GB2487574A

GB2487574A - Fault Current Limiter

Info

Publication number: GB2487574A
Application number: GB1101475.0A
Authority: GB
Inventors: Carsten Buehrer; Jens Mueller; Jan Wiezoreck; Juergen Kellers
Original assignee: Zenergy Power GmbH
Current assignee: Zenergy Power GmbH
Priority date: 2011-01-28
Filing date: 2011-01-28
Publication date: 2012-08-01
Also published as: GB201101475D0; AU2011201156A1

Abstract

A fault current limiter has a ferromagnetic core 1 which in at least one region has an alternating current (AC) coil winding 4.1 and another region a direct current (DC) coil winding 3 for magnetic saturation of the core 1 in the normal condition. To compensate for the effects of magnetic leakage flux, to further reduce inductive reactance, the cross-sectional area of the region with the AC coil winding 4.1 is reduced in relation to the cross-sectional area of the region carrying the DC coil winding 2 so that the magnetic flux density in the AC coil winding region 4.1 in the normal operating condition reaches the value required for magnetic saturation. The DC coil can be superconducting and have an axis that is mutually orthogonal to that of the AC coil. The AC coil region can comprise a ferromagnetic material whose saturation field strength is less than that of the DC coil region and decrease in area as the distance from the DC coil winding increases. The DC coil core region can have an air gap. The core can comprise two transverse limbs 1.3,1.4 and longitudinal limbs 1.1,12 which the AC coils jointly surround. For a three-phase network the core can either have three mutually separate and parallel AC windings, having numbers of turns that increase with the distance from the DC coil, or include three closed yokes (fig 2 21,22,23) which adjoin, the DC coil surrounding the joint yokes.

Description

I

Fault Current Limiter The invention concerns a fault current limiter comprising an at least S substantially annularly closed ferromagnetic core which in at least one region has at least one coil winding through which alternating current flows and in at least one region a coil winding through which direct current flows for magnetic saturation of the iron core in the normal operating condition A fault current limiter of that kind and its mode of operation are known from Wa 2007/029224 Al. In the normal operating condition it has an inductive reactance (impedance) which is admittedly low but dependent on the instantaneous value of the alternating current and which consequently leads to unwanted non-linear distortions of the sinusoidal configuration of the alternating current. At the same time the voltage drop across the alternating current coil is increased and has to be compensated by an increase in the operating voltage.

The invention is based on the problem of still further reducing the O inductive reactance of a fault current limiter of the general kind set forth in

the opening part of this specification.

O 20 According to the invention that problem is solved in that the cross-sectional area of the region the core, that carries the alternating current coil winding, is reduced in relation to the cross-sectional area of the region of the core, that carries the coil winding through which direct current flows, to such an extent that the magnetic flux density in the region carrying the alternating current coil winding in the normal operating condition reaches there at least the value required for magnetic saturation.

The invention is based on the realisation that, in a ferromagnetic core, hereinafter referred to as an iron core, non-negligible magnetic leakage fluxes occur in the saturation condition. Because of those leakage fluxes the magnetic flux density in the iron core is reduced with increasing distance from the region in which the coil winding through which direct current flows is disposed and therefore in the region of the alternating current coil winding it is of a lower value than in the region of the direct current coil winding. The consequence of this is that the fault current limiter has a reactance which can no longer be disregarded for high alternating current amplitudes or instantaneous values which however are still in the normal operating range. That amplitude-dependent reactance of the fault current limiter causes the above-mentioned distortion of the sinusoidal shape of the alternating current and the unwanted additional voltage drop.

The fault current limiter according to the invention counteracts those effects Preferably the difference in the cross-sectional areas in the region of the alternating current coil winding and in the region of the direct current coil winding is such that it compensates for the magnetic leakage flux in order to maintain the saturation condition of the iron core even in its region carrying the alternating current coil winding.

The region of the iron core, that carries the alternating current coil winding, can comprise a ferromagnetic material whose saturation field strength is less than that of the other regions of the iron core. That also (\j counteracts the detrimental effect of the magnetic leakage fluxes.

O To minimise the transformatory coupling of the alternating coil winding with the direct current coil winding the axes of the coil winding O 20 through which direct current flows and the alternating current coil winding are preferably in mutually orthogonal relationship.

For the same reason the iron core can be interrupted by an air gap in the region of the coil winding through which direct current flows. That increases the magnetic resistance of the iron core so that the current which is high in a fault situation on the alternating current side causes a smaller flux change in the iron core.

Preferably the fault current limiter is of a geometry in which the iron core comprises a yoke with at least two longitudinal limbs and at least two transverse limbs and the alternating current coil winding jointly surrounds the two longitudinal limbs. For the direct current coil winding the iron core acts like a closed yoke, when an air gap is present like a substantially closed yoke, but for the alternating current coil winding it acts like a bar-shaped common iron core.

In an embodiment for a three-phase alternating current network the iron core has three mutually separate alternating current coil windings which are preferably arranged in parallel to one another. That embodiment is distinguished by its compact structure because it requires only a single direct current coil winding.

The proportion of the magnetic leakage fluxes increases with increasing distance from the coil winding through which direct current flows. To compensate for that effect the alternating current coil windings can have an increasing number of turns, with an increasing distance from the coil winding through which direct current flows.

Alternatively or additionally magnetic saturation of the iron core can also be achieved in the region of the alternating current coil windings that are further away from the direct current coil winding, by the cross-sectional area of the region of the iron core, that carries the alternating current windings, decreasing with increasing distance from the direct current coil winding. The cross-sectional area can be reduced stepwise from one alternating current coil winding to the next.

O A further embodiment of the fault current limiter for a three-phase alternating current network provides that the iron core includes three at O 20 least substantially closed yokes which adjoin each other with a respective one of their transverse limbs and the coil winding through which direct current flows jointly embraces those three transverse limbs of the three yokes. This embodiment has the advantage that the alternating current coil windings are symmetrical in relation to the direct current coil winding so that the cross-sectional areas of the yoke limbs in question and the numbers of turns can be the same in each case.

A considerable direct current feed power for operation of the fault current limiter can be saved if the coil winding through which direct current * flows is superconducting.

Diagrammatically simplified fault current limiters according to embodiments of the invention are shown in the drawing in which: Figure 1 shows a fault current limiter for a 3-phase alternating current network, Figure 2 shows a further embodiment of a fault current limiter for a 3-phase alternating current network, and Figure 3 shows a view in section along line 111-111 in Figure 2 of the fault current limiter of Figure 2.

In the embodiment shown in Figure 1 a fault current limiter includes a ferromagnetic core 1, preferably of layered iron plates, which is in the shape of a rectangular yoke comprising two parallel longitudinal limbs 1.1 and 1,2, an upper transverse limb 1.3 and a lower transverse limb 1.4 which is wider in relation to the width of those three limbs. With the same limb thickness (not visible in the Figure) consequently the lower transverse limb 1.4 is of the largest cross-section. The gradation in the widths of the longitudinal limbs 1.1 and 1.2 and the different cross-sections resulting therefrom are described in greater detail hereinafter. The lower transverse limb is divided by a small air gap 2. Mounted on that lower transverse limb 1.4 is a coil winding 3 which is only diagrammatically indicated and through which direct current flows.

The two longitudinal limbs 1.1 and 1.2 are each jointly surrounded O by a first alternating current coil winding 4.1, a second alternating current coil winding 4.2 and a third alternating current coil winding 4.3. Those O 20 alternating current coil windings are also only diagrammatically indicated.

An important feature however is the direction, that is orthogonal to the winding axis of the direct current coil 3, of the common angle axis of the alternating current coil windings, for that minimises the transformatory coupling of the alternating current coils with the direct current coil. The numbers of turns of the alternating current coil windings 4.1 to 4.3 are different, more specifically they increase with an increasing distance from the coil winding 3 through which direct current flows. Each of the three alternating current coil windings is disposed in a respective phase of a 3-phase network between the feed side and the load or consumer side.

As is known, in the case of a fault current limiter of that structure, the number of turns of the direct current winding 3 and the direct current flowing therethrough as well as the numbers of turns of the alternating current coils 4.1, 4.2 and 4.3 are such that the core 1 which acts as a ring core for the direct current coil 3 is in a condition of magnetic saturation up to a predetermined value in respect of the currents in the alternating current colts 4.1 to 4.3, for which the core 1 acts as a bar core.

Consequently the alternating current coil windings act substantially only as ohmic resistors between the feed side and the load side of the 3-phase network. When the predetermined current is exceeded in at least one of the three phases, that is to say in particular in the event of a short-circuit on the load side, in contrast the magnetic flux which is induced by the alternating current coil winding in question and which in each half-wave is directed in opposite relationship to the magnetic unidirectional flux alternately in the one longitudinal limb 1.1 and the other longitudinal limb 1.2 becomes so great that the core I is no longer in a condition of magnetic saturation at least during a part of the alternating current wave. As a result the alternating current coil winding in question acts as an inductive resistor which limits the current.

As a consequence of magnetisation as far as saturation very much higher magnetic leakage fluxes occur in the core 1, in particular in the O region of the corners of the yokes, than in the case of magnetisation below the saturation level. The reduction in the cross-sections of the longitudinal O 20 limbs 1.1 and 1,2 and optionally also the cross-section of the upper transverse limb 1.3 in relation to the cross-section of the lower transverse limb 1.4 ensures that particularly in the region of the two longitudinal limbs 1.1 and 1.2, in normal operation, the magnetic flux required for magnetic saturation is nonetheless achieved. The cross-sections of the longitudinal limbs LI and 1.2 are graduated. They decrease from the alternating current coil winding 4.1 to the alternating current coil winding 4.3. In addition, the numbers of turns which are indicated in the drawing and which increase from the alternating current coil winding 4.1 to the alternating current coil winding 4,3 provide that the inductive reactance thereof is substantially the same (on the assumption of overcurrents of equal magnitude on all three phases).

In the case of a single-phase embodiment the alternating current coils 4.2 and 4.3 would be omitted and the core or the yoke would be of a correspondingly shorter (lower) structure.

Figures 2 and 3 show another embodiment of a fault current limiter operating on the same principle. It includes three similar yokes 21, 22, 23 which are so assembled that they adjoin each other with their limbs 21.4, 22.4 and 23.4. They can be divided by a narrow air gap similarly to the air gap 2 in Figure 1. Those three mutually adjoining limbs are enclosed by a common coil winding 24 through which direct current flows, and are each of a larger iron cross-section than the other three limbs of each of the yokes 21, 22, 23. The rectangular shape of the three yokes and the star-shaped arrangement thereof, displaced through 120°, are only by way of example.

Each two parallel limbs of the yokes 21, 22, 23 are jointly enclosed by an alternating current coil 20.1, 20.2 and 20.3. In this embodiment the three alternating current coils have the same numbers of turns and the limbs of the yokes, carrying them, are of the same cross-sections.

The direct current coil 3 in Figure 1 and the direct current coil 24 in C Figure 2 can have a superconducting winding to save on exciter power and are then enclosed by a conventional cryocontainer.

O 20 Although particular embodiments have been described herein, it will be appreciated that the invention is not limited thereto and that many modifications and additions thereto may be made within the scope of the invention. For example, various combinations of the features of the following dependent claims could be made with the features of the independent claim without departing from the scope of the present invention.

Claims

CLAIMS1. A fault current limiter comprising an at least substantially annularly closed ferromagnetic core (1) which in at least one region has at least one coil winding (4.1) through which alternating current flows and in at least one region a coil winding (3) through which direct current flows for magnetic saturation of the core (1) in the normal operating condition, characterised in that the cross-sectional area of the region (1.1, 1.2) of the core (1), that carries the alternating current coil winding (41), is reduced in relation to the cross-sectional area of the region (14) of the core (1), that carries the coil winding (3) through which direct current flows, to such an extent that the magnetic flux density in the region (1.1, 1.2) carrying the alternating current coil winding (4.1) in the normal operating condition reaches there at least the value required for magnetic saturation. r
2. A fault current limiter according to claim 1 characterised in that (4 the coil winding (3, 24) through which direct current flows is O superconducting.

O
3. A fault current limiter according to claim 1 or claim 2 characterised in that the difference in the cross-sectional areas is such that it compensates for the magnetic leakage flux to maintain the saturation condition of the core even in the region thereof carrying the alternating current coil winding.
4. A fault current limiter according to one of claims I to 3 characterised in that the region (1.1, 1.2) of the core (1), that carries the alternating current coil winding (4.1), comprises a ferromagnetic material whose saturation field strength is less than that of at least the region (1.4) carrying the coil winding (3) through which direct current flows.
5. A fault current limiter according to one of claims 1 to 4 characterised in that the axes of the coil winding (3) through which direct current flows and the alternating current coil winding (4.1) are in mutually orthogonal relationship.
6. A fault current limiter according to one of claims 1 to S characterised in that the core (1) is interrupted by an air gap (2) in the region of the coil winding (3) through which direct current flows.
7. A fault current limiter according to one of claims I to 6 characterised in that the core (1) comprises a yoke having at least two longitudinal limbs (1.1, 1.2) and at least two transverse limbs (1.3, 1.4) and the alternating current coil winding (4.1) jointly surrounds the two longitudinal limbs (1.1, 1.2).
8. A fault current limiter according to one of claims 1 to 7 for a 3-phase alternating current network characterised in that the core has three mutually separate, alternating current coil windings (4.1, 4.2, 4.3) arranged parallel to one another.
9. A fault current limiter according to claim 8 characterised in that 0 the alternating current coil windings (4.1, 4.2, 4.3) have numbers of turns which increase with an increasing distance from the coil winding (3) through which direct current flows.
10. A fault current limiter according to claim 8 or claim 9 characterised in that the cross-sectional area of the region of the core (1), that carries the alternating current coil windings (4.1, 4,2, 4.3), decreases with increasing distance from the coil winding (3) through which direct current flows.
11. A fault current limiter according to one of claims 1 to 10 for a 3-phase alternating current network characterised in that the iron core includes three at least substantially closed yokes (21, 22, 23) which adjoin each other with a respective one of their limbs (21.4, 22.4, 23.4) and the coil winding (24) through which direct current flows jointly surrounds those three limbs of the three yokes.
12. A fault current limiter substantially as herein described, with reference to the accompanying figures. r r c\J wt