[001] The invention relates to the joint use of a long-distance (i.e. longer than 50 km) shared corridor for underground high-voltage power lines and natural gas transmission pipelines. Specifically, it refers to a long-distance underground system comprising high voltage lines laid out in parallel with a natural gas pipeline (NGP), this arrangement using the corridor of the natural gas pipeline.

[002] High voltage direct current (HVDC) represents a technical solution for high capacity power transmission over long distances. HVDC cable transmission systems are used in applications such as: transmission of highest electricity capacities over long distances, enabling interconnection of power grids between different countries via submarine cables, wind farms (offshore) connection to on shore power grids. The HVDC solution has also started to be used inside some national power grids (on shore), connecting excess energy grid areas to poor energy areas. HVDC systems will assure the main high-capacity transmission over long-distance in a mixed HVDC - HVAC solution.

[003] An overhead high voltage alternative current (HVAC), if laid out in the corridor of underground metallic infrastructure, would require a safety corridor of at least 200 m between pipelines and the overhead HVAC due to the electromagnetic induction phenomenon of alternative current. Collocation of HVAC with underground metal pipelines, especially underground gas pipelines, is avoided in order to prevent a pipeline corrosion and/or short-circuit potential residual currents.

[004] HVDC overhead power lines solution in parallel with the NPG has as main disadvantage the design and construction of two separate infrastructure systems, one overhead and the other underground, with own operational and safety corridor, having a direct impact on the environment. [005] The underground placement of electric cables in HVDC technology along the technical corridor of the NPG has not been carried out before due to the lack of studies on phenomena that can occur in normal and fault conditions. Even small voltage variations induced by HVDC cables operation can cause degradation of the metallic structures. When the induced voltage occurs, the current passing through the metal structure causes corrosion in time, leading to accidents, such as explosions, if the metal structure is a NGP. To maintain a safe operation for both gas and power transmission systems, the invention also analyzed protection systems of the pipeline (corrosion protection) and HVDC cable (including station protection).

[006] The system according to the invention provides the following advantages:

- ensures the fast, cheap and safe application of the HVDC solution by coexisting with NGPs, while also having a minimal impact on the environment, by laying out the cables without additional land usage,

- the use of HVDC system significantly reduces the technological losses comparing to the HVAC solution,

- generates significant energy savings,

- provides solutions to avoid corrosion phenomena on the NGP when operating in parallel with HVDC electric cables, ensuring the functioning of a high-capacity power transmission system (2 - 2.5 GW per cable) using NGP corridor.

[007] The underground natural gas supply and power transmission system according to the invention comprises:

- a pipeline for natural gas supply, provided at various intervals with surge arrester devices (such as adapted cathodic protection devices) and stray current drainage devices, and

- a first pair of HVDC cables for power transmission, arranged on the same side of the pipeline, wherein

- the pipeline and the cables of the first pair of HVDC cables are parallel to each other and buried underground, the pipeline at a depth in the range of 1 .0 m - 1 .2 m and the cables at a depth in the range of 1 .0 m - 1 .8 m, - the cables of the first pair of HVDC cables are arranged, one in respect to the other, at a spacing in the range of 2 m - 4 m, and

- the cable the nearest to the pipeline is arranged in respect to the pipeline at a distance in the range of 6 m - 16 m.

The surge arrester devices and the stray current drainage devices ensure, on one hand, the protection against corrosion, and on the other hand, mitigation measures in case of HVDC cables faults.

[008] In a preferred embodiment of the system according to the invention, the system further comprises:

- a second pair of HVDC cables for power transmission, wherein:

- the pipeline being arranged between the first and the second pair of HVDC cables,

- the cables of the second pair of HVDC cables and the pipeline are parallel to each other, the cables being buried underground at a depth in the range of 1 .0 m - 1 .8 m,

- the cables of the second pair of HVDC cables are arranged, one in respect to the other, at a spacing in the range of 2 m - 4 m,

- the cable of the second pair of HVDC cables that is the nearest to the pipeline, is arranged in respect to the pipeline at a distance in the range 6 m - 16.

In a preferred embodiment, when supplied with power, the cable of the first pair of HVDC cables that is arranged the nearest to the pipeline, has the same polarity as that of the cable of the second pair of HVDC cables that is arranged the nearest to the pipeline; in other words, this cable arrangement ensures the same polarity in the pipeline vicinity. When supplied with power, the cables belonging to the same pair of HVDC cables have opposite polarity in relation to each other.

[009] In a preferred embodiment of the system according to the invention, the system further comprises:

- a first DMR (Dedicated Metallic Return) cable for power transmission, arranged between the cables of the first pair of HVDC cables, parallel to and at the same depth as the HVDC cables, and wherein - a respective spacing between the first DMR cable and each of the cables of the first pair of HVDC cables is 2 m.

[0010] In a preferred embodiment of the system according to the invention, the system further comprises:

- a second DMR cable for power transmission, arranged between the cables of the second pair of HVDC cables, parallel to and at the same depth as the HVDC cables, and wherein

- a respective spacing between the second DMR cable and each of the cables of the second pair of HVDC cables is 2 m.

[0011] In a preferred embodiment of the system according to the invention, the distance between the pipeline and the cable(s) the nearest to the pipeline is preferably in the range of 6 m - 12 m, more preferably in the range of 6 m - 10 m, the most preferably 6 m.

[0012] In a preferred embodiment of the system according to the invention, the HVDC cables and, if present, the DMR cable(s), are provided with groundings, preferably every 5 km - 10 km and in areas situated nearby pipeline valves and nearby other metallic structures such as railroad lines.

[0013] In a preferred embodiment, the system according to the invention further comprises a single monitoring system for the pipeline, for the HVDC cables and, if present, for the DMR cable(s), and wherein the monitoring system comprises an optical fiber arranged parallel to the pipeline and to the cables, along the whole length thereof.

[0014] The invention will be better understood from the following embodiments, explained in detail and based on the figures, that represent:

Figures 1a and 1b: cross-sectional front view and cross-sectional top view, respectively, of a first embodiment of the power supply system, according to the invention; Figures 2a and 2b: cross-sectional front view and cross-sectional top view, respectively, of a second embodiment of the power supply system, according to the invention;

Figures 3a and 3b: cross-sectional front view and cross-sectional top view, respectively, of a third embodiment of the power supply system, according to the invention;

Figures 4a and 4b: cross-sectional front view and cross-sectional top view, respectively, of a fourth embodiment of the power supply system, according to the invention;

Figures 5a and 5b: cross-sectional front view and cross-sectional top view, respectively, of a fifth embodiment of the power supply system, according to the invention;

Figure 6: diagram showing an example of a practical implementation of the power supply system, according to the invention.

[0015] Figures 1a and 1b represent a first embodiment of the system according to the invention, namely an underground natural gas supply and power transmission system, comprising:

- a pipeline P for the supply of natural gas; the pipeline P is provided at various intervals with surge arrester devices and stray current drainage devices (not shown in the figures), and

- a first pair of power transmission HVDC cables 1.1 , 1.2, arranged on a same side of the pipeline P (meaning that one HVDC cable 1.1 is situated between the pipeline P and the other HVDC cable 1.2). The pipeline and the HVDC cables form the underground shared infrastructure.

The pipeline P has a standardized inner diameter, suitable for long-distance gas supply, for example (but not limited to these values): 48” (i.e. 1219 mm) or 40” (i.e. 1016 mm).

The pipeline P and the HVDC cables 1.1 , 1.2 and are parallel to each other and buried underground. The pipeline P is buried at a depth d in the range of 1.0 m - 1.2 m, the depth d being always measured between the ground surface and the nearest point (in respect to the ground surface) of the outer surface of the pipeline P.

The cables 1.1 , 1.2 are buried at a depth d1 in the range of 1.0 m - 1.8 m, the depth d1 being always measured between the ground surface and the center (when viewed in cross-section) of the HVDC cables 1.1 , 1.2.

[0016] The HVDC cables 1.1 , 1.2 are arranged, one in respect to the other, at a spacing c1 of 2 m. The spacing c1 is always measured between the centers of the HVDC cables 1.1 , 1.2 (when viewed in cross-section).

The HVDC cable 1.1 the nearest to the pipeline P is arranged in respect to the pipeline P at a distance g1 that is in the range 6 m - 16 m, preferably in the range 6 m - 12 m, more preferably in the range 6 m - 10 m, the most preferably 6 m.

The distance g1 is always measured between the nearest point (in respect to the HVDC cable 1.1) of the outer surface of the pipeline P and the center (when viewed in cross-section) of the HVDC cable 1.1.

[0017] Figures 2a and 2b represent a second embodiment of the system according to the invention, which, in respect to the first embodiment, further comprises a second pair of HVDC cables 2.1 , 2.2, such that the pipeline P is arranged between the first and the second pair of cables.

The cables 2.1, 2.2 of the second pair of HVDC cables and the pipeline P are parallel to each other, the cables 2.1 , 2.2 being buried underground at a depth d2 in the range of 1.0 m - 1.8 m. The depth d2 is always measured between the ground surface and the center (when viewed in cross-section) of the HVDC cables

2.1 , 2.2.

[0018] The HVDC cables 2.1, 2.2 of the second pair of cables are arranged, one in respect to the other, at a spacing c2 of 2 m. The spacing c2 is always measured between the centers (when viewed in cross-section) of the HVDC cables 2.1 , 2.2.

The HVDC cable 2.1 of the second pair of cables, that is the nearest to the pipeline P, is arranged in respect to the pipeline P at a distance g2 that is in the range 6 m - 16 m, preferably in the range 6 m - 12 m, more preferably in the range 6 m - 10 m, the most preferably 6 m.

The distance g2 is always measured between the nearest point (in respect to the HVDC cable 2.1) of the outer surface of the pipeline P and the center (when viewed in cross-section) of the HVDC cable 2.1.

Preferably, when supplied with power, the HVDC cable 1.1 of the first pair of cables that is arranged the nearest to the pipeline P, has the same polarity as that of the HVDC cable 2.1 of the second pair of cables that is arranged the nearest to the pipeline P.

When supplied with power, the HVDC cables 1.1 , 1.2; 2.1 , 2.2 belonging to the same pair of cables have opposite polarity in relation to each other. This polarity arrangement ensures the further minimization of the induced voltage in the pipeline P (compared to the situation where the cables 1.1 and 2.1 have opposite polarities).

[0019] Figures 3a and 3b represent a third embodiment of the system according to the invention, which, in respect to the first embodiment, further comprises a first DMR cable 3A for power transmission, arranged between the HVDC cables 1.1 , 1.2 of the first pair of cables, parallel to and at the same depth d1 as the cables 1.1 , 1.2.

The depth d1 of the DMR cable 3A is always measured between the ground surface and the center (when viewed in cross-section) of the DMR cable 3A.

A respective spacing cT between the first DMR cable 3A and each of the HVDC cables 1.1 , 1.2 is 2 m (thus, the spacing c1 is in this case 4m).

The spacing cT is always measured between the centers (when viewed in crosssection) of the DMR 3A and the respective HVDC cable 1.1 , 1.2.

[0020] Figures 4a and 4b show a fourth embodiment of the system according to the invention, which, in respect to the second embodiment, further comprises a first DMR cable 3A for power transmission, arranged between the HVDC cables 1.1, 1.2 of the first pair of cables, parallel to and at the same depth d1 as the HVDC cables 1.1 , 1.2. The depth d1 of the DMR cable 3A is always measured between the ground surface and the center (when viewed in cross-section) of the DMR cable 3A.

The spacing cT between the first DMR cable 3A and each of the HVDC cables 1.1 , 1.2 is 2 m (thus, the spacing c1 is in this case 4m).

The spacing cT is always measured between the centers (when viewed in crosssection) of the DMR 3A and the respective HVDC cable 1.1 , 1.2.

[0021] Figures 5a and 5b represent a fifth embodiment of the system according to the invention, which, in respect to the fourth embodiment, further comprises a second DMR cable 3B for power transmission, arranged between the HVDC cables 2.1 , 2.2 of the second pair of cables, parallel to and at the same depth d2 as the HVDC cables 2.1, 2.2.

The depth d2 of the second DMR cable 3B is always measured between the ground surface and the center (when viewed in cross-section) of the second DMR cable 3B.

The spacing c2' between the second DMR cable 3B and each of the HVDC cables 2.1, 2.2 of the second pair of cables is 2 m (thus, the spacing c2 is in this case 4m).

The spacing c2’ is always measured between the centers (when viewed in crosssection) of the DMR 3B and the respective HVDC cable 2.1 , 2.2.

[0022] Just for example, a preferred (but not limited to) arrangement of the fourth embodiment has the following particulars:

- the pipeline P has an inner diameter of 40” and is buried underground at a depth d of 1 .2 m,

- the HVDC cables 1.1, 1.2 and the DMR cable 3A are all buried underground at a

- depth d1 of 1 .7 m,

- the HVDC cables 2.1 , 2.2 are buried underground at a depth d2 of 1 .7 m,

- the spacings c1’ are each 2 m (thus, the spacing c1 is 4 m),

- the spacing c2 is 2 m,

- the distance g1 is 6 m, and

- the distance g2 is 6 m. [0023] In all five embodiments according to the invention, the voltage induced in the pipeline P is below 300 V, well below the admissible threshold. The minimum value (i.e. 6 meters) of the distance g1 and, where applicable, of the distance g2, between the pipeline P and the nearest HVDC cable 1.1 , 2.1 , provides the minimum overall width of the common gas/power corridor, as well as facilitating access for service and maintenance.

The distance between two neighbouring cables (depending on the embodiment, either between two HVDC cables or between a HVDC cable and a DMR cable) is in all cases 2 m, which is the minimum distance required by industry standards and which ensures an effective heat dissipation, as well as an adequate ease of access for service and maintenance.

[0024] Figure 6 shows an example of a simplified general diagram of power transmission, specifically an example of a practical way of system implementation according to the invention within an energy infrastructure project. The diagram also comprises other known components, such as (but in practice not limited to):

- end-of-line line converter stations S1 - S4 and

- intermediate converter stations 11 - I6 which carry out the DC - AC energy exchange to guarantee energy uptake or release I extraction or injection of energy and a maximum energy flow over long distances.

This design ensures full use of the capacity of HVDC cables 1.1 , 1.2; 2.1 , 2.2, with small variations in the power transmission. Thus, the design gives a uniform mode of coexistence with the gas pipeline P (i.e. acceptable induced voltage in the gas pipeline P).

[0025] In the example presented in figure 6, the system according to the invention is the one according to the fourth embodiment (i.e. from figures 4a, 4b - comprising a gas pipeline P, a first pair of HVDC cables 1.1 , 1.2 with a DMR cable 3A between them and a second pair of HVDC cables 2.1 , 2.2). However, the system may be chosen according to any of the five embodiments disclosed above. [0026] The first pair of HVDC cables 1.1 , 1.2 with the DMR cable 3A are connected to an end-of-line converter station S1, continuing to a plurality of intermediate converter stations I5, I6 and finally to a further end-of-line converter station S3. Similarly, the second pair of HVDC cables 2.1 , 2.2 is connected to another end-of-line converter station S2, from there to another plurality of intermediate converter stations 11 , I2, I3, I4 and finally to another further second end-of-line converter station S4. The energy carried by each group of cables (1.1 , 1.2, 3A and respectively 2.1 , 2.2) is, in this particular example, 2.5 GW.

[0027] In all embodiments, for safety reasons, at various intervals (for example every 5 km - 10 km) and in areas located nearby pipeline valves or other metal structures such as railroad lines, the HVDC 1.1 , 1.2; 2.1 , 2.2 and DMR 3A, 3B cables may be provided with groundings.

[0028] In all embodiments, for enhanced protection against corrosion, the pipeline P is provided with surge arrester devices and stray current drainage devices at different intervals (for example every 5 km - 10 km).

[0029] The design presented in Figure 6 has the advantage of using a single monitoring system (not shown in the figure) both for the gas pipeline P and for the electrical cables 1.1, 1.2; 2.1 , 2.2; 3A, 3B. For example, the mentioned monitoring system may comprise an optical fiber arranged parallel to the pipeline P and to the cables 1.1 , 1.2; 2.1 , 2.2; 3A, 3B, along the whole length thereof, a transmitter and a plurality of receivers.

[0030] When a control signal sent by the transmitter through the optical fiber is received by the last receiver, it means that the optical fiber is intact and in principle the cables 1.1, 1.2; 2.1 , 2.2; 3A, 3B and the pipeline P are also intact.

If the control signal is received only by the first “n” receivers, it means that an unwanted event (unintentional excavation, landslide, etc.) occurred in the “n+1” receiver region and damaged the optical fiber. This is an indication that the pipeline P and/or the cables 1.1 , 1.2; 2.1 , 2.2; 3A, 3B could also have been damaged in said “n+1” region, so an on-site inspection should be carried out to determine the exact cause and prevent possible disasters.

[0031] Although the invention has been described in connection with particular illustrative embodiments, it will be clear that it is in no way limited to these embodiments and that it covers all the technical equivalents of the means described and combinations thereof, insofar as in which the same function is achieved.

References list

1.2, 1.2, 2.1 , 2.2 : HVDC cables

3A, 3B : DMR cables

P : natural gas pipeline

S1 , S2, S3, S4 : end-of-line converter stations

11 , 12, 13, 14, 15, 16 : intermediate converter stations d : underground pipeline depth d1, d2 : underground cables depths c1 , c2 : spacings between a pair of HVDC cables c1’, c2’ : spacings between a DMR and a neighboring HVDC cable gi , g2 : distances between the NGP and proximal HVDC cables