Space charge behavior of acrylic monomer-grafted polyethylene

This report describes space charge behavior of polyethylene (PE) grafted with acrylic monomers such as acrylic acid (AA), methacrylate (MA), ethylacrylate (EA) and n-butyl acrylate (NBA). The specimens were 0.7 mm thick with semicon electrodes, and vacuum degassed. Heterocharge is found in ungrafted control PE. This heterocharge decreases with the increase of AA content when AA is grafted onto the PE chains. This was attributed to an enhanced charge trapping at carbonyl sites provided by AA. When other acrylic monomers are grafted, heterocharge of the control PE increases first at low graft ratios and then decreases at higher graft ratios. This change was attributed to the combined effects of alkoxy groups acting as electron donating species and carbonyls acting as charge trapping sites. It seems that charge trapping at the carbonyls is sacrificed in part by the alkoxy groups at low graft ratios. All grafted polyethylenes are found at low fields below 1×10⁵ V/cm to obey the space-charge limited conduction (SCLC) mechanism. Effective charge mobilities of PE-g-AA and PE-g-NBA decrease with the increase of graft ratio while those of others remain unchanged     

Charge distribution in polyethylene/ethylene vinylacetate laminatesand blends

The formation of space charge in polyethylene/ethylene vinylacetate copolymer (PE/EVA) laminates was investigated using a pulsed electroacoustic (PEA) method. The formation of heterocharge was observed in PE while positive charge (holes) was distributed over the EVA specimen. In the laminates, however, a large amount of charge accumulates at the interface between PE and EVA, resulting in an increase of the electric field in the PE layer. When the thickness of the EVA layer is smaller than that of the PE layer, negative charge dominates in the PE layer due to enhanced charge injection caused by the increased electric field in the PE layers. When these polymers are blended, the amount of heterocharge in the PE/EVA blends varies inversely with the vinylacetate content by the enhancement of homocharge trapping at PE/EVA interfaces and/or by the enhancement of charge injection via an increased electric field in the PE component     

Effects of sample preparation conditions and short chains on spacecharge formation in LDPE

The effects of chemical change during sample preparation and low molecular weight chains of low density polyethylene (LDPE) have been investigated in order to clarify the origins for heterocharge in LDPE which does not contain antioxidants. The carbonyls formed at the surface of polyethylene (PE) sheet prepared at 180°C with polyethylene terephthalate backing films were found to contribute to the increase of heterocharge in LDPE. All aspects of the processes used for the preparation of samples must be studied carefully to ascertain possible effects on the properties of samples. The type of charge accumulated in LDPE was also found to depend on the concentration of short PE chains. Details of experimental results and their possible causes are described     

Generation and Dissipation of Negative Heterocharges in XLPE and EPR

Charge generation in cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR) under dc stress are influenced strongly by inclusions. The inclusions influencing charge generation are cross-linking byproducts, antioxidants, and moisture. These inclusions particularly influence the generation of negative heterocharges observed near positive electrodes, which produce high electric stress regions in XLPE cable insulation under dc voltage. Because a high electric field might cause dielectric breakdown of high-voltage equipment, it is important to minimize negative heterocharge accumulation. Consequently, the authors performed experiments to clarify the negative heterocharge generation mechanism in XLPE and EPR. The authors first studied the influence of cross-linking byproduct and moisture on negative heterocharge generation to clarify the mechanism. The authors next performed experiments to elucidate the effect of antioxidants on heterocharge generation, which revealed that the negative heterocharge is generated in XLPE insulation containing sulfur-containing phenolic antioxidant, or sulfur-type antioxidant. This heterocharge is presumed to be created by the combined effect of the antioxidant and acetophenone. Furthermore, the authors studied the dissipation of negative heterocharges in XLPE and EPR, clarifying that the dissipation of negative heterocharges in EPR is much faster than that in XLPE. This paper presents results of these studies.     

Comparative laboratory evaluation of TR-XLPE and XLPE cables with Super-smooth conductor shields

This paper provides data on four commercial tree retardant crosslinked polyethylene (TR-XLPE) and one cross-linked polyethylene (XLPE) insulated 15 kV cables supplied by three manufacturers. The cables have "super-smooth" conductor shields and "extra-clean" insulation and insulation shields. AC and impulse voltage breakdown and selected other characterization data are presented for cables that were aged immersed in room temperature water (15-30/spl deg/C) up to 24 months of a planned 48 months aging program. The five cables have high ac voltage breakdown strength, three of the TR-XLPE cables, actually increased in breakdown strength during aging. The one TR-XLPE cable that had the lowest ac voltage breakdown had vented trees at the insulation shield and high dissipation factor, which the other cables did not have. The impulse voltage breakdown strength of all cables decreased during aging; the cable with the lowest ac voltage breakdown also has the lowest impulse voltage breakdown. The dissimilar performance of the TR-XLPE cables and the excellent performance of the XLPE cable indicates evaluations at longer times are required to differentiate between modern TR-XLPE and XLPE insulated cables.     

Interfacial properties of XLPE/EPDM laminates

Interfacial properties such as space charge accumulation and breakdown characteristics in crosslinked polyethylene (XLPE)/ethylene propylene diene terpolymer (EPDM) laminates were investigated. Homocharge is observed in EPDM containing 1,4-hexadiene while heterocharge is observed in EPDM containing 5-ethylene-2-norbornene. Interfacial charge develops when the EPDM is laminated with XLPE. The polarity of this interfacial charge reverses at less than a few tens of hours heat treatment at 80°C dc breakdown voltage shows a sequential change of an increase, a decrease and a slight increase as a function of heat treatment time, which holds true for both interfacial and volume breakdown voltages. A maximum breakdown voltage is observed at 20 to 24 h heat treatment. After long heat treatment, silicone grease used in the interface shows lower interfacial breakdown voltage than silicone oil for the dc case, which was attributed to the additives in the grease and the molecular weight of silicone molecules in oil and grease. Details of the results are given and their origins discussed     

Electrical properties of silane crosslinked polyethylene incomparison with DCP crosslinked polyethylene

The electrical properties such as water tree length, electrical conduction, ac breakdown strength and space charge of silane crosslinked polyethylene (SXLPE) were investigated, with a purpose to compare this material with ordinary dicumyl peroxide (DCP) crosslinked polyethylene (XLPE). Experimental results show that SXLPE has a smaller water tree length, much lower conduction current density, a little bit higher ac breakdown strength than XLPE, and on SXLPE only a very small homocharge is seen. The better behaviour of SXLPE under electric stress is attributed to the absence of residual curing byproducts present in XLPE and the change of chemical structure by grafting and curing reactions     

抑制聚乙烯中水树生长的试验研究

本文研究了乙烯丙酸共聚物（EAA）作为一种改性添加剂对聚乙烯（PE）中水树生长的抑制作用。通过添加不同含量的EAA,在交流电场下进行水树生长试验,找到了交流电场EAA抑制水树生长的最佳含量,并对试样进行了不同的热处理及机械性能的测试,探讨了结晶形态对PE中水树生长抑制的机理。     

Influence of post-manufacturing residual mechanical stress andcrosslinking by-products on dielectric strength of HV extruded cables

Residual mechanical stresses from manufacturing always exist in freshly extruded HV cables. These stresses are not uniformly distributed in the cable insulation bulk material. Five different HV cables were analyzed with respect to residual mechanical stresses and dielectric breakdown strength. Photoelastic measurements have been carried out and show that maximum residual stresses range from 4.5 to 6 MPa and are located near the conductor shield. Breakdown strength measurements with respect to mechanical stresses have also been performed up to, and above the maximum stresses observed. A significant decrease in ac breakdown strength was observed for stresses higher than 6 MPa. Typical crosslinking by-products from dicumyl peroxide (DCP) have been measured by FTIR spectroscopy at five radial positions in the insulation bulk. Measured by-products consisted in acetophenone and cumyl alcohol. FTIR measurements show that crosslinking by-products content is higher in the bulk of the insulation than near the conductor and insulation shields thus showing a diffusion process. Moreover, residual amount of cumyl alcohol is generally between 1 and 3 times that of acetophenone. No important effect of both by-products on the ac dielectric breakdown strength was observed     

Interfacial charge in polyethylene/ethylene vinylacetate laminates

Space charge formation in polyethylene (PE)/ethylene vinylacetate copolymer (EVA) laminates has been investigated using a pulsed electroacoustic method. The PE shows heterocharge while the EVA shows a broad distribution of positive charge over the sample. The positive charge in EVA decreases with the increase of vinylacetate (VA) content in EVA. Interfacial charge is found in all laminates and remains unchanged by heat treatment at 100°C to 1 h and by coating chemicals such as silicone oil and trimethylolpropane trimethacrylate at the interfaces, This interfacial charge distorts the electric field distribution in the direction of increasing the electric field in the PE layers. The charge distribution in PE/EVA blends has been measured and is explained via the results obtained with the laminates     

Comparative Laboratory Evaluation of Premolded Joints for Medium Voltage Cables

This paper provides information on laboratory performance of 15 and 35 kV premolded joints from three manufacturers. Data on partial discharge, ac and hot impulse voltage breakdown tests, and dissection of breakdowns are presented for joints that were tested immediately after their assembly and after one year aging immersed in room temperature water (15-30degC), under operating voltage. The test program was repeated twice using joints acquired with a two-year interval. The two batches showed similar performance indicating that all three manufacturers provide consistent quality joints. AC breakdown strength of the joints practically did not change with aging. In contrast, their impulse breakdown strength decreased noticeably. Joints having partial discharge levels up to 5000 pC were included in the study. No obvious correlation was found between the presence of partial discharge and the joints ultimate dielectric strength.     

茂金属聚乙烯改善低密度聚乙烯的介电性能

针对高压、超高压直流塑料电缆中存在的空间电荷效应,大多数直流电力电缆常采用改性方法提高其介电性能,如空间电荷特性、体积电阻率和击穿强度等。常用的改性方法主要有添加剂、共混、接枝和二元共聚4种。共混技术较广泛用以改性聚乙烯电缆,提高其介电性能。因此用电声脉冲法(PEA)测量了MPE与LDPE共混试样中的空间电荷分布;用高阻计测量了共混物的体积电阻率,用阶梯电压测量了共混物的交流击穿场强。试验结果表明,1%MPE与LDPE共混能有效降低空间电荷效应,提高交流击穿场强7.9%,略降低体积电阻率。最后讨论了共混物的物质结构、电荷陷阱及介电性能间的关系。     

Changes of thermoplastic PE cable insulation properties caused bythe overload current

Experiments were conducted to determine structural changes (degree of crystallinity) of PE (polyethylene), the development of treeing (velocity, shape), and changes of tan δ and breakdown strength as a result of thermal aging. The results show that thermal aging causes increased separation of spherulites. Overheating leads to the appearance of amorphous regions with dimensions exceeding the diameter of the neighboring spherulites. It is shown that the treeing channels develop faster in amorphous thermoplastic PE insulation than in a partially crystalline one and that tan δ increases, but the breakdown decreases with the time of thermal aging     

Weibull statistical analysis of area and volume effects on thebreakdown strength in liquid nitrogen

In order to examine the area and the volume effects on breakdown strength in liquid nitrogen (LN₂), we measured dc and ac breakdown voltages in LN₂ with sphere to plane and coaxial cylindrical electrode configurations. We also carried out statistical analysis of the experimental results using the Weibull distribution and discussed with the statistical stressed electrode area (SSEA) and/or liquid volume (SSLV). The dc and ac breakdown strength in LN₂ decreased to 1/5 with increasing SSEA and SSLV over the wide range from 10⁰to 10⁵ mm² and from 10^-1 to 10⁵ mm³, respectively. The Weibull shape parameter m for the sphere to plane and the coaxial cylindrical electrodes was estimated to be 6 to 8 and 11 to 13, respectively; the breakdown strength in LN₂ saturated with increasing SSEA and SSLV. From these results, it was verified that the area and the volume effects definitely determined the breakdown strength in LN₂. The results of the Weibull statistics were compared with the conventional concept of the stressed electrode area (SEA) and liquid volume (SLV). Consequently, SSEA or SSLV was found to be nearly equal to 80 to 85% of SEA or SLV     

Effects of DC testing on AC breakdown strength of XLPE insulatedcables subjected to laboratory accelerated aging

The results of a study performed on crosslinked polyethylene (XLPE) insulated cables tested in the laboratory under accelerated conditions are described. URD-type cables rated at 15 kV were aged at 60 Hz, 6 kV/mm, (150 V/mil) and load cycled daily to 90°C conductor, with water inside the pipe and outside of the samples, for periods of up to 3 years. DC testing was performed before and during the aging. An anticipated significant reduction in the AC breakdown strength is observed in control cables (not DC tested), from 44 kV/mm (1100 V/mil) for unaged cables, to 10 kV/mm (250 V/mil), for aged cables. However, there was no further reduction in the AC breakdown strength of cables subjected to DC testing as compared to those that are not tested with DC. It is concluded that AC breakdown strength is not an effective diagnostic tool for determining the effect of DC testing of URD cables aged under the conditions described     

Investigation of Water Tree Degradation in Dry‐Cured and Extruded Three‐Layer (E‐E Type) 6.6‐kV Removed XLPE Cables

Dry‐cured and extruded three‐layer (E‐E type) 6.6‐kV cross‐linked polyethylene (XLPE) cables were introduced into electric power systems more than 30 years ago, but they do not experience failures because of water tree degradation. Also, the degradation index of water treeing for these cables has not been established. Therefore, investigating results of residual breakdown voltage and water tree degradation of these cables will help us plan for cable replacement and determine water tree degradation diagnosis scheduling, and will be fundamental data for cable lifetime evaluation. In this study, the authors measured the ac breakdown voltages of dry‐cured and E‐E type 6.6‐kV XLPE cables removed after 18 to 25 years of operation and observed the water trees in their XLPE insulation. As a result, it was observed that breakdown voltages were larger than the maximum operating voltage (6.9 kV) and the ac voltage for the dielectric withstanding test (10.3 kV). Water trees were mainly bow‐tie water trees and their maximum length was approximately 1 mm. Although the number of measured cables was limited, the lifetime of this type of cable was estimated to be approximately 40 years, even experiencing water immersion.     

The residual ac breakdown voltage of 6.6 kV dry-cured XLPE powercable under wet-accelerated aging tests

The residual ac breakdown voltage decreased to about half of the initial breakdown voltage after 3 months of accelerated aging test under immersion. This decrease was found to be caused mainly by a trace amount of moisture existing in the insulation. However, the breakdown voltage did not further decrease with increasing moisture content. In order to clarify the influence of moisture, the state of water existing in the insulation was investigated using FTIR (Fourier transform infrared spectroscopy) and optical microscopy. From these results, it was concluded that the moisture in the amorphous region and/or at the interface next to the crystal region caused the reduction of the ac breakdown voltage     

Electrical strength of paper-oil insulation subjected to compositevoltages

The results of a composite voltage, ac with lightning impulse (LI) tests of a paper-oil insulation model are presented. It was demonstrated that the presence of the background ac voltage leads to a decrease in LI breakdown voltage. This is particularly conspicuous at opposite polarities. The dispersion of LI breakdown voltages decreases save for the case when the LI is superimposed on the opposite polarities. The dispersion of LI breakdown voltages decreases save for the case, when the LI is superimposed on the opposite polarity peak value of the ac sinusoid. In this critical case the combined breakdown voltage (ac+LI) may be even lower than the ac one     

Prebreakdown and breakdown phenomena under uniform field in liquid nitrogen and comparison with mineral oil

This paper presents a study of breakdown and prebreakdown phenomena (streamers) in liquid nitrogen and mineral oil under quasi uniform electric field, under ac and impulse voltage. Streamers preceding breakdown are studied up to 0.5 MPa by high-speed visualization and recording of emitted light. In these conditions, breakdown in LN/sub 2/ is mainly due to negative streamers, initiated at lower voltage than the corresponding positive voltage. Hydrostatic pressure has a limited effect on breakdown voltage, such as in mineral oil. It is shown that the ratio of impulse to ac breakdown voltage in LN/sub 2/ is surprisingly low (close to 1), whereas in the same conditions ac breakdown voltage in mineral oil is lower than impulse breakdown voltage. Practical consequences for the design of HV insulation in superconducting systems are discussed.     

Area and volume effects on breakdown strength in liquid nitrogen

We measured dc and ac breakdown voltages in liquid nitrogen (LN ₂) with a sphere-to-plane electrode configuration. Experimental results revealed that the breakdown voltage in LN₂ did not increase monotonously but partially decreased as the sphere diameter increased at a constant gap length. Thus, the existence of the area and the volume effects on the breakdown voltage in LN₂ was verified quantitatively; the breakdown strength decreased when increasing the {SEA}₉₀ (90% stressed electrode area) and the {SLV}₉₀ (90% stressed liquid volume). By changing the experimental conditions, it was verified that both area and volume effects, having a mutual correlation, simultaneously lead to the degradation of the breakdown strength in LN₂. In order to examine the area and the volume effects for the larger {SEA}₉₀ and {SLV}₉₀, we also measured the breakdown voltage with a coaxial cylindrical electrode. It was concluded that the dc and ac breakdown strengths in LN₂ decreased as the {SEA}₉₀ and the {SLV}₉₀ increased varying widely from 10⁰ to 10⁵ mm² and from 10 ^-1 to 10⁵ mm³, respectively