Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
4 SECTION 2 LITERATURE REVIEW 2.1 INTRODUCTION Polymer insulated cables are widely used all over the world. As a result of the rapid developments in solid dielectric extruded cable manufacturing techniques, the polymeric insulated cables are very popular for HV applications due to their great technical and economic advantage. The main polymer insulation material that is dominating the market is XPLE, which has been used for low, medium and high voltage cables for up to 500kV. Consequently, since the early 1960's, great quantities of XLPE cables have been installed around the world leading to improved reliability, simplicity and overall system economy. Although XLPE has a very high intrinsic dielectric strength ( 1000 kV/mm), the actual operating stress in practical cables are much lower than this value. This is due to different defects which occur during extrusion as well as various factors which influence aging and breakdown processes that are inherent in actual cables when used in practical environment. Treeing phenomena is generally responsible for most of the XLPE cable's insulation failures. In treeing, there are two types of processes, which are termed as electrical treeing and water treeing. Both of these treeing processes have distinct characteristics. The electrical treeing produces clearly visible tree shaped degradation paths which are caused mainly by high electrical stress in the polymer and their growth can be observed experimentally. Thus, an electrical tree can be defined as formation of a degradation path in a solid dielectric due to high electric stress. It consists of tree-shaped hollow channels created through repeated partial dielectric breakdowns occurring locally in a region that is small but has a very high electric stress. Electrical treeing can also be defined as the growth process of hollow channels in the solid dielectric caused by electrical pre-breakdown phenomena. Usually electrical treeing is the dominant cause of cable failures in High and Extra High Voltage 5 transmission class cables. At Medium Voltage primary distribution class cables, water treeing is the predominant cause of cable insulation failures. When the XLPE insulation was introduced in the early 1950s, an overall belief existed that this material was immune to most of the chemicals and water available in soil. Consequently, early construction of medium voltage cables in the United States was without any protective covering. However, field experience showed the shortcoming of this construction: many cable failures, which had occurred, were mainly due to phenomenon called water treeing. Water trees are regions of microcavitation and filamentary damage in a polymer such as XLPE caused by the penetration of aqueous salts in the presence of an electric field. The determination of the extent of the damage to polymeric cable insulation is a significant issue in the assessment of the cable’s residual life. The various other failure reasons / mechanisms of XPLE cables related to electrical, chemical and thermal stresses and as studied by various researchers [4, 20, 24, 37, 39, 51, 72, 73] are briefly reviewed in this section of this chapter. This review mainly covers the work reported over the last decade and compliments the extensive review reported earlier [304]. Moreover, the review mainly focuses on important issues related to medium voltage XLPE cables which form the backbone of Saudi Arabia’s distribution network and completes Task 1 of this applied research project. 2.2 WATER TREEING During the last 15 years, significant amount of research work has been reported in the literature on different aspects of water treeing. Table 2.1 presents a summary of such publications regarding water treeing. It also classifies various sub topics and gives pertinent references for each sub topic for an easy access to the earlier work. Water and electrical treeing are complex phenomena involving several processes with many synergistic effects. Water treeing is a well-known phenomenon responsible for failures of the underground cables using polymeric insulation. The phenomenon of water treeing is both phenomenal in its economic impact and fascinating from scientific point of view. This is reflected in the vast number of papers that have appeared since water trees were first discovered in Japan [24,39,51,72,73,81,136,147,173,304]. Still, the subject requires attention in order to maintain and increase the cable’s insulation reliability. Water trees are degradation structures in cables that are permanent, have 6 grown due to at least humidity and electric field, have lower residual electric field strength and are substantially more hydrophilic than the original insulation material. Table 2.1: Recent Literature on Water Treeing. Topic General background, review type, mechanisms Mechanical, chemical and thermal aspects Residual charge and space charge effects Dielectric spectroscopy and analysis Investigations using micro-PIXI, FTIR and electron spectroscopy Fractal behavior and analysis Effect on electric-field Dielectric properties , r Breakdown voltage Diagnostics of water trees using: Capacitance, tan Loss current waveform and its nonlinearity with voltage BD strength Space charge Broadband spectroscopy Criterion of diagnostic DC leakage, polarization / depolarization currents Current pulses during treeing Time domain spectroscopy Thermally stimulated currents Non standard test voltages Return / recovery voltage measurements Dielectric response measurements Ultrasound measurements Time domain reflectrometery Absorption current measurements Relaxation current Reference Number 4, 20, 24, 39, 51, 72, 73, 81, 136, 147, 173, 304 2, 4, 12, 18, 39, 77, 90,97 1, 3, 8, 17, 22, 31, 53, 62, 93, 116, 135, 141, 146, 184, 185, 230, 246 5, 35, 47, 55, 56, 63, 67, 74, 87, 89, 121, 122, 124, 138, 153, 157, 251 6, 19, 32, 121 10, 28, 36 1, 11, 33, 53, 73 14, 44, 86, 92, 128, 162 45, 74, 75, 304 67 7, 54, 174, 249, 304 15, 17, 40, 74, 88, 89, 118 45, 74, 83, 95, 300, 304 17, 105, 135, 141, 144 103 152, 153 47, 82, 97, 99, 119, 151 26, 76 149 27 43, 66 60, 85, 110, 115, 154, 155 5, 55, 75, 104, 107, 108 139 140 101 145 7 Table 2.1: Recent Literature on Water Treeing (Contd..) Topic Suppression methods using: Tree-retardant materials Cross linking Additives Barriers Powders Accelerated aging and assessment methods Benefits of TR – XLPE cables Blue water trees Factors affecting the growth of water trees: Bow-tie trees Zero crossings of voltage Frequency of voltage Additives in insulation Jacket and moisture barrier Cross-linking methods Irradiation Ions and oxidation Non standard voltages Semi conducting screen compounds Low temperature Effect of water without electrical stress on ac breakdown voltage Statistical analysis of water tree lengths Estimation of water tree length Restoration of water treed insulation Effect of cable processing on: Impurity content Electrical properties Water absorption Cable performance Reference Number 57, 68, 156, 168 49 42, 71, 94 38, 123, 304 123 16, 30, 46, 57, 59, 68, 102, 114, 115, 120, 158, 158, 159, 294, 298, 304 98 23 70 21, 64 37, 41 13, 71, 94 38, 304 18, 49, 111 80 12, 41, 61, 96, 165, 292, 299 58, 65, 134 297, 301 9 143 50, 61, 69, 302, 304 52 45, 130 106, 150 111, 113, 160 112, 161 127, 245 8 Table 2.1: Recent Literature on Water Treeing (Contd..) Topic Effect of conductor water on PD and BDV Water tree simulation and modeling Conversion of water tree to electrical tree Propagation of electrical tree from water tree Economic diagnostic and life cycle cost evaluation of cables On site testing and diagnostics of cables Characterization of un-aged cables Life modeling of cables Service experience of cables and accessories and reliability evaluation and failure modes Reference Number 117, 143 34, 78, 100, 109, 164 25, 79 190 29, 84, 142 35, 40, 91, 129, 145, 110 148 163, 241 125, 137, 172, 176 Till now no investigation could give a clear and precise explanation of water treeing mechanism. However, all the studies have in common the factors that contribute to the initiation of water trees and their growth. Different mechanisms responsible for water tree propagation [24] can be broadly classified as: - Processes related to electrical forces on water - Diffusion of hydrophilic species - Electrochemical processes like oxidation phenomenon - Condition dependent degradation A brief discussion to some of these issues is provided next. 2.2.1 Mechanical, Thermal and Chemical Aspects In the mechanical process, some molecular bonds are broken by mechanical stresses, which eventually result in micro cracks that are filled with water and finally result in a water tree [2,4,18,39]. It is known that water droplets at high ac field could generate local pressures that are well above the local mechanical strength of the 9 molecular chains. Amorphous polymers being more flexible and thus resisting better to fatigue should then grow smaller water trees [39]. Temperature is neither really a chemical nor a mechanical parameter. However, even then experiments show that water tree length increases when samples are wetted on both sides whereas it decreases when samples are wetted only on one side. The chemical theories rely on the interaction between water and contaminants, additives, and O2. Since many cables are directly buried in the ground which may contain almost all the chemical species, hence such chemicals can easily contaminate the cables. In this case, the water treeing is a two steps process: first, penetration of water or ions assisted by Maxwell stresses and second, chemical deterioration leading to permanent micro voids in the insulation [77, 90, 97]. 2.2.2 Investigation Using Micro-PIXI, FTIR and Electron Spectroscopy The water tree consists of micro voids which are connected by micro channels or tracks. The density of micro voids (number per unit volume) decreases from the root to the front of the water tree. The micro voids contain the water and ions which have penetrated the polymer under the action of the electric field [6,19]. Reference [32] illustrates the range of water tree structures observed in medium voltage cable insulation using the scanning electron microscope (SEM) and also high resolution imaging system based on the transmission electron microscope (TEM). Recently, the nonlinear dielectric response of water treed XLPE cables has been explained by voltage assisted water ingress into water treed regions [121]. Water content of up to 2% was found in vented water tree structures. Some sections of the tree branches were, however, found to be hydrophilic, indicating water being bound to the polymer. The water content was highest in the tree branches and lowest at the tip of the trees as well as in the root region close to the insulation screen. The length of these regions was typically 200-300 μm. Scanning with the micro-PIXE technique was employed to analyze water trees in the XLPE insulation of a field-aged underground HV cable [6]. X-ray spectra of bow tie and vented water trees, the inner and outer semi conductive compounds, and an insulation spot free from any water tree were acquired. Simultaneously, twodimensional elemental distribution profiles across the water trees were also measured. Various trace element impurities were identified in the analyzed spots and their possible sources were suggested. Differences in elemental distribution profiles in the scanned 10 areas were observed. This study demonstrated the effectiveness of the micro-PIXE facility in analyzing water trees in underground power cables. Traditionally, water and ions in the water tree had been examined separately, by different methods, such as infrared spectroscopy, neutron activation analysis or X-ray analysis. In reference [20] , the authors presented a study of water and ions distribution in the same water tree by FTIR micro spectroscopy and tried to correlate their results with the proposed mechanisms for the growth of water trees. 2.2.3 Fractal Behavior and Analysis Water trees arise from the penetration of water into the XLPE dielectric of medium and HV power cables when operating in a humid or wet environment. They appear in the insulation of service aged power cables and can lead to electrical failure. The visual appearance of water trees suggests underlying fractal characteristics. The calculation of the fractal dimension can help to confirm the fractal character and to understand the physical mechanisms that lead to water tree formation and growth [10]. The growth process and shape of water tree in XLPE dielectric under ac electric field is random and branched and cannot be described using Euclidean geometry. However, the fractal dimension and the branching tendency of water tree in XLPE dielectric can be estimated based on partial electric breakdown and the fractal theory [28,36]. The shapes and structures of water trees observed by an optical microscope proved to have fractal characteristics, and it is concluded in these studies that water trees are fractal objects. 2.2.4 Effects of Water Tree on Electric Field It is reported that ac breakdown strength of the aged XLPE is reduced as the growth length of water tree is increased and the trend of the reduction has been similar to that of water tree growth rate. It has been found that the homo-charge is introduced due to treeing which is heavily concentrated at the tip of the water tree path while a little hetero-charge is gathered in front of electrode contacted to the water tree in virgin and aged cables. By the electric field calculation using the space charge distribution, the local field enhancement at the tip of water tree path has also been verified. Based on the experimental results and analysis, it has been shown that because of the increase in length and permittivity of the water tree, the electric field in front of the degraded area 11 is amplified because of the homo-charge concentration at the tip of water tree path causing the reduction of the breakdown strength of XLPE [1,11,33]. 2.2.5 Dielectric Properties The dielectric properties of the XLPE insulation are affected by water treeing as reported in the literature. A brief review of these aspects is given next. 2.2.5.1 Dielectric constant and loss tangent The good electrical and dielectric properties of PE remain largely unchanged during cross-linking process. XLPE cables therefore, like PE cables have a very small and insignificant temperature dependent dielectric constant (εr) and loss tangent (Tan δ). As a result, the dielectric loss of XLPE cables is small in comparison with those of PVC and paper insulated cables. Hence XLPE cables are specially suitable for long cable routes and high voltages since in both cases the dielectric loss is of great significance [14,44]. Due to the dependence of ion mobility on the presence of water, the wet water trees have much higher conductivities than the dry water trees and than the PE [44]. It has also been reported that the conductivity of water-treed polyethylene is more than 106 times higher than that of non-treed polyethylene, while the permittivity increase is only a few times [86,92,128]. The Maxwell-Wagner-Sillars (MWS) model proved a reasonable fit to the experimental data providing that the conductivity in water treed region lies between 10-3 S/m and 10-2 S/m. 2.2.5.2 Breakdown voltage The breakdown strength of XLPE insulation is degraded due to water treeing. Due to this reason, the breakdown voltage of cable insulation reduces as the length of water tree increases [304]. However, if an insulation fluid such as silicone oil is injected in the water-treed region of the insulation, the dielectric strength can be restored to some extent. The effects of silicone restoration liquid on water treed XLPE power cable insulation materials under ac and impulse breakdown strengths showed that the shortterm effect of restoration is to strongly increase the ac and impulse breakdown strength. It also increases low-frequency dielectric loss [45,74]. This effect is a non linear function of time and voltage. 12 2.2.5.3 Conductivity The conductivity of water trees is high at power-frequency ac voltages because charges were observed around the tip of water trees and on the electrode at the nondegraded side, but not on the electrode at the water-treed side. Whereas at high frequency of applied voltage such as 2 kHz, the conductivity of water trees is slightly low due to charge deposition on the electrode at the water-treed side [1,11,33,53,73]. 2.3 DIAGNOSTIC OF WATER TREEING Various techniques have been reported in the literature for diagnostics of water trees in the cable insulation. Hvidsten et. al. [67] show that the degree of water treeing in XLPE cables rated at 36 kV and above is strongly dependent upon the cable design and the system voltage. The water trees are typically relatively short, but their density is similar to that observed in medium voltage cables with similar design. The results from nondestructive diagnostic testing indicated that it can be possible to detect water treeing also in old high voltage cables extending diagnostic techniques developed for medium voltage cables. These diagnostic techniques are based on measurements of various parameters as outlined next. 2.3.1 Capacitance and Loss Tangent The temperature dependence of loss tangent and capacitance is a very significant parameter. Hence, the differential variation of loss tangent and capacitance and its dependence on the temperature can be used as a measure of water tree degradation of polymer insulated medium voltage cables as suggested in several recent reports [7,54,174,249]. 2.3.2 Loss Current Waveform and its non-linearity with Voltage It has been reported that water trees introduce higher harmonics into the insulation loss current waveform and that the harmonic distortion of the loss current can be correlated with the length of the water trees. Current waveforms (current comparative technique [118]) for insulation free of water trees or insulation in which the water trees were completely dried out showed negligible harmonic distortion. Insulation with partially dried water trees maintained significant harmonic distortion of the loss 13 current. Harmonic components arise as a result of the nonlinear voltage-current characteristic of the water treed insulation [89]. The results confirm that water treeing causes reduced residual ac breakdown strength and high and nonlinearly increasing lowfrequency dielectric loss [74]. 2.3.3 Space Charge Space charge behavior is also affected by the degradation of insulating materials by water treeing. The charge distribution at any phase angle of the applied voltage inside water-treed polyethylene was examined using pulsed electro acoustic method (PEA) in [144]. Thus several studies have attempted to understand the space charge behavior of water treed insulation and the measurement of space charge as a diagnostic tool [17,105,135,141,144]. .