Electrical power distribution system

Abstract

An electrical power distribution system comprising a power cable and a shunt reactor electrically connected to a conductor line of the power cable, the shunt reactor comprising a winding having an open core such that, when submerged in water, the open core is water filled.

Description

[0001] ELECTRICAL POWER DISTRIBUTION SYSTEM

[0002] Technical Field

[0003] The present invention relates to an electrical power distribution system and its use for transporting power in an underwater environment.

[0004] Electrical cables have an operating (e.g., a line) capacitance, which is typically of the order of a few pico-farads per metre. For relatively short lengths of cable (<10m), the total operating (e.g., line) capacitance is small and its effects are negligible. For longer lengths of cable (e.g., > 1km), the total operating capacitance can become significant, resulting in considerable charging currents towards the ends of the cable, reactive power losses and potentially unstable voltage profiles. This is, in particular, a problem with distributing or transmitting alternating current, AC, power to and from remote offshore facilities using submarine cables.

[0005] An improved system, which is able to at least partially mitigate the effects of capacitance (e.g., phase to ground), is desirable.

[0006] According to an aspect of the present invention, there is provided an electrical power distribution or transmission system. The electrical power distribution system comprises a power cable and a shunt reactor electrically connected to a conductor line of the power cable. The shunt reactor comprises a winding having an open core such that, when submerged in water, the open core is water filled. Such a shunt reactor, being water filled in use, can be much more lightweight and potentially easier to install underwater than conventional shunt reactors, which have bulky iron cores and which are placed within pressure-compensated oil-filled housings.

[0007] The power cable may be a pigtail cable which is arranged to electrically connect to a conductor line of a submarine power cable. The power cable may also be a submarine power cable.

[0008] Optionally, the system further comprises a non-magnetic support for securing the winding and for maintaining its shape.

[0009] Optionally, the winding of the shunt reactor contains no electrical shielding (i.e., no electromagnetic shielding). That is, the winding of the shunt reactor may be electrically insulated from the surrounding seawater (if it is submerged therein) but there is no electrically conducting shielding (e.g., a sheath) arranged around the electrical insulation.

[0010] Optionally, the winding of the shunt reactor is absent a protective armour. The winding may be made of an electrically insulated copper conductor, which is absent any magnetic or electrically conducting protective armour.

[0011] The shunt reactor may be electrically connected to the submarine power cable at a midpoint of the submarine power cable.

[0012] In some examples, the submarine power cable comprises a plurality of cable segments spliced together, and the shunt reactor is electrically connected to the conductor line of the cable at one of the splice connections. In such examples, the shunt reactor is optionally electrically connected to the cable at the splice connection nearest to the midpoint of the cable.

[0013] Optionally, the system further comprises a ground terminal and the shunt reactor electrically connects the ground terminal to the conductor line. The ground terminal may be non-magnetic.

[0014] The shunt reactor may be further electrically connected to a sheath of the corresponding conductor line of the submarine power cable.

[0015] In some examples, the submarine power cable is a first cable and the shunt reactor is a first shunt reactor, and the system further comprises: a plurality of first cables; and a plurality of first shunt reactors, each first shunt reactor being electrically coupled to a conductor line of a corresponding one of the first cables. The submarine power cable may include three or more cables for three-phase power distribution. Each submarine power cable (i.e. , the first cable) is then a single phase power cable.

[0016] In some examples, the conductor line is a first conductor line and the shunt reactor is a first shunt reactor, and the system further comprises: a plurality of first conductor lines; and a plurality of first shunt reactors, each first shunt reactor being electrically connected to a corresponding one of the first conductor lines. The submarine power cable may include three or more conductor lines for three-phase power distribution. The submarine power cable is then a three phase power cable.

[0017] Optionally, the system further comprises a plurality of non-magnetic supports, each nonmagnetic support being configured to secure the winding of the corresponding shunt reactor and for maintaining its shape.

[0018] The or each winding of the shunt reactor may be provided with electrical insulation.

[0019] The or each submarine power cable may have a length greater than 1km, 3km, 50km, or 100km.

[0020] The pigtail cable may have a length in the range of 5m to 1 km.

[0021] In some examples, the or each non-magnetic support is comprised from concrete.

[0022] Preferably, although not necessarily, the winding of the or each shunt reactor has an inductance greater than that of the cable. This enables the shunt reactor to compensate for all or part of the cable capacitance.

[0023] In use, the system may be submerged underwater and the system may include a plurality of shunt reactors. A minimum distance between any pair of different shunt reactor windings may be no less than 5 metres. This helps to avoid unbalanced interference.

[0024] In use, the system may be submerged underwater and includes three shunt reactors.

[0025] Each of the shunt reactors may be arranged at a centre of a notional equilateral triangle. Brief of the

[0026] Figure 1 is a schematic illustration of a transverse cross section of a submarine power cable.

[0027] Figure 2 is an equivalent circuit diagram of an electrical power distribution system.

[0028] Figure 3 is an equivalent circuit diagram of an electrical power distribution system, according to an embodiment.

[0029] Figure 4 is a schematic illustration of an electrical power distribution system, according to an embodiment.

[0030] Figure 5 is a schematic illustration of an electrical power distribution system, according to an embodiment.

[0031] Figure 6A and Figure 6B are schematic illustrations of support structures for a shunt reactor.

[0032] Detailed

[0033] In general terms, the present disclosure proposes an electrical power distribution system comprising a submarine power cable, for use in an underwater environment. The conductor line of the submarine power cable is electrically coupled to a shunt reactor that comprises a winding with an open core which, when in use underwater, is filled with water. A winding with an open core means that the shunt reactor facilitates the flow of water into, through and out from the core of the winding, when in use in an underwater environment. Advantageously, the presence of the water within the core means that no coolant or heat exchanger system is required for the shunt reactor (the underwater environment acts as an infinite heat sink) and electrical shielding (i.e. , without a sheath for providing electromagnetic shielding) for the windings of the shunt reactor is not necessary (water is slightly conductive and so confines the electric field relatively effectively). Saturation effects are also absent when water is used as the core material.

[0034] Unlike traditional shunt reactors, which can be bulky and cumbersome to handle, the shunt reactors of this disclosure can be made much more lightweight. For example, the shunt reactor winding may comprise only an inner conductor which is surrounded by an electrical insulation. The terms electrical power “distribution” and “transmission” are used interchangeably throughout this disclosure to mean that electrical power is transmitted from one part of a system to another part of the system.

[0035] For a shunt reactor having an inductive power rating of 50MVAR (at an AC phase to ground voltage of 74kV at 50Hz), the cross sectional area of the windings, A, may be around 100 to 500mm2, preferably 200 to 300 mm2, the winding radius may be around 1.5 to 2.5 metres, and the number of winding turns, N, may be around 150 to 450. These numerical values are illustrative only. The skilled reader will understand that a 50MVAR inductive power rating, or any other power rating, can be achieved using different combinations of A and N, such that AN2remains generally constant.

[0036] The present disclosure is especially useful if the system is used for distributing power over large distances (> 10km, more preferably > 50km, even more preferably greater than 100 km). Specific use cases include the electrification of remote offshore facilities (either by providing power from an onshore site or distributing power amongst offshore facilities), and the transmission of generated power from a wind farm, for example, to an onshore site to service the grid.

[0037] Figure 1 is a schematic illustration of a transverse cross section of a submarine power cable 100 (also referred to herein as a submarine cable). A submarine cable refers to a power transmission cable intended for and suitable for transporting electrical power underwater (be that fresh or salt water). As shown in Figure 1 , the submarine cable includes a conductor line 102 arranged concentrically with respect to a sheath 106. An electrical insulation 104 fills the space between the conductor line and sheath. The cable shown in Figure 1 includes three conductor lines, each with their own respective or corresponding sheaths and electrical insulation. The cable shown in Figure 1 is, therefore, suitable for three-phase alternating current, AC, power transmission. The conductor lines are embedded within further electrical insulation 110 and the cable is protected by an outer armour 108. In an embodiment, the conductor line is made up of a bundle of metallic wires (e.g., copper wires), the electrical insulation 104, 110 is usually a cross-linked polyethylene, XLPE, and the armour comprises a lead sheath arranged around a ring formed from a plurality of metallic wires (e.g., stainless steel), which are embedded in bitumen. In some embodiments (not shown), the cable further includes one or more optical lines (e.g., an optical fibre or optical fibre assembly) so that it is able to transmit both data and power. The optical lines can be run through the electrical insulation 110.

[0038] In an alternative embodiment (not shown), the submarine cable includes a single conductor line only. An assembly of three such “single conductor” cables may then be used for three-phase AC power transmission.

[0039] Figure 2 is an equivalent circuit diagram for an electrical power distribution system. The system comprises an AC power supply 202, an intended load 206 for the power, and a conductor line 208a of a submarine cable 204 for coupling the power supply to the load. The submarine cable is of sufficient length (e.g., > 1 km) such that its line capacitance and the effects thereof are appreciable.

[0040] The submarine cable is represented by distributed series elements (inductive element 204a and resistive element 204b) and distributed shunt elements (capacitive element 204c which connects the conductor line 208a to a return line 208b). The return line may be regarded as electrical ground. The submarine cable is notionally segmented into a plurality of sections, each section having its own respective or corresponding series and shunt elements. In Figure 2, four sections are shown but the skilled reader will understand that, in general, the submarine cable may be modelled as having one or more of these sections.

[0041] During use, the capacitive shunt elements 204c of the submarine cable 204 supply reactive power (current leads voltage by TT / 2 radians) and the inductive series elements absorb reactive power (current lags behind voltage by TT / 2 radians). The supply of reactive power from the capacitive shunt elements is expected to exceed the reactive power that can be absorbed by the submarine cable. This can lead to a number of undesirable effects, namely the development of capacitive charging currents towards both ends of the power cable but other effects such as voltage instabilities are envisaged. The line capacitance of the submarine cable 204 also draws its own current (referred to as a charging current). The charging current for submarine cables (>100km) can become significant and make power transmission impractical. Figure 3 is an equivalent circuit diagram for an electrical power distribution system 300 according to an embodiment. In addition to the components described with reference to Figure 2, the system further comprises a shunt reactor 302 electrically connected between the conductor line 208a and the return line 208b.

[0042] For a three-phase AC power transmission cable or cable assembly, at least one shunt reactor is provided for each of the three phase carrying conductor lines. These shunt reactors are referred to as single phase shunt reactors.

[0043] The shunt reactor is an inductor, such as a winding or a coil, with an inductance sufficiently high to absorb surplus reactive power supplied by the line capacitance of the submarine cable. The shunt reactor functions to minimise voltage instabilities and to compensate for charging current. Preferably, although not necessarily, a shunt reactor 302 is arranged at the midpoint of the submarine cable. This configuration optimises the benefits of the shunt reactor.

[0044] Figure 4 is a schematic illustration of an electrical power distribution system 400 according to an embodiment. The electrical power distribution system includes a submarine cable 402 and three single phase shunt reactors 404a, 404b, 404c. Each shunt reactor 404a, 404b, 404c is electrically connected between a corresponding one of the conductor lines 406a, 406b, 406c and electrical ground 408. In the example shown, electrical ground is provided by a metallic plate. Preferably, although not necessarily, the metallic plate is non-magnetic (i.e., pr“1) and suitable for use in an underwater environment. Copper alloys are especially suitable.

[0045] Each of the shunt reactors comprises a winding or a coil with an open core. The winding or coil may be formed from a high-voltage, HV, cable, comprising an inner conductor (e.g., a copper wire bundle) surrounded by an electrical insulation (e.g., XLPE). Unlike the submarine power cable, however, the HV cable does not include a sheath (i.e., electrical or electromagnetic shielding) arranged concentrically with respect to the inner conductor and / or a protective outer armour (as for example shown in Figure 1). While the shunt reactor coil is depicted as having a circular cross section in Figure 4, the skilled reader will understand that the term coil encompasses other shapes in cross section too. Each shunt reactor 404a, 404b, 404c connects to its conductor line 406a, 406b, 406c via a corresponding pigtail cable 410a, 410b, 410c. In some examples, the pigtail cable can be thought of as a short extension of the submarine cable. In other examples, the pigtail cable may comprise only an inner conductor, which is surrounded by electrical insulation. That is, in some examples, the pigtail cable can be thought of as an extension of the shunt reactor. In practice, this pigtail cable can be from 5m to 1 km in length. Longer pigtail cables (e.g., >300m) can advantageously allow installation of the submarine cable and the shunt reactor separately. This is because, while the submarine cable is being installed, the shunt reactor can remain on a surface vessel via the pigtail cable. The length of the pigtail cable is then greater than the water depth at which the submarine cable is installed. Each shunt reactor connects to the ground plate 408 via a corresponding terminating cable 412a, 412b, 412c. As with the shunt reactor, it may comprise only an inner conductor, which is surrounded by electrical insulation. The electrical insulation of the terminating cable or pigtail cable is absent or tapers away at the intended contact point between the shunt reactor and ground or conductor line in order to establish a good electrical contact.

[0046] In use (i.e., when the system is submerged), the core of the shunt reactor coil is open to water. A shunt reactor with an open core allows water to flow into, through, and out from the core of the winding, when in use underwater. Conventional shunt reactors used “on land” typically have iron cores and are contained within bulky housings, which are often oil-filled to provide cooling. The iron core is prone to magnetic saturation at high voltages. These conventional shunt reactors would be impractical to install, operate, and repair in an underwater environment. For example, surge arresters would need to be installed in order to protect the shunt reactor against overvoltage and saturation scenarios. Alternative solutions based on wet mateable connections are not qualified for HV operations (>100 kV).

[0047] The inventors have realised that water from the underwater environment in which the system is submerged during use can address the problems associated with conventional “land-based” shunt reactors. In particular, water (i) serves as an effective coolant for the shunt reactor; (ii) electrically shields the coil (water, and in particular, salt water is slightly electrically conductive); and (iii) does not saturate at HV. Ultimately, this means the shunt reactor does not require a protective outer armour to avoid water ingress or a sheath for electrical shielding. Surge arresters at the terminals of the power distribution system are also not needed.

[0048] In Figure 4, all the shunt reactors are shown electrically connected to the same ground plate 408. In an alternative example (not shown), each shunt reactor is electrically connected to a different ground plate. As already been noted, the ground plate(s) are non-magnetic in order to minimise inductive coupling effects.

[0049] Figure 5 is a schematic illustration of an electrical power distribution system 500 according to an embodiment. The electrical power distribution system is similar to the system from Figure 4, except that each single phase shunt reactor 504 is electrically connected between one of the conductor lines 406a and its corresponding sheath 508a (denoted as 106 in Figure 1) via a corresponding pair of pigtail cables (510a, 512a). That is, the sheath 508a substitutes the ground plate from Figure 4, and pigtail cable 512a substitutes the terminating cable from Figure 4. It will be understood that a corresponding single phase shunt reactor and pigtail cable pairs will be provided for each of the other conductor lines 406b, 406c but these have been omitted for the sake of clarity in Figure 5.

[0050] As with Figure 4, the pigtail cables may comprise only an inner conductor, which is surrounded by electrical insulation (i.e., absent any electrical or electromagnetic shielding or protective armour). The electrical insulation may be absent or taper away at the intended contact point between the shunt reactor and the corresponding sheath 508a in order to establish a good electrical contact. The electrical insulation of the pigtail cable in the vicinity of the intended contact point between the shunt reactor and the corresponding conductor line 406a may temporarily be absent or taper away to facilitate good electrical contact therebetween. After this electrical connection is made however, an electrical insulation (e.g., XLPE) will be provided around the contact point in order to protect the system from the surrounding seawater, when in use. As has already been noted, each of the pigtail cables can be from 5m to 1 km in length.

[0051] In use, the submarine power cable 204 and shunt reactors 404, 504 rest on the floor or bed of the underwater environment (e.g., at the sea bed or lake bed) and each of the single phase shunt reactors 404, 504 may generate a magnetic field according to the alternating current that it carries. As water is non-magnetic (i.e., pr“1), magnetic fields can penetrate several metres in water before reducing to background levels (i.e. the earth’s magnetic field strength). To minimise the effects of inductive coupling between the shunt reactor coils, each single phase shunt reactor coil may be positioned no closer than 5 metres from any of the other shunt reactor coils. In some examples, the three shunt reactors are arranged so that they define an equilateral triangle (i.e. each shunt reactor is centred at a corner of the notional equilateral triangle). In this triangular configuration, the interaction between any pair of shunt reactors is the same as the interaction between the other pairs of shunt reactors. This negates any adverse effects on the system that might be caused by inductive coupling.

[0052] Submarine cables are typically made up from a plurality of cable sections, which are spliced together at their ends during the cable laying procedure. Splicing typically takes place in a dry environment. The cable laying and splicing procedure is known to the skilled reader, per se. To facilitate assembly, the location of these splices is conveniently used as the location in which to make the join between the shunt reactors 404, 504 and their respective or corresponding conductor lines 406a, 406b, 406c. The pigtail cables 410a; 510a, 512a may be joined to the conductor lines before or after they are connected to the shunt reactor coil. For a submarine cable comprising an odd number of identical cable sections, a splice connection does not coincide with the midpoint of the assembled submarine cable. In such embodiments, the shunt reactor is electrically connected to the splice connection nearest to the midpoint of the cable.

[0053] In some examples, shunt reactors 404, 504 are also arranged at each or one of the endpoints of the submarine cable (assuming each or the endpoint is located at an offshore location following cable laying). If either endpoint is located at an onshore location following cable laying, any shunt reactor known to the skilled reader may be used. The skilled reader will understand that any number of shunt reactors 404, 504 may be installed along the assembled submarine cable and that these shunt reactors may be spaced uniformly or non-uniformly (e.g., clustered near the midpoint) along its length.

[0054] Figure 6A and Figure 6B are schematic illustrations of shunt reactors 600 having a support structure 602 for securing the coil 404, 504 of a single phase shunt reactor and maintaining its shape during use. Preferably, although not necessarily, the support structure is non-magnetic (i.e., pr“1) to minimise losses. In Figure 6A and 6B, the support structure takes the form of a tubular defining a single hole. The tubular may be closed at one end, although the presence of more than one open end provides more effective cooling for the winding, when in use. In some embodiments (not shown), the tubular defines a respective or corresponding hole for each single phase shunt reactor. In an embodiment, the support structure is made from concrete.

[0055] Referring to Figure 6A, the inner diameter of the support structure 602 is marginally less than the outer diameter of the shunt reactor coil 404, 504. The shunt reactor coil, when inserted within the support structure, is therefore biased against the inner surface of the support structure.

[0056] Referring to Figure 6B, the outer diameter of the support structure 602 may be marginally greater than the inner diameter of the shunt reactor coil. The shunt reactor coil, when applied around the exterior of the support structure, is therefore biased against the exterior surface of the support structure. Alternatively, the coil may be wound around the support structure in order to deform it into the shape. The winding may comprises a plurality of turns, which are arranged in layers stacked on top of one another (i.e., the winding may be, at least partly, wound on itself). That is, some turns of the winding may be in direct contact with the support structure, whereas other turns of the winding may be in direct contact with other turns of the winding. It will be understood that a shunt reactor coil 404, 504 applied to the support structure 602 in Figure 6B is still regarded to have an open core because the majority of its core does not include the support structure 602 (i.e., remains open for water and allows the flow of water into, through and out from the coil core).

[0057] Specific Example 1

[0058] According to a specific example, the shunt reactor is a coil, with a coil height of 2 metres and a coil radius of 2 metres and having 300 turns. At an AC phase to ground voltage of 74kV and a frequency of 50 Hz, the reactive power absorbed by the shunt reactor is around 25MVAr. The wire making up the coil has a cross sectional area of 100mm2to 500mm2, more preferably 240 to 250mm2. The wires forming the coil may overlap or be wound about one another. Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments only. Features from different examples may be combined as appropriate to form other working examples.

Claims

CLAIMS:1 . An electrical power distribution system comprising a power cable and a shunt reactor electrically connected to a conductor line of the power cable, the shunt reactor comprising a winding having an open core such that, when submerged in water, the open core is water filled.

2. The system according to claim 1 , wherein the power cable is a pigtail cable which is arranged to electrically connect to a conductor line of a submarine power cable.

3. The system according to claim 1 , wherein the power cable is a submarine power cable.

4. The system according to any one of claims 1 to 3, further comprising a non-magnetic support for securing the winding and for maintaining its shape.

5. The system according any one of claims 1 to 4, wherein the winding of the shunt reactor contains no electromagnetic shielding.

6. The system according to any one of the preceding claims, in which the winding of the shunt reactor is absent a protective armour.

7. The system according to any one of claims 3 to 6, in which the shunt reactor is electrically connected to the submarine power cable at a midpoint of the submarine power cable.

8. The system according to any one of claims 3 to 7, in which the submarine power cable comprises a plurality of cable segments spliced together, and the shunt reactor is electrically connected to the conductor line of the cable at one of the splice connections.

9. The system according to claim 8, in which the shunt reactor is electrically connected to the cable at the splice connection nearest to the midpoint of the cable.

10. The system according to any one of the preceding claims, further comprising a nonmagnetic ground terminal and the shunt reactor electrically connects the non-magnetic ground terminal to the conductor line.11 . The system according to any one of claims 3 to 9, wherein the shunt reactor is further electrically connected to a sheath of the corresponding conductor line of the submarine power cable.

12. The system according to any one of claims 3 to 11 , wherein the submarine power cable is a first cable and the shunt reactor is a first shunt reactor, and the system further comprises: a plurality of first cables; and a plurality of first shunt reactors, each first shunt reactor being electrically coupled to a conductor line of a corresponding one of the first cables.

13. The system according to any one of claims 1 to 10, wherein the conductor line is a first conductor line and the shunt reactor is a first shunt reactor, and the system further comprises: a plurality of first conductor lines; and a plurality of first shunt reactors, each first shunt reactor being electrically connected to a corresponding one of the first conductor lines.

14. The system according to any one of claims 12 or 13, comprising: a plurality of non-magnetic supports, each non-magnetic support being configured to secure the winding of the corresponding shunt reactor and for maintaining its shape.

15. The system according to any one of the preceding claims, in which the or each winding is provided with electrical insulation.

16. The system according to any one of claims 3 to 15, in which the or each submarine power cable has a length greater than 3km, preferably more than 50km, even more preferably greater than 100km.

17. The system according to any one of claims 2, 4 to 6, 10 and 13, in which the pigtail cable has a length in the range of 5m to 1 km.

18. The system according to any one of the preceding claims, in which the or each nonmagnetic support is comprised from concrete.

19. The system according to any one of the preceding claims, in which the winding of the or each shunt reactor has an inductance greater than that of the cable.

20. The system according to any one of the preceding claims, in which the system is submerged underwater and includes a plurality of shunt reactors, wherein a minimum distance between any pair of different shunt reactor windings is no less than 5 metres.

21. The system according to any one of the preceding claims, in which the system is submerged underwater and includes three shunt reactors, wherein each of the shunt reactors is arranged at a centre of a notional equilateral triangle.

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