Power cable health monitoring method

Abstract

A power cable health monitoring method for determining degradation of a mechanical property of a power cable using distributed optical fiber sensing is disclosed. The method includes receiving an input signal from an optical fiber (3) coupled to a power cable (1) conducting a current in the presence of a bias magnetic field (11). The input signal is a measurement of strain on the optical fiber. The method includes receiving a current signal from a current having a fundamental frequency, measuring a first characteristic of the input signal at the fundamental frequency of the current signal and / or an odd harmonic thereof, and measuring a second characteristic of the input signal on the optical fiber at a second harmonic frequency of the current signal or an associated harmonic thereof. The measured characteristic is either amplitude or phase. The method includes extracting a numerical value of the first characteristic and a numerical value of the second characteristic, generating a degradation rate that is a ratio of the numerical value of the first characteristic to the numerical value of the second characteristic, and providing an output signal including the degradation rate over time.

Description

[Technical Field]

[0001] The present invention relates to identifying mechanical faults in power cables using signals provided by a fiber optic cable mechanically coupled to the power cable. [Background technology]

[0002] Transmission power cables are critical and expensive infrastructure, and failures are expensive to locate and repair. This is especially true for buried submarine power cables. Typically, the failure process is slow. The outer jacket is worn or damaged by impact, and then abrasion or corrosion on the armor wire begins. As shown in Figure 2, the outer jacket wears away, thereby exposing the armor wire (7). Fatigue can also be an issue, with highly loaded strands of the armor wire (7) breaking and transferring stress to other wires in an ever-increasing number of cycles until failure.

[0003] If this gradual degradation of the cable can be detected, measures can be put in place to slow or prevent the process. This detection is difficult, and often the first knowledge of this gradual failure process is when catastrophic failure occurs. Those skilled in the art will appreciate that the construction of high voltage transmission lines is complex, with water-blocking, shielding, strengthening, packing, and conducting components (2). This must be considered in the following discussion of measuring strain in a cable and how that strain relates to the gradual failure of the cable.

[0004] One method for measuring strain in power cables is to use distributed fiber optic sensing. This is particularly useful for this application because many existing power cables contain fiber optic cables within the cable structure for other purposes. While several different fiber sensing techniques exist for measuring strain, any of these techniques that take longer than about 10 seconds to measure (e.g., distributed strain sensing, DSS) have very limited application due to the nature of the fiber cable structure. Fibers within power cables are typically in a "loose tube" configuration, which means that they do not adequately couple to the mechanical strain state of the power cable over longer timescales.

[0005] Rayleigh backscattering techniques can be used to measure changes in strain state very rapidly (>1000 times per second) with high sensitivity. This technique is commonly known as distributed acoustic sensing, or DAS, and is most applicable to the methods described below. However, those skilled in the art will understand that other fiber sensing techniques, such as fiber Bragg gratings or high-speed DSS, can be used, and the use of DAS for the following examples is not intended to limit the ideas to DAS. It is also possible that special cables can be designed in which the fiber is mechanically well coupled to the cable structure. This type of fiber optic cable is known as a "tight buffer," and this structure is equally applicable to the invention proposed below.

[0006] Power cables are typically constructed with a loose-tube configuration of fiber. In these situations, strain changes are the only realistic method for analyzing the cable. Prior art exists that uses DAS to measure strain changes in power cables to determine some information about the cable, such as burial depth or curvature. One example of dynamic strain-related measurements performed on comparable structures is suspension bridge cabling. Here, the acoustic signature of individual strand breaks in the load-bearing cable can be picked up with a microphone or DAS; this approach can also work for subsea cables. This signal is transient in nature and can be difficult to measure in, for example, a noisy subsea environment. This is not the approach presented here.

[0007] While power cables are subject to forces from various sources in the environment, the forces used in this disclosure arise from the current carried by the cable itself. In a three-phase cable, current flow is not unidirectional, but power (5) is delivered in one direction. The forces of interest here are related to the frequency of current variation in the cable. This frequency is called the fundamental frequency, F0, and is typically 50 Hz or 60 Hz for AC power cables. In cables that conduct DC current, the current is always unidirectional (unlike AC cables), but there is also variation in the amplitude of the current at a known frequency, which is typically 100 Hz or 120 Hz. Both AC and DC cables are suitable for use with the present invention below, and the analysis of the AC example is not intended to limit the discussion to AC.

[0008] In a cable with multiple conductors, if both conductors carry current, a force (magnetic) will be generated between any two of those conductors. For known currents and cable dimensions, this force is very well defined and easily calculated. In the case of a typical three-core AC cable, it will exude radial (4) and tangential (6) forces acting on the individual conductors, which will be transmitted oscillatorily (if present) to the fibers in a complex but detectable manner. The radial and tangential forces manifest themselves as axial strains in a manner determined by the effective Poisson's ratio of the entire cable structure.

[0009] If the cable is bent or otherwise deformed, the magnitude of this strain transmission to the fiber can be expected to vary, and this has been the subject of previous patent applications. It is important to understand that all of these magnetic forces have a minimum frequency of twice the fundamental frequency. Due to the distortion resulting from the nonlinearity of deformation within the cable, harmonics at n times the fundamental frequency (where n is a positive integer) are present, but no signal on the fiber at the fundamental frequency is expected. Also, there is no axial magnetic force component.

[0010] Current flow in the cable can also generate internal forces through another mechanism, namely magnetostriction of the cable armor, which produces a measurement signal that is smaller than that resulting from the force of the current.

[0011] Magnetostriction is a magnetic effect that can occur in ferromagnetic materials, whose dimensions depend on the local strength of a magnetic field. It is due to the rotation of magnetic domains within the material when a field is applied, and occurs effectively instantaneously. The steel armor wire (7) present in most power cables exhibits this magnetostrictive property and is the cable's primary load-bearing component. The use of steel armor (7) to detect magnetic field changes in fiber optic cables has been the subject of a previous patent. The magnetostrictive effect is symmetric with respect to the magnetic field, B = 0, so that, in the absence of a bias field, an alternating current at the fundamental frequency is expected to generate magnetostriction at twice the fundamental frequency. The armor magnetostriction transmits a dominant axial strain (8) through the power cable structure to the fiber. The detectable signal at twice the fundamental frequency is the sum of all of these individual contributions, which oscillate in unison at that frequency (10) in response to the forces experienced by the fiber from the relationship between each conductor (9).

[0012] In this specification, a frequency that is twice the fundamental frequency (or first harmonic frequency) is referred to as the second harmonic frequency. Summary of the Invention

[0013] According to a first aspect of the present invention, there is provided a power cable health monitoring method for determining degradation of a mechanical property of a power cable having distributed optical fiber sensing, the method comprising: The method comprises: receiving an input signal from an optical fiber coupled to a power cable, the power cable conducting a current in the presence of a bias magnetic field, the input signal being a measurement of strain on the optical fiber; receiving a current signal having a fundamental frequency from a current; measuring a first characteristic of the input signal at a fundamental frequency of the current signal and / or an odd harmonic thereof, and measuring a second characteristic of the input signal on the optical fiber at twice the fundamental frequency of the current signal or an associated harmonic thereof, the characteristic measured being either amplitude or phase; extracting a value of a first characteristic and a value of a second characteristic; generating a deterioration rate that is a ratio of the value of the first characteristic to the value of the second characteristic; and providing an output signal comprising the rate of deterioration over time.

[0014] Optionally, the input signal includes multiple channel input signals. Optionally, each channel input signal is a measurement d of strain at a different specific location along the length of the optical fiber. Optionally, for each channel input signal, one first characteristic and one second characteristic are measured. Optionally, numerical values ​​are extracted to generate one degradation rate for each specific location. Optionally, the output signal includes one degradation rate for each specific location along the length of the optical fiber over time.

[0015] In some cases, the bias magnetic field is the ambient magnetic field from the Earth.

[0016] In some cases, the bias field results from passive, accidental, or intentional magnetization of components of the cable.

[0017] In some cases, the bias field results from a net current flow in the cable being analyzed.

[0018] In some cases, the bias field is generated external to the cable.

[0019] Optionally, the reference current signal is used to increase the gain of the fundamental frequency of the current signal.

[0020] Optionally, the reference current signal is from another sensor, which may be another optical fiber, which may be associated with or coupled to the power cable.

[0021] In some cases, the reference current signal is a current signal from one particular location along the length of the optical fiber.

[0022] In some cases, the first characteristic and the second characteristic are amplitudes.

[0023] Optionally, the amplitude is normalized by the current signal.

[0024] Optionally, the fundamental frequency is measured at each location to provide a location-specific fundamental frequency signal.

[0025] Optionally, the amplitudes are normalized to either or both the average of all of the location-specific fundamental frequency signals and / or the average of the location-specific second harmonic frequency signals.

[0026] Optionally, the amplitude is normalized to a selected one or more of the location-specific fundamental frequency signal and / or the location-specific second harmonic frequency signal.

[0027] Optionally, the fundamental frequency signal and the second harmonic frequency signal each have an amplitude. Optionally, the amplitudes of the fundamental frequency signal and the second harmonic frequency signal are first normalized by the amplitude at each particular location at (fundamental frequency)-m and (2×fundamental frequency)-m, respectively, where m represents a small frequency shift away from the electrically dominant frequency and subtracts the contribution of background noise.

[0028] In some cases, the contribution from the Earth's magnetic field is first subtracted, leaving the locally induced variations.

[0029] Optionally, the deterioration rate increases as the power cable deteriorates. Optionally, the output signal activates an alert when the deterioration rate reaches a predetermined deterioration rate alert level.

[0030] Optionally, the amplitudes are summed over multiple periods, which may be tidal periods, to improve the signal-to-noise ratio.

[0031] According to another aspect of the invention, there is provided a computer program product comprising instructions which, when executed by a computer, cause the computer to carry out the method described above.

[0032] According to another aspect of the present invention, there is provided a computer readable medium having stored thereon a computer program product as described above.

[0033] Optionally, the method includes any of the features or steps set forth in any of the preceding or following descriptions.

[0034] Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: [Brief explanation of the drawings]

[0035] [Figure 1] The power cable (1) contains three copper conductors (2) carrying three electrical phases and a monitoring fiber optic cable (3). When power (5) is delivered, a circumferential magnetic field (6) is generated, generating forces in both the tangential (6) and radial (4) directions. [Figure 2] Power cables typically feature a steel spiral armor made up of many individual strands (7). In the presence of a fluctuating magnetic field, these exhibit magnetostrictive characteristics, which induce axial forces (8) within the cable that are transmitted by coupling to the fiber optic cable (3). [Figure 3] The generation of three-phase power generates a magnetic field (9) from each of the cores that can be measured at any point around the circumference of the cable. This generates a varying strain (10) in the fiber optic cable through a magnetostrictive response, characterized by a periodic response at twice the fundamental frequency of the generation. [Figure 4] The cable may experience the presence of a bias field (11) that provides a DC offset to the dynamic magnetic field (shown as the Earth's magnetic field (11), but additionally formed from a range of sources including passive or intentional magnetization of the armor), or the presence of external ferromagnetic materials. [Figure 5]The magnetic field (12) at the cable surface is modified by the presence of a DC bias offset (13), and the resulting axial distortion from the magnetic distortion now experiences both the original double fundamental frequency signal and an additional fundamental frequency component, as shown by the difference in height of the peak (14). [Figure 6] Varying the strength of the magnetic bias field in a typical cable changes the axial strain response, which exhibits a double fundamental frequency behavior in the absence of bias (15), a bias equivalent to the typical Earth's magnetic field (0.05 mT), and the emergence of a stronger fundamental frequency signal (16) as the field is increased to 0.5 mT and 1 mT (17, 18) by additional means such as cable magnetization (16, 17). [Figure 7] A plot of the amplitude of the fundamental (50 Hz) and second harmonic (100 Hz) signals along the fiber length is shown. There is a section of cable with joints, junction boxes, and ferromagnetic infrastructure (19), where the 50 Hz magnitude signal is clearly stronger. The beach begins at channel 230 (20), and after this point, the 50 Hz signal can be seen to become much lower. The 100 Hz signal, while showing complex behavior, is relatively unchanged by comparison. [Figure 8] The plots show the variation of the amplitude of the 50 Hz (top) and 100 Hz (bottom) signals over one tidal cycle (12 hours). Two areas are highlighted: the free span area (21) and the repaired section of the cable (22). In the free span area (21), a strong 50 Hz signal (darker trace) is visible between hours 4 and 10, indicating an increasing level of armor stress affecting the 50 Hz signal. The repaired section (22), exposed to tidal currents, shows similar behavior. The 100 Hz signal remains relatively constant over time. This indicates a static power level, and these results confirm the validity of the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0036] In the drawings, where multiple instances of the same or similar features are present, for clarity, only a representative one or some of the instances of the feature are provided with a numerical reference.

[0037] A power cable health monitoring method for determining degradation of a mechanical property of a power cable (1) with distributed optical fiber sensing (3) is disclosed. The method includes receiving an input signal from an optical fiber (3) coupled to the power cable (1) conducting a current (5) in the presence of a bias magnetic field (11). The input signal is a measurement of strain on the optical fiber. The method includes receiving a current signal from a current having a fundamental frequency, measuring a first characteristic of the input signal at the fundamental frequency of the current signal and / or an odd harmonic thereof, and measuring a second characteristic of the input signal on the optical fiber at a second harmonic frequency of the current signal or an associated harmonic thereof. The measured characteristic is either amplitude or phase. The method includes extracting a numerical value of the first characteristic and a numerical value of the second characteristic, generating a degradation rate that is a ratio of the numerical value of the first characteristic and the numerical value of the second characteristic, and providing an output signal including the degradation rate over time.

[0038] The inventors recognized that in the presence of a local magnetic bias field, the induced magnetic asymmetry (12) leads to an additional fundamental frequency component (14) in the dynamic strain signal measured in the fiber. If present, this fundamental frequency signal is related only to the axial cable force. Calculations and measurements reveal that bias fields of various origins and strengths exist in real cables. This is due in part to the Earth's magnetic field (11) and in part to residual or induced magnetism in the steel.

[0039] The inventors further recognized that the magnitude of the magnetostriction is highly dependent on the axial tensile stress of the armor wire. Any change to the fundamental frequency strain signal (measured as either an amplitude or phase measurement) indicates a change in either the bias field or the tensile stress of the armor wire (cable strength member). Because the amplitude of the signal at twice the fundamental frequency (i.e., the second harmonic frequency) is largely unaffected by the bias state, the ratio change in the signals at one and two times the fundamental frequency can be used as an indicator of the change in the axial strain state. This ratio can be used to indicate the deterioration of the condition of the electrical cable, and is therefore referred to as the deterioration rate. Usefully, a higher deterioration rate indicates increased deterioration of the power cable.

[0040] The order of the modulus provides a different perspective on the force interaction since the modulus contains harmonics of the fundamental.

[0041] The mechanism by which armor tensile loading affects the magnitude of magnetostrictive strain is well documented in the literature. Increasing tensile stress significantly reduces the magnitude of magnetostrictive strain for a given magnetic field change. Compressive stress has a much smaller effect. A sudden increase in tensile load on a section of cable (e.g., from a broken armor wire) causes a sudden decrease in the magnetostrictive strain component. This is primarily manifested in the fundamental frequency band, where this signal is not obscured by other, larger factors. This signal change can be used to notify power cable owners of areas of high stress requiring maintenance before cable failure occurs. Those skilled in the art will recognize that any odd multiple of the fundamental frequency can also be used to track these changes. These harmonics exist due to the nonlinearity of the system's response to magnetic field changes.

[0042] Varying the level of the DC bias signal (Figure 6) modulates the fundamental frequency component. This example shows a typical power conduction situation as the bias field is increased from 0 T (15) to 0.050 mT (16) (approximately equivalent to the Earth's magnetic field), and then to larger biases of 0.5 mT (17) and 1 mT (18), indicating some residual or induced magnetization in the cable. In an installation, the contribution from the Earth's magnetic field may be subtracted, leaving locally induced variations, or alternatively, may be trended over time to reveal the changing state of cable stress.

[0043] Several signal processing methods can be used to extract the phase and / or amplitude of either the fundamental frequency signal (e.g., 50 Hz) or harmonics (e.g., the second harmonic (100 Hz) etc.) from the background noise, or to improve the signal-to-noise ratio.

[0044] In one example, one method is to compare the amplitude of the signal within a narrow frequency window centered around the fundamental frequency with the amplitude of a signal from a nearby but distinct frequency window that is not associated with electrical conduction activity. This distinct "noise" window can then be subtracted from the signal window.

[0045] In another example, another method is to compare to a known reference signal. A reference electrical signal from either a separate sensor and / or a defined section of the power cable in which the fiber is embedded can be used to determine the gain of the fundamental frequency of the electrical signal. The amplitude of the signal received from the fiber sensor can be normalized by the known current flowing in the cable. Alternatively, an averaged signal from all or a portion of the fiber can be used for normalization.

[0046] Another normalization technique is to take the amplitude of the fundamental frequency and twice the fundamental frequency signals and normalize each fiber distance segment channel by the amplitude at (fundamental frequency)-m and (2×fundamental frequency)-m, where m represents a small frequency shift away from the electrically dominant frequency to remove the contribution of background noise.

[0047] Furthermore, the signal-to-noise ratio can be improved by a stacking process, whereby time slices corresponding to similar expected stimuli (e.g., slices per tidal cycle to extract tidal-cycle-induced distortions) are summed. This signal-to-noise ratio gain technique is not limited to tidal stacking; first-time stacking is one approach, and stack segments experiencing similar current flow or weather are another.

[0048] There are other ways to extract the reference signal, and the examples presented here are not intended to limit the scope of the present invention.

[0049] Example 1 In this Example 1, data is acquired from a live subsea power cable operating at a fundamental electrical frequency of 50 Hz. A fiber optic cable is embedded within the power cable structure, and a distributed acoustic sensing system is used to extract tension data at all points along the fiber. The measured section of cable is 30 kilometers long. When the magnitude of the 50 Hz signal from the fiber optic cable is extracted from approximately one hour's worth of data and averaged, its variation over distance along the fiber can be examined. As shown in Figure 7, this signal varies with position along the cable. The section of cable from which this data is extracted contrasts with the cable laid on land, where there are several channels with large signals corresponding to junction boxes, joints, and other ferromagnetic infrastructure that affected the local field bias (19). Once the cable exits channel 230 (20) and heads out to sea, these external influences disappear and the 50 Hz signal drops to a much lower level. The amplitude of the 100 Hz signal shows a different pattern; in this case, the cable's sensitivity to radial and tangential forces is of primary importance over the local bias field.

[0050] Example 2 A particular section of the cable described in Example 1 was found to be free-span, i.e., unsupported for a significant distance in the subsea section. During changes in tidal current velocity, the stress on this cable varied due to the drag exerted by the flowing water. Nearby was a section of cable that had previously been broken and subsequently repaired; this repair was exposed to the seabed and therefore also subject to forces from the flowing water. Figure 8 plots the amplitudes of the 50 Hz and 100 Hz signals over a 12-hour period from the free-span (21) and one of the cable repair (22) locations. While the amplitude of the 100 Hz signal remained stable over the tidal cycle (approximately 6 hours during high flow), the same cannot be said for the 50 Hz signal. This change in amplitude can be attributed to changes in armor stress and, therefore, the magnitude of the magnetostrictive response.

[0051] Example 3 The inventors have also discovered that the method can utilize phase related signals to determine degradation of the power cable.

[0052] Changes in the detected phase of the magnetic signature directly correlate with changes in the mechanical condition of the cable. In a typical trefoil design for power cables, the fiber optic cable must remain in the same position relative to the conductors along the entire length of the power cable unless the electrical phase or position of the fiber is physically changed.

[0053] Each conductor generates a magnetic field that can be indirectly detected using the fiber optic cable, but the magnitude and phase of the contribution from each conductive element is different. During generation, the phase is carefully maintained at 120 degrees between each conductor, and the magnitude from each conductor is determined by a number of factors, including the distance between the conductor and the fiber optic cable, and the contribution of the armor to the overall stiffness of the cable.

[0054] The overall signal detected using the fiber is the sum of these contributions from each conductor. The contribution of each conductor to the overall detectable signal is strongly dependent on the distance between the conductor and the fiber, so that as this distance changes, the summed signal also changes in both magnitude and phase.

[0055] Either magnitude or phase can be used to detect changes in the position of the conductive element relative to the fiber optic cable, but phase, as opposed to magnitude, generally and advantageously contributes to extracting the change signal because it is believed to be time-invariant.

[0056] This temporal stability means that a long averaging time can be used to extract the changing signal. This approach means that small deformations in the cable can be detected as local changes in phase relative to the average phase measured from other parts of the cable, using either optical fiber or other sensors. Another aspect that changes the total summed signal that can be detected by the fiber is if the armor begins to deteriorate or break down. This therefore also leads to changes in the phase and magnitude of the detectable signal.

[0057] Discussion of the invention Embodiments of the present invention relate to measurement methods and devices necessary for monitoring mechanical degradation of power cables. Degradation is related to failure of strength components of the power cable due to fatigue or overstress. Additionally or alternatively, degradation can be due to a reduction in the cross-sectional area of ​​the strength members due to, for example, wear or corrosion.

[0058] In one embodiment, the Earth's magnetic field is used to provide the magnetic bias. This field, while small, is well known. As the orientation of the cable relative to the magnetic field changes, the bias changes, and therefore the amplitude of the signal at the fundamental frequency. Following a length of cable with a spatially varying path, this provides a gradually varying background to the fundamental frequency signal proportional to the local Earth's magnetic field, which can be normalized to account for local variations that depend on cable strain, anomalies, and tensile stresses, compared to a relatively constant double fundamental frequency signal.

[0059] In a further embodiment, the current load of the axially stressed cable induces some degree of permanent magnetism in the spatially coupled ferromagnetic armor strands at the location of maximum strain, thereby creating a magnetic bias and thus affecting the fundamental frequency signal.

[0060] In some embodiments, the bias field may be intentionally created or enhanced, for example, by magnetizing the armor during deployment. The response of the cable to the magnetic field may also be tailored through material selection and armor treatment.

[0061] In one embodiment, the current conducted by the cable itself can generate a bias field. In DC cables, the current itself generates a very significant bias field, and DC ripple can be used to generate distortion signals at the fundamental frequency.

[0062] In a further embodiment, the current can be measured using a separate instrument and the fiber sensing signal correlated with the electrical signal of one of the conductors in the cable to improve the signal-to-noise ratio. This methodology is one example, although others exist, of an approach that can also reveal phase shifts over time. Phase shifts can also indicate changes in the mechanical state of the cable, as this means that the mechanical load is distributed differently around the cable's circumference. An alternative to using a separate sensor to correlate the signal is to use an average value of all the cable and / or several specifically selected reference channels. For example, the reference channel could be near the beginning of the cable, close to a noisy transformer.

[0063] These embodiments described above represent examples of possible ways to exploit the asymmetry created by the bias field. Those skilled in the art can envision other methodologies for exploiting this effect. These examples do not limit the scope of the invention.

[0064] Any of the features or steps of any of the embodiments shown or described may be combined in any suitable manner within the scope of the overall disclosure herein.

[0065] Final findings Thus, a method for monitoring degradation of inaccessible remote power cables is provided. The method allows for long distance monitoring of degradation. Advantageously, in some embodiments, no additional hardware is required beyond what is already present in terms of sensors. Furthermore, the method provides a simple output to identify the level of degradation.

Claims

1. 1. A power cable health monitoring method for determining degradation of a mechanical property of a power cable using distributed optical fiber sensing, comprising: receiving an input signal from an optical fiber coupled to a power cable, the power cable conducting a current in the presence of a bias magnetic field, the input signal being a measurement of strain on the optical fiber; receiving a current signal having a fundamental frequency from the current; measuring a first characteristic of the input signal at the fundamental frequency of the current signal and / or an odd harmonic thereof, and measuring a second characteristic of the input signal on the optical fiber at twice the fundamental frequency of the current signal or a related harmonic thereof, wherein the measured characteristic is either amplitude or phase; extracting a value of the first characteristic and a value of a second characteristic; generating a deterioration rate that is a ratio between the value of the first characteristic and the value of the second characteristic; providing an output signal comprising said rate of deterioration over time; The method comprising:

2. 2. The method of claim 1, wherein the input signal comprises a plurality of channel input signals, each channel input signal being a measurement of strain at a different specific location along the length of the optical fiber, and for each channel input signal, one first characteristic and one second characteristic are measured and numerical values ​​are extracted to generate one degradation rate for each specific location, and the output signal comprises one degradation rate for each specific location along the length of the optical fiber over time.

3. The method of claim 1 or 2, wherein the bias magnetic field is the ambient magnetic field from the Earth.

4. The method of claim 1 or 2, wherein the bias field results from passive, accidental, or intentional magnetization of components of the cable.

5. 3. The method of claim 1, wherein the bias magnetic field results from a net current flow in the cable being analyzed.

6. The method of claim 1 or 2, wherein the bias magnetic field is generated externally to the cable.

7. 10. A method according to any one of the preceding claims, wherein a reference current signal is used to increase the gain of the fundamental frequency of the current signal.

8. The method of claim 7 , wherein the reference current signal is from another sensor that may be coupled to the power cable.

9. The method of claim 7 , wherein the reference current signal is a current signal from one particular location along the length of the optical fiber.

10. 10. The method of any one of the preceding claims, wherein the first and second characteristics are amplitudes, and the amplitudes are normalized by the current signal.

11. The method of claim 2 , wherein the first characteristic and the second characteristic are amplitudes and the fundamental frequency is measured at each location to provide a location-specific fundamental frequency signal.

12. 12. The method of claim 11, wherein the amplitudes are normalized to either or both an average value of all the location-specific fundamental frequency signals and / or an average value of the location-specific second harmonic frequency signals.

13. The method of claim 11 , wherein the amplitude is normalized to a selected one or more of the location-specific fundamental frequency signal and / or the location-specific second harmonic frequency signal.

14. 3. The method of claim 2, wherein the fundamental frequency signal and the second harmonic frequency signal each have an amplitude, and the amplitudes of the fundamental frequency signal and the second harmonic frequency signal are first normalized by the amplitude at a particular location, (fundamental frequency)-m and (2×fundamental frequency)-m, respectively, where m represents a small frequency shift away from an electrically dominant frequency to subtract out contributions of background noise.

15. 4. A method according to claim 3 or any one of the claims dependent thereon, wherein the contribution from the Earth's magnetic field is first subtracted, leaving locally induced variations.

16. 10. The method of any preceding claim, wherein the deterioration rate increases with deterioration of the power cable.

17. 10. A method according to any one of the preceding claims, wherein the output signal activates an alert when the deterioration rate reaches a predetermined deterioration rate alert level.

18. 10. A method according to any one of the preceding claims, wherein the first and second characteristics are amplitudes, and the amplitudes are summed over multiple periods to improve the signal to noise ratio.

19. A computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of the preceding claims.

20. A computer readable medium having stored thereon the computer program product of the preceding claims.

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