Electrical treeing from needle implants in XLPE during very low frequency (VLF) voltage testing

Abstract

This paper presents results from experimental investigations performed in order to characterize electrical treeing as a function of frequency and magnitude of the applied ac voltage. All tests were performed using steel needles molded into 2 mm thick samples of cross-linked polyethylene (XLPE) with an insulation distance of 1 mm. The formation of electrical trees from the energized needle tips were studied using optical microscopy video techniques and characterized with respect to growth rate and maximum tree length.The effect of different ac voltages up to 14 kVrms values and frequencies in the range of 0.1 -50 Hz were examined.The results are discussed with respect to the electromechanical theory of electrical treeing. A theoretical model is used to qualitatively explain the frequency dependence of the observations,considering the dissipated partial discharge power and the resistive and capacitive voltage distribution along the tree channel. The experiments confirmed that the effect of reducing the voltage frequency below 1 Hz was to reduce the ratio of the applied voltage distributed along the tree channel, toreduce thedegree of branchingand to facilitate a growth rate proportional to the frequency.

Electrical treeing in XLPE cable insulation at cryogenic temperature under harmonic AC voltages

“…The cause for fibrillar type damage could be due to high packing density of the material (due to shrinkage of material) at low temperature and fast quenching of local temperature rise occurred due to partial discharge activity, the amount of damage to the insulating material gets reduced thereby forming thin fibrillar type damage than voluminous damage that occurs in same material at room temperature [17]. It is also observed with higher order harmonics with increased THD, the diameter of the main branch is reduced and more of fibrillar type structure forms.…”

“…To identify the formation of any defects during laying of high voltage cables, AC and DC tests were carried out [16] fundamental issue with the DC test is the formation of space charge in cable insulation and very low frequency test is identified as an alternative [17]. It is essential to understand the electrical tree formation in XLPE polymer insulation at low temperature under VLF voltages as well the rate of failure of XLPE polymer insulation with trees and its growth under VLF voltages.…”

Analysis of Partial Discharges in Electrical Tree Growth Under Very Low Frequency (VLF) Excitation Through Pulse Sequence and Nonlinear Time Series Analysis

“…It is known that with frequency reduction, electrical trees grow slower and their shape is also affected. It has been reported that the time to breakdown at 0.1 Hz was three times longer than at 1 Hz [9] and trees grown at VLF (0.1 Hz) can be almost five times slower than industrial frequency (50 Hz) in test samples at the same high voltage level [11], showing that the tree growth rate is not linear with respect to the frequency. Moreover, electrical trees grown at VLF have a less bushy/dense structure compared to those grown at industrial frequencies [9], [11].…”

“…Electrical tree growth is affected by the frequency of the voltage excitation that is stressing the system [9]- [13]. Nowadays, electrical components of a power grid, and thus the dielectric systems that conform them, are subjected to a variety of frequencies due to the grid operation and nonlinear loads [14], or due to tests needed for commissioning and maintenance of equipment [15], [16].…”

“…Electrical tree growth has been mainly studied for industrial frequency, i.e. alternating current of 50-60 Hz [4], [8], [17], while the study of other excitation frequencies are gaining interest [9], [10]. One particular case is the use of very low frequency (VLF) excitation, which corresponds to an alternating voltage with frequencies between 0.01-1 Hz.…”