WO2006054932A1

WO2006054932A1 - Phase compensation

Info

Publication number: WO2006054932A1
Application number: PCT/SE2005/001562
Authority: WO
Inventors: Bror Axelsson
Original assignee: Stri Ab
Priority date: 2004-11-17
Filing date: 2005-10-19
Publication date: 2006-05-26
Also published as: SE527870C2; SE0402812D0; SE0402812L

Abstract

System comprising a number of compensating devices (1) for the reactance per unit length of an overhead cable (9) in a power line. One compensating device comprises a capacitor device (4) for serial compensation for the inductance of the overhead cable and a transformer device provided with an iron core (6), a primary winding (2) and a secondary winding (5). The capacitor device is arranged to be magnetically connected to a phase conductor (3) of an overhead cable via the transformer device by an electrical serial connection of the capacitor device (4) in the secondary winding (5), and an electrical serial connection of the ends of the primary winding (2) to the phase conductor (3). This causes the magnetic saturation property of the iron core (6) in the transformer device to form an over-voltage protection for the capacitor device at excess currents in the phase conductor.

Description

Phase Compensation

TECHNICAL FIELD

This invention relates to a compensating device for the reactance per unit length of an overhead cable in a power line, and a compensating system comprising such compensating devices.

BACKGROUND OF THE INVENTION AND PRIOR ART

A power line for transmitting electrical power from a power station to the electricity subscribers comprises a power transmission network for high voltages (10OkV - 400 kV) , and a power distribution network with stepped-down voltages in the size of 1OkV - 100 kV. The power line transfers a three-phased alternating current, AC, mainly over a so-called overhead cable, or an aerial line, which is an electrical cable comprising three phase conductors mounted in suspension insulators or standoff insulators on strongly supported poles made of e.g. metal, concrete of wood. The design of the phase conductors is standardized, and they may be manufactured by e.g. aluminium or copper. They may also, be provided with a steel armouring to increase their mechanical strength. An overhead cable has an upper limit for the capacity to carry an electrical load due to its construction, and this upper limit will vary depending on the weather conditions. Since the power lines are an important part of the infrastructure, it is very important that they have a high capacity, in order to transfer high-voltage power with a minimum of power losses. The gradual increase of the price of electricity leads to larger costs for the power losses in a cable, and it becomes more and more important to reduce the losses.

A sinusoidal alternating current, AC, in an overhead cable results in a power loss, and the size of the power loss depends on the impedance per unit length of the conductor, composed of the resistance per unit length of the cable, R, and its reactance per unit length, X, which is normally indicated in the unit Ohm/km within the field of power engineering. The reactance per unit length gives rise to a phase shift of the current relative the voltage.

The resistance in an overhead cable gives rise to a resistive power loss to be minimized, while the inductance gives rise to a reactive voltage drop. The line voltage can be increased by a reduction of the reactive line voltage drop, wherein the phase current can be reduced, since the same power can be transferred with a lower current. Thereby, the resistive loss is reduced, as well, since the resistive power loss is proportional to the square of the current value, according to 'the following well- known relationship: P (Power)=R(Resistance) x I²(Current) .

Prior art in reducing the reactance per unit length in a overhead cable involves an electrical connection of a serial capacitor station in the three phases of the overhead cable, on one or a few locations, wherein a connected high-voltage serial capacitance compensates for the inductance of the overhead cable. The dimensioning of the capacitance of a serial capacitor station may be based on the desired degree of compensation, i.e. the part of the positive reactance (i.e. inductance) of the overhead cable to be compensated for by means of the negative reactance (serial capacitance) added by the connected high- voltage serial capacitor, normally having a nominal voltage measured in kilovolt, kV. Thus, the term "degree of compensation" can be defined as the part of the inherent reactance per unit area of the overhead cable that is compensated for by means of a compensating serial capacitance.

Prior art within this technical field is described e.g. in the Swedish patent No. SE 457 588, describing a serial capacitor equipment for reducing the reactance per unit length in a power line electrical conductor by compensating for the inductance of the conductor by means of the capacitance of the serial capacitors.

However, the above-described expensive and complicated prior art is only worthwhile to apply in the reduction of the resistive losses at very high voltages and high powers. Thus, there is a need of a simpler, less expensive and more flexible compensation of the reactance per unit length in an overhead cable.

DESCRIPTION OF THE INVENTION

The object of the present invention is to facilitate a flexible compensation for the reactance per unit length in an overhead cable, at a low cost. This and other objects are achieved by the invention according to the appended claims, which relate to a compensating device and a compensating system comprising several such compensating devices.

The compensating device according to the invention is arranged to compensate for the reactance per unit length of an overhead cable in a power line, and it comprises a capacitor device for serial compensation for the inductance of the overhead cable, and a transformer device provided with a iron core, a primary winding and a secondary winding. The capacitor device is arranged to be magnetically connected to a phase conductor in the overhead cable via said transformer device by an electrical serial connection to the secondary winding, and an electrical serial connection of the ends of the primary winding to said phase conductor. Thereby, the magnetic saturation property of the iron core in the transformer device forms an over-voltage protection for the capacitor device at excess currents in the phase conductor. The compensating device may comprise a mounting arrangement for installation in any of the phase conductors at mounting locations distributed over the length of the overhead cable, and the mounting arrangement may comprise electrically conducting retainers and electrically insulating spacers designed to enable one phase conductor to carry the total weight of the compensating device.

The compensating device according to the invention may be intended to be a part of a compensating system comprising several compensating devices, wherein the degree of compensation of the compensating system for an overhead cable with a given length is adjustable by a change of the total number of installed compensating devices in said overhead cable. The compensating device may be provided with mounting arrangements for installation in the phase conductor of said overhead cable at suitable mounting locations distributed over the length of the overhead cable.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail below, with reference to the figures 1 - 4, of which:

figure 1 illustrates the electrical connection of a compensating device according to the invention, at a mounting location in a phase conductor,

figure 2 illustrates schematically the components of a compensating device according to the invention, and how they are connected to each other and to the phase conductor,

figure 3 illustrates an embodiment of a compensating device provided with a mounting arrangement for installation at a mounting location in a phase conductor, and

figure 4 illustrates a section of a three phase-conducting cable having three mounting locations for compensating devices according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Thus, this invention provides a simpler, less expensive and more flexible compensation for the inherent reactance per unit length of an overhead cable in a power line, and can be implemented to a much larger extent than prior art. This invention can reduce the resistive losses in a cost-efficient way, by compensating for the reactance per unit length of the overhead cable. The reduced reactance per unit length of the overhead cable leads to a slightly lower voltage drop over the line, wherein the line voltage increases and the phase current is slightly reduced. The reduced phase current leads to a reduced resistive power loss, which is proportional to the square of the phase current.

According to this invention, the compensation for the reactance per unit length of an overhead cable is performed by means of a small-sized compensating device comprising a serial capacitance and designed to magnetically connect a serial capacitor to an existing phase conductor via a current transforming device, instead of a direct electrical serial connection of a high- voltage capacitor device, according to prior art within the technical field. Further, a compensating device is installed in each one of the three phase conductors, at suitable mounting locations distributed over the length of the overhead cable, instead of by means of one or a few large, high-voltage and expensive serial capacitors, according to prior art. A compensating device according to this invention comprises a small-sized current transforming device having an iron core, a primary winding to be electrically serial connected with a phase conductor, and an insulated secondary winding, provided with a simple and inexpensive serial capacitor load. A discharging resistor is connected in parallel with the capacitor, and is normally an integrated part of said capacitor. The capacitor is protected from all kinds of excess currents or surge currents in the phase conductor, occurring e.g. at short circuiting and earth faults on the line, by the saturation of the iron core, wherein the strong magnetic connection between the primary winding and the secondary winding is interrupted and the capacitor voltage is limited, which is an important advantage with the present invention compared to prior art within the field. Due to the magnetic connection of the capacitor to the phase conductor by means of the current transforming device, a low-cost, low-voltage capacitor can be used, without requiring any additional surge protection, which is an important advantage compared to prior art serial capacitor element, which are designed to be directly serial connected with a phase conductor, and may be exposed to large over-voltages. According to this invention, the iron saturation of the iron core of the transformer in reality functions as an over-voltage protection for the capacitor.

Figure 1 shows a compensating device 1, according to this invention, connected to a phase conductor 3, and illustrates the electromagnetic connection of a serial capacitor to a phase conductor, the serial capacitor comprised in a compensating device 1 according to this invention. A primary winding 2 of the compensating device 1 is electrically connected directly in the phase conductor 3, wherein the ordinary phase current flows through the low-resistive primary winding. A capacitor 4 is electrically connected in a low-resistive secondary winding 5. An iron core 6 surrounds both the primary winding 2 and the secondary winding 5 and inter-connects them magnetically, wherein a current is induced in the secondary winding when a phase current flows through the primary winding 2.

Figure 2 illustrates schematically how the primary winding 2, the iron core β, the secondary winding 5 and the capacitor 4 are interconnected. Accordingly, the capacitor 4 is connected via a current transformer, i.e. connected magnetically, to the primary winding 2 via the secondary winding 5 and the iron core 6. When a surge current occurs in the phase conductor, e.g. due to short-circuiting, the current flows through the primary winding 2 of the compensating device. The iron core is saturated at a certain current level, depending on the properties and the design of the iron core, thereby limiting the induced current in the secondary winding 5 and protecting the serial capacitor 4 against harmful over-voltages.

In preferred embodiments of the compensating device according to the invention, the primary winding 2 comprises up to some twenty or thirty windings. Since both ends of the primary winding is electrically serial connected to the phase conductor 3, the ordinary phase current of the line flows through the primary winding. The secondary winding 5 has between some twenty or thirty and a few hundred windings, and one or a few capacitors 4 are connected in parallel in the secondary winding, the capacitors having a capacitance in the magnitude between some twenty or thirty μF and a few hundred μF and a nominal voltage of between some twenty or thirty volt and a few hundred volt. A high-efficient iron core .6 surrounds both windings, interconnecting the primary and secondary windings via a current transformer, i.e. in a magnetic manner, thereby connecting the capacitor device to the phase conductor, whereby its capacitance loads the conductor and compensates for the inductance of the overhead cable. However, the capacitor will not be exposed to the high voltages caused by momentary current surges in the phase conductor, since the iron core of the transformer device will be saturated and, thereby, interrupt the strong magnetic connection between the primary winding and the secondary winding.

Since a compensating device according to this invention is adapted to be mounted on a phase conductor of an overhead cable in a power line, the components must be assembled by means of parts providing an appropriate insulation, as well as an appropriate electrical and mechanical connection. Thus, the compensating device and its mechanical installation must be arranged to withstand the electrical and mechanical stress that may be present in the vicinity of an en overhead cable. It is also advantageous to be able to perform the installation on a live conductor. Preferably, the compensating device together with the mounting arrangement will have a total weight that is low enough to enable the phase conductor to carry said compensating device without any additional supporting arrangement. A compensating device provided with a mounting arrangement according to a preferred embodiment of the invention is illustrated in figure 3, in which a compensating device 1 is connected to a phase conductor 3 at a mounting location of an overhead cable. The mounting arrangement comprises two strong and electrically conducting carriers 7a, 7b, which are mechanically interconnected via en strong and armoured spacer 8, which is electrically insulating. The first ends of the carriers 7a, 7b are fastened to the phase conductor 3 and their other ends are fastened to the ends of the compensating device 1 to balance the mechanical load, by means of the carriers 7a, 7b and the spacer 8 carrying the mechanical stress. Hereinafter, a preferred method of performing the mechanical mounting of the compensating device on the phase conductor by means of the mounting arrangement shown in figure 3 is described. First, the firs.t ends, 7a and 7b, of the electrically conducting carriers, including the strong and armoured spacers, 8, are fastened in the phase conductor 3. Thereafter, the compensating device 1 is fastened to the two other ends of the carriers 7a, 7b, and, finally, the part of the phase conductor located between the two connection points in the phase conductor is removed in a suitable way, e.g. by means of a special tool designed to be used on a live conductor. The mounting arrangement is preferably designed such that the total size and weight of the mounting arrangement together with the compensating device will allow the mounting arrangement to be installed at a mounting location in any of the phase conductors of an overhead cable without any additional supporting device.

Figure 4 illustrates mounting locations on a section of a three- phase conductor, according to an embodiment of the compensating system comprising several compensating devices according to this invention. The conductor 9 comprises three phase conductors 3a, 3b, 3c, and this section of the conductor is provided with a total or nine compensating devices Ia,b, c,d,e, f, g,h, i. Those are mounted in each of the phase conductors, respectively, at each of the three mounting locations 10, 11 and 12, respectively, which are distributed over the length of the overhead cable. The distance Dl between the two mounting locations 10 and 11 is larger than the distance D2 between the two mounting locations 11 and 12. Thus, as illustrated in this figure, the intervals (i.e. Dl and D2) between the different mounting locations do not have to be the same, and they may vary depending on the position of a suitable mounting location. Suitable mounting locations may e.g. be in the vicinity of a road, but not over watercourse, etc. One embodiment of a compensating device according to the invention will now be described in more detail below in order to illustrate the degree of compensation that can be obtained by a particular design of the compensating device. This embodiment also intends to illustrate how the degree of compensation is affected by the dimensioning of the compensating system according to this invention, comprising several compensating devices intended to be mounted at suitable mounting locations distributed over the length of the overhead cable, with a compensating device mounted in each of the phase conductors in every mounting location.

By using this embodiment of the invention, a compensation for the inherent reactance per unit length is performed in a phase conductor being a so called "Waxwing", i.e. a standard ACSR (Aluminum Conductor Steel Reinforced) conductor, which comprises 18 aluminum wires and one steel wire, having a total diameter of 15.46 mm. The maximum absolute value of the current is estimated to 300 Amperes (1.6 A/mm²) , corresponding to the phase current at a nominal load.

The compensating device according to this embodiment is designed with a primary winding of 15x15 mm and comprising 20 windings of aluminum wire, and a secondary winding comprising 520 windings of copper wire, having a wire diameter of 2 mm. The capacitor connected in the secondary winding has a capacitance, C, of 133.9 μF and a nominal voltage of 275 V, which is a realistic value for one or several parallel connected low-voltage, low- cost standard capacitors. The discharging resistance of the capacitor is estimated to approximately 50 kilo-ohm, giving a nominal power of 1.5 W at a nominal load. The iron core in the compensating device is made of electroplating- with a cylindrical shape, having an inner and outer diameter of 120 mm and 160 mm, respectively, and a length of 60mm. The relative permeance of the electroplating is estimated to approximately 10000, and it is dimensioned for a magnetic flux density of less than 2 Vs/m², corresponding to the effective value of 1.41 Vs/m² at a maximum current of 300 ampere (effective value) at a nominal load. Saturation is assumed to take place at a flux density of 2.2 till 2.4 Vs/m²' i.e. at a flux density value that is between 10% and 20% larger than compared to a nominal load. The winding voltage at the frequency of 50 Hz is calculated to 0.53 V from the above data, wherein the voltage on the primary winding is 20 x 0.53 = 10. β V, and the voltage on the secondary winding, i.e. over the capacitor, is 520 x 0.53 = 275 V.

The intrinsic losses in a compensating device according to this embodiment can be calculated to between 130 W and 150 W, which is negligible in comparison to the intrinsic losses of the phase conductor, which can be estimated to between 20000 W and 25000 W per km and phase. The total weight of the compensating device, including a mounting arrangement, can be calculated to between 10 kg and 15 kg, and this load can be handled by the phase conductor, whose intrinsic weight, as a comparison, is approximately 430 kg/km.

Since the inherent reactance of an overhead cable comprises an inductance that can be estimated to e.g. 0.3 ohm per phase/ kilometre, the reactive (i.e. inductive) voltage drop is 300 x 0.3 = 90 V /kilometre in each phase at a nominal load (phase current 300 A) .

According to his embodiment, four compensating devices are installed per kilometre in each phase of an overhead cable, i.e. one compensating device in each of the phase conductors at mounting locations distributed over the length of the overhead cable, with an average interval of 250 metres. If the compensating devices are designed and dimensioned according to this embodiment, an addition of 4 x 10.6 = 42.4 V capacitive voltage per kilometre is obtained in each phase. This compensates for a part of the inductive voltage drop caused by the reactance of the overhead cable. The degree of compensation obtained in this embodiment is 42.4 / 90 = 0.47, i.e. 47%, thereby reducing the reactive (i.e. inductive) voltage drop per phase and kilometre with almost 50%. This leads to a higher line voltage and a lower phase current, whereby the resistive losses are reduced in the overhead cable in direct proportion to the square of the reduction of the phase current.

This embodiment illustrates how the degree of compensation is affected by the dimensioning of the components of a compensating device according to this invention. One advantage with this invention is to facilitate the use of inexpensive, low-voltage, standard capacitors, without requiring any additional over- voltage protection. This embodiment also illustrates that a higher degree of compensation is obtainable, and a higher line voltage and even lower resistive losses, by an increase of the number of compensating device distributed over the length of the overhead cable, i.e. with a smaller interval, on average, between the mounting locations.

A suitable dimensioning of a compensating system according to this invention, comprising a compensating device having a particular design, can thus be performed based on the desired degree of compensation by installing a larger number of compensating devices in an overhead cable with a given length. A compensating device having a low total weight together with the mounting arrangement will result in a compensating system having a high degree of flexibility, since the desired degree of compensation is easily obtainable by means of a closer mounting, i.e. by choosing smaller intervals, on an average, between the mounting locations. However, the length of the individual intervals between the mounting location do not have to be the same, and suitable mounting locations on the overhead cable can be chosen with a large liberty, considering e.g. the geography and the location of buildings.

Since the method of mounting the compensating devices in a phase conductor can be arranged to be performed directly on a live conductor, it is easy to replace a compensating device for the system maintenance purposes, but also to flexibly increase or reduce the number of compensating devices per unit length, thereby achieving a desired degree of compensation.

The present invention is not limited to the embodiments described above, but can be modified within the scope of the appended claims.

Claims

1. A compensating device (1) for the reactance per unit length of an overhead cable (9) in a power line, the compensating device comprising a capacitor device (4) for serial compensation for the inductance in the overhead cable, and a transformer device provided with an iron core (6), a primary winding (2) and a secondary winding (5), characterised in that said capacitor device (4) is arranged to be magnetically connected to a phase conductor (3) of the overhead cable via said transformer device, by an electrical serial. connection of said capacitor device (4) in the secondary winding (5) and an electrical serial connection of the ends of the primary winding (2) to said phase conductor (3) , wherein the magnetic saturation property of the iron core (6) in the transformer device forms a over-voltage protection for the capacitor device (4) at excess currents in the phase conductor.

2. A compensating device according to claim 1, characterised in that it is provided with a mounting arrangement (7a, 7b, 8) for installation in a phase conductor (3a, 3b, 3c) at mounting locations (11, 12, 13) distributed over the length of the overhead cable (9) .

3. A compensating device according to claim 2, characterised in that the mounting arrangement comprises electrically conducting retainers (7a, 7b) and electrically insulating spacers (8) designed to enable one phase conductor to carry the total weight of the compensating device.

4. A compensating device (1) according any of the preceding claims, characterised in that it is intended to be part of a compensating system for the reactance per unit length in an overhead cable (9) in a power line.

5. A compensating system for the reactance per unit length of an overhead cable in a power line, the system comprising several compensating device (1) according to any of the preceding claims, characterised in that the degree of compensation of the compensating system for an overhead cable (9) having a given length is adjustable by a change of the number of installed compensating devices (1) in said overhead cable.

6. A compensating system according to claim 5, characterised in that the said compensating devices (1) are provided with a mounting arrangement (7a, 7b, 8) for installation in a phase conductor (3a, 3b, 3c) of said overhead cable at suitable mounting locations (11, 12, 13) distributed over the length of the overhead cable.