IEEE Transactions on Dielectrics and Electrical Insulation Vol. 22, No. 6; December 2015 3177 DOI 10.1109/TDEI.2015.005022 Electrical Treeing in XLPE Cable Insulation under Harmonic AC voltages R. Sarathi, K. H. Oza, C. L. G. Pavan Kumar Department of Electrical Engineering, IIT Madras, Chennai- 600 036, India. and T. Tanaka IPS Research Center, Waseda University, Kitakyushu, Japan ABSTRACT The shape of electrical trees gets altered due to applied voltage profile. Bush type electrical trees forms under harmonic voltages with lower THDs. With increase in harmonic level with increased THDs fibrillar type electrical trees were formed. Weibull distribution studies indicate that increase in harmonic level (odd/even) with higher THDs accelerates the tree growth in cable insulation causing early failure of cable insulation. The frequency content of the Ultra High Frequency (UHF) signals radiated due to tree formation lies in the frequency band of 0.5-2 GHz. Adoption of spectral power ratios to UHF signal can classify the shape of electrical trees that has incepted and propagating in the insulating material under different voltages. Phase Resolved Partial Discharge (PRPD) analysis using UHF signal indicates that discharges occurs under harmonic voltages where the rate of rise of voltage is high thereby accelerating the tree growth. During tree growth discharges occur in the entire phase of the triangular voltage. Partial discharges occur at the peak with the low frequency AC voltage and at the raising portion when the supply voltage is increased. Index Terms - Treeing, voltage harmonics, UHF signal, XLPE cables, Weibull analysis, power spectral analysis. 1 INTRODUCTION HIGH voltage AC power transmission and distribution through underground cables is more prominent in recent times. XLPE insulation is one of the best insulation for cables as they have high breakdown strength, low loss tangent, good thermal properties and high mechanical strength. One of the causes for failure of underground cable is due to presence of conducting/non-conducting defect in the insulating material formed at the time of manufacturing or during laying of cables, causing formation of electrical treeing [1, 2]. In recent times, in distribution system, the supply voltage is highly polluted with harmonic voltages due to power supplied to variety of large non-linear loads. Large amount of database are available with electrical treeing formation under AC and DC voltages in XLPE cable specimen. [3-5]. Sarathi et al, studied tree pattern formed in XLPE cable insulation and have concluded that harmonic AC voltages with higher THDs can accelerate the tree growth causing early failure of cable insulation [6]. Early literature on influence of harmonic AC voltages of electrical tree growth clearly indicates that rate of rise of AC voltage have high influence on PD occurrence during tree growth [7, 8]. IEEE 519-1992 standards has indicated allowable THD of 1.5% in high voltage networks and 2.5–5% THD in medium voltage networks [9]. Dionise et al, studied voltage distortion in a distribution network and observed the presence of harmonic voltages with THD as high as 60.6% [10]. Florowski et al, have mentioned that knowledge of harmonic content of applied voltage is essential and critical in proper assessment of PD impact [11]. Kanao et al., clearly explained three different mechanisms by which even harmonic voltages occur in the power system network. In addition, with given harmonics and THD, the shapes of the harmonic AC voltages are different [12]. In real practice, the underground cables connected with this network will be stressed with such voltage profiles. In addition, very low frequency test is found to be alternative to DC test in underground cables especially as after laying test. Thus it is essential to understand tree growth in underground cable insulation under very low frequency AC voltage. In practice, it is important to use reliability tools and techniques to identify the cause for low failure probabilities rather than estimating mean time to failure. Thus the more conventional technique adopted for estimating life of insulating material under electrical stress, is by use of two parameter Weibull distribution studies [7, 13]. Treeing process forms partial discharges radiating UHF signals in the frequency range of 300– 3000 MHz [6]. It is essential for one to understand the characteristics of UHF Manuscript received on 17 October 2014, in final form 20 April 2015, accepted 30 April 2015.