GB2583712A

GB2583712A - Distributed acoustic sensor applications

Info

Publication number: GB2583712A
Application number: GB1905892.4A
Authority: GB
Inventors: Bishop Luke; Wyles Simon
Original assignee: ELECTRICITY DISTRIB SERVICES Ltd
Current assignee: ELECTRICITY DISTRIB SERVICES Ltd
Priority date: 2019-04-26
Filing date: 2019-04-26
Publication date: 2020-11-11
Anticipated expiration: 2039-04-26
Also published as: GB201905892D0; GB2583712B

Abstract

A discharge 108 in a high voltage transmission line 100 is identified by applying an optical signal to an optical fibre 104 along the power line, sensing reflections of the optical signal, processing the reflections to identify reflections that show a discharge in the power line, and estimating a location of the discharge based on the propagation time of the reflections. Reflections of the optical signal due to an acoustic signal applied at a known location on the line are detected and the actual location of the discharge is determined based on the known location, the estimated discharge location and the detected reflections due to the acoustic pulse. The discharge may be intentionally caused by applying a high power electrical pulse to the power line. The optical fibre may form part of a distributed acoustic sensing system mounted onto the power cable and linked with a station at the end of the cable. The power cable may be deployed in a subsea environment where thumping and making measurements from the surface as a way of fault finding is impractical.

Description

DISCHARGE DETECTION METHOD

[0001] This invention relates to a method of detecting and locating short circuits in high voltage cables, in particular for subsea power cables.

BACKGROUND

[0002] Technology that uses optical fibres to detect minute mechanical effects, such as vibration, pressure and impact, has been developed and in existence for a number of years. Applications for this technology encompass oil and gas pipeline monitoring, for damage, leaks, and impacts, to security systems for detecting traffic and pedestrians. This technology, referred to as Distributed Acoustic Sensing (DAS), works by injecting light from a low energy laser into a pair of fibres looped at the remote end or into a single fibre with a terminating splice at the end to limit reflection. Light from the continuously active low energy laser is returned to the source by a mechanism referred to as Rayleigh back scatter when any mechanical force, such as vibration, impact, pressure, etc., is exerted upon the optical fibre.

This back scattered light is detected by sensors within the same unit that contains the laser. The back scattered light has a frequency spectrum characteristic of the attributes of mechanical force attributes, which can be analysed in a known manner.

[0003] In addition, the speed of light within the optical fibre is determined by the refractive index of the fibre material. Knowing the refractive index of the material and hence the propagation speed of the light allows the unit to calculate the distance the back scattered light has travelled, and thus give a location of the mechanical force in terms of distance along the optical fibre.

[0004] The spectrum of the scattered light may be visually displayed to an operator using a 'waterfall' plot on a user display apparatus, such as a computer monitor. In a typical waterfall plot, the cable length is plotted along the x-axis split into 10m sections across the entire length of the cable circuit; thus a typical cable of 40km is displayed by 4,000 virtual sensors. Snapshot rows of time periods, approximately 1 second apart, are displayed on the y-axis and gradually move from top to bottom of the display, hence the waterfall description. The magnitude of the mechanical force exerted on each 10m section for a given time period is displayed according to a low-high colour scale from black, through blue, green, and yellow to red. In addition, algorithms may be configured to automatically identify different types and levels of force.

[0005] The present invention seeks to provide at least an alternative to fault location techniques of the prior art.

BRIEF SUMMARY OF THE DISCLOSURE

[0006] In accordance with the present invention viewed from a first aspect there is provided a method of identifying a discharge in a high voltage transmission line.

The method comprises applying an optical signal to an optical fibre associated with the transmission line, sensing reflections of the optical signal received from the optical fibre, and processing the reflections of the optical signal whereby to identify reflections of the optical signal characteristic of a discharge in the transmission line.

[0007] Thus, in accordance with the present invention distributed acoustic sensing can be used to identify a discharge in a high voltage transmission line. Typically, such a discharge will be caused by a fault in the insulation surrounding the transmission line. When such a fault is present in a transmission line (or cable) at least part of the energy transmitted down the transmission line escapes the transmission line through a fault in the insulation and goes to ground potential. The escaping energy causes the discharge.

[0008] Electricity networks are connected by transmission lines in the form of cables and/or overhead lines. In particular, some High Voltage (HV) networks are connected by cables, either partially or entirely, with optical fibres embedded within the HV cable, or routed in close proximity to the HV cable. Cable systems with either embedded optical fibres, or optical fibres running adjacent to the main cables, are becoming increasingly available in power systems.

[0009] Fault location is notoriously difficult on cable systems, and has been the subject of many studies, investigations and many methods subsequently being developed and used, all with advantages and disadvantages. One particular technique is called 'cable thumping'. When a high voltage, high current pulse is transmitted along a faulty HV cable, the discharge of the pulse at the fault location via a high current arc causes a 'bang' at the location of the fault. On underground cables, the noise of this underground bang can be loud enough to be heard above ground.

Repeated cable thumping to pinpoint the location of the fault can cause damage to the insulation of the HV cable at locations away from the fault location. Since only the insulation at the fault location will be repaired, this can increase the likelihood of further cable faults developing at other locations in the future as a direct result of the cable thumping.

[0010] The present invention identifies that the existing DAS technology can be used conveniently to identify a discharge due to a cable fault in a transmission line. The 'bang' associated with the discharge is sufficient to generate a characteristic reflection of the optical signals in the optical fibre, which can be identified as a discharge. For example, a transmission line may be monitored continuously in accordance with the invention, so that should a fault occur, an immediate notification can be generated in response to identification of the reflections characteristic of a discharge.

[0011] However, the method may further comprise the step of applying an electrical pulse to the transmission line, whereby to cause the discharge in the high voltage transmission line. Thus, the method may comprise the step of 'thumping' the cable in order to cause the discharge. Ranges of wavelength can be excluded from the display to only show a desired spectrum range specific to 'thumping' or electromechanical forces associated with cable faults, and thus reduce/remove display misinterpretation, false alarms and incorrect location. Reflected spectra including phase information may be sensed to enhance the reliability of the identification of the characteristic reflected optical signals.

[0012] The method may comprise the step of determining an estimated location in the transmission line of the discharge, based on the propagation time of the reflections of the optical signal. Thus, the time at which the reflections characteristic of the discharge are sensed can be used relative to the time at which the corresponding optical signal was applied to the optical fibre can be used, with knowledge of the speed of light in the optical fibre, to calculate the distance along the optical fibre of the discharge. This can assist in quickly identifying the location of a cable fault.

[0013] The method may further comprise the steps of detecting reflections of the optical signal due to an acoustic pulse applied to at least one known location on the transmission line, and determining an actual location of the discharge based on the known location, the estimated location and the detected reflections due to the acoustic pulse. Thus, an acoustic pulse, for example a noise from a ship in the vicinity of the cable, can be used to determine an actual location of the discharge.

For example, the source of the acoustic pulse may be located proximate the cable at a position along its length. When the acoustic pulse is generated, reflections of the optical signal applied to the optical fibre are caused by the mechanical interaction of the acoustic pulse with the optical fibre. The position at which the acoustic pulse interacts with the optical fibre can be determined from the propagation time of the optical signal and the reflection thereof. With the knowledge of the geographical location of the source of the acoustic pulse a geographical location of the position of mechanical interaction of the acoustic pulse with the optical fibre can be determined. The relative location of the discharge from this geographical location can then be determined by a comparison of the distance along the optical fibre of the discharge and the distance along the optical fibre of the acoustic pulse. If desired, the source of the acoustic pulse can be moved, for example until the distance along the optical fibre of the discharge and the distance along the optical fibre of the acoustic pulse are substantially the same. At this point, the actual location of the discharge will correspond geographically to location of the source of the acoustic pulse.

[0014] The optical fibre may be mounted to the transmission line. This assists in providing good transmission of vibration between the transmission line and the optical fibre. In embodiments of the invention, the high voltage transmission line is a high voltage cable. 'High voltage' in this context means, typically, a voltage greater than 1,000 volts. The high voltage cable may comprise the optical fibre. Thus, the optical fibre may be embedded in the material of the high voltage cable, for example the insulation material.

[0015] Viewed from a further aspect, the invention provides fault-finding apparatus configured to identify a discharge in a high voltage transmission line. The apparatus comprises a laser configured for coupling to an end of an optical fibre associated with the transmission line, and configured to apply an optical signal to the optical fibre. The apparatus further comprises a sensor configured for coupling to the end of the optical fibre, and configured to sense reflections of the optical signal received from the optical fibre and to generate an output characteristic of the sensed reflections. The apparatus further comprises a data processor configured to process the output of the sensor, whereby to identify reflections of the optical signal characteristic of a discharge in the transmission line. The data processor may be any suitable data processing device, such as a general-purpose computer, a microprocessor or the like.

[0016] The apparatus may comprise a signal generator configured for applying an electrical pulse to the transmission line, whereby to cause the discharge in the high voltage transmission line. Thus, a signal generator may 'thump' the cable.

[0017] The data processor may be configured to determine an estimated location in the transmission line of the discharge, based on the propagation time of the reflections of the optical signal. The data processor may be configured to identify in the output of the sensor reflections of the optical signal due to an acoustic pulse applied to at least one known location on the transmission line and to determine an actual location of the discharge based on the known location, the estimated location and the identified reflections due to the acoustic pulse. Thus the data processor may be configured to implement the method as described above.

[0018] The invention extends to computer software which programs a data processor to operate as the data processor in the fault finding apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: Figure 1 is an illustration of a cross-section through a high voltage cable; Figure 2 is an illustration of an embodiment of a high voltage transmission line system according to the present disclosure; and Figure 3 is a diagram representing steps in a method for identifying a discharge in a high voltage transmission line.

DETAILED DESCRIPTION

[0020] Figure 1 is an illustration of a cross-section through a high voltage cable. A high voltage transmission line in the form of a high voltage subsea cable 100 may comprise three conductors 102 spaced within and running along the high voltage cable 100. An optical fibre bundle 104 is also provided within the high voltage cable 100. The optical fibre bundle 104 comprises a plurality of optical fibres (not shown).

Other high voltage transmission lines may be three separate power cables and a fourth optical fibre laid adjacent to which this method is equally applicable. The conductors 102 and the optical fibre bundle 104 are all contained within a protective sheath 106 which surrounds and protects the components within the high voltage cable 100 from the environment outside the high voltage cable 100.

Typically, the conductors 102 and the optical fibre bundle 104 run the length of the high voltage cable 100. It will be appreciated that although the high voltage cable 100 has been described as having a plurality of optical fibres, high voltage cables are also possible having only a single optical fibre running within the high voltage cable. A bundle of optical fibres provides redundancy, however, in the event that any single optical fibre becomes damaged. The high voltage cable 100 may be used to transfer energy from one location to another. The route of the high voltage cable 100 may involve some parts that are subterranean and other parts that are subsea, for example.

[0021] Figure 2 is an illustration an embodiment of a high voltage transmission line system according to the present disclosure. The high voltage cable 100 is substantially as described in relation to Figure 1 above. As shown in Figure 2 a point on the high voltage cable 100 is subject to a fault 108. The fault 108 is characteristic of damage to an insulation layer (not shown) around one or more of the conductors 102 in the high voltage cable 100 and can result in arcing between the conductors 102 of the cable 100.

[0022] In the system of Figure 2, the conductors 102 are connected to a pulse generating unit 200. At least one optical fibre in the optical fibre bundle 104 is connected to a distributed acoustic sensing unit 300. The pulse generating unit 200 (commonly referred to in the electricity industry as a 'thumper') injects a high energy pulse into the cable conductor 102 and the DAS unit 300 is used to detect a subsequent 'bang' as the energy discharges into the surrounding earth/sea at the point of fault 108. The 'bang' that is emitted from the fault 108 is an acoustic pulse, which results in an impact shockwave impinging upon the bundle of optical fibres 104. Thus, when high voltage power is applied to the faulty high voltage cable 100, discharge of current at the location of the fault 108 causes a 'bang', which creates an acoustic signal. Although a pulse generating unit 200 has been described as providing the high energy pulse needed to cause a 'bang' at the location of the fault 108, it will be appreciated that any suitable unit which produces a high energy signal can be used. Indeed, in accordance with the present disclosure, the DAS unit 300 can monitor the cable 100 continuously to detect a spontaneous 'bang' as a fault emerges.

[0023] Figure 3 is a diagram representing steps in a method for identifying a discharge in a high voltage transmission line. In order to locate a fault 108 in a high voltage cable 100, an optical pulse 402 is applied to an optical fibre of the optical fibre bundle 104 running through the high voltage cable 100. The impact shockwave resulting from the 'bang' at the fault 108 perturbs the bundle of optical fibres 104 at the location of the fault 108. Accordingly, a reflection 404 of the optical signal 402 travelling in the optical fibres 104 is generated at the reflection point 109 which corresponds to the location of the fault 108. The reflection 404 will typically have a reduced amplitude compared to the optical signal 402, as only some of the optical signal 402 will be reflected. Further spectral transformations to the optical signal 402 may also be present in the reflection 404 as a result of the interaction of the impact shockwave with the optical fibre bundle 104 at the time the optical signal 402 is travelling down the optical fibre.

[0024] Thus, the DAS technology can be applied to HV power cables, with embedded or adjacently routed optical fibres, to pre-locate cable faults. Pre-location is the description used by fault engineers to give an approximate location of a cable fault, based on the distance along the cable route of a low energy electrically reflected pulse.

[0025] The DAS unit 300 comprises three main components which convert standard optical fibre cables into an array of virtual sensors along the cable's length. An interrogator unit comprises the laser source that sends a pulse of light into the optical fibre. The backscattered light returns down the same optical fibre to a sensor in the interrogator unit. A processing unit receives data from the interrogator unit and processes the acoustic information. The processing unit monitors each 10 metre channel continuously to identify the presence of specific acoustic events characteristic of a discharge event using pre-programmed algorithms. The interrogator unit and processing unit are typically located together. The DAS unit 300 further comprises a user interface which displays the real time data that the processing unit has processed into clear and understandable information. Alerts may be displayed when a discharge event has taken place. The discharge event may be characterised by one or more of the following attributes: threshold, i.e. the detected signals are above a defined value; signature, i.e. the acoustic profile is used to classify the discharge event; behaviour, i.e. the reflected signals are measured over time to determine threat levels.

[0026] The user interface could be a monitor, tablet or phone, for example. There could be one or many and located locally or anywhere remotely from the interrogator and processing units. In some embodiments, the user interface may comprise text alerts sent to a mobile phone.

[0027] Using known techniques, it is possible to estimate the pre-location position to within approximately 10 metres of the optical fibre route. However, the optical fibre route may be a different length from the main HV cable route due to maintenance loops on the optical fibre, slight route deviations etc. A more precise pin-point estimate of the location of the fault 108 can be found by causing artificial acoustic signals, such as shockwaves, at the pre-located position by stamping on the ground above the cable fault or causing an underwater noise next to the subsea cable by piloting a boat over the cable, etc. Since the artificial acoustic signals are created from a known location, the detection and estimated location of the artificial signals by the DAS unit 300 can be used to correct or refine the estimated fault location. In some embodiments, the acoustic signals may be caused by a propulsion motor of a boat.

Accordingly, the boat may not even need specialist emitting equipment to be able to be used in the presently described fault locating system. It will be appreciated that the artificial acoustic signals may be generated in a number of different ways. For example, by divers or remotely operated vehicles or underwater speakers in the vicinity of the high voltage cable.

[0028] It is anticipated that this technique can be used in a large range of applications where high voltage power cables can develop faults. In particular, railways, windfarms, solar parks, and other generators of electricity, to the much more extensive Distribution Network Operators (DN0s) formerly Regional Electricity Companies (RECs), the National Grid and other global transmission/distribution system operators.

[0029] The DAS unit 300 may be used to detect other characteristics of the HV cable 100, such as oscillation due to wave motion or perturbation due to the proximity of vehicles.

[0030] Although the presently described system has involved using a pulse generating unit to provide the high voltage pulse to cause the 'bang' at the location of the fault, it will be appreciated that the 'bang' may also be detected at the time the fault first occurs. To do this, the distributed acoustic sensing unit may be connected to the optical fibre during normal operation of the high voltage cable. The embodiment may be referred to as use of DAS for continuous online monitoring of cables. In this way, an operator of the cable may be provided with live pre-location data almost immediately after a fault occurs. This is achieved by sensing the mechanical shock caused by the sudden discharge of fault energy in a similar way to using the thumper in the offline fault finding application. It will be apparent that the benefit of the online system is the much reduced fault location time by almost immediately knowing the pre-location position of the fault. This allows the process of mobilising the appropriate assets (people and/or equipment) to pin point the fault location to begin as soon as possible after detection of the fault. Not only does this remove the delay of pre-location work, but also the delay introduced by not knowing which specialist resource is needed to fix the fault according to the fault location i.e. offshore/onshore staff, traffic management, landowner's permission etc. The operators of subsea cables are likely to benefit particularly from this reduced delay, which implies significantly reduced maintenance and repair costs.

[0031] By providing information about fault location determined by online monitoring to insurance companies and warranty surveyors, this may provide a useful source of evidence for settling disputes and attributing costs for damage caused to high voltage power cables during other works in the same area.

[0032] In summary, a method of identifying a discharge 108 in a high voltage transmission line 100 comprises applying an optical signal to an optical fibre 104 associated with the transmission line, sensing reflections at specific spectra of the optical signal received from the optical fibre, and processing the reflections of the optical signal whereby to identify reflections of the optical signal characteristic of a discharge in the transmission line.

[0033] Throughout the description and claims of this specification, the words 'comprise' and 'contain' and variations of them mean 'including but not limited to', and they are not intended to (and do not) exclude other components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0034] Features, integers, characteristics or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0035] The present disclosure also extends to the following numbered clauses: 1. A method of identifying a discharge in a high voltage transmission line, the method comprising: applying an optical signal to an optical fibre associated with the transmission line; sensing reflections of the optical signal received from the optical fibre; and processing the reflections of the optical signal whereby to identify reflections of the optical signal characteristic of a discharge in the transmission line.

2. A method as defined in clause 1, the method further comprising the step of: applying an electrical pulse to the transmission line, whereby to cause the discharge in the high voltage transmission line.

3. A method as defined in clause 1 or 2 further comprising the step of determining an estimated location in the transmission line of the discharge, based on the propagation time of the reflections of the optical signal.

4. A method as defined in clause 3 further comprising the steps of: detecting reflections of the optical signal due to an acoustic pulse applied to at least one known location on the transmission line; and determining an actual location of the discharge based on the known location, the estimated location and the detected reflections due to the acoustic pulse.

5. A method as defined in any preceding clause, wherein the optical fibre is mounted to the transmission line.

6. A method as defined in any preceding clause, wherein the high voltage transmission line is a high voltage cable.

7. A method as defined in clause 6, wherein the high voltage cable comprises the optical fibre.

8. Fault-finding apparatus configured to identify a discharge in a high voltage transmission line, the apparatus comprising: a laser configured for coupling to an end of an optical fibre associated with the transmission line, and configured to apply an optical signal to the optical fibre; a sensor configured for coupling to the end of the optical fibre, and configured to sense reflections of the optical signal received from the optical fibre and to generate an output characteristic of the sensed reflections; and a data processor configured to process the output of the sensor, whereby to identify reflections of the optical signal characteristic of a discharge in the transmission line.

9. Fault-finding apparatus as defined in clause 8, further comprising a signal generator configured for applying an electrical pulse to the transmission line, whereby to cause the discharge in the high voltage transmission line.

10. Fault-finding apparatus as defined in clause 8 or 9, wherein the data processor is configured to determine an estimated location in the transmission line of the discharge, based on the propagation time of the reflections of the optical signal.

11. Fault-finding apparatus as defined in clause 10, wherein the data processor is configured to identify in the output of the sensor reflections of the optical signal due to an acoustic pulse applied to at least one known location on the transmission line and to determine an actual location of the discharge based on the known location, the estimated location and the identified reflections due to the acoustic pulse.

12. Computer software which programs a data processor to operate as the data processor in the fault finding apparatus of any of clauses 8 to 11.

Claims

CLAIMS1. A method of identifying a discharge in a high voltage transmission line, the method comprising: applying an optical signal to an optical fibre associated with the transmission line; sensing reflections of the optical signal received from the optical fibre; processing the reflections of the optical signal whereby to identify reflections of the optical signal characteristic of a discharge in the transmission line; determining an estimated location in the transmission line of the discharge, based on the propagation time of the reflections of the optical signal; detecting reflections of the optical signal due to an acoustic pulse applied to at least one known location on the transmission line; and determining an actual location of the discharge based on the known location, the estimated location and the detected reflections due to the acoustic pulse.
2. A method as claimed in claim 1, the method further comprising the step of: applying an electrical pulse to the transmission line, whereby to cause the discharge in the high voltage transmission line.
3. A method as claimed in any preceding claim, wherein the optical fibre is mounted to the transmission line.
4. A method as claimed in any preceding claim, wherein the high voltage transmission line is a high voltage cable.
5. A method as claimed in claim 4, wherein the high voltage cable comprises the optical fibre.
6. Fault-finding apparatus configured to identify a discharge in a high voltage transmission line, the apparatus comprising: a laser configured for coupling to an end of an optical fibre associated with the transmission line, and configured to apply an optical signal to the optical fibre; a sensor configured for coupling to the end of the optical fibre, and configured to sense reflections of the optical signal received from the optical fibre and to generate an output characteristic of the sensed reflections; and a data processor configured to: process the output of the sensor, whereby to identify reflections of the optical signal characteristic of a discharge in the transmission line; determine an estimated location in the transmission line of the discharge, based on the propagation time of the reflections of the optical signal; identify in the output of the sensor, reflections of the optical signal due to an acoustic pulse applied to at least one known location on the transmission line; and determine an actual location of the discharge based on the known location, the estimated location and the identified reflections due to the acoustic pulse.
7. Fault-finding apparatus as claimed in claim 6 further comprising a signal generator configured for applying an electrical pulse to the transmission line, whereby to cause the discharge in the high voltage transmission line.
8. Computer software which programs a data processor to operate as the data processor in the fault finding apparatus of claim 6 or claim 7.