Monitoring of power cables with distributed fibre optic sensing

Patent metadata

Field	Value
Publication number	US-10775425-B2
Application number	US-201615738849-A
Country	US
Kind code	B2
Filing date	Jun 23, 2016
Priority date	Jun 29, 2015
Publication date	Sep 15, 2020
Grant date	Sep 15, 2020

Abstract

This application relates to methods and apparatus for monitoring power cables (100) carrying multiple AC phases to detect deformation of the power cable. A distributed fibre optic interrogator unit (302) is used to interrogate a sensing optical fibre (301) coupled to the power cable to provide a measurement signal from each of a plurality of longitudinal sensing portions of the sensing optical fibre. An analyser (602) is configured to analyse the measurements signals to detect a characteristic of an imbalance in magnetic fields. The characteristic may be a signal component with a characteristic frequency related to the power frequency and number of AC phases, the sensing optical fibre may be sensitised to magnetic fields and the characteristic frequency may be 2n times the power frequency where n is the number of phases, e.g. six times the power frequency for three phase AC.

First claim

The invention claimed is:

1. A method of monitoring a power cable carrying multiple AC (alternating current) phases to detect deformation of the power cable, the method comprising:
performing distributed fibre optic sensing on a sensing optical fibre to provide a measurement signal from each of a plurality of longitudinal sensing portions of the sensing optical fibre, the sensing optical fibre being coupled to the power cable; and
analysing said measurements signals to detect a characteristic of an imbalance in magnetic fields wherein said characteristic of an imbalance in magnetic fields is a signal component at a characteristic frequency related to a power frequency of the AC phases.

2. A method as claimed in claim 1 wherein the measurement signal is a signal indicative of strains acting on the power cable and the characteristic frequency is n times the power frequency, where n is the number of AC phases.

3. A method as claimed in claim 1 wherein the sensing optical fibre comprises a magnetically sensitised optical fibre and wherein the measurement signal from each longitudinal sensing portion is indicative of the extent of any variations in magnetic field acting on that longitudinal sensing portion.

4. A method as claimed in claim 3 , wherein said characteristic of an imbalance in magnetic fields is a signal component at a characteristic frequency related to a power frequency of the AC phases and wherein the characteristic frequency is 2n times the power frequency, where n is the number of AC phases.

5. A method as claimed in claim 3 wherein the sensing optical fibre comprises an optical fibre mechanically coupled to a magnetostrictive material.

6. A method as claimed in claim 1 wherein the method comprises detecting a signal component at the characteristic frequency above a threshold level.

7. A method as claimed as claimed in claim 1 wherein the method is performed when a test signal is applied to the power cable, the test signal being a multiple phase AC signal at a voltage lower than the nominal operating voltage for the power cable.

8. A method as claimed as claimed in claim 7 wherein the method comprises identifying the existence of the characteristic of an imbalance in magnetic fields in the measurement signal from a longitudinal sensing portion as a possible defect in the power cable and determining the location of the defect along the power cable from the location of the relevant sensing portion.

9. A method as claimed in claim 7 wherein the method is performed with the test signal prior to energising the power cable to its nominal operating voltage.

10. A method as claimed in claim 1 wherein the method is performed in operation of the power cable and the method comprises detecting whether the measurement signal from a sensing portion exhibits any variation in characteristic of an imbalance in magnetic fields.

11. A method as claimed in claim 10 , wherein the method comprises detecting any variation in any signal component at the characteristic frequency.

12. A method as claimed in claim 10 wherein the method comprises identifying any variation in the characteristic of an imbalance in magnetic fields in the measurement signal from a sensing portion as an indication that the power cable is moving at the location of the relevant sensing portion.

13. A method as claimed in claim 12 wherein the method comprises analysing the variation in characteristic of an imbalance in magnetic fields to determine a characteristic of the movement of the power cable.

14. A method as claimed in claim 1 wherein the power cable is deployed, at least partly, as a submarine cable.

15. An apparatus for monitoring a power cable carrying multiple AC (alternating current) phases to detect deformation of the power cable, the apparatus comprising:
a distributed fibre optic interrogator unit for interrogating a sensing optical fibre coupled to the power cable to provide a measurement signal from each of a plurality of longitudinal sensing portions of the sensing optical fibre; and
an analyser configured to analyse said measurements signals to detect a characteristic of an imbalance in magnetic fields wherein said characteristic of an imbalance in magnetic fields is a signal component at a characteristic frequency related to a power frequency of the AC phases.

16. An apparatus as claimed in claim 15 wherein the sensing optical fibre comprises a magnetically sensitised optical fibre and wherein the measurement signal from each longitudinal sensing portion is indicative of the extent of any variations in magnetic field acting on that longitudinal sensing portions.

17. An apparatus as claimed in claim 16 wherein the sensing optical fibre comprises an optical fibre mechanically coupled to a magnetostrictive material.

18. An apparatus as claimed in claim 15 wherein the analyser is configured to detect a signal component at the characteristic frequency above a threshold level.

Assignees

• Optasense Holdings Ltd

Inventors

• Godfrey Alastair

Classifications

• G02B6/4416

  Heterogeneous cables · CPC title

• G01R31/083 Primary

  in cables, e.g. underground · CPC title

• G01R33/0327 Primary

  with application of magnetostriction · CPC title

• G01R33/072

  Constructional adaptation of the sensor to specific applications · CPC title

• G01D5/12

  using electric or magnetic means (G01D5/06 takes precedence) · CPC title