FR2719125A1 - Internal arc detection appts. for gas filled cables - Google Patents

Abstract

High tension multi-core gas filled electrical cables are fabricated in a number of sections (1) separated by sealed compartments. Each section contains a number of parallel conductors (4). The conductors are insulated by gas under pressure. Electrical discharge between conductors is accompanied by emission of light which is detected by an optical fibre (14,15,34,35) in two strands running the full length of the cable, passing through the compartments between sections and forming a closed loop. The first strand collects light emitted by an electrical discharge. The second strand is sheathed and is opaque to external light.Analysis of light received at each end of the loop allows the fault to be detected and its position within the cable to be located.

Description

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  INTERNAL ARC DETECTION DEVICE FOR HIGH CABLE

GAS INSULATED VOLTAGE

  The present invention relates to the detection of internal arcs in cables & high voltage & gas insulation and in particular for buried cables. For buried cables, it is absolutely necessary to be able to locate with certainty the section of cable

  damaged during a fault causing an internal arc.

  There has been described in French patent application No. 9315355 of December 21, 1993, an embodiment of cable & gas insulation, usable for long distance electrical connections, and comprising sections separated by watertight compartments. Each section comprises conductors in the form of tubes arranged parallel to each other. The envelope surrounding these conductors is a cylindrical steel envelope; a magnetic screen consisting of an aluminum envelope is arranged between the conductors and the steel envelope. The

  length of a section is between 50 and 100 meters.

  It was pointed out in the aforementioned document the possible use of optical fibers for the transmission of information; the use of optical fibers as sensors inside the cable has also been mentioned. In the event of an internal fault, an electric arc occurs which damages the elements inside the section (conductors, screen) and even the outer steel casing; it is then necessary to replace the damaged section. It is noted that the gas, polluted by the electric arc, cannot propagate beyond the faulty section due

  the presence of watertight compartments.

  An object of the invention is to define a device for detecting and locating a fault in a cable of great length (several kilometers). The subject of the invention is a device for locating a faulty section in an electric cable & gas insulation comprising a plurality of sections separated by watertight compartments, each section comprising conductors, an outer steel casing and a magnetic screen. disposed between the conductors and the outer envelope, the target being isoé16 by a pressurized gas, the defect causing the appearance of an electric arc, characterized in that it comprises an optical fiber arranged in a loop whose two strands each extend along the target by sealingly crossing the watertight compartments, the loop being open at one of the ends of the target, a first strand being arranged to collect and transmit light generated by the defect, the other strand being sheathed so as to be opaque to light, the device comprising means for collecting light & each of the ext remitted from the loop and measure the time between the appearance of the

  light & the end of each of said strands.

  In a first embodiment of the invention, the light is that of the electric arc appearing during the fault. In a second embodiment of the invention, light is generated from the overvoltage appearing during a fault between said outer envelope and said screen. The invention will be clearly understood on reading the

  description of various embodiments of the invention, with reference to the accompanying drawing in which:

  - - Figure 1 is a schematic view of a section of an electrical & high voltage target with gas insulation,

  - Figure 2 is a sectional view along line II-

  II of Figure 1, - Figure 3 shows an open loop optical fiber for the detection and localization of a fault in a high voltage target & gas insulation, - Figure 4 is a diagram of a device for the calculation of the distance from the fault & from the information given by the two strands of the optical fiber, - Figure 5 represents an associated optical fiber & a light concentrator according to a first embodiment, - Figure 6 represents an optical fiber associated with a light concentrator according to a second embodiment, - Figure 7 shows an optical fiber associated with a light concentrator according to a third embodiment, - Figure 8 illustrates a device capable of emitting light when voltage threshold is reached between

  the steel envelope of the cable and the screen.

  Figure 1 shows a section of a cable & high voltage & gas insulation. The section i comprises an outer casing 2 of steel, an inner casing or magnetic screen 3, preferably of aluminum and conductors 4 for the passage of the three-phase current; the

  conductors are held by insulating support arms 5.

  The conductors 4, the screen 3 and 1 the outer casing 2 are formed of elements having for example a length of 10 meters and which are welded together respectively. The section has for example a length of 50 & 100 meters; a cable can have a length of several tens of kilometers. 25 The insulating arms 5 serving to hold the conductors 4, are spaced apart for example every 10 meters; these arms allow free circulation of the insulating gas which fills the interior volume 6 of the section; they also allow the passage of light throughout the

section.

  Space 6A between 1 the outer envelope 2 and the screen

  3 is also filled with insulating gas.

  The insulating gas is preferably nitrogen under a pressure of the order of 12 hectopascals.

  Between the outer casing 2 and the screen 3 are arranged teflon strips 7, which allow the centering of the heat screen 3 and its free expansion & inside the casing 2. At each end of the section is installed a dismantling compartment referenced 8 for the left compartment in FIG. 1 and 8 'for the right compartment. The compartment comprises a watertight partition formed by a metal crown 10 & which are fixed three closed insulating cones 11 gas tight & through which the conductors 4 pass; one end 3A of the screen 3 is welded & the casing 2, the other end 3B is free and can slide on an insulating jacket 12. The end 3B is therefore isolated from the steel casing 2. The screen magnetic aluminum 3 reduces the field intensity in the outer casing 2

  magnetic created by three-phase currents.

  To detect the arc appearing during a fault & ground (arc 13 in Figure 2) or between two conductors (arc 13A in Figure 2), an optical fiber is used which is arranged in an open loop and whose two strands 14 and 15 extend all along the target (Fig. 3). The optical fiber is preferably silica in the case of large targets

  length, to ensure good light transmission. The strands 14 and 15 pass through the metal rings 10 through holes, the seal being obtained by plugging the holes using an epoxy resin or a silicone or a sealed cable gland.

  -One of the strands 14 is intended to receive the light generated by the fault, the other strand is used to convey the information. An electro-optical device 16, connected to the respective ends A1 and A2 of the strands 14 and 15, makes it possible to measure the difference between the instants of arrival of the light signal coming from the strands 14 and 15 respectively; this difference makes it possible to calculate the distance between the device 16 and the location of the fault and consequently to identify the section

in default.

  An embodiment of the device 16 is given

in figure 4.

  The signals from the strands 14 and 15 are received respectively by photodetectors D1 and D2, supplying electrical signals amplified by amplifiers G1 and G2; the signals from the amplifiers are shaped as a voltage pulse by triggers T1 and T2. The signal from strand 14 (shortest optical path) triggers a time count by a counter C; this counting is stopped by the signal from branch 15. The digital output of counter C gives the time difference At between the two signals, which is a function of the path traveled and therefore of the location where the fault occurred. A CALC calculator calculates the distance between the

  location of fault and device 16.

  If the location of the fault is designated by xl (see figure 3), the signal sent in xl takes tl seconds to be detected by D1. this same signal takes a time tl + 2t2 to be detected by D2, t2 being the time taken by the light to

  go from place xl & end B of the opposite loop to end A1A2.

The measurement provides At.

  In addition, At = tl + 2t2 -tl = 2t2 As we know the length of the loop, we know

  tl + t2 and it is therefore easy to deduce ti from which we draw x1.

  In a first embodiment of the invention, the light generated by the fault is the light from the electric arc itself. The light enters the core of the strand 14 at a determined location, for example in the vicinity of one end of the section. To this end, the fiber is stripped over a small portion so as to reveal the core of the optical fiber. As shown in FIG. 5, a light concentrator 20, formed of a transparent cylinder (made of glass or plexiglass for example) provided with a spherical end, is placed in line with the fiber; in a first embodiment, the stripped portion of strand 14 is fixed to a cradle 21 engaged in a notch at the base of the cylinder; the attachment of the part 21 to the cylinder, as well as the optical continuity, are obtained by means of an optical gel 22 of the same index as the core of the fiber. The cradle is advantageously curved and convex to give a certain radius of curvature & this part of stripped fiber in order to better capture the parallel light rays & the axis of

1 envelope.

  Alternatively, shown in Figure 6, the strand 14 is cut and the two severed ends 14A and 14B are stripped and bonded by an optical gel 22 to the concentrator

  of light 20, at two diametrically opposite points.

  In another variant, represented in FIG. 7, the fiber 14 is cut and the two sectioned ends are provided with optical connectors 23A and 23B provided with an external thread and engaging in threaded holes of the light concentrator 20, arranged at points

  diametrically opposite. The bottom of the holes is lined with optical gel 22.

  The light concentrators have been shown in FIG. 3, regularly arranged in each of the sections of the strand 14 of the optical fiber. The second fault detection and location mode according to the invention uses a spark gap, for example a

  spark gap & gas with transparent glass wall.

  Note first of all that this second fault detection and localization mode can be installed in place of the previous device, but that it is preferable to provide it in the cable with the previous device, which makes it possible to obtain redundancy and confirmation of the location determined by the previous device. The second mode is based on the observation that in normal operation, the potential difference between the screen 3 and the outer envelope 2 is practically zero. When a fault occurs, an overvoltage appears between the screen 3 and the casing 2 and the earth by the sudden setting at high voltage of the screen 3 via the electric arc. It is sufficient to connect, between the screen 3 and the outer envelope 2, a device capable of emitting light when a fixed threshold of overvoltage is reached; as before, this light is received by a strand of a fiber arranged in a loop along

  of the cable and equipped as described above.

  In this embodiment, the optical fiber will preferably be placed in the upper part of the cable, between the outer envelope 2 and the screen 3; the two strands 34 and

  of such a fiber are shown in Figure 2.

  A device capable of emitting light when a voltage threshold is detected is represented in FIG. 8. It is a spark gap & gas 40 (rare gas such as argon or neon) connected between the metal casing outer 2 of the cable and the screen 3. Such spark gaps are marketed and miniature models whose dimensions do not exceed 5mm are available. The spark gap 40 with transparent walls is housed in the part 20 in plexiglass in

  face of the partially stripped strand 34.

  Of course, all the devices described above are protected according to known techniques: the optical fibers are suitably sheathed and arranged with play to avoid any risk of breakage, the devices

  electronics are protected against magnetic disturbances.

  It should be noted that with this arrangement of fibers

  optical, we can recheck, if necessary after the fault, the location of the damaged location.

  Indeed, the electric arc, by its intensity, will destroy the optical fibers in the damaged section and there will therefore be a break in the optical fibers.35 It will suffice (see Figure 3) to emit a light signal, either from the the end A1-A2 of the line on one or the other of the strands using light sources S1 or S2, or from the end B of the line using a source S3. The light is reflected at the location of the defect and returns to its origin. We will then measure the time 5 back and forth put by the light to locate the fault. The advantage of this technique, which can be implemented

  with a device similar to that of FIG. 4, is that it does not require permanent measurement and recording. This thus provides a third mode of fault location.

  The invention finds application, not only for cables & three-phase gas insulation as described

  above, but also in electrical connections & gas insulation in which each phase has its own gas-insulated envelope.

Claims (6)

  1 / Device for locating a faulty section in a gas-insulated electrical target comprising a plurality of sections (1) separated by watertight compartments (8), each section comprising conductors (4), an outer steel casing (2) and a magnetic screen (3) disposed between the conductors (4) and the outer casing (1), the cable being insulated6 by a gas under pressure, the defect causing the appearance of an electric arc, characterized in what it includes an optical fiber (14-15, 34-35) arranged in a loop, the two strands of which each extend along the target by sealingly crossing the sealed compartments, the loop being open to one from the ends of the target, a first strand (14, 34) being arranged to collect and transmit light generated by the defect, the other strand (24, 34) being sheathed so as to be opaque to light, the device comprising means (16) p or collect the light at each end of the loop and measure the time between the appearance of the light & the end of each of said strands.   2 / Device according to claim 1, characterized in that the light is that of the electric arc (13, 13A) appearing during the fault.   3 / Device according to claim 1, characterized in that the light is generated from the overvoltage appearing during a fault between said outer casing (2) and said screen (3). 4 / Device according to claim 2, characterized in that said first strand (14) is associated with a light concentrator comprising a transparent cylinder (20) provided with a spherical end. 5 / Device according to claim 4, characterized in that said first strand is partially stripped, the stripped part being fixed & a cradle (21) engaging in an oo   indentation of said cylinder (20), an optical gel (22) being disposed between said part (21) and said cylinder (20).   6 / Device according to claim 5, characterized in that the cradle (21) is curved and convex.   7 / Device according to claim 4, characterized in that said first strand (14) is cut and its severed ends (14A, 14B) partially stripped and glued respectively on the outer wall of said cylinder (20) at two diametrically opposite points. 10 8 / Device according to claim 4, characterized in that said first strand (14) is cut, its ends   sectioned being partially stripped and provided with an optical connector (23A, 23B) & external thread, said connectors being engaged in tapped holes made in the cylinder (20) in two diametrically opposite locations.   9 / A device according to claim 3, characterized in that said light is generated by a spark gap (40) connected between said outer envelope (2) and said screen (3). 20 10 / Device according to claim 9, characterized in that said distributor is disposed in a concentrator of light (20).