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Tackling sheath problems: Latest research developments in solving operational sheath problems in underground power transmission cables
Electric Power Systems Research 77 (2007) 1449–1457
Review
Tackling sheath problems: Latest research developments in solving operational sheath problems in underground power transmission cables X.H. Wang a,∗ , Y.H. Song b,1 , C.K. Jung c,2 , J.B. Lee c,2 a
Faculty of Computing, Information Systems and Mathematics, Kingston University, Kingston upon Thames, Surrey KT1 2EE, UK b School of Engineering and Design, Brunel University, Middlesex UB8 3PH, UK c Advanced T&D Research Centre, Department of Electrical Engineering, Wonkwang University, South Korea Received 23 March 2006; received in revised form 29 August 2006; accepted 10 October 2006 Available online 15 November 2006
Abstract In a power transmission cable system, the uniformly configuration of the cables between sections is sometimes difficult to achieve because of the geometrical limitation. This will cause the increase of sheath circulating current which results in the increase of sheath loss and the decrease of permissible current of the power transmission system. While the cable is on operation, because of cable aging and other unexpected reasons, sheath fault always occurs which leads to the further damage of the insulation layer and the life of the cable. These two problems were particularly addressed in this paper. Over last few years, two research groups in South Korea and the United Kingston have worked together extensively to solve these problems and made some great achievements. For tackling the problem of the increase of sheath circulating current, one special device was designed to measure this kind of current; an effective measure was proposed to reduce the current and the new measure was well protected from any further threats like cable fault and lightning. Regarding the problem with sheath fault, one data acquisition system was designed to monitor the sheath fault on field; a new fault phenomenon was discovered for the cable buried in sand; based on the new discovery, the fault was characterised, modelled and simulated; furthermore advanced signal processing techniques were introduced to extract the fault signals from the sheath, detect and locate the fault. These new solutions have either been successfully applied in practical operation of the power cable system or used in further research to propose more advanced solutions. The paper reviewed the latest developments of these new solutions. © 2006 Elsevier B.V. All rights reserved. Keywords: Sheath circulating current; Sheath fault; Underground power cables
Contents 1. 2. 3. 4. 5.
6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Equipment design for sheath study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reduction of sheath circulating current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterisation of sheath fault and modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sheath fault detection and location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Signal processing in fault detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Fault location based on travelling wave method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
∗
Corresponding author. Tel.: +44 20 85472000x62495; fax: +44 20 85477197. E-mail addresses: [email protected] (X.H. Wang), [email protected] (Y.H. Song), [email protected] (C.K. Jung), [email protected] (J.B. Lee). 1 Tel.: +44 1895 265932. 2 Tel.: +82 63 850 6735; fax: +82 63 850 6735. 0378-7796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsr.2006.10.004
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1. Introduction Sheath in underground power transmission cables is serving as a layer to prevent the moisture ingress into the insulation layer and provide a path for earth return currents. Nowadays, owing to the maturity of manufacturing technologies, normally there
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are no problems for the quality of the sheath itself. However, after the cable is laid in the cable tunnel and operating as part of the transmission network, due to network construction and some unexpected accidents, such as construction damage and cable immersion into water because of heavy rain, there maybe cause some problems to the sheath. These include the increase of the sheath current because of the un-uniform construction of the network and the damage of the sheath because of some mechanical effects and some critical weather conditions. The consequence is the increased loss of power and the damage of the cable eventually. However, there is little research work to investigate these two particular problems. Our research work particularly addressed these two problems over the last few years. Some great achievements have been made and applied in practice. This paper will review these developments and solutions to the problems. In Section 2, the phenomena of the sheath circulating current and sheath fault were explained. New devices designed for the measurement of the sheath circulating current and monitoring of sheath fault were introduced. Section 3 discussed a new proposed effective method to reduce the sheath circulating current and how to protect the new device. Section 4 investigated the characteristics of the sheath fault for the cable buried in sand and its modelling and simulation. Section 5 studied how to detect and locate the sheath fault by applying some advanced signal processing techniques, such as adaptive signal processing and wavelet analysis. Section 6 concluded the paper and introduced other relevant ongoing research works. 2. Equipment design for sheath study While the underground power transmission cable is in operation, the sheaths are crossbonded at each end of the cable to suppress the induced voltages in the sheath. The details of the
crossbonding can be referred to IEEE guide [1]. The crossbonding of the sheaths produces a returning path of the induced current from other phase cables. This current is known as “sheath circulating current” which produces “sheath circulating loss”. In addition to the sheath circulating loss, another type of loss in the sheath is called “sheath eddy loss”, which is due to the induced eddy current flowing circumferentially in the sheaths of a three-core cable or in the sheaths of a system of three singlecore cables. But in many instances this loss is small and can be disregarded. Sheath current can cause sheath loss and reduce the permissible current of the power transmission system. If the cables are uniformly constructed in a transmission network and working normally, the induced voltage and the sheath circulating current will remain at a reasonable level that will not cause any problems. However, in practice, because of the geographical limitations, such as obstruction of buildings, bridges and rivers, and some other effects, the task of uniformly construction is not easy to achieve. In this case, a high sheath current will be produced. In order to precisely study this kind of current, a special device was designed. It is named as “Power Cable Current Analyser” [2]. A photograph of the whole system is shown in Fig. 1. The equipment consists of three parts: a sensor, processing unit and ancillary devices. The sensor is a current transformer (CT) which measures the sheath circulating current of each phase. The processing unit can calculate the measured currents, display them in magnitude and phase angle, analyse the current in frequency domain by fast Fourier transform (FFT) and display the 3rd, 5th, 7th and 9th harmonic components. The equipment has a connecting port to a digital oscilloscope to display the current waveforms, and a port to a printer to print the result. The equipment has the capability to measure up to 9 currents simultaneously. Other specifications of the equipment are:
Fig. 1. Photograph of power cable current analyser.
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Fig. 2. Photograph of testing site.
1. Maximum current measurement: 500 A (±1%) at power frequency of 60 Hz. 2. Minimum current measurement: 0.1 A (±3%) at power frequency. 3. Operation voltage: DC 24 V (battery). 4. Saving function: it can save the amplitude and phase angle of current, including the harmonic component. This device is designed to measure the sheath circulating current. In order to study fault in sheaths, a monitoring system was also developed [3]. The system involves the data acquisition, signal conditioning and signal processing. It is powered by Labview. It monitors the fault voltage and current of three cycles every hour in order to reduce the data volume but keep the record at various conditions over long term. Fig. 2 shows the site where the system is located to monitor the fault condition in ambient environment.
structure of the cable is shown in Fig. 4. It is a single core oilfilled cable. The studied sheath layer is between insulation and oversheath. This circuit is very complicated. On the one hand, the length of each is totally different. On the other hand, the burying formation between joints #8 and #10 is different from the others. In most of the sections, the cables are buried in trefoil formation. However, the minor section between joints #9 and #10 is buried in duct formation, and the minor section between joints #8 and #9 is buried with mixed trefoil and duct formation. It is discovered from practical operation that there was a huge sheath current produced in this circuit. The sheath currents are
3. Reduction of sheath circulating current As discussed in Section 2, when the cables constructed asymmetrically, a high sheath circulating current will be produced. Fig. 3 shows such a practical circuit in South Korea. It is a double-circuit underground transmission cable system. The
Fig. 4. Structure of single-core oil-filled cable.
Fig. 3. Diagram of 154 kV double-circuit underground power cable system.
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Fig. 5. Measured and simulated sheath circulating currents.
shown in Fig. 5. Fig. 5 shows the sheath circulating current on Phase C on each minor section. Line marked with square is the measured current, while diamond is the simulated current by ATP. While the cables were simulated via ATP, the Constant Parameter Distributed Line model was applied throughout the paper [4]. As shown in the figure, there is an apparent increase of the current at sections 9 and 12. That happens on the major section where the cables are laid in mixed formation with trefoil and duct. The mixed formation is the main reason of the increase of the current. The current among other sections differs as well because of the different length between them, but the effect is not significant. The currents between sections 9 and 12 are extremely high. They reach up to 100 A that is not negligible for the power loss. This current should be reduced to a low level as the same as those between section 1 and 8. In order to reduce the sheath circulating current, one possible method is to increase the spacing between the cables and keeping the cable balanced. However, as the spacing increases, the construction of the cable channel will be increased as well. That
Fig. 6. Connection diagram of reactor.
will increase the cost of the system. This is not an effective way to reduce the sheath circulating current. A new method to reduce the sheath circulating current was proposed in laboratory and in practical application [2,5,6]. That is to reduce it by connecting a reactor or resistor on the cross bonding lead. Both situations were extensively studied. This paper will only review the implementation of the reactor. The reactor is serially connected to the cross bonding lead on the insulating joint where the sheath circulating current increases. The connection circuit and the implementation in the actual system are shown in Figs. 6 and 7. The reactor is a 2.65 mH reactor box. It could be installed at either joint #10 or joint #11. Simulation results show that it is more effective to reduce the current. The results are shown in Table 1. As the result shows it can reduce the sheath circulating current averagely by 89.6%. This is so effective.
Fig. 7. Photograph of the installed reactor in actual system.
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Table 1 Reduction effect of reactor through measurement and simulation J/B
Phase
Sheath circulating current [A] Measurement
Reduction rate [%]
With
Without
10.1 7.3 9.4
50.6 78.9 80.95
80 90.7 88.4
Simulation
Reduction rate [%]
With
Without
5.6 16.9 25.7
61.59 81.98 97
90.9 79.4 73.5
#9 (NJ)
A B C
#10 (IJ)
A–C B–A C–B
7.2 5.7 7.2
56.1 72 69.9
87.1 92.1 86.7
2.7 3.61 7.17
53.9 64.3 82.3
94.99 94.38 91.3
#11 (IJ)
A–C B–A C–B
1.7 1.6 1.4
61.6 83.8 78.35
97.2 98.1 98.2
8.24 12.46 7.12
75.2 92.7 64.9
89.04 86.56 89.03
#12 (NJ)
A B C
9.5 8.6 8.8
56.8 72.4 62.55
83.3 88.1 86
3.43 16.7 15.4
75 44.7 56.4
95.43 62.64 72.7
Average [%]
89.6
One possible problem by installing the reactor on the bonding lead is to increase the induced sheath voltage. As the reactor is serially connected to bonding lead, it actually increases the impedance of the bonding lead. That will increase the induced voltage on the sheath from the core. It is also realized that the new device on the cable system could be damaged when the sheath is hit by high voltages, such as cable fault and lightning stroke. Therefore, the device needs proper protection. A new device, called reduction device protector (RDP), was designed to protect the reactor against the transient overvoltage [2,5]. Its characteristic, shown in Fig. 8(b), is similar as sheath voltage limiter (SVL), shown in Fig. 8(a). The difference between SVL and RDP is that RDP has a lower initial voltage than SVL so that the RDP can act before SVL when an overvoltage strikes on the sheath. RDP is connected to the reactor in parallel, but in two conditions. One is one end of RDP is grounded, the other is RDP without grounding. The schematic graphs of these two methods are shown in Fig. 9. The effectiveness of the two protection methods of RDP were extensively studied by ATP simulation. From the study [2,5], it is found that the RDP without grounding will provide a better protection against fault and lightning strike. The combination of the above design of current reduction and protection can effectively reduce the sheath circulating current and protect it against any possible threats. These new measures have been implemented in the above transmission system.
84.98
in the development of new reliable non-destructive detection and location technique, sheath to earth faults under ac voltage have been fully investigated. For this work, a cable test assembly was constructed and various fault conditions applied. A single core, XLPE insulated 275 kV power cable is laid in a simulated trough with sand. After initial investigation, a monitoring system is established to monitor the fault over long time against different weather conditions. The monitoring system is discussed in Section 2.
4. Characterisation of sheath fault and modelling After the cable is laid in operation, the oversheath (jacket) will be inevitably damaged because of the cable aging, mechanical and immersion in water. This causes sheath to earth fault that is simply referred to sheath fault. Sheath fault has being talked among scientists and engineers over the years [7–10], but no one knows exactly what kind of characteristics it has. To help
Fig. 8. V–I characteristic curve of SVL and RDP. (a) V–I characteristic of SVL. (b) V–I characteristic of RDP.
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Fig. 9. Protection methods of reduction device.
The fault shows a strange characteristic and this characteristic has been confirmed by monitoring at different conditions. In summery, if a sine wave is applied to the system the fault characteristic manifests three different conditions. Firstly when the applied voltage on the fault is low, at this stage, the fault current is still a sine wave. Secondly, when the applied voltage on the fault is up to a certain voltage, a spike appears from the top of positive cycle. It is opposite to the applied voltage. We regard the voltage at this point as the threshold voltage or inception voltage. Thirdly, any further increase in the applied voltage leads to another spike appears at the bottom of negative cycle. Its direction is still opposite to the applied voltage. At the same time, a flat current appears within the spikes both on positive and negative cycles when the applied voltage is increased further more, as shown in Fig. 10. And the magnitude of flat current in positive cycle is less than in negative one. Strangely this threshold voltage is not high. For the cable in normal working conditions, on most of the cases it is below the induced voltage on the sheath. Some explanations have been proposed [3]. Unfortunately, these explanations are not enough without further experimental confirmation. However, based on this discovery, further detection and location methods can be developed. In order to speed up the research, a fault model is established and implemented in simulation soft-
ware ATP so that later research can be done by simulation instead of experiment. When the fault is implemented in ATP, the model is presented as shown in Fig. 11. R1 represents the fault characteristics of non-linear resistance in positive cycle of applied voltage. R2 is the non-linear resistance in negative cycle. R3 and C are linear resistance and capacitance, respectively. k1 , k2 and k3 are three TACS-controlled switches which control the opening and closing of the three branches. If the applied voltage is below the threshold voltage, the switch k3 is closed while k1 and k2 stay open. Then the linear branch works. When the applied voltage reaches the inception voltage in positive cycle, then switch k1 is closed while k2 and k3 open. When the applied voltage is above the threshold voltage in negative cycle then switch k2 is closed while the other two open. The model presented in Fig. 11 produces a good simulation result comparing the experimental investigation [11]. However, in ATP software sometimes the non-linear resistance will cause oscillation. The parameters of the non-linear resistance and other control parameters should be chosen carefully to avoid this. In order to make the model work more efficiently, we neglect the spikes of the fault and regard the flat fault currents as two current sources for the simplicity. Then the simplified fault model is proposed as shown in Fig. 12, in which the two diodes control
Fig. 10. Fault current under higher applied voltage.
Fig. 11. Fault model in ATP.
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Fig. 13. Single-frequency adaptive noise canceller.
Fig. 12. Simplified sheath fault model.
the opening and closing of the two current sources for nonlinear fault above threshold voltage and the linear resistance and capacitance represent the linear fault below threshold voltage. For the simplicity of simulation, the sheath fault is supposed to be symmetrical, that is the magnitude of the threshold voltage and the flat currents on both positive and negative cycles are the same. This model is easy to be implemented in ATP and still gives reliable result. Thus, it is applied to simulate the sheath fault in the transmission network.
in fault detection and location. Other advantages include easy control of bandwidth, an infinite null, and the capability of adaptively tracking the exact frequency of the interference. Fig. 14(b) shows the filtered fault current extracted from the sheath current in Fig. 14(a) after applying ANC technique. After the extraction of the fault current, the fault detection becomes an easy job.
5. Sheath fault detection and location 5.1. Signal processing in fault detection It is realized from experiment that the sheath fault for the cable laid in sand trough is a high impedance fault even after the sheath is broke down. For high impedance fault, the fault signal is very weak. They are difficult to be detected from the strong background noises. In this case, the sheath fault current is very low, at milli-ampere level. However, normally the current on the sheath will be at around few or dozen of ampere level. To extract such a small current from the high background current is not easy. It is also realized that the background current has single frequency component, while the fault current is non-linear with multiple frequency components. Thus, a signal processing technique called adaptive noise cancelling (ANC) was adopted to extract the weak signal [12]. The schematic figure is shown in Fig. 13. For ANC technique, the received signals, including power fundamental frequency signal (60 Hz in this situation) and transient signals were applied to Primary Input end. A 60 Hz signal with a random given amplitude was regarded as noise signal and applied to the Reference Input. Then the fundamental frequency components from the received signals would be removed to leave only the transient signals. This technique has several advantages. It has the ability to adjust its own parameters automatically, and its design requires little or no priori knowledge of signal or noise characteristics. The noise can be eliminated without signal distortion, which is a very important characteristic needed
Fig. 14. Sheath and fault current. (a) Sheath current with hided fault current. (b) Filtered fault current.
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5.2. Fault location based on travelling wave method For non-destructive and quick fault detection and location on power cable, an ideal method is to detect fault generated high frequency transients at one or two ends of the cable [13–15]. For this method, two parameters need to be determined. One is the velocity of the transients travelling on the line. The other is the arrival times of these transients at one end or both ends of the line. The velocity of the travelling wave is determined by the wave frequency and the properties of the line itself, and its surrounding environment. For the signals on the sheath, they are propagating in a complicated way because of the crossbonding of the sheath from three phases at both ends of each minor section. From the theoretical study and simulation by ATP, it is found that there are six propagation modes of the signals on cable [16]. Its propagation velocities versus frequency are plotted in Fig. 15. In the figure, both the frequency and the velocity are plotted after taking the natural logarithm. The first three modes are associated with waves travelling on the sheath. Mode 1, marked with diamond at the bottom, is the sheath-earth mode. Mode 2, marked with square and its velocity just being below 16.5, is an inter-sheath mode between sheaths on central cable and side cables (in case of the flat layout of the cables in my study). Mode 3, marked with triangle and being just above the 16.5, is also an inter-sheath mode but between the sheath of side cables. Modes 4–6 are waves travelling in the cable conductors. They have similar returning loops as modes 1–3. However, in practice, the generated fault transients on the sheath are not propagating in all three modes, and cannot be detected all. For the case of flat layout of the three-phase cables, if the fault occurs in the side cable, the transients will propagate in all three modes. However, if the fault occurs in the central cable, the transients will propagate in only two modes which are sheath-earth mode (mode 1) and inter-sheath mode between central sheath and side sheaths (mode 2). In addition, the high impedance of the earth causes high attenuation of signals trav-
Fig. 16. Fault generated transients. (a) Fault transients for fault in side cable. (b) Fault transients for fault in central cable.
elling in mode 1 which makes it impossible to be detected. Therefore, only two traveling modes of signals can be detected if the fault is in side cable, and one mode for the fault in central cable. This can be reflected from Fig. 16. Overall, transients travelling in mode 2 is selected to locate the fault on all conditions. Details on how to discriminate the propagation mode and locate the fault can be referred to paper [17]. The arrival times of the transients at the end of the cable were located by the wavelet analysis. Wavelet analysis has the ability to discriminate the transient signals and locate when the signal occurred. This makes it a powerful tool in fault location. After the velocity and the arrival time are known, the distance of the fault to the end of the cable can be easily determined by the famous Bewley’s lattice diagram [18]. 6. Conclusions
Fig. 15. Wave velocity versus frequency.
The above are the research results that have been achieved over the last few years. Some of the research results, such as designed new equipment, reduction method of the sheath circulating current and their protection measures, have been implemented in power system. Other research work, such as fault
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characterisation, modelling and the fault detection and location, can contribute to the knowledge in power system and improve the development of new solutions to the problems. Research work is still going on at both laboratories on fault location by applying wavelet analysis technology, particular on noisy environment and mixed circuit. Some achievements have been achieved. New results will be published soon. Acknowledgements The authors would like to express their great gratitude to National Grid, UK and Wonkwang University for their supports in both finance and provision of facilities and testing site. References [1] IEEE Guide for the Application of Sheath-Bonding Methods for SingleConductor Cables and the Calculation of Induced Voltages and Currents in Cable Sheaths. [2] C.K. Jung, J.B. Lee, J.W. Kang, X.H. Wang, Y.H. Song, The characteristic of the sheath circulating current and its reduction in underground power cable systems, Int. J. Emerging Electric Power Syst. (IJEEPS) 1 (2004) Article 1. [3] X.H. Wang, Characterization, detection and location of sheath fault on underground power transmission cables, Ph.D. Thesis, Department of Electronic and Computer Engineering, Brunel University, London, 2001. [4] Alternative Transients Program—Rule Book, Leuven EMTP Center, 1990. [5] C.K. Jung, J.B. Lee, J.W. Kang, T.I. Jang, Analysis and reduction methods of sheath circulating current in underground transmission systems, KIEE Trans. 50A (11) (2001) 537–545. [6] J.W. Kang, H.W. Yang, A study on the characteristic and rising cause of sheath circulating current by analysis and measurement, KIEE Trans. 51A (10) (2002) 525–533.
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[7] H.T. Gooding, T.A. Briant, Location of serving defects in buried cables, IEE Proc. (1961) 124–125. [8] P.F. Gail, Cable fault location by impulse current method, Proc. IEE 122 (4) (1975) 403–408. [9] T.V. Lathrop, Locating sheath breaks in wet PIC cable, in: Proceedings of the International Conference on Communications, Part II, vol. 19, New York, USA, 1976, pp. 4–9. [10] J.T. Bejaminsen, A. Bruaset, H. Faremo, Sheath fault testing-a cost effective tool in order to extend cable life, in: Proceedings of the Nordic Insulation Symposium, Technical University, Denmark, 1999, pp. 101– 108. [11] X.H. Wang, Y.H. Song, Sheath fault characteristics and modelling on underground power transmission cables, ETEP Eur. Trans. Electr. Power 11 (2) (2001) 137–140. [12] X.H. Wang, Y.H. Song, Adaptive noise cancelling technique in sheath fault detection on power cables, in: 5th International Power Engineering Conference (IPEC2001), Singapore, 2001. [13] Z.Q. Bo, G. Weller, M.A. Redfern, Accurate fault location technique for distribution system using fault generated high-frequency transient voltage signals, IEE Proc. Gen. Transm. Distrib. 146 (1) (1999) 73–79. [14] Z. Chen, Z.Q. Bo, F. Jiang, G.A. Weller, Fault generated high frequency current transients based protection scheme for series compensated lines, in: IEEE Power Engineering Society Winter Meeting, Piscataway, USA, 2000. [15] A.T. Johns, R.K. Aggarwal, Z.Q. Bo, Non-unit protection technique for EHV transmission systems based on fault-generated noise. Part 1: Signal measurement, IEE Proc. Gen. Transm. Distrib. 141 (2) (1994) 133– 140. [16] X.H. Wang, Y.H. Song, C. Ferguson, Wave propagation characteristics on crossbonded underground power transmission cables and sheath fault location, ETEP Eur. Trans. Electric Power 13 (2) (2003) 127–131. [17] X.H. Wang, Y.H. Song, Sheath fault detection and classification based on wavelet analysis, ETEP Eur. Trans. Electric Power 16 (4) (2006). [18] A.I. Ramirez, A. Semlyen, R. Iravani, Modeling nonuniform transmission lines for time domain simulation of electromagnetic transients, IEEE Trans. Power Deliv. 18 (3) (2003) 968–974.
Review
Tackling sheath problems: Latest research developments in solving operational sheath problems in underground power transmission cables X.H. Wang a,∗ , Y.H. Song b,1 , C.K. Jung c,2 , J.B. Lee c,2 a
Faculty of Computing, Information Systems and Mathematics, Kingston University, Kingston upon Thames, Surrey KT1 2EE, UK b School of Engineering and Design, Brunel University, Middlesex UB8 3PH, UK c Advanced T&D Research Centre, Department of Electrical Engineering, Wonkwang University, South Korea Received 23 March 2006; received in revised form 29 August 2006; accepted 10 October 2006 Available online 15 November 2006
Abstract In a power transmission cable system, the uniformly configuration of the cables between sections is sometimes difficult to achieve because of the geometrical limitation. This will cause the increase of sheath circulating current which results in the increase of sheath loss and the decrease of permissible current of the power transmission system. While the cable is on operation, because of cable aging and other unexpected reasons, sheath fault always occurs which leads to the further damage of the insulation layer and the life of the cable. These two problems were particularly addressed in this paper. Over last few years, two research groups in South Korea and the United Kingston have worked together extensively to solve these problems and made some great achievements. For tackling the problem of the increase of sheath circulating current, one special device was designed to measure this kind of current; an effective measure was proposed to reduce the current and the new measure was well protected from any further threats like cable fault and lightning. Regarding the problem with sheath fault, one data acquisition system was designed to monitor the sheath fault on field; a new fault phenomenon was discovered for the cable buried in sand; based on the new discovery, the fault was characterised, modelled and simulated; furthermore advanced signal processing techniques were introduced to extract the fault signals from the sheath, detect and locate the fault. These new solutions have either been successfully applied in practical operation of the power cable system or used in further research to propose more advanced solutions. The paper reviewed the latest developments of these new solutions. © 2006 Elsevier B.V. All rights reserved. Keywords: Sheath circulating current; Sheath fault; Underground power cables
Contents 1. 2. 3. 4. 5.
6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Equipment design for sheath study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reduction of sheath circulating current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterisation of sheath fault and modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sheath fault detection and location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Signal processing in fault detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Fault location based on travelling wave method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
∗
Corresponding author. Tel.: +44 20 85472000x62495; fax: +44 20 85477197. E-mail addresses: [email protected] (X.H. Wang), [email protected] (Y.H. Song), [email protected] (C.K. Jung), [email protected] (J.B. Lee). 1 Tel.: +44 1895 265932. 2 Tel.: +82 63 850 6735; fax: +82 63 850 6735. 0378-7796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsr.2006.10.004
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1. Introduction Sheath in underground power transmission cables is serving as a layer to prevent the moisture ingress into the insulation layer and provide a path for earth return currents. Nowadays, owing to the maturity of manufacturing technologies, normally there
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are no problems for the quality of the sheath itself. However, after the cable is laid in the cable tunnel and operating as part of the transmission network, due to network construction and some unexpected accidents, such as construction damage and cable immersion into water because of heavy rain, there maybe cause some problems to the sheath. These include the increase of the sheath current because of the un-uniform construction of the network and the damage of the sheath because of some mechanical effects and some critical weather conditions. The consequence is the increased loss of power and the damage of the cable eventually. However, there is little research work to investigate these two particular problems. Our research work particularly addressed these two problems over the last few years. Some great achievements have been made and applied in practice. This paper will review these developments and solutions to the problems. In Section 2, the phenomena of the sheath circulating current and sheath fault were explained. New devices designed for the measurement of the sheath circulating current and monitoring of sheath fault were introduced. Section 3 discussed a new proposed effective method to reduce the sheath circulating current and how to protect the new device. Section 4 investigated the characteristics of the sheath fault for the cable buried in sand and its modelling and simulation. Section 5 studied how to detect and locate the sheath fault by applying some advanced signal processing techniques, such as adaptive signal processing and wavelet analysis. Section 6 concluded the paper and introduced other relevant ongoing research works. 2. Equipment design for sheath study While the underground power transmission cable is in operation, the sheaths are crossbonded at each end of the cable to suppress the induced voltages in the sheath. The details of the
crossbonding can be referred to IEEE guide [1]. The crossbonding of the sheaths produces a returning path of the induced current from other phase cables. This current is known as “sheath circulating current” which produces “sheath circulating loss”. In addition to the sheath circulating loss, another type of loss in the sheath is called “sheath eddy loss”, which is due to the induced eddy current flowing circumferentially in the sheaths of a three-core cable or in the sheaths of a system of three singlecore cables. But in many instances this loss is small and can be disregarded. Sheath current can cause sheath loss and reduce the permissible current of the power transmission system. If the cables are uniformly constructed in a transmission network and working normally, the induced voltage and the sheath circulating current will remain at a reasonable level that will not cause any problems. However, in practice, because of the geographical limitations, such as obstruction of buildings, bridges and rivers, and some other effects, the task of uniformly construction is not easy to achieve. In this case, a high sheath current will be produced. In order to precisely study this kind of current, a special device was designed. It is named as “Power Cable Current Analyser” [2]. A photograph of the whole system is shown in Fig. 1. The equipment consists of three parts: a sensor, processing unit and ancillary devices. The sensor is a current transformer (CT) which measures the sheath circulating current of each phase. The processing unit can calculate the measured currents, display them in magnitude and phase angle, analyse the current in frequency domain by fast Fourier transform (FFT) and display the 3rd, 5th, 7th and 9th harmonic components. The equipment has a connecting port to a digital oscilloscope to display the current waveforms, and a port to a printer to print the result. The equipment has the capability to measure up to 9 currents simultaneously. Other specifications of the equipment are:
Fig. 1. Photograph of power cable current analyser.
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Fig. 2. Photograph of testing site.
1. Maximum current measurement: 500 A (±1%) at power frequency of 60 Hz. 2. Minimum current measurement: 0.1 A (±3%) at power frequency. 3. Operation voltage: DC 24 V (battery). 4. Saving function: it can save the amplitude and phase angle of current, including the harmonic component. This device is designed to measure the sheath circulating current. In order to study fault in sheaths, a monitoring system was also developed [3]. The system involves the data acquisition, signal conditioning and signal processing. It is powered by Labview. It monitors the fault voltage and current of three cycles every hour in order to reduce the data volume but keep the record at various conditions over long term. Fig. 2 shows the site where the system is located to monitor the fault condition in ambient environment.
structure of the cable is shown in Fig. 4. It is a single core oilfilled cable. The studied sheath layer is between insulation and oversheath. This circuit is very complicated. On the one hand, the length of each is totally different. On the other hand, the burying formation between joints #8 and #10 is different from the others. In most of the sections, the cables are buried in trefoil formation. However, the minor section between joints #9 and #10 is buried in duct formation, and the minor section between joints #8 and #9 is buried with mixed trefoil and duct formation. It is discovered from practical operation that there was a huge sheath current produced in this circuit. The sheath currents are
3. Reduction of sheath circulating current As discussed in Section 2, when the cables constructed asymmetrically, a high sheath circulating current will be produced. Fig. 3 shows such a practical circuit in South Korea. It is a double-circuit underground transmission cable system. The
Fig. 4. Structure of single-core oil-filled cable.
Fig. 3. Diagram of 154 kV double-circuit underground power cable system.
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Fig. 5. Measured and simulated sheath circulating currents.
shown in Fig. 5. Fig. 5 shows the sheath circulating current on Phase C on each minor section. Line marked with square is the measured current, while diamond is the simulated current by ATP. While the cables were simulated via ATP, the Constant Parameter Distributed Line model was applied throughout the paper [4]. As shown in the figure, there is an apparent increase of the current at sections 9 and 12. That happens on the major section where the cables are laid in mixed formation with trefoil and duct. The mixed formation is the main reason of the increase of the current. The current among other sections differs as well because of the different length between them, but the effect is not significant. The currents between sections 9 and 12 are extremely high. They reach up to 100 A that is not negligible for the power loss. This current should be reduced to a low level as the same as those between section 1 and 8. In order to reduce the sheath circulating current, one possible method is to increase the spacing between the cables and keeping the cable balanced. However, as the spacing increases, the construction of the cable channel will be increased as well. That
Fig. 6. Connection diagram of reactor.
will increase the cost of the system. This is not an effective way to reduce the sheath circulating current. A new method to reduce the sheath circulating current was proposed in laboratory and in practical application [2,5,6]. That is to reduce it by connecting a reactor or resistor on the cross bonding lead. Both situations were extensively studied. This paper will only review the implementation of the reactor. The reactor is serially connected to the cross bonding lead on the insulating joint where the sheath circulating current increases. The connection circuit and the implementation in the actual system are shown in Figs. 6 and 7. The reactor is a 2.65 mH reactor box. It could be installed at either joint #10 or joint #11. Simulation results show that it is more effective to reduce the current. The results are shown in Table 1. As the result shows it can reduce the sheath circulating current averagely by 89.6%. This is so effective.
Fig. 7. Photograph of the installed reactor in actual system.
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Table 1 Reduction effect of reactor through measurement and simulation J/B
Phase
Sheath circulating current [A] Measurement
Reduction rate [%]
With
Without
10.1 7.3 9.4
50.6 78.9 80.95
80 90.7 88.4
Simulation
Reduction rate [%]
With
Without
5.6 16.9 25.7
61.59 81.98 97
90.9 79.4 73.5
#9 (NJ)
A B C
#10 (IJ)
A–C B–A C–B
7.2 5.7 7.2
56.1 72 69.9
87.1 92.1 86.7
2.7 3.61 7.17
53.9 64.3 82.3
94.99 94.38 91.3
#11 (IJ)
A–C B–A C–B
1.7 1.6 1.4
61.6 83.8 78.35
97.2 98.1 98.2
8.24 12.46 7.12
75.2 92.7 64.9
89.04 86.56 89.03
#12 (NJ)
A B C
9.5 8.6 8.8
56.8 72.4 62.55
83.3 88.1 86
3.43 16.7 15.4
75 44.7 56.4
95.43 62.64 72.7
Average [%]
89.6
One possible problem by installing the reactor on the bonding lead is to increase the induced sheath voltage. As the reactor is serially connected to bonding lead, it actually increases the impedance of the bonding lead. That will increase the induced voltage on the sheath from the core. It is also realized that the new device on the cable system could be damaged when the sheath is hit by high voltages, such as cable fault and lightning stroke. Therefore, the device needs proper protection. A new device, called reduction device protector (RDP), was designed to protect the reactor against the transient overvoltage [2,5]. Its characteristic, shown in Fig. 8(b), is similar as sheath voltage limiter (SVL), shown in Fig. 8(a). The difference between SVL and RDP is that RDP has a lower initial voltage than SVL so that the RDP can act before SVL when an overvoltage strikes on the sheath. RDP is connected to the reactor in parallel, but in two conditions. One is one end of RDP is grounded, the other is RDP without grounding. The schematic graphs of these two methods are shown in Fig. 9. The effectiveness of the two protection methods of RDP were extensively studied by ATP simulation. From the study [2,5], it is found that the RDP without grounding will provide a better protection against fault and lightning strike. The combination of the above design of current reduction and protection can effectively reduce the sheath circulating current and protect it against any possible threats. These new measures have been implemented in the above transmission system.
84.98
in the development of new reliable non-destructive detection and location technique, sheath to earth faults under ac voltage have been fully investigated. For this work, a cable test assembly was constructed and various fault conditions applied. A single core, XLPE insulated 275 kV power cable is laid in a simulated trough with sand. After initial investigation, a monitoring system is established to monitor the fault over long time against different weather conditions. The monitoring system is discussed in Section 2.
4. Characterisation of sheath fault and modelling After the cable is laid in operation, the oversheath (jacket) will be inevitably damaged because of the cable aging, mechanical and immersion in water. This causes sheath to earth fault that is simply referred to sheath fault. Sheath fault has being talked among scientists and engineers over the years [7–10], but no one knows exactly what kind of characteristics it has. To help
Fig. 8. V–I characteristic curve of SVL and RDP. (a) V–I characteristic of SVL. (b) V–I characteristic of RDP.
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Fig. 9. Protection methods of reduction device.
The fault shows a strange characteristic and this characteristic has been confirmed by monitoring at different conditions. In summery, if a sine wave is applied to the system the fault characteristic manifests three different conditions. Firstly when the applied voltage on the fault is low, at this stage, the fault current is still a sine wave. Secondly, when the applied voltage on the fault is up to a certain voltage, a spike appears from the top of positive cycle. It is opposite to the applied voltage. We regard the voltage at this point as the threshold voltage or inception voltage. Thirdly, any further increase in the applied voltage leads to another spike appears at the bottom of negative cycle. Its direction is still opposite to the applied voltage. At the same time, a flat current appears within the spikes both on positive and negative cycles when the applied voltage is increased further more, as shown in Fig. 10. And the magnitude of flat current in positive cycle is less than in negative one. Strangely this threshold voltage is not high. For the cable in normal working conditions, on most of the cases it is below the induced voltage on the sheath. Some explanations have been proposed [3]. Unfortunately, these explanations are not enough without further experimental confirmation. However, based on this discovery, further detection and location methods can be developed. In order to speed up the research, a fault model is established and implemented in simulation soft-
ware ATP so that later research can be done by simulation instead of experiment. When the fault is implemented in ATP, the model is presented as shown in Fig. 11. R1 represents the fault characteristics of non-linear resistance in positive cycle of applied voltage. R2 is the non-linear resistance in negative cycle. R3 and C are linear resistance and capacitance, respectively. k1 , k2 and k3 are three TACS-controlled switches which control the opening and closing of the three branches. If the applied voltage is below the threshold voltage, the switch k3 is closed while k1 and k2 stay open. Then the linear branch works. When the applied voltage reaches the inception voltage in positive cycle, then switch k1 is closed while k2 and k3 open. When the applied voltage is above the threshold voltage in negative cycle then switch k2 is closed while the other two open. The model presented in Fig. 11 produces a good simulation result comparing the experimental investigation [11]. However, in ATP software sometimes the non-linear resistance will cause oscillation. The parameters of the non-linear resistance and other control parameters should be chosen carefully to avoid this. In order to make the model work more efficiently, we neglect the spikes of the fault and regard the flat fault currents as two current sources for the simplicity. Then the simplified fault model is proposed as shown in Fig. 12, in which the two diodes control
Fig. 10. Fault current under higher applied voltage.
Fig. 11. Fault model in ATP.
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Fig. 13. Single-frequency adaptive noise canceller.
Fig. 12. Simplified sheath fault model.
the opening and closing of the two current sources for nonlinear fault above threshold voltage and the linear resistance and capacitance represent the linear fault below threshold voltage. For the simplicity of simulation, the sheath fault is supposed to be symmetrical, that is the magnitude of the threshold voltage and the flat currents on both positive and negative cycles are the same. This model is easy to be implemented in ATP and still gives reliable result. Thus, it is applied to simulate the sheath fault in the transmission network.
in fault detection and location. Other advantages include easy control of bandwidth, an infinite null, and the capability of adaptively tracking the exact frequency of the interference. Fig. 14(b) shows the filtered fault current extracted from the sheath current in Fig. 14(a) after applying ANC technique. After the extraction of the fault current, the fault detection becomes an easy job.
5. Sheath fault detection and location 5.1. Signal processing in fault detection It is realized from experiment that the sheath fault for the cable laid in sand trough is a high impedance fault even after the sheath is broke down. For high impedance fault, the fault signal is very weak. They are difficult to be detected from the strong background noises. In this case, the sheath fault current is very low, at milli-ampere level. However, normally the current on the sheath will be at around few or dozen of ampere level. To extract such a small current from the high background current is not easy. It is also realized that the background current has single frequency component, while the fault current is non-linear with multiple frequency components. Thus, a signal processing technique called adaptive noise cancelling (ANC) was adopted to extract the weak signal [12]. The schematic figure is shown in Fig. 13. For ANC technique, the received signals, including power fundamental frequency signal (60 Hz in this situation) and transient signals were applied to Primary Input end. A 60 Hz signal with a random given amplitude was regarded as noise signal and applied to the Reference Input. Then the fundamental frequency components from the received signals would be removed to leave only the transient signals. This technique has several advantages. It has the ability to adjust its own parameters automatically, and its design requires little or no priori knowledge of signal or noise characteristics. The noise can be eliminated without signal distortion, which is a very important characteristic needed
Fig. 14. Sheath and fault current. (a) Sheath current with hided fault current. (b) Filtered fault current.
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5.2. Fault location based on travelling wave method For non-destructive and quick fault detection and location on power cable, an ideal method is to detect fault generated high frequency transients at one or two ends of the cable [13–15]. For this method, two parameters need to be determined. One is the velocity of the transients travelling on the line. The other is the arrival times of these transients at one end or both ends of the line. The velocity of the travelling wave is determined by the wave frequency and the properties of the line itself, and its surrounding environment. For the signals on the sheath, they are propagating in a complicated way because of the crossbonding of the sheath from three phases at both ends of each minor section. From the theoretical study and simulation by ATP, it is found that there are six propagation modes of the signals on cable [16]. Its propagation velocities versus frequency are plotted in Fig. 15. In the figure, both the frequency and the velocity are plotted after taking the natural logarithm. The first three modes are associated with waves travelling on the sheath. Mode 1, marked with diamond at the bottom, is the sheath-earth mode. Mode 2, marked with square and its velocity just being below 16.5, is an inter-sheath mode between sheaths on central cable and side cables (in case of the flat layout of the cables in my study). Mode 3, marked with triangle and being just above the 16.5, is also an inter-sheath mode but between the sheath of side cables. Modes 4–6 are waves travelling in the cable conductors. They have similar returning loops as modes 1–3. However, in practice, the generated fault transients on the sheath are not propagating in all three modes, and cannot be detected all. For the case of flat layout of the three-phase cables, if the fault occurs in the side cable, the transients will propagate in all three modes. However, if the fault occurs in the central cable, the transients will propagate in only two modes which are sheath-earth mode (mode 1) and inter-sheath mode between central sheath and side sheaths (mode 2). In addition, the high impedance of the earth causes high attenuation of signals trav-
Fig. 16. Fault generated transients. (a) Fault transients for fault in side cable. (b) Fault transients for fault in central cable.
elling in mode 1 which makes it impossible to be detected. Therefore, only two traveling modes of signals can be detected if the fault is in side cable, and one mode for the fault in central cable. This can be reflected from Fig. 16. Overall, transients travelling in mode 2 is selected to locate the fault on all conditions. Details on how to discriminate the propagation mode and locate the fault can be referred to paper [17]. The arrival times of the transients at the end of the cable were located by the wavelet analysis. Wavelet analysis has the ability to discriminate the transient signals and locate when the signal occurred. This makes it a powerful tool in fault location. After the velocity and the arrival time are known, the distance of the fault to the end of the cable can be easily determined by the famous Bewley’s lattice diagram [18]. 6. Conclusions
Fig. 15. Wave velocity versus frequency.
The above are the research results that have been achieved over the last few years. Some of the research results, such as designed new equipment, reduction method of the sheath circulating current and their protection measures, have been implemented in power system. Other research work, such as fault
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characterisation, modelling and the fault detection and location, can contribute to the knowledge in power system and improve the development of new solutions to the problems. Research work is still going on at both laboratories on fault location by applying wavelet analysis technology, particular on noisy environment and mixed circuit. Some achievements have been achieved. New results will be published soon. Acknowledgements The authors would like to express their great gratitude to National Grid, UK and Wonkwang University for their supports in both finance and provision of facilities and testing site. References [1] IEEE Guide for the Application of Sheath-Bonding Methods for SingleConductor Cables and the Calculation of Induced Voltages and Currents in Cable Sheaths. [2] C.K. Jung, J.B. Lee, J.W. Kang, X.H. Wang, Y.H. Song, The characteristic of the sheath circulating current and its reduction in underground power cable systems, Int. J. Emerging Electric Power Syst. (IJEEPS) 1 (2004) Article 1. [3] X.H. Wang, Characterization, detection and location of sheath fault on underground power transmission cables, Ph.D. Thesis, Department of Electronic and Computer Engineering, Brunel University, London, 2001. [4] Alternative Transients Program—Rule Book, Leuven EMTP Center, 1990. [5] C.K. Jung, J.B. Lee, J.W. Kang, T.I. Jang, Analysis and reduction methods of sheath circulating current in underground transmission systems, KIEE Trans. 50A (11) (2001) 537–545. [6] J.W. Kang, H.W. Yang, A study on the characteristic and rising cause of sheath circulating current by analysis and measurement, KIEE Trans. 51A (10) (2002) 525–533.
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[7] H.T. Gooding, T.A. Briant, Location of serving defects in buried cables, IEE Proc. (1961) 124–125. [8] P.F. Gail, Cable fault location by impulse current method, Proc. IEE 122 (4) (1975) 403–408. [9] T.V. Lathrop, Locating sheath breaks in wet PIC cable, in: Proceedings of the International Conference on Communications, Part II, vol. 19, New York, USA, 1976, pp. 4–9. [10] J.T. Bejaminsen, A. Bruaset, H. Faremo, Sheath fault testing-a cost effective tool in order to extend cable life, in: Proceedings of the Nordic Insulation Symposium, Technical University, Denmark, 1999, pp. 101– 108. [11] X.H. Wang, Y.H. Song, Sheath fault characteristics and modelling on underground power transmission cables, ETEP Eur. Trans. Electr. Power 11 (2) (2001) 137–140. [12] X.H. Wang, Y.H. Song, Adaptive noise cancelling technique in sheath fault detection on power cables, in: 5th International Power Engineering Conference (IPEC2001), Singapore, 2001. [13] Z.Q. Bo, G. Weller, M.A. Redfern, Accurate fault location technique for distribution system using fault generated high-frequency transient voltage signals, IEE Proc. Gen. Transm. Distrib. 146 (1) (1999) 73–79. [14] Z. Chen, Z.Q. Bo, F. Jiang, G.A. Weller, Fault generated high frequency current transients based protection scheme for series compensated lines, in: IEEE Power Engineering Society Winter Meeting, Piscataway, USA, 2000. [15] A.T. Johns, R.K. Aggarwal, Z.Q. Bo, Non-unit protection technique for EHV transmission systems based on fault-generated noise. Part 1: Signal measurement, IEE Proc. Gen. Transm. Distrib. 141 (2) (1994) 133– 140. [16] X.H. Wang, Y.H. Song, C. Ferguson, Wave propagation characteristics on crossbonded underground power transmission cables and sheath fault location, ETEP Eur. Trans. Electric Power 13 (2) (2003) 127–131. [17] X.H. Wang, Y.H. Song, Sheath fault detection and classification based on wavelet analysis, ETEP Eur. Trans. Electric Power 16 (4) (2006). [18] A.I. Ramirez, A. Semlyen, R. Iravani, Modeling nonuniform transmission lines for time domain simulation of electromagnetic transients, IEEE Trans. Power Deliv. 18 (3) (2003) 968–974.