The invention relates to a reactor for use in HVDC power transmission equipment.

HVDC (high-voltage direct current) electrical power transmission uses direct current for the transmission of electrical power. This is an alternative to alternating current electrical power transmission which is more common. There are a number of benefits to using HVDC electrical power transmission.

In order to use HVDC electrical power transmission, it is typically necessary to convert alternating current (AC) to direct current (DC) and back again. To date most HVDC transmission systems have been based on line commutated converters (LCCs), such as a six-pulse bridge converter using thyristor valves. LCCs use elements such as thyristors that can be turned on by appropriate trigger signals and remain conducting as long as they are forward biased. In LCCs the converter relies on the connected AC voltage to provide commutation from one valve to another.

It is known to provide individual thyristors, or groups of thyristors within a thyristor valve, with a reactor to mitigate against inrush current which may otherwise damage a thyristor when the valve is turned on. Such reactors may comprise an inductor wound with a primary coil, and may be provided with a resistor in parallel with the reactor for damping current oscillation.

According to an aspect of the invention, there is provided a reactor for use in HVDC power transmission equipment, comprising a core structure and a primary coil wound around the core structure; wherein the core structure comprises a primary core element and an auxiliary core element stacked along a stack direction of the core structure; wherein each one of the core elements is a laminate comprising a plurality of laminations, the laminations of each core element having a respective lamination thickness; and wherein the lamination thickness varies between the primary core element and the auxiliary core element. The lamination thickness of laminations in the primary core element may be less than the lamination thickness of laminations in the auxiliary core element. Accordingly, the eddy current loss may be greater in the auxiliary core element than in the primary core element. The relatively higher energy loss in the or each auxiliary core element may provide current oscillation damping.

When the lamination thickness of laminations in the auxiliary core element is greater than the lamination thickness of laminations in the primary core element, the damping resistance may be primarily determined by properties of the or each auxiliary core element, in particular by the level of eddy current loss which is a function of the lamination thickness. It will be appreciated that the damping resistance of the core as a whole may be determined by the properties of (and eddy current losses in) both the primary core element and the auxiliary core element. Nevertheless, by providing an auxiliary core element having a greater lamination thickness, a greater proportion of eddy current loss may be concentrated in the auxiliary core element, and thus the design of the auxiliary core element may primarily determined by the damping resistance as a whole.

The primary core element may comprise a gap. The gap may be a gap in a flux path of the primary core element ("a flux path gap") - i.e. a gap in the ferromagnetic material forming the magnetic flux path around the core element. The gap may be occupied by a spacer, for example a polymeric spacer such as nylon. In some examples, the (or each) auxiliary core element may comprise a gap.

The reactance of a core element may be primarily determined by the size of a gap in the core element. The variation in size of the gap in the core element may have a greater influence on the reactance of the reactor than the variation of other parameters (such as lamination thickness) within a suitable design envelope. The reactance of the reactor as a whole will depend on the reactance of each core. When the primary core element comprises a gap, the reactance of the core, and thereby the reactor, may be primarily controlled or determined by the size of the gap. When the lamination thickness of laminations in the auxiliary core element is greater than the lamination thickness of laminations in the primary core element, the reactance of the reactor may be primarily determined and adjusted by the properties of the primary core element, in particular by the gap - It will be appreciated that the reactance of the core as a whole may be determined by the properties of both the primary core element and the auxiliary core element. Nevertheless, variation in the size of the gap in the primary core may have a greater influence on the reactance of the core (and/or the reactor) as a whole than variations of other parameters (such as lamination thickness) within suitable design envelopes.

The gap may be variable. Accordingly, the reactance of the core may be varied by varying the dimensions of the gap.

The auxiliary core element may be continuous at the or each location corresponding to the gap of the primary core element. In other words, the outer core elements may not have a gap corresponding to the gap of the central core element. Gaps in one core element may not correspond with gaps in another core element. Alternatively, the auxiliary core element may comprise a gap at the or each location corresponding to the gap of the primary core element. The core elements may be insulated from each other by an elastomeric membrane. Accordingly, mechanical vibrations in use may be mitigated. Such mechanical vibrations may be due to electromotive force generated in the reactor in use.

The reactor may further comprise a cooling plate disposed adjacent the auxiliary core element in the stacking direction and configured to transfer heat away from the auxiliary core element. The reactor may further comprise a cooling pathway for conveying a cooling medium to cool the primary coil and/or core structure. The cooling pathway may be part of a cooling circuit for circulating the cooling medium.

The cooling pathway may include a coil portion extending through the primary coil and a core portion thermally coupled to one or more cooling plates configured to transfer heat from the core structure. The coil portion may be in series with the core portion.

The core portion may have a plurality of parallel flow portions for cooling respective cooling plates configured to transfer heat from respective regions of the core structure.

There may be two auxiliary core elements, one on either side of the primary core element, such that the primary core element has a central position within the core structure.

There may be at least two cooling plates adjacent the respective auxiliary core elements in the stacking direction and configured to transfer heat away from the auxiliary core element.

The reactor may further comprise a structural support having support elements disposed on opposite sides of the core structure in the stacking direction and configured to clamp the cooling plates and core structure therebetween. The primary coil may be disposed within a housing having attachment points for attaching to the structural support. The housing may be a cast housing. The housing may comprise a composite material.

The reactor may be a saturable reactor.

The reactor may comprise any combination of the features and/or limitations referred to herein, except combinations of such features as are mutually exclusive. The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows a perspective view of a reactor;

Figure 2 schematically shows a core structure and primary coil of the reactor of Figure 1;

Figure 3 schematically shows a perspective view of the core structure of Figure 2;

Figure 4 schematically shows a perspective view of a further example core structure;

Figure 5 schematically shows laminations of a primary core element of the core structure; Figure 6 schematically shows laminations of an auxiliary core element of the core structure;

Figure 7 schematically shows a further example core element;

Figure 8 schematically shows a cross-sectional view of the primary core element of the reactor; and Figure 9 diagrammatically shows an example cooling pathway of the reactor.

Figure 1 shows an example reactor 100 for a thyristor valve. In this example, the reactor 100 is a saturable reactor, which means that the core of the reactor can be saturated after an initial period of current flow so that the inductance of the reactor su bsequently reduces. This may be advantageous when used in conjunction with a thyristor valve, as the inductive properties of the reactor may only be required for a short period after turn-on of the thyristor valve, after which time it may be desirable for the reactor to have a low inductance. The reactor 100 comprises a structural support 110, a core structure 130, a primary coil 150 and a cooling assembly 160.

In this example, the core structure 130 has the shape of a rectangular frame, as will be described in detail below. It will be appreciated that the core structure may be of any suitable shape.

The following description makes use of relative terms associated with the orientation of the rectangular core structure as shown in Figures 1 to 6. In particular, as shown, the rectangular frame of the core structure 130 has upper and lower portions coupled by upright arms at either end of the upper and lower portions. The core structure 130 has front and rear faces, each of which have an upper region and a lower region corresponding to the upper and lower portions of the core structure respectively. The upper and lower portions and regions are elongate along a lateral direction of the structure (i.e. the direction along which they are elongate is the lateral direction of the structure). The front and rear faces are separated along a stacking direction of the core structure (as will be further described below). The upright arms extend along an upright direction of the core structure. The lateral, upright and stacking directions described above are orthogonal with respect to each other. Whilst the terms lateral, upright and stacking directions have been used herein to aid understanding with respect to the particular examples, it will be appreciated that the reactor can be used in any orientation. Accordingly, the lateral and upright directions may be referred to as first and second directions which, together with the stacking direction, are three orthogonal directions of the reactor.

As shown in Figure 1, the primary coil 150 is wound around the core structure 130. In this particular example, the primary coil 150 is wound around first and second upright arms of the rectangular frame of the core structure 130.

In this example, the primary coil 150 comprises an aluminium bar or duct having a wound shape and which comprises a central conduit 174 for conveying a cooling medium, as will be described below. The aluminium bar therefore constitutes the functional winding of the coil. It is housed in a cast housing, so that the functional winding is physically and electrically protected from the external environment, and so that each successive turn of the winding is linearly separated from an adjacent turn. In this particular example, the cast housing is a composite housing formed by laying up fibre reinforcement material, such as glass fibre, around the winding. The fibre reinforcement may be "pre-preg" glass fibre (i.e. pre-impregnated with a resin matrix material), or a resin matrix material may subsequently applied to the fibre reinforcement, for example in a die. The fibre reinforcement and resin is then cured to form the casting. Conductive connector pads 154 are provided at each end of the aluminium bar, outside of the cast housing.

In this example, the cooling assembly comprises both a cooling pathway including a coil portion 174 and a core portion. The coil portion 174 is formed by the central conduit within the primary coil 150. In this example, the core portion comprises a network of flow conduits configured to convey a cooling medium. The flow conduits are thermally coupled to cooling plates 162 of the cooling assembly, which in turn are thermally coupled to the core structure 130 such that in use heat is transferred from the core structure 130 to the cooling medium. In this example, there are six cooling plates 162 including four lateral cooling plates extending along the upper and lower regions on the front and rear faces of the core structure 130; and two upright cooling plates on the outer side faces of the upright arms of the core structure 130. However, in other examples there may be fewer cooling plates, for example lateral cooling plates only or upright plates only.

As shown in Figure 1, in this example the cooling assembly 160 is configured to receive an inlet flow of a cooling medium at an inlet manifold 176 of the core portion which is at the lower right side of the core structure 130 as shown in Figure 1. The cooling medium flows through parallel pathways within the core portion and is collected at an opposing outlet manifold 178 of the core portion, from where it is discharged to the coil portion 174 via a hose 180. In this example, the core portion and the core portion 174 are arranged in series with one another such that the cooling medium flows through the coil portion 174 after flowing through the core portion. In other examples, the core portion and the coil portion 174 may be arranged in parallel or may be provided with separate cooling circuits.

The structural support 110 comprises an upper pair and a lower pair of lateral support members 112, each pair having opposing front and rear members 112 coupled by clamping bands 114 so as to clamp a respective upper portion or lower portion of the core structure 130 therebetween. The lateral support members 112 are configured to clamp the lateral cooling plates 162 against the respective portions of the core structure. In this example, the lateral support members each have an L-shape cross section, with a first limb of the L-shape configured to act against the core structure, and a second limb of the L-shape configured to rest on and be coupled to the housing of the primary coil 150. As shown in Figure 1, in this example the first limb is oriented in the vertical plane (normal to stacking direction), whereas the second limb is oriented in the horizontal plane (normal to upright direction). Further, the upper and lower lateral support members 112 on each face (i.e. the front or rear face) of the core structure 130 are coupled by an upright support member 116 extending between the respective second limbs and coupled thereto. The lateral support members 112, clamping bands 114 and upright support members 116 are coupled to form the structural support 110 by mechanical fasteners, such as bolts.

The structural support 110 further comprises attachment portions for coupling to the primary coil 150. In particular, as shown in Figure 1, the second limbs of the lateral support members 112 include fixing holes for fixing the lateral support member 112 to the primary coil 150 with a mechanical fastener, for example a bolt or screw. The primary coil 150 is manufactured with corresponding attachment points, for example by machining after forming the cast housing. In this example, a plurality of bolts extend through the lateral support members 112 and into the housing of the primary coil 150 to couple the primary coil 150 to the structural support 110 and core structure 130.

Figure 2 shows a schematic front side view of the core structure 130 and the winding 152 of the primary coil 150 without the cast housing. As shown, the winding 152 is wound around first and second upright arms 132, 134 of the core structure. Figure 2 also shows in outline the location of upper and lower cooling plates 162 on the upper and lower regions of the front face of the core structure 130 (i.e. the front face of the upper and lower portions of the rectangular frame shape of the core structure). In this example, the cooling plates 162 extend over the major part of the upper and lower regions.

Figure 3 schematically shows an example configuration of the core structure 130 in which there is a primary core element 136 and two auxiliary core elements 138 arranged side by side in the stacking direction. Each of the core elements has su bstantially the same cross-sectional profile with respect to the stacking direction (i.e. the rectangular frame shape described above), with the exception of a gap in the primary core element 136 as will be described in further detail below. In other examples, the core elements may have different cross-sectional profiles (e.g. they may be of different shapes or sizes), and both or neither may have a core gap. In this particular example, the primary core element 136 has a central position between two auxiliary core elements 138 disposed on either side with respect to the stacking direction. In other examples, there may only be a single auxiliary core element.

An elastomeric membrane is disposed between the primary core element 136 and each of the auxiliary core elements 138. The front and rear faces of the auxiliary core elements 138 may also be provided with respective elastomeric membranes. Providing one or more elastomeric membranes between the core elements and between the core elements and other components of the reactor 100 may mitigate against vibrations in the reactor 100 owing to induced electromotive force in use.

Each of the core elements 136, 138 comprises a plurality of laminations. In this example, the laminations are provided in a concentric arrangement so that they form of ribbons having a thickness corresponding to the thickness of the respective core element 136, 138. The laminations therefore extend circumferentially around the stacking direction. Accordingly, a thickness direction of the laminations is orthogonal to the stacking direction, and varies around the circumference.

As shown in Figure 3, in this example the primary core element comprises laterally- extending gaps in the upright arms, which are occupied by spacers 144 as will be described in detail below. In the example of Figure 3, the auxiliary core elements 138 are continuous (i.e. there is no gap) at the locations of the gaps in the primary core element.

Figure 4 shows a further example of a core structure 130 which is similar to the core structure 130 described above with respect to Figure 3. In this example, both the primary core element 136' and the auxiliary core elements 138' have laterally- extending gaps at corresponding locations in the upright arms, occupied by spacers 144', 145' respectively. In yet further examples, both the primary and auxiliary core elements may have gaps, but the gaps may be located at different positions around the respective core elements (i.e. they may not align with each other).

Figure 5 schematically shows laminations 140 of the primary core element in a concentric arrangement. Figure 6 schematically shows laminations 142 of an auxiliary core element. As shown, in this example the lamination thickness of the laminations 140 in the primary core element is lower than the lamination thickness of the laminations 142 in the auxiliary core element. In this example, each lamination 140, 142 is manufactured by winding a ribbon of ferromagnetic material (e.g. stainless steel) around a former to produce a pre-form, and then the pre-form is shaped (i.e. by a press-forming operation under elevated temperature and pressure) to the desired shape for the lamination. Each lamination may be rolled to a desired thickness.

In this example, each lamination is coated with a silicon layer on both sides to provide an insulation layer between laminations. In other examples, other insulating materials could be used, and may be provided as a separate membrane rather than a coating.

In this example, the primary core element 136 comprises laminations 140 having a first lamination thickness, and the auxiliary core elements 138 comprise laminations 142 having a greater second lamination thickness. For example, the second lamination thickness may be at least double the first lamination thickness. I n one example, the first lamination thickness may be 0.8mm, and the second lamination thickness may be 1.8mm. I n a further example, the first lamination thickness may be 1.1mm and the second lamination thickness may be 0.28mm.

In the example shown in Figures 5 and 6, each of the plurality of laminations 140 forming the primary core element 136 is of the first lamination thickness, and each of the plurality of laminations 142 forming the auxiliary core elements 138 is of the second lamination thickness.

Figure 7 schematically shows a further example of a core structure 600 which has a su bstantially rectangular profile with rounded corners. The core structure 600 may comprise primary and auxiliary core elements as described above. Owing to the curved profile of the core structure and core elements, the laminations may be easily wound around a former for the core element. After winding, minimal or no pressing may be required prior to assembly with other core elements.

Referring now to Figure 8, in this example, each core element is cut after winding the laminations to enable assembly of the primary coil 150 onto the core structure 130. In this particular example, the core elements 136, 138 are cut along a horizontal plane (i.e. normal to the upright direction) at a mid-point of the upright arms. Accordingly, the core structure 130 can be separated into respective halves along the cut line so that the primary coil 150 can be pre-formed and slid onto one half of the core structure 130, the other half can then be inserted into the centre of the primary coil 150 windings and the core structure 130 re-assembled.

As shown in Figure 8 (and also in Figure 3), the primary core element 136 is cut and re-assembled so that there is a gap between the cut surfaces which in this example is occupied by a spacer 144. The gap and spacer 144 separate the magnetic flux pathway around the primary core element 136. The gap is variable by using spacers of different sizes. By varying the size of the gap, the reactance of the core structure 130 as a whole can be varied. In this example, there is a gap and corresponding spacer 144 on each lateral side of the primary core element 136. In this example, the gap and spacer 144 have a gap dimension (i.e. the dimension separating the ferromagnetic surfaces) of 0.1mm.

In this example, the auxiliary core elements 138 do not have gaps corresponding to the gap in the primary core element as described above. Accordingly, the ferromagnetic material of the auxiliary core elements 138 is continuous in the region of the gap in the core element. However, in other examples, an auxiliary core element may have a core gap.

Figure 9 diagrammatically shows the cooling assembly 160 of the reactor 100 which is briefly described above with respect to Figure 1. As shown, the cooling assembly 160 comprises the core part 172 and the coil part 174 in series. The inlet manifold 176 of the core part 172 is configured to receive a cooling medium from an inlet port and convey it along four parallel lateral pathways associated with respective lateral cooling plates 162. As shown in Figure 6, the inlet manifold 176 is directly coupled to two of the parallel lateral pathways (the two upper pathways in this example), and is indirectly coupled to the other lateral pathways by an upright conduit. There is a similar arrangement at the outlet manifold 178. The outlet manifold 178 is fluidically connected to the coil part 174 which is formed by the internal flow pathway through the aluminium bar of the primary coil 150.

In use, the reactor 100 can be installed in HVDC equipment, for example as part of a thyristor valve. When the thyristor valve is turned on, the reactor 100 acts to limit a rise in current. An in-rush current flows through the primary coil 150, producing a magnetic field in the core 130 which generates electromotive force to oppose the flow of current through the primary coil 150.

Eddy currents develop in the laminations of the primary and auxiliary core elements 136, 138. Since the laminations are thicker in the auxiliary core elements (in this example), relatively larger eddy currents are established in the auxiliary core elements 138 than in the primary core element 136, with corresponding heat generation in the auxiliary core elements. Since the auxiliary core elements are located outside of the primary core element with respect to the stacking direction, they are adjacent to the cooling plates 162. Accordingly, this arrangement provides for efficient heat transfer away from the auxiliary core elements, where heat generation is concentrated.

By allowing relatively larger eddy currents to be established in the auxiliary core elements 138, a relatively high proportion of energy is dissipated through the auxiliary core elements 138, thereby effectively providing resistive damping to damp the su bsequent oscillating inrush current in the thyristor valve. The eddy currents in the auxiliary core elements 138 may therefore be equivalent to providing a resistor in parallel with a reactor. It will be appreciated that relatively smaller eddy currents are also established in the primary core element 136, but resulting in relatively smaller energy dissipation and contribution to the resistive damping of the reactor as a whole. The reactor acts to limit the current growth in the thyristor valve, particularly over a short period (of approximately 5 to 10 microseconds) after turn-on of the thyristor valve in which the individual thyristors would be vulnerable to damage by excessive current. Further, by damping the subsequent oscillating currents, the reactor helps ensure that the current flow does not reverse, which would automatically turn off the individual thyristors.

The profile of current in the thyristor valve over time after turn-on is determined by the properties of the reactor 100, in particular, based on the effective damping resistance and the saturation of the core structure. In particular, it may be desirable to initially limit the rate of current growth (for example, during the first 5 to 10 microseconds) to a first threshold, and to subsequently allow greater current growth. A particular reason for this is that the semiconductor material of the thyristors is only progressively activated after turn-on, such that the amount of semiconductor material that can conduct current in each thyristor is initially very low. Accordingly, it is desirable to limit the current growth for an initial period.

The applicant has found that a core element having thinner laminations will typically saturate before a core element having thicker laminations. In particular, it is considered that a magnetic field penetrates each individual laminations from both respective sides simultaneously. It is considered that saturation occurs when the magnetic field from both sides meet. It is considered that, with relatively thicker laminations, the magnetic field takes relatively longer to penetrate and therefore saturation occurs later. Accordingly, the profile of current over time after turn-on may have a first increase in the rate of current growth corresponding to saturation of the core element with thinner laminations, and subsequently a second increase in the rate of current growth corresponding to the subsequent saturation of the or each core element with thicker laminations.

By providing at least two core elements including a primary core element and an auxiliary core element having laminations of different lamination thicknesses with respect to one another (i.e. different lamination thickness in the primary core element as compared with the auxiliary core element), the performance of the reactor may be optimised. For example, an improved output profile of current from the reactor over time after turn-on may be achieved by having different saturation points (i.e. different times at which saturation occurs) for the primary core element and the or each auxiliary core elements. Further, the lamination thickness of the auxiliary core elements can be selected to result in a target level of eddy current loss in the reactor as a whole in response to a predetermined inrush current. Accordingly, in the design of a reactor, the level of damping resistance may be effectively determined by the configuration of the or each auxiliary core element whilst maintaining the configuration of the primary core element. Nevertheless, it will be appreciated that the level of damping resistance depends on the properties of both primary and auxiliary core elements.

Other properties of the reactor may be primarily determined by the configuration of the primary core element, for example, the reactance of the reactor may be primarily determined by the sizing of the gap in the primary core element. Accordingly, it may be possible to optimize such properties in a reactor by suitable design of the primary core element, separately from optimizations of other properties which are primarily influenced by other design features (such as the lamination thickness in the auxiliary core element which may provide a significant proportion of resistive damping of the reactor as a whole). Further, by using separate primary and auxiliary core elements, heat generation due to eddy currents (resistive damping) may be concentrated in a particular area of the reactor and therefore cooling can be directed specifically to this area.

Further, it will be appreciated that laminations are typically bought off the shelf and are available in a limited range of thicknesses. Accordingly, whilst it may be possible to achieve an optimal balance of performance using laminations of a single optimal thickness, it may be unfeasible to source or manufacture laminations having that thickness. Accordingly, by providing a primary core element and at least one auxiliary core element having different lamination thicknesses, suitable performance may be achieved using off-the-shelf or readily available laminations, whilst particular performance requirements can be balanced by selecting an appropriate number (total thickness) of each type of lamination.