FI3753082T3

FI3753082T3 - Electrical circuit for reactive power compensation

Info

Publication number: FI3753082T3
Application number: FIEP18716956.0T
Authority: FI
Inventors: Jonas Pinkwart; Kwok Tung Wong
Original assignee: Siemens Energy Global Gmbh & Co Kg
Priority date: 2018-03-29
Filing date: 2018-03-29
Publication date: 2023-09-27
Also published as: PL3753082T3; WO2019185151A1; EP3753082A1; EP3753082B1

Claims

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2017P27181WE

Description

Electrical circuit for reactive power compensation

The invention relates to an electrical circuit, comprising a load and at least one voltage source convertor for reactive power compensation, which is connected in series with the load.

The invention furthermore relates to a device comprising the electrical circuit.

The invention furthermore relates to a method for reactive power compensation which uses the electrical circuit.

Reactive power often occurs in single-phase or polyphase AC power supply systems, in particular in three-phase AC power supply systems, for example during the use of arc furnaces at a three- phase feeding power supply system.

Reactive power is understood to mean energy per time which does not contribute to the active power of an electrical consumer, which is a load in the technical sense, and goes beyond the energy or active power actually converted per time by the electrical consumer.

Large electrical consumers have to bear costs not only for the active power drawn, but also for the drawing of reactive power.

Moreover, the reactive power is fed back into the power supply system that feeds the electrical consumer, or is drawn from said power supply system, where it often causes additional loadings of electrical components of the feeding power supply system and often causes considerable voltage fluctuations.

Both power supply system operators and large electrical consumer industries are therefore interested in reducing the reactive power demand to the greatest possible extent.

Reactive power compensation, for example, can be used for this purpose.

In this case, inductive or capacitive reactive power can be compensated for by means of capacitive or inductive consumers.

By way of

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2017P27181WE example, compensation can be effected using capacitors or inductors or impedances.

Reactive power compensation is effected for example with the use of STATCOM installations in electrical three-phase circuits.

The latter comprise, inter alia, a voltage source convertor, also referred to as power electronic freguency convertors or self-commutated convertors.

In known embodiments, voltage source convertors for reactive power compensation in a series or parallel connection comprise DC capacitors besides semiconductor elements.

The DC capacitors have to be charged before operation and their voltages have to be kept in a tolerance band during operation.

In order that a voltage source convertor can operate reliably, it is desirable for the capacitor voltages to be in a permissible range.

A charging and discharging method does already exist for voltage source convertors connected in parallel with the load.

In this case, the DC capacitors are charged by the current from the feeding power supply system.

For voltage source convertors that are connected in series with the load for compensation purposes, without further precautions no charging or discharging current can flow if the load is an open circuit, i.e. the electrical circuit is interrupted.

Since the voltage source convertor is connected in series with the load, however, this current flow cannot always be guaranteed.

In particular directly after the switching on of the voltage source convertor, which results in precharging, and during times in which the load carries no current, it is thus not possible to control the capacitor voltages.

It is known to realize the serial compensation of a rapidly variable nonlinear load by means of a so-called "Smart Predictive Line Controller” (SPLC). The latter is based on a thyristor- controlled inductor which acts like a variable inductance.

The total impedance and accordingly also the reactive power thus

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2017P27181WE remain approximately constant.

This can result in an improved performance with regard to power supply system reactions.

The European patent application EP 3 142 239 Al discloses equipping a modular multilevel power converter with an auxiliary charging energy supply unit in order to precharge the capacitors of the power converter cells.

An auxiliary control energy supply unit is additionally used in order to provide control energy for controlling the semiconductor switches of the power converter cells during the charging process.

The European patent application EP 0 982 827 Al discloses a series compensation device having a power converter and a filter circuit, the filter circuit consisting of two inductor coils and a capacitor.

A further electrical circuit is known from EP2947766A1.

The object is to provide an electrical circuit which improves the usability of the electrical circuit with the voltage source convertor.

The invention discloses an electrical circuit having the features of Claim 1 and a method having the features of Claim 7.

The invention provides an electrical circuit in which the voltage source convertor is configured to be functional in a zero-load state of the load, in which the load does not carry any current.

The circuit according to the invention has the advantage that it enables the voltage source convertors to be controlled even during times in which the load carries no current, while maintaining the advantages of a circuit with the voltage source convertor.

The inventors have recognized that the usability of the electrical circuit with voltage source convertor mentioned in

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2017P27181WE the introduction can be improved in a particularly simple manner by virtue of the fact that the operation of the voltage source convertor is controllable even in the zero-load state of the load.

In this regard, in particular large, rapidly variable nonlinear loads can be operated even at power supply systems with stringent limit values for voltage quality.

Conventional parallel compensation, such as e.g.

SVC, can probably be dispensed with.

A smooth current flow into the load enables optimized operation and thus higher production and/or lower loading of the operating elements.

The voltage source convertor comprises a capacitor arrangement and the electrical circuit is configured to control the charge and/or voltage of the capacitor arrangement in the zero-load state of the load.

Preferably, the voltage source convertor is a polyphase voltage source convertor in the case of a polyphase feeding power supply system, particularly preferably a three- phase voltage source convertor in the case of a three-phase feeding power supply system.

Alternatively, however, electrical circuits are also provided, in principle, in which a single- phase voltage source convertor is provided, particularly in the case of a single-phase feeding power supply system.

In embodiments, a single-phase voltage source convertor is provided per phase present.

Voltage source convertors with a capacitor arrangement are known in principle.

Hitherto, however, they have not been configured to control the charge and/or voltage of the capacitor arrangement in the zero-load state of the series- connected load.

The use of a voltage source convertor which is known in principle can nevertheless be particularly simple.

Said voltage source convertor is known to the person skilled in the art in terms of its principle as an electrical component and merely has to be configured accordingly to control the capacitor arrangement in the zero-load state.

The capacitor arrangement can comprise one or more capacitors.

The invention makes it possible to precharge the capacitors of

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2017P27181WE a voltage source convertor if the serial load to be compensated for has not yet been switched on or is not carrying a current for some other reason.

The electrical circuit comprises an impedance configured to provide a circulating current for charging and discharging the capacitor arrangement.

The impedance is preferably a polyphase impedance, particularly preferably a three-phase impedance.

The number of phases of the impedance preferably corresponds to a number of phases of the feeding power supply system.

The impedances are then arranged for charging and/or discharging the capacitor arrangement in the zero-load state of the load.

Embodiments provide for the impedance to be embodied between the phases or in relation to a star point.

In some embodiments, the impedance is embodied with low losses.

The impedance is interposed between the load and the voltage source convertor.

In some embodiments, the impedance comprises a plurality of individual impedance elements.

Three individual impedance elements are preferably provided in the case of a three-phase impedance.

A number of the impedance elements preferably corresponds to the number of phases of the feeding power supply system.

In some embodiments, an embodiment of the individual impedance elements in a star connection or a delta connection is provided.

Preferably, an individual impedance element is assigned to each phase output of the voltage source convertor.

Embodiments provide for the impedance to be embodied as a filter circuit.

In some embodiments, said filter circuit performs not only the task of carrying circulating currents, but also, during the operation of the load, the task of reactive power compensation and/or the filtering of harmonics.

The advantage of this possibility is that no switchable elements are required because the losses are low.

18716956.0 - 0 - 2017P27181WE Embodiments comprise the filter circuit embodied in a star connection or in a delta connection. Preferably, the impedance of the filter circuit is embodied between the phases or in relation to a star point. The load is preferably a three-phase load. The installation is preferably a three-phase installation,

i.e. preferably configured for feeding by a three-phase feeding power supply system. In some embodiments, a star connection can be particularly advantageous. Alternatively, a delta connection is also possible. Some embodiments provide for determining the power and the tuning frequency of the filter circuit in the design of the overall installation. In some embodiments, the filter circuit is damped. The filter circuit is preferably provided with one or more resistors. The device can thereby be damped. Some embodiments comprise one or more further filter circuits that are preferably connected in parallel with the load at the feeding power supply system. That preferably means that said one or more further filter circuits are installed in parallel with the apparatus at the feeding power supply system, also referred to as rail. Some embodiments comprise a convertor. In some embodiments, the convertor connects the load to an AC voltage power supply system. Embodiments provide a control unit for the convertor. In some embodiments, control parameters of the load serve as input data for the control unit. Some embodiments provide for the convertor comprising the voltage source convertor. Some embodiments additionally provide for the convertor to comprise a series inductor, which, in some embodiments, is connected in series upstream of the voltage source convertor in the direction of the feeding power supply system. In embodiments, the electrical circuit comprises one or more measurement transducers. Preferably, provision is made of one

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2017P27181WE measurement transducer per phase.

The measurement transducer is preferably configured to provide voltage and current measurement values for controlling the electrical circuit.

Preferred measurement transducers are current transformers and voltage transformers.

If the measurement transducer is the current transformer, then it is preferably connected in series with the voltage source convertor.

If the measurement transducer is the voltage transformer, it is preferably connected either between the phases or between one phase and ground.

The invention additionally provides a device comprising an electrical circuit according to the invention.

The voltage source convertor in the device is configured to be functional in the zero-load state of the load, in which the load does not carry any current.

The device according to the invention thus has the advantage that, by way of the electrical circuit, it enables the voltage source convertor to be controlled even during times in which the load carries no current, while maintaining the advantages of a circuit with voltage source convertor.

By way of example, a harmonized current flow into the load of the device can enable a higher productivity.

In some embodiments, the device is a device which feeds an arc furnace.

In some embodiments, a furnace voltage of the arc furnace can be provided by the electrical circuit according to the invention in a manner exhibiting greater stability than without serial compensation.

This can result in a higher power input into contents of the furnace, such as scrap, for example, and/or less wear on electrodes of the arc furnace and thus overall in an increase in productivity.

By way of example, current-carrying elements of the arc furnace, such as electrodes or heating elements and the arc itself, can thus be the load of the electrical circuit.

18716956.0 - 8 - 2017P27181WE The invention additionally provides a method for reactive power compensation which uses an electrical circuit according to the invention.

In the method, the voltage source convertor in the electrical circuit functions in the zero-load state of the load, in which the load carries no current.

The voltage source convertor is thus functional independently of the state of the load, even in the zero-load state of the load.

The method according to the invention thus has the advantage that, by way of the electrical circuit, it enables the voltage source convertors to be controlled even during times in which the load carries no current, while maintaining the advantages of a circuit with voltage source convertor.

The above-described properties, features and advantages of this invention and the way in which they are achieved will become clearer and more clearly understood in association with the following description of the exemplary embodiments which are explained in greater detail in association with the drawings, in which: Figure 1 shows an electrical circuit in accordance with the prior art without additional impedances, and Figure 2 shows an electrical circuit in accordance with one embodiment of the invention, in which a voltage source convertor is configured to be functional in a =zero- load state of a load, in which the load does not carry any current.

Figure 1 shows an electrical circuit for reactive power compensation in accordance with the prior art.

The circuit is part of a device 1, in the present case for feeding a load 2, here an arc furnace.

In this exemplary embodiment, the electrical circuit comprises a three-phase load 2, which is electrically connected to a respective convertor 6a-c via a first conductor

18716956.0 - 9 - 2017P27181WE 3, a second conductor 4 and a third conductor 5. Each convertor 6a-c electrically connects the respective conductor 3, 4, 5 to a respective first, second and third electrical phase 13, 14, 15 of a feeding power supply system 16 at a respective first, second and third transfer point 10, 11, 12 via a respective first, second and third electrical connection 7, 8, 9. The circuit and hence the device 1 is thus electrically connected to the feeding power supply system 16 as described. The first conductor 3 is thus electrically connected to the first phase 13, the second conductor 4 is electrically connected to the second phase 14, and the third conductor 5 is electrically connected to the third phase 15. The electrical circuit furthermore comprises a control unit 17, for example an SPLC. The control unit 17 is connected to the respective one of the three phases 13, 14, 15 of the feeding power supply system 16 via a first, a second and a third voltage transformer 18, 19, 20, for detecting respective voltages. The control unit 17 is electrically connected to the respective convertor 6a-c of each phase via three connecting lines 21, 22,

23. The control unit 17 is designed to the effect that control parameters of the load 2 serve as input data for the control unit 17. The control unit 17 is furthermore configured to the effect that, for reactive power compensation, it transmits control signals to the respective convertor 6a-c of each phase via the first, second and third connecting lines 21, 22, 23. In figure 1 and in figure 2, for example, the feeding power supply system 16 is a medium-voltage power supply system having a voltage of between 10 and 35 kV for feeding the load 2. Figure 2 shows an electrical circuit in accordance with one embodiment of the invention, which is realized in a device 1 that feeds an arc furnace. The electrical circuit comprises a load 2, in this case once again a three-phase load 2. Furthermore, the electrical circuit comprises, in this case for each phase of the load 2, a voltage source convertor 24a-c, which is connected

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2017P27181WE in series with the load 2. In the embodiment of the invention shown, a respective series inductor 25a-c is connected upstream of the voltage source convertor 24a-c of each phase 13, 14, 15. The series inductor 25a-c is merely optional, however.

The respective series inductor 25a-c and the respective voltage source convertor 24a-c of each of the three phases 13, 14, 15 form a convertor 6a-c for the respective phase 13, 14, 15.

As is explained in greater detail below, the voltage source convertor 24a-c is configured to be functional in a zero-load state of the load 2, in which the load 2 carries no current.

Fach of the three voltage source convertors 24a-c, one per phase 13, 14, 15, comprises a capacitor arrangement, which is not illustrated.

In contrast to the electrical circuit from figure 1, in the electrical circuit from figure 2 provision is made for the electrical circuit to be configured to control the charge and/or voltage of the capacitor arrangement in the zero-load state of the load 2.

For this purpose, the electrical circuit comprises an impedance 26, which is a three-phase impedance 26 in this exemplary embodiment of the invention.

The three-phase impedance 26 is configured to provide a circulating current for charging and discharging the capacitor arrangement.

It is configured to do so even if the load 2 is in the zero-load state.

Therefore, it is possible to precharge the capacitors of the respective voltage source convertor 24a-c if the serial load to be compensated for has not yet been switched on and to control the capacitor voltage even during times in which the load 2 carries no current.

In order to achieve this, the three-phase impedance 26 is respectively interposed between the load 2 and the voltage source convertor 24a-c.

To put it more precisely, an individual one of a total of three impedance elements 27a-c of the three-phase impedance 26 is assigned to each phase 13, 14, 15. A respective

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2017P27181WE impedance element 27a-c is interposed between the respective phase of the load 2 and the assigned convertor 6a-c.

The three- phase impedance 26 is embodied as a filter circuit, for example.

As is shown in figure 2, the three-phase impedance 26 is electrically connected in each case to a phase between voltage source convertor 24a-c and load 2. In the present exemplary embodiment, the three impedance elements 27a-c of the three- phase impedance 26 are embodied in a star connection.

In embodiments that are not shown, the three impedance elements 27a-c are embodied in a delta connection.

The embodiment of the invention as shown in figure 2 additionally comprises measurement transducers, here current transformers and voltage transformers.

The current transformers are connected in series with the voltage source convertor.

The voltage transformers are connected between the phases.

In order to simplify the illustration, the measurement transducers are not illustrated.

In the exemplary embodiment in accordance with figure 2, the filter circuit is eguipped with resistors in order to achieve a damping.

The resistors are likewise not illustrated in order to simplify the illustration.

In embodiments that are not shown, the electrical circuit comprises at least one further filter circuit connected in parallel with the load 2. Said further filter circuit is then installed at the power supply system rail, that is to say the feeding power supply system 16.

The invention is illustrated with reference to figure 2 for the case of a three-phase feeding power supply system 16, a three- phase impedance 26 having three individual impedance elements 27a-c and a three-phase load 2. However, embodiments of the invention that are not illustrated are single-phase, for example, with the corresponding modifications, such that the elements for

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2017P27181WE the second phase 14 and the third phase 15 are omitted in each case.

The electrical circuit illustrated in figure 2 and also the abovementioned embodiments that are not shown can be used for a method for reactive power compensation according to the invention.

The invention accordingly relates to an electrical circuit, comprising a load 2, and at least one voltage source convertor 24a-c for reactive power compensation, which is connected in series with the load 2. The voltage source convertor 24a-c is configured to be functional in a zero-load state of the load 2, in which the load 2 does not carry any current.

The invention additionally relates to a device 1 and furthermore relates to a method for reactive power compensation.

The solution for reactive power compensation according to the invention is able to be realized and usable in a particularly simple manner.

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List of reference signs

1 Device

2 Load

3 First conductor

4 Second conductor

Third conductor

6a-c Convertor

7 First electrical connection 8 Second electrical connection 9 Third electrical connection First transfer point

11 Second transfer point

12 Third transfer point

13 First phase

14 Second phase

Third phase

16 Feeding power supply system 17 Control unit

18 First step-down transformer 19 Second step-down transformer Third step-down transformer 21 First connecting line

22 Second connecting line

23 Third connecting line

24a-c Voltage source convertor 25a-c Series inductor

26 Three-phase impedance

27a-c Impedance element