Technical field and prior art of the invention

The invention relates to an electrical inductive device and, more particularly, to an air core reactor. The invention also relates to a process of manufacturing an air core reactor.

It is a further object of the invention to provide an air core reactor capable of providing a high impedance with minimized dimensions and having a plurality of series-connected coils to thereby allow the utilization for high voltage applications.

Air core reactors (ACR) are applied to many different applications in Power Systems requiring inductors (current limiting, harmonic filtering, among others), mainly in the field of reactive compensation.

The construction of power reactors (inductors) with dry-type technology without an iron- core yields a diversity of benefits, such as reduced costs, robustness, construction simplicity, reliability, low maintenance requirements, linearity, etc. Air Core Reactors are the preferable technology for a number of applications in Power Systems, such as:

SVC (Static Var Compensation) reactors for controlled reactive compensation; - Shunt reactors for switched reactive compensation;

Harmonic filtering ;

Smoothing reactors at HVDC systems ; Neutral grounding reactors ;

Current limiting reactors (including arc-furnace reactors, inrush reactors at shunt capacitor banks, discharge reactors at series capacitor banks , etc . )

In order to improve reliability of power transmission systems, the utilization of single-phase switching (SPS) and auto-reclosing on EHV transmission line is a common practice. However, in some cases, it is necessary to apply special means like neutral grounding reactor (NGR) to improve the conditions for secondary arc extinction for successful auto-reclosing. This kind of reactor is typically a high impedance, high voltage and low current reactor, which results in an equipment with prohibitive dimensions.

Although reaching high inductive power output, the utilization of ACR' s as power inductors is limited to relatively low impedance, due to the absence of an iron core. Some applications, such as neutral grounding reactors (NGR) with high impedance/low current, are usually implemented through classical iron-core oil-filled reactors. The utilization of ACR construction principle for high impedance coils results in very large dimensions and costs that overcome the benefits of the dry-type technology. Specially the large dimensions required at substations hinder the utilization of ACR' s as high impedance neutral grounding reactors (NGR) .

Iron-core oil-filled reactors are sophisticated and expensive equipment, with high cost and many technical issues concerning maintenance, oil treatment, etc. That makes the solution very expensive to this very simple application.

Typically, air core reactors present a series of advantages against oil-filled reactors for this application. Main advantages are:

Very lower cost (price) , typically less than 50 % of oil-filled reactors.

Reduced dimensions and weight.

Easiness of installation, due to simplicity of construction.

Environmental friendly, no need for installations to retain oil leakages.

Almost free of maintenance.

Even though the above mentioned advantages, normal air core reactors presents very large dimensions, which makes it a non-preferred solution for high impedance neutral grounding. A 800Ω/15Α reactor, for example, may result in an external diameter of around 3m. Typical 2000Ω/10Α reactor exceeds the manufacturing capabilities of ACR facilities, due to the need of very large dimensions.

Air core reactors manufactured with fibreglass encapsulation are dry-type power reactors, where the coils are made of round insulated wires, encapsulated by fiberglass in several concentric winding cylinders, and clamped together by two cross- arms, which provides also the terminal connections. The figures 1A and IB represent an example of air core reactor according to the prior art. The figure IB is a detailed view of figure 1A. The air core reactor comprises six windings W1-W6 parallel connected, thus forming operationally a unique coil with high current capability. The six windings W1-W6 are divided among three encapsulation packs EP1, EP2, EP3 made of fiberglass, each encapsulation pack comprising two windings parallel connected. Two cross-arms Al, A2 constitute the terminals of the air core reactor. One of the two cross-arms (cross-arm Al on figure 1A) is mounted on four insulators 11-14. The example of prior art air core reactor manufactured with fibreglass encapsulation of figures 1A and IB comprises six windings and three encapsulation packs. Generally, prior art air core reactors manufactured with fiberglass encapsulation comprise a number of windings varying typically from one to one hundred and a number of encapsulation packs varying from 1 to twenty. A problem of such air core reactors is that there are limited to relatively low impedance due to the absence of an iron core.

The device of the invention does not have such drawbacks. The device of the invention allows the construction of high impedance air core reactors with significant reduction of dimensions and costs, when it comes to limited steady-state current, if comparing to the prior art technology. Description of the invention

Indeed, the invention relates to an air core reactor comprising a coil connected between a first terminal and a second terminal, characterized in that the coil is made of a succession of bundles of conductor connected in series along an axis between the first terminal and the second terminal, each bundle of conductor being made of one wire wound around said axis so as to form a multi-layer winding having a cross- section made of N winding layers in a direction perpendicular to the axis, from a winding layer of rank 1 which is the closest to the axis to a winding layer of rank N which is the furthest from the axis, each winding layer in the direction perpendicular to the axis comprising several winding layers in the direction of the axis, the number of winding layers in the direction of the axis of the winding layer of rank j (j=2, N) in the direction perpendicular to the axis being equal or less than the number of winding layers in the direction of the axis of the winding layer of rank j-1 in the direction perpendicular to the axis.

According to another feature of the invention, the cross section of each bundle of conductor is in a trapezoidal form, or a rectangular form, or a square form, or a triangular form.

According to another feature of the invention, the wire conductor is encapsulated in an encapsulation pack made of fiberglass. According to a first embodiment of the invention, the succession of bundles of conductors is made with a same wire.

According to a second embodiment of the invention, each bundle of conductor is made with a different wire and the bundles of conductor are connected in series by means of electrical connections.

The invention also relates to a process of manufacturing an air core reactor comprising a coil connected between a first terminal and a second terminal , characterized in that it comprises at least one step of winding one wire around an axis so as to form a multi-layer winding having a cross-section made of N winding layers in a direction perpendicular to the axis, from a winding layer of rank 1 which is the closest to the axis to a winding layer of rank N which is the furthest from the axis, each winding layer in the direction perpendicular to the axis comprising several winding layers in the direction of the axis, the number of winding layers in the direction of the axis of the winding layer of rank j (j=2, N) in the direction perpendicular to the axis being equal or less than the number of winding layers in the direction of the axis of the winding layer of rank j-1 in the direction perpendicular to the axis.

According to another feature, the process of the invention comprises at least two steps of winding along the axis, said at least two steps of winding being made continuously with a same wire. According to another feature, the process of the invention comprises at least two steps of winding, each step of winding being made separately from the other with a different wire and the multi- layer windings obtained from said at least two steps of winding are series-connected along the axis.

The winding system of the invention is feasible to be applied to typical reactors using fiberglass encapsulated design, resulting in much higher inductance, despite the reduced heat dissipation properties which are not necessary to high impedance neutral grounding reactors .

In conventional air core reactors, all winding layers are parallel-connected, resulting in large conductive section, which allows reaching high rated power. The winding system of the invention is based on a series-connected winding, using at least one conductor. The turns are simply wound in accumulated layers which cross-section forms a stable shape in square, rectangular, triangular or trapezoidal form, thus forming a bundle of conductor to be series- connected with a succession of equal or different bundles. The cross-section with a square, rectangular, triangular or trapezoidal form allows keeping the structure stable for winding at common winding machines for air core reactors. The separation of winding in several bundles allows the complete coil to withstand high BIL (BIL for "Basic Impulse Level") to work at high voltage systems. Brief description of the figures

Other characteristics and advantages of the invention will become clearer upon reading a preferred embodiment of the invention made in reference to the attached figures among which:

- Figures 1A and IB show an example of an air core reactor according to the prior art;

- Figures 2A, 2B and 2C show an example of a first embodiment of an air core reactor according to the invention;

- Figure 3 shows an example of the way a wire conductor is wound to form an air core reactor according to the first embodiment of the invention;

- Figure 4 shows a first example of cross- section of an air core reactor of the invention;

- Figure 5 shows a second example of cross- section of an air core reactor of the invention;

- Figure 6 shows a third example of cross- section of an air core reactor of the invention;

- Figures 7-9 show examples of side view of bundles of conductor according to the invention.

In all the figures, the same references designate the same elements.

Detailed description of a preferred embodiment

Figure 2A shows an overall view of an example of air core reactor according to the invention while figures 2B and 2C show two detailed views of figure 2A.

The air core of the invention comprises two cross-arms Al, A2, and a coil C2 made of a wire conductor W encapsulated in a fiberglass encapsulation E (see figures 2A and 2B) .

According to the first embodiment of the invention, the winding sytem of the air core reactor uses only one round wire W as conductor. The turns of the round wire conductor W are accumulated to form several bundles of conductor Bl, B2, B3,..., BN.

The Figure 3 illustrates an example of the way the wire conductor is wound to form two successive bundles Bi, B . The first bundle Bi is made by winding the wire eleven times around an axis so as to form a first layer of conductor in the direction of the axis, then by winding the wire ten times around the axis upon the first layer so as to form a second layer of conductor, and so on until a fifth layer of conductor made by winding the wire seven times around the axis upon a fourth layer. When the first bundle has been achieved, the second bundle Bj is made in the same way than the first bundle Bi from a first layer to a fifth layer. As an example, each bundle has a trapezoidal cross-section (see Figure 4) . However, other forms of cross-section are concerned by the invention as, for example, a rectangular form (see Figure 5) or a triangular form (see Figure 6) .

More generally, as it can be seen on figure 3, the wire conductor is wound on different levels to form a first bundle of conductor from a bottom level to an upper level and, when the upper level of the first bundle is reached, the wire conductor is taken back to the bottom level to form a second bundle of conductor next to the first bundle, and so on. A first terminal of the wire conductor is connected to one of the two cross-arms while the other terminal of the wire conductor is connected to the second cross-arm.

According to a second embodiment of the invention, each bundle of conductor is made separately and the different bundles are series-connected afterwards by means of electrical connections. The figures 7-9 show side views of elementary bundles of conductor made separately. The figure 7 corresponds to a rectangular cross-section. The figure 8 corresponds to a trapezoidal cross section. The figure 9 corresponds to a triangular cross section.

Whatever is the embodiment, the air core reactor of the invention joins the benefits of air-core reactor technology with a very significant reduction in dimensions in comparison with the prior art. For example, a 800Ω/15Α reactor can be built with very reduced weight and an external diameter of 1,28m, which is far lower than the typical dimensions of air-core reactors of the prior art.

The table below shows the comparison among three solutions:

(1) prior art iron core oil-filled reactor, - (2) prior art air core reactor, (3) air core reactor according to the invention .

With W for Width, H for Height and D for

Diameter .

More generally, the air core reactor of the invention presents better characteristics in all points, when compared to all other available present solutions in the market.

A main advantage of the invention is to provide a high impedance air core reactor. It is therefore possible to achieve very high impedance air core reactors (as high as lOkQ for example) , thus replacing oil-filled reactors with all benefits of the air core reactor technology, i.e. much lower cost, weight and dimensions, leackage and maintenance free, robustness and simplicity of mounting and installation.