit can only be changed in uniform steps with the same interconnection of the current limiting reactors.

According to a preferred embodiment of the invention, each current limiting choke is composed of sub-coils, with two sub-coils being arranged next to one another between two metal plates to form a sub-coil pair, and each sub-coil being equipped with connection terminals. This construction of the current limiting chokes has proven to be particularly simple and robust. In addition, this design enables 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 partial coil pairs, which are arranged one above the other, is exactly as great as the thickness of the lower or upper metal plate of the stack. If two adjacent pairs of coil sections are active, almost no magnetic flux can be measured in this metal plate, so that the flux of the lower and upper pairs of coil sections cancel each other out. The metal plates with their constant thickness, are necessary if an adjacent pair of sub-coils is not active, i.e. does not generate a magnetic field.

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

The partial coils of a partial coil pair are advantageously connected in series or in parallel with one another. This advantageous further development makes it possible to reduce the number of partial coil pairs required, so that material and structural volume can be 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 of identical design. According to a further development in this regard, partial coils of different partial coil pairs have different numbers of turns and conductor cross-sections. In relation to the stack detailed above, this means that the partial coils arranged between two metal plates are identical to one another. However, the partial coils of a partial coil pair arranged further above or below in the stack can have a lower number of turns, but a larger conductor cross-section in order to be able to carry the higher currents which 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 useful 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, the same reference symbols referring to components with the same effect and the

Figure 1

an embodiment of the device according to the invention,

Figure 2

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

Figure 3

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

Figures 4.5

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

Figure 1 shows an 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 a high-voltage network carrying AC voltage with a nominal voltage of 325 kV. In the circuit 2, a converter unit 4 can be seen in series with the compensation winding 3, which converter unit is designed, for example, as a diode. A diode can also be referred to as a non-controllable power semiconductor.

The converter unit 4 enables current to flow through it in only one direction, which is defined by the tip of the triangle (i.e. in Figure 1 from left to right) is indicated. The grounded circuit 2 also has switching branches 5 ₁ , 5 ₂ , 5 ₃ ... 5 _n-1 , 5 _n connected in parallel. In each switching branch, a switching unit 6 ₁ , 6 ₂ , 6 ₃ ... 6 _n-1 , 6 _n and a current limiting throttle 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 shown embodiment realized as thyristors, which take over the function of the figuratively shown diode 4. The diode 4 is therefore part of the switching unit and integrated into it. In other words, the rectifying effect of a converter unit is also taken over by the electronic switching unit. After ignition, a thyristor allows current to flow through it in only one direction.

In this case, thyristor 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 aid 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, a signal is provided which corresponds to the direct flux component and which is transferred to the control unit 8.

The compensation winding 3 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 via the circuit 2. The size of this current flowing in the circuit is dependent on the number of active and parallel switching branches. The more switching branches are actively switched by igniting the thyristor assigned to it, the greater the current flowing in circuit 2, which is also established in compensation winding 3, so that due to the inductive coupling, a direct flux component in the core, which is caused by direct current flowing through the high voltage and / or low voltage winding is caused, is compensated.

The control unit 8 is a particularly simple control unit, since it only connects switching branches in parallel by continuously igniting the thyristors or by omitting continuous ignition, 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, virtually error-free and robust device for direct current compensation is provided.

Figure 2 illustrates the structure of an inductance 6 ₁ , ... 6 _n as an example. It can be seen that the current limiting reactor 6 _{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 figures. The two partial coils 10 and 11 form a partial coil pair 13. An inductance constructed in this way can be put together with other inductances 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 of the metal plates 13 of the stack 14 have the same thickness. Only a metal plate is necessary between two pairs of coil sections 13 arranged one above the other. The metal plates are preferably designed to be identical to one another. The metal plates are preferably plates made of thin electrical steel sheets.

The number of coil pairs required can be limited by means of an appropriate interconnection. In Figure 4 two sub-coils 10 and 11 are connected in series as an example. Figure 5 shows the two partial coils 10 and 11 of a partial coil pair 13 connected in parallel. Since the partial coil pairs according to Figure 4 and Figure 5 otherwise not differentiate further, the partial coil pair 13 according to FIG Figure 5 only a quarter of the inductance of the partial coil pair according to Figure 4 on. If you designate partial coil pairs with full inductance with L1, see Figure 4 , and partial coil pairs according to Figure 5 that are connected in parallel with each other and thus have a quarter of the inductance of L1 with L2, 12 switching stages can be implemented with eight partial choke pairs, as can be seen from Table 1. Table 1: n 1 2 3 4th 5 6th 7th 8th L1 L2 L1 L2 L1 L2 L1 L2 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 4th 1 1 1 0 0 0 0 0 5 1 1 0 1 0 0 0 0 6th 1 1 1 1 0 0 0 0 7th 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 throttles differently. If n is the number of partial coil pairs with different inductances, then the number of possible switching stages results from 2 ⁿ -1. Ideally, the partial coil pairs are graded in such a way that the following partial coil pair only has half the inductance. This means that with n partial coil pairs, 2 ⁿ -1 switching stages can be implemented with the same increments.

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. As a result of the parallel connection, the inductances of the partial choke pairs can now be quartered again, so that, according to this further development of the invention, four partial choke pairs with different inductances L1, L2, L4 and L8 result.

It can be seen that only four partial coil pairs are required to achieve 15 switching steps, 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 2 3 4th L1 L2 L4 L8 1 1 0 0 0 2 0 1 0 0 3 1 1 0 0 4th 0 0 1 0 5 1 0 1 0 6th 0 1 1 0 7th 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 14th 0 1 1 1 15th 1 1 1 1

Claims (14)

Device (1) for suppressing a magnetic direct current component in the magnetizable core of an electrical device, with - a compensation winding (3) for generating a magnetic flux in the core, the effect of which is opposite to the direct flux component, - A circuit (2) in which the compensation winding (3) is arranged, - At least one converter unit (4) arranged in the circuit (2) and in series with the compensation winding (3), which enables a current to flow through it in only one direction, - A number of switching branches (5 ₁ , 5 ₂ , ... 5 _n ) which are arranged in the circuit parallel to one another and each in series with the compensation winding (3) and with the converter unit (4), with each switching branch (5 ₁ , 5 ₂ , ... 5 _n ) a switching unit (6 ₁ , 6 ₂ , ... 6 _n ) and a current limiting choke (7 ₁ , 7 ₂ , ... 7 _n ) are connected in series, and - one with each switching unit (6 _1, 6 _2, ... 6 _n) associated control unit (8), the (... 6 _n 6 _1, 6 ₂₎ is adapted to operate each switching unit, such that a current flow via so many switching branches (5 ₁ , 5 ₂ , ... 5 _n ) is made possible that an input signal of the control unit (8) is minimized. Device (1) according to claim 1,
characterized in that
each switching unit is an electronic switching unit (6 ₁ , 6 ₂ , ... 6 _n ). Device (1) according to claim 2,
characterized in that
every electronic switching unit is a thyristor (6 ₁ , 6 ₂ , ... 6 _n ). Device (1) according to claim 2 or 3,
characterized in that
each switching unit (6 ₁ , 6 ₂ , ... 6 _n ) has at least two thyristors connected in parallel in opposite directions. Device (1) according to claim 3 or 4,
characterized in that
the thyristors (6 ₁ , 6 ₂ , ... 6 _n ) serve at least partially as a current direction unit (4). Device (1) according to one of the preceding claims,
characterized in that
the current limiting chokes (7 ₁ , 7 ₂ , ... 7 _n ) are identical or different from one another. Device (1) according to claim 6,
characterized in that
each current limiting choke (7 ₁ , 7 ₂ , ... 7 _n ) is composed of partial coils (10, 11), two partial coils (10, 11) being arranged next to one another between two metal plates (12) to form a partial coil pair (13) and each sub-coil (10, 11) is equipped with connection terminals. Device (1) according to claim 7,
characterized in that
the partial coil pairs (13) are arranged one above the other, so that a coil stack (14) is formed. Device (1) according to claim 7 or 8,
characterized in that
the partial coils (10, 11) of a partial coil pair (13) are connected in series or parallel to one another. Device (1) according to one of claims 7 to 9,
characterized in that
the partial coils (10, 11) of a partial coil pair (13) are identical to one another. Device (1) according to claim 10,
characterized in that
Partial coils (10, 11) of different partial coil pairs (13) have different numbers of turns and conductor cross-sections. Electrical device having a core and at least one winding which is set up to generate a magnetic flux in the core, and a device (1) according to one of Claims 1 to 8 which is inductively coupled to the core. Electrical device according to claim 9,
characterized in that
a sensor (9) is provided for determining the direct current component in one or the winding, the sensor (9) being connected on the output side to the control unit (8). A method for suppressing a direct current component during the operation of 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 a compensation winding arranged in a circuit (2) and inductively coupled to the core, the Circuit (2) at least one converter unit (4) arranged in series with the compensation winding (3), which enables current to flow through it in only one direction, and has a number of switching branches (5 ₁ , 5 ₂ , ... 5 _n ), in the circuit (2) parallel to each other and are each connected in series with the compensation winding (3) and the inverter unit (4), wherein in each switching branch (5 _1, 5 _2, ... 5 _n) a switching unit (6 _1, 6 ₂ , ... 6 _n ) and a current limiting choke (7 ₁ , 7 ₂ , ... 7 _n ) are connected in series, in which - the output signal of a sensor (9), which detects the direct current component in one or the winding, is fed to a control unit (8), - The control unit (8) _{switches on so many switching units (6 1} , 6 ₂ , ... 6 _n ) that a current flow is established in the circuit (2) which minimizes the direct flux component.

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