Journal of ELECTRICAL ENGINEERING, VOL. 62, NO. 2, 2011, 73–79 UNDERSTANDING ELECTRICAL TREEING PHENOMENA IN XLPE CABLE INSULATION ADOPTING UHF TECHNIQUE ∗ ∗ ∗∗ Ramanujam Sarathi — Arya Nandini — Michael G. Danikas A major cause for failure of underground cables is due to formation of electrical trees in the cable insulation. A variety of tree structure can form from a defect site in cable insulation viz bush-type trees, tree-like trees, fibrillar type trees, intrinsic type, depending on the applied voltage. Weibull studies indicate that a higher applied voltage enhances the rate of tree propagation thereby reducing the life of cable insulation. Measurements of injected current during tree propagation indicates that the rise time and fall time of the signal is of few nano seconds. In the present study, an attempt has been made to identify the partial discharges caused due to inception and propagation of electrical trees adopting UHF technique. It is realized that UHF signal generated during tree growth have signal bandwidth in the range of 0.5–2.0 GHz. The formation of streamer type discharge and Townsend type discharges during tree inception and propagation alters the shape of the tree formed. The UHF signal generated due to partial discharges formed during tree growth were analyzed adopting Ternary plot, which can allow one to classify the shape of tree structure formed. K e y w o r d s: power cables, cable insulation, partial discharge, UHF signal, electrical breakdown, failure analysis, XLPE, ternary plot 1 INTRODUCTION The conventional method of identifying the discharges is through identification of luminous discharges from the In recent times, cross linked polyethylene (XLPE) ma- defect site due to tree propagation, acoustic signal radi- terial is used as insulation material in high voltage AC ated during tree propagation and by measuring the par- and DC cables. With the advancement in polymer tech- tial discharge magnitude [7–10]. Ueno et al , studied tree- nology, it has become essential to identify a material for ing phenomena in epoxy resin insulating material and cable insulation with high dielectric strength, low per- they could identify the point of inception and propaga- mittivity, low loss tangent and with good mechanical tion adopting UHF technique [11]. However, adoption of strength. However, there is also considerable evidence UHF technique for identification of incipient discharges in that defects in the insulation structure can cause pre- underground cable insulation diagnostics is at an infancy mature failure of insulation structure. The failure of un- stage. derground cable in service can cause long interruption Hence in the present work, a methodical experimental in supply of power to the customers. Early detection of study was carried out to understand the feasibility of us- failure can prevent such failures and by adopting pre- ing broad band sensors, to identify any UHF signal radi- ventive measures, it would be possible to improve the ated during partial discharges causing tree growth and for power system reliability and its operation. The life ex- classification of shape of tree structure that has formed. pectancy of the underground cables is adversely affected Generation of electrical trees in actual cables is a cum- by Electrical Treeing, a pre-breakdown phenomenon and bersome process. It is not possible to visualize the tree accounts for its pre-mature failure [1]. In underground growth. The cable insulation has to be microtomed and cables, under normal operating voltage, the partial dis- have to visualized with sophisticated equipments. Hence charges incepts pre-breakdown channels from a region of it is essential to identify the point of inception using non- defect site present in the form of gas cavities or conduct- intrusive type sensor and to provide information regard- ing inclusions or intrusions, in the insulation structure, ing the shape of tree structure formed. The sensor signal due to field enhancement. These pre-breakdown channels generated due to incipient discharges during tree growth emanated from the defect site in the insulation structure was used for constructing the ternary diagram for classi- resemble branches of a tree and, hence, the name “tree- fying the different shapes of tree structure that forms. ing” is given to the damage process and since such an occurrence is purely due to electrical stress the mecha- nism is termed as “Electrical Treeing” [2–5]. In general, 2 EXPERIMENTAL STUDIES the partial discharge injected current pulses involve rise and fall-times of about 1 ns or less, exciting signals in the The experimental setup used for generating the electri- UHF range of 300-3000MHz [6]. cal trees and measurement of UHF signal generated due ∗ ∗∗ Department of Electrical Engineering, IIT Madras, Chennai-600 036, India; [email protected] Department of Electrical and Computer Engineering Democritus University of Thrace, Xanthi, Greece. DOI: 10.2478/v10187-011-0012-4, ISSN 1335-3632 c 2011 FEI STU 74 R. Sarathi — A. Nandini — M.G. Danikas: UNDERSTANDING ELECTRICAL TREEING PHENOMENA IN XLPE CABLE ... Fig. 1. Experimental Setup near to the pin tip causing localized discharges/damage near the needle tip. The needle used had a nominal tip ra- dius of curvature, 5 µm. The effective thickness between the tip of the needle electrode and ground electrode was maintained between 3–5 mm. The space between the pin and the dielectric was effectively sealed with the cold set- ting araldite and the specimens were immersed in filtered, degassed mineral transformer oil ready for voltage appli- cation. 2.3 UHF sensor The UHF sensor is a non-intrusive type sensor which can provide the information about source of partial dis- Fig. 2. Frequency response of the UHF sensor charge occurrences. One of the fundamental requirements of the UHF sensor for identification of partial discharges is that, it must have a broadband response. Also the sen- to partial discharges in the cable insulation system dur- sor should have minimum sensitivity in the range 500 to ing the electrical tree formation, under the AC voltage is 1500 MHz [12]. Martin et al , have carried out a detailed shown in Fig. 1. The experimental setup could be section- study on partial discharges in transformers using the sen- alized in to three parts. The first, second and third part of sors (that are) used in gas insulated systems and realized the experimental setup covers the high AC voltage source, that the same sensor could be utilized for monitoring in- sample preparation and methodology of testing and the cipient discharges in transformers [13, 14]. The sensitivity UHF sensor respectively. The details of each section are response of the broad band sensor is shown in Fig. 2. as follows. In the present study, UHF sensor used for detection of partial detection activity it is adopted to identify 2.1 High voltage source the tree growth process by placing the sensor at a dis- tance of 20 cm away from the sample holder. The out- The high AC voltages were generated by using 100 kV, put of the UHF sensor is connected to the spectrum an- 5 kVA discharge free test transformer. The applied AC alyzer/high bandwidth digital storage oscilloscope. The voltage was measured using a capacitance divider. The UHF signals were captured using a digital storage os- AC voltage was increased at a rate of 300 V/s up to the cilloscope (LeCroy, 4 channel, 3 GHz bandwidth, oper- required test voltage level. ated at 10 GSa/s) with an input impedance of 50 ohms. The spectrum analyzer Hewlett Packard E4402B ESA-E- 2.2 Sample preparation and methodology of Series was used to measure the signal in zero span mode testing with 1GHz centre frequency. In the present work, electrical trees were generated in 3 RESULTS AND DISCUSSION a 33 kV XLPE cable insulation. The cable insulation is cut in to regular size of 1 cm thick. A conducting defect 3.1 Influence of Electrical Stress on the Size and was simulated by inserting a sharp metallic needle into Shape of Tree Growth the insulation, which is connected to the high voltage terminal. The centre conductor of the cable is connected The major cause of failure of underground cable insu- to the ground. Since the needle tip is sharp and near to lation is due to formation of electrical trees from a defect the ground terminal, the local field enhancement occurs site in it. The defect in the insulation structure can be Journal of ELECTRICAL ENGINEERING 62, NO. 2, 2011 75 Fig. 3. Electrical Tree in XLPE cable Insulation under AC voltage (a) Bush Type Tree (b) Tree-Like-Tree (c) Fibrillar type tree (d) Intrinsic type tree (e) Tree Followed with Breakdown insulation a conducting/non-conducting defect. The size and shape injection of charges from the high voltage electrode to of the defect vary the local electric field, which alter the the insulation structure were trapped near the defect site shape of tree and its mode of propagation. To generate formed and the charges gets deposited in the surface of electrical trees in an insulation structure, selecting the ap- the damaged zone causing local reaction with the applied plied voltage magnitude is very important. If the stress is field reducing the electric field in the zone. Sometimes, the of larger magnitude intrinsic type, failure can occur with homo or hetero charges were also injected into the insula- few pre-breakdown channels and if it is less, no tree can tion structure through the defect formed zone-enhancing form in the insulation structure. field at one point causing further enlargement of chan- Mason [15] suggested that electric field (Emax ) at the nel resulting in Tree-Like- Tree structure. Otherwise lo- tip of the needle electrode can calculated as cal discharges will occur causing increased diameter of the damage zone forming bush Type of electrical tree.