AU2012203278A1

AU2012203278A1 - Directional detection of a resistive earth fault and of breaking of a medium-voltage conductor

Info

Publication number: AU2012203278A1
Application number: AU2012203278A
Authority: AU
Inventors: Nathalie Baumes; Stephane Sinistro; Guillaume Vereneau
Original assignee: Schneider Electric Industries SAS
Current assignee: Schneider Electric Industries SAS
Priority date: 2011-06-07
Filing date: 2012-06-04
Publication date: 2013-01-10
Anticipated expiration: 2032-06-04
Also published as: FR2976363B1; AU2012203278B2; EP2533060A1; RU2583452C2; RU2012123514A; CN102818969A; PL2533060T3; FR2976363A1; CN102818969B; ES2511644T3; EP2533060B1

Abstract

DIRECTIONAL DETECTION OF A RESISTIVE EARTH FAULT AND OF BREAKING OF A MEDIUM-VOLTAGE CONDUCTOR To detect a resistive earth fault and/or breakage of a conductor on a medium-voltage power system, information relative to the voltages in the low-voltage system are used. By means of the reverse voltages, amplitudes, etc., the invention makes it possible to perform diagnosis of the fault and to locate the fault with respect to the performed measurements. (FIGURE 4) 15 15 c Fig. I

Description

Pool Section 29 Regulation 3.2(2) AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Application Number: Lodged: Invention Title: Directional detection of a resistive earth fault and of breaking of a medium-voltage conductor The following statement is a full description of this invention, including the best method of performing it known to us: P1 11ABAU/0610 1 DIRECTIONAL DETECTION OF A RESISTIVE EARTH FAULT AND OF BREAKING OF A MEDIUM-VOLTAGE CONDUCTOR 5 TECHNICAL FIELD The invention relates to fault detection on an electric power distribution grid, in particular a medium-voltage grid. In particular, the invention proposes a principle for detection of resistive faults between a medium-voltage electric conductor and earth, a fault caused for 10 example by breaking of said conductor, as well as to an appropriate device. STATE OF THE ART As illustrated in figure 1, power distribution grids 1 can be broken down into different 15 levels, with a first very high-voltage and high-voltage VHV/HV transportation and sub transmission grid 2 (from 35 to more than 200 kV) used for transporting or distributing electric power from electricity production plants over large distances. A medium-voltage MV distribution grid 5, usually between 1 and 35 kV, more precisely 11 kV line-to-neutral voltage in France, takes over for transportation on a smaller scale to customers of 20 industrialist type or sub-stations 10, 20, 30 transforming the medium voltage into low voltage LV (in particular 0.4 kV in France). The low voltage 15, 25, 35 supplies clients having low power demands via three phase conductors 15A, 15B, 15c, and a neutral conductor 1 5 N. 25 The MV power grid 5 can be composed of overhead lines and/or underground cables. Whatever the solution, the grid 5 is subject to various faults which it is important to detect and locate in order to palliate the problems arising therefrom: power outage, impaired strength of insulation materials, or safety of persons. Among these faults 7, the most frequent are single-phase faults located outside the source substation, in which a phase is in 30 contact with the ground, or breakage of an overhead cable in particular in case of inclement weather conditions.

2 These faults 7, just like multiphase faults which concern several phases, are of short-circuit type and are at the origin of high currents which may reach several thousands or tens of thousands of amps, whereas the conductors and/or equipment are generally designed to withstand a few hundred amps in nominal operation. For example, when the transformer 5 neutral N is directly earthed, the fault current corresponds to the voltage of the grid 5 divided by the sum of the resistances of the circuit, which is very low. One option to detect this type of fault is to measure the current that is flowing or parameters that are relative thereto. These measurements do however have to be completed 10 by measurements on the three-phase voltages if the relative direction (line-side or load side) with respect to the fault detection equipment is to be given. However, the MV voltage of the grid 5 gives rise to a complication in access to the measuring points, and problems of insulation of the electronic equipment: this type of directional detection is difficult to implement. 15 Furthermore, detection of the short-circuit current itself can become complex. In particular, grounding of the MV grid is henceforth generally performed by an impedance: a limiting element of resistance or compensation coil type is inserted between the secondary of the neutral point N of the transformer 3 and earth in order to increase the global impedance of 20 the fault current flow path, and to therefore reduce the current. This alleviates the stress on the components of the grid 5, and also provides protection of persons. Finer current measurements (sensitive earth fault) then have to be made. Other elements are moreover liable to limit the value of the fault current and complicate 25 detection: the resistance of the ground is a factor to be taken into account, as is the nature of the fault 7. However, although the parameters of the ground can be taken into account by adjusting the settings of the protection and detection equipment at the time installation is performed, the same is not the case for the characteristics of the fault, which are not predictable. Among the faults that are the most difficult to detect, there is in particular 30 breaking of an MV conductor, which can take place with or without contact with the ground.

3 For these very resistive faults, detection via the voltage is therefore implemented. For example, the document EP 1,603,211 concerns a communicating equipment unit fitted at the ends of the lines. Detection of conductor breakage is performed by simple detection of line voltage loss. More theoretical studies indicate the possibility of using the reverse 5 voltage and/or the zero sequence voltage on the MV grid 5. However, the problems inherent in measurement of the voltage on previously mentioned medium-voltage phase conductors remain totally unresolved. It is thus apparent that detection and location of resistive faults on a medium-voltage grid, 10 in particular in the case of breakage of conductors, has hardly been developed on account of the use of measurements that are complex to implement, in particular the voltage taps on the medium-voltage conductors and precise measurements of the short-circuit flowing in the latter. 15 SUMMARY OF THE INVENTION Among other advantages, the object of the invention is to palliate some shortcomings of existing detection systems and to propose preferably directional detection of resistive earth faults in medium voltage, with a specific focus on broken conductors. In particular, the 20 invention proposes the use of measurements on the low-voltage power distribution system, load-side from the MV/LV substation, to detect and identify a fault on the medium-voltage side. According to one feature, the invention relates to a method for detecting resistive earth 25 faults, in particular faults due to breakage of a conductor, on a medium-voltage power grid supplying a plurality of low-voltage feeders. The method comprises determination, for each feeder, of the reverse voltage corresponding to the symmetric component of the phase voltages, comparison of the amplitude of these reverse voltages with a threshold which is advantageously adjustable, and indication of the occurrence of a fault when a threshold 30 overshoot occurs at least once. According to another feature, the detection method is associated with relative location of the detected fault. For this purpose, the phase voltages are processed and the result of the 4 processing enables it to be determined whether the detected fault is line-side or load-side from the measuring point. In particular, the standard or the amplitude of the phase voltages of each feeder is determined, as are their mean value and their minimum value for each feeder. The mean value is compared with the amplitudes, and the minimum value is 5 compared with a threshold. According to another feature, the invention relates to a device suitable for the detection method, and preferably to the above-mentioned directional detection method, suitable for a medium-voltage power grid that comprises at least one medium-voltage / low-voltage 10 transformer substation defining a feeder providing a plurality of low-voltage phase conductors. According to one feature of the invention the above-mentioned device is associated in a system to enable the medium-voltage power grid to be monitored. For this purpose, the 15 device receives signals representative of the voltages of each conductor for each feeder and is able to process them, or a plurality of devices are implemented at the level of each feeder, or any other combination with for example a device designed to receive the information relative to a branch of the power grid. The system further comprises means for indicating occurrence of the fault, and preferably the relative location, line-side or load 20 side, of the latter with respect to the points for obtaining the signals received by the system. In particular, the device for detecting a resistive earth fault comprises means for receiving signals representative of the voltages of each conductor of a low-voltage feeder, means for obtaining signals representative of the phase voltages from the received voltage signals, 25 means for determining the symmetric component of the reverse voltage of the feeder, means for comparing the reverse voltage with a tripping threshold, and advantageously means for adjusting the tripping threshold. The detection device is advantageously fitted on each of the supplied low-voltage feeders, in particular located in the transformer substation, and the system formed in this way comprises means for indicating the 30 occurrence of a fault in the medium-voltage power grid when the comparison means of one of the detection devices give a result in which the module of the reverse voltage exceeds the tripping threshold.

5 Alternatively, the system for detecting a resistive fault in a medium-voltage power grid according to the invention comprises at least one detection device having means which are able to receive signals representative of the voltages of each conductor of each low-voltage feeder, to obtain signals representative of the phase voltages for each feeder, to determine 5 the symmetric component of the reverse voltage from the latter, and to compare the module of each reverse voltage with the tripping threshold, as well as means for indicating the occurrence of a fault in the medium-voltage power grid when the comparison means give a result in which a reverse voltage exceeds the tripping threshold. 10 In the case where the neutral of the medium-voltage / low-voltage transformer substations is earthed, the system according to the invention further comprises voltage sensors on each phase conductor and on the neutral conductor. The device according to the invention is advantageously designed for directional detection 15 of a resistive fault, further comprising processing means of the signals representative of the phase voltages, and means for interpreting the results of processing of the signals to determine whether the fault is line-side or load-side from the measuring point of the signals received by said device. In particular, the processing means comprise means for computing the norm of the phase voltages, means for computing the mean of the norms, means for 20 determining the minimum of the norms, first means for comparing the norm of the phase voltages with their mean, and second means for comparing the minimum of the norms with a threshold. BRIEF DESCRIPTION OF THE DRAWINGS 25 Other advantages and features will become more clearly apparent from the following description of particular embodiments of the invention, given for illustrative and in no way restrictive example purposes only, represented in the appended figures. 30 Figure 1 illustrates a power grid on which detection according to the invention is used. Figures 2A and 2B show consequences of different faults on the voltages of the LV lines.

6 Figure 3 schematically represents the steps of a method according to an embodiment of the invention. Figure 4 represents a directional detection device according to a preferred embodiment of 5 the invention. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT When a resistive earth fault 7 occurs in the MV power grid 5, the voltage flowing in the 10 latter is disturbed at least in the involved phase, and these disturbances can be identified at the level of the LV power systems 15, 25, 35 distributed by the MV power grid 5. In the multiphase power system 1 previously described in figure 1, detection of a fault according to the invention is implemented by means of directional detection devices 100, 200, 300 at the level of each LV feeder. 15 In the illustrated preferred embodiment and preferred use of the invention, the device 100 is associated with each transformer 10, which comprises three phase conductors 15A, 1 5 B, 1 5 c and a neutral conductor 15N. It is however possible to depart from this ideal situation, and the power system can comprise another number of phases, and in particular the neutral 20 can be compensated. In case of a resistive fault 7 on the MV power grid 5, three options present themselves directional detection devices 100 are associated with healthy feeders 15, devices 200 are situated line-side from the fault 7, and devices 300 are located load-side from the fault 7. 25 The fault 7 creates an evident disturbance on the voltage V of the phases that it affects load-side from said fault. However, in certain cases, other phases may be impaired, as may be the line-side LV power systems, or even the healthy feeders 15. In particular, faults in the case of the three-phase overhead power systems are illustrated in 30 figures 2 - the same graphic representation has been kept, with the line-side of the fault located to the left in the diagram. Six types of faults can be identified (the situation of three broken conductors is discarded, as no information of electric nature is then available, and only simultaneous faults are considered): 7 i) breaking at the level of a jumper, an element that enables the insulators to be passed on each side of the post which supports the lines (i.e. the fault 7 is of infinite resistance with no earth conductor); ii) breaking of a single conductor with line-side earthing; 5 iii) breaking of a single conductor with load-side earthing; iv) breaking of two conductors, one being earthed on the line-side and the other on the load-side; v) breaking of two conductors with load-side earthing; vi) breaking of two conductors with line-side earthing. 10 In the other cases, for example an earthed conductor and a broken jumper, one of the two phenomena in fact precedes the other and will have been detected beforehand. In the first four cases i), ii), iii) and iv), as illustrated in figure 2A, only the voltage V load side of the fault 7 is affected. One of the phases remains invariant and the other two phase 15 voltages see their amplitude decrease. In case v) of breaking of two conductors with in addition load-side earthing (figure 2B), only the load-side voltage is affected, with a decrease of the voltage of each phase, one of them even being annulled. On the other hand, in case vi) of breaking of two conductors with line-side earthing, there are also modifications of the line-side voltage 25, and even on the healthy feeder 15, with complete 20 load-side annulation 35. For each of the measuring points, it is possible to compute the symmetric components of the voltage (Vd forward voltage, Vi reverse voltage and V, zero sequence voltage for the phase voltages VAN, VBN, VCN) via a Fortescue matrix: V Ia a2 VAN 2n 25V -1 a2 a iB- wt ~ T 33 V. 1 1 1 _VcN_ In all the above-mentioned fault cases, it can be noted that the module of the reverse voltage Vi exceeds a threshold Sd that can be fixed between 4 and 20% of the rated current, preferably 12%, at one of the measuring points 100, 200, 300 at least. It is thus possible to detect the presence of a fault by computing this symmetric component. In the case of a 30 power system with n phases, the above formulas, in particular that of the Fortescue matrix, have to be adjusted by replacing the figure 3 by n.

8 The detected fault is load-side from the measuring point with the exception of a situation vi) where only the line-side feeders 25 and healthy feeders 15 indicate its presence. To locate the fault 7 relatively with respect to the measuring point 100, 200, 300, it is 5 therefore important to identify the last case vi) in which Vi 300 < Sd. It can be ascertained that, for the first four cases i), ii), iii), iv) only and not in the problematic case vi), for the load-side measuring point of the fault, the mean R between the amplitudes of the phase voltages is greater than two nominal values ||VXN |. Furthermore, to discriminate the remaining two cases v) and vi) in which the mean between the amplitudes of the phase 10 voltages is lower than two nominal values whether it be line-side or load-side of the fault 7, it can be ascertained that the measuring point load-side from the fault indicates that at least one of the phase voltages is annulled, i.e. that the minimum Vmin of the phase voltage amplitudes ||VXN is lower than a threshold Sv. It is therefore possible, only by knowing the phase voltages, to determine whether the detected fault is line-side or load-side from 15 the measurement. The detection and location principle according to the invention is thus based on measurement in the distributed LV power systems 15, 25, 35 of the phase voltages solely to detect a resistive fault of conductor break type via the reverse voltage, and to then 20 relatively locate the fault via the amplitudes. In a first step, as illustrated in figure 3, the voltage VA, VB, Vc of each phase is measured on the LV power system 15, 25, 35, as is the voltage of the neutral VN. The reverse symmetric component Vi is determined by the suitable formula. If the reverse voltage Vi does not exceed a tripping threshold Sd, no fault is detected in the power system directly supplying the feeder, and measurement is resumed 25 at defined intervals for continuous monitoring. When the reverse voltage Vi exceeds the tripping threshold Sd, a resistive fault is identified and the second step of the method, i.e. directional location, can be implemented. Simple detection of the occurrence of a fault can be notified to the user by lighting of a diode, or 30 another warning system. To locate the fault 7 line-side or load-side of the measuring point of the voltages VXN, the amplitudes ||VXN of the voltages, for example the root mean square value or other, are 9 determined for each phase X and their arithmetic mean pt is computed. If at least two values of norms ||VXN exceed the mean p, then the fault 7 is line-side from the measuring point 300. If not, the minimum value Vmin of the phase voltage amplitudes |1 VXN| is calculated, and if this minimum value Vmin exceeds a threshold Sv set for 5 example at 5% of the nominal value of the power system, a fault 7 is line-side from the measuring point 300, if not it is load-side. Locating the fault 7 load-side from the measuring point does in fact concern both a healthy feeder 15 and a transformer 20 situated in the faulty line. It is however then easy to identify 10 the branch of the MV power grid 5 that is at fault, given that it is the only feeder for which at least one installed detector 300 does not indicate a fault. A fault is in fact located L between two measuring points, one complying with all the above criteria and the other not complying with the latter. 15 The result D, L can be indicated either directly at the level of the transformer substations, by lighting of diodes, in particular of different colour, or via a centralized system of remote type on a functional diagram of the power grid 1, for example by remote indication to a control centre. 20 In conclusion, by means of this indicating principle, it is possible to locate the fault 7 which, in all cases and whatever the situation, is located between a "lit" detector and an "unlit" detector. Detection of a resistive earth fault, of breaking of an MV conductor, is performed without current sensors (and therefore without a low detection limit), by measurements of voltage installed in LV, which, in addition to ease of installation, provide 25 a better precision of measurement. Detection means 50 of voltage sensor type are therefore fitted on each LV feeder, in particular at the LV level of the transformers 10, 20, 30 supplied by the MV power grid 5. They are associated with a monitoring system of the MV power grid, comprising 30 directional detection devices 100, 200, 300 enabling the information from each feeder to be processed, said devices 100, 200, 300 being able to be integrated in each transformer 10, 20, 30 or form part of a central system to which the information from each sensor 50 is transmitted. The data from these directional detection devices 100, 200, 300 will enable the 10 relative location of the fault to be identified, between two devices. In a preferred embodiment of a device suitable for implementation of the method according to the invention is illustrated in figure 4. 5 The directional detection device according to the invention 100 comprises means 110 for obtaining signals representative of the phase voltages VA, VB, VC, VN provided by the sensors 50 and received by suitable means of the device 100. In the preferred embodiment, the signals, advantageously filtered by suitable means 112 such as an analog filter, VAf, VBf, VCf, VNf, are in addition conditioned and the means for obtaining representative 10 signals 110 comprise a sampling module 114, operating in particular at more than 1 kHz, thereby providing filtered sampled signals VAf*, VBf*, Vcr* to means for computing complex phasors 116 enabling the phase voltages VAN, VBN, VCN to be obtained. It should be noted that if the neutral of the LV power system is not distributed, using the measurements of the voltages between phases UAB, UBC, UCA enables an artificial neutral 15 to be reconstituted by delta-star transformation. The signals VXN originating from the means for obtaining 110 are transmitted to a detection unit 120 of a fault 7, which in particular comprises means 122 for determining at least one Vi of the symmetric components of the voltage, in particular by means of a 20 Fortescue matrix. The reverse voltage module |Vi l is determined by the suitable means 124. The detection unit 120 comprises means 126 for comparing the reverse voltage module |Vi l with a detection threshold Sd - if the threshold is exceeded, then a fault D is detected and a processing unit 130 is activated. Advantageously, means 128 are provided for adjusting the tripping threshold Sd, in particular between 4 and 20 % of the rated value 25 in France. The processing unit 130 receives the signals VAN, VBN, VCN from the means for obtaining 110. It comprises means for computing 132 their norm (or amplitude, rms value or other) 11 VAN 1, VBN 11, VCN 1. The value obtained is transmitted to first means for comparing 30 134 the minimum of the norm Vmin with a threshold Sv. In parallel manner, the norms obtained 11 VAN 11, VBN 1, VCN are successively transmitted to means for computing 136 the arithmetic mean [t of the three input data, and to second means for comparing 138, which comprise a fourth input corresponding to the computed mean p. The means for 11 comparing 132, 138 are connected to interpretation means 140 the output of which is a directional detection signal L of an earth fault load-side or line-side from the sensors 50 according to the result of the interpretation. 5 The second means for comparing 138 compare each of the norm values 1 VXN with their mean p and advantageously provide a binary signal according to the direction of comparison to the interpretation means 140. In a preferred embodiment, the interpretation means 140 or the second means for comparing 138 comprise means for summing the binary results of the comparison. 10 The elements 110, 120, 130 of the directional detection device can be grouped together within an apparatus, which can for example be coupled with four sensors 50 and located in an MV/LV transformer substation. They can also be separated, with for example the means for obtaining 110 associated with the sensors 50 in a transformer substation, and the 15 detection and processing units integrated in a control centre, the phase voltages being transmitted to these units in automatic manner or not. In particular, it is possible for each of the units 110, 120, 130 to be able to process a plurality of feeders, either simultaneously or sequentially, and to comprise suitable storage memories. 20 Thus, according to the invention, means for measuring the voltage of each conductor are fitted on the LV feeders, preferably at the level of each transformer supplied by the MV power grid to be monitored. The means for measuring are designed to measure the voltage of each phase and the voltage of the neutral. The signals obtained are transmitted to one or more detection devices which are designed to determine the resulting reverse voltage and 25 the amplitude. The solution according to the invention presents the advantage of considering the problem of breakage of MV conductors by placing oneself from the point of view of the voltages in the LV power system in which the measurement instrumentation is simpler, and by considering in exhaustive manner the whole range of possible situations for one or more broken conductors (for a three-phase power system). 30 The system or the method according to the invention can be associated with protection devices of the power grid 5, for example a protection relay which trips and isolates the branch of the power system that is faulty in case of detection of a fault in said branch.

12 According to another option, deferred indication alone can be envisaged, for example by means of an events log. Any type of direct, local or remote, immediate or deferred indication, or any more radical type of action can be associated with detection according to the invention. 5 Although the invention has been described with reference to an overhead power grid, it is in no way limited thereto. The invention finds the same applications and the same embodiment modes for a partially or totally underground power grid in which cable breakage may occur subsequent to damage or wear to insulators or breakage of other 10 components which may be concerned by the invention. Furthermore, the elements presented to perform the above-mentioned functions can be replaced by equivalents. In particular, the voltage can be measured by any type of sensor (resistive, capacitive, electric field,...), and the signal processing means can take suitable forms, in particular for computation of the reverse voltage. 15

Claims

1. A device (100) for detecting a resistive earth fault (7) in a medium-voltage power grid (5) which comprises at least one medium-voltage / low-voltage transformer 5 substation (10) defining a feeder (15) providing a plurality of low-voltage phase conductors (15A, 15B, 1 5 c), said device (100) comprising: - means for receiving signals representative of the voltages of each conductor of a low-voltage feeder (15); - means for obtaining (110) the signals representative of the phase voltages (VAN, 10 VBN, VCN) from the received voltage signals; - means (122) for determining a symmetric reverse voltage component (Vi) of the feeder; - means (126) for comparing the reverse voltage (Vi) with a tripping threshold (Sd). 15

2. The detection device according to claim 1 further comprising means (128) for adjusting the tripping threshold (Sd).

3. A system for detecting a resistive fault (7) in a medium-voltage power grid (5) which comprises a plurality of medium-voltage / low-voltage transformer substations (10, 20 20, 30) each defining a feeder (15, 25, 35) providing a plurality of low-voltage phase conductors (15A, 15B, 15c), said system comprising: - at least one detection device (100) according to one of the foregoing claims, the means (110, 122, 126) of said detection device (100) being designed to receive signals representative of the voltages of each conductor of each low-voltage feeder 25 (15, 25, 35), to obtain signals representative of the phase voltages for each feeder (15, 25, 35), to determine the symmetric reverse voltage component (Vi) therefrom, and to compare the module of each reverse voltage (Vi) with the tripping threshold (Sd), and - means for indicating (D) the occurrence of a fault (7) in the medium-voltage power 30 grid (5) when the comparison means (126) give a result in which a reverse voltage (Vi) exceeds the tripping threshold (Sd). 14

4. A system for detecting a resistive fault (7) in a medium-voltage power grid (5) which comprises a plurality of medium-voltage / low-voltage transformer substations (10, 20, 30) each defining a feeder (15, 25, 35) providing a plurality of low-voltage phase conductors, said system comprising a detection device (100, 200, 300) according to 5 one of claims I or 2 associated with each feeder (15, 25, 35) and means for indicating (D) the occurrence of a fault (7) in the medium-voltage power grid (5) when the comparison means (126) of one of the detection devices (300) give a result in which the module of the reverse voltage (Vi) exceeds the tripping threshold (Sd). 10

5. The detection system according to claim 4 wherein each detection device (100, 200, 300) is located in the transformer substation (10, 20, 30).

6. The detection system according to one of claims 3 to 5 wherein the neutral of the medium-voltage / low-voltage transformer substations (10, 20, 30) is earthed, and 15 further comprising voltage sensors on each phase conductor and on the neutral conductor.

7. A device (100) for directional detection of a resistive fault (7) in a medium-voltage power grid (5) which comprises at least one medium-voltage / low-voltage 20 transformer substation (10) defining a feeder (15) providing a plurality of low voltage phase conductors (1 5 A, 1 5 B, 15c), said device comprising a detection device according to one of claims 1 to 2, processing means (130) of the signals representative of the phase voltages (VAN, VBN, VCN), and means (140) for interpreting the results of processing of the signals to determine whether the fault is 25 line-side or load-side from the measuring point of the signals received by said device (100).

8. The directional detection device according to claim 7 wherein the processing means (130) comprise: 30 - means (132) for computing the norm of the phase voltages (jVAN I, jjVBN I, 11VCN 11) - means (136) for computing the mean (p) of the norms (11 VAN 11, IIVBN I, VCNII); - means for determining the minimum of the norms (Vmin); 15 - first means (138) for performing comparison of the norm of the phase voltages with their mean (u); - second means (134) for performing comparison of the minimum of the norms with a threshold (Sv). 5

9. A system for detection and location of a resistive fault (7) in a medium-voltage power grid (5) which comprises a plurality of medium-voltage / low-voltage transformer substations (10, 20, 30) each defining a feeder (15, 25, 35) providing a plurality of low-voltage phase conductors, said system comprising a detection system 10 according to one of claims 3 to 6 wherein the detection devices (100, 200, 300) are directional detection devices according to one of claims 7 or 8, and wherein the means for indicating (D) the occurrence of a fault (7) in the medium-voltage power grid (5) are designed to indicate whether the fault is line-side or load-side from the measuring point of the signals received by said detection devices. 15

10. A detection method of a resistive earth fault (7) on a medium-voltage power grid (5) comprising obtaining voltages from each phase of the low-voltage feeders (15, 25, 35) supplied by the medium-voltage power grid (5), determination of the reverse voltage (Vi) of the low-voltage feeders, comparison of the modules of the reverse 20 voltages (Vi) with a threshold (Sd), and indication of the occurrence of a fault (7) when at least one comparison indicates that the threshold (Sd) has been overshot.

11. The detection method according to claim 10 further comprising adjustment of the thresholds (Sd). 25

12. A method for detection and location of a resistive earth fault (7) on a medium voltage power grid (5) comprising the detection method according to one of claims 10 or 11, determination of the norm of the phase voltages (I VAN I, VBN II, VCN 11), processing of the norms of the phase voltages (11 VAN 11, VN '11, VCN II), 30 and determination of the location of the detected fault line-side or load-side from the measuring point. 16

13. The method for detection and location of a resistive earth fault according to claim 12 wherein processing of the norms comprises computation of the arithmetic mean, comparison of the amplitude of the voltages with their mean, and comparison of the voltage minimum with a threshold.