TECHNICAL FIELD The present invention relates to a current limiting arrangement for limiting a current in an electrical power system, which current limiting arrangement comprises an induction winding for the current, surrounding a magnetic flux circuit comprising a ferromagnetic or semi-ferromagnetic material, which material exhibits a residual magnetic field after a first current limiting effort.

The present invention also relates to an electrical power system comprising such an arrangement, as well as the employment of such an arrangement.

A ferromagnetic or semi-ferromagnetic material herein denotes a magnetic material with a coercive field intensity exceeding 3 kA/m.

An electrical power system herein denotes a system for transmission of distribution of electric energy in the form of alternating current at voltages exceeding 1 kV.

BACKGROUND ART It is known, for instance through the East German patent 209 313 or Alger, P. L. (1995),"Induction Machines- Their Behaviour and Uses", 3rd ed., pp. 10-33, to utilize the non-linear magnetic characteristics of a ferromagnetic or semi-ferromagnetic material for current-limiting purposes. Such an arrangement comprises

an induction winding, surrounding a magnetic flux circuit, the ferromagnetic or semi-ferromagnetic material forming part thereof. The current breaking arrangement is arranged in a current-carrying electric system so that the current that is to be limited by the current limiting arrangement flows through the induction winding. The current limiting arrangement acts so that the current in the induction winding generates a magnetizing field, H, with a field intensity that is proportional to the current. The H field magnetizes the ferromagnetic or semi-ferromagnetic material and generates a magnetic flux in the magnetic flux circuit.

The inductance of the induction winding is substantially proportional to the magnetic flux and is thus influenced by the degree of magnetization of the ferromagnetic or semi-ferromagnetic material. At currents below a critical value, corresponding to an H field with a field intensity equal to the coercive field intensity, Hc, of the ferromagnetic or semi-ferromagnetic material, the ferromagnetic or semi-ferromagnetic material exhibits relatively weak magnetization, which slowly increases linearly or substantially linearly to the current. In other words, in this linear area, the induction winding exhibits relatively low inductance. However, at currents exceeding said critical value, the ferromagnetic or semi-ferromagnetic material exhibits a magnetization that abruptly increases with the current in a non-linear way to be saturated, finally, at a magnetization level that is somewhat higher than the remanence, Br, of the ferromagnetic or semi-ferromagnetic material. In other words, in this non-linear area, the induction winding exhibits a swiftly increasing inductance, which reaches a maximum value when the ferromagnetic or semi- ferromagnetic material is saturated, after which point

the inductance slowly decreases with any further increase in the current. To this end, the current limiting arrangement is dimensioned so that it operates within the linear area when a normal operational current flows through the induction winding, corresponding to the nominal current of the electrical system. However, if a fault occurs in the electrical system generating a fault current exceeding said critical value, the current limiting arrangement will switch to operating in the non-linear current limiting area. An example of such faults is a short circuit generating a short-circuit current in the electrical system.

However, the ferromagnetic or semi-ferromagnetic material in a current limiting arrangement of the kind described above exhibits considerable hysteresis for fault currents corresponding to an H field with a field intensity exceeding He. After a fault in the electrical power system that has triggered a current limiting effort by the current limiting arrangement, significant residual magnetization can remain in the ferromagnetic or semi-ferromagnetic material. When the next fault occurs, causing a further current limiting effort by the current limiting arrangement, the residual magnetization can result in premature saturation of the ferromagnetic or semi-ferromagnetic material, which constitutes a problem as this, in turn, results in asymmetry in the fault current and impairs the current limiting capacity of the current limiting arrangement.

DESCRIPTION OF THE INVENTION The object of the present invention is to eliminate the problems described above and to provide a current

limiting arrangement of the kind specified above that exhibits good current limiting capacity even after a current limiting effort by the current limiting arrangement has resulted in residual magnetization of the ferromagnetic or semi-ferromagnetic material.

The current limiting arrangement and the electrical power system in accordance with the invention are characterized in that -the magnetic flux circuit is divided into a first flux-circuit part, comprising a first portion of said ferromagnetic or semi-ferromagnetic material, and a second flux-circuit part, comprising a second portion of said ferromagnetic or semi- ferromagnetic material, which portions are separated from each other, -the induction winding is divided into a first winding part, surrounding the first flux-circuit part, and a second winding part, surrounding the second flux-circuit part and connected in series with the first winding part, and -a circuit-changing member arranged to reverse the direction of the residual magnetic field in the first portion relative to the magnetic field generated in the first flux-circuit part by the current in the first winding part, said reversal taking place between said first current limiting effort and a second subsequent current limiting effort.

By reversing the direction of the residual magnetic field in the first portion, a current limiting arrangement is obtained that is invulnerable to residual magnetization, as this ensures that either the first or

the second portion containing ferromagnetic or semi- ferromagnetic material is not saturated by the second current limiting effort.

DESCRIPTION OF THE DRAWINGS The invention will be further explained with reference to the drawings.

Figure 1 shows, schematically, an electrical power system comprising a conventional current limiting arrangement.

Figure 2 shows a graph illustrating the relationship between the field intensity of a magnetizing field, H, and the magnetic flux density, B, in the current limiting arrangement in accordance with Figure 1.

Figures 3 and 4 show a first embodiment of a current limiting arrangement in accordance with the invention, which current limiting arrangement comprises a switch reverser.

Figure 5 shows a graph illustrating the relationship between the field intensity of a magnetizing field, H, and the magnetic flux density, B, in the current limiting arrangement in accordance with Figures 3 and 4.

Figure 6 shows a second embodiment of the current limiting arrangement in accordance with the invention, which current limiting arrangement comprises a rotary device.

DESCRIPTION OF PREFERRED EMBODIMENTS

To facilitate understanding the invention, the operation of a conventional current limiting arrangement will be described by way of introduction in connection with Figure 1, which shows, schematically, an electrical power system comprising a conventional current limiting arrangement 1, and also Figure 2, which illustrates the relationship between the field intensity of a magnetizing field, H, and the magnetic flux density, B, in a magnetic flux circuit 2 in the current limiting arrangement 1. In addition to the current limiting arrangement 1, the electrical power system comprises a voltage source, U, for instance a generator, and a load, Z. The voltage source, U, the current limiting arrangement 1 and the load, Z, are series-connected and carry the same alternating current, I. The current limiting arrangement 1 comprises an induction winding 3, surrounding said magnetic flux circuit 2, which comprises a ferromagnetic or semi-ferromagnetic material 4. During normal operation of the electrical power system, when it carries a relatively small alternating current, corresponding to an H field with a field intensity that is less than the coercive field intensity, Hc, of the magnetic material, the ferromagnetic or semi-ferromagnetic material 4 exhibits relatively weak magnetization. With such an alternating current, the magnetic flux density, B, in the magnetic flux circuit 2 varies substantially linearly to the field intensity of the H field, as illustrated by the linear curve portion 5 in Figure 2. In these circumstances, the induction winding 3 exhibits a relatively small inductance, which only to a modest extent affects the current in the electrical power system. In connection with a fault in the electrical

power system, for instance a short circuit of the load, Z, when the current in the electrical power system suddenly increases greatly and a fault current flows through the electrical power system, the magnetization of the magnetic material 4 increases swiftly and abruptly. The maximum magnetization of the ferromagnetic or semi-ferromagnetic material 4 is dependent upon the saturation magnetization of the material 4. This is usually somewhat greater than the remanence, Br, of the ferromagnetic or semi-ferromagnetic material.

Preferably, the current limiting arrangement is designed such that the ferromagnetic or semi-ferromagnetic material is not saturated in the event of a fault. The relationship between the field intensity of the H field and the magnetic flux density, B, during a sudden increase in the current is illustrated by the curve portion 6 in Figure 2. The inductance of the induction winding 3 increases abruptly because of the swift and abrupt magnetization of the ferromagnetic or semi- ferromagnetic material. In addition, the ferromagnetic or semi-ferromagnetic material exhibits severe hysteresis for as long as the fault in the electrical power system remains, that is for as long as the fault current flows through the electrical power system, as illustrated by the curve portion 7 in Figure 2.

Consequently, the impedance of the current limiting arrangement increases sharply during said sudden increase in the current, which impedance increase has an inductive component, dependent on the inductance increase of the induction winding 3, and also a resistive component, arising from the hysteresis. The fault current is limited to a predetermined maximum value, by means of the impedance increase, until the fault is disconnected from the electrical power system,

which usually occurs within two or three periods or current cycles. However, a significant residual magnetization can occur in the ferromagnetic or semi- ferromagnetic material 4 after the fault current is disconnected. The residual magnetization corresponds to a residual magnetic flux, Bk, which can assume a value between-Br and Br. In Figure 2, this is illustrated by the broken-line curve 8, which is displaced by the distance Bk corresponding to the flux density of the residual magnetic flux. In the event of a second fault in the electrical power system following the first fault, which triggers a further current limiting effort by the current limiting arrangement 1, the residual magnetic flux will cause a premature saturation of the ferromagnetic or semi-ferromagnetic material 4 in one of the directions of the fault current, which causes asymmetry in the fault current and results in impairment of the current limiting capacity of the current limiting arrangement.

Figures 3 and 4 show a first embodiment of a current limiting arrangement in accordance with the invention.

Figure 3 shows the current limiting arrangement immediately after a first current limiting effort and Figure 4 shows the current limiting arrangement prior to a subsequent second current limiting effort. The current limiting arrangement has a first and a second connection device 9,10 for connecting the current limiting arrangement to an electrical power system (not shown).

The current limiting arrangement comprises an induction winding 3, divided into a first and a second winding part 11,12. The current limiting arrangement further comprises a magnetic flux circuit 2, divided into a first and a second flux-circuit part 13,14. The first

winding part 11 surrounds the first flux-circuit part 13, which comprises a first portion 15 containing ferromagnetic or semi-ferromagnetic material. The second winding part 12 similarly surrounds the second flux- circuit part 14, which comprises a second portion 16 containing ferromagnetic or semi-ferromagnetic material.

Preferably, the flux-circuit parts 13 and 14 are fashioned in the same way with respect to the mass of ferromagnetic or semi-ferromagnetic material in the portions 15 and 16 as well as the geometric design of the same. The winding parts 11,12 are series-connected between the connection devices 9,10. The winding parts 11,12 are thus arranged so that the same current flows through both parts. The current limiting arrangement further comprises a circuit-changing member in the form of a switch reverser 17, comprising four terminal contacts 18,19,20,21. The first winding part 11 has a first and a second end 22 and 23, which are connected to the terminal contacts 18 and 19, respectively, of the switch reverser 17. The second winding part 12 similarly has a first and a second winding end 24 and 25, which second winding end 25 is connected to the terminal contact 20 of the switch reverser 17. The first winding end 24 of the second winding part 12 is connected to the connection device 10, and the terminal contact 21 of the switch reverser 17 is connected to the connection device 9. The switch reverser 17, and thus the current limiting arrangement, can assume a first position in accordance with Figure 3 and a second position in accordance with Figure 4. In the first position, the terminal contacts 18 and 21 are connected to each other, as are the terminal contacts 19 and 20. The other end 23 of the first winding part 11 is thereby connected to the other end 25 of the second winding part 12 ; and the first end

22 of the first winding part 11 is connected to the first connection device 9. In the second position, the terminal contacts 18 and 20 are connected to each other, as are the terminal contacts 19 and 21, whereby the first end 22 of the first winding part 11 is connected to the other end 25 of the second winding part 12, and the other end 23 of the first winding part 11 is connected to the first connection device 9 of the current limiting arrangement. When the switch reverser 17 is switched from the first position to the second position, or vice versa, the direction of the current in the first winding part 11 is thus reversed relative to the direction of the current in the second winding part 12. In the following, it is assumed that the current limiting arrangement performs a first current limiting effort when it is in its first position, i. e. in accordance with Figure 3, and that a residual magnetization in the form of a residual magnetic flux with a flux density Bk thereby occurs in each of the two portions 15,16 containing ferromagnetic or semi- ferromagnetic material. The direction of the residual magnetic flux in the portions 15,16 is dependent on the direction of the current through the winding parts 11, 12 at the time of the disconnection of the fault. After the first current limiting effort, the current through the first winding part 11 is caused to change its direction with the aid of the switch reverser 17. In other words, the switch reverser 17 is switched from said first position to said second position, or vice versa, whereby the phase angle of the current through the first winding part 11 is displaced 180° relative to the phase angle for the current through the second winding part 12. The direction of the residual magnetic field in the portion 15 is thereby reversed relative to

the magnetic field generated by the current through the first winding part 11. The effect of this on the magnetization of the portions 15,16 containing ferromagnetic or semi-ferromagnetic material during a second current limiting effort subsequent to the first current limiting effort is evident from Figure 5. The solid curve 26 illustrates the magnetization of the ferromagnetic or semi-ferromagnetic material in the portion 15 and the broken-line curve 27 illustrates the magnetization of the ferromagnetic or semi-ferromagnetic material in the portion 16. Because of the reversal of the direction of current in the first winding part 11, the difference between the residual magnetic flux density in the two portions 15,16 is 2Bk, as indicated in Figure 5. Depending on the direction of the current during the second current limiting effort, either the ferromagnetic or semi-ferromagnetic material in the portion 15 or the ferromagnetic or semi-ferromagnetic material in the portion 16 will be saturated at an early stage during the second current limiting effort. In the present example, this applies to the ferromagnetic or semi-ferromagnetic material in the portion 16, as illustrated by the curve 27 in Figure 5. However, the magnetization of the ferromagnetic or semi-ferromagnetic material in the portion 15 will increase abruptly without reaching saturation, as illustrated by the curve 26. The magnetization increase in the portion 15 thus leads to a limitation of the current through the first winding part 11 and thereby also through the second winding part 12. If the fault remains during several periods, the magnetization in both portions 15 and 16 will follow the hysteresis curve 28 illustrated in Figure 5. By means of the reversal of the residual magnetic field in the portion 15 relative to the

magnetic field generated by the current through the first winding part 11 during the second current limiting effort, the current limiting capacity of the current limiting arrangement in accordance with the invention is thus independent of any residual magnetic field in the portions 15,16 containing ferromagnetic or semi- ferromagnetic material.

An alternative way to reverse the residual magnetic field in the portion 15 relative to the magnetic field generated by the current through the first winding part 11 is illustrated in Figure 6, which shows a current limiting arrangement in accordance with a second embodiment of the invention. As the current limiting arrangement described in connection with Figures 3 and 4, the current limiting arrangement comprises two series-connected winding parts 11,12 and two flux- circuit parts 13,14, each of which comprises a portion 15,16 containing ferromagnetic or semi-ferromagnetic material. The current limiting arrangement further comprises a circuit-changing member in the form of a rotary device 29, comprising a rotatable shaft on which the portion 15 containing the ferromagnetic or semi- ferromagnetic material is rotatably journalled. The rotary device is arranged to rotate the portion 15 containing ferromagnetic or semi-ferromagnetic material half a turn, between two consecutive current limiting efforts, in the direction of the flux-circuit part 13, as indicated in Figure 6. As with the switching of the current in the first winding part described in connection with Figures 3 and 4, this procedure results in the residual magnetic field in the portion 15 being reversed relative to the magnetic field generated by the current through the winding part 11. Consequently, in

the same way as in the first embodiment of the invention, it is ensured that one of the portions 15,16 containing ferromagnetic or semi-ferromagnetic material are not saturated during the second current limiting effort.

However, a problem with known current limiting arrangements comprising ferromagnetic or semi- ferromagnetic material is that the magnetization exhibited by many such magnetic materials is too great even at currents corresponding to an H field less than the coercive field intensity, He, of the ferromagnetic or semi-ferromagnetic material. This causes the induction winding to phase-shift the operational current in an unacceptable way during normal operation. Although there are ferromagnetic or semi-ferromagnetic materials that exhibit relatively slight magnetization at a normal operational current, for instance an alloy commercially known as ALNICO and comprising aluminium, nickel and cobalt, these are very costly, which is a disadvantage, as large volumes of the ferromagnetic or semi- ferromagnetic material are needed in a current limiting arrangement. Accordingly, the use of known ferromagnetic of semi-ferromagnetic materials exhibiting slight magnetization at a normal operation current would be very expensive.

To reduce the above problem and enable the use of ferromagnetic or semi-ferromagnetic material exhibiting relatively great magnetization at a normal operational current, that is in the above-mentioned linear area, the current limiting arrangement preferably comprises a capacitor, connected in series with the induction winding. The reactance of the capacitor is Xc=l/(wC),

where w is the angular frequency of the electric power system and C is the capacitance of the capacitor, and the reactance of the induction winding is X=wL, where L is the inductance of the induction winding. By selecting the capacitance of the capacitor such that wC=1/wLo, where Lo is the inductance of the nominal current of the electric power system, the current limiting arrangement obtains a reactance that is zero at a normal operation current.

In accordance with one embodiment of the invention, the above-mentioned ferromagnetic or semi-ferromagnetic material is ferromagnetic ferrite, which, although it provides an induction winding exhibiting relatively high inductance at a normal operational current, exhibits other properties that are desirable in the context of current limiting, such as energy density and resistivity. In addition, ferromagnetic ferrite is relatively inexpensive, which further increases the attraction of such a material.

The invention has been described above in relation to a free-standing current limiting arrangement for a single phase. However, it will be appreciated that the invention also is applicable to other types of current limiting arrangements, for instance those that are built into transformers, where the ferromagnetic or semi- ferromagnetic material forms a magnetic flux circuit between the primary and the secondary windings of the transformer. It will also be appreciated that it is fully possible to unite the flux-circuit parts 13,14 described above with a single magnetic flux circuit.