WO2010119095A1

WO2010119095A1 - Monitoring temperature of an overhead electrical line

Info

Publication number: WO2010119095A1
Application number: PCT/EP2010/054960
Authority: WO
Inventors: Leif Bjerkan; Tarun Kumar Gangopadhyay; Kamal Dasgupta; Somnath Bandyopadhyay; Palas Biswas; Shyamal Bhadra; H. S. Maiti
Original assignee: Sintef
Priority date: 2009-04-15
Filing date: 2010-04-15
Publication date: 2010-10-21
Also published as: NO20091450L; NO333161B1

Abstract

The invention relates to a system and a probe unit for monitoring on-line temperature of high voltage cables in air stretches, comprising of at least one optical fibre attached to the cable, and said optical fibre comprising of at least one Bragg grating (FBG) with known reflection characteristics, a light source for transmitting light within a known range of wavelengths into said optical fibre, and measuring devices for detection of light reflected from said Bragg grating(s) in the fibre and for recognizing light reflected from each Bragg grating based on their known reflection characteristics and their unique correspondence with temperature of the Bragg grating. The Bragg grating is mounted in a probe, the probe being mounted in and thermally coupled to a housing being mounted on and thermally coupled to the cable, wherein the probe has a cylindrical shape and is mounted in the housing in an opening having essentially the same cross section as the probe but a length exceeding the length of the probe, so as to allow for mutual variations in the temperatures between the probe and housing without subjecting the Bragg grating to strain.

Description

MONITORING TEMPERATURE OF AN OVERHEAD ELECTRICAL LINE

The present invention relates to a system and probe unit for monitoring on-line temperature of high voltage cables in air stretches, comprising of at least one optical fibre attached to the cable including fibre Bragg grating based sensors. More specifically the present invention relates to a system and device that makes it possible to register temperature of overhead high voltage conductors in real time.

A system for monitoring high voltage cables in air stretches with emphasis on strain and vibrations is described in the Norwegian patent, NO 310125. The method is based on fibre-optic Bragg grating sensors that are integrated in an optical fibre. The Bragg grating sensors as such are well known and are described in several patents; for example U.S patent no. 4,725,110 by Glenn et. al. "Methods for impressing gratings within fibre optics" and U.S. patent no. 4,807,950 by Glenn et. al. "Optical fibre with impressed reflection gratings".

In patent publication EP1496369 a fiber based probe is suggested to measure the temperature of a wire. The probe is either mounted in the wire or in a protective casing being clamped on the outside of the wire making it vulnerable for damage and reducing the accuracy of the temperature measurements, and also the shape and mounting method is not suitable for high voltage applications as it will generate corona effects.

The sensors can be positioned anywhere along the conductor, and when operating in the low loss 1550 nm window of standard optical fibres, it is also possible to access remote and inaccessible areas.

Overhead lines are subject to mechanical loads from their environment like snow and ice accretion, extreme temperatures and wind loads like aeolian vibrations and galloping. Vibrations cause wear at the clamps and reduced lifetime, while galloping can lead to short circuiting and damage to the lines. In extreme situations environmental loads can lead to power outage and severe maintenance work with significant economical consequences. An on-line surveillance system for critical line spans can be a valuable tool for providing information on real-time loads and load history. Thus, corrective measures can be undertaken before damage occurs. Although several technologies are available to determine the condition of overhead lines and associated components, there is still need for improvement and introduction of new technology to improve the existing ones and get access to parameters that so far have been unavailable. Better surveillance of the mechanical and thermal loads on power lines contributes to a more efficient utilization of the transmission capacity, enhances reliability and provides better knowledge of the condition of the power lines and their remaining lifetime.

Conventional techniques for monitoring environmental and thermal loads are often based on indirect methods to measure the selected parameters. A common feature with many conventional methods is that they cannot measure the conductor loads directly, but parameters that are more or less indirectly related to the load in question. One reason for this is the presence of high voltage.

In many industrialized nations the power utilities are presently constructing fewer new lines. The emphasis has moved towards extending the life and upgrading of existing installations. However, the demand for power is steadily increasing, and the simplest way to increase power flow in existing lines is to increase the electric current. However, this procedure has two major limitations:

A higher current causes an increased temperature of the power line. The consequence is that the conductors may age prematurely and in worst case fail because of too high temperature.

A higher temperature leads to elongation of the conductor by thermal expansion, which again leads to increased sagging of the conductor. This effect causes hazards to the ground below, and may in worst case ignite fires.

The following factors are the most essential for the temperature of a power transmission line; conductor resistivity, solar heat radiation, solar absorption, solar emissivity coefficients, velocity of wind flowing, type of conductor used etc. These parameters appear statistically in different weather conditions. A standard adopted by the Institute of Electrical & Electronics Engineer (IEEE, Std. 738-1993), on June 17, 1993 provides description and estimates of these parameters. So far, the maximum permissible power loads are determined from these estimates which are quite conservative and include considerable safety margins. Therefore, the maximum power flow capacity is not utilized. On the other hand, if the power flow exceeds the safe limits, the consequences may be damage to the conductor caused by excess heating and serious hazards to the ground below from extensive conductor sagging. In any case, an on-line monitoring system like the one suggested here solves the problems, and should be highly recommended in particular for some critical spans. With a reliable monitoring system, the utility can increase the power flow to safe levels without serious risks.

Thus it is an object of this invention to provide a system for monitoring the temperature of a high voltage cable, thus increase the life length of the cable by controlling the sagging and load on the cable based on the temperature measurements. This is obtained as disclosed in the accompanying claims.

This way a monitoring system and probe unit is provided which may provide a robust temperature monitoring in high voltage cables without being subject to strain from the cable or from the temperature differences between the cable, housing and/or the sensor. The sensors and the housings may also be removed without affecting the operation of the cable.

The present invention is aimed at use in high voltage environments, e.g. up to 40OkV, with cables having circular cross sections of winded, solid metal threads and a sensor housing and probe containing the fibre preferably being made from the same material as the high voltage cable.

The invention will be described below with reference to the accompanying drawings, illustrating the invention by way of examples. Figure 1 Illustrates a schematic diagram for a Fibre Bragg gratings sensor system in series multiplexing.

Figure 2 Illustrates a schematic diagram for a Fibre Bragg gratings sensor system in four branches and parallel multiplexing. Figure 3 Illustrates a schematic diagram for a Fibre Bragg gratings sensor system in four branches with a switching arrangement. Figure 4 Illustrates an assembly of a probe unit with a sensor housing with two holes for two sensors according to the invention. Figure 5 Illustrates an assembly of a sensor housing with four holes for four sensors multiplexing. Figure 6 Illustrates two versions of a sensor probe according to the invention comprising optic fibres with FBGs. The sensor fibre is spliced with the fibre cable and splicing joint is protected inside the aluminium probe tube. Figure 7 Illustrates the calibration mount of a housing with sensors according to the invention..

The sensor medium is based on optical fibres, preferably standard single-mode fibres made of quartz (CCITT G652) with a primary protective coating. The sensors are Fibre Bragg gratings (FBG) that can be inscribed permanently in the fibre at arbitrary positions. The gratings are recoated preferably with polyimide after inscription. The length of a FBG is typically around one centimetre, so in this case the system can operate as a set of discrete point sensors. The centre wavelength of the FBG is fixed during FBG fabrication or as per the purchased FBG specification. The peak reflectivity and FWHM (full width half maximum) can be tailored to the resolution, dynamics, the spectrum of the light source and the distance from the light source to the sensors. For long distances it is an advantage to operate in the low loss transmission window around 1550 nm where availability of standard telecommunication components provides additional benefits. The fibre serves two functions: A transport path for the light signals and as signals from the sensor positions. For this application it is an advantage to operate the sensors in the reflection mode so that both transmission and reception of signals is served by the same fibre enabling access to remote locations. In addition, all instrumentation can be located in the same place.

For protection the fibres are laid in a metal free cable that is compatible with high voltage environments and can be attached to the conductor by using a spinning machine or using tape or nylon strips depending on the distance from the tower to the sensors. For remote sensing where the registration instrument is located far away from the sensor positions the connection cables can be wrapped around the conductor in the same way as for ordinary communication cables. For simultaneous measurement of several sensor positions the FBG's can be configured in series along the same fibre as illustrated in figure 1 , where a broadband light source 1 transmits light through a coupler 2 to a number og FBGs along the same fiber 14, and the light reflected from the FBGs is transmitted trough the 3dB coupler 2 to a detector 3 and measuring unit 4. , Figure 2 illustraes a corresponding system where an interrogation unit 5 is coupled to a number of fibers thus providing parallel measurements, each fiber having a series of FBGs, thus providing a combination of parallel and serial measurements.. Furthermore, with a switch 6 as illustrates in figure 3, in the measurement control unit it is possible to switch measurements among parallel sensor lines.

Bragg gratings respond to strain and temperature variations so they can work as both strain and temperature sensors. Changes in strain or temperature are recorded as changes in the wavelength position of the reflected peak. A FBG that is integrated with an object will measure strain or temperature variations of the object. Since an optical fibre is a dielectric, the high voltage environment will not have any influence on the measurements.

Functionally, this invention covers the following aspects of climatic stresses that overhead power conductors are exposed to:

On-line registration of conductor temperature by using the FBG as a temperature sensor.

Determine the on-line sag of the conductor and the safe distance to the ground below based on the temperature measurements.

Utilize the results for optimized performance of power flow along the conductor within safe limits.

For direct temperature measurements the FBG sensors are embedded in a housing in direct contact with the conductor and positioned as close to the conductor as possible inside the housing. The FBG sensors must be configured in the housing in such a way that they will not be influenced by strain. Figure 4 shows a sketch of the preferred embodiment of the arrangement. With reference to Figure 4 the entire housing 7 is made in two equal halves that are fastened together with screws or bolts 20 to the conductor or cable 21 to be monitored. A hole in the center 8 matches the diameter of the conductor 21. The housing is egg-shaped or rounded in order to prevent unwanted effects (corona) from sharp edges. For each sensor another hole 9 is made on the mount close to the center hole 8 so as to have essentially the same temperature as the cable or conductor 21. This assembly of sensor mount can be designed with sensor holes as per the number of the sensors to be used in the same mount. An arrangement is shown in Figure 4 to hold two sensors and Figure 5 is to hold four sensors.

A thin rod 19 of the same material, e.g. aluminium, as the housing is made as illustrated in figure 6a and 6b, to fit the size of the hole 9 in figures 4,5 and 7. A groove 17,18 is carved out longitudinally in the rod 19 for assembly of the fibre 14 including one or more FBG sensors 13. The groove 17,18 is widened at the ends 17 to accommodate the thickness of the secondary fibre coating 11. The depth of the hole 9 is made longer than the length of the rod 19 containing the sensor 13 so that the fibre-optic cable 10 can fit into it for robust mounting. The diameter of the hole receiving the probe rod is somewhat larger at the entrance so that the fibre in the cable connected to the sensor can enter the assembly without bends since optical fibres in cables are stranded around a centre member and, thus, not located in the centre of the cable. Splicing joint 12,15 with the fibre cable is also protected inside the groove 17,18 of probe 19.

In figure 6a the fibre sensor 13,14 is in a fiber end, for example in a parallel measuring system, while in figure 6b the fibre 14 extends through the probe 19 and a serial measuring system may be used. In that case the hole C in the housing extends through the housing and the fiber sensor is spliced into the fiber extending through the hole.

In Figure 4 a probe unit with two FBG sensor probes located opposite to each other is shown as an example, but several sensor locations can be accommodated around the perimeter. In Figure 5 a probe unit with four sensor probes located in two halves of the sensor mount. For a serial configuration the hole 9 runs through the entire length of the housing 7, and the rod containing the fibre is positioned in the centre. The end configurations of the rods are made symmetrical in order to fit input and output cable connections. For a single sensor the hole 9 is terminated within the housing 7 as illustrated in the drawings.

For assembling the unit for a series configuration, one part of the fibre-optic cable is threaded through the hole 9, which in that case extends through the housing, and the FBG sensor is spliced to one fibre in the cable. In case of a series arrangement of several sensors the other end of the FBG is spliced to the fibre cable at the other end. Such an arrangement constituted by an assembly of FBG in aluminium sensor probe is shown in Figure 6, in an embodiment where the fiber end in a parallel configuration. If only one sensor is needed the other end can be laid loose in the fitted clamp hole.

The FBG's can be laid loosely in the groove of the rods 19 or fixed in the groove 17,18 with an epoxy. The latter configuration is preferable since the effective thermal expansion coefficient will be much larger than that of the bare glass which enhances the measurement resolution. In order to reduce measurement noise and undesired reflections from the fibre ends, the end cleaves should be irregular or covered with some epoxy.

After all parts are assembled the cable entrances are sealed with epoxy for protection and prevention of moisture penetration as well as making the assembly robust for mounting. Prior to installation on a power conductor the sensors in the final assembly may be calibrated, i.e. the reflection wavelength vs. temperature for each sensor is obtained. This can be performed in a temperature chamber or similar heating devices with a controlled temperature. A laboratory based accurate temperature controlled resistive heating rod 25 in the place of the cable to mimic a current carrying conductor is fabricated as shown in figure 7. This device is also used for final calibration of the device before field use. In figure 7 the cable and environmental temperatures T1,T2 are compared with the temperature in the sensors 9 are monitored in a monitoring unit 22, and a calibration of the signals may be calculated according to this. In addition to the sensors the calibration unit also comprises an insulator 23 and a connection box 24.

The sensor assembly 7 can be easily assembled on the conductor by tightening the two housing halves with screws 20 (Figures 4 and 5) at any location on the conductor span. The connection cable is wrapped around or attached with other means to the conductor towards one of the towers and taken down to ground. In general, one can use guidelines that apply to wrapping of communication cables along power conductors. Between the conductor end and the tower a fibre-optic insulator system is employed in order to reduce risk of damage from creepage currents. Such devices are commercially available. The fibre cable must be secured to the tower to avoid damage from wind, snow or ice. The far end of the cable can be terminated at a convenient location where the measurement equipment is placed.

The lifetime of an overhead power conductor is typically around 40 years, and the sensor system should be able to function over long time. Several investigations indicate that FGB' s do not degrade with time.

The total system consists of one or more fibre-optic cables each containing a suitable number of fibres. The measurement unit consists of an Optical Sensing Analyzer (here FBG interrogator), appropriate data storing device (PC) and processing units (with special software). The wavelength spacing between FBG' s in a serial configuration must be chosen large enough to accommodate the expected signal variations and a safety margin in order to ensure a unique identification of each sensor. Several methods are available for FBG interrogation. Several such interrogation systems are commercially available and can be chosen according to desired resolution, speed and dynamic range. In addition there are several patents describing various measurement principles like U.S. Patent 5,397,891, U.S. Patent 5,426,297, U.S. Patent 5,646,401 and U.S. Patent 5,380,995. These methods are applicable for interrogation of the sensors for this purpose. To summarize the invention relates to a system and probe unit for monitoring on-line temperature and sag of high voltage cables in air stretches, comprising of at least one optical fibre attached to the cable, and said optical fibre comprising of at least one Bragg grating (FBG) with known reflection characteristics. In addition, a light source for transmitting light within a known range of wavelengths into said optical fibre, and measuring devices for detection of light reflected from said Bragg grating(s) in the fibre and for recognizing light reflected from each Bragg grating based on their known reflection characteristics and their unique correspondence with temperature of the surrounding medium.

Preferably the system comprises one optical fibre being in its longitudinal direction provided with a number of Bragg gratings and the Bragg gratings are mounted in a protective tube with a groove in the longitudinal direction where it is laid loose or fixed with epoxy. The tube and housing is preferably made in the same material as the cable or at least a material having essentially the same thermal characteristics, usually aluminium, as this is the usual material on high voltage cables, but steel is also used in some cases.

The protective tube is mounted in an egg-shaped or rounded protective housing which is designed as two equal halves with a longitudinal hole in the middle so that it will fit the diameter of the overhead cable when mounted together and secured with screws. Holes are drilled through for fixing the said tubes into the said housing, the holes having a length allowing for length variations of the tube relative to the housing so as to avoid strain introduced by temperature differences between the housing and the tube. The cross section of the hole should on the other hand be comparable to the cross section of the tube, so as to ensure thermal coupling between the housing and the tube. The cross section will usually be circular but other shapes may be used.

The housing is fastened to the cable, e.g. being constituted by two halves enclosing the cable when fitted together with thermal coupling to the cable. The tube inside the housing thus obtains a temperature being within measurement uncertainty from the temperature of the cable. The invention is aimed at a use of an optical measurement system comprising of at least one optical fibre in a cable designed for high voltage environments, the optical fibre containing at least one Bragg grating with known reflection characteristics and calibrated temperature response at a chosen position along the fibre, and the Bragg grating is secured in protective housings. The system also comprises a light source for light emission with a known wavelength range into the optical fibre containing the Bragg grating(s) and measurement devices for recognition of their reflection characteristics and their conversion to temperature. The Bragg grating is positioned in the protective tubes and housing discussed above and in close contact with the high voltage cable at chosen positions along the cable. The number of sensors and distribution along the cable may be chosen depending on the local conditions, calculated sag of the cable, length between supports etc.

The exemplified embodiment of the housing and probe in the drawings may have the following dimensions: The housing having a length of 140mm and a diameter of 59mm, being constituted by two halves joined together with bolts thus enclosing the cable, having a central channel with a diameter corresponding to the cable diameter of 32mm. The probe tube having a length of 107mm and a 5mm diameter, while the groove has a wide part 17 at 2mm and a narrow part 18 with approximately 1 mm.

Claims

C l a i m s

1. System for monitoring on-line temperature of high voltage cables in air stretches, comprising of at least one optical fibre attached to the cable, and said optical fibre comprising of at least one Bragg grating (FBG) with known reflection characteristics, a light source for transmitting light within a known range of wavelengths into said optical fibre, and measuring devices for detection of light reflected from said Bragg grating(s) in the fibre and for recognizing light reflected from each Bragg grating based on their known reflection characteristics and their unique correspondence with temperature of the Bragg grating, wherein the Bragg grating is mounted in a probe, the probe being mounted in and thermally coupled to a housing being mounted on and thermally coupled to the cable, wherein the probe has a cylindrical shape and is mounted in the housing in an opening having essentially the same cross section as the probe but a length exceeding the length of the probe, so as to allow for mutual variations in the temperatures between the probe and housing without subjecting the Bragg grating to strain.

2. System according to claim 1 wherein the probe has a circular cross section.

3. System according to claim 1 wherein the probe is a protective tube with a groove in the longitudinal direction where the fiber Bragg grating is positioned.

4. System according to claim 3 wherein the fibre Bragg grating is fixed in the grove, e.g. with epoxy.

5. System according to claim 3, wherein the groove has a first, inner part having dimensions corresponding to the optical fiber in which the Bragg grating is made and the second, outer part having dimensions corresponding to the optical fiber having a chosen cladding.

6. System according to claim 1, wherein the housing is an egg-shaped protective housing which is designed as two equal halves with a longitudinal hole in the middle so that it will fit the diameter of the overhead cable when mounted together, also having at least one hole adapted to receive at least one probe.

7. System according to claim 1, wherein the housing and probe is made from the same material, e.g. aluminum.

8. System according to claim 1 comprising a chosen number of Bragg gratings positioned in a number of housings distributed along a cable.

9. System according to claim 1, wherein each housing is adapted to receive a number of Bragg gratings.

10. Use of a system according to claim 1 for measuring the temperature of power cable wherein the system comprises analyzing means of wavelength shift due to temperature rise at the Bragg gratings.

11. Use of a system according to claim 1 for calculating the sagging of a voltage cable from known relations between the sagging and the measured temperatures at the Bragg gratings.

12. Probe unit for monitoring on-line temperature of high voltage cables in air stretches, comprising of at least one optical fibre said optical fibre comprising of at least one Bragg grating (FBG) with known reflection characteristics, wherein the Bragg grating is mounted in a probe, the probe being mounted in and thermally coupled to a housing being mounted on and thermally coupled to the cable, wherein the probe has a cylindrical shape and is mounted in the housing in an opening having essentially the same cross section as the probe but a length exceeding the length of the probe, so as to allow for mutual variations in the temperatures between the probe and housing without subjecting the Bragg grating to strain.

13. System according to claim 12 wherein the probe has a circular cross section.

14. System according to claim 12 wherein the probe is a protective tube with a groove in the longitudinal direction where the fiber Bragg grating is positioned.

15. System according to claim 14 wherein the fibre Bragg grating is fixed in the grove, e.g. with epoxy.

16. System according to claim 14, wherein the groove has a first, inner part having dimensions corresponding to the optical fiber in which the Bragg grating is made and the second, outer part having dimensions corresponding to the optical fiber having a chosen cladding.

17. System according to claim 12, wherein the housing is an egg-shaped protective housing which is designed as two equal halves with a longitudinal hole in the middle so that it will fit the diameter of the overhead cable when mounted together, also having at least one hole adapted to receive at least one probe.

18. System according to claim 12, wherein the housing and probe is made from the same material, e.g. aluminum.