2015 17th UKSIM-AMSS International Conference on Modelling and Simulation Online Localisation of Partial Discharge Using n Parameters in Medium Voltage Cable Network Tauqeer Ahmed Shaikh, Abdulrehman Al-Arainy, Nazar Malik, Nissar R.Wani. Department of Electrical Engineering, King Saud University Riyadh, Saudi Arabia {shtauqeer, aarainy, nmalik, nrasool}@ksu.edu.sa Abstract In medium voltage power cable, detection and localization of partial discharge (PD) is a challenging process due to presence of joints and ring main units that are located at various places in the network. The propagation of PD pulses in a network system are multifarious because of various cable lengths and impedances. This paper is an attempt to investigate localization of PD pulses source in medium voltage cable by using pulse propagation parameters such as pulse width and rise time. Various experiments were performed in order to find the effect on pulse width and rise time of different cable network system and then pulse parameters were used as basic tool in localization of PD signals. These experiments show that PD pulses attenuation depends on pulse width, rise time and fall time. Incorporation of the proposed technique over a wider network can be very useful as an efficient diagnostic tool for improving the reliability of power supply system. defective section can be repaired or replaced before the complete breakdown occurs [2]. One of the basic standard for online measurement is by using the concept of pulse propagation away from the source in power cable and the time to travel through the cable. Time domain reflectometry (TDR) is the most commonly used method in order to locate partial discharge sources during online measurements [3]. The main idea of this method is to measure the difference in the time of arrival between the direct and the reflected pulses as illustrated in Figure -1. Keywords - Power cables, On-line Partial discharge, n, width, Rise Time, Time-domain Reflectometry. I. INTRODUCTION Power cables plays an important role in power transmission and distribution systems. The reliability and availability of a power cable system are determined by the health condition of the cables and its components (accessories) [1]. The insulation of cable and accessories may become deteriorated during installation and in the period of its use. PD are one of the major reasons for deterioration and degradation of cable system insulations in service. There are mainly two different methods for detection of partial discharge, off-line and on-line. In off-line method, entire cable is removed from the live system, then connected to a coupling capacitor in parallel, and tested by applying voltage from an external source. This method is difficult and takes more time to perform, but is accurate in PD measurement and can measure PD in long power cables as well. In the on-line testing method, a non-intrusive high frequency current transformer (HFCT) sensor is installed at the ground wire of the cable system without de-energizing the power supply network. Some of the plus point for using online partial discharge detection are ease of detection of the PD signals and possibility of multiple measurements at the same time. However, the on-line partial discharge measurements also have some negative aspects as some times the HFCT is affected by the noise from the neighboring sources, which can lead to error in the results of the PD signals. The measurement technique can help in locating the PD source so that the Figure 1. Principle of time-domain reflectometry When the propagation velocity of the partial discharge pulse is known, then distance α can be calculated from the following equation. = + Where, l c is total length of cable, t 1 is time of reflection from the measuring end and v is the propagation velocity of the pulse on the cable. If the propagation velocity is not known, then the distance α can be calculated by using the three-times reflected pulses. Therefore, in this case the following equation is valid for calculating the distance α. (1) = (2) 978-1-4799-8713-9/15 $31.00 2015 IEEE DOI 10.1109/UKSim.2015.48 396
Here, l c is the total length of cable, t 1 is time of reflection from the measuring end and t 2 is the time of reflection from the far end. If we use transponder at the far end to get a proper reflected pulse from the source of partial discharge, then we can use the following formula to detect the source location as percentage of the cable length. % = 1 100 (3) The aim of this research is to explore the use of a simple two-stage localization technique for cable networks. The first stage is to detect the pulse width and rise time for a simple non-jointed cable and then by plotting the trend for pulse width and rise time, idea about the PD trend for simple cable is obtained. The second stage is to determine the fault location on the cable network system. Here the trend of pulse width and rise time for simple cable is taken as a base reference and the variation in the pulse width and rise time is compared and then by using approximation the source location is performed using the PD pulse width characteristics. II. PULSE PROPAGATION IN XLPE CABLE The propagating partial discharge pulses is a traveling wave of short duration (1ns 1µs) which travel along the length of the cable network system (100m 100km). Such broadband pulses traveling along the transmission line have a group velocity (υg) which is dependent upon the derivative of the angular frequency and the wave number given by the following equation. = The first-order differential equations, known as the telegraph equations allow for dispersive travelling wave solutions. These describe how pulses are distorted in power cables and general transmission lines and are derived from the Maxwell s equations. magnitude attenuation, and broadening are described by the real, and imaginary parts of the frequency-dependent propagation constant γ(ω) which appears in the exponent of the transfer function H(ω) associated with pulses measured at distances l and x, given in the following equation. (4) Typically, PD pulses are of far shorter duration than the noise and interference due to the electron avalanche process that causes the insulation breakdown. Thus understanding how PD pulses distort as they propagate through MV cable networks is important for identifying the PD source location [4] [5]. III. EXPERIMENTAL SETUP Experiments were performed to study the behavior of transient pulses as they propagate along the power cables. All these experiments were carried in High Voltage Laboratory of King Saud University that is well equipped with all the tools to perform online PD testing. The main purpose of this work is to understand how the alteration of PD pulses takes place as these propagate on power cable system. Based on the experimental work, a measurement-based model was developed and the result of this model may be applied to the actual field systems. The experiments were perform on a 20-meter long XLPE sample cable. This XLPE cable is a medium voltage, single core, 50mm 2, 15 kv, with plain annealed copper stranded conductor, extruded semi-conducting compound as a conductor screen, XLPE insulation, plain annealed copper wires concentrically applied over the insulation screen with an open helix copper tape serving as a binder and acting as a metallic screen. In this cable screen, the pulse injection tests were performed by using a mini pulse generator of type MPG001-HVPD-Longshot and then the result were analyzed for pulse width and rise time. This test indicates how to obtain multiple pulse reflections from the open-circuit end of a 20- metre cable specimen, as the open circuit reflection coefficient (Г) is unity, resulting in a total reflection of the incident pulse. Figure. 2 shows a single line diagram were two HFCT are connected to the screen of the cable in which one of the HFCT is used to inject the pulse using mini pulse generator and the other HFCT is used for measuring the pulse propagation through the cable. The bandwidth of both of these HFCTs are 76 khz to 16 MHz with transfer impedance of 4.0 Ω. HV Transformer AC Source XLPE cable HFCT H(ω) = = (5) = ( + ωl)( + ωl) (6) Here, α(ω) is pulse magnitude attenuation (Np/m), β(ω) is pulse dispersion (rad/m), R is resistance (Ω/m), L is inductance (H/m), G is conductance (S/m), C is capacitance (F/m), j is (-1) and ω is frequency (radians/s). Generator HVPD Longshort Oscilloscope n graph Figure 2. injected in a 20-meter simple non jointed cable. 397
As the pulse propagates through the cable, it is measured to determine the pulse width and rise time. Table-I summarizes results of such measurement of pulse width and rise time for an injected pulse of 100ns width and 1 volt amplitude using the mini pulse generator. The HFCT provide a reliable measurement up to 360 meters in steps of 40 meters which is twice the cable length. TABLE I. PULSE WIDTH AND RISE TIME FOR PULSE INJECTED OF 100NS AND 1 VOLT Similarly the results obtained for some other pulse widths and rise times were also measured and then simulated in a matlab program for initial pulse width of 200, 500, 1000ns. These are represented in the Figures 4 and 5. As the pulse propagates, its amplitude attenuates whereas the pulse width and rise times increase and it disperses along the distance propagated, due to the semi conducting layers and conductors losses, which increase with frequency. Sr. No Distance propagation (m) width (ns) Rise time (ns) Slope for width Slope for Rise time 1 0 100 99 0 0 2 40 105 100 0.1250 0.025 3 80 111 102 0.1500 0.05 4 120 115 108 0.1000 0.15 5 160 118 111 0.0750 0.075 6 200 125 117 0.1750 0.15 7 240 127 119 0.0500 0.05 8 280 131 122 0.1000 0.075 Figure 4. width and rise time trend for injected pulse of 200ns. 9 320 133 126 0.0500 0.1 10 360 142 128 0.2250 0.05 Average slope 0.105 0.0725 The results obtained from pulse width and rise time of initial pulse of 100 ns and 1 volt were simulated in a matlab program and the results are shown in Figure 3. Then by applying slope point formula on two consecutive points of pulse width and rise time a liner approximation was obtained. This indicates that the pulse width and rise time are approximately a linear function of the distance propagated. Figure 3. width and rise time trend for injected pulse of 100ns. Figure 5. width and rise time trend for injected pulse of 500 ns and 1000ns pulse widths. 398
Joints RMU 15 m 5m AC HFCT-0 HFCT-1 HFCT-2 HFCT-3 HFCT-4 Generator n Graph HVPD Longshort Oscilloscope Figure 6. injected test on a Laboratory devloped Cable network of 60 meter. IV. EXPERIMENTAL WORK ON NETWORK SYSTEM Experimental work was also carried on laboratory developed cable network system. This system consists of 60- meter long cable, which receives supply from AC source through transformer (this end is called near end). The cable consists of 2 joints at 20m and 35m and a Ring Main Unit (RMU) which is at distance of 55 meter from the near end. This cable network system is developed in such a way that at the ends, at joints and at incomer of RMU, HFCT are connected to the screen of the cable. The system is not only analogous to the real distribution systems but also easy to access for any measurements purposes. On this cable network system, pulse propagation tests were performed by injecting pulse of 1 volt and 100ns width using the mini pulse generator on the screen from the source end. The other HFCTs connected at various locations help in the measurement of propagating pulse. With the help of pulse characteristics, different data were obtained from the pulse prorogation and then velocity of propagation for the different cable sections were measured. Table II provides details of the calculated velocity of propagation, which confirms the cable lengths and positions of the HFCTs. Figure 7. represent the various pulses waveforms, representing the injected pulse in the cable network and during the measurements by the HFCTs connected at different locations along the cable. The propagation of pulse along the HFCT-1 to HFCT-2, HFCT-1 to HFCT-3, HFCT-1 to HFCT- 4 are measured which also reflects the distance between the HFCTs along the cable length. TABLE II. PULSE WIDTH AND RISE TIME FOR PULSE INJECTED OF 100NS AND 1 VOLT. Sr. N o 1 2 3 n Path HFCT-1 to HFCT-2 HFCT-1 to HFCT-3 HFCT-1 to HFCT-4 Distance between n Path (m) n time (ns) n Speed(m/µs ) 15 187 80.17 35 431 81.33 40 500 80 187 ns 431 ns 500 ns Figure 7. propagation of all HFCT s for 60 meter cable network system. 399
V. ANALYSIS AND DISCUSSIONS The 60m cable network system was tested for localization of PD when a discharging fault was created at the joint-1 of the system. As discharging joint is located at 20m from the near end of cable terminal, then the pulse shapes were measured, so as to assess the frequency response of the cable and to measure the effect of the HFCT connectors upon the pulse characteristics. To compare parts of the experimental work which were perform on 20m non-jointed cable to that of the direct measurement on the 60m cable network system, a pulse of 100 ns pulse width and 1 volt amplitude was injected and measured upon both the cable systems. The result obtained shows how the pulse width and rise time alter as function of distance propagated through the network system. These measurements give a linear trend of increment which is plotted in Figure 8. Finally, an analytical study is carried out under the direct measurement of the pulse propagation for 60m cable network system and the pulse propagation measurements obtained from the 20m non-jointed cable. These measurements are based on the pulse width and rise time. As the 60m cable network system is energized with 7.6 kv of AC source power and using a mini pulse generator, pulse of 1 volt and 100ns width was injected in the screen simultaneously. At this point of time, PD pulse starts reflecting from the joint-1 which is at 20m from the near end. Figure 10 represents the PD signal obtained from different HFCT s connected across the network and Figure 11 represents the PD pulse shape located at 20- meters through HFCT. Figure 10. The PD signal obtained from all HFCT s in cable network. Figure 11. PD Shape located at 20-meters through HFCT Figure 8. width for Direct measurement and HFCT measurement at initial pulse width of 100ns and 1 volt. Similarly, Figure 9 represents the alteration of rise time with respect to propagated distance in both the network system and the non-jointed cable and it also has a linear trend of increment. PD pulse obtained in Figure 10, 11 for network cable have its pulse width and rise time as a function of cable length, which have a linear trend and that can be approximated using matlab code. In addition, this interdependent relationship of pulse width, and rise time to cable length can also be determined by using Table 1. The relationship obtained for pulse width, rise time to cable length is give in the form of equation (7) and (8). ( ) = 0.105 + 100 (7) ( ) = 0.0725 + 98 (8) Figure 9. Rise time for Direct measurement and HFCT measurement at initial pulse width of 100ns and 1 volt. The HFCT measurement on 60-meter cable network at various locations and the approximated linear trend measured for 20-meter cable (obtained from Figure 8 and 9) have its pulse width and rise time values shown in Table 3. 400
TABLE III. Parameter PULSE CHARACTRISTIC IN 60M CABLE NETWORK HFCT measurement (ns) Approximated linear trend (ns) width 102.1 102 Rise time 99.45 99 Therefore, Table III. indicates that the measurement of pulse width and rise time show a linear trend are almost equivalent to each other. The minor variation in pulse measurement may due to the multiple reflection of PD pulse that can cause the pulse shape to distort and delay or due to the limitation of measurement bandwidth or the approximations applied. VI. CONCLUSIONS Online PD monitoring in MV cables is an important tool for the maintenance programs. In this paper pulse propagation parameter (pulse width and rise time) has been used for localization of PD source. These parameters can be easily obtained during the regular monitoring of cable network system. The pulse propagation parameters variations with distance can form an effective and reliable tool in localization of PD sources in the cable networks. Experimental work on MV cable shows that the pulse propagates along the cable length, it attenuates in its amplitude, whereas the pulse width and rise times increase linearly with respect to distance propagated. Thus by understanding these parameters as the pulse propagates along the cable length, PD localization may be possible for MV power cables. Both pulse width and rise time can be used as a function of distance propagated effectively to locate PD sources along cable network system. [4] Ammar Anwar Khan, Nazar Malik, Abdulrehman Al-Arainy, Saad Alghuweinem, Online Partial Discharge Monitoring in Underground Power Cables - Challenges and Oversights, Journal of International Review on Modelling and Simulations(IREMOS). vol. 5, pp. 2582, December 2012. [5] D. Clark, R. Mackinlay, R. Giussani., L. Renforth, R. Shuttleworth, Partial discharge pulse propagation, localisation and measurements in medium voltage power cables, Power Engineering Conference (UPEC), 2013 48th International Universities', pp.1,6, 2-5 September. 2013 [6] Ammar Anwar Khan, Nazar Malik, Abdulrehman Al-Arainy, Saad Alghuweinem,"Investigation of attenuation characteristics of PD pulse during propagation in XLPE cable," Power and Energy Society General Meeting (PES), 2013 IEEE, pp.1,5, 21-25 July 2013. [7] S. Boggs, A. Pathak, and P. Walker, Partial discharge. XXII. High frequency attenuation in shielded solid dielectric power cable and implications thereof for PD location, IEEE Electrical Insulation Magazine,, vol. 12, no. 1, pp. 9 16, 2006. [8] M. Shafiq, L. Kutt, F. Mahmood, G. A. Hussain, M. Lehtonen, "An improved technique to determine the wave propagation velocity of medium voltage cables for PD diagnostics," Environment and Electrical Engineering (EEEIC), 2013 12th International Conference on, pp.539,544, 5-8 May 2013 [9] J. D. Glover, M. S. Sarma and T. J. Overbye, Power System Analysis and Design, 5thEdition, 2011. [10] Y. Tian, P. L. Lewin, J. S. Wilkinson, S. J. Sutton, S. G. Swingler, Continuous on-line monitoring of partial discharges in high voltage cables, Conference Record of the IEEE International Symposium on Electrical Insulation, pp: 454 457, 2004. ACKNOWLEDGMENT The authors acknowledge the College of Engineering Research Center and Deanship of Scientific Research at King Saud University in Riyadh for the financial support to carry out the research work reported in this paper. REFERENCES [1] Ammar Anwar Khan, Nazar Malik, Abdulrehman Al-Arainy, Saad Alghuweinem, A Review of Condition Monitoring of Underground Power Cables, IEEE International Conference on Condition Monitoring and Diagnosis, Bali, Indonesia, pp, 23-27, September, 2012. [2] Ammar Anwar Khan, Nazar Malik, Abdulrehman Al-Arainy, Saad Alghuweinem, Online Partial Discharge Monitoring in Underground Power Cables - Challenges and Oversights, Journal of International Review on Modelling and Simulations(IREMOS). vol. 5, pp. 2582, December 2012. [3] Ammar Anwar Khan, Nazar Malik, Abdulrehman Al-Arainy, Saad Alghuweinem, A Review of Condition Monitoring of Underground Power Cables, IEEE International Conference on Condition Monitoring and Diagnosis, Bali, Indonesia, pp, 23-27, September, 2012. 401