Method for detecting cable joint faults in power distribution cables

Abstract

A method of detecting a cable joint fault in an electrical distribution cable of an electrical power network, the electrical distribution cable comprising at least one electrically conductive core, the method comprising: (a) disconnecting the electrical distribution cable from the electrical power network; (b) applying a DC voltage to one end of the electrically conductive core for a predetermined duration; (c) obtaining a polarization current in the electrical distribution cable during the application of the DC voltage; and (d) plotting a curve of the polarization current against time; (e) wherein an increase in the polarization current over time is indicative of a water ingress in a cable joint in the electrical distribution cable; and (f) wherein a non-monotonic change in the polarization current over time is indicative of a water ingress in a cable joint in the electrical distribution cable.

Description

Technical Field

[0001] This disclosure relates to a method for detecting cable joint faults in power distribution cables, and more specifically, to a method for detecting water ingress in cable joints of power distribution cables. Background Technology

[0002] Distribution networks typically use medium-voltage power cables, such as 22kV and 6.6kV power cables. An exemplary medium-voltage power cable is a cross-linked polyethylene (XLPE) power cable, which uses XLPE as the insulation material around the conductors in the cable. For long-distance continuous current, cable joints are used to electrically connect discrete segments of cable to form a continuous distribution cable. Figure 1 An exemplary cable connector 10 connecting two cable segments 21, 22 is shown. Each cable 21, 22 is shown as typically including a conductor 15 insulated from a conductor insulation material 13, such as XLPE. The insulation material 13 is typically surrounded by semiconductor shielding elements 11, 16. In the cable connector 10, the conductors 15 of both cables 21, 22 are electrically connected to a connector 14 disposed between the cables 21, 22. The connector 14 and the connection ends of the cables 21, 22 are encapsulated using a reinforcing insulation element 12, typically comprising silicone rubber (SR). Within the reinforcing insulation encapsulation of the cable connector 10, there is an interface 18 between the reinforcing insulation element 12 and the conductor insulation material 13. This interface 18 is a point where water ingress from the surrounding environment in which the cable connector 10 is located can occur. When water enters the interface 18 of the cable connector, this can cause cable degradation, which can lead to undesirable failures in the distribution cable.

[0003] Existing methods for detecting cable faults include using an Oscillating Wave Test System (OWTS) with accompanying software to precisely locate potential cable or joint faults. However, this requires expensive equipment (typically SGD 160,000), and each test using an OWTS takes approximately 45 minutes to perform. Another method for testing cable faults is VLF (Very Low Frequency) cable testing, where test times can range from 15 to 60 minutes. While the small size of VLF test equipment is advantageous for field testing, conclusive results may be difficult to obtain with VLF testing if there is significant water damage to the cable insulation. VLF testing using very high test voltages can also introduce unwanted space charges in some types of cable insulation, such as extruded polyethylene rubber (EPR) insulation.

[0004] Therefore, it is desirable to provide a low-cost and rapid method for testing cable joints in power distribution cables for water ingress, a method that does not suffer from the drawbacks found in existing cable testing methods. Summary of the Invention

[0005] According to a first aspect, a method for detecting cable joint faults in a power distribution cable of a power supply network is provided, the power distribution cable including at least one conductive core, the method comprising: (a) Disconnect the power distribution cable from the power supply network; (b) Applying a DC voltage to one end of the conductive core for a predetermined duration; (c) Obtaining the polarization current in the distribution cable during the application of the DC voltage; and (d) Plot the curve of the polarization current as a function of time; (e) wherein the increase of the polarization current over time indicates water ingress into the cable joints of the distribution cable; and (f) wherein the non-monotonic change of the polarization current over time indicates water ingress into the cable joints of the distribution cable.

[0006] The method may also include fully discharging the power distribution cable after step (a) and before step (b).

[0007] The power distribution cable may include a plurality of conductive cores, and the method includes performing steps (b) to (d) on each of the plurality of conductive cores.

[0008] The predetermined duration can range from 50s to 100s.

[0009] The DC voltage can have a value between 2kV and 6kV.

[0010] The power distribution cable may include a medium-voltage power cable insulated with cross-linked polyethylene (XLPE). Attached Figure Description

[0011] In order to fully understand and readily put the invention into practice, exemplary embodiments of the invention will now be described by way of non-limiting example only, with reference to the accompanying illustrative drawings.

[0012] Figure 1 This is a schematic diagram of a cable connector structure.

[0013] Figure 2(a) is a schematic diagram of a single-layer insulation before voltage is applied.

[0014] Figure 2(b) is a schematic diagram of the single-layer insulator of Figure 2(a) after a voltage is applied during the polarization process.

[0015] Figure 2(c) shows the response characteristics in the current-time curve of the polarization process in Figure 2(b).

[0016] Figure 3 It is a physical model of a double-layer insulation component.

[0017] Figure 4 It is a physical model of a double-layer nonlinear insulator connected in series.

[0018] Figure 5 It is a graph showing the polarization current as a function of the applied voltage for all three phases.

[0019] Figure 6(a) is the polarization current-time curve, showing the abnormal increase in total current over time.

[0020] Figure 6(b) is the polarization current-time curve, showing the non-monotonic change of the total current over time.

[0021] Figure 7(a) shows the experimental polarization and depolarization current (PDC) test results for the first circuit, illustrating the anomalous increase in polarization current over time for one of the three phases.

[0022] Figure 7(b) shows the experimental PDC test results of the second circuit, illustrating the normal decrease of polarization current over time for all three phases.

[0023] Figure 8 This is a function diagram of a time-domain reflectometry (TDR) test performed on a power cable, showing the injected and reflected waveforms.

[0024] Figure 9 This is a schematic diagram of the TDR waveforms obtained for power cables under different test conditions.

[0025] Figure 10(a) shows the TDR waveform results obtained from the TDR test performed on the first circuit.

[0026] Figure 10(b) shows the TDR waveform results obtained from the TDR test performed on the second circuit.

[0027] Figure 11 This is a flowchart of an exemplary method for detecting cable joint faults in power distribution cables. Detailed Implementation

[0028] Reference Figures 1 to 11 An exemplary embodiment of a method 100 for detecting cable joint faults in power distribution cables is described.

[0029] The method 100 disclosed herein includes performing polarization and depolarization current (PDC) testing on one end of a power cable that has been switched off, de-energized, and isolated. The PDC test can be performed using an insulation resistance (IR) tester, such as a Megger® S1-1568 DC IR tester. Since the cable circuit consists of the cable body and the cable joint, the PDC test results can simultaneously reflect the condition of both the cable body and the joint. The PDC test can diagnose the condition of the cable joint based on the polarization current results.

[0030] Polarization current characteristics of single-layer insulators

[0031] In PDC testing, when a DC test voltage is applied to a single-layer insulator, a current with more than one component flows through the insulator. As shown in Figure 2(a), the single-layer insulator 20 typically includes dipoles 21, which are randomly oriented before the voltage is applied to the electrodes 22, with one electrode placed on each side of the single-layer insulator 20. Once the voltage is applied to the insulator 20 as shown in Figure 2(b), the dipoles 21 align themselves according to the polarity of the voltage, resulting in the formation of bound charges 24 on the electrodes 22. Free charges 23 are also formed on the surface of the electrodes 22, thereby generating a current in the external circuit, which is initially large but rapidly decreases to zero and is almost unmeasurable. The resulting current is called the capacitive charging current because the bulk insulator 20 behaves as a charging capacitor. Figure 2(c) shows the corresponding current generated by the behavior of the charges formed during polarization. The total current consists of three component currents: capacitive charging current, absorption current, and conduction current.

[0032] As shown in Figure 2(c), the absorption current decays to near zero at a decreasing rate over a period of time. This current is mainly due to the alignment of polarized molecules under a DC electric field. The conduction current, or leakage current, flows steadily through the insulator 20 and can be measured when the insulator is fully charged and complete absorption has occurred.

[0033] As can be seen from the above, capacitive current, absorbance current, and conduction current are the three main response characteristics of the single-layer insulation 20 under DC voltage. However, due to the rapid dissipation of capacitive current, the total current measured does not include capacitive current. Therefore, in PDC testing, only absorbance current and conduction current are used to diagnose the health status of cables and joints.

[0034] Polarization current characteristics of double-layer insulators

[0035] For double-layered insulators, according to the Maxwell-Wagner polarization theory described in the publication 'Suo, Changyou et al., "Dynamic characteristics analysis on interface polarization and depolarization of nonlinear double-layered dielectrics", IEEE Transactions on Dielectrics and Electrical Insulation 24.3 (2017): 1511-1526, if each layer of the double-layered insulator is linear, meaning that the conductivity and dielectric constant are constant, the physical model of the double-layered insulator can be simplified, such as... Figure 3 As shown. It is worth noting that the polarization current of the linear double-layer interface polarization also decreases over time, similar to the case of the single-layer insulator 20 shown in Figure 2(b).

[0036] However, the conductivity of nonlinear insulators in power cable systems is a function of temperature and electric field E. Hyperbolic sine waves and power functions are commonly used to describe the electric field E and the conductivity of nonlinear insulators. The relationship between them. In some cases, within a small electric field E, this relationship can be approximated as linear, where conductivity can be approximately described by the following equation (1). Relationship with the applied electric field E: (1) in, E is the electrical conductivity of the insulating component, and E is the electric field.

[0037] Therefore, based on nonlinear insulators with a+bE type nonlinear conductivity (i.e., the conductivity of the insulator) (It is a linear function of the applied electric field E), which can be used to establish an interface polarization model for a double-layer nonlinear insulator, and represent it using a physical model of a series-connected double-layer nonlinear insulator, such as... Figure 4 As shown.

[0038] exist Figure 4 In the model, it is assumed that the double-layer insulation or dielectric is homogeneous and is referred to as dielectric 1 and dielectric 2. and d1 and d2 are the dielectric constants of dielectric 1 and dielectric 2, respectively, in farads per meter (F / m). They are almost independent of the electric field E, so they can be considered constants. d1 and d2 are the thicknesses of dielectric 1 and dielectric 2, respectively, in meters (m). and The conductivity of dielectric 1 and dielectric 2 is expressed in Siemens per meter (S / m). If the temperature is constant, the conductivity is a function of the electric field strength, as shown in equation (2) below: (2)

[0039] Under a DC step voltage U(t), based on the principle of current continuity and the loop voltage law, the following equations (3) and (4) can be obtained. (3) (4)

[0040] It was found that when water enters the cable joint, the conductivity will be higher than usual, and in PDC testing, it increases almost linearly with the applied voltage, such as... Figure 5 As can be seen, each data point is the average of the data obtained from the measurements of the last two minutes. This yields the polarization current curve of the water-infiltrated cable, which shows anomalies, as seen in Figures 6(a) and 6(b). The total polarization current of the double-insulated cable with a+bE type conductivity changing over time shows two possible anomalies. One is a monotonically increasing current followed by a tendency towards a steady state, as shown in Figure 6(a). The other is a non-monotonic change in current over time, such as the monotonically decreasing current followed by a tendency towards a steady state curve shown in Figure 6(b).

[0041] Based on the observed abnormal polarization current curves, a PDC test was developed to diagnose cable joint faults, such as water ingress in cable joint 10. The experimental PDC cable joint test and results are described below.

[0042] Experiments and Results

[0043] In the experiments conducted, two circuits were tested: Circuit 1 and Circuit 2. Each circuit comprised a 6.6kV XLPE distribution cable used in the Singapore power grid. Each distribution cable was a three-core cable with three conductive cores, each corresponding to one of the three phases of the distribution cable. Each distribution cable comprised multiple cable segments connected by multiple joints, as detailed in the circuit specifications given in Tables 1 and 2 below. Table 1 - Specifications of Circuit 1 Table 2 - Specifications of Circuit 2

[0044] Using a Megger® S1-1568 DC IR tester remotely controlled by a laptop computer, PDC tests were performed on circuits 1 and 2 using a test DC voltage of 5 kV and a test duration of 70 s, according to the IR tester operation guide. Performing the PDC test involved first disconnecting the distribution cable from the power supply network (101). Next, a DC voltage was applied to one end of the conductive core of the distribution cable for 70 s (102). The polarization current in the distribution cable during the application of the DC voltage was obtained (103), and the polarization current curve of that conductive core was plotted against time (104). PDC tests were performed on all three conductive cores of the distribution cable to obtain the polarization curves for all three phases of the distribution cable, as shown in Figures 7(a) and 7(b). The polarization current obtained in the first ten seconds is omitted due to the influence of the initial response of the test equipment. Preferably, the disconnected distribution cable was fully discharged before the DC voltage was applied.

[0045] As can be seen from Figure 7(b), the polarization currents of all three phases in circuit 2 decrease with time, following the above reference... Figures 2(a) to 2(c) The polarization current characteristics of the single-layer insulation are described. However, for the yellow phase of circuit 1, the polarization current increases with time. As shown above with reference to Figure 6(a), the increase in polarization current over time is an abnormal trend, which can indicate cable joint failure due to water ingress.

[0046] Although a 5kV test voltage was used in the experiments conducted, the DC voltage applied in Method 100 can be in the range of 2kV to 6kV. In addition to the 70s test duration used in the experiments, the test duration can range from 50s to 100s. Distribution cables may include varying numbers of conductive cores, and all conductive cores in the cable should be subjected to PDC testing.

[0047] To confirm that the obtained abnormal PDC curve indicates a cable joint failure due to water ingress, a time-domain reflectometry (TDR) test was performed. In the TDR test, the distribution cable was taken offline, and a pulse of known low-voltage energy was injected into the offline cable. The amplitude of the reflected waveform at any impedance discontinuity can be determined by the following equation (5) and expressed as the reflection coefficient. : (5)

[0048] Where Z0 is the characteristic impedance of the cable; Z d Z is the impedance along any discontinuity in the cable (such as a defect or cable joint); for a cable short circuit, Z... d = 0, for open paths, Z d = From equation (1), Between -1 and 1. The injection and reflection waveforms of the TDR test device are as follows: Figure 8 As shown. Typical waveforms obtained from TDR testing under different conditions are as follows. Figure 9 As shown. In TDR testing, any impedance discontinuity, such as cable joints and suspected defects, will produce reflections and cause distortion of the original pulse. By capturing the injected and response signals in the time domain, the location of abnormal cable joints and defects can be identified.

[0049] Figures 10(a) and 10(b) show the TDR waveforms obtained from the TDR tests of circuit 1 and circuit 2, respectively. According to... Figure 9 As shown in Figure 10(a), the abnormal TDR waveforms of all three phases (red, yellow, and blue) of Circuit 1 occur around 230m near connector 1, as shown in Table 1. Note that only the yellow phase of Circuit 1 shows an abnormal trend in the PDC test results, while all three phases of Circuit 1 show abnormalities in the TDR waveforms. This is likely due to the very low IR value of the yellow phase of Circuit 1, causing abnormal polarization current curves due to water entering connector 1. As shown in Figure 10(b), the TDR waveforms of all three phases of Circuit 2 are normal, indicating that there is no problem with the cable body or cable connector in Circuit 2.

[0050] The results of the PDC tests for circuits 1 and 2, and the TDR verification of the PDC test results, show that the PDC test of the distribution cable is sufficient to provide an indication of cable joint failure in the distribution cable, which could be the result of water entering the cable joint in the distribution cable.

[0051] While exemplary embodiments of the invention have been described above, those skilled in the art will understand that many changes can be made to the details of design, construction, and / or operation without departing from the invention. It should be understood that many further changes, modifications, and substitutions can be made to various aspects of the described embodiments within the spirit and scope of the appended claims. References to any existing publications (or information derived therefrom) or any known content in this specification are not, and should not be, considered as an acknowledgment or endorsement or any form of suggestion that such publications (or information derived therefrom) or known content form part of common general knowledge in the field to which this specification pertains.

Claims

1. A method for detecting cable joint faults in a power distribution cable of a power supply network, the power distribution cable comprising at least one conductive core, the method comprising: (a) Disconnect the power distribution cable from the power supply network; (b) Applying a DC voltage to one end of the conductive core for a predetermined duration; (c) During the application of the DC voltage, obtain the polarization current in the distribution cable; as well as (d) Plot the curve of the polarization current as a function of time; Wherein, the increase of the polarization current over time indicates water ingress into the cable joints of the power distribution cable; and The non-monotonic change of the polarization current over time indicates water ingress into the cable joints of the power distribution cable.

2. The method according to claim 1, further comprising: After step (a) and before step (b), the power distribution cable is fully discharged.

3. The method according to claim 1 or 2, wherein, The power distribution cable includes a plurality of conductive cores, and the method includes performing steps (b) to (d) on each of the plurality of conductive cores.

4. The method according to any one of the preceding claims, wherein, The predetermined duration ranges from 50s to 100s.

5. The method according to any one of the preceding claims, wherein, The DC voltage has a value between 2kV and 6kV.

6. The method according to any one of the preceding claims, wherein, The power distribution cable includes a medium-voltage power cable insulated with cross-linked polyethylene (XLPE).

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