WO2013075886A2

WO2013075886A2 - Three phase voltage control system

Info

Publication number: WO2013075886A2
Application number: PCT/EP2012/070394
Authority: WO
Inventors: Christian M. Hartmann; Bjørnar S. JOHANSEN
Original assignee: Magtech As
Priority date: 2011-11-21
Filing date: 2012-10-15
Publication date: 2013-05-30
Also published as: NO333687B1; NO20111594A1; WO2013075886A3

Abstract

The invention relates to a three phase voltage control system for connection to a three phase power supply line. The system comprises first, second and third autotransformer devices (AT1, AT2, AT3) and first, second and third magnetic controllable inductor devices (M1, M2, M3). The magnetic controllable inductor devices (M1, M2, M3) each comprises a primary winding, a secondary winding and a control winding. The first primary terminal (M1p1) of the first magnetic controllable inductor device (M1) is connected to first secondary terminal (M3s1) of the third magnetic controllable inductor device (M3), the first primary terminal (M2p1) of the second magnetic controllable inductor device (M2) is connected to first secondary terminal (M1s1) of the first magnetic controllable inductor device (M1), the first primary terminal (M3p1) of the third magnetic controllable inductor device (M3) is connected to first secondary terminal (M2s1) of the second magnetic controllable inductor device (M2), he second secondary terminal (M1s2) of the first magnetic controllable inductor device (M1) is connected to the second parallel terminal (AT2p2) of the second autotransformer (AT2), the second secondary terminal (M2s2) of the second magnetic controllable inductor device (M2) is connected to the second parallel terminal (AT3p2) of the third autotransformer (AT3) and the second secondary terminal (M3s2) of the third magnetic controllable inductor device (M3) is connected to the second parallel terminal (AT1p2) of the first autotransformer (AT1).

Description

Three phase voltage control system

FIELD OF THE INVENTION

The present invention relates to a three phase voltage control system for connection to a three phase power supply line. BACKGROUND OF THE INVENTION

Undersized lines for electric power transmission, also referred to as "weak lines", have too small a conductor cross section in relation to the load requirements and a relatively high resistance. Excessive voltage drop will result from the losses caused by undersized conductors. The excessive voltage drop results in inadequate voltage levels for the electric power connected to the lines.

A transformer is a static unit which supplies a fixed voltage determined by the number of windings on the primary and secondary sides, i.e., the transformer ratio. A fixed transformer ratio may result in a voltage that is too low when the load is high, and a voltage that is too high when the load is low. Because the load is dependent at all times on the highly variable requirements of individual electric power consumers, fixed ratio transformers are often inadequate to serve a dynamic load.

Another problem related to weak lines occurs when there is distributed power generation along the line so that the net power flow has direction toward the supplying transformer. In this case the voltage will increase as a function of the distance from the supplying transformer, hence creating a problem that is opposite of what was described for weak lines with net power consumers. The voltage of the supplying transformer will be too high at high amounts of power fed back. Again, a fixed ratio transformer will often be inadequate to serve a dynamic load. The erroneous voltage level can be compensated for by changing the voltage in steps at the transformer that is supplying the line. In one prior art approach, the voltage level is controlled by means of a load tap changer on the transformer which is connected to the individual phase at the location where the voltage reaches an unacceptable level. At present, the problem of weak lines is often solved by replacing existing lines with new lines having a larger cross section and correspondingly lower resistive losses.

Presently, several methods are employed for upgrading the line. If there is room on the existing pole, a new line can be installed on the other side of the pole in parallel with the weak line. Once the new line is installed, the old one is disconnected and removed from service. This approach allows the power transmission system to be upgraded without a noticeable interruption in service. Another method involves installing hardware for securing new lines to the existing poles, disconnecting the weak lines, and quickly installing the new lines. This approach results in a longer interruption when compared with the preceding approach. Alternatively a new route is established. Such construction involves the installation of new poles and new conductors. Significantly, before construction begins, the new route may have to be approved by local government and property owners.

In another prior art approach to voltage regulation, a mechanically controlled variac (i.e., a transformer with variable transformer ratio) is used in connection with a transformer. However, a mechanically controlled variac is rarely used anymore due to the frequent service needed on the mechanical components.

Another method that is currently employed consists of relocating the electric lines closer to users and connecting a new transformer to the relocated line where it will be closer to users. This approach is also undesirable because of the large scope of work required to relocate electric lines and the high cost associated with such a project.

US 3,409, 822 (Wanlass) describes a voltage regulator that includes a device with an AC or load winding and a DC or control winding wound on a ferromagnetic core. In a portion of the core, a DC generated flux component and an AC generated flux component are provided along the same path but with an opposite direction at all times. As a result in these portions, the flux components are subtracted and the core has a permeability that, to a limited extent, corresponds to the resulting flux. In other portions, but not the entire core, the fluxes are orthogonal to one another. For example, Wanlass shows a regulator based on flux control in the core's legs via addition or subtraction of magnetic fluxes lying in the same path (coincident fluxes with opposite signs). However, the power handling capability of the device is limited because the regulator described in Wanlass is meant for operation in the non-saturated area of the core, and the permeability range is limited to the linear region of the core.

In WO 2004053615 it is shown a system for voltage stabilization of a power supply line. The system comprises an autotransformer comprising a series winding and a parallel winding; a reactor with variable inductance, hereinafter referred to as a magnetic controllable inductor device, connected to the autotransformer, the magnetic controllable inductor device comprising; a magnetic core, a main winding wound around a first axis, and a control winding wound around a second axis, wherein the first axis and the second axis are orthogonal axes so that when the main winding and the control winding are energized, orthogonal fluxes are generated in the magnetic core; and a control system for controlling the permeability of the magnetic core to compensate automatically for voltage variations in the supply line. In a three phase system, one such voltage boosting device is used per phase, either in a star configuration or in a delta configuration.

There are some disadvantages of the above technology. The magnetic controllable inductor device of the voltage boosting device is dimensioned for the maximum power for the network it is being used in. Moreover, the magnetic controllable inductor device is dimensioned to handle maximum phase voltage in star connected networks and maximum line voltage in delta connected networks. Both of these criteria make it necessary to increase the size of the magnetic controllable inductor devices in the voltage boosting device.

The object of the invention is to provide a three phase voltage control system for voltage stabilization where the dimensioning maximum voltage over the magnetic controllable inductor devices is decreased. This allows for reducing the size of the magnetic controllable inductor devices without increasing the risk of failure of the system.

SUMMARY OF THE INVENTION

The invention relates to a three phase voltage control system for connection to a three phase power supply line, comprising:

- first, second and third autotransformer devices, each comprising a first series terminal, a second series terminal, a first parallel terminal and a second parallel terminal;

- first, second and third magnetic controllable inductor devices, each comprising:

- a magnetic core;

- a first primary terminal and a second primary terminal; - a primary winding wound around a first axis of the magnetic core and connected between the respective first primary terminal and the respective second primary terminal;

- a control winding wound around a second axis of the magnetic core, where the first axis and the second axis are orthogonal axis, and where the control winding is connected to a control system for controlling the permeability of the magnetic core to compensate for voltage variations in the power supply line;

characterized in that

- the first, second and third magnetic controllable inductor devices each comprises: - a first secondary terminal and a second secondary terminal;

- a secondary winding wound around the first axis of the magnetic core and connected between the respective first secondary terminal and the second secondary terminal; - the first primary terminal of the first magnetic controllable inductor device is connected to first secondary terminal of the third magnetic controllable inductor device;

- the first primary terminal of the second magnetic controllable inductor device is connected to first secondary terminal of the first magnetic controllable inductor device;

- the first primary terminal of the third magnetic controllable inductor device is connected to first secondary terminal of the second magnetic controllable inductor device;

- the second secondary terminal of the first magnetic controllable inductor device is connected to the second parallel terminal of the second autotransformer;

- the second secondary terminal of the second magnetic controllable inductor device is connected to the second parallel terminal of the third autotransformer;

- the second secondary terminal of the third magnetic controllable inductor device is connected to the second parallel terminal of the first autotransformer. In one aspect, the first, second and third autotransformer devices each comprises a series winding connected between the respective first series terminal and the second series terminal and a parallel winding connected between the respective first parallel terminal and the second parallel terminal, and where there is a

transformational relationship between the respective series winding and the parallel winding.

In one aspect, the first, second and third autotransformer devices each comprises a balancing winding connected to each other in a delta configuration.

In one aspect, each balancing winding comprises a first balance winding terminal and a second balance winding terminal, where:

- the first balancing winding terminal of the first balancing winding is connected to the secondt balance winding terminal of the second balancing winding;

- the first balancing winding terminal of the second balancing winding is connected to the second balance winding terminal of the third balancing winding; - the first balancing winding terminal of the third balancing winding is connected to the second balance winding terminal of the first balancing winding.

In one aspect, the respective first series terminals and the first parallel terminals are connected to each other. In one aspect, the second primary terminal of the first magnetic controllable inductor device, the second primary terminal of the second magnetic controllable inductor device and the second primary terminal of the third magnetic controllable inductor device are connected to each other.

In one aspect, the second primary terminal of the first magnetic controllable inductor device is connected to the first parallel terminal of the second

auto trans former; the second primary terminal of the second magnetic controllable inductor device is connected to the first parallel terminal of the third

autotransformer; the second primary terminal of the third magnetic controllable inductor device is connected to the first parallel terminal of the first

auto trans former .

DETAILED DESCRIPTION

Embodiments of the invention will now be described with reference to the enclosed drawings, wherein: Fig. 1 illustrates a prior art three phase star connected voltage stabilization system;

Fig. 2 illustrates a prior art three phase delta connected voltage stabilization system;

Fig. 3a and 3b illustrate equivalent symbols for the magnetic controllable inductor device;

Figs. 4a-c illustrate the terminals of the autotransformer; Figs. 5a-c illustrate the terminals of the magnetic controllable inductor device;

Fig. 5d illustrates the magnetic controllable inductor device of fig. 5a by means of the drawing principle of fig. 3b;

Fig. 6 illustrates a first embodiment of the invention;

Fig. 7 illustrates a second embodiment of the invention; Fig. 8a illustrates some voltages of fig. 7;

Fig. 8b illustrates the phase vectors corresponding to the voltages of fig. 8a when the load is zero; Fig. 8c illustrates the phase vectors corresponding to the voltages of fig. 8a at nominal load;

Fig. 9a illustrates a third embodiment of the invention;

Figs. 9b-d illustrate the terminals of the autotransformer in fig. 9a;

Two prior art three phase voltage stabilisation systems for connection to a three phase power supply line are shown in fig. 1 and 2. These systems are known from the abovementioned WO 2004053615. Fig. 1 is a star connected voltage

stabilization system and fig. 2 is a delta connected voltage stabilization system. It is now referred to fig. 3a and 3b. The symbol in fig. 3a is used in fig. 1 and 2 for a magnetic controllable inductor device. In WO 2004053615, the symbol shown in fig. 3b where used for the same device.

The magnetic controllable inductor device in fig. 3a and fig. 3b comprises a magnetic core, a primary winding pw with two terminals Mpl and Mp2, and a control winding C with control terminals CI and C2. The primary winding pw is wound around a first axis of the core and the control winding C is wound around a second axis of the core, where the first axis and the second axis are orthogonal axes so that when the main winding and the control winding are energized, orthogonal fluxes are generated in the magnetic core. The two terminals Mp l and Mp2 are provided for connection to a power supply line and the control terminals C 1 and C2 are provided for connection to a control system. By changing the current through the control winding, the permeability of the magnetic core may be changed to compensate for voltage variations in the power supply line. This is considered to be prior art and will not be described in detail herein. It is now referred to fig. 6 illustrating a first embodiment of the invention. The dashed box 1 illustrates the three phase voltage compensation system for connection to a three phase power supply line.

The system 1 comprises first, second and third autotransformer devices ATI , AT2, AT3 and first, second and third magnetically controllable inductor devices Ml , M2, M3.

Each of the autotransformer devices ATI , AT2, AT3 comprises four terminals. The reference number for these terminals are shown in fig. 4a, 4b and 4c, but are omitted from fig. 6 for clarity.

In fig. 4a it is shown that the first autotransformer device ATI comprises a first series terminal ATlsl , a second series terminal ATl s2, a first parallel terminal

ATlp l and a second parallel terminal ATlp2. The first autotransformer device ATI further comprises a series winding SW1 connected between the first and second series terminals ATI si and ATl s2, and a parallel winding PWl connected between the first and second parallel terminals ATlpl and ATlp2.

In fig. 4b it is shown that the second autotransformer device AT2 comprises a first series terminal AT2sl , a second series terminal AT2s2, a first parallel terminal AT2p l and a second parallel terminal AT2p2. The second autotransformer device AT2 further comprises a series winding SW2 connected between the first and second series terminals AT2sl and AT2s2, and a parallel winding PW2 connected between the first and second parallel terminals AT2p l and AT2p2. In fig. 4c it is shown that the third autotransformer device AT3 comprises a first series terminal AT3sl , a second series terminal AT3s2, a first parallel terminal AT3p l and a second parallel terminal AT3p2. The third autotransformer device AT3 further comprises a series winding SW3 connected between the first and second series terminals AT3sl and AT3s2, and a parallel winding PW3 connected between the first and second parallel terminals AT3p l and AT3p2.

It should be noted that there is a transformational relationship between the respective series winding and the parallel winding, i.e. between the series winding SW1 and the parallel winding PWl , between the series winding SW2 and the parallel winding PW2 and between the series winding SW3 and the parallel winding PW3. The first, second and third autotransformer devices are considered known for a skilled person and will not be described in detail herein.

Each of the first, second and third magnetically controllable inductor devices Ml , M2, M3 comprises four terminals. The reference number for these terminals are shown in fig. 5a, 5b and 5c, but are omitted from fig. 6 for clarity. In fig. 5a it is shown that the first magnetic controllable inductor device Ml comprises a first primary terminal Mlpl , a second primary terminal Mlp2, a first secondary terminal Mlsl and a second secondary terminal Mls2. In the present invention, the first magnetic controllable inductor device Ml comprises a primary winding and a secondary winding. The primary winding pwl is wound around the magnetic core and is connected between the first primary terminal Mlp l and the second primary terminal Mlp2. The secondary winding swl is wound around the magnetic core and is connected between the first secondary terminal Mlsl and the second secondary terminal Mls2.

In fig. 5b it is shown that the second magnetic controllable inductor device M2 comprises a first primary terminal M2pl , a second primary terminal M2p2, a first secondary terminal M2sl and a second secondary terminal M2s2. In the present invention, the second magnetic controllable inductor device M2 comprises a primary winding and a secondary winding. The primary winding pw2 is wound around the magnetic core and is connected between the first primary terminal M2p 1 and the second primary terminal M2p2. The secondary winding sw2 is wound around the magnetic core and is connected between the first secondary terminal M2sl and the second secondary terminal M2s2. In fig. 5c it is shown that the third magnetic controllable inductor device M3 comprises a first primary terminal M3pl , a second primary terminal M3p2, a first secondary terminal M3sl and a second secondary terminal M3s2. In the present invention, the third magnetic controllable inductor device M3 comprises a primary winding and a secondary winding. The primary winding pw3 is wound around the magnetic core and is connected between the first primary terminal M3pl and the second primary terminal M3p2. The secondary winding sw3 is wound around the magnetic core and is connected between the first secondary terminal M3sl and the second secondary terminal M3s2.

It should be noted that the first, second and third magnetic controllable inductor device each comprises a control winding wound around the second axis of the magnetic core and is connected to a control system for controlling the permeability of the magnetic core to compensate for voltage variations in the power supply line, as described above with reference to fig. 1 and 2. This is illustrated in fig. 5d. Here, both the primary winding and the secondary winding are wound around the first axis of the core, and the control winding C is wound around the second axis of the core. The first axis and the second axis are orthogonal axes so that when the main winding and the control winding are energized, orthogonal fluxes are generated in the magnetic core.

By changing the current through the control winding, the permeability of the magnetic core may be changed to compensate for voltage variations in the power supply line. By lowering the permeability of the magnetic core, the impedance of the magnetic controllable inductor device is reduced. The voltage over the magnetically controllable device is correspondingly reduced. This results in a larger part of the input voltage being occupied by the autotransformers' windings which again lead to an increased output voltage.

It is now referred to fig. 6. Inside the dashed box representing the system 1 , it is shown that the first primary terminal Mlp l of the first magnetic controllable inductor device Ml is connected to first secondary terminal M3sl of the third magnetic controllable inductor device M3. The first primary terminal M2pl of the second magnetic controllable inductor device M2 is connected to first secondary terminal Ml si of the first magnetic controllable inductor device Ml . The first primary terminal M3pl of the third magnetic controllable inductor device M3 is connected to first secondary terminal M2sl of the second magnetic controllable inductor device M2. Moreover, the second secondary terminal Mls2 of the first magnetic controllable inductor device Ml is connected to the second parallel terminal AT2p2 of the second autotransformer AT2. The second secondary terminal M2s2 of the second magnetic controllable inductor device M2 is connected to the second parallel terminal AT3p2 of the third autotransformer AT3. The second secondary terminal M3s2 of the third magnetic controllable inductor device M3 is connected to the second parallel terminal ATlp2 of the first autotransformer ATI .

In addition, the respective first series terminals ATlsl , AT2sl , AT3sl and first parallel terminals ATlp l , AT2p l , AT3pl are connected to each other, i.e. the first series terminal ATlsl is connected to the first parallel terminal ATlpl , the first series terminal AT2sl is connected to the first parallel terminal AT2pl and the first series terminal AT3sl is connected to the first parallel terminal AT3pl .

In fig. 6, it is shown that the second primary terminal Mlp2 of the first magnetic controllable inductor device Ml , the second primary terminal M2p2 of the second magnetic controllable inductor device M2 and the second primary terminal M3p2 of the third magnetic controllable inductor device M3 are connected to each other.

Hence, this is considered to be a star connected configuration.

The system 1 in fig. 6 is connected to a three phase power supply line in the following way: A first input phase R is connected to the first series terminal ATl sl of the first autotransformer device ATI , a second input phase S is connected to the first series terminal AT2sl of the second autotransformer device AT2 and a third input phase S is connected to the first series terminal AT3sl of the third autotransformer device AT3. A first output phase U is connected to the second series terminal ATl s2 of the first autotransformer device ATI , a second output phase V is connected to the second series terminal AT2s2 of the second autotransformer device AT2 and a third output phase W is connected to the second series terminal AT3s2 of the third autotransformer device AT3.

Moreover, the second primary terminal Mlp2 of the first magnetic controllable inductor device Ml , the second primary terminal M2p2 of the second magnetic controllable inductor device M2 and the second primary terminal M3p2 of the third magnetic controllable inductor device M3 are connected to a neutral phase N.

It is now referred to fig. 7. The components and the connections inside the dashed box is similar to those inside the dashed box of fig. 6 and will not be described here in detail. Also in fig. 7, the respective first series terminals ATlsl , AT2sl , AT3sl and first parallel terminals ATlpl , AT2pl , AT3pl are connected to each other, i.e. the first series terminal ATlsl is connected to the first parallel terminal ATlpl , the first series terminal AT2sl is connected to the first parallel terminal AT2pl and the first series terminal AT3sl is connected to the first parallel terminal AT3pl .

In fig. 7, the second primary terminal Mlp2 of the first magnetic controllable inductor device Ml is connected to the first parallel terminal AT2p l of the second autotransformer AT2. Moreover, the second primary terminal M2p2 of the second magnetic controllable inductor device M2 is connected to the first parallel terminal AT3pl of the third autotransformer AT3. The second primary terminal M3p2 of the third magnetic controllable inductor device M3 is connected to the first parallel terminal ATlp l of the first autotransformer ATI . Hence, this is considered to be a delta connected configuration.

The system 1 in fig. 7 is connected to a three phase power supply line in the following way:

A first input phase R is connected to the first series terminal ATI si of the first autotransformer device ATI , a second input phase S is connected to the first series terminal AT2sl of the second autotransformer device AT2 and a third input phase S is connected to the first series terminal AT3sl of the third autotransformer device AT3.

A first output phase U is connected to the second series terminal ATl s2 of the first autotransformer device ATI , a second output phase V is connected to the second series terminal AT2s2 of the second autotransformer device AT2 and a third output phase W is connected to the second series terminal AT3s2 of the third

autotransformer device AT3.

It is now referred to fig. 8a, 8b and 8c. In fig. 8a, voltage vectors VI - V6 are indicated, where: VI represents the voltage between phase R and neutral N;

V2 represents the voltage between phase U and neutral N;

V3 represents the voltage over the series winding SW1 of the first autotransformer device AT 1 ;

V4 represents the voltage over the primary winding pwl of the furst magnetic controllable inductor device Ml ;

V5_represents the voltage contribution from the secondary winding SW3 of the third magnetic controllable inductor device M3;

V6 represents the voltage over the parallel winding PWl of the first autotransformer device AT 1. The magnitude and direction of the voltage vectors are illustrated in figure 8b and 8c. They relate to the star connected system in figure 6 and 8a. The voltage vectors for the delta connected system in fig. 7 is in principle identical.

In fig. 8b the phase vectors are shown for phase R when the load is zero. In symmetrical operation, the corresponding phase vectors for phase S and T are similar, but displaced by 240 and 120 degrees. The important thing to note here is that the voltages V4 and V5 are considerably lower than VI and V2. In prior art these voltages would be equal to each other.

In fig. 6a and fig. 8, the voltage VI is divided between the primary winding of the first magnetic controllable inductor device Ml (i.e. voltage V4) and the secondary winding of the second magnetic controllable device M2 (i.e. voltage V5).

Consequently, the highest dimensioning voltage is reduced for the magnetic controllable inductor devices.

In fig. 8c, the corresponding phase vectors are shown at nominal load. Here, the voltage vectors V4 and V5 are small because the permeability of the magnetic controllable inductor devices is small. Correspondingly, the voltage V6 over the parallel winding PW1 of the auto trans former device ATI is large. Since there is a transformational relation between the parallel winding PW1 and series winding SWl of the auto trans former device ATI , the voltage over the series winding SWl is also large. This leads to the output voltage V2 being larger than the input voltage VI . This is in contrast to the situation in figure 8b, where VI and V2 were equal.

By replacing the prior art (fig. 1 and fig. 2) with the proposed topology, the magnetic controllable inductor devices need only be designed for approximately 57% of the voltage compared to prior art. The reduction in design voltage would automatically lead to a reduction in the minimum inductance of the magnetic controllable inductor devices to 57%. The reduction in minimum inductance is highly desirable since it is inversely proportional to the power capacity of the system.

In practice, the reduction of minimum inductance to 57% is not quite enough to give an equal power rating compared to prior art. In practice, the proposed topologies lead to magnetic controllable inductor devices that are reduced to about 60%>-80%> of the original size in prior art. The delta connected system (fig. 7) generally gives a larger size reduction compared to prior art.

An alternative embodiment will now be described with reference to fig. 9. The system in fig. 9 comprises first, second and third auto trans former devices ATI , AT2, AT3 and first, second and third magnetic controllable inductor devices Ml , M2, and M3 in a similar configuration as in fig. 7. In addition, the first, second and third autotransformer devices ATI , AT2, AT3 each comprises a balancing winding BW1 , BW2, BW3 connected to each other in a delta configuration.

More specifically, each balancing winding BW1 , BW2, BW3 comprises a first balance winding terminal ATlbl , AT2bl , AT3bl and a second balance winding terminal AT lb2, AT2b2, AT3b2, where:

- the first balancing winding terminal ATlbl of the first balancing winding BW1 is connected to the second balance winding terminal AT2b2 of the second balancing winding BW2;

- the first balancing winding terminal AT2bl of the second balancing winding BW2 is connected to the second balance winding terminal AT3b2 of the third balancing winding BW3;

- the first balancing winding terminal AT3bl of the third balancing winding BW3 is connected to the second balance winding terminal AT lb2 of the first balancing winding BW1. Hence, if there are differences between the voltages over the different

autotransformer devices, these voltage differences will be balanced out, or cancelled. Hence, better symmetry in the three phase system will be achieved.

It should be noted that balance windings can be added to the autotransformer devices in fig. 6 as well.

Claims

1. Three phase voltage control system for connection to a three phase power supply line, comprising:

- first, second and third autotransformer devices (ATI , AT2, AT3), each comprising a first series terminal (ATlsl , AT2sl , AT3sl), a second series terminal (ATls2, AT2s2, AT3s2), a first parallel terminal (ATlp l , AT2pl , AT3p l) and a second parallel terminal (ATlp2, AT2p2, AT3p2);

- first, second and third magnetic controllable inductor devices (Ml , M2, M3), each comprising:

- a magnetic core;

- a first primary terminal (Mlp l , M2pl , M3p l) and a second primary terminal (Mlp2, M2p2, M3p2);

- a primary winding (pwl , pw2, pw3) wound around a first axis of the magnetic core and connected between the respective first primary terminal (Mlp l , M2pl , M3pl) and the respective second primary terminal (Mlp2, M2p2, M3p2);

- a control winding (C) wound around a second axis of the magnetic core, where the first axis and the second axis are orthogonal axis, and where the control winding (C) is connected to a control system for controlling the permeability of the magnetic core to compensate for voltage variations in the power supply line;

characterized in that

- the first, second and third magnetic controllable inductor devices (Ml , M2, M3) each comprises:

- a first secondary terminal (Mlsl , M2sl , M3sl) and a second secondary terminal (Mls2, M2s2, M3s2);

- a secondary winding (swl , sw2, sw3) wound around the first axis of the magnetic core and connected between the respective first secondary terminal (Mlsl , M2sl , M3sl) and the second secondary terminal (Mls2, M2s2, M3s2);

- the first primary terminal (Mlpl) of the first magnetic controllable inductor device (Ml) is connected to first secondary terminal (M3sl) of the third magnetic controllable inductor device (M3);

- the first primary terminal (M2pl) of the second magnetic controllable inductor device (M2) is connected to first secondary terminal (Mlsl) of the first magnetic controllable inductor device (Ml); - the first primary terminal (M3pl) of the third magnetic controllable inductor device (M3) is connected to first secondary terminal (M2sl) of the second magnetic controllable inductor device (M2);

- the second secondary terminal (Ml s2) of the first magnetic controllable inductor device (Ml) is connected to the second parallel terminal (AT2p2) of the second auto trans former (AT2);

- the second secondary terminal (M2s2) of the second magnetic controllable inductor device (M2) is connected to the second parallel terminal (AT3p2) of the third autotransformer (AT3); - the second secondary terminal (M3s2) of the third magnetic controllable inductor device (M3) is connected to the second parallel terminal (ATlp2) of the first autotransformer (ATI).

2. Three phase system according to claim 1 , where the first, second and third autotransformer devices (ATI , AT2, AT3) each comprises a series winding (SW1 , SW2, SW3) connected between the respective first series terminal (ATlsl , AT2sl , AT3sl) and the second series terminal (ATls2, AT2s2, AT3s2) and a parallel winding (PWl , PW2, PW3) connected between the respective first parallel terminal (ATlpl , AT2pl , AT3pl) and the second parallel terminal (ATlp2, AT2p2, AT3p2), and where there is a transformational relationship between the respective series winding and the parallel winding.

3. Three phase system according to claim 1 or 2, where the first, second and third autotransformer devices (ATI , AT2, AT3) each comprises a balancing winding (BW1 , BW2, BW3) connected to each other in a delta configuration.

4. Three phase system according to claim 3, where each balancing winding (BW1 , BW2, BW3) comprises a first balance winding terminal (ATlbl , AT2bl , AT3bl) and a second balance winding terminal (ATlb2, AT2b2, AT3b2), where:

- the first balancing winding terminal (ATlb l) of the first balancing winding (BW1) is connected to the secondt balance winding terminal (AT2b2) of the second balancing winding (BW2); - the first balancing winding terminal (AT2bl) of the second balancing winding (BW2) is connected to the second balance winding terminal (AT3b2) of the third balancing winding (BW3);

- the first balancing winding terminal (AT3bl) of the third balancing winding (BW3) is connected to the second balance winding terminal (ATlb2) of the first balancing winding (BW1).

5. Three phase system according to any one of the above claims, where the respective first series terminals (ATlsl , AT2sl , AT3sl) and the first parallel terminals (ATlpl , AT2p l , AT3p l) are connected to each other.

6. Three phase system according to any one of the above claims, where the second primary terminal (Mlp2) of the first magnetic controllable inductor device (Ml), the second primary terminal (M2p2) of the second magnetic controllable inductor device (M2) and the second primary terminal (M3p2) of the third magnetic controllable inductor device (M3) are connected to each other.

7. Three phase system according to any one of claims 1 - 5, where: - the second primary terminal (Mlp2) of the first magnetic controllable inductor device (Ml) is connected to the first parallel terminal (AT2pl) of the second auto transformer (AT2);

- the second primary terminal (M2p2) of the second magnetic controllable inductor device (M2) is connected to the first parallel terminal (AT3pl) of the third autotransformer (AT3);

- the second primary terminal (M3p2) of the third magnetic controllable inductor device (M3) is connected to the first parallel terminal (ATlpl) of the first autotransformer (ATI).