Devices for monitoring the condition of power transmission media

Abstract

A device (20) for monitoring the condition of a power transmission medium (22) is provided, the device (20) comprising: a signal source (24) for transmitting a signal (30) traveling along the power transmission medium (22); and a monitoring device (26) configured to detect one or more reflected signals (34) in the power transmission medium (22), wherein the monitoring device (24) is configured to use the or each detected reflected signal (34) to determine a change in capacitance of the power transmission medium (22).

Description

Technical Field

[0001] The present invention relates to an apparatus and method for monitoring the condition of a power transmission medium. Background Art

[0002] In power transmission systems, electricity is typically transmitted via overhead lines, submarine cables, and / or underground cables. Summary of the invention

[0003] According to a first aspect of the present invention, there is provided an apparatus for monitoring the condition of a power transmission medium, the apparatus comprising: a signal source for transmitting a signal travelling along the power transmission medium; and a monitoring device configured to detect one or more reflected signals in the power transmission medium, wherein the monitoring device is configured to determine a change in capacitance of the power transmission medium using the or each detected reflected signal.

[0004] For a variety of reasons, the capacitance of the power transmission medium may change during its lifetime. For example, the capacitance of the power transmission medium may change due to aging (such as thermal aging and high voltage aging), particularly at higher frequencies. Such changes may result in conditions of the power transmission medium that are out of an acceptable operating range, and therefore, performance of the power transmission medium that is out of an acceptable operating range.

[0005] The inventors have discovered that time domain reflectometry (TDR) can be used to monitor and determine changes in the capacitance of a power transmission medium, which affects the characteristic impedance (otherwise known as surge impedance) and propagation rate or velocity of the power transmission medium. By successfully determining changes in the capacitance of the power transmission medium, it is possible to accurately monitor the condition of the power transmission medium to allow operators to make any necessary adjustments to maintain its performance within a target operating range and identify any need for maintenance or repair.

[0006] The present invention is particularly useful and cost effective for monitoring the condition of long power transmission media which may be tens or hundreds of kilometers in length. This is because the use of a TDR means that, as opposed to having multiple devices connected at various points along the length of the power transmission medium, a device only needs to be connected to one end of the power transmission medium in order to perform the function of determining a change in the capacitance of the power transmission medium.

[0007] Furthermore, the present invention is readily applicable to any type of power transmission medium used in any type of power transmission system, regardless of the topology.

[0008] In order to determine a change in capacitance of the power transmission medium, the monitoring device may be configured to use the time of arrival of the or each detected reflected signal and / or to use the magnitude or amplitude of the or each detected reflected signal.

[0009] There are different ways of configuring the apparatus of the present invention in order to obtain a wide range of information about changes in the capacitance of the power transmission medium and thereby provide more specific details about the condition of the power transmission medium.

[0010] In embodiments of the invention, the monitoring device may be configured to use the or each detected reflection signal to determine a change in capacitance of the power transmission medium over the length of the power transmission medium.

[0011] Configuring the monitoring device in the above manner enables determination of changes in capacitance across the length of the power transmission medium.

[0012] It may be the case that a change in capacitance only affects a section of the power transmission medium and not all of it, which means that it would be desirable to obtain information about the affected section.

[0013] The use of TDR not only enables the configuration of the apparatus of the invention to obtain information about changes in capacitance of the affected segment, but also enables obtaining the information without being in the vicinity of the affected segment. This is useful when the affected segment is in a remote location.

[0014] In further embodiments of the invention, the monitoring device may be configured to use the or each detected reflection signal to determine a position of a section of the power transmission medium corresponding to a change in capacitance of the power transmission medium.

[0015] Configuring the monitoring device in the above manner is useful for remotely pinpointing the location of an affected section.

[0016] The monitoring device may be configured to use a reference reflected signal based on a reference position in the power transmission medium, and the monitoring device may be configured to compare the or each detected reflected signal with the reference reflected signal to determine the position of the section of the power transmission medium corresponding to the change in capacitance of the power transmission medium. This enables a more accurate determination of the position of the affected section.

[0017] In yet further embodiments of the invention, the monitoring device may be configured to use the or each detected reflection signal to determine a length of a section of the power transmission medium corresponding to a change in capacitance of the power transmission medium.

[0018] Configuring the monitoring device in the above manner enables remote determination of the length of the affected section.

[0019] The monitoring device may be configured to use a reference reflection signal based on a reference position in the power transmission medium, and the monitoring device may be configured to compare the or each detected reflection signal with the reference reflection signal to determine the length of the section of the power transmission medium corresponding to the change in capacitance of the power transmission medium. This enables a more accurate determination of the length of the affected section.

[0020] Preferably, the reference position in the power transmission medium is the position of a discontinuity of characteristic impedance in the power transmission medium. The discontinuity of characteristic impedance may be, for example, a joint, a junction or a terminal in the power transmission medium.

[0021] The monitoring device may be configured to use the or each detected reflected signal to determine changes in capacitance of the power transmission medium over time. This enables the collection of time-varying information about changes in capacitance of the power transmission medium. Such time-varying information can be used not only to schedule any necessary adjustments, maintenance or repairs of the power transmission medium in advance, but also to predict future changes in the condition of another similar power transmission medium.

[0022] In a preferred embodiment of the present invention, the power transmission medium may be, but is not limited to, a power transmission line or cable. Examples include, but are not limited to, overhead lines, submarine cables, and / or underground cables.

[0023] In each of the foregoing embodiments, the apparatus may include circuitry, a processor and / or a computing device configured to determine a change in capacitance of the power transmission medium using the or each detected reflected signal. The apparatus may also include software or computer instructions implementing an algorithm for determining a change in capacitance of the power transmission medium using the or each detected reflected signal.

[0024] According to a second aspect of the invention, there is provided a method for monitoring the condition of a power transmission medium, the method comprising the steps of: transmitting a signal travelling along the power transmission medium; detecting one or more reflected signals in the power transmission medium; and determining a change in capacitance of the power transmission medium using the or each detected reflected signal.

[0025] The features and advantages of the apparatus of the first aspect of the invention and its embodiments apply mutatis mutandis to the method of the second aspect of the invention and its embodiments.

[0026] The method of the invention may comprise the step of using the time of arrival of the or each detected reflected signal to determine a change in capacitance of the power transmission medium.

[0027] The method of the invention may comprise the step of using the magnitude or amplitude of the or each detected reflected signal to determine a change in capacitance of the power transmission medium.

[0028] The method of the invention may comprise the step of using the or each detected reflection signal to determine a change in capacitance of the power transmission medium over the length of the power transmission medium.

[0029] The method of the invention may comprise the step of using the or each detected reflection signal to determine the position of a section of the power transmission medium corresponding to a change in capacitance of the power transmission medium.

[0030] The method of the present invention may comprise the steps of: using a reference reflection signal based on a reference position in the power transmission medium; and comparing the or each detected reflection signal with the reference reflection signal to determine the position of a section of the power transmission medium corresponding to a change in capacitance of the power transmission medium.

[0031] The method of the invention may comprise the step of using the or each detected reflection signal to determine the length of a section of the power transmission medium corresponding to a change in capacitance of the power transmission medium.

[0032] The method of the present invention may comprise the steps of: using a reference reflection signal based on a reference position in the power transmission medium; and comparing the or each detected reflection signal with the reference reflection signal to determine the length of a section of the power transmission medium corresponding to a change in capacitance of the power transmission medium.

[0033] In the method of the invention, the reference position in the power transmission medium is preferably the position of a discontinuity in the characteristic impedance in the power transmission medium.

[0034] The method of the invention may comprise the step of using the or each detected reflection signal to determine a change in capacitance of the power transmission medium over time.

[0035] In the method of the present invention, the power transmission medium may be, but is not limited to, a power transmission line or cable. As mentioned above, examples include, but are not limited to, overhead lines, submarine cables, and / or underground cables. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Preferred embodiments of the present invention will now be described by way of non-limiting examples with reference to the accompanying drawings, in which:

[0037] Figure 1 Schematically illustrates a device according to an embodiment of the present invention;

[0038] Figure 2 shows the change in dielectric constant of power cables due to aging;

[0039] Figure 3shows the change in characteristic impedance of a power cable due to aging;

[0040] Figure 4 shows the change in propagation velocity of a power cable due to aging;

[0041] Figure 5 A model showing a π-segment representation of a power cable;

[0042] Figure 6 Show Figure 1 simulation of devices; and

[0043] Figures 7 to 12 Show Figure 4 The simulation results. DETAILED DESCRIPTION

[0044] The following embodiments of the invention are described with respect to power cables, but it will be appreciated that the following embodiments of the invention are applicable mutatis mutandis to other types of power transmission media, some examples of which are described in the present specification.

[0045] The device according to the embodiment of the present invention is Figure 1 1 and generally designated by reference numeral 20. Apparatus 20 is configured for monitoring a condition of a power cable 22 configured for power transmission, as shown below.

[0046] The apparatus 20 comprises a signal source 24 and a monitoring device 26. In use, each of the signal source 24 and the monitoring device 26 is connected to an end 28 of the power cable 22, which end 28 is referred to herein as the transmitting end 28.

[0047] The signal source 24 in this embodiment is configured as a voltage source. In use, the voltage source generates an electrical signal 30, which is transmitted into the transmitting end 28 so that the transmitted signal 30 travels along the power cable 22. If the characteristic impedance of the power cable 22 is uniform throughout its length, there will be no reflection of the transmitted signal 30. However, when there is a discontinuity 32 in the characteristic impedance of the power cable 22, a portion of the transmitted signal 30 is reflected from the discontinuity 32, and thereby the reflected signal 34 travels back toward the transmitting end 28. At the same time, the remaining portion 36 of the transmitted signal 30 continues to travel along the power cable 22 in the original direction. If the remaining portion 36 of the transmitted signal 30 further encounters another discontinuity in the characteristic impedance along the power cable 22, this will result in the generation of another reflected signal that travels back toward the transmitting end 22.

[0048] The monitoring device 26 includes an electrical sensor, which in this embodiment is a voltage sensor. When the reflected signal 34 arrives at the transmitting end, the voltage sensor detects the voltage amplitude of the reflected signal 34. In addition, by associating time with the detection of the reflected signal 34, the voltage sensor detects the arrival time of the reflected signal 34 at the transmitting end 28. This in turn enables the monitoring device 26 to determine the total travel time of the reflected signal 34 based on the difference between the departure time of the transmitted signal 30 from the transmitting end 28 and the arrival time of the reflected signal 34 at the transmitting end 28.

[0049] Multiple reflections of the transmitted signal 30 may occur in the power cable 22 , resulting in multiple reflected signals being detected by the monitoring device 26 .

[0050] During use, the power cable 22 may experience aging, such as heat aging and high voltage aging. The capacitance of the power cable 22 decreases at a higher frequency due to aging. Figure 2 In the figure, the Figure 2 The change in the power cable dielectric constant is shown, which directly corresponds to the change in the power cable capacitance due to aging. In an ideal case where an exemplary power cable undergoes thermal aging, the drop in the power cable dielectric constant is approximately 10 5 However, in reality, the same power cable undergoes both thermal aging and high voltage aging, which results in a decrease in the dielectric constant of the power cable of approximately 10 compared to the decrease for an ideal case. 4 In both cases, the decrease in the dielectric constant of the power cable due to aging occurs at a relatively high frequency.

[0051] The reduction in capacitance of the power cable 22 has the effect of increasing the characteristic impedance and propagation velocity of the power cable 22, which in turn affects the amplitude and arrival time of the reflected signal.

[0052] Figure 3 The change in characteristic impedance of the power cable 22 due to aging is shown, where the value of the characteristic impedance of the power cable 22 starts at or near the lowest curve and moves toward the highest curve due to aging. The legend shows the value associated with each curve (1000 to 0.1), each curve representing the conductivity (S / m) of the semiconducting shielding layer of the power cable 2.

[0053] Figure 4The change in the propagation velocity of the power cable 22 due to aging is shown, where the value of the propagation velocity of the power cable 22 starts at or near the lowest curve and moves toward the highest curve due to aging. The legend shows the value associated with each curve (1000 to 0.1), which represents the conductivity (S / m) of the semiconductor shielding layer of the power cable 22.

[0054] The inventors have discovered that changes in the capacitance of the power cable 22 can be determined by using TDR, in particular by using the voltage amplitude and arrival time of the detected reflected signal 34 .

[0055] In this embodiment, the apparatus comprises a processor for determining a change in capacitance of the power transmission medium using the or each detected reflected signal.

[0056] Using TDR to determine the change in capacitance of the power cable 22 Figures 5 to 12 As described exemplarily below, Figures 5 to 12 The simulations performed are illustrated in order to show how a TDR can be used to detect changes in capacitance of the power cable 22. The simulations were performed using a MATLAB model in PSCAD.

[0057] The power cable 22 is modeled as a π-segment comprising a parallel arrangement 38 of a plurality of resistor-inductor series connections connected between a pair of shunt impedances 40. Each shunt impedance 40 comprises a series connection of a plurality of resistor-capacitor parallel connections. Figure 5 The modeled π segment is shown as a frequency-dependent representation of the shunt impedance 40 .

[0058] The power cable 22 includes an inner semiconductive shielding layer, an outer semiconductive shielding layer and a cross-linked polyethylene (XLPE) insulation layer in the middle. The resistivity of the inner semiconductive shielding layer and the outer semiconductive shielding layer is typically lower than 1000 Ωm and 2000 Ωm respectively due to the requirements of the International Electrotechnical Commission, while the resistivity of the insulation layer is usually more than 1000 G Ωm. It is assumed that the semiconductive shielding layer is extruded (extruded) by the same material and using the same process as the insulation layer, and additives in the form of carbon nanoparticles are added to the semiconductive shielding layer.

[0059] The overall change in capacitance of the power cable 22 due to aging is represented by an increase in the resistance Rsco, Rsci of the semiconductor shielding layer. Analysis of the power cable impedance shows that during aging, the semiconductor shielding layer becomes more resistive, which may have occurred due to the movement or chemical changes of the nanoparticles. The resistivity of the semiconductor shielding layer may increase by about 3 orders of magnitude due to aging, which is still lower than the resistivity of the insulating layer.

[0060] Figure 6 A comparison between the estimated and simulated capacitance of the power cable 22 is shown. Expansion coefficients for the insulation and semiconducting shielding layers have been included to enable estimation and simulation of changes in the capacitance of the power cable due to aging at different temperatures. Figure 6 It is shown in that the estimated capacitance and the simulated capacitance are sufficiently similar in value. As a result, the model is reliable for accurately simulating changes in power cable capacitance due to aging.

[0061] The effectiveness of TDR in determining changes in capacitance of the power cable 22 was tested by using a first model representing an unaged power cable and a second model representing an aged power cable. For the power cable in both models, the series impedance remained the same (i.e., the value of the parallel arrangement of multiple resistor-inductors connected in series remained the same), while the resistance Rsco, Rsci of the semiconductor shield was changed to simulate unaged and aged to fit the estimated capacitance.

[0062] Figure 7 PSCAD simulation is shown. The signal source is represented as a 50 V voltage source of a circuit breaker with a pulse width of a control signal. A standard 20 µs pulse width is used, where the circuit breaker is closed at 0.02 s and is disconnected again at 0.02002. Each of the blocks marked as "cable" represents a 50 km length of power cable consisting of 40 π sections, each of which includes a parallel arrangement of 50 resistor-inductor series connections. A large number of π sections and parallel resistor-inductor series connections are used to represent power cables well from DC up to 1 MHz. Fewer resistor-capacitor parallel connections can be used for shunt impedance because they produce a good fit in the same frequency range. In this case, each π section includes 3 resistor-capacitor parallel connections.

[0063] Four aging stages are considered with Figure 8 The first aging stage 44 is based on an unaged power cable, the second aging stage 46 is based on the power cable at the beginning of aging, the third aging stage 48 is based on an aged power cable, and the fourth aging stage 50 is based on a power cable that has been aged to a greater extent than the third stage. Figure 8 As shown in FIG. 4 , there is no decrease in the capacitance of the power cable 22 for the first stage 44, no decrease in the capacitance of the power cable 22 occurs until after a frequency of 1 MHz for the second aging stage 46, and a decrease in the capacitance of the power cable 22 for the third stage 48 at a frequency of about 10 MHz. 4 Hz, and the decrease in the capacitance of the power cable 22 at about 10 3The decrease in capacitance of the power cable 22 occurs at Hz.

[0064] Fig. 9 shows the simulated TDR results for the first reflected pulse and the second reflected pulse at the transmitting end 28, and Fig.10 The pulse amplitude and arrival time changes for the same simulated TDR results are shown. The pulse amplitude and arrival time of the first reflected pulse and the second reflected pulse are similar to the first stage 44 and the second stage 46, which is expected in view of having Figure 8 44, 46. The pulse amplitudes of the first reflected pulse and the second reflected pulse for the third stage 48 are lower when compared with the first stage and the second stage 44, 46, and the pulse amplitudes of the first reflected pulse and the second reflected pulse for the fourth stage 50 are lower when compared with the first stage, the second stage, and the third stage 44, 46, 48. Similarly, the arrival time of the first reflected pulse and the second reflected pulse for the third stage 48 is earlier when compared with the first stage and the second stage 44, 46, and the pulse amplitudes of the first reflected pulse and the second reflected pulse for the fourth stage 50 are earlier when compared with the first stage, the second stage, and the third stage 44, 46, 48. These simulated TDR results are similar to those with Figure 6 This is because a reduction in the power cable capacitance due to aging has the effect of increasing the characteristic impedance and propagation velocity of the power cable 22, which results in a reduced pulse amplitude and an earlier arrival time of the reflected pulse.

[0065] Fig.11 Simulated TDR results are shown for four power cables of the same type and length but with different aging characteristics. The length of each power cable is 150 km. The first power cable 52 is new and thus not aged, the second power cable 54 is aged throughout its length, the third power cable 56 is aged only in its central 50 km section, and the fourth power cable 58 is aged in its central 6.5 km section. Uneven aging of the power cables may occur due to, for example, hot spots.

[0066] exist Fig.11 It is shown in FIG. 1 that the pulse amplitude and arrival time of the reflected pulse are different for four power cables 52 , 54 , 56 , 58 having different aging characteristics.

[0067] The pulse amplitude of the reflected pulse for the second power cable 54 is lower than the pulse amplitude of the reflected pulse for the first power cable 52. The arrival time of the reflected pulse for the second power cable 54 is earlier than the arrival time of the reflected pulse for the first power cable 52. This is caused by the fact that the first power cable 52 has not aged throughout its length and the second power cable 54 has aged throughout its length.

[0068] The pulse amplitude of the reflected pulse for the third power cable 56 is lower than the pulse amplitude of the reflected pulse for the first power cable 52. The arrival time of the reflected pulse for the third power cable 56 is earlier than the arrival time of the reflected pulse for the first power cable 52. This is caused by the fact that the first power cable 52 has not aged throughout its length and the third power cable 56 has not aged throughout its length except for its central 50 km section.

[0069] The pulse amplitude of the reflected pulse for the second power cable 54 is lower than the pulse amplitude of the reflected pulse for the third power cable 56. The arrival time of the reflected pulse for the second power cable 54 is earlier than the arrival time of the reflected pulse for the third power cable 56. This is caused by the second power cable 54 aging throughout its length and the third power cable 56 not aging throughout its length except for its central 50 km section.

[0070] Additionally, with respect to the third power cable 58, an additional pulse 60 is observed just after the 0.0205 second mark, which means that the apparatus 20 is able to detect changes in capacitance of the aged central 50 km section.

[0071] The pulse amplitudes and arrival times of the reflected pulses of the first and fourth power cables 52, 58 are almost identical except for an additional pulse 62 just before the 0.021 second mark. Fig.12 A close-up of an additional pulse 62 for the fourth power cable is shown in FIG. This is due to the fact that the first power cable 54 has not aged throughout its length and the fourth power cable 58 has not aged throughout most of its length except for the central 6.5 km section which is aged. The presence of the additional pulse 62 means that even though the central 6.5 km is significantly shorter than the total length of the 150 km power cable, the device 20 is able to detect changes in capacitance of the relatively short central 6.5 km section.

[0072] The simulated TDR results show that it is possible to perform TDR using the apparatus 20 to detect different aging characteristics and thereby distinguish between different aging characteristics of the four power cables 52 , 54 , 56 , 58 .

[0073] Further information about changes in the capacitance of the power cable 22 can be obtained by comparing the pulse amplitudes and arrival times of the reflected pulses at different points in time (e.g. during commissioning and during aging) in order to monitor the aging of the power cable 22 over time and, therefore, the change in capacitance of the power cable 22 over time.

[0074] The arrival time of the additional pulses 60, 62 can be used to determine the location and length of the aging sections of the third and fourth power cables 56, 58. This is based on the positive pulse amplitude corresponding to the increase in characteristic impedance, and the negative pulse amplitude corresponds to the decrease in characteristic impedance. When the transmitted signal 30 first travels from the unaged section with a lower characteristic impedance to the aged section with a higher characteristic impedance, the resulting first reflected pulse has a positive pulse amplitude. When the transmitted signal 36 continues to travel from the aged section with a higher characteristic impedance to the unaged section with a lower characteristic impedance, the resulting second reflected pulse has a negative pulse amplitude. Therefore, the location of the aging section along the power cable 22 can be identified, and in addition, the aging along the full length of the power cable 22 can be determined. The length of the aging section can be determined by comparing the arrival time of the first reflected pulse with the arrival time of the second reflected pulse.

[0075] To help determine the location and / or length of the aged section, the monitoring device 26 can use a reference reflection signal based on a known reference location in the power cable 22. The known reference location in the power cable 22 can be a joint, a junction or a terminal in the power cable 22 or any other known location of a discontinuity 32 of the characteristic impedance in the power cable 22. By comparing the detected reflected pulse with the reference reflection signal, the monitoring device 26 is able to use more information to help determine the location and / or length of the aged section of the power cable 22.

[0076] Thus, the device 20 of the present invention enables the use of TDR to obtain information about changes in the capacitance of the power cable 22. The information obtained can be used to monitor the condition of the power cable 22. More specifically, the information obtained can be used as an input to determine whether the condition and performance of the power cable 22 are within an acceptable operating range, and to determine whether any adjustment, maintenance, or repair of the power cable 22 is required. Moreover, the use of TDR to monitor changes in the capacitance of the power cable 22 is beneficial for long power cables because the device 20 only needs to be connected to one end 28 of the power cable 22.

[0077] It will be appreciated that each value and property given for the illustrated embodiments is selected merely to help illustrate the operation of the invention and may be replaced by another value or property.

Claims

1. A device for monitoring the condition of a power transmission medium, the device comprising: a signal source for transmitting a signal to a transmitting end of the power transmission medium to travel along the power transmission medium; and a monitoring device configured to detect one or more reflected signals in the power transmission medium, wherein the monitoring device is configured to detect arrival times of the one or more reflected signals at the transmitting end, and to determine a change in capacitance of the power transmission medium using the arrival time of each detected reflected signal, wherein the monitoring device is configured to compare each detected reflected signal with a reference reflected signal to determine a position and / or length of a section of the power transmission medium corresponding to the change in capacitance of the power transmission medium, and wherein the reference reflected signal is based on a reference position in the power transmission medium, and wherein the reference position in the power transmission medium is a position of a discontinuity in a characteristic impedance in the power transmission medium.

2. The device according to claim 1, wherein: The monitoring device is configured to compare a first arrival time of a first reflected signal with a second arrival time of a second reflected signal to determine a length of an aged segment along the power transmission medium.

3. The device according to claim 1 or claim 2, wherein: The monitoring device is configured to use the magnitude or amplitude of each detected reflected signal to determine the change in capacitance of the power transmission medium.

4. The device according to claim 1 or claim 2, wherein: The monitoring device is configured to use each detected reflection signal to determine the change in capacitance of the power transmission medium over the length of the power transmission medium.

5. The apparatus according to claim 1 or claim 2, wherein: The monitoring device is configured to use each detected reflected signal to determine the change in capacitance of the power transmission medium over time.

6. The apparatus according to claim 1 or claim 2, wherein: The power transmission medium is a power transmission line.

7. The device according to claim 6, wherein: The power transmission line is a power cable.

8. A method for monitoring a condition of a power transmission medium, the method comprising the steps of: transmitting a signal to a sending end of the power transmission medium to travel along the power transmission medium; detecting one or more reflected signals in the power transmission medium; and detecting arrival times of the one or more reflected signals at the transmitting end, and using the arrival time of each detected reflected signal to determine a change in capacitance of the power transmission medium, wherein the method further comprises the steps of: comparing each detected reflected signal with a reference reflected signal to determine a position and / or length of a section of the power transmission medium corresponding to the change in capacitance of the power transmission medium, and wherein the reference reflected signal is based on a reference position in the power transmission medium, and wherein the reference position in the power transmission medium is a position of a discontinuity in a characteristic impedance in the power transmission medium.

9. The method of claim 8, comprising the step of comparing a first arrival time of a first reflected signal with a second arrival time of a second reflected signal to determine the length of an aged section along the power transmission medium.

10. A method according to claim 8 or claim 9, comprising the step of using the magnitude or amplitude of each detected reflected signal to determine the change in capacitance of the power transmission medium.

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