Polyethylene sheathed cables offer good resistance to oils, water, chemicals and abrasions making them commonly specified in heavy industry.
This thermoplastic material is referred to as PE, or occasionally the more specific compounds MDPE (medium density polyethylene) and HDPE (high-density polyethylene), and can be found on low voltage, medium voltage, and high voltage cables owing to its high dielectric strength.
Commonly specified for external applications, PE cables can sit in cable ducts including those that become waterlogged. Medium Voltage cables with PE sheathing are often available with additional waterblocking layers to provide additional protection in these circumstances. Direct burial is also applicable as an installation method when deemed appropriate under prevailing Wiring Regulations. When applied to instrumentation and data cables, the PE sheathing often sees the cables referred to as 'duct grade'. Cables are usually black in colour and are UV resistant, although this should be confirmed against individual technical datasheets if specifically required or in a location where cables are subject to elevated solar temperatures.
Generally, the breakdown strength of the polymer depends on its morphology. In intraspherulitic regions, E bd is higher than in interspherulitic regions and a change in the disorder within the spherulites and/or the interspherulitic region can affect the voltage endurance and, of course, the breakdown strength [ 13 ]. Since the nominal voltages of the power cables are increasing (exceeding 500 kV), polymer materials with a higher breakdown strength are required for insulation. The incorporation of inorganic nanofillers into insulation can increase the E bd of the materials, depending on the filler concentration, their shape, size and surface modifications with different agents, material homogeneity, electrical properties of the fillers, sample preparation routes and so forth [ 267 , 268 , 269 ].
The breakdown of a dielectric occurs at a high concentration of charge carriers such as free electrons in the conduction band and/or holes in the valence band, particularly in the specimen transition in a conductive state [ 133 ]. Macroscopically, the breakdown involves a sudden increase in the conduction current passing through an insulator. The minimum value of the electric field strength, at which the breakdown of the dielectric occurs, is called breakdown strength E bd and the minimum voltage applied to the electrodes is called breakdown voltage U bd . The values of dielectric breakdown depend on many factors such as chemical nature of the polymer and its physical structure, the specimen dimensions, the temperature, the frequency, the duration of the applied electric filed, humidity, type and form of electrode and so forth. Depending on the physical processes that cause the breakdown of a solid insulating material, breakdown mechanisms are classified as thermal, electronic, electromechanical, free-volume, by PDs and by treeing (electrical and electrochemical).
The performance of PE/SiO2 nanocomposites in high electric fields is presented in numerous papers. A rather detailed experimental study was carried out by CIGRE WG D1.24 [105]. The authors used plate samples of pure XLPE and XLPE with 5 wt % of fumed nano-SiO2 fillers, either unfunctionalized or surface-functionalized by a specific chemical coupling agent [105]. Some of the samples were heated at 60 °C in vacuum for 8 d. Three types of cells were used to perform the tests, namely with spherical and cylindrical electrode systems for AC breakdown tests and with needle to plane electrode system for impulse voltage tests. It was observed that (i) the addition of nanofillers enhanced the AC breakdown strength, (ii) the functionalized nanofillers seemed to cause only a slight increase of Ebd, (iii) heat treatment seemed to increase the Ebd values, and (iv) correspondingly, the highest values of Ebd were obtained in samples with surface-functionalized nanofillers after heat treatment.
A comparative experimental study on the influence of silica micro- and nanoparticles on breakdown strength and voltage endurance of PE was performed by Roy et al. on plate samples of XLPE and SiO2/XLPE [13,122,270] ( ). As filler, 5 wt % of nano-SiO2 were used, either untreated or surface-modified with triethoxyvinylsilane (TES), N-(2-aminoethyl) 3-aminopropyl-trimethoxysilane (AEAPS) or hexamethyldisilazane (HMDS). It was shown that (i) there was a significant increase in breakdown strength for nanofilled composites compared to unfilled XLPE, (ii) the samples with modified nano-SiO2 had a more significant increase in Ebd compared to the untreated ones, (iii) the TES nanocomposite samples that were stored at 80 °C exhibited the highest breakdown strength; (iv) the breakdown strength had lower values in micro- than in nanocomposites. A similar finding was also reported by Reading and Vaughan [271]. In order to explain the increase of DC breakdown strength and voltage endurance of the nanocomposites compared to the unfilled and microfilled composites, the authors stated that the large number of small particles acted as scattering centres and that the nanoparticles altered the crystalline morphology of XLPE (providing another scattering mechanism). It was suggested that the nanoparticles within the crystalline phase could disturb the continuity of the path provided to the charge carriers, which increased the breakdown strength values. Furthermore, the smaller values of Ebd in microcomposites (compared to nanocomposites) were argued to be due to the higher number of defects in microcomposites [272]. In order to explain the existence of higher Ebd values in nanocomposites due to surface functionalization of nanoparticles, Roy argued the emergence of &#;quasi-conductive&#; layers [273] in nanocomposites with untreated particles that locally reduce the electric field and do not appear in composites with surface-treated nanoparticles. In addition, polar surface treatments (by, for example, AEAPS and HMDS) have very deep trap sites, which reduce the mobility of charge carriers and increase the breakdown strength [106]. This explanation also takes into account the increase of the interfacial area and the reduction of the free volume in nanocomposites [122,270]. In the case of some polymers, the change in free volume due to the introduction of nanofillers is relatively low and does not excessively affect the breakdown strength [274].
Pleşa studied the influence types and contents of inorganic nanoparticles on the performance of plane-shaped LDPE-based nanocomposites in high fields [99]. As inorganic fillers, SiO2, Al2O3 and TiO2 were used ( ; ). The highest breakdown strength was obtained with LDPE/SiO2 samples that showed comparable breakdown strength of approx. 40 kV·mm&#;1, which, however, were lower than those of unfilled LDPE (~ 46 kV·mm&#;1). In general, the decrease of the breakdown strength in LDPE nanocomposites with inorganic nanoparticles is quite low, which could be correlated with the change in the degree of crystallinity in the nanostructured materials.
Lau et al. studied the influence of nano-SiO2 with particle sizes of 10 to 20 nm on the breakdown strength of blends containing 80 wt % LDPE and 20 wt % HDPE [153,275]. Tests on the AC and DC breakdown behavior of PE blends upon the addition of different contents of nanofillers (2, 5 and 10 wt %) and surface modifications (untreated particles and particles surface-treated with trimethoxy(propyl)silane were performed [275]. The addition of 2 and 10 wt % of untreated nano-SiO2 reduced the DC Ebd from 480 (unfilled polymer) to 278 and to 307 kV·mm&#;1, respectively. In composites with surface-treated nano-SiO2, the DC breakdown strength was higher than that of composites with untreated nanoparticles. For samples with 2, 5 and 10 wt % of treated nano-SiO2, the Ebd values were 58, 55 and 21 kV·mm&#;1 higher than those with untreated nano-SiO2. In all cases, Ebd of the filled PE was lower than that of the unfilled analogue [275]. The authors explained the reduction of Ebd by the accumulation of space, the increase in charge mobility and nanoparticle agglomeration effects [153]. Tests performed in AC showed a slight reduction of Ebd values in samples with a filler content of 5 wt % of untreated nano-SiO2 and a reduction of over 30 kV·mm&#;1 for those containing 10 wt % of filler compared to the unfilled samples [275]. In the case of samples containing surface-treated nano-SiO2, the AC Ebd values were higher than those of non-filled samples, particularly if the nano-SiO2 content amounted to 10 wt %. In those samples, the sizes of agglomerates were much smaller, which could be the reason for the improved AC breakdown strength [275].
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The influence of the matrix structure and the nanofiller content on the breakdown strength of nanocomposites was described by Sami et al. [144]. For the tests, the authors used plate samples with a height of 250 μm based on nanocomposites of the composition HDPE/SiO2 and LDPE/SiO2 with 0, 1, 2, 4 and 5 wt % of spherical nanoparticles. The cumulative probability of the electrical failure P was analysed using the two-parameter Weibull function ( ) [276]. Ebd increased with increasing nanofiller content, both for LDPE as well as for HDPE nanocomposites. The highest Ebd values were obtained for HDPE/SiO2 with 4 to 5 wt % of nanoparticles [144]. The dielectric strength growth was measured in AC on LDPE/SiO2 and XLPE/SiO2 nanocomposites in comparison with the unfilled PE and was reported in additional publications [277,278].
The influence of humidity on the breakdown strength of nanocomposites was studied by Hui et al. on XLPE nanocomposites with unfunctionalized (UN) and vinylsilane-functionalized (VS) SiO2 fillers [123] at loadings of 5 and 12.5 wt % [125]. Multi-recess samples were prepared and exposed to a humid environment of 100% r.h. and 75% r.h. at 50 °C for 14 d. The nanocomposites had a much higher moisture uptake compared to the crosslinked PE resin [125]. The presence of nanoparticles caused an increase of the breakdown strength; under humidity and heat, the Ebd values were reduced for all types of samples. In some tests of the nanocomposites, lower Ebd values were obtained than in unfilled PE [153,173]. This may be due to the space charge accumulation inside the samples and the local enhancement of the electric field [153].
The change of breakdown strength in samples based on LDPE filled with MMT and MA was presented by Gao et al. [145]. It was shown that the introduction of MMT increased the breakdown strength 1.35 to 1.70 times. The authors explained the increase of Ebd of the nanocomposite by the increase of polarity (MA) and the decrease of the free volume (MMT). Both processes intensify the electron scattering and decrease the initiation probability of a breakdown channel. Using nanocomposites based on LDPE and the MMT cloisite 15A in 5 wt % quantity, Guastavino et al. noted an increase of the breakdown strength as well [279]. Green et al. investigated samples of a blend of a high density linear polyethylene HDLPE (10 wt %), a branched low-density polyethylene LDBPE (90 wt %) and a PE/MMT masterbatch MB, which contained 40 wt % of MMT functionalized with dimethyldi(hydrogenated tallow) quaternary amine [280]. The introduction of masterbatch MB led to an increase in dielectric strength from 143 ± 9 MV·m&#;1 (in the absence of MB) to 171 ± 3 MV·m&#;1 (in the case of 20 parts MB) [280]. A very important role in manipulating the breakdown strength is the way how samples were prepared. If the dispersion of MMT nanoparticles into the polymer matrix was inhomogeneous, the values of the breakdown strength were considerably reduced compared to samples with very well dispersed nanocomposites [281,282]. Shah et al. studied the influence of the MMT content on the breakdown strength of nanocomposites based on HDPE [283]. The authors prepared samples of HDPE with MMT modified by hexadecyl trimethyl ammonium bromide, untreated or treated with 3-aminopropyl triethoxy silane and titanium acetylacetonate in quantities of up to 10 wt %. The results revealed that the breakdown strength of the nanocomposites increased with increasing clay content of up to 5 wt %. If the concentration exceeded 5 wt %, the breakdown strength slightly decreased. The Ebd values were also influenced by the type of nanoparticles and amounted to 30 kV·mm&#;1 for unfilled HDPE, 48 kV·mm&#;1 for HDPE with unmodified MMT and 54 kV·mm&#;1 for organoclay nanocomposites [283]. These changes were explained by the fact that exfoliated and intercalated clay platelets altered the electric field repartition and increased the path length for the breakdown.
The use of POSS as filler in LDPE yields composites with breakdown strength different from that of the LDPE matrix. Guo et al. demonstrated that the breakdown strength decreased if octamethyl POSS was applied in 5 wt % quantity, whilst the use of iso-octyl POSS in the same amount lead to a decrease of the breakdown strength [284]. The introduction of 0.3 wt % of functionalized graphene within LDPE improved the Ebd, while the addition of graphene oxide did not provide any significant change at a content of up to 0.3 wt %.
In a series of papers, the influence of MgO nanofillers on the breakdown strength of PE is detailed. Reddy et al. [285,286] studied the DC breakdown strength of LDPE nanocomposites with small amounts of MgO nanoparticles. Maximum thermal voltage MTV calculations, also known as thermal breakdown voltage, were carried out. The results showed that the values of the MTV increased with nanofiller contents of up to about 5 wt %, for which Ebd exhibited a maximum and decreased afterwards. Similar results have been obtained by Tanaka et al. [230], Masuda et al. [97], Murakami et al. [98] and Murata et al. [178]. Peng et al. used plate samples based on LLDPE and MgO nanoparticles surface-modified with (3-aminopropyl) triethoxysilane in concentrations of 0.1, 1, 2 and 5 wt % [19]. The results revealed that, at low nanoparticle loading, the LLDPE/MgO interface produced a large number of shallow traps, which increased the LDPE/MgO interface traps in the nanocomposites. Thus, the number and energy levels of free charges was reduced, enhancing the nanocomposites&#; breakdown strength. The introduction of 1 wt % of MgO nanoparticles into LLDPE increased the breakdown strength from 346.8 to 380.2 kV·mm&#;1 [19].
The introduction of TiO2 into PE also increases the breakdown strength. This effect was highlighted by Ma et al. [287] who conducted a study on LDPE-based nanocomposites. The presence of water on the nanoparticle surfaces decreased the Ebd: Samples prepared with dried nanoparticles exhibited 50% higher values than those filled with as-received TiO2 nanoparticles. Samples filled with surface-functionalized TiO2 showed an approx. 40% increase of Ebd [287]. The surface modification was carried out with N-(2-aminoethyl) 3-aminopropyl-trimethoxysilane (AEAPS) as a coupling agent.
An experimental study by Tian et al. on plate samples of LDPE filled with untreated ZnO nanoparticles with a mean diameter of 50 nm showed that the breakdown strength of the nanocomposites with a content of less than 1 wt % was higher than those of unfilled PE [177]. If the content was higher than 1 wt %, the Ebd values were lower than in unfilled LDPE [177].
From the studies described above, it can be concluded that nanoparticles can influence the breakdown strength of nanocomposites by different mechanisms such as (i) serving as heterogeneous nucleation agents (that accelerate the formation of crystalline areas and prohibit the formation of large spherulites), (ii) becoming scattering sources for electrons, (iii) decreasing the mobility of electrons by forming a large amounts of charge traps, (iv) hindering the polymer from the erosion of partial discharge, (v) inducing large void defects as well as distorting the local electric field (due of the large permittivity of nanoparticles), (vi) forming conductivity pathways and (vii) increasing the tunnelling current between nanoparticles.
As was mentioned above, the introduction of inorganic nanofillers into PE leads to the improvement of its electrical properties. On the other hand, some nanoparticles such as SiO2, ZnO and MgO cause an increase in the aging resistance (respectively, an increase in lifetime) of the nanocomposites compared to unfilled polyethylene [288,289].
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