NO20200055A1

NO20200055A1 - Power supply system

Info

Publication number: NO20200055A1
Application number: NO20200055A
Authority: NO
Inventors: Ole Johan Bjerknes; Ole A Heggdal; Leif Arne Müller
Original assignee: Aker Solutions As
Priority date: 2020-01-16
Filing date: 2020-01-16
Publication date: 2021-07-19

Description

P<OWER SUPPLY SYSTEM>

Technical Field

[0001] The present invention relates to a power supply system configured to provide electric power to electric loads at a remote location and to heat a pipeline.

Background Art

[0002] Various power supply systems have been disclosed, which are interchangeable between a 3-phase motor powering mode, and a heating mode for heating of a subsea pipeline. A common heating solution is known as direct electric heating (DEH). With DEH, a current is carried in a DEH cable adjacent the pipeline and enters the pipeline at a remote location. Heat is generated in the pipeline partly due to induction, i.e. due to the alternating magnetic field of the DEH cable, and partly due to the current flowing through the pipeline itself.

[0003] Publication US6617556 describes a system with a topside 3-phase power supply and a subsea cable leading to a wellhead platform. The system can be switched between a heating mode and a powering mode. In the heating mode, electric power is used to heat a subsea pipeline arranged adjacent the subsea cable. In the power mode, the subsea cable conveys electric power to subsea loads. The system comprises power switches at both ends of the electric cable.

[0004] EP1836375 also presents a system having a topside power supply that can be switched between a pipeline heating mode, and a motor power mode. In one mode, a frequency converter connected to a three-phase power source delivers power, through a subsea cable, to a subsea motor. The subsea cable is arranged adjacent a subsea pipeline. In a heating mode, a symmetry and power factor compensation unit connects to the three-phase power source and delivers, through the subsea cable, electric current ito the pipeline (DEH – direct electric heating). Topside switches and a subsea switch are used to choose between the modes.

[0005] Publication EP2964993 relates to an electric power supply system configured for providing a single-phase supply for DEH and 3-phase supply for an electric motor. A switching section is provided for interchanging between the single-phase mode and the 3-phase mode.

Summary of invention

[0006] According to a first aspect of the present invention, there is provided a power supply system configured to supply electric power from a first location to an electric load at a second location and to heat a steel pipe extending at least partly between the first and second locations. The power supply system comprises an electric power supply arrangement arranged at the first location, comprising a first set of at least a first and second phase line and a second set of at least a first and second phase line. The power supply system further comprises a cable arrangement extending along the steel pipe. The cable arrangement has a plurality of clusters of cable lines that equals the number of phase lines in each set. The cable lines are arranged along and external to the steel pipe and angularly distributed about an axial center axis of the steel pipe. Furthermore, the respective cable lines of one cluster are connected to one output phase line of each respective set. According to the invention, the mutual angular distance between cable lines in respective clusters is less than the angular distance between adjacent clusters, with respect to the axial center axis of the steel pipe.

[0007] Notably, it is the characteristics of the electric power fed to the cable lines that governs how the power will be used. That is, whether the electric power results in heating the steel pipe and/or supplying the electric load at the second location, will be determined by how the electric power is adapted. This contrasts with known systems, where switching between different power modes is not done by adapting the electric power characteristics, but rather conveyed to different end loads by means of switching means. With the power supply system according to the invention, the operator avoids using switches at the second, remote location for choosing the desired power mode. The second location can typically be a subsea location.

[0008] Advantageously, the power supply system is interchangeable between at least a load power only mode, wherein the currents in respective cable lines in each respective cluster are oppositely angularly aligned and of same amperage, and a load power and heating mode, wherein the currents in respective cable lines in each respective cluster are not oppositely angularly aligned and of same amperage.

[0009] When in the load power only mode, since generated fields of the cable lines of each respective cluster will substantially cancel each other out, there will be substantially no induction heating of the steel pipe. Furthermore, when in the load power and heating mode, the generated fields of the cable lines in the respective clusters will not cancel each other out, so that there is an alternating field that will induce heat in the steel pipe. A load at the second location may still be provided with electric power.

[0010] In some embodiments, the power supply system may further comprise, at the remote location, a remote transformer. The remote transformer can comprise output windings configured to power a remote load. The remote load can for instance be an electric motor. The remote transformer can further comprise a first set of input windings and a second set of input windings, wherein the respective input windings of the first and second sets of input windings that are on a common transformer leg are oppositely wound.

[0011] Thus, typically, one transformer leg can have one output winding to deliver electric power to the load, and two oppositely wound input windings. With such a winding configuration, currents flowing into two such respective input windings with the same direction and amperage will each generate an opposite flux in the transformer leg, the sum of which is zero (the fluxes cancel each other out). Such a situation will be present in a heating only mode, since no electric power will be transmitted to the output windings.

[0012] Thus, with such embodiments, in addition to the load power only mode and the load power and heating mode, the power supply system is configured to be operated in a heating only mode.

[0013] Furthermore, with such embodiments, the electric power supply arrangement can comprise one or more variable speed drives (VSD).

[0014] The power supply arrangement can advantageously comprise a transformer that comprises input windings, a first set of output windings and a second set of output windings. The respective output windings of the first and second sets of output windings that are on a common transformer leg can advantageously be oppositely wound.

[0015] In some embodiments, the power supply system may further comprise a power line distribution arrangement at the first location, wherein the configuration of the phase lines are grouped into said clusters. It may further comprise a power line redistribution arrangement at the second location, wherein the configuration of the cable lines of the clusters are redistributed into the configuration before the power line distribution arrangement.

[0016] In some embodiments, at least one of the sets of at least a first and second phase line is configured to be constantly electrically connected to cable lines, independent of being operated in the load power only mode or the load power and heating mode.

[0017] This means that at least one of the sets of multiphase outputs needs not be connected to the cable lines through a switching means.

[0018] In some embodiments of the power supply system, the length of the steel pipe and the length of the cable arrangement, which extend between the first location and the second location, are more than 3 km, 10 km, or even more than 50 km.

[0019] According to a second aspect of the invention, there is provided an electric load assembly configured to power an electric load, comprising a transformer with transformer legs, output windings wound about respective transformer legs, and a first set of input windings. The transformer further comprises a second set of input windings wound on the same transformer legs as the respective input windings of the first set of input windings. The respective input windings on each transformer leg are wound in opposite directions.

[0020] The electric load assembly can in some embodiments be a subsea assembly, wherein the transformer is a subsea transformer, meaning that it is located subsea. The skilled person will appreciate that in embodiments described herein involving a remote transformer, such as a subsea transformer, having two sets of input windings, the number of turns of the two input windings on the same transformer legs are substantially identical. Consequently, two identical currents flowing into or out of the input windings through the inlet lines will generate oppositely directed fluxes in the transformer leg that will cancel each other out.

[0021] According to a third aspect of the invention, there is provided an electric power supply assembly, comprising a transformer with transformer legs, input windings, a first set of output windings and a second set of output windings. The respective transformer legs comprise one input winding, one output winding of the first set of output windings, and one output winding of the second set of output windings. The electric power supply assembly further comprises a multiphase power supply connected to the input windings, at least a first pair and a second pair of phase lines exiting the transformer and connected to the output windings on respective transformer legs. According to the third aspect of the invention, on the respective transformer legs, the respective one output winding of the first set of output windings and one output winding of the second set of output windings are wound in opposite directions.

[0022] By winding the two output windings of the respective sets of output windings in opposite directions, current induced in the respective windings from the multiphase power supply will flow in opposite directions through the pairs of phase lines.

[0023] The number of turns in the respective output windings of the first and second sets of output windings will be identical or substantially identical, so that the same power or current is induced through the phase lines connected to them.

[0024] In some embodiments of the third aspect of the invention, the electric power supply assembly can further comprise a switching arrangement having a first passage line set and a second passage line set. The first passage line set connects to the output windings of the first set of output windings, and the second passage line set connects to the output windings of the second set of output windings.

Furthermore, in such embodiments, the output lines of the second passage line set can be configured to assume the following connection modes:

a) a connection mode where the output lines of the second passage line set are connected to the output windings of the second set of output windings; and b) a connection mode where the output lines of the second passage line set are connected to the respective lines of the first passage line set.

[0025] Moreover, in some embodiments the second passage line set can further be configured to assume the following connection mode:

c) a connection mode where the output lines of the second passage line set are disconnected.

[0026] When the output lines of the second passage line set are disconnected, no electric power will be transmitted through the second passage line set.

Detailed description of the invention

[0027] While various aspects of the invention have been discussed in general terms above, some non-limiting example embodiments are discussed in the following with reference to the drawings, in which

[0028] Fig. 1 depicts a typical application of the system according to the invention, where a remote subsea load is powered from a topside facility;

[0029] Fig. 2 is a schematic illustration of an embodiment of the invention;

[0030] Fig. 3 is a cross section through an integrated production umbilical (IPU) which may be used as a part of the power supply system;

[0031] Fig. 4a to Fig.4c illustrate various vector diagrams that represent different modes of operation of the power supply system;

[0032] Fig. 5 is a cross section through another IPU, suited for transmitting threephase electric power;

[0033] Fig. 6a to Fig.6b illustrate various vector diagrams that represent different modes of operation of the power supply system;

[0034] Fig. 7a illustrates an embodiment of the invention, including a three-phase power supply, when in a power only mode;

[0035] Fig. 7b illustrates the same embodiment as Fig.7a, however when in a heating mode, in which a steel pipeline is heated by induction;

[0036] Fig. 8a illustrates components of a power supply system at a first location, comprising a transformer, and when in a power only mode;

[0037] Fig. 8b illustrates a portion of the components shown in Fig.8a, however when in a heating mode;

[0038] Fig. 8c illustrates the same components as in Fig.8b, however when in a combined heating and load-powering mode;

[0039] Fig. 9 illustrates, in addition to some components at the first location, a transformer and an electric load at the second location;

[0040] Fig. 10 depicts a multiphase power supply in the form of a VSD, connected to a tunable transformer.

[0041] Fig. 1 depicts a typical example of application of the power supply system according to the present invention. A surface structure 1 is arranged on the sea surface 3. The surface structure 1 can for instance be a ship, a floating installation or an installation resting on the seabed 5. The surface structure 1 is arranged at a first location 10.

[0042] In the shown embodiment, the first location 10 is an offshore location.

However, the first location may in other embodiments be an onshore location.

[0043] At a second location 20, which in the present example is a subsea location, there is arranged a subsea pump 7. The subsea pump 7 is mechanically powered with an electric power consumer 9, in the form of an electric motor. Also arranged at the subsea location 20 is a subsea transformer 11, to which the electric motor 9 is connected.

[0044] A riser 13 extends between the seabed 5 and the surface structure 1. The riser 13 comprises electric conduits, through which power to the electric motor 9 is transferred.

[0045] At the seabed, the riser 13 connects to a cable arrangement 15. The cable arrangement 15 comprises electrical cable lines CA1, CA2, CB1, CB2 (cf. Fig.2) for provision of electric power to the electric motor 9. This will be discussed in further detail below. The cable arrangement 15 may be several kilometers long, for instance several tens of kilometers. In some embodiments, the cable arrangement 15 may be more than 50 km. The horizontal distance between the first location 10 and the second location 20 may thus be several kilometers, even more than 50 km.

[0046] In the embodiment shown in Fig.1, a steel pipe 17, here in the form of a steel pipeline, extends between the first location 10 and the second location 20. Typically, the steel pipeline 17 will carry produced hydrocarbons originating from a hydrocarbon well (not shown). As is well known to the skilled person in the art, such pipelines may need to be heated to prevent formation of wax and hydrates inside the pipeline.

[0047] Fig. 2 depicts a schematic view of a power supply system 100 according to the invention. An electric power supply arrangement 19 has a first power supply A and a second power supply B. Each of the power supplies A, B have a plurality of output lines. In the shown embodiment, the power supplies A, B are two-phase power supplies. Thus, the first power supply A has a first phase line LA1 and a second phase line LA2. The second power supply B has, correspondingly, a first phase line LB1 and a second phase line LB2. In Fig.2, these output phase lines LA1, LA2, LB1, LB2 are schematically depicted exiting the power supply arrangement 19.

[0048] In an embodiment according to the one shown in Fig.1, the power supply arrangement 19 could be arranged on the surface structure 1. However, in other embodiments the power supply arrangement 19 could be installed onshore.

[0049] Also schematically depicted in Fig.2 is a steel pipe 17. The steel pipe 17 could typically be a steel pipeline conducting a hydrocarbon-containing fluid.

[0050] Also shown in Fig.2 is a cable arrangement 15, which comprises four cable lines CA1, CA2, CB1, CB2. The four cable lines are grouped in a first cluster CA and a second cluster CB, thus having two cable lines in each cluster.

[0051] The first output line LA1 of the first power supply A and the first output line LB1 of the second power supply B are connected to the two cable lines in the first cluster CA. Furthermore, the second output line LA2 of the first power supply A and the second output line LB2 of the second power supply B are connected to the two cable lines CB1, CB2 of the second cluster CB.

[0052] Also schematically depicted in Fig.2, on the second location 20, there is a first electric load 9a and a second electric load 9b. The first electric load 9a has a first and second input line IA1, IA2. Correspondingly, the second electric load 9b has a first and second input line IB1, IB2. In other embodiments, such as will be discussed further below, there may be one electric load, or more than two loads.

[0053] The first and second input lines IA1, IA2 of the first electric load 9a are connected to the first phase line LA1 and the second phase line LA2, respectively, of the first power supply A. As appears from Fig.2, the input lines IA1, IA2 of the first electric load 9a connect to the first power supply A via one cable line CA1 of the first cluster CA and one cable line CB1 of the second cluster CB. Correspondingly, the input lines IB1, IB2 of the second electric load 9b connect to the second power supply B via one cable line CA2 in the first cluster CA and one cable line CB2 of the second cluster CB.

[0054] Fig. 3 shows a cross section through an integrated production umbilical (IPU) 21. The IPU 21 may typically be arranged on the seabed 5 (cf. Fig.1), extending between the first location 10 and the second location 20. It could, however, also be arranged onshore between two onshore locations. The IPU 21 has an inner steel pipe 17 configured to transport a hydrocarbon-containing fluid. Radially outside the steel pipe 17 there is arranged a filler material 23, and on the outside of the filler material 23 there is a sheath 25. Embedded in the filler material 23, between the steel pipe 17 and the sheath 25, there is advantageously arranged electric and/or optic control cables, hydraulic cables, etc., which are common in the art.

[0055] Embedded in the filler material 23, between the steel pipe 17 and the sheath 25, are the cable lines CA1, CA2 of the first cluster CA (cf. Fig.2) and the cable lines CB1, CB2 of the second cluster CB. As appears from Fig.3, the first cluster CA and the second cluster CB are distributed substantially 180 degrees apart, with respect to an axial center axis of the IPU 21. Thus, the first and second clusters CA, CB are arranged on respective sides of the steel pipe 17. Compared to the angular distance between the first and second clusters CA, CB, the mutual angular distance between the cable lines CA1, CA2, CB1, CB2 in the respective clusters, is small. In other words, the mutual angular distance between the cable lines CA1, CA2 and CB1, CB2 in respective clusters is less than the mutual angular distance between respective clusters CA, CB.

[0056] The embodiment discussed with reference to Fig.2 and Fig.3 is suitable for two two-phase power supplies, such as the first power supply A and the second power supply B schematically shown in Fig.2. A two-phase power supply can typically be represented with the vector diagram shown in Fig.4a, which depicts the output on the first phase line LA1 and the second phase line LA2 of the first (2-phase) power supply A. The vectors are of identical length and have a mutual angle of 90 degrees.

[0057] Fig. 4b depicts the power output from the first power supply A and the second power supply B, shown in the same vector diagram. In the shown situation, the output from the first power supply A is 180 degrees apart from the output from the second power supply B. Consequently, the angle between the first phase line LA1 of the first power supply A and the first phase line LB1 of the second power supply B is 180 degrees. Provided that these two phase lines LA1, LB1 carry an identical amount of electric current, in opposite directions, their combined electromagnetic fields will substantially cancel each other out. This is because the two currents are conveyed in the same cluster CA, in which the cable lines CA1, CA2 are arranged close to each other. As a result, their electromagnetic fields will induce substantially no heat in the steel pipeline 17.

[0058] The corresponding effect will occur in the second cluster CB, where power from the second phase line LA2 of the first power supply A and the second phase line LB2 of the second power supply B is transmitted.

[0059] As the skilled person will appreciate, if the two outputs from one of the first or second power supplies A, B, would be transmitted in one single cluster without being split as shown in Fig.2, an electromagnetic field would be generated and induce heat in the steel pipeline 17. Thus, with the present invention, the operator can transmit electric power from the power supply arrangement 19 at the first location 10 to the second location 20 without inducing heat in the steel pipeline 17.

[0060] In some situations, the operator may want to heat the steel pipe 17. Such situations may be where there is a risk of hydrate formation, which may reduce or even block the flow path in the steel pipe 17. The operator may then adjust the output from the first and/or second power supply A, B, such that the two currents flowing in the two cable lines in the cluster are not opposite. Such a situation is shown in Fig. 4c.

[0061] In the situation shown in Fig.4c, the output from the second power supply B is turned 180 degrees, so that it coincides with the output of the first power supply A. As shown in Fig.4c, the first phase line LA1 of the first power supply A and the first phase line LB1 of the second power supply B have the same angle. Thus, since these two currents are carried by the cable lines CA1, CA2 of the same first cluster CA, an electromagnetic field will induce heat in the steel pipe 17. The same will occur with the currents carried by the second cluster CB (i.e. LA2 and LB2).

[0062] Notably, with the situation shown in Fig.4c, even though the operator uses electric power to heat the steel pipe 17, the operator may still provide power to the first and second electric loads 9a, 9b. Thus, with the power supply system 100 according to the invention, and exemplified with the present embodiment, the operator may switch between a load power only mode, as shown in Fig.4b, and a load power and heating mode, as shown in Fig.4c.

[0063] The first and second power supplies A, B may advantageously comprise variable speed drives (VSD). Using VSDs enables the operator to rotate the phase and power of the output. Other types of multiphase power supplies are also feasible, for instance a motor-generator assembly, a transformer with two sets of output windings and a switch.

[0064] Reference is now made to Fig.5 and Fig.6a to Fig.6c. These drawings illustrate an embodiment where the first and second power supplies A, B are threephase power supplies. Using a power supply having more than two phases, e.g. a rotating three-phase power supply, is advantageous when driving electric motors at the second location 20.

[0065] Fig. 6a illustrates two three-phase outputs that are in opposite phase. The first power supply A has a first phase line LA1, a second phase line LA2 and a third phase line LA3, which transmit power 120 degrees apart. Correspondingly, the second power supply B has first, second and third phase lines LB1, LB2, LB3. As shown in Fig.6a, the first phase line LA1 of the first power supply A has the same magnitude as, and an angle of 180º, with respect to the first phase line LB1 of the second power supply B. The same applies to the second and third phase lines of the two multiphase power supplies A, B.

[0066] In the IPU 21 shown in Fig.5, there is a first, second and third cluster CA, CB, CC that each have two cable lines CA1, CA2, CB1, CB2, CC1, CC2. Like the embodiment shown in Fig.2, the two cable lines CA1, CA2 of the first cluster CA are connected to the first phase lines LA1, LB1 of the first and second multiphase power supplies A, B. The second phase lines LA2, LB2, and the third phase lines LA3, LB3 are correspondingly connected to the cable lines CB1, CB2, CC1, CC2 of the second and third clusters CB, CC respectively.

[0067] In Fig.5 there are indicated two angles. One angle is the mutual angle α between two clusters CA, CC. The other angle is the mutual angle β between two cable lines CC1, CC2 in one cluster CC. As appear from Fig.5, the angular distance β between two cable lines is less than the angular distance α between two clusters.

[0068] In the embodiment shown in Fig.5, however also possible in other embodiments, there are arranged three clusters CA, CB, CC and the mutual angular distance or angle α between each cluster is 120º.

[0069] In the embodiment shown in Fig.5, however also possible in other embodiments, the mutual angular distance or angle β between two cable lines in one cluster is less than 30º, preferably less than 20º or even 10º, as measured at the center axis of the cable lines.

[0070] Thus, in the situation illustrated by Fig.6a, the operator runs the power supply system 100 in power-only-mode, since the pairs of opposite phases are arranged together in the respective clusters CA, CB, CC.

[0071] If the operator wants to heat the steel pipe 17, it is possible to turn the mutual angle of between the multiphase power supplies A, B. This is shown in Fig.6b. In this situation, the same amount of power may be delivered from the power supplies A, B, however some power will be transmitted to the steel pipe 17 due to induction. This will generate heat in the steel pipe 17.

[0072] Another way of transferring power (heat) to the steel pipe 17 is to reduce the vector length of one of the multiphase power supplies. This is shown in Fig.6c. In this situation, the power output from the second multiphase power supply B is reduced, without shifting the phase angle. As a result, since the two currents in the respective clusters are oppositely directed, however not of the same magnitude, an electric field will be induced in the steel pipe 17, thus heating it.

[0073] Reference is now made to Fig.7a and Fig.7b. This embodiment corresponds in many ways to the embodiment shown in Fig.5 and Fig.6a to Fig.6c. However, while the previous embodiment included an integrated production umbilical (IPU) 21, the embodiment shown in Fig.7a and Fig.7b involves a steel pipe in form of a steel pipeline 17. The cable lines CA1, CA2, CB1, CB2, CC1, CC2 are piggybacked onto the steel pipeline 17.

[0074] Moreover, instead of or in addition to a power supply arrangement 19 having multiphase power supplies A, B in form of VSDs, the power supply arrangement 19a is in the form of a transformer. As shown in Fig.7a, the transformer 19a has a threephase input winding and two three-phase output windings. In the situation shown in Fig. 7a, three phase lines LA1, LA2, LA3 exit from the first multiphase power supply in the form of a first output winding arrangement A. Moreover, the remaining three phase lines LB1, LB2, LB3 exit the second multiphase power supply in the form of a second output winding arrangement B. The output winding arrangements A, B are arranged such that the three-phase output from the first output winding arrangement A is shifted 180 degrees with respect to the three-phase output from the second output winding arrangement B. This situation corresponds to Fig.6a discussed above.

[0075] Hence, as illustrated in Fig.7a, the two three-phase outputs are distributed in three clusters CA, CB, CC that each has two cable lines CA1 CA2, CB1 CB2, and CC1 CC3. As with the preceding embodiment, this enables the operator to transfer electric power to the second location 20 without inducing heat in the pipeline 17.

[0076] Should the operator wish to induce heat in the pipeline 17, all the pairs of cable lines CA1, CA2, CB1, CB2, CC1, CC3 can be connected to e.g. the first output winding arrangement A, as shown in Fig.7b. This can be performed by means of a switch (not shown) at the first location 10. When connected as illustrated in Fig.7b, the currents in each respective pair of cable lines in the respective clusters CA, CB, CC will be in phase and thus induce an electric field in the steel pipeline 17. This situation corresponds to the situation depicted in Fig.6b.

[0077] Fig. 8a is a schematic diagram illustrating a possible winding configuration of a transformer 19a used as a power supply arrangement, such as in the embodiment shown in Fig.7a and Fig.7b. A multiphase power supply 141, for instance a VSD, connects to the transformer. As shown in Fig.8a, the two three-phase power supplies A, B, namely the phase lines LA1, LA2, LA3, LB1, LB2, LB3 enter a switching arrangement 27. The switching arrangement 27 comprises a first passage line set 43 and a second passage line set 45. When in the mode shown in Fig.8a, electric power from the phase lines LA1, LA2, LA3, LB1, LB2, LB3 is conveyed through the first and second passage line sets 43, 45.

[0078] The switching arrangement 27 connects to a power line distribution arrangement 29, in which the phase lines LA1, LA2, LA3, LB1, LB2, LB3 are distributed into the respective pairs of cable lines CA1, CA2, CB1, CB2, CC1, CC2 of the clusters CA, CB, CC. As shown with the arrows on the respective cable lines in Fig. 8a, the two current in the respective clusters CA, CB, CC are in opposite directions. Hence, in the situation shown in Fig.8a, the power supply system 100 is in power only mode.

[0079] Fig. 8b schematically depicts the switching arrangement 27 when in the situation corresponding to Fig.7b, in which the pipeline 17 is heated by induction heating. In this situation, all the cable lines are connected to the first multiphase (3-phase) power supply A. As indicated with the arrows on the cable lines, the two currents in the respective clusters CA, CB, CC flow in the same direction. In this mode, the output lines 51 of the second passage line set 45 of the switching arrangement 27 are connected to the lines of the first passage line set 43.

[0080] Fig. 8c depicts another mode, wherein only one cable line CA1, CB1, CC1 in each cluster transmits current. This will also induce power in the steel pipeline 17. In this mode, the lines of the second passage line set 45 are disconnected.

[0081] The way the operator switches between power only mode and heat-andpower mode, will depend on the connections and electric loads on the second location 20. For instance, when the switching arrangement 27 is in the position shown in Fig.8c, the operator might want to run one electric motor 9a on the second location, while simultaneously provide some heat to the steel pipeline 17.

[0082] Notably, the transformer 19a and the multiphase power supply 141 constitutes an electric power supply assembly 300. The electric power supply assembly 300 may also comprise a switching arrangement, such as the switching arrangement 27 shown in Fig.8a, Fig.8b, and Fig. 8c.

[0083] Fig. 9 depicts a subsea transformer 11 at the second location 20. The components shown in Fig.9 can be part of the same power supply system 100 as shown in Fig.8a. At the second location 20, the power supply system 100 comprises a power line redistribution arrangement 31. In the power line redistribution arrangement 31, the phase lines LA1, LA2, LA3, LB1, LB2, LB3 are redistributed back into the configuration that corresponds to the output from the transformer 19a. After the redistribution arrangement 31, the lines enter the subsea transformer 11.

[0084] The subsea transformer 11 is part of a subsea electric load assembly 200, which is configured to receive electric power from at least a first and a second set of phase lines, and to transmit such power to an electric subsea load, such as the motor M. In the shown embodiment, the subsea transformer has three transformer legs 39a, 39b, 39c about which there are wound respective output windings 33a, 33b, 33c.

[0085] Furthermore, the subsea transformer 11 comprises a first set of input windings 35a, 35b, 35c and a second set of input windings 37a, 37b, 37c. One input winding of respective set of input windings is wound about one of the respective transformer legs 39a, 39b, 39c. Hence, in the shown embodiment, each transformer leg has two input windings, of which one is connected to a phase line of the first set of phase lines LA1, LA2, LA3, and one is connected to a phase line of the second set of phase lines LB1, LB2, LB3. Moreover, these two input windings are oppositely wound about their common transformer leg, as shown in Fig.9.

[0086] In the embodiment shown in Fig.8a and Fig.9, the transformer 19a on the first location 10 and the subsea transformer 11 at the second location 20 have some similarity. As appears from the winding diagram shown in Fig.8a, the one threephase input that is fed to the transformer 19a is split into two three-phase outputs, connected to the six output windings. These two three-phase outputs have a mutual angle of 180º. The subsea transformer 11 has, however, six input lines leading to the input windings and three output lines connected to the output windings.

[0087] When in the power only-mode, as shown in Fig.8a, the currents in the input windings 35a, 35b, 35c, 37a, 37b, 37c of the subsea transformer 11 will induce power to the three output windings 33a, 33b, 33c, thus enabling operation of the motor M at the second location 20.

[0088] However, when the switching arrangement 27 is switched as shown in Fig. 8b, the two sets of three phase lines that is supplied to the subsea transformer 11 are angularly aligned. Due to the configuration of the input windings of the subsea transformer 11, the current in the windings will induce opposite fields / flux in the transformer legs, which are cancelling each other out. Hence, when the power supply system 100 comprises a transformer at the second location, which is configured as the subsea transformer 11, the operator can run the power supply system 100 in a heating only-mode. This is because no power is transmitted to the output windings of the transformer at the second location end. Moreover, the current in each cable line in the respective clusters CA, CB, CC have the same direction, thus generating a field that induces heat in the steel pipeline 17.

[0089] Consequently, with such a transformer at the second location, the operator can choose between a power only-mode (cf. Fig.8a), a combined power and heating-mode (cf. Fig.8c), and a heating-only mode (cf. Fig.8b).

[0090] An advantage with the shown power supply system 100 is that no switching means is necessary at the remote, second location. When the operator switches between the different modes, all switching occurs at the first location 10.

[0091] Fig. 10 depicts a multiphase power supply in the form of a VSD 141, connected to a tunable transformer 19a. These components can for instance constitute the power supply 141 and the transformer 19a shown in Fig.8a and/or Fig. 7a and Fig.7b.

Claims

1. A power supply system (100) configured to supply electric power from a first location (10) to an electric load (9, 9a, 9b, M) at a second location (20) and to heat a steel pipe (17) extending at least partly between the first and second locations, comprising

- an electric power supply arrangement (19) arranged at the first location (10), comprising a first set of at least a first and second phase line (LA1, LA2, LA3) and a second set of at least a first and second phase line (LB1, LB2, LB3);

- a cable arrangement (15) extending along the steel pipe (17);

wherein the cable arrangement (15) comprises a plurality of clusters (CA, CB, CC) of cable lines (CA1, CA2, CB1, CB2, CC1, CC2) that equals the number of phase lines in each set of phase lines,

wherein the cable lines are arranged along and external to the steel pipe (17) and angularly distributed about an axial center axis of the steel pipe (17),

wherein the respective cable lines (CA1, CA2; CB1, CB2; CC1, CC2) of one cluster are connected to one output phase line (LA1, LB1; LA2, LB2; LA3, LB3) of each respective set, and

wherein the mutual angular distance (β) between cable lines (CA1, CA2, CB1, CB2) in respective clusters (CA, CB) is less than the angular distance (α) between adjacent clusters, with respect to the axial center axis of the steel pipe (17).

2. A power supply system (100) according to claim 2, characterized in that it is interchangeable between at least

- a load power only mode, wherein the currents in respective cable lines (CA1, CA2; CB1, CB2; CC1, CC2) in each respective cluster (CA, CB, CC) are oppositely angularly aligned and of same amperage; and

- a load power and heating mode, wherein the currents in respective cable lines (CA1, CA2; CB1, CB2; CC1, CC2) in each respective cluster (CA, CB, CC) are not oppositely angularly aligned and of same amperage.

3. A power supply system (100) according to claim 1 or claim 2, characterized in that it further comprises, at the remote location (20), a remote transformer (11) that comprises

- output windings (33a, 33b, 33c) configured to power a remote load (M);

- a first set of input windings (35a, 35b, 35c) and a second set of input windings (37a, 37b, 37c), wherein the respective input windings (35a, 37a; 35b, 37b; 35c, 37c) of the first and second sets of input windings that are on a common transformer leg (39a, 39b, 39c) are oppositely wound.

4. A power supply system (100) according to claim 3, characterized in that the electric power supply arrangement (19) comprises one or more variable speed drives (VSD).

5. A power supply system (100) according to claim 3 or claim 4, characterized in that the power supply arrangement (19) comprises a transformer (19a), said transformer comprising

- input windings (133a, 133b, 133c);

- a first set of output windings (135a, 135b, 135c) and a second set of output windings (137a, 137b, 137c), wherein the respective output windings (135a, 137a; 135b, 137b; 135c, 137c) of the first and second sets of output windings that are on a common transformer leg are oppositely wound.

6. A power supply system (100) according to one of the preceding claims, characterized in that it further comprises

- a power line distribution arrangement (29) at the first location (10), wherein the configuration of the phase lines (LA1, LA2, LA3, LB1, LB2, LB3) are grouped into said clusters (CA, CB, CC);

- a power line redistribution arrangement (31) at the second location (20), wherein the configuration of the cable lines (CA1, CA2; CB1, CB2; CC1, CC2) of the clusters (CA, CB, CC) are redistributed into the configuration as before the power line distribution arrangement (29).

7. A power supply system (100) according to claim 2 or claim 2 and one of claims 3 to 6, characterized in that at least one of the sets of at least a first and second phase line (LA1, LA2, LA3) is configured to be constantly electrically connected to cable lines (CA1, CA2, CB1, CB2), independent of being operated in the load power only mode or the load power and heating mode.

8. A power supply system (100) according to one of the preceding claims, characterized in that the length of the steel pipe (17) and the length of the cable arrangement (15), extending between the first location (10) and the second location (20), are more than 3 km, 10 km, or even more than 50 km.

9. An electric load assembly (200), configured to power an electric load (M), comprising a transformer (11) with transformer legs (39a, 39b, 39c), output windings (33a, 33b, 33c) wound about respective transformer legs, and a first set of input windings (35a, 35b, 35c),

characterized in that the transformer (11) further comprises a second set of input windings (37a, 37b, 37c) wound on the same transformer legs as the respective input windings (35a, 35b, 35c) of the first set of input windings, wherein the respective input windings (35a, 37a; 35b, 37b; 35c, 37c) on each transformer leg (39a, 39b, 39c) are wound in opposite directions.

10. An electric power supply assembly (300), comprising

- a transformer (19a) with transformer legs (139a, 139b, 139c), input windings (133a, 133b, 133c), a first set of output windings (135a, 135b, 135c) and a second set of output windings (137a, 137b, 137c), wherein the respective transformer legs comprise one input winding, one output winding of the first set of output windings, and one output winding of the second set of output windings;

- a multiphase power supply (141) connected to the input windings;

- at least a first pair (LA1, LB1) and a second pair (LA2, LB2) of phase lines (LA1, LA2, LA3, LB1, LB2, LB3) exiting the transformer (19a) and connected to the output windings on respective transformer legs;

characterized in that

- on the respective transformer legs, the respective one output winding of the first set of output windings and one output winding of the second set of output windings are wound in opposite directions.

11. An electric power supply assembly (300) according to claim 10, characterized in that it further comprises a switching arrangement (27) having a first passage line set (43) and a second passage line set (45), wherein the first passage line set (43) connects to the output windings (135a, 135b, 135c) of the first set of output windings, and the second passage line set (45) connects to the output windings (137a, 137b, 137c) of the second set of output windings,

and wherein the output lines (51) of the second passage line set (45) is configured to assume the following connection modes:

a) a connection mode where the output lines (51) of the second passage line set (45) are connected to the output windings (137a, 137b, 137c) of the second set of output windings;

b) a connection mode where the output lines (51) of the second passage line set (45) are connected to the respective lines of the first passage line set (43).

12. An electric power supply assembly (300) according to claim 11, characterized in that the second passage line set (45) is further configured to assume the following connection mode:

c) a connection mode where the output lines (51) of the second passage line set (45) are disconnected.