METHOD FOR LOCATING A FAULT POINT ON A HIGH-VOLTAGE THREE-VOLTAGE CABLE AND SYSTEM FOR LOCATING A FAULT POINT

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for locating a fault point on a high-voltage three-phase AC cable and to a system for locating a fault point.PRIOR ART

[0002] Methods for detecting a fault on high-voltage three-phase AC cables, i.e., that a short circuit has occurred between a conductor and a shield of said cable, are known.

[0003] Methods which, in addition to detecting a fault, locate the point where said fault has occurred, are also known.

[0004] For example, US20210072302A1 relates to a method for locating a fault point on a high-voltage cable based on providing at least two current measuring apparatuses arranged at the cable at a given distance and having timers synchronized with one another, and a measuring device for detecting an electric current flowing in the shield and / or an earthing line connecting the shield and earth. The measuring apparatus transmits pairs of current measurement values and associated values of the timer to an analysis unit which, upon the occurrence of a current exceeding a threshold value and / or of a current profile over time that satisfies specific stipulations, feeds to a calculation unit the associated values of the timers and also an indication about the location of the measurement, for which first effects of the short circuit occurred. The calculation unit calculates the location of the fault from the known distance between the measuring apparatuses and a difference between the values of the synchronized timers.

[0005] The document "MINGZHEN LI ET AL: "A novel fault localization method based on monitoring of sheath current in a cross-bonded HV cable system",2017 IEEE ELECTRICAL INSULATION CONFERENCE (EIC), IEEE, 11 June 2017 (2017-06-11), pages 123-126, XP033138823,DOI: 10.1 109 / EIC.2017.8004678ISBN: 978-1-5090-3965-4" discloses using the fault current in the metal sheath, between the instant of fault occurring and the moment the fault is cleared, to realise fault localization in a high-voltage three-phase AC sheathed cable. An equivalent two-port network of the cable as a whole is employed.DISCLOSURE OF THE INVENTION

[0006] The object of the invention is to provide a method for locating a fault point on a high-voltage three-phase AC cable, and a system for locating a fault point on a high-voltage three-phase AC cable, as defined in the claims.

[0007] A first aspect of the invention relates to a method for locating a fault point on a high-voltage three-phase AC cable.

[0008] The cable in which the method of the invention is applied comprises a first end, a second end, and at least one main section extending between said ends, the cable comprising one conductor per phase and three shields associated with said conductor, said shields being attached to one another and connected to ground at both ends of each main section. Therefore, the method of the invention is configured for being applied in cables with cross bonding connection systems of the shields and in cables with solid bonded connection systems.

[0009] The method comprises the following steps measuring the conductor current circulating through each conductor both at the first end and at the second end of the cable, measuring the shield currents in the vicinity of the connection points to ground of the shields, determining the conductor in fault based on the previously measured conductor currents, in the event that the cable comprises more than one main section, determining the main section in fault based on the previously measured shield currents, and locating the fault point by means of a model, also known as digital twin, of the shields of the main section in fault, taking into account that the shields are attached to one another at the ends of said main part in fault, such that three closed nets in which the sum of voltages is zero are formed in the model, the only unknown being the distance to the fault point.

[0010] A second aspect of the invention relates to a system for locating a fault point on a cable such as the one described in the first aspect of the invention, the system comprising: a first current measuring equipment configured for measuring the conductor current circulating through each conductor both at the first end and at the second end of the cable, a second current measuring equipment configured for measuring the shield current of each shield in the vicinity of the ground connection points, and a processor configured for executing the method described in the first aspect of the invention based on the current measurements performed by said current measuring equipment.

[0011] The method for locating and the system for locating of the invention offer a rapid solution for locating a fault point on a high-voltage three-phase AC cable, with the result virtually being obtained in real time.

[0012] Even though a model of the shields is required for locating the exact fault point, said model is not very sensitive to the type of the terrain in which the cable is arranged, since it affects the different parameters of the shield model in a similar manner, and therefore, the influence thereof is virtually cancelled.

[0013] Furthermore, the fault resistance, i.e., the resistance between the conductor in fault and the shield in fault at fault point F, nor is it necessary to know the ground resistance of the connections to ground of the shields.

[0014] These and other advantages and features of the invention will become apparent in view of the figures and of the detailed description of the invention.DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 schematically shows a system for locating a fault point on a cable according to a first embodiment of the invention applied on a cable having a first configuration. Figure 2 shows the main section in fault of the cable of Figure 1. Figure 3 shows the model of the shields of the main section in fault of the cable of Figure 1. Figure 4 shows in detail the model of the shield that is in fault of the main section in fault of the cable of Figure 1. Figure 5 shows the model of the shield of Figure 4, in which the mutual inductances are depicted as generators. Figure 6 shows the model of one of the shields that is not in fault of the main section in fault of the cable of Figure 1, in which the mutual inductances are depicted as generators. Figure 7 schematically shows a system for locating a fault point on a cable according to a second embodiment of the invention applied on a cable having a second configuration. DETAILED DISCLOSURE OF THE INVENTION

[0016] A first aspect of the invention relates to a method for locating a fault point F on a high-voltage three-phase AC cable 1.

[0017] The method of the invention is configured for locating a fault point F on a cable 1 comprising a first end 10, a second end 11, and at least one main section MP 1 , MP 2 , MP 3 extending between said ends 10, 11. The cable 1 comprises one conductor R, S, T for each phase and three shields S A , S B , S C ; S R , S S , S T associated with said conductors R, S, T. The shields S A , S B , S C ; S R , S S , S T are attached to one another and connected to ground G at both ends of each main section MP 1 , MP 2 , MP 3 , preferably by means of a corresponding grounding device 3, 4. Therefore, the method of the invention is configured for being applied in cables 1 with shield cross bonding connection systems, and in cables with solid bonded connection systems.

[0018] As discussed above, fault point F is considered the point where one of the conductors R, S, T, of the cable 1 has electrical contact with the corresponding shield S A , S B , S C ; S R , S S , S T , such that a short circuit occurs between said conductor R, S, T and the corresponding shield S A , S B , Sc; S R , S S , S T at said fault point F. Furthermore, in the context of the invention, main section in fault will be considered the main section MP 1 , MP 2 , MP 3 of cable 1 at which the fault point F is arranged.

[0019] The method of the invention comprises the following steps: measuring the conductor current I R1 , I S1 , I T1 , I R2 , I S2 , I T2 circulating through each conductor R, S, T both at the first end 10 and at the second end 11 of the cable 1, measuring the shield currents I SA1 , I SB1 , I SC1 , I SA2 , I SB2 , I SC2 , I SA3 , I SB3 , I SC3 , I SA4 , I SB4 , I SC4 ; I SR1 , I SS1 , I ST1 ; I SR2 , I SS2 , I ST2 in the vicinity of the connection points to ground G of the shields S A , S B , S C , determining the conductor R, S, T in fault based on the previously measured conductor currents I R1 , I S1 , I T1 , I R2 , I S2 , I T2 , in the event that the cable 1 comprises more than one main section MP 1 , MP 2 , MP 3 , determining the main section MP 1 , MP 2 , MP 3 in fault based on the previously measured shield currents I SA1 , I SB1 , I SC1 , I SA2 , I SB2 , I SC2 , I SA3 , I SB3 , I SC3 , I SA4 , I SB4 , I SC4 ; I SR1 , I SS1 , I ST1 ; I SR2 , I SS2 , I ST2 , and locating the fault point F by means of a model of the shields S A , S B , S C ; S R , S S , S T of the main section MP 1 , MP 2 , MP 3 in fault, taking into account that the shields S A , S B , S C ; S R , S S , S T are attached to one another at the ends of said main section MP 1 , MP 2 , MP 3 in fault, such that three closed nets in which the sum of voltages is zero are formed in the model, the only unknown being the distance x to the fault point.

[0020] Preferably, for determining the conductor R, S, T in fault, the conductor current I R1 ; I S1 ; I T1 measured at the first end 10 of the cable 1 for each conductor R, S, T is compared with the conductor current I R2 ; I S2 ; I T2 measured at the second end 11 of the cable 1, determining that the conductor R, S, T in fault will be the conductor in which the conductor current I R1 , I R2 ; I S1 , I S2 ; I T1 , I T2 at said ends 10, 11 is different.

[0021] Preferably, when the cable 1 comprises more than one main section MP 1 , MP 2 , MP 3 , the main section MP 1 , MP 2 , MP 3 in fault is determined taking into account that at the ends of said main section MP 1 , MP 2 , MP 3 in fault, the shield currents I SA1 , I SB1 , I SC1 ; I SA2 , I SB2 , I SC2 ; I SA3 , I SB3 , I SC3 ; I SA4 , I SB4 , I SC4 are not in phase.

[0022] It should be pointed out that in the case of cables 1 with cross bonding connection systems, although the measurement of shield currents I SA1 , I SB1 , I SC1 , I SA2 , I SB2 , I SC2 at the ends 10, 11 of the cable 1 is exclusive to the corresponding main section MP 1 , MP 3 (only shield current measurement), usually the measurement of shield currents I SA3 , I SB3 , I SC3 ; I SA4 , I SB4 , I SC4 in the intermediate connections to ground G does not present said separation, since the shields S A , S B , Sc are connected to ground by means of a coaxial cable 30, as shown for example in Figure 1, said coaxial cable being connected to the corresponding shield S A , S B , S C of the main sections MP 1 , MP 2 , MP 3 on both sides of the connection to ground of the shield S A , S B , S C , the measurement of said shield currents I SA3 , I SB3 , I SC3 ; I SA4 , I SB4 , I SC4 being performed in the coaxial cable, and therefore being a measurement that combines the shield current on both sides of the connection to ground of the shield S A , S B , S C . Therefore, when there are no single shield current measurements for all the main sections MP 1 , MP 2 , MP 3 , for determining the main section MP 1 , MP 2 , MP 3 in fault, the shield currents I SA1 , I SB1 , I SC1 ; I SA2 , I SB2 , I SC2 ; I SA3 , I SB3 , I SC3 ; I SA4 , I SB4 , I SC4 are successively analyzed starting from one of the ends 10, 11 of the cable 1 until observing that the shield currents I SA1 , I SB1 , I SC1 ; I SA2 , I SB2 , Isc2; I SA3 , I SB3 , Isc3; I SA4 , I SB4 , Isc4 are not in phase, determining that the main section MP 1 , MP 2 , MP 3 in fault will be the main section MP 1 , MP 2 , MP 3 after the connection point to ground G in which it is detected that said shield currents I SA1 , I SB1 , I SC1 ; I SA2 , I SB2 , I SC2 ; I SA3 , I SB3 , I SC3 ; I SA4 , I SB4 , I SC4 are not in phase. To analyze whether or not the shield currents I SA1 , I SB1 , I SC1 ; I SA2 , I SB2 , I SC2 ; I SA3 , I SB3 , I SC3 ; I SA4 , I SB4 , I SC4 are in phase, it can be performed, for example, by observing the positive, negative, and homopolar sequences of said currents, although the person skilled in the art can do this in any other way known in the state of the art. In other possible embodiments, in which in the intermediate points where the shields are grounded, the shield currents referring exclusively to the corresponding main section are measured, the main section in fault will be determined by observing said shield currents and seeing at which point they are out of phase, or alternatively detecting the main part in which for one of the shields the shield current at both ends of the main section in fault is different.

[0023] In the case of cables 1 with a cross bonding connection system, each main section MP 1 , MP 2 , MP 3 comprises three minor sections mp 1 , mp 2 , mp 3 , the method comprising determining the minor section mp 1 , mp 2 , mp 3 in fault once the main section MP 1 , MP 2 , MP 3 in fault has been determined.

[0024] Preferably, for determining the minor section mp 1 , mp 2 , mp 3 in fault, first the shield S A , S B , S C in fault is determined by comparing for each shield S A , S B , S C the shield current I SA1 , I SB1 , I SC1 ; I SA2 , I SB2 , I SC2 ; I SA3 , I SB3 , I SC3 ; I SA4 , I SB4 , I SC4 measured at one of the ends of the main part MP 1 , MP 2 , MP 3 in fault with the shield current I SA1 , I SB1 , I SC1 ; I SA2 , I SB2 , I SC2 ; I SA3 , I SB3 , I SC3 ; I SA4 , I SB4 , I SC4 measured at the other end of the main part MP 1 , MP 2 , MP 3 in fault, the shield S A , S B , S C in fault being the shield S A , S B , Sc whose current I SA1 , I SB1 , I SC1 ; I SA2 , I SB2 , I SC2 ; I SA3 , I SB3 , I SC3 ; I SA4 , I SB4 , I SC4 at both ends of the main section MP 1 , MP 2 , MP 3 in fault is different. The minor section mp 1 , mp 2 , mp 3 in fault will be the minor section mp 1 , mp 2 , mp 3 in which the conductor R, S, T in fault and the shield S A , S B , S C in fault coincide. It should be pointed out that the case of the cables with a cross bonding connection system, the minor part mp 1 , mp 2 , mp 3 in fault is determined univocally, since the determination of said minor part mp 1 , mp 2 , mp 3 in fault is performed from the short circuit values, without the need for models or suppositions.

[0025] Preferably, in cables 1 with a cross bonding connection system, the model of the shields S A , S B , S C of main section MP 1 , MP 2 , MP 3 in fault of each shield S A , S B , S C comprises per minor section mp 1 , mp 2 , mp 3 its own resistance R SA , R SB , R SC and its own self-inductance L SA , L SB , Lsc of the corresponding shield S A , S B , S C , the mutual inductance M SB-SA , M SC-SA ; M SA-SB , M SC-SB ; M SA-SC , M SB-SC between said shield S A , S B , S C and the rest of the shields S A , S B , S C , and the mutual inductance M R-SA , M S-SA , M T-SA ; M R-SB , M S-SB , M T-SB ; M R-SC , M S-SC , M T-SC between said shield S A , S B , S C and the conductors R, S, T.

[0026] Preferably, to simplify the calculations needed for locating the fault point F, when defining the model of the shield for each shield S A , S B , S C the capacitive resistance between said shield and the rest of the shields and the conductors is not taken into account, since in case of fault, as the currents are very high and the frequency is the nominal frequency of the facility, they are completely negligible. In the same way, preferably, the model of shield of each shield S A , S B , S C does not include the admittance between said shield and the rest of the shields and the conductors, since they are values that contribute nothing in fault situations.

[0027] Therefore, preferably the parameters that are considered to define the model of the shield for each shield S A , S B , S C are the following: the resistivity of the shield R SA , R SB , R SC . It is a known but temperature-dependent term. It is possible to improve the algorithm if the operating temperature of the cable 1 at the time of the fault is estimated or known. Direct measurements of said temperature with optical or distributed sensors can help to improve said estimate, the self-inductance of the shield L SA , L SB , L SC , and the mutual inductance between the shield and the rest of the shields M SB-SA , M SC-SA ; M SA-SB , M SC-SB ; M SA-SC , M SB-SC and conductors M R-SA , M S-SA , M T-SA ; M R-SB , M S-SB , M T-SB ; M R-SC , M S-SC , M T-SC . These impedances can ultimately be considered generators, whose magnitude depends on the currents circulating through the shield S A , S B , S C or the corresponding conductor R, S, T. This coupling is dependent on geometric considerations of both the cable 1 and of how said cable 1 is deployed throughout the facility, whether resting on the ground or in trefoil, etc.

[0028] Therefore, preferably, when defining the model of the shields only those parts small as they may be, which contribute significantly within the model due to the high level of short circuit currents, are considered.

[0029] As previously discussed, the shields S A , S B , Sc are attached and connected to ground at both ends of each main section MP 1 , MP 2 , MP 3 . For this reason, in the model of the shields S A , S B , Sc of main section MP 1 , MP 2 , MP 3 in fault the ends of the model corresponding to each shield S A , S B , S C are attached to one another, such that three nets in which the sum of voltages is zero are formed, the only unknown being the distance x to the fault point F.

[0030] Therefore, when the cable 1 in which the method of the invention is applied has a cross bonding configuration, preferably the location of the fault point F is calculated by means of the following equations: V SF = V SNF V SF = V SF _ mpf + V SF _ mpnf 1 + V SF _ mpnf 2 V SF mpf = xI SF 1 R SF mpf + xjwI SF 1 L SF mpf + xjwI CF 1 M CF − SF mpf + xjwI CNF 1 M CNF 1 − SF mpf + xjwI CNF 2 M CNF 2 − SF mpf + xjwI SNF 1 M SNF 1 − SF mpf + xjwI SNF 2 M SNF 2 − SF mpf − 1 − x I SF 2 R SF mpf − 1 − x jwI SF 2 L SF mpf − 1 − x jwI CF 2 M CF − SF mpf + 1 − X jwI CNF 1 M CNF 1 − SF mpf + 1 − X jwI CNF 2 M CNF 2 − SF mpf + 1 − X jwI SNF 1 M SNF 1 − SF mpf + 1 − X jwI SNF 2 M SNF 2 − SF mpf V SF mpnf = I SF R SF mpnf + jwI SF L SF mpnf + jwI CF M CF − SF mpnf + jwI CNF 1 M CNF 1 − SF mpnf + jwI CNF 2 M CNF 2 − SF mpnf + jwI SNF 1 M SNF 1 − SF mpnf + jwI SNF 2 M SNF 2 − SF mpnf V SNF = V SNF _ mpf + V SNF _ mpnf 1 + V SNF _ mpnf 2 V SNF mpf = xI SNF R SNF mpf + xjwI SNF L SNF mpf + xjwI CF 1 M CF − SNF mpf + xjwI CNF 1 M CNF 1 − SNF mpf + xjwI CNF 2 M CNF 2 − SF mpf + xjwI SF 1 M SF − SNF mpf + xjwI SNF 2 M SNF 2 − SNF mpf − 1 − x I SNF R SNF mpf + 1 − x jwI SNF L SNF mpf − 1 − x jwI CF 2 M CF − SNF mpf + 1 − X jwI CNF 1 M CNF 1 − SNF mpf + 1 − X jwI CNF 2 M CNF 2 − SNF mpf − 1 − X jwI SF 2 M SF − SNF mpf + 1 − X jwI SNF 2 M SNF 2 − SNF mpf V SNF mpnf = I SNF R SNF mpnf + jwI SNF L SNF mpnf + jwI CF M CF − SNF mpnf + jwI CNF 1 M CNF 1 − SNF mpnf + jwI CNF 2 M CNF 2 − SNF mpnf + jwI SF M SF − SNF mpnf + jwI SNF 2 M SNF 2 − SNF mpnf wherein SF= shield in fault, SNF= shield that is not in fault, SNF1= one of the shields that is not in fault, SNF2= the other shield that is not in fault, CNF1= one of the conductors that is not in fault, CNF2= the other conductor that is not in fault, mpf= the minor section in fault, mpnf= the minor section that is not in fault, I SF1 = The current of the shield in fault on one side of the fault point, I SF2 = The current of the shield in fault on the other side of the fault point, I SF = The current circulating through the shield in fault in the section in fault. If the current flows from the first end to the second end, it will be considered positive; in contrast, if it flows from the second end to the first end, it will be considered negative, I CF = The current circulating through the conductor in fault. If the current flows from the first end to the second end, it will be considered positive; in contrast, if it flows from the second end to the first end, it will be considered negative, I CF1 = The current of the conductor in fault on one side of the fault point, and I CF2 = The current of the conductor in fault on the other side of the fault point.

[0031] When the method of the invention is applied in a cable with connection system solid-bonded, the cable 1 comprises a single main section MP 1 . The model of the shields S R , S S , S T of each shield S R , S S , S T comprises its own resistance R SR , R SS , R ST and self-inductance L SR , L SS , L ST , the mutual inductance M SS-SR , M ST-SR ; M SR-SS , M ST-SS ; M SR-ST , M SS-ST between said shield S R , S S , S T and the rest of the shields S R , S S , S T and the mutual inductance M R-SR , M S-SR , M T-SR ; M R-SS , M S-SS , M T-SS ; M R-ST , M S-ST , M T-ST between said shield S R , S S , S T and the conductors R, S, T.

[0032] Like in the model of shields of the cable with cross bonding connection system and for the same reasons, preferably, when defining the model of shields of the cable with solid bonded connection system, only those parts, small as they may be, which contribute significantly within the model due to the high level of short circuit currents, are considered.

[0033] As previously discussed, the shields S R , S S , S T are attached and connected to ground at both ends of the main section MP 1 . For this reason, the ends of the models corresponding to each shield S R , S S , S T are attached to one another, such that three nets in which the sum of voltages is zero are formed, the only unknown being the distance x to the fault point F.

[0034] Therefore, preferably, when the cable 1 in which the method of the invention is applied has a solid bonded configuration, preferably the location of the fault point F is calculated by means of the following equations: V SF = V SNF 1 = V SNF 2 V SF = xI SF 1 R SF + xjwI SF 1 L SF + xjwI CF 1 M CF − SF + xjwI CNF 1 M CNF 1 − SF + xjwI CNF 2 M CNF 2 − SF + xjwI SNF 1 M SNF 1 − SF + xjwI SNF 2 M SNF 2 − SF − 1 − x I SF 2 R SF − 1 − x jwI SF 2 L SF − 1 − x jwI CF 2 M CF − SF + 1 − x jwI CNF 1 M CNF 1 − SF + 1 − x jwI CNF 2 M CNF 2 − SF + 1 − x jwI SNF 1 M SNF 1 − SF + 1 − x jwI SNF 2 M SNF 2 − SF V SNF = xI SNF R SNF + xjwI SNF L SNF + xjwI CF 1 M CF − SNF + xjwI CNF 1 M CNF 1 − SNF + xjwI CNF 2 M CNF 2 − SNF + xjwI SF 1 M SF − SNF + xjwI SNF 2 M SNF 2 − SNF + 1 − x I SNF R SNF + 1 − x jwI SNF L SNF − 1 − x jwI CF 2 M CF − SNF + 1 − x jwI CNF 1 M CNF 1 − SNF + 1 − x jwI CNF 2 M CNF 2 − SNF − 1 − x jwI SF 2 M SF − SNF + 1 − xjwI SNF 2 M SNF 2 − SNF wherein SF= shield in fault, SNF= shield that is not in fault, SNF1= one of the shields that is not in fault, SNF2= the other shield that is not in fault, CF= conductor in fault, CNF1= one of the conductors that is not in fault, CNF2= the other conductor that is not in fault, I SF1 = The current of the shield in fault on one side of the fault point, I SF2 = The current of the shield in fault on the other side of the fault point, I CF1 = The current of the conductor in fault on one side of the fault point, and I CF2 = The current of the conductor in fault on the other side of the fault point.

[0035] A second aspect of the invention relates to a system for locating a fault point F on a high-voltage three-phase AC cable 1.

[0036] The system of the invention is configured for locating a fault point F on a cable 1 comprising a first end 10, a second end 11, and at least one main section MP 1 , MP 2 , MP 3 extending between said ends 10, 11. The cable 1 comprises one conductor R, S, T per phase and three shields S A , S B , S C ; S R , S S , S T associated with said conductors R, S, T. The shields S A , S B , S C ; S R , S S , S T are attached to one another and connected to ground G at both ends of each main section MP 1 , MP 2 , MP 3 .

[0037] The system of the invention comprises a first current measuring equipment configured for measuring the conductor current I R1 , I S1 , I T1 , I R2 , I S2 , I T2 circulating through each conductor R, S, T both at the first end 10 and at the second end 11 of the cable 1, a second piece of measuring equipment configured for measuring the shield current I SA1 , I SB1 , I SC1 ; I SA2 , I SB2 , I SC2 ; I SA3 , I SB3 , I SC3 ; I SA4 , I SB4 , I SC4 ; I R1 , I S1 , I T1 , I R2 , I S2 , I T2 of each shield S A , S B , S C ; S R , S S , S T in the vicinity of the connection points to ground G of the shields S A , S B , S C ; S R , S S , S T , and a processor configured for executing the method described in the first aspect of the invention based on the current measurements performed by said current measuring equipment.

[0038] Figure 1 schematically shows a first embodiment of the system of the invention, said system being applied on a high-voltage three-phase AC cable 1 having a first cable configuration.

[0039] The cable 1 according to the first configuration has a cross bonding connection system. The cable 1 comprises a first end 10, a second end 11, and three main sections MP 1 , MP 2 , MP 3 extending between said ends 10, 11. The shields S A , S B , S C are attached to one another and connected to ground G at both ends of each main section MP 1 , MP 2 , MP 3 . Each main section MP 1 , MP 2 , MP 3 comprises three minor sections mp 1 , mp 2 , mp 3 which are defined by the points where the shields S A , S B , S C are cross bonded.

[0040] In the first embodiment, the system comprises a first current measuring equipment configured for measuring the conductor current I R1 , I S1 , I T1 , I R2 , I S2 , I T2 circulating through each conductor R, S, T both at the first end 10 and at the second end 11 of the cable 1. The first current measuring equipment comprises six current sensors 50, 51, 52, 53, 54, 55, with one current sensor 50, 51, 52, 53, 54, 55 being arranged at each end of each conductor R, S, T.

[0041] In the first embodiment, the system comprises a second current measuring equipment configured for measuring the shield current I SA1 , I SB1 , I SC1 ; I SA2 , I SB2 , I SC2 ; I SA3 , I SB3 , I SC3 ; I SA4 , I SB4 , I SC4 of each shield S A , S B , S C in the vicinity of the connection points to ground G of the shields S A , S B , S C . The second current measuring equipment comprises twelve current sensors 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, with one current sensor 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71 being arranged in the vicinity of the connection points to ground G of each shield S A , S B , S C . As previously discussed, although the measurement of shield currents I SA1 , I SB1 , I SC1 , I SA2 , I SB2 , I SC2 at the ends 10, 11 of the cable 1 is exclusive to the corresponding main section MP 1 , MP 3 (only shield current measurement), the measurement of shield currents I SA3 , I SB3 , I SC3 ; I SA4 , I SB4 , I SC4 in the intermediate connections to ground G does not present said separation, since the shields S A , S B , S C are attached to the corresponding grounding device 3, 4 through a coaxial cable 30, as shown for example in Figure 1.

[0042] The system of the invention comprises a processor, not shown in the figures, configured for executing the method of the invention based on the current measurements performed by said current measuring equipment.

[0043] A fault between the conductor S and the shield S B in the main section MP 2 and the minor section mp 2 has been depicted in Figures 1 and 2. In order to locate said fault point F, the method of the invention would be applied as follows: Measuring the conductor current I R1 , I S1 , I T1 , I R2 , I S2 , I T2 circulating through each conductor R, S, T both at the first end 10 and at the second end 11 of the cable 1 by means of the first current measuring equipment.

[0044] Measuring the shield current I SA1 , I SB1 , I SC1 , I SA2 , I SB2 , I SC2 , I SA3 , I SB3 , I SC3 , I SA4 , I SB4 , I SC4 circulating through each shield S A , S B , S C in the vicinity of the connection points to ground G of the shields S A , S B , S C .

[0045] Comparing the conductor current I R1 ; I S1 ; I T1 measured at the first end 10 of the cable 1 for each conductor R, S, T with the conductor current I R2 ; I S2 ; I T2 measured at the second end 11 of the cable 1. In this case, the conductor current I S1 of the conductor S at the first end 10 of the cable 1 will be different from the conductor current I S2 at the first end 11, such that it will be determined that conductor S is the conductor that is in fault.

[0046] Likewise, for determining the main section MP 1 , MP 2 , MP 3 in fault, the shield currents I SA1 , I SB1 , I SC1 ; I SA2 , I SB2 , I SC2 ; I SA3 , I SB3 , I SC3 ; I SA4 , I SB4 , I SC4 are successively analyzed starting from one of the ends 10, 11 of the cable 1, for example starting from the first end 10. After verifying that the shield currents I SA1 , I SB1 , I SC1 at the first end 10 are in phase, it is ruled out that the main section MP 1 is the section that is in fault, and therefore the shield currents I SA2 , I SB2 , I SC2 in the following connection point to ground G are analyzed. Upon analyzing said shield currents I SA2 , I SB2 , I SC2 , it will be observed that they are not in phase, determining that the main section MP 1 , MP 2 , MP 3 in fault will be the main section MP 2 after said connection point to ground G. If instead of starting from the first end 1 of the cable the analysis starts from the second end 11 of the cable 1, the result would be the same. It should be pointed out that since the shields S A , S B , Sc are attached to one another at both ends of each main section MP 1 , MP 2 , MP 3 , in the main section MP 2 in fault the shield currents I SA3 , I SB3 , I SC3 ; I SA4 , I SB4 , I SC4 , are out of phase, but once the shield currents I SA3 , I SB3 , I SC3 ; I SA4 , I SB4 , I SC4 reach the end of the main section MP 2 in fault, in the rest of the main sections MP 1 , MP 3 the distribution of the shield currents I SA1 , I SB1 , I SC1 ; I SA2 , I SB2 , I SC2 is the same for all the shields S A , S B , S C .

[0047] Next, the minor section mp 1 , mp 2 , mp 3 is determined. To that end, first the shield S A , S B , S C in fault is determined by comparing for each shield S A , S B , S C the shield current I SA2 , I SB2 , I SC2 measured at one of the ends of the main part MP 2 in fault with the shield current I SA3 , I SB3 , I SC3 measured at the other end of the main part MP 2 in fault. In this case, it will be determined from the comparison that the shield current I SB3 at the first end of the main section MP 2 in fault will be different from the shield current I SB4 at the second end of the main section MP 2 in fault, determining that the shield S B is the section that is in fault. The minor section mp 1 , mp 2 , mp 3 in fault will be the minor section mp 2 in which the conductor S in fault and the shield S B in fault coincide.

[0048] It should be pointed out that up to this point, locating is based solely on the measurements performed by the current measuring equipment, and therefore, determining the minor section mp 1 , mp 2 , mp 3 in fault is performed without having to estimate any parameter or establish any model, and therefore it is performed univocally.

[0049] Once the conductor S in fault, the main section MP 2 in fault, and the minor section mp 2 in fault are determined, the model of the shields S A , S B , S C of main section MP 2 in fault is defined. The model of the shields S A , S B , S C of the main section MP 2 in fault are observed in Figure 3. As can be observed in said figure, since the shields S A , S B , S C are attached to one another at the ends of the main part MP 2 in fault, three nets S A- S B , S A- S C , S B- S C in which the sum of the voltages is equal to zero, or in other words, the voltage at the ends of the shields is the same for the three shields, are created. V SA = V SB = V SC

[0050] Actually, since there is only one unknown (the distance x to the fault point), with the model of the shield S B which is in fault and the model of another one of the shields S B , S C is sufficient to calculate the distance to the fault point.

[0051] Figure 4 shows in detail the model of the shield S B which is in fault, whereas Figure 5 shows said model of the shield S B which is in fault in which the mutual inductances are depicted as generators.

[0052] Figure 6 shows the model of the shield S A which is not in fault in which the mutual inductances are depicted as generators.

[0053] Therefore, the model depicted of each shield S A , S B , S C comprises per minor section mp 1 , mp 2 , mp 3 its own resistance R SA , R SB , Rsc and its own self-inductance L SA , L SB , Lsc of the corresponding shield S A , S B , S C , the mutual inductance M SB-SA , M SC-SA ; M SA-SB , M SC-SB ; M SA-SC , M SB-SC between said shield S A , S B , S C and the rest of the shields S A , S B , S C , and the mutual inductance M R-SA , M S-SA , M T-SA ; M R-SB , M S-SB , M T-SB ; M R-SC , M S-SC , M T-SC between said shield S A , S B , S C and the conductors R, S, T.

[0054] Therefore, the location of the fault point depicted in Figures 1 and 2 is calculated by means of the following equations: V SB = I SB 3 R SB 1 + jwI SB 3 L SB 1 + jwI R M R − SB 1 + jwI S 1 M S − SB 1 + jwI T M T − SB 1 + jwI SA 3 M SA 1 − SB 1 + jwI SC 3 M SC 1 − SB 1 xI SB 3 R SB 2 + xjwI SB 3 L SB 2 + xjwI R M R − SB 2 + xjwI S 1 M S − SB 2 + xjwI T M T − SB 2 + xjwI SA 3 M SA 2 − SB 2 + xjwI SC 3 M SC 2 − SB 2 − 1 − x I SB 4 R SB 2 − 1 − x jwI SB 4 L SB 2 + 1 − x jwI R M R − SB 2 − 1 − x jwI S 2 M S − SB 2 + 1 − x jwI T M T − SB 2 + 1 − x jwI SA 3 M SA 2 − SB 2 + 1 − x jwI SC 3 M SC 2 − SB 2 − I SB 4 R SB 3 − jwI SB 4 L SB 3 + jwI R M R − SB 3 − jwI S 2 M S − SB 3 + jwI T M T − SB 3 + jwI SA 3 M SA 3 − SB 3 + jwI SC 3 M SC 3 − SB 3 V SA = I SA 3 R SA 1 + jwI SA 3 L SA 1 + jwI R M R − SA 1 + jwI S 1 M S − SA 1 + jwI T M T − SA 1 + jwI SB 3 M SB 1 − SA 1 + jwI SC 3 M SC 1 − SA 1 xI SA 3 R SA 2 + xjwI SB 3 L SA 2 + xjwI R M R − SA 2 + xjwI S 1 M S − SA 2 + xjwI T M T − SA 2 + xjwI SB 3 M SB 2 − SA 2 + xjwI SC 3 M SC 2 − SA 2 + 1 − x I SA 3 R SA 2 + 1 − x jwI SA 3 L SA 2 + 1 − x jwI R M R − SA 2 − 1 − x jwI S 2 M S − SA 2 + 1 − x jwI T M T − SA 2 + − 1 − x jwI SB 4 M SB 2 − SA 2 + 1 − x jwI SC 3 M SC 2 − SA 2 − I SA 2 R SA 3 − jwI SA 3 L SA 3 + jwI R M R − SA 3 − jwI S 2 M S − SA 3 + jwI T M T − SA 3 + jwI SB 4 M SB 3 − SA 3 + jwI SC 3 M SC 3 − SA 3 V SA = V SB = V SC

[0055] Figure 7 schematically shows a second embodiment of the system of the invention, said system being applied on a high-voltage three-phase AC cable 1 having a second cable configuration.

[0056] The cable 1 according to the second configuration has a solid bonded connection system. The cable 1 comprises a first end 10, a second end 11, and a single main section MP 1 extending between said ends 10, 11. The cable 1 comprises one conductor R, S, T and one shield S R , S S , S T per phase. The shields S R , S S , S T are attached to one another and connected to ground G at both ends of the main section MP 1 .

[0057] In the first embodiment, the system comprises a first current measuring equipment configured for measuring the conductor current I R1 , I S1 , I T1 , I R2 , I S2 , I T2 circulating through each conductor R, S, T both at the first end 10 and at the second end 11 of the cable 1. The first current measuring equipment comprises six current sensors 50, 51, 52, 53, 54, 55, with one current sensor 50, 51, 52, 53, 54, 55 being arranged at each end of each conductor R, S, T.

[0058] In the first embodiment the system comprises a second current measuring equipment configured for measuring the shield current I SR1 , I SS1 , I ST1 ; I SR2 , I SS2 , I ST2 ; of each shield S R , S S , S T in the vicinity of the connection points to ground G of the shields S R , S S , S T . The second current measuring equipment comprises six current sensors 60, 61, 62, 63, 64, 65, with one current sensor 60, 61, 62, 63, 64, 65 being arranged in the vicinity of the connection points to ground G of each shield S R , S S , S T .

[0059] The system of the invention comprises a processor, not shown in the figures, configured for executing the method of the invention based on the current measurements performed by said current measuring equipment 50, 51, 52, 53, 54, 55, 60, 61, 62, 63, 64, 65.

[0060] A fault between the conductor S and the shield Ss has been depicted in Figure 7. In order to locate said fault point F, the method of the invention would be applied as follows: Measuring the conductor current I R1 , I S1 , I T1 , I R2 , I S2 , I T2 circulating through each conductor R, S, T both at the first end 10 and at the second end 11 of the cable 1 by means of the first current measuring equipment 5.

[0061] Measuring the shield current I SR1 , I SS1 , I ST1 ; I SR2 , I SS2 , I ST2 circulating through each shield S A , S B , Sc in the vicinity of the connection points to ground G of the shields S R , S S , S T .

[0062] Comparing the conductor current I R1 ; I S1 ; I T1 measured at the first end 10 of the cable 1 for each conductor R, S, T with the conductor current I R2 ; I S2 ; I T2 measured at the second end 11 of the cable 1. In this case, the conductor current I S1 of the conductor S at the first end 10 of the cable 1 will be different from the conductor current I S2 at the first end 11, such that it will be determined that conductor S is the conductor that is in fault.

[0063] Once the conductor S in fault has been determined, the model of the shields S R , S S , S T is defined.

[0064] The model of each shield S R , S S , S T comprises its own resistance R SS , R SR , R SC and its own self-inductance L SS , L SR , L ST of the corresponding shield S R , S S , S T , the mutual inductance Mss-S R , M ST-SR ; M SR-SS , M ST-SS ; M SR-ST , M SS-ST between said shield S R , S S , S T and the rest of the shields S R , S S , S T , and the mutual inductance M R-SR , M S-SR , M T-SR ; M R-SS , M S-SS , M T-SS ; M R-ST , M S-ST , M T-ST between said shield S R , S S , S T and the conductors R, S, T.

[0065] As previously discussed, the shields S R , S S , S T are attached and connected to ground at both ends of the main section MP 1 . For this reason, in the model of the shields S R , S S , S T the ends of the models corresponding to each shield S A , S B , Sc are attached to one another, such that three nets in which the sum of voltages is zero are formed.

[0066] Figure 8 shows the detailed model of the shields S R and Ss.

[0067] Therefore, the location of the fault point depicted in Figures 1 and 2 is calculated by means of the following equations: V SS = xI SS 1 R SS + xjwI SS 1 L SS + xjwI R M R − SS + xjwI S 1 M S − SS + xjwI T M T − SS + xjwI SR M SR − SS + xjwI ST M ST − SS − 1 − x I SS 2 R SS − 1 − x jwI SS 2 L SS + 1 − x jwI R M R − SS − 1 − x jwI S 2 M S − SS + 1 − x jwI T M T − SS + 1 − x jwI SR M SR − SS + 1 − x jwI ST M ST − SS V SR = xI SR R SR + xjwI SR L SR + xjwI R M R − SR + xjwI S 1 M S − SR + xjwI T M T − SR + xjwI SS 1 M SS − SR + xjwI ST M ST − SR − 1 − x I SR R SR − 1 − x jwI SR L SR + 1 − x jwI R M R − SR − 1 − x jwI S 2 M S − SR + 1 − x jwI T M T − SR − 1 − x jwI SS 2 M SS − SR + 1 − x jwI ST M ST − SR V SR = V SS = V ST

Claims

1. Method for locating a fault point (F) on a high-voltage three-phase AC cable (1) comprising a first end (10), a second end (11), and at least one main section (MP1, MP2, MP3) extending between said ends (10, 11), the cable (1) comprising one conductor (R, S, T) per phase and three shields (SA, SB, SC; SR, SS, ST) associated with said conductors (R, S, T), said shields (SA, SB, SC; SR, SS, ST) being attached to one another and connected to ground (G) at both ends of each main section (MP1, MP2, MP3), the method comprising the following steps: - measuring the conductor current (R1, IS1, IT1, IR2, IS2, IT2) circulating through each conductor (R, S, T) both at the first end (10) and at the second end (11) of the cable (1), - Measuring the shield current (ISA1, ISB1, ISC1, ISA2, ISB2, ISC2, ISA3, ISB3, ISC3, ISA4, ISB4, ISC4; ISR1, ISS1, IST1; ISR2, ISS2, IST2) in the vicinity of the connection points to ground (G) of the shields (SA, SB, SC), - determining the conductor (R, S, T) in fault based on the previously measured conductor currents (IR1, IS1, IT1, IR2, IS2, IT2), - in the event that the cable (1) comprises more than one main section (MP1, MP2, MP3), determining the main section (MP1, MP2, MP3) in fault based on the previously measured shield currents (ISA1, ISB1, ISC1, ISA2, ISB2, ISC2, ISA3, ISB3, ISC3, ISA4, ISB4, ISC4; ISR1, ISS1, IST1; ISR2, ISS2, IST2), and - locating the fault point (F) by means of a model of the shields (SA, SB, SC; SR, SS, ST) of the main section (MP1, MP2, MP3) in fault, taking into account that the shields (SA, SB, SC; SR, SS, ST) are attached to one another at the ends of said main part (MP1, MP2, MP3) in fault such that three closed nets in which the sum of voltages is zero are formed in the model, the only unknown being the distance (x) to the fault point (F).

2. Method according to claim 1, wherein for determining the conductor (R, S, T) in fault, the conductor current (IR1; IS1; IT1) measured at the first end (10) of the cable (1) for each conductor (R, S, T) is compared with the conductor current (IR2; IS2; IT2) measured at the second end (11) of the cable (1), determining that the conductor (R, S, T) in fault will be the conductor in which the conductor current (IR1, IR2; IS1, IS2; IT1, IT2) at said ends (10, 11) is different.

3. Method according to claim 1 or 2, wherein the cable (1) comprises at least two main sections (MP1, MP2, MP3), the main section (MP1, MP2, MP3) in fault being determined taking into account that at the ends of said main section (MP1, MP2, MP3) in fault the shield currents (ISA1, ISB1, ISC1; ISA2, ISB2, ISC2; ISA3, ISB3, ISC3; ISA4, ISB4, ISC4) are not in phase.

4. Method according to claim 3, wherein for determining the main section (MP1, MP2, MP3) in fault the shield currents (ISA1, ISB1, ISC1; ISA2, ISB2, ISC2; ISA3, ISB3, ISC3; ISA4, ISB4, ISC4) are successively analyzed starting from one of the ends (10, 11) of the cable (1) until observing that the shield currents (ISA1, ISB1, ISC1; ISA2, ISB2, ISC2; ISA3, ISB3, ISC3; ISA4, ISB4, ISC4) are not in phase, determining that the main section (MP1, MP2, MP3) in fault will be the main section (MP1, MP2, MP3) after the connection point to ground (G) in which it is detected that said shield currents (ISA1, ISB1, ISC1; ISA2, ISB2, ISC2; ISA3, ISB3, ISC3; ISA4, ISB4, ISC4) are not in phase.

5. Method according to any of claims 1 to 4, wherein the cable (1) has a cross bonding configuration, each main section (MP1, MP2, MP3) comprising three minor sections (mp1, mp2, mp3), the method comprising determining the minor section (mp1, mp2, mp3) in fault once the main section (MP1, MP2, MP3) in fault has been determined, wherein for determining the minor section (mp1, mp2, mp3) in fault, first the shield (SA, SB, SC) in fault is determined by comparing for each shield (SA, SB, SC) the shield current (ISA1, ISB1, ISC1; ISA2, ISB2, ISC2; ISA3, ISB3, ISC3; ISA4, ISB4, ISC4) measured at one of the ends of the main part (MP1, MP2, MP3) in fault with the shield current (ISA1, ISB1, ISC1; ISA2, ISB2, ISC2; ISA3, ISB3, ISC3; ISA4, ISB4, ISC4) measured at the other end of the main part (MP1, MP2, MP3) in fault, the shield (SA, SB, SC) in fault being the shield (SA, SB, SC) whose current (ISA1, ISB1, ISC1; ISA2, ISB2, ISC2; ISA3, ISB3, ISC3; ISA4, ISB4, ISC4) at both ends of the main section (MP1, MP2, MP3) in fault is different, and the minor section (mp1, mp2, mp3) in fault being the minor section (mp1, mp2, mp3) in which the conductor (R, S, T) in fault and the shield (SA, SB, SC) in fault coincide.

6. Method according to claim 5, wherein the model of the shields (SA, SB, SC) of the main section (MP1, MP2, MP3) in fault of each shield (SA, SB, SC) comprises per minor section (mp1, mp2, mp3) its own resistance (RSA, RSB, Rsc) and its own self-inductance (LSA, LSB, Lsc) of the corresponding shield (SA, SB, SC), the mutual inductance (MSB-SA, MPSC-SA; MSA-SB, MPSC-SB; MPSA-SC, MPSB-SC) between said shield (SA, SB, SC) and the rest of the shields (SA, SB, SC), and the mutual inductance (MR-SA, MS-SA, MT-SA; MR-SB, MS-SB, MPT-SC; MPR-SC, MPS-SC, MPT-SC;) between said shield (SA, SB, SC) and the conductors (R, S, T).

7. Method according to claim 6, wherein the location of the fault point (F) is calculated by means of the following equations: V SF = V SNF V SF = V SF _ mpf + V SF _ mpnf 1 + V SF _ mpnf 2 V SF mpf = xI SF 1 R SF mpf + xjwI SF 1 L SF mpf + xjwI CF 1 M CF − SF mpf + xjwI CNF 1 M CNF 1 − SF mpf + xjwI CNF 2 M CNF 2 − SF mpf + xjwI SNF 1 M SNF 1 − SF mpf + xjwI SNF 2 M SNF 2 − SF mpf − 1 − x I SF 2 R SF mpf − 1 − x jwI SF 2 L SF mpf − 1 − x jwI CF 2 M CF − SF mpf + 1 − X jwI CNF 1 M CNF 1 − SF mpf + 1 − X jwI CNF 2 M CNF 2 − SF mpf + 1 − X jwI SNF 1 M SNF 1 − SF mpf + 1 − X jwI SNF 2 M SNF 2 − SF mpf V SF mpnf = I SF R SF mpnf + jwI SF L SF mpnf + jwI CF M CF − SF mpnf + jwI CNF 1 M CNF 1 − SF mpnf + jwI CNF 2 M CNF 2 − SF mpnf + jwI SNF 1 M SNF 1 − SF mpnf + jwI SNF 2 M SNF 2 − SF mpnf V SNF = V SNF _ mpf + V SNF _ mpnf 1 + V SNF _ mpnf 2 V SNF mpf = xI SNF R SNF mpf + jxwI SNF L SNF mpf + xjwI CF 1 M CF − SNF mpf + xjwI CNF 1 M CNF 1 − SNF mpf + xjwI CNF 2 M CNF 2 − SNF mpf + xjwI SF 1 M SF − SNF mpf + xjwI SNF 2 M SNF 2 − SNF mpf + 1 − x I SNF R SNF mpf + 1 − x jwI SNF L SNF mpf − 1 − x jwI CF 2 M CF − SNF mpf + 1 − X jwI CNF 1 M CNF 1 − SNF mpf + 1 − X jwI CNF 2 M CNF 2 − SNF mpf − 1 − X jwI SF 2 M SF − SNF mpf + 1 − X jwI SNF 2 M SNF 2 − SNF mpf V SNF mpnf = I SNF R SNF mpnf + jwI SNF L SNF mpnf + jwI CF M CF − SNF mpnf + jwI CNF 1 M CNF 1 − SNF mpnf + jwI CNF 2 M CNF 2 − SNF mpnf + jwI SF M SF − SNF mpnf + jwI SNF 2 M SNF mpnf wherein SF= shield in fault, SNF= shield that is not in fault, SNF1= one of the shields that is not in fault, SNF2= the other shield that is not in fault, CNF1= one of the conductors that is not in fault, CNF2= the other conductor that is not in fault, mpf= the minor section in fault, mpnf= minor section which is not in fault, ISF1 = The current of the shield in fault on one side of the fault point, ISF2 = The current of the shield in fault on the other side of the fault point, ISF = the current circulating through the shield in fault in the section in fault, if the current flows from the first end to the second end, it will be considered positive; in contrast, if it flows from the second end to the first end, it will be considered negative, ICF = The current circulating through the conductor in fault. If the current flows from the first end to the second end, it will be considered positive; in contrast, if it flows from the second end to the first end, it will be considered negative, ICF1 = The current of the conductor in fault on one side of the fault point, and ICF2 = The current of the conductor in fault on the other side of the fault point.

8. Method according to claim 1 or 2, wherein the cable (1) has a solid bonded configuration such that it comprises a single main section (MP1), the model of the shields (SR, SS, ST) of each shield (SR, SS, ST) comprising its own resistance (RSR, RSS, RST) and self-inductance (LSR, LSS, LST), the mutual inductance (MSS-SR, MST-SR; MSR-SS, MST-SS; MSR-ST, MSS-ST) between said shield (SR, SS, ST) and the rest of the shields (SR, SS, ST), and the mutual inductance (MR-SR, MS-SR, MT-SR; MR-SS, MS-SS, MT-SS; MR-ST, MS-ST, MT-ST;) between said shield (SR, SS, ST) and the conductors (R, S, T).

9. Method according to claim 8, wherein the location of the fault point (F) is calculated by means of the following equations: V SF = V SNF V SF = xI SF 1 R SF + xjwI SF 1 L SF + xjwI CF 1 M CF − SF + xjwI CNF 1 M CNF 1 − SF + xjwI CNF 2 M CNF 2 − SF + xjwI SNF 1 M SNF 1 − SF + xjwI SNF 2 M SNF 2 − SF − 1 − x I SF 2 R SF − 1 − x jwI SF 2 L SF − 1 − x jwI CF 2 M CF − SF + 1 − x jwI CNF 1 M CNF 1 − SF + 1 − x jwI CNF 2 M CNF 2 − SF + 1 − x jwI SNF 1 M SNF 1 − SF + 1 − x jwI SNF 2 M SNF 2 − SF V SNF = xI SNF R SNF + xjwI SNF L SNF + xjwI CF 1 M CF − SNF + xjwI CNF 1 M CNF 1 − SNF + xjwI CNF 2 M CNF 2 − SNF + xjwI SF 1 M SF 1 − SNF + xjwI SNF 2 M SNF 2 − SNF + 1 − x I SNF R SNF + 1 − x jwI SNF L SNF − 1 − x jwI CF 2 M CF − SNF + 1 − x jwI CNF 1 M CNF 1 − SNF + 1 − x jwI CNF 2 M CNF 2 − SNF − 1 − x jwI SF 2 M SF − SNF + 1 − xjwI SNF 2 M SNF 2 − SNF wherein SF= shield in fault, SNF= shield that is not in fault, SNF1= one of the shields that is not in fault, SNF2= the other shield that is not in fault, CF= conductor in fault, CNF1= one of the conductors that is not in fault, CNF2= the other conductor that is not in fault, ISF1 = The current of the shield in fault on one side of the fault point, ISF2 = The current of the shield in fault on the other side of the fault point, ICF1 = The current of the conductor in fault on one side of the fault point, and ICF2 = The current of the conductor in fault on the other side of the fault point.

10. System for locating a fault point (F) in a high-voltage three-phase AC cable (1) comprising a first end (10), a second end (11), and at least one main section (MP1, MP2, MP3) extending between said ends (10, 11), the cable (1) comprising one conductor (R, S, T) per phase and three shields (SA, SB, SC; SR, SS, ST) associated with said conductors (R, S, T), said shields (SA, SB, SC; SR, SS, ST) being attached to one another and connected to ground (G) at both ends of each main section (MP1, MP2, MP3), the system comprising - a first current measuring equipment configured for measuring the current of conductor (IR1, IS1, IT1, IR2, IS2, IT2) circulating through each conductor (R, S, T) both at the first end (10) and at the second end (11) of the cable (1), - a second current measuring equipment configured for measuring the shield current (ISA1, ISB1, ISC1, ISA2, ISB2, ISC2, ISA3, ISB3, ISC3, ISA4, ISB4, ISC4; ISR1, ISS1, IST1; ISR2, ISS2, IST2) in the vicinity of the connection points to ground (G) of the shields (SA, SB, SC), and - a processor configured for executing the method according to any of claims 1 to 9 based on the current measurements performed by said current measuring equipment.