WO2019004897A1

WO2019004897A1 - A variable shunt reactor

Info

Publication number: WO2019004897A1
Application number: PCT/SE2018/050609
Authority: WO
Inventors: Agne Fälldin
Original assignee: Kkm Ab
Priority date: 2017-06-28
Filing date: 2018-06-12
Publication date: 2019-01-03
Also published as: SE1750841A1; SE541456C2

Abstract

Described is, among other things,a variable shunt reactor(100) comprising at least three cores (101, 102, 103, 104) and at least two AC windings (106, 108) wound around at least a respective first core and a second core. The variable shunt reactor further comprises at least one DC winding(109) wound around at least one third core, and configured to, when a DC current is applied to the DC winding, generate or alter a magnetic flow in cores wound by an AC winding. Hereby a variable shunt reactor can be formed wherein a varying DC current in the DC winding will alter the current in the AC windings. This in turn will vary the rating of a shunt reactor formed by the AC windings wound around the cores. Hereby it becomes possible to with a short response time vary the reactive power compensation of the shunt reactor. Further, the reactive power compensation can be made stepless since the DC current can be made to vary in a stepless manner.

Description

A variable shunt reactor

TECHNICAL FIELD

The present disclosure relates to methods, devices and systems used in and associated with variable shunt reactors. The disclosure also extends to a variable capacitor and method and systems associated therewith.

BACKGROUND

A shunt reactor is an absorber of reactive power and is the device most commonly used for reactive power compensation. The main function of a shunt reactor is to provide voltage stability in high voltage networks. A high voltage network or power grid can for example be an electrical network having a higher voltage than 1000 V. The shunt reactor can be directly connected to the power line or to a tertiary winding of a three-winding transformer. A shunt reactor can be seen as an inductive device that compensate for capacitive generation in a high voltage power transmission system. The shunt reactor could be permanently connected or switched via a circuit breaker. By tradition, shunt reactors have been made with a fixed rating. Recently, shunt reactors with variable rating under load (VSR) have been introduced. Variable Shunt Reactors (VSR) are typically used in high voltage energy transmission systems to stabilize the voltage during load variations. The rating of a VSR can be changed in steps. The maximum regulation range typically is a factor of two, e.g. from 100-200 Mvar. The regulation speed is normally in the order seconds per step and around a minute from max to min rating. VSRs are today available for voltages up to 550 kV. The VSR can typically be formed by a so-called gapped core, see for example WO2010083924. The variability of a VSR brings several benefits compared to a traditional, fixed, shunt reactors. The VSR can continuously compensate reactive power as the load varies and thereby securing voltage stability. Other important benefits can be reduced voltage jumps resulting from switching in and out of traditional fixed reactors, and flexibility for future (today unknown) load.

There is a constant desire to improve systems and components used for power transmission. Hence, there is a need for an improved shunt reactor and devices used for compensation in power transmission.

SUMMARY

It is an object of the present invention to provide an improved shunt reactor and system components used for power transmission.

This object and/or others are obtained by the device as set out in the appended claims.

As has been realized, existing VSRs although being somewhat flexible in that they can change rating under load suffer from only being changed in pre-defined steps, where the steps can be relatively large in size. Also, the change from one step to another is relatively slow and typically in the order of several seconds. It would be advantageous if the reactive compensation to compensate for capacitive generation in high voltage power transmission networks could be made stepless. Also, it would be advantageous if the response time to apply a changed reactive compensation power could be reduced.

In accordance with the present invention a variable inductor is provided. The variable inductor can be used as a shunt reactor that can provide stepless compensation under load. The shunt reactor in accordance with the invention can further apply a new reactive compensation power in a very short time thereby increasing the performance of the reactive compensation by reducing the response time to changed conditions in a high voltage power transmission system.

In accordance with a first aspect of the invention, a variable shunt reactor comprising at least three cores and at least two AC windings wound around at least a respective first core and a second core is provided. The variable shunt reactor further comprises at least one DC winding wound around at least one third core, and configured to, when a DC current is applied to the DC winding, generate or alter a magnetic flow in said first core and said second core. Hereby a variable shunt reactor can be formed wherein a varying DC current in the DC winding will alter the current in the AC windings. This in turn will vary the rating of a shunt reactor formed by the AC windings wound around the cores. Hereby it becomes possible to with a short response time vary the reactive power compensation of the shunt reactor. Further, the reactive power compensation can be made stepless since the DC current can be made to vary in a stepless manner.

In accordance with one embodiment the variable shunt reactor comprises a second core and a third core. The variable shunt reactor further comprises a DC winding wound around a first part of the second core and a first part of the third core. The variable shunt reactor also comprises a first core and a first AC winding wound around a first part of the first core and a second part of the second core. Further, the variable shunt reactor comprises a fourth core and a second AC winding wound around a first part of the fourth core and a second part of the third core. Hereby an efficient variable shunt reactor can be formed. The variable shunt reactor can be made symmetric and can easily be controlled with a stepless DC current fed to the DC winding.

In accordance with another alternative embodiment the variable shunt reactor comprises a first core and a fourth core and a first DC winding wound around a first part of the first core. The variable shunt reactor further comprises a second DC winding wound around a first part of the fourth core. The variable shunt reactor further comprises a second core and a first AC winding wound around a first part of the second core and around the first part of the first core. Further, the variable shunt reactor comprises a third core and a second AC winding wound around a first part of the third core and the first part of the fourth core. Hereby an alternative construction of a variable shunt reactor can be obtained that comprises more than one DC winding that can be used to control the rating of the variable shunt reactor.

In accordance with yet another embodiment the variable shunt reactor comprises a first core and a third core and also a first DC winding wound around a first part of the first core and a second DC winding wound around a first part of the third core. The variable shunt reactor further comprises a second core and a first AC winding wound around a first part of the second core and a second part of the first core. Further, the variable shunt reactor comprises a second AC winding wound around a second part of the second core and a second part of the third core. Hereby an alternative construction of a variable shunt reactor can be obtained that comprises only three cores to form the variable shunt reactor.

In accordance with some embodiments the variable shunt reactor comprises at least one pair of AC windings connected in anti-phase. Hereby an efficient configuration of a shunt reactor that can be controlled in a stepless manner is obtained. In accordance with one embodiment the at least two AC windings can be provided symmetrically in the variable shunt reactor.

In accordance with a second aspect of the invention a variable shunt reactor arrangement comprising three variable shunt reactors as described herein is provided. Hereby, a variable shunt reactor arrangement that can be used to compensate reactive power in a three-phase power grid can be achieved. In accordance with some embodiments the three variable shunt reactors are connected in a Y configuration. Further, a Petersen coil connected to a common output terminal of the three variable shunt reactors can be connected. In an alternative embodiment, the three variable shunt reactors are connected in a delta configuration. In accordance with a third aspect of the invention a variable shunt reactor system

comprising a variable shunt reactor according or a variable shunt reactor arrangement comprising three variable shunt reactors is provided. The system can comprise at least one DC current generator connected at least one DC winding and at least one DC current controller operatively connected to the DC current generator for controlling the current generated by the DC current controller. Hereby a system that can provide a controllable reactive power compensation is achieved.

In accordance with some embodiments the DC current controller is configured to control the DC current to a set point where a predetermined condition is met. Hereby a robust control system is achieved that always strives to uphold a balance in an electrical network to which the system can be connected. For example, the DC current controller can be configured to control the DC current based on a control signal where the control signal is an electrical network frequency and/or a phase angle of an electrical network.

In accordance with some embodiments when there are at least two separate DC windings provided, the system can comprise a separate DC current generator for each at least two separate DC winding. Also, if a variable shunt reactor comprises more than one DC winding a separate current generator could be provided for each DC winding. As an alternative, if one single variable shunt reactor comprises more than one DC winding, the Dc windings could be connected in series. When more than one variable shunt reactor is provided, it is possible to use one DC current generator for each variable shunt reactor such that each variable shunt reactor can be controlled individually. However, one DC current

controller can be configured to control multiple, in particular all, DC current generators of a variable shunt reactor system.

In another aspect of the invention a variable inductor comprising at least one DC core having a winding configured to be supplied with a DC current is provided. The inductor further comprises a number, at least one and typically at least two, additional cores connected to the at least one core provided with the DC winding. The variable inductor is configured to move AC magnetization from the at least one core with DC winding to said additional cores in response to a DC current supplied to the winding of the at least one DC core. Thus, a variable inductor having additional cores connected to the core provided with the DC winding is provided. This has the technical effect that any AC magnetization moved from the core with the DC winding will generate a magnetization in the core not having the DC winding. In other words, the magnetization by a DC current will move the AC magnetic field to the other cores. The invention also extends to methods for controlling a variable shunt reactor and other arrangements as described herein.

The variable inductor as describe herein can also be used to provide a variable capacitor or more generally a device providing a variable reactance. This can be obtained by

supplementing the variable inductor with at least one capacitor. The capacitor can be provided serially and or in parallel to the variable inductor. The magnitude of the capacitor can be selected to meet different implementation needs.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail by way of non-limiting examples and with reference to the accompanying drawings, in which:

- Fig. 1 is a view illustrating a variable shunt reactor according to a first embodiment,

- Fig. 2 is a view illustrating a variable shunt reactor according to a second embodiment, - Fig. 3 is a view illustrating a variable shunt reactor according to a third embodiment,

- Fig. 4 is a view illustrating connection of a variable shunt arrangement to a 3-phase power transmission line in accordance with a first configuration,

- Fig. 5 is a view illustrating connection of a variable shunt arrangement to a 3-phase power transmission line in accordance with a second configuration, - Fig. 6 is a view illustrating connection of a variable shunt arrangement to a 3-phase power transmission line in accordance with a third configuration,

- Fig. 7 is a view illustrating connection of a variable shunt arrangement to a 3-phase power transmission line also showing a control system, and

- Figs. 8a - 8c illustrates the working principle of a stepless variable shunt reactor.

- Figs. 9a - 9b illustrates an arrangement for providing a variable reactance.

DETAILED DESCRIPTION

In the following a power transmission system will be described and components, in particular a variable shunt reactor, used in a power transmission system. In the figures, the same reference numerals designate identical or corresponding elements throughout the several figures. It will be appreciated that these figures are for illustration only and are not in any way restricting the scope of the invention. Also, it is possible to combine features from different described embodiments to meet specific implementation needs. In particular, the different embodiments of the variable shunt reactor can be used in any shunt reactor arrangement configuration. Further, the term AC winding will refer to a winding or a coil configured to receive an alternating current. Similarly, the term DC winding will refer to a winding or a coil configured to receive a direct current. In accordance with the description variable shunt reactor is designed to provide a stepless compensation of reactive power within its specified range. The reactive power

compensation can typically be varied within a range of 25 - 100% of the rating of the variable shunt reactor. This is achieved by a reactor comprising a core arrangement comprising at least three cores. The cores can be said to be connected. The term "connected core" is used herein to describe two cores that are magnetically coupled such that a magnetic flux in a first core generates or alters a magnetic flux in a second core. A pair of such a first core and a second core are said to be connected cores. When more than two cores are used in a core arrangement, the cores can likewise interact magnetically. The variable shunt reactor further comprises at least one pair of AC (Alternate Current) windings provided in the core arrangement. The AC windings are connected to the power

transmission line to be compensated for generating a compensation of reactive power. In particular, the compensation of reactive power can be used in an electrical network, such as a power grid. The reactive power compensation in the variable shunt reactor is varied by providing at least one DC (Direct Current) winding in the core arrangement. By supplying a controlled DC current to the DC winding, the reactive power compensation of the reactor formed by the core arrangement and the at least one pair of AC windings is controlled. The controlled variation of reactive power compensation varied is achieved by transposing the magnetic flux generated by the AC winding in the core arrangement by altering the magnetic flux in the core arrangement with the controlled DC current in the DC winding.

In other words, additional cores connected to the core(s) provided with the DC winding(s) are provided. This has the effect that any AC magnetization moved from the core with the DC winding(s) will generate a magnetization in the core not having the DC winding. In other words, the magnetization by a DC current will move the AC magnetic field to the other cores. Hereby a variable inductor is provided that can be controlled by the DC current.

Below a number of different alternatives for constructing such a variable shunt reactor are described. In Fig. 1, a view illustrating a variable shunt reactor 100 according to a first embodiment is shown. The variable shunt reactor 100 comprises four cores 101, 102, 103, and 104 for carrying a magnetic flux. A DC winding 109 is wound around a first part of the second core 102 and also around a first part of the third core 103. The variable shunt reactor 100 further comprises a first core 101 and a first AC winding 106 wound around a first part of the first core 101 and a second part of the second core 102. The variable shunt reactor 100 further comprises a fourth core 104 and a second AC winding 108 wound around a first part of the fourth core 104 and a second part of the third core 103. The pair of AC windings 106 and 108 are connected to each other and the free ends of the AC windings form an AC input terminal and an AC output terminal, respectively of the variable shunt reactor 100. The reactive power compensation provided by the variable shunt reactor 100 can be varied by varying a DC current supplied to the DC winding 109 as will be described in more detail below. In other words, by feeding a controlled DC current through the DC winding 109 the rating of the variable shunt reactor 100 can be controlled. In Fig. 2, a view illustrating a variable shunt reactor 140 according to a second, alternative, embodiment is shown. In Fig. 2 a variable shunt reactor 140 also having four cores 101,

102, 103, and 104 as the embodiment of Fig. 1 is shown. However, as a difference to the configuration of the variable shunt reactor 100 in Fig. 1, the variable shunt reactor 140 is configured with two DC windings 111 and 114. In Fig. 2, a first DC winding 111 is wound around a first part of the first core 101. The variable shunt reactor 140 further comprises a second DC winding 114 wound around a first part of the fourth core 104. The variable shunt reactor 140 further comprises a first AC winding 112 wound around a first part of the second core 102 and also around the first part of the core 101. The variable shunt reactor 140 further comprises a second AC winding 113 wound around a first part of the third core 103 and also around the first part of the fourth core 104. The pair of AC windings 112 and 113 are connected to each other and the free ends of the AC windings form an AC input terminal and an AC output terminal, respectively of the variable shunt reactor 140. The DC windings 111 and 114 can be separate windings but are preferably connected in series. The reactive power compensation provided by the variable shunt reactor 140 can be varied by varying a DC current supplied to the DC windings 111 and 114 as will be described in more detail below.

In Fig. 3, a view illustrating a variable shunt reactor 160 according to a third embodiment is shown. The variable shunt reactor 160 comprises only three cores 101, 102, and 103 and has two DC windings 121, 124 and a pair of AC windings 122, 123. In Fig. 3, a first DC winding 121 is wound around a first part of the first core 101. The variable shunt reactor 160 further comprises a second DC winding 124 wound around a first part of the third core

103. The variable shunt reactor 160 further comprises a first AC winding 122 wound around a second part of the first core 101 and a first part of the second core 102. The variable shunt reactor 160 further comprises a second AC winding 123 wound around a second part of the second core 102 and also wound around a second part of the third core 103. The pair of AC windings 122 and 123 are connected to each other and the free ends of the AC windings form an AC input terminal and an AC output terminal, respectively of the variable shunt reactor 160. The DC windings 121 and 124 can be fed with separate DC currents but are advantageously connected in series. The reactive power compensation provided by the variable shunt reactor 160 can be varied by varying a DC current supplied to the DC windings 121 and 124 as will be described in more detail below. In the above exemplary embodiments described in conjunction with Figs. 1 - 3 one pair of AC windings are shown. It is however possible to use multiple pairs of AC windings.

Further, it can be advantageous to connect the AC windings such that a pair of AC windings are connected in anti-phase, i.e. being wound in opposite directions. In particular, the number of turns in one direction for a winding can be equal to the number of turns in another winding in an opposite direction. When multiple windings are used it is preferred that the total number of turns in one direction is equal to the number of turns in the opposite direction. The cores can be made of any known suitable material for manufacturing a shunt reactor such as an iron material. The AC windings and the DC windings can be

manufactured by any suitable material used in manufacturing shunt reactors, such as a copper material. The cores can be physically separated from each other by an air gap. In accordance with some embodiments the air gap between different cores can be different from each other. For example, in the exemplary embodiment in accordance with Fig. 1, the air gap between core 102 and core 103 can be small or zero and the air gap between core 101 and core 102 and between core 103 and 104, respectively can be bigger than the air gap between core 102 and core 103. By altering the air gaps between the different cores in a core arrangement, different properties with regard to the reactive compensation provided can be achieved. In particular, the rating of the variable shunt reactor can be altered. Further, the cores can be formed in any suitable geometric shape. In Figs 1 - 3 the cores are formed in a rectangular shape making it easy to manufacture closed cores. Also, the cores can be placed in a common plane as is shown in Figs. 1 - 3.

In accordance with some embodiments, the variable shunt reactor formed by a number of cores, in particular closed cores, a number of AC windings and a number of DC windings is made symmetric with respect to a plane A in the middle of the variable shunt reactor as is the case for the implementations shown in Figs. 1 - 3. The symmetry can in accordance with some embodiments be with regard to all parameters relating to the individual cores and AC/DC windings and including the number of turns for the AC windings and DC windings.

In Fig. 4 a view illustrating connection of a variable shunt reactor arrangement 172 to a 3- phase power transmission line in accordance with a first configuration is shown. The arrangement comprises three variable shunt reactors here generally represented by the reference numeral 100, one for each phase LI, L2 and L3. It is to be understood that any variable shunt reactor as described herein could be used. The shunt reactors 100 are connected in a Y configuration with a common output connected to ground or to neutral point of the system. The configuration according to Fig. 4 can be particularly advantageous for a so-called positive sequence in the electrical system to be compensated for reactive power.

In Fig. 5 is a view illustrating connection of a variable shunt reactor arrangement 174 to a 3- phase power transmission line in accordance with a second configuration is shown. The configuration of Fig. 5 is similar to the configuration of Fig. 4. However, a Petersen coil 200 is inter-connected between the output of the variable shunt reactors 100 and ground/ a neutral point of the system. It is to be understood that any variable shunt reactor as described herein could be used. The Petersen coil 200 is used as a grounding reactor in alternating-current power transmission systems. It can be designed and used to limit the current flowing to ground at the location of a fault almost to zero by setting up a reactive current to ground that balances the capacitive current to ground flowing from the electrical transmission power lines. The configuration in Fig. 5 can advantageously be used in a three- phase electrical system with so-called zero-sequence.

In Fig. 6, a view illustrating connection of a variable shunt reactor arrangement 176 to a 3- phase power transmission line in accordance with a third configuration is shown. The configuration of shunt reactors 100 in Fig. 6 is a so-called delta configuration. It is to be understood that any variable shunt reactor as described herein could be used.

In Fig. 7, a view illustrating a variable shunt reactor system 600. In Fig. 7 the variable shunt system 600 comprises a variable shunt reactor here represented by a variable shunt reactor 160. For a three phase electrical power system three identical variable shunt reactors can be used, one for each phase and be connected for example as is shown in Figs. 4 - 6. Here only one phase is shown to simplify the description. Thus, the system 600 can then preferably comprise three identical arrangements as depicted in Fig. 7, one for each phase of a power transmission system. The system 600 comprises a DC current generator 300 that supplies a DC current to the DC winding(s) of the variable shunt reactor 160. Further, the DC current generator 300 can be controlled by a controller 400. The controller 400 is configured to control the DC current such that the variable shunt reactor generates a desired reactive power compensation. The desired reactive power compensation is typically achieved when the frequency in the network is at its nominal frequency such as for example 50 Hz or 60 Hz (or within a range around such a value) and/or when the phase angle is zero (or within a range around zero). Then there are no or only small inductance losses in the network and the system provides the correct network frequency. Hence, the DC current is controlled to a set point where a predetermined condition is met based on a control signal. The control signal can be the phase angle and/or the frequency in the electrical network. The predetermined condition can typically be when the network frequency is within a set range or at a frequency set point and/or the inductance in the network is with a set range or at an inductance set point. One single controller 400 can be used to control multiple DC current generators 300. In Figs. 8a - 8c, the working principles of the stepless reactive power compensation is described in more detail. As is clear from the above, providing a DC current to a DC winding provided in a core arrangement of connected cores will cause a current in an AC winding also provided in the core arrangement. The example in Figs. 8a - 8c is for illustration purposes only and numerous other configurations are envisaged including the ones described herein. In Fig. 8a the magnetic flux generated by the current in the AC windings are illustrated by the white arrows. Assuming a symmetric configuration where the AC windings are provided in pair (s) and where each winding in a pair has the same number of turns and the windings of a pair are connected in anti-phase, the magnetic flux generated by the AC windings will be zero in DC winding, and no current will be generated in the DC winding. In Fig. 8b, a DC current is applied. The DC current will also generate a magnetic flux as is illustrated by the black arrows in Fig. 8b. By increasing the DC current, the magnetic flux generated by the DC current will transpose the magnetic flux, and thus the current generated by the AC windings to other parts of the variable shunt reactor here to other connected cores. This is illustrated by the large arrows in Fig. 8c. Thus, by applying a DC current in a DC winding a larger current in a connected AC winding can be generated thereby increasing the reactive power compensation. Because the DC current can be varied stepless, the reactive power compensation can be made stepless. Further, because the response time between when an increased DC current is applied to when this will generate an increased current in the AC winding is short, the variable shunt reactor can react very quickly to changed conditions in an electrical network to which the variable shunt reactor is connected. The variable inductor as described herein can also be used to provide a variable capacitor or more generally a device for generating a variable reactance.

In Fig 9a variable inductor 100 is shown. The variable inductor 100 can be any variable inductor configured to move an AC magnetic field from a core provided with a DC winding to another core based on a DC current supplied to the DC winding. For example, any of the shunt reactors described herein could be used. The variable inductor 100 is connected in series with a capacitor 180. This will form an arrangement 190 that can produce a variable reactance.

In Fig. 9b another embodiment of an arrangement 190 for providing a variable reactance is shown. In Fig. 9b the capacitor 180 is connected in parallel with the variable inductor 100. Other combinations of a variable inductor connected to a capacitor are also envisaged to meet different implementation needs.

By providing a variable inductor 100 as described herein connected to a capacitor 180 an arrangement 190 that can provide a variable reactance can be provided. By selecting the capacitance of the capacitor, different types of variable reactance arrangements can be provided. For example, if the capacitance is selected large in relation to the maximum inductance that the variable inductor can generate, a variable capacitor is formed. Such an arrangement can be useful when a variable capacitance is desired. In such an

implementation the capacitance value of the capacitor can be selected equal to or larger than the maximum inductance that can be generated by the variable inductor. When the inductance of the variable inductor 100 is varied the capacitance value of the arrangement will vary.

On the other hand, if the capacitor is selected smaller than the maximum magnitude of the inductor, an arrangement that can vary the reactance to implement both a variable capacitor and a variable inductor is formed. Such an arrangement can be useful when the reactance should be possible to vary between a capacitance value and an inductive value. In such an implementation the capacitance value of the capacitor can be selected to about half of the maximum inductance that can be generated by the variable inductor. For example, the capacitance value of the capacitor can be selected to be in the range of 10 - 90% or in the range of 30 - 70% of the maximum inductance that can be generated by the variable inductor. When the inductance of the variable inductor 100 is varied the reactance value of the arrangement will vary between a capacitance value and an inductance value.

Further, depending on if the arrangement is desired to have band-stopping frequency properties or band passing properties, the arrangement according to Fig. 9a or 9b can be selected. When the arrangement according to Fig. 9a is selected, the arrangement will have band passing properties. When the arrangement according to Fig. 9b is selected, the arrangement will have band stopping properties.

Claims

CLAIMS 1. A variable inductor (100, 140. 160) comprising at least one DC core having a winding configured to be supplied with a DC current, the inductor further comprising a number of additional cores connected to the at least one core provided with the DC winding, the variable inductor being configured to move AC magnetization from the at least one core with DC winding to said additional cores in response to a DC current supplied to the winding of the at least one DC core.

2. A variable shunt reactor (100, 140, 160), having a variable inductor according to claim 1 comprising:

- at least three cores (101, 102, 103),

- at least two AC windings (106, 108; 112, 113; 122, 123) wound around at least a respective first core and a second core, and

- at least one DC winding (109; 111, 114; 121, 124) wound around at least one third core, and configured to, when a DC current is applied to the DC winding, generate or alter a magnetic flow in said first core and said second core.

3. A variable shunt reactor (100) according to claim 2, comprising:

- a second core (102) and a third core (103),

- a DC winding (109) wound around a first part of the second core (102) and a first part of the third core (103),

- a first core (101),

- a first AC winding (106) wound around a first part of the first core (101) and a second part of the second core (102),

- a fourth core (104), and - a second AC winding (108) wound around a first part of the fourth core (104) and a second part of the third core (103).

4. A variable shunt reactor (140) according to claim 2 comprising:

- a first core (101) and a fourth core (104),

- a first DC winding (111) wound around a first part of the first core (101),

- a second DC winding (114) wound around a first part of the fourth core (104),

- a second core (102),

- a first AC winding (112) wound around a first part of the second core (102) and around the first part of the first core (101),

- a third core (103), and

- a second AC winding (113) wound around a first part of the third core (103) and the first part of the fourth core (104).

5. A variable shunt reactor (160) according to claim 2 comprising:

- a first core (101) and a third core (103),

- a first DC winding (121) wound around a first part of the first core (101),

- a second DC winding (124) wound around a first part of the third core (103),

- a second core (102),

- a first AC winding (122) wound around a first part of the second core (102) and a second part of the first core (101), and

- a second AC winding (123) wound around a second part of the second core (102) and a second part of the third core (103).

6. A variable shunt reactor according to any one of claims 2 - 5, wherein at least one pair of said at least two AC windings are connected in anti-phase.

7. The variable shunt reactor according to any one of claims 2 - 6, wherein said at least two AC windings are provided symmetrically in the variable shunt reactor.

8. A variable shunt reactor arrangement (172, 174, 176) comprising three variable shunt reactors according to any of claims 2 - 7.

9. A variable shunt reactor arrangement according to claim 8, wherein the three variable shunt reactors are connected in a Y configuration.

10. A variable shunt reactor arrangement according to claim 9, further comprising a Petersen coil (200) connected to a common output terminal of the three variable shunt reactors.

11. A variable shunt reactor arrangement according to claim 8, wherein the three variable shunt reactors are connected in a delta configuration.

12. A variable shunt reactor system (600) comprising a variable shunt reactor according to any of claims 2 - 7 or a variable shunt reactor arrangement according to any of claims 8 - 11, the system comprising at least one DC current generator (300) connected to said at least one DC winding and at least one DC current controller (400) operatively connected to the DC current generator for controlling the current generated by the DC current controller.

13. A variable shunt reactor system according to claim 12, wherein the DC current controller is configured to control the DC current to a set point where a predetermined condition is met.

14. A variable shunt reactor system according to claim 13, wherein the DC current controller is configured to control the DC current based on a control signal where the control signal is an electrical network frequency and/or a phase angle of the electrical network.

15. A variable shunt reactor system according to claim 13 or 14, when there are at least two separate DC windings provided, the system comprising a separate DC current generator for each at least two separate DC winding.

16. A variable shunt reactor system according to claim 15 wherein only one DC current controller is configured to control the DC current generators.

17. An arrangement (190) for generating a variable reactance comprising a variable inductor (100) according to claim 1 or a variable shunt reactor (100) according to any of claims 2 - 7 and at least one capacitor (180) connected to the variable inductor or to the variable shunt reactor.

18. The arrangement according to claim 17 wherein the at least one capacitor is connected in series to the variable inductor or to the variable shunt reactor.

19. The arrangement according to claim 17 wherein the at least one capacitor is connected in parallel to the variable inductor or to the variable shunt reactor.

20. The arrangement according to any one of claims 17 - 19 wherein the at least one capacitor is selected to have a capacitance with a magnitude equal to or larger than the maximum inductance of the variable inductor or the variable shunt reactor.

21. The arrangement according to any one of claims 17 - 19 wherein the at least one capacitor is selected to have a capacitance with a magnitude in the range of 10 -90 % of the maximum inductance of the variable inductor or the variable shunt reactor.

22. The arrangement according to claim 21, wherein the at least one capacitor is selected to have a capacitance with a magnitude in the range of 30 -70 % of the maximum inductance of the variable inductor or the variable shunt reactor.