Patent Description:
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.

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

For example, <CIT> 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.

The document "<NPL>" discloses, in a high voltage <NUM>-phase AC cable system, the location of the section where a fault has occured by measuring and comparing the sheath currents in the relevant sections. The fault is simulated.

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.

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

The cable in which the method of the invention is applied comprises a first end, a second end, and at least one section extending between said ends, the cable comprising a conductor and a respective shield per phase. The cable comprises a single point connection system, i.e., the shields are connected to a surge arrester device grounded at one end of said at least one section, and the shields are connected to a grounding element at the other end of said at least one section. The cable comprises an earth continuity conductor establishing a common ground connecting the ground connections of the grounding element(s) and the surge arrester device(s).

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:.

The points at which the shields are connected to a surge arrester device are free for the purpose of conductivity, i.e., as if they were in an open circuit, such that it complicates the possibility of using a model of the shields to be able to calculate the fault point, since the voltage is unknown at said points. The voltage would have to be measured at said points to be able to use a model of the shields, and this may generate many problems. This problem is avoided by using the model of the earth continuity conductor since it is a model the ends of which are grounded.

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.

Even though a model of the earth continuity conductor 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 model in a similar manner, and therefore, the influence thereof is virtually cancelled.

Furthermore, the fault resistance, i.e., the resistance between the conductor in fault and the shield in fault at fault point F, does not need to be known to locate the fault point.

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.

A first aspect of the invention relates to a method for locating a fault point F on a high-voltage three-phase AC cable <NUM>.

The method of the invention is configured for locating a fault point F on a cable <NUM> extending between a first end <NUM> and a second end <NUM>, said cable <NUM> comprising at least one section P<NUM>, P<NUM> extending between said ends <NUM>, <NUM>. The cable <NUM> comprises one conductor R, S, T per phase and a shield SR, SS, ST associated with each conductor R, S, T.

As discussed above, fault point F is considered the point where one of the conductor R, S, T, of the cable <NUM> has electrical contact with the corresponding shield SR, SS, ST, such that a short circuit occurs between said conductor R, S, T and the corresponding shield at said fault point F. Furthermore, in the context of the invention, section in fault will be considered section P<NUM>, P<NUM> of cable <NUM> at which the fault point F is arranged.

The method of the invention is configured for being applied on cables <NUM> with single point connection systems, i.e., on cables <NUM> in which the shields SR, SS, ST are connected to a surge arrester device <NUM>, <NUM> connected to ground G at one end of each section P<NUM>, P<NUM>, shields SR, SS, ST being connected to a grounding element at the other end of each section P<NUM>, P<NUM>. Cables <NUM> of this type with single point connection systems comprise an earth continuity conductor ECC establishing a common ground G of the system connecting the ground connection(s) <NUM> of the grounding element(s) and the ground connection(s) <NUM>, <NUM> of the surge arrester device(s) <NUM>, <NUM> of the cable <NUM>.

The method of the invention comprises the following steps:.

Preferably, for determining the conductor R, S, T in fault, the conductor current IR1; IS1; IT1 measured at the first end <NUM> of the cable <NUM> for each conductor R, S, T is compared with the conductor current IR2; IS2; IT2 measured at the second end <NUM> of the cable <NUM>, 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 <NUM>, <NUM> is different.

Preferably, for determining section P<NUM>, P<NUM> in fault, the shield currents ISR1, ISS1, IST1; ISR2, ISS2, IST2, of each section P1, P2 are analyzed, determining that section P<NUM>, P<NUM> in fault will be the conductor in which one of the shield currents ISR1, ISS1, IST1; ISR2, ISS2, IST2 has an overcurrent value. The shield current ISR1, ISS1, IST1; ISR2, ISS2, IST2 circulating through the shields SR, SS, ST when there is no fault is a low-value eddy current, for example a current of up to 10A. However, when a fault occurs, an overcurrent that is much higher than the eddy current, for example a current higher than <NUM>,000A, circulates through the corresponding shield SR, Ss, ST. Therefore, when a fault occurs, simply looking at the shield currents ISR1, ISS1, IST1; ISR2, ISS2, IST2 of each section P<NUM>, P<NUM> is sufficient to determine which of the sections P<NUM>, P<NUM> is in fault. It should be pointed out that since each section P<NUM>, P<NUM> is connected at one of its ends to ground G, the overcurrent circulating through the shield SR, SS, ST in fault circulates towards ground G such that the rest of the sections P<NUM>, P<NUM> are not affected by said fault.

Preferably, the model of the earth continuity conductor ECC of section P<NUM>, P<NUM> in fault takes into account the self-resistance RECC and the self-inductance LECC of the earth continuity conductor ECC, the ground resistance Rg of the ground connections <NUM>, <NUM>, <NUM> of the grounding element and the corresponding surge arrester device <NUM>, <NUM>, the mutual inductance MR, MS, MT between said earth continuity conductor ECC and each of the conductors R, S, T, and the mutual inductance MSR, MSS, MST between said earth continuity conductor ECC and the shield in fault SR, SS, ST from the fault point F to the point where the shields are connected to ground G.

When defining the model of the earth continuity conductor ECC, values relevant for the purpose of current have been taken into account, and capacitive inputs have been disregarded since they are of little value in fault situations.

Furthermore, mutual inductance between the shields SR, SS, ST which are not in fault and earth continuity conductor ECC have been ignored for defining the model since they input scarcely any coupling on the cable ECC and are therefore negligible values, and the same occurs with the shield in fault to fault point F.

Preferably, the location of the fault point is calculated using the following formula: <MAT> wherein,.

Preferably when the cable <NUM> comprises more than one section P<NUM>, P<NUM>, ground resistance Rg is determined by means of a model of the earth continuity conductor ECC of a section P<NUM>, P<NUM> which is not in fault, said model comprising the self-resistance RECC and the self-inductance LECC of the earth continuity conductor ECC, the ground resistance Rg of the ground connections <NUM>, <NUM>, <NUM> of the grounding element and the corresponding surge arrester device <NUM>, <NUM>, the mutual inductance MR, MS, MT between said earth continuity conductor ECC and each of the conductors R, S, T, the only unknown being said ground resistance Rg.

Preferably, ground resistance Rg is calculated by means of the following formula: <MAT> wherein,.

Conversely, when the cable <NUM> comprises a single section P<NUM>, preferably the ground resistance Rg of the ground connections <NUM>, <NUM> of the grounding element and the surge arrester device <NUM> is estimated, for example, taking into account parameters such as: ground humidity, ground temperature, etc..

A second aspect of the invention relates to a system for locating a fault point F on a high-voltage three-phase AC cable <NUM>.

The system of the invention is configured for locating a fault point F on a cable <NUM> extending between a first end <NUM> and a second end <NUM>, comprising said cable <NUM> at least one section P<NUM>, P<NUM> extending between said ends <NUM>, <NUM>. The cable <NUM> comprises one conductor R, S, T per phase and a shield SR, SS, ST associated with each conductor R, S, T.

The system of the invention is configured for being applied on cables <NUM> with single point connection systems, i.e., on cables <NUM> in which the shields SR, SS, ST are connected to a surge arrester device <NUM>, <NUM> connected to ground G at one end of each section P<NUM>, P<NUM>, being the shields SR, SS, ST connected to a grounding element at the other end of each section P<NUM>, P<NUM>. Cables <NUM> of this type with single point connection systems comprise an earth continuity conductor ECC establishing a common ground G of the system connecting the ground connection(s) <NUM> of the grounding element(s) and the ground connection(s) <NUM>, <NUM> of the surge arrester device(s) <NUM>, <NUM> of the cable <NUM>.

The system of the invention comprises a first current measuring equipment <NUM> configured for measuring the conductor current IR1, IS1, IT1, IR2, IS2, IT2 circulating through each conductor R, S, T both at the first end <NUM> and at the second end <NUM> of the cable <NUM>.

The system of the invention also comprises a second current measuring equipment <NUM> configured for measuring the shield current ISR1, ISS1, IST1, ISR2, ISS2, IST2 of each shield SR, SS, ST in each section P<NUM>, P<NUM> at a point close to the grounding element <NUM>.

Furthermore, the system of the invention also comprises a third current measuring equipment <NUM> configured for measuring the earth continuity conductor current IECC1, IECC2 of the earth continuity conductor ECC in each section P<NUM>, P<NUM> at a point close to the grounding element <NUM>.

The system of the invention comprises 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 <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

<FIG> schematically show a first embodiment of the system of the invention, said system being applied on a high-voltage three-phase AC cable <NUM> having a first cable configuration.

The cable <NUM> according to the first configuration comprises one conductor R, S, T per phase and a shield SR, SS, ST associated with each conductor R, S, T and it extends between a first end <NUM> and a second end <NUM>.

Furthermore, the cable <NUM> according to the first configuration has a single point connection system, the connection to ground G of the shields SR, SS, ST being performed at an intermediate point <NUM> by means of a grounding element, with the cable <NUM> being divided into two sections P1, P2. Furthermore, the shields SR, SS, ST are connected to a surge arrester device <NUM>, <NUM> connected to ground G both at the first end <NUM> and at the second end <NUM> of the cable <NUM>.

The cable <NUM> according to the first configuration also comprises an earth continuity conductor ECC connecting the ground connection <NUM> of the grounding element and the ground connections <NUM>, <NUM> of the devices surge arrester <NUM>, <NUM> establishing a common ground G of the system.

In the first embodiment, the system comprises a first current measuring equipment <NUM> configured for measuring the conductor current IR1, IS1, IT1, IR2, IS2, IT2 circulating through each conductor R, S, T both at the first end <NUM> and at the second end <NUM> of the cable <NUM>. As shown in <FIG>, the first current measuring equipment <NUM> comprises six current sensors <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, with one current sensor <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> being arranged at each end of each conductor R, S, T.

In the first embodiment, the system comprises a second current measuring equipment <NUM> configured for measuring the shield current ISR1, ISS1, IST1, ISR2, ISS2, IST2 of each shield SR, SS, ST in each section P<NUM>, P<NUM> at a point close to the grounding element. As shown in <FIG>, the second current measuring equipment <NUM> comprises six current sensors <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, with one current sensor <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> being arranged in each shield SR, SS, ST in each section P<NUM>, P<NUM> at a point close to the grounding element.

Furthermore, in the first embodiment the system comprises a third current measuring equipment <NUM> configured for measuring the earth continuity conductor current IECC1, IECC2 of the earth continuity conductor ECC in each section P<NUM>, P<NUM> at a point close to the grounding element. As shown in <FIG>, the third current measuring equipment <NUM> comprises two current sensors <NUM>, <NUM>, with one of the current sensors <NUM> being arranged in section P<NUM> of the earth continuity conductor ECC at a point close to the grounding element, with the other current sensor <NUM> being arranged in section P<NUM> of the earth continuity conductor ECC at a point close to the grounding element.

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 <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

A fault between the conductor T and its shield ST in the first section P<NUM> has been depicted in <FIG>. In order to locate said fault point F, the method of the invention would be applied as follows:
Measuring the conductor current IR1, IS1, IT1, IR2, IS2, IT2 circulating through each conductor R, S, T both at the first end <NUM> and at the second end <NUM> of the cable <NUM> by means of the first current measuring equipment <NUM>.

Also, measuring the shield current ISR1, ISS1, IST1, ISR2, ISS2, IST2 circulating through each shield SR, SS, ST in each section P<NUM>, P<NUM> at a point close to the grounding element by means of the second current measuring equipment <NUM>. Also, measuring the earth continuity conductor current IECC circulating through each section P<NUM>, P<NUM> at a point close to the grounding element by means of the third current measuring equipment.

Comparing the conductor current IR1; IS1; IT1 measured at the first end <NUM> of the cable <NUM> for each conductor R, S, T with the conductor current IR2; IS2; IT2 measured at the second end <NUM> of the cable <NUM>. In this case, the conductor current IR1 of the conductor T at the first end <NUM> of the cable <NUM> will be different from the conductor current IR2 at the first end <NUM>, such that it will be determined that conductor T is the conductor which is in fault.

Also, analyzing the shield currents ISR1, ISS1, IST1; ISR2, ISS2, IST2 of each section P<NUM>, P<NUM>. In this case, the shield current IST1 will be observed as having an overcurrent value, determining that section P<NUM> is the section which is in fault.

Once conductor T in fault and section P<NUM> in fault have been determined, defining the model of the earth conductor ECC shown in <FIG> and <FIG>.

The net representing section P<NUM> which is not in fault comprises the self-resistance RECC and the self-inductance LECC of the earth continuity conductor ECC, the ground resistance Rgm, Rg2 of the ground connections <NUM>, <NUM> of the grounding element and the surge arrester device <NUM>, and the mutual inductance MR, MS, MT between said earth continuity conductor ECC and each of the conductors R, S, T. Therefore, all the parameters of the net of section P<NUM> which is not in fault can be calculated by knowing the characteristics of the cable <NUM>, the conductor currents IR2, IS2, IT2 circulating through the conductor R, S, T at the second end <NUM> of the cable <NUM>, and the earth continuity conductor currents ECC IECC1, IECC2 circulating through both sections P<NUM>, P<NUM>, the only unknown being the ground resistances Rgm, Rg2. All the ground resistances Rg, Rg2, Rgm, Rg2 of the model of the earth conductor ECC are considered equal.

Therefore, in this case ground resistance Rg is calculated using the following formula: <MAT> wherein,.

The net representing section P<NUM> in fault comprises the self-resistance RECC and the self-inductance LECC of the earth continuity conductor ECC, the ground resistance Rg of the ground connection <NUM> of the grounding element and of the ground connection <NUM> of the surge arrester device <NUM>, the mutual inductance MR, MS, MT between said earth continuity conductor ECC and each of the conductors R, S, T, and the mutual inductance MST between said earth continuity conductor ECC and the shield ST in fault from the fault point F to the point where the shields are connected to ground G. Taking into account that the ground resistance Rg is obtained from the net of section P<NUM> which is not in fault, all the parameters of the net of section P<NUM> in fault can be calculated by knowing the characteristics of the cable <NUM>, the conductor currents IR1, IS1, IT1 circulating through the conductor R, S, T at the first end <NUM> of the cable <NUM>, the shield current IST1 circulating through the shield ST in fault in section P<NUM> in fault and the currents of earth continuity conductor ECC IECC1, IECC2 circulating through both sections P<NUM>, P<NUM>, the only unknown being the distance x to the fault point F.

Therefore, the location of the fault point is calculated using the following formula: <MAT> wherein,.

<FIG> schematically shows a second embodiment of the system of the invention, said system being applied on a high-voltage three-phase AC cable <NUM> having a second cable configuration.

The cable <NUM> according to the first configuration comprises one conductor R, S, T per phase and a shield SR, SS, ST associated with each conductor R, S, T and extends between a first end <NUM> and a second end <NUM>.

Furthermore, the cable <NUM> according to the first configuration has a single point connection system, the connection to ground G of the shields SR, SS, ST being performed at the first end <NUM> of the cable <NUM>, with shields SR, SS, ST being connected to a surge arrester device <NUM> connected to ground G at the second end <NUM> of the cable <NUM>. The cable <NUM> thereby comprises a single section P<NUM>.

The cable <NUM> according to the first configuration also comprises an earth continuity conductor ECC connecting the ground connection <NUM> of the grounding element and the connection <NUM> of the surge arrester device <NUM> establishing a common ground G of the system.

In the second embodiment, the system comprises a first current measuring equipment <NUM> configured for measuring the conductor current IR1, IS1, IT1, IR2, IS2, IT2 circulating through each conductor R, S, T both at the first end <NUM> and at the second end <NUM> of the cable <NUM>. The first current measuring equipment <NUM> comprises six current sensors <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, with one current sensor <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> being arranged at each end of each conductor R, S, T.

In the second embodiment, the system comprises a second current measuring equipment <NUM> configured for measuring the shield current ISR1, ISS1, IST1 of each shield SR, SS, ST at a point close to the grounding element <NUM>. The second current measuring equipment <NUM> comprises three current sensors <NUM>, <NUM>, <NUM>, with one current sensor <NUM>, <NUM>, <NUM> being arranged in each shield SR, SS, ST at a point close to the grounding element <NUM>.

Furthermore, in the second embodiment the system comprises a third current measuring equipment <NUM> configured for measuring the earth continuity conductor current IECC1 of the earth continuity conductor ECC at a point close to the grounding element <NUM>. The third current measuring equipment <NUM> comprises a single current sensor <NUM> arranged at a point close to the grounding element <NUM>.

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 <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>.

Claim 1:
Method for locating a fault point (F) on a high-voltage three-phase AC cable (<NUM>) comprising a first end (<NUM>), a second end (<NUM>), and at least one section (P<NUM>, P<NUM>) extending between said ends (<NUM>, <NUM>), the cable (<NUM>) comprising a conductor (R, S, T) and a respective shield (SR, SS, ST) per phase, shields (SR, SS, ST) being connected to a surge arrester device (<NUM>, <NUM>) connected to ground (G) at one end of said at least one section (P<NUM>, P<NUM>), and shields (SR, SS, ST) being connected to a grounding element (<NUM>) at the other end of said at least one section (P1, P2), the cable (<NUM>) comprising an earth continuity conductor (ECC) establishing a common ground (G) connecting the ground connections (<NUM>, <NUM>, <NUM>) of the grounding element(s) and the surge arrester device(s) (<NUM>, <NUM>), the method comprising the following steps:
- measuring the conductor current (IR1, IS1, IT1, IR2, IS2, IT2) circulating through each conductor (R, S, T) both at the first end (<NUM>) and at the second end (<NUM>) of the cable (<NUM>),
- measuring the shield current (ISR1, ISS1, IST1, ISR2, ISS2, IST2) circulating through each shield (SR, SS, ST) in each section (P<NUM>, P<NUM>) at a point close to the grounding element,
- measuring the earth continuity conductor current (IECC1, IECC2) circulating through each section (P<NUM>, P<NUM>) at a point close to the grounding element,
- 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 (<NUM>) comprises more than one section (P<NUM>, P<NUM>), determining section (P<NUM>, P<NUM>) in fault based on the previously measured shield currents (ISR1, ISS1, IST1, ISR2, ISS2, IST2), and
- locating the fault point (F) by means of a model of section (P<NUM>, P<NUM>) in fault of the earth continuity conductor (ECC), the only unknown being the distance (x) to the fault point (F) from an end of said section (P<NUM>, P<NUM>) in fault.