Method for measuring impedance of electric cable, coupler structure therefor, and use thereof

Abstract

To provide a method and coupler structure for non-galvanic monitoring of an electric cable.SOLUTION: The present invention relates to a method for cable condition monitoring of an electric cable. The method comprises the steps of: inducing a test current in the electric cable; measuring a magnetic field in the vicinity of the electric cable to measure a resulting current in a measurement point of the electric cable; measuring an electric field in the vicinity of a cable to thereby calculate a resulting voltage in the measurement point of the electric cable; calculating the impedance of the electric cable on the basis of the resulting current and the resulting voltage by a line resonance analysis system; and analysing the impedance by the line resonance analysis system, and checking the condition of the electric cable. A coupler structure is also disclosed which provides a non-galvanic connection to the cable.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] Introduction The present invention relates to a method and a coupler configuration for non-galvanic monitoring of electrical cables, and to the use of the present invention. Measurements can be carried out on cables with the connection removed, as well as on cables during power transmission.

Background Art

[0002] Background Electrical cables, both power cables and signal cables, are used in power generation, distribution and transmission as cables in the process industry, in the aerospace industry, and in onshore, offshore and underwater installations.

[0003] The cable insulation of electrical cables ages over time. The rate of deterioration of cable insulation increases due to harsh environmental conditions such as high temperature, humidity, and radiation. Cable insulation materials may also deteriorate locally due to mechanical effects or local abnormal environmental conditions. Such overall and local deterioration of cable insulation, and even power line disconnection, occur in all types of electrical cables for both power and signal applications.

[0004] There are several cable fault detection techniques for detecting and monitoring insulation degradation and wire breaks in electrical cables. Examples include partial discharge (PD) analysis, which requires use online with the maximum voltage on the line, time domain reflectometry (TDR), spread spectrum TDR (SSTDR), conductor resistance (CR), and insulation resistance (IR). Some methods are based on transmission line theory, which measures the cable voltage as a function of time and attempts to locate local cable faults (it is not possible to assess overall degradation) by evaluating the time delay from the incident wave to the reflected wave from the error point.

[0005] Examples of methods based on transmission line theory can be found in U.S. Patent No. 4,307,267 (Patent Document 1) and U.S. Patent No. 4,630,228 (Patent Document 2), as well as U.S. Patent Application Publication No. 2004 / 0039976 (Patent Document 3) and U.S. Patent Application Publication No. 2005 / 0057259 (Patent Document 4).

[0006] Other examples based on transmission line theory can be found in U.S. Patent No. 7,966,137 B2 (Patent Document 5) and International Publication No. 2014 / 065674 A1 (Patent Document 6), which are based on line resonance analysis (LIRA) that provides condition monitoring and real-time diagnosis of electrical cables.

[0007] To monitor the condition of an electrical cable, these systems are connected to the free end of the electrical cable to send a signal into the cable and measure the reflected signal. The LIRA method measures the voltage V(d) of the electrical cable and the corresponding current I(d) flowing into the electrical cable. The line impedance Z of the cable measured at a distance d from the cable termination d (which is a complex variable) is TIFF2025161846000002.tif11128. This impedance is strongly frequency dependent, and the condition of the cable is analyzed based on the observed resonant frequency.

[0008] Traditionally, monitoring has only been performed on disconnected cables by making a direct galvanic connection to the cable via the cable termination. Disconnecting a cable is undesirable because it involves additional work, including disconnecting and connecting, and may involve shutting down the entire power system. These procedures are costly and time-consuming. In critical applications, such as nuclear power plants or power distribution lines, shutting down the system may involve undesirable risks. This also has the effect of such critical electrical cables not being monitored for condition as frequently as would be desirable. Therefore, many systems would benefit from the possibility of continuously monitoring cable defects and potential critical cable failures. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Patent No. 4,307,267 [Patent Document 2] U.S. Patent No. 4,630,228 [Patent Document 3] U.S. Patent Application Publication No. 2004 / 0039976 [Patent Document 4] U.S. Patent Application Publication No. 2005 / 0057259 [Patent Document 5] U.S. Patent No. 7,966,137B2 [Patent Document 6] International Publication No. 2014 / 065674A1 Summary of the Invention

[0010] The present invention provides a method and coupler configuration for non-galvanic monitoring of electrical cables.

[0011] The present invention provides a method for cable condition monitoring of an electric cable, the method including the steps of inducing a test current in the electric cable, measuring a resulting current at a measurement point on the electric cable by measuring the electric near-field of the electric cable, and measuring a resulting voltage at the measurement point on the electric cable by measuring the electric near-field of the cable. A line resonance analysis system can calculate the impedance of the electric cable based on the resulting current and the resulting voltage. The impedance can be analyzed by the line resonance analysis system to confirm the condition of the electric cable.

[0012] The near field may be measured by a capacitive sensor. The near field may be measured by an inductive sensor. The inductive sensor may be shielded from the near field of an electric cable. The shielding increases the sensitivity of the measurement. Alternatively, the near field may be measured by using a solid-state sensor. The sensor for measuring the near field may be a remotely located sensor.

[0013] The method may further include synchronously detecting the output from the inductive sensor and the output from the capacitive sensor. Prior to performing the amplification and synchronous detection, the output from the inductive sensor and the output from the capacitive sensor may be filtered for the power line frequency of the electrical cable. The method may further include amplifying the output from the inductive sensor and the output from the capacitive sensor prior to performing the synchronous detection.

[0014] The method may further include using an inductive or capacitive coupler to induce a test current in the electrical cable.

[0015] The inductive and capacitive sensors may be clamped to the electrical cable.

[0016] The condition of the electrical cable may be ascertained based on the resulting measured current and the resulting measured voltage. The complex impedance of the electrical cable may be calculated based on the resulting measured current and the resulting measured voltage.

[0017] The electrical cable may be a powered (live) electrical cable or an unpowered electrical cable.

[0018] The present invention further provides a coupler arrangement for condition monitoring of an electric cable, the coupler arrangement comprising a first sensor for measuring a current at a measurement point on the electric cable by measuring a near-field field of the electric cable, and a second sensor for measuring a voltage at the measurement point on the electric cable by measuring a near-field field of the electric cable.

[0019] The coupler arrangement is adapted to connect an electrical cable condition monitoring system to a monitored electrical cable, the electrical cable condition monitoring system being a line resonance analysis system based on impedance measurements of the electrical cable.

[0020] The first sensor may be an inductive sensor. The inductive sensor may include a transformer. The first sensor may be electrically shielded from the near field of the electric cable. The inductive sensor may include an electrostatic screen that electrically shields the inductive sensor from the near field of the electric cable. The second sensor may be a capacitive sensor. The capacitive sensor may include an insulated capacitor electrode. The capacitor electrode may be designed to avoid localized partial discharges. The insulated capacitor electrode may be adapted to contact the electric cable insulation around at least a portion of the circumference of the electric cable. The first sensor may be positioned at a distance from the electric cable.

[0021] The coupler may further comprise a signal coupler for inducing a test current in the electrical cable, which may be an inductive coupler or a capacitive coupler.

[0022] The coupler arrangement may further be adapted to be clamped to an electrical cable, and the inside of the coupler arrangement may be adapted to contact the cable insulation or a semi-conductive layer on the cable insulation.

[0023] The coupler configuration may be used to connect a cable condition monitoring device to an electrical cable. The cable monitoring device may be a cable fault analysis device. The cable monitoring device may be a line resonance analysis system based on impedance measurements of the electrical cable.

[0024] The above method or the above coupler configuration may be used to measure noise from electrical cables.

[0025] The present invention further provides a method for measuring the impedance of an electric cable, the method comprising detecting a near field of the electric cable by using a non-galvanic connection to the electric cable, and calculating the impedance of the electric cable based on the detected near field of the electric cable.

[0026] The present invention provides a method and coupler configuration for monitoring the condition of an electrical cable. Errors and aging of the electrical cable can be monitored by measuring the near-field caused by a test current induced in the cable. The present invention also provides a method for measuring the impedance of an electrical cable by detecting the near-field of the electrical cable using a non-galvanic connection to the cable. The method and coupler configuration enable non-galvanic measurement of the complex line impedance at freely selected points on the electrical cable.

[0027] The above method is based on detecting the near-field of an electric cable using a near-field magnetic field to measure current and a near-field electric field to measure line voltage. Because electric and magnetic fields propagate freely through cable insulation, this method is inherently non-galvanic. Measurements can be performed on disconnected (de-energized) cables as well as energized cables. A coupler configuration can be used to connect a cable monitoring system to energized electric cables to analyze cable condition without requiring any direct galvanic connection to the electric cable. The coupler configuration can be designed to be installed on electric cables under maximum line power. For high-voltage power systems, this can typically be several kV and hundreds of amperes. The present invention enables condition monitoring of electric cables without shutting down the power system, enabling cost-effective fault analysis of electric cables under maximum operating voltages and currents. The present invention also applies to the broader use of LIRA (Line Resonance Analysis) methods and systems.

[0028] The present invention also applies to partial discharge (PD) measurements on live electrical cables, as the method and coupler configuration allows for full spectrum analysis of current and voltage noise on the cable under maximum power conditions. [The present invention 1001] inducing a test current in the electrical cable; measuring the resulting current at a measurement point on the electrical cable by measuring the near magnetic field of the electrical cable; and measuring the resulting voltage at the measurement point of the electrical cable by measuring the near field of the cable; calculating, by a line resonance analysis system, the impedance of the electrical cable based on the resultant current and the resultant voltage; and Analyzing the impedance using the line resonance analysis system to confirm the state of the electric cable. 1. A method for cable condition monitoring of an electrical cable, comprising: [The present invention 1002] 1001. The method of claim 1001, further comprising measuring said near field with a capacitive sensor. [The present invention 1003] The method of invention 1001 or invention 1002, further comprising the step of measuring said near magnetic field by an inductive sensor. [The present invention 1004] 1004. The method of claim 1003, further comprising the step of shielding said inductive sensor from said near field of said electrical cable. [The present invention 1005] The method according to any one of claims 1002 to 1004, further comprising the step of synchronously detecting the output from the inductive sensor and the output from the capacitive sensor. [The present invention 1006] 1005. A method according to claim 10, further comprising filtering the output from the inductive sensor and the output from the capacitive sensor for a power line frequency of the electrical cable before performing amplification and synchronous detection. [The present invention 1007] The method of invention 1005 or invention 1006, further comprising the step of amplifying said output from said inductive sensor and said output from said capacitive sensor before performing synchronous detection. [The present invention 1008] The method of any one of claims 1001 to 1007, further comprising the step of using an inductive coupler to induce a test current in said electrical cable. [The present invention 1009] The method of any one of claims 1001 to 1007, further comprising the step of using a capacitive coupler to induce a test current in said electrical cable. [The present invention 1010] The method of any one of claims 1002 to 1007, further comprising the step of clamping said inductive sensor and said capacitive sensor to said electrical cable. [The present invention 1011] The method of claim 1001 or 1002, further comprising the step of measuring the near magnetic field with a solid-state sensor. [The present invention 1012] The method of any one of claims 1001 to 1011, further comprising the step of measuring the near field by using a remotely located sensor. [The present invention 1013] The method of any one of claims 1001 to 1012, further comprising the step of calculating a complex impedance of the electrical cable based on the measured resulting current and the measured resulting voltage. [The present invention 1014] The method according to any one of inventions 1001 to 1013, wherein the electric cable is an electric cable to which power is being supplied (current is being applied) or an electric cable to which no power is being supplied. [The present invention 1015] A coupler arrangement (20, 50) for condition monitoring of an electric cable, the coupler arrangement being adapted to connect an electric cable condition monitoring system to an electric cable (1) to be monitored, the electric cable condition monitoring system being a line resonance analysis system based on impedance measurements of the electric cable (1); The coupler configuration comprises: a first sensor (23, 52) for measuring the current at a measurement point of the electric cable (1) by measuring the magnetic field near the electric cable; a second sensor (24, 54) for measuring the voltage at the measurement point of the electric cable (1) by measuring the electric field near the electric cable; A coupler arrangement (20, 50) comprising: [The present invention 1016] The coupler configuration of the present invention 1015, wherein the first sensor (23, 52) is an inductive sensor. [The present invention 1017] The coupler configuration of the present invention 1016, wherein the inductive sensor comprises a transformer (31, 33). [The present invention 1018] The coupler configuration of any one of claims 1015 to 1017, wherein the first sensor is electrically shielded from the near field of the electrical cable. [The present invention 1019] 10. The coupler configuration of claim 1016 or 1017, wherein the inductive sensor (23, 52) comprises an electrostatic screen (53) that electrically shields the inductive sensor from the near field of the electrical cable. [The present invention 1020] The coupler configuration of any one of inventions 1015 to 1019, wherein the second sensor (24, 54) is a capacitive sensor. [The present invention 1021] A coupler configuration of the present invention 1020, wherein the capacitive sensor comprises an insulated (42) capacitor electrode (41) designed to avoid localized partial discharges. [The present invention 1022] 1021. A coupler arrangement according to claim 1021, wherein said insulated (42) capacitor electrode (41) is adapted to contact said electrical cable insulation (11) over at least a portion of the circumference of said electrical cable. [The present invention 1023] The coupler configuration of any one of inventions 1015 to 1022, wherein the first sensor (23, 52) is arranged at a certain distance from the electric cable. [The present invention 1024] The coupler configuration of any one of claims 1010 to 1023, further comprising a signal coupler (21, 51) for inducing a test current in the electrical cable. [The present invention 1025] The signal coupler is an inductive coupler, in accordance with the present invention. [The present invention 1026] The signal coupler is a capacitive coupler, and the coupler configuration of the present invention is 1024. [The present invention 1027] A coupler configuration according to any one of claims 1015 to 1026, adapted to be clamped to the electrical cable. [The present invention 1028] 1027. A coupler arrangement according to claim 1027, wherein an inside of said coupler arrangement is adapted to contact a cable insulation or a semi-conductive layer on said cable insulation. [The present invention 1029] Use of any of the methods of inventions 1001 to 1014 or use of any of the coupler configurations of inventions 1015 to 1028 for measuring noise from an electrical cable. [The present invention 1030] detecting the near field of the electrical cable by using a non-galvanic connection to the electrical cable; and calculating the impedance of the electrical cable based on the detected near field from the electrical cable; 1. A method for measuring impedance of an electrical cable, comprising: [Brief explanation of the drawings]

[0029] Exemplary aspects of the present invention will now be described with reference to the following drawings.

[0030]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6a

Figure 6b

Figure 6c

Figure 6d

[0031] Detailed Description The present invention will now be described with reference to the drawings.

[0032] FIG. 1 shows a coupling arrangement 20 for monitoring an electric cable connected to an electric cable 1 (power line or signal cable). The power line 1 under test in FIG. 1 has an insulating layer (not shown). The insulation may include a semiconductive outer layer 36 (FIG. 2). The cable 1's possible outer sheath and cable screen 12 are removed to expose the cable insulation 11 in the measurement area of ​​the coupling arrangement 20. The measurement area is a freely selected point on the cable. The cable screen 12 is connected to a common earth with the electronic monitoring system (22, 27, 25). The electric cable 1 may be a powered (live) electric cable or an unpowered electric cable. The coupling arrangement in FIG. 1 includes a signal coupler 21, a current sensor 23, and a cable voltage sensor 24. The signal coupler 21 may be an inductive or capacitive coupler for inducing a test signal in the electric cable. The test signal passes through the electric cable under the influence of the cable condition. Cable condition is monitored by measuring the current and voltage in the cable resulting from an induced test signal at measurement points on the cable. The current and voltage generate a near-field electromagnetic field around the electrical cable. The voltage is measured by sensing the near-field electric field, and the current is measured by sensing the near-field magnetic field of the electrical cable. Measuring the near-field electromagnetic field around the cable allows for a non-galvanic connection to the electrical cable.

[0033] The signal coupler 21 may be located separately from the current sensor 23 and the voltage sensor 24, and the test signal may be induced at a different location on the cable. Therefore, the signal coupler 21 may be a separate component and not part of the coupler configuration 20. The current sensor 23 may be an inductive sensor that measures the resulting current at a measurement point on the electric cable by measuring the cable's near-field. The near-field of the electric cable may also be measured by a solid-state sensor, typically a Hall element. The sensor for measuring the near-field may be located a certain distance from the electric cable. The distance at which the measurement can be performed depends on the near-field strength. The voltage sensor 24 may be a capacitive sensor that measures the voltage between the center conductor and the screen at the measurement point. The cable screen 12 of the electric cable 1 is connected to a common ground 14. The electronics are connected to the common ground 14.

[0034] The impedance of an electrical cable at a particular measurement point can be measured by inducing a test current in the electrical cable and measuring the resulting current and resulting voltage at the measurement point, as described above. In this regard, the measurement point will have some extent along the cable length, depending on the cable dimensions and the voltage carried in the cable. The sensors 23, 24 for measuring the resulting current and resulting voltage, respectively, should be positioned near each other, and together their positions define the measurement point on the cable. The size of the measurement point can vary from several meters for very long high-voltage cables to several millimeters for short low-voltage signal cable applications.

[0035] The signals from the current sensor 23 and the voltage sensor 24 are fed to an electronic detection system. The electronic detection system includes a synchronous detector 25. The synchronous detector 25 separates the current sensor signal and the voltage sensor signal into an in-phase portion and a quadrature portion, respectively. The synchronous detector receives its frequency and phase reference for the measurements from a signal source 22 that drives the signal combiner 21. The output from the synchronous detector is sent to a computer 26 for further analysis. The computer may calculate a complex cable impedance based on the measured current and voltage signals. The complex cable impedance can be used to analyze the condition of the electrical cable 1.

[0036] Using the coupling configuration shown in Figure 1, several cable monitoring systems can be connected to a live power cable. One example is a line resonance analysis system (LIRA), as described in detail in U.S. Patent No. 7,966,137 B2 and WO 2014 / 065674 A1. As shown in Figure 1, signals from a signal source (generator), current sensor, and cable voltage sensor are fed to a synchronous detector, digitized, and then fed to a computer for further analysis. The computer may be the LIRA system described in detail in U.S. Patent No. 7,966,137 B2 and WO 2014 / 065674 A1. The method and coupler configuration may also be used with monitoring systems that analyze cable condition based on partial discharge (PD) measurements.

[0037] One embodiment of an inductive coupler 30 with a transformer is shown in FIG. 2. The inductive coupler is based on a toroidal transformer with a core material 31. The core material 31 may be a ferromagnetic core material or air. The toroidal transformer is divided into two halves. An air gap 34 filled with a non-magnetic isolation material is provided between the two halves. A coil 33 is wound at least partially around one of the two toroidal halves. In FIG. 2, the coil has three turns, but it may have additional or fewer turns. The core material and number of turns may vary depending on the coupler design and application. Protective insulation 32 is provided on both the outside and inside of the ferromagnetic core 31. The protective insulation 32 also covers the coil 33. Inductive couplers may be used to induce test signals in electrical cables. The electrical cable conductor 10 in FIG. 2 includes a cable insulation 11 further comprising a semiconducting layer 36.

[0038] The inductive coupler of FIG. 2 can also be used as a sensor for measuring the near-field around an electric cable, and therefore for measuring the current in the electric cable. However, when used as a sensor for measuring current, the coil 33 can be electrically shielded 35 from the near-field of the electric cable 10. The shielding in FIG. 2 is provided by an electrostatic screen 35. The electrostatic screen 35 is located inside the protective insulation 32 inside the ferromagnetic core 31. The electrostatic screen 35 covers a sectoral area that is larger than the sectoral area covered by the coil 33. Shielding the inductive sensor from the near-field of the electric cable improves the sensitivity of the inductive sensor for measuring the near-field of the cable.

[0039] FIG. 3 shows an embodiment of a voltage sensor in the form of a capacitive coupler 40 for measuring line voltage from near-fields near an electric cable insulation 11. In the embodiment of FIG. 3, the capacitive coupler includes an insulated capacitor electrode 43. The capacitive coupler is adapted to contact the electric cable around at least a portion of the power line's circumference. The capacitor coupler includes an insulated semi-cylindrical metal plate 43. In the embodiment of FIG. 3, the insulated semi-cylindrical metal plate is pressed against the electric cable insulation 11 to form a capacitive pickup toward the conductor 10. In FIG. 3, the capacitor electrode includes rounded ends. This design, which avoids sharp edges, reduces the possibility of localized electric field concentrations that could cause localized partial discharges. To avoid such localized electric field concentrations, the capacitor electrode may also include a semi-conductive component. Avoiding localized electric field concentrations is particularly important in high-voltage applications and for cables with a semi-conductive layer on the underside of the cable screen. The capacitor coupler 40 includes an insulating portion 41 and is disposed within an insulating housing that also includes an electronics box 57. A flexible conductor 44 connects the semi-cylindrical metal plate to the input filter of the electronic detection system in the electronics box 57 .

[0040] One embodiment of a coupler configuration is shown in Figures 4a-4b. As shown in Figure 4a, a coupler configuration 50 is clamped around an electric cable 1 outside the cable insulation 11. The electric cable 1 has a cable insulation 11 with a cable screen 12 and an outer sheath 13, which are removed before clamping the coupler configuration around the electric cable. In Figure 4b, for illustrative purposes, the removed cable screen section is shown bent away. Figure 4b shows a cross-sectional view of only the main components of the coupler configuration 50 of Figure 4a: the inductive sensor 52, the capacitive sensor 54, and the inductive injector 51. As can be seen in Figure 4b, the inductive coupler 51 (inductive injector) is positioned a certain distance away from the inductive pickup (sensor) 52 for measuring the current flowing through the cable and the capacitive pickup (sensor) 54 for measuring the line voltage from the magnetic field near the electric cable insulation. The inductive coupler 51 for injecting the test signal into the cable may inject the test signal into the cable at a location other than the measurement point on the cable, as described above, and may be a separate component from the coupler arrangement.

[0041] The inductive pickup device in the coupler configuration of Figures 4a-4b is based on the inductive coupler principle described above for Figure 2, while the capacitive pickup device is based on the capacitive coupler principle described in Figure 3. The inductive pickup device in Figure 4b is electrically shielded from the cable using a grounded electrostatic screen 53. The inductive sensor measures the cable's near-field, and the use of the grounded electrostatic screen 53 reduces error signals associated with the near-field. An electronic detection system for signal filtering and / or signal conditioning is provided inside the electronics box 57. The inductive sensor 52 and capacitive sensor 54 are located inside the sensor housing. The inductive injector 51 is located inside the injector housing. The coupler configuration of Figure 4b is isolated and compatible with high-voltage systems, allowing the inductive signal injector, inductive sensor, and capacitive sensor to all operate even with full power on the cable.

[0042] The coupler configuration 50 of FIG. 4b is clamped around the electrical cable insulation in intimate contact with the cable insulation. Because the coupler configuration is tightly clamped around the cable insulation, it provides a stable geometry for both the inductive and capacitive sensors in terms of the cable geometry. The stable geometry improves measurement stability and enhances measurement sensitivity. A protective insulator (58) is provided inside the coupler configuration. The thickness of the insulator may be adjusted depending on the cable dimensions and the magnitude of the near-field electric and magnetic fields being measured to ensure that the sensor is close enough to the electrical cable to achieve adequate signal strength from the near-field. A stable geometry can also be achieved in applications where the current and voltage sensors are located a certain distance from the cable.

[0043] A block diagram of an embodiment of an electronic detection system using a coupler configuration is shown in Figure 5. An oscillator 62 with a power amplifier 63 drives the signal coupler 21 to induce a test signal in the electrical cable. The oscillator frequency, and therefore the frequency of the test signal, may be varied over a wide range. The frequency or frequency range of the test signal is adapted to the monitoring purpose and cable characteristics, such as length. The oscillator frequency may be controlled by an external computer, analyzer, or controller and input to the oscillator 62 via a signal frequency command 72. The signal from the current sensor is filtered 64 to the power line frequency, amplified 27, and fed to the synchronous detector 25. Correspondingly, the signal from the voltage sensor is also filtered 64 and amplified 27 before the resulting signal is synchronously detected. The power line frequency is typically 50 or 60 Hz. The reference frequency 65 for detecting the signals from the current and voltage sensors comes from the oscillator 62 as in-phase (I) and quadrature (Q) signals in the form of sine waves or square waves with a 90° offset. The signals 66, 67 from the sensors 23, 24 are multiplied by the in-phase (I) and quadrature (Q) reference signals 65 in corresponding multipliers (X) 68. The multiplication products (X) of the signals 66, 67 from the sensors and the in-phase (I) and quadrature (Q) reference signals are integrated in corresponding integrators (Int) 70 with an integration time T equal to the full signal period of the test signal. The synchronous detection described above can be performed by analog components or digitally within a signal processor. Finally, the in-phase and quadrature measurement signal outputs 71 from the integrators 70 are sent 73 to the computer 26 for further analysis. The computer may also be, for example, an analyzer, a cable monitoring system (e.g., an LIRA system), or a partial discharge (PD) signal analyzer.

[0044] If the monitoring system is a LIRA system, the in-phase and quadrature signals output from the synchronous detector 25 are sent to a computer 26 for line impedance calculation and error analysis, as discussed in U.S. Pat. No. 7,966,137 B2 and WO 2014 / 065674 A1.

[0045] The method for cable condition monitoring of an electrical cable may also be implemented by a coupler arrangement connected to a commercially available high voltage cable connector. Figures 6a-6d show exemplary embodiments of a coupler arrangement connected to a commercially available high voltage cable connector.

[0046] FIG. 6a shows a schematic cross-sectional view of a high-voltage cable connector 80 connected to an electric cable 1. The high-voltage cable connector has a connection structure 91 with a male portion adapted to be placed on the electric cable termination. The high-voltage cable connector has a connection area 81 and a bolt 82 that holds the connected male portion of the structure in place on the electric cable termination. The area on the high-voltage cable connector 80 suitable for connecting injection capacitors in a coupler configuration is sealed with a packer 87 and a fixing cap 86. The connection structure 91 includes a crimp terminal 88 that is applied to the end of the electric cable conductor. The electric cable separator 11 begins immediately after the crimp connector 88. An additional insulating sleeve 89 is provided on the outside of the electric cable separator 11. The high-voltage cable connector 80 has a connector housing 92 that is located outside the connection structure 91 and extends over the outside of the additional insulating sleeve 89. The connector housing 92 has an inner semiconductive layer 83, followed by an insulating layer 84 and an outer semiconductive layer 85. When placed on the electrical cable termination, the inner semi-conductive layer 83 partially covers the additional insulation sleeve 89. Therefore, the insulation layer 84 is also exposed to a portion of the additional insulation sleeve 89, as shown in Figure 6a. The outer semi-conductive layer 85 is connected to the system ground 69 by the cable 90. The cable shield 93 is folded back and connected to the system ground 69. The high-voltage cable connector 80 is rigidly connected to the electrical cable termination to ensure good electrical contact between the bolt 82 and the electrical cable conductors, and a close fit between the connector housing 92, the additional cable insulation sleeve 89, and the inner cable screen 12.

[0047] FIG. 6b shows an embodiment of a coupler configuration including a sensor assembly and a capacitive injector 101 connected to a high-voltage cable connector 80. The retaining cap 86 and packer 87 have been removed, and an injection capacitor has been inserted and is tightly fitted with the bolt 82. The injection capacitor comprises a high-voltage capacitor and may include safety and frequency shaping circuitry. The sensor assembly 100 fits tightly or partially around the connector housing 92. FIG. 6c shows the sensor assembly 100 in cross section. The sensor assembly 100 includes a capacitive sensor plate 102 and a pickup coil 105, both of which are connected to electronics 103. In the embodiment of FIG. 6c, the pickup coil 105 is a Rogowski coil, although other pickup coils can be used. The electronics 103 can be as described above and shown in FIG. 5. The capacitive sensor plate 102 is cylindrical and covers at least a portion of the periphery of the connector housing 92 above the additional insulator sleeve 89. The capacitive sensor plate 102 is in electrical contact with the outer semiconducting layer 85. Isolation rings 106 on the top and bottom of the sensor assembly 100 isolate the grounded sensor assembly housing 104 from the outer semiconductive layer 85 .

[0048] An embodiment of a coupler configuration including a sensor assembly 100 and an inductive injector connected to a high-voltage cable connector 80 is shown in Figure 6d. The sensor assembly 100 is as described above with respect to Figures 6b and 6c. The injector for injecting a reference signal into the electrical cable is an injection coil 108 in the embodiment of Figure 6d. The injector is positioned at least partially around the connector housing 92. In Figure 6d, the injector is positioned over a portion of the area with the crimp terminals 88, although other locations are possible as long as a test signal can be injected into the electrical cable.

[0049] The coupler configuration may be externally powered via the cable, or alternatively, may be powered by an internal battery. The signal coupler 21 can be used to charge the internal battery from the cable's magnetic field during periods when the system is not making measurements. Communication with the computer can be via electrical cable, fiber optic cable, or digital wireless communication. The use of fiber optic cable or digital wireless communication improves the safety of the computer and signal electronics and reduces power line noise.

[0050] Although preferred embodiments of the present invention have been described, it will be apparent to those skilled in the art that other embodiments incorporating this concept may also be used. These and other examples of the present invention described above are intended as examples only, and the actual scope of the invention should be determined from the appended claims.

Claims

1. inducing a test current in the electrical cable; measuring a resultant current at a measurement point on the electrical cable by measuring a near-field of the electrical cable induced by the test current; and measuring a resultant voltage at the measurement point of the electrical cable by measuring the near field of the cable induced by the test current; calculating, by a line resonance analysis system, the impedance of the electrical cable based on the resultant current and the resultant voltage; and Analyzing the impedance using the line resonance analysis system to confirm the state of the electric cable.

1. A method for cable condition monitoring of an electrical cable, comprising:

2. The method of claim 1 , further comprising measuring the near field with a capacitive sensor.

3. 3. The method of claim 1 or claim 2, further comprising measuring the magnetic proximity field with an inductive sensor.

4. The method of claim 3 , further comprising shielding the inductive sensor from the near field of the electrical cable.

5. The method of any one of claims 2 to 4, further comprising the step of synchronously detecting an output from the inductive sensor and an output from the capacitive sensor.

6. 6. The method of claim 5, further comprising filtering the output from the inductive sensor and the output from the capacitive sensor for a power line frequency of the electrical cable before performing amplification and synchronous detection.

7. 7. The method of claim 5 or claim 6, further comprising amplifying the output from the inductive sensor and the output from the capacitive sensor before performing synchronous detection.

8. 8. The method of claim 1, further comprising using an inductive coupler to induce a test current in the electrical cable.

9. The method of any one of claims 1 to 7, further comprising using a capacitive coupler to induce a test current in the electrical cable.

10. The method of any one of claims 2 to 7, further comprising clamping the inductive sensor and the capacitive sensor to the electrical cable.

11. 3. The method of claim 1 or claim 2, further comprising measuring the near field with a solid-state sensor.

12. The method of any one of claims 1 to 11, further comprising measuring the magnetic near field by using a remotely located sensor.

13. 13. The method of claim 1, further comprising the step of calculating a complex impedance of the electrical cable based on the measured resultant current and the measured resultant voltage.

14. 14. The method of any one of claims 1 to 13, wherein the electrical cable is a powered (live) electrical cable or a non-powered electrical cable.

15. A coupler arrangement (20, 50) for condition monitoring of an electric cable, the coupler arrangement being adapted to connect an electric cable condition monitoring system to an electric cable (1) to be monitored, the electric cable condition monitoring system being a line resonance analysis system based on impedance measurements of the electric cable (1); The coupler configuration comprises: a first sensor (23, 52) for measuring a current at a measurement point of the electric cable (1) by measuring a magnetic field near the electric cable caused by a test current induced in the electric cable; a second sensor (24, 54) for measuring a voltage at the measurement point of the electric cable (1) by measuring a near-field of the electric cable caused by a test current induced in the electric cable; A coupler arrangement (20, 50) comprising:

16. 16. The coupler arrangement of claim 15, wherein the first sensor (23, 52) is an inductive sensor.

17. 17. The coupler arrangement of claim 16, wherein the inductive sensor comprises a transformer (31, 33).

18. 18. The coupler arrangement of claim 15, wherein the first sensor is electrically shielded from the near field of the electrical cable.

19. 18. A coupler arrangement according to claim 16 or 17, wherein the inductive sensor (23, 52) comprises an electrostatic screen (53) for electrically shielding the inductive sensor from the near field of the electrical cable.

20. The coupler arrangement of any one of claims 15 to 19, wherein the second sensor (24, 54) is a capacitive sensor.

21. 21. The coupler arrangement of claim 20, wherein the capacitive sensor comprises insulated (42) capacitor electrodes (41) designed to avoid localized partial discharges.

22. 22. The coupler arrangement of claim 21, wherein the insulated (42) capacitor electrode (41) is adapted to contact the insulation (11) of the electrical cable over at least a portion of the circumference of the electrical cable.

23. A coupler arrangement according to any one of claims 15 to 22, wherein the first sensor (23, 52) is located at a distance from the electrical cable.

24. A coupler arrangement according to any one of claims 15 to 23, further comprising a signal coupler (21, 51) for inducing a test current in the electrical cable.

25. 25. The coupler arrangement of claim 24, wherein the signal coupler is an inductive coupler.

26. 25. The coupler arrangement of claim 24, wherein the signal coupler is a capacitive coupler.

27. 27. A coupler arrangement according to any one of claims 15 to 26, adapted to be clamped to the electrical cable.

28. 28. The coupler configuration of claim 27, wherein an inside of the coupler configuration is adapted to contact a cable insulation or a semiconducting layer on the cable insulation.

29. Use of the method of any one of claims 1 to 14 or the coupler arrangement of any one of claims 15 to 28 for measuring noise from an electric cable.

30. 1. A method for measuring the impedance of an electrical cable, comprising detecting a near field of said electrical cable by using a non-galvanic connection to said electrical cable, the method further comprising the steps of: inducing a test current in the cable; measuring a resultant current at a measurement point on the electrical cable by measuring a near magnetic field of the electrical cable caused by the test current; measuring the resulting voltage at the measurement point on the electrical cable by measuring the near field of the cable induced by the test current; and Calculating the impedance of the electrical cable based on the detected near field from the electrical cable.

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