Less-flammable liquids used in transformers

Two main types of replacement liquid have been developed, high molecular-weight hydrocarbons and polydimethylsiloxane (silicone). These liquids are formulated to withstand fairly large amounts of electrical arcing, and are less flammable than mineral oils. These properties qualify the liquids for use in installations where less-flammable liquids are required and where the use restrictions associated with the products are followed. The requirements of the National Electrical Code are described and the advantages of using less-flammable liquids are also described. The fault mechanisms of transformers are discussed. The UL investigation of transformer liquids and the limitations of use are also discussed     

Vegetable oils for liquid-filled transformers

Several billion litres of transformer oil are used in transformers worldwide. To meet the challenges posed by environmental concerns, fully biodegradable vegetable oils have been developed for use in electrical equipment, particularly in transformers. Against this background the author discusses the development of vegetable oils for transformer use, their biodegradability, decomposition products, functional life test, fire hazard tests and the special challenges presented by cold weather. Further exploitation of these fluids for use in capacitors and cables needs further study and tests     

The behavior of different transformer oils relating to the generation of fault gases after electrical flashovers

Dissolved gas analysis is a common technique in the supervision and maintenance of transformers. It can give hints on defects and faults in oil-immersed transformers by determination and quantification of gases arising in consequence of thermal or electrical stress. Different algorithms for the assessment of transformers, which employ data obtained from dissolved gas analysis, have been published so far. However, only few investigations, dealing with the influence of the oil’s composition towards the oil’s tendency of releasing gases, exist. This research study presents a comparison of three commercially available transformer oils of different composition regarding that aspect. The results indicate that an oil with high percentages of paraffinic hydrocarbons forms gases to a lesser extent than naphthenic oils when it is exposed to electrical stress. This phenomenon can be explained either by a variable solubility of gases in the medium oil, or by the presumption that cyclic hydrocarbons are the main producers of gases.     

Characterization of static electrification in power transformers

Static electrification due to oil now causes many field failures of large forced-oil cooled power transformers. Also, in practice under normal operating conditions, the oil volume in these power transformers is reduced due to the internal heat generated (from the energized windings) which accelerates the chemical processes occurring inside. To complete filling of the oil tanks of such transformers, either new oil of the same or different type is used. Laboratory analogs of these transformers; namely a closed and an open cycle, have been used for investigating this phenomenon. Such tests have been performed upon different types of fresh and aged oils and oil mixtures to measure the electrification current. Investigations of the effect of oil temperature, oil velocity, frequency and type of the applied voltage, type of the solid-phase material, and the length of the oil gap are introduced. Also, a 200 kVA, 3.3 kV/380 V distribution power transformer is used where the oil is forced and heated externally to examine and demonstrate this phenomenon for unenergized and energized cases and for both fresh and aged oils     

Fundamental study of the development of a transformer with low‐viscosity silicone liquid

Silicone liquid is less flammable and produces less environmental pollution compared with mineral oil. Applicability of 20‐cSt silicone liquid to transformers was investigated from the viewpoints of insulation, cooling performance, and the less‐flammability evaluation. The results showed that the basic characteristics of 20‐cSt silicone liquid were verified in application to a less‐flammable transformer. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 165(2): 21–28, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20483     

Dielectric characterization of power transformer oils as a diagnostic life prediction method

The monitoring of the complex permittivity of transformer oils as a function of frequency and temperature provides information concerning the state of the insulation. In this work, it has been investigated as a quality control method, providing service life estimations of power transformations. The formation of databases recording polarization effects in a wide frequency and temperature range may be used effectively as a working tool for service engineers. Reliable insulation monitoring reduces accidents in aged HV transformers, some of which have been in operation since the 1960s. These transformers always impose an environmental threat because of possible accidental leaks or fires, especially when containing traces of PCBs.     

植物绝缘油纸浸渍模型与试验研究

植物绝缘油已成功应用于配电变压器中,但尚未在更高等级电力变压器中得到应用。由于植物绝缘油的粘度比传统矿物油的粘度更高,有必要研究植物绝缘油在厚纸板中的浸渍特性。在分析油纸绝缘浸渍机理的基础上,设计菜籽绝缘油油纸绝缘浸渍试验模型,讨论影响油纸绝缘浸渍过程的主要因素,通过试验得到菜籽绝缘油油纸绝缘浸渍长度随时间的关系曲线,并与矿物油纸绝缘浸渍长度与时间的实测曲线和理论计算曲线进行对比分析。研究结果表明,通过提高浸渍温度,植物绝缘油油纸绝缘的浸渍效果可以达到矿物绝缘油的标准。     

Evaluation of the Long-Term Thermal Capability of a High-Temperature Insulation System Using Silicone Liquid as a Dielectric Coolant

The functional thermal evaluation of test transformers insulated with silicon fluid is described. Studies have shown that polydimethylsiloxane fluids maintain good electrical properties at elevated temperatures, but system evaluation is required to determine their capability in a transformer. A transformer test facility meeting the requirements of IEEE Standard 262 is described and was used for the accelerated thermal testing of the transformer models. The test results are summarized and an indication is given of how silicone fluid can be used in the design of high-reliability industrial transformer systems.     

500 kV SF6 电流互感器内部盆式绝缘子缺陷电场分析和实验研究

近年来,涉及SF₆气体绝缘电流互感器故障次数较以往明显增多。为此,对一起故障的500 kV SF₆电流互感器进行解体事故分析,发现事故是由运行过程中内部盆式绝缘子产生缺陷造成的。为深入了解气体绝缘电流互感器绝缘特性,从源头上防止突发性故障,依据SF₆气体绝缘电流互感器事故分析结果,对盆式绝缘子有气泡、裂缝、金属污染等缺陷的情况进行了三维电场仿真计算分析和局放实验研究。研究结果显示：有缺陷的盆式绝缘子电场发生畸变,尤其是有金属污染时电场畸变最为严重,局放电压急剧下降。研究得出的数据和结论为SF₆气体绝缘电流互感器的绝缘设计、结构优化、运行维护和故障分析提供了理论参考依据。     

Unusual cases of gassing in transformers in service

This paper presents some unusual cases of gassing in transformers in service without an actual fault. These cases reveal that hydrogen production caused by stray gassing may be due to strongly hydro-treated oils, but is also very strongly influenced by transformer materials. Stray gassing with overproduction of saturated hydrocarbons with no production of hydrogen and no increased rates of hydrogen, ethylene, and other fault gases has probably always existed and is possibly related to oxidation. The overproportional development of methane and especially ethane proceeds in the first years of service and reaches a plateau after considerable consumption of oxygen. Possibly, this process is also catalytically triggered by transformer materials. Catalytic phenomena and stray gassing can be distinguished from a transformer fault through regular checks not only of the absolute DGA values but also the rates of gas increases.     

Protection of a Single-Phase Transformer from Interwinding Failure in Windings of Integral Magnetic Transformers

A simple means of protection of a powerful single-phase transformer with integral magnetic transformers has been suggested. The protection has a high sensitivity to the interwinding failure and is based on the measurements of unsymmetry of the leakage magnetic fluxes of transformer windings. This magnetic unsymmetry is determined by the difference of electromotive forces of windings of two magnetic transformers placed symmetrically with respect to the symmetry plane of the protected transformer. It has been suggested to produce the magnetic transformers in the form of a single-layer winding with an alternating pitch, which is wound around a textolite round or rectangular body. The length of a magnetic transformer is taken to be equal to the length of a magnetic core, which allows one to provide independence of protection sensitivity on the location of closed turns on the transformer rod. To measure the difference of the emf of magnetic transformers, the circuit of comparison with a responsive element in the form of a TT-40/0.2 or RT1-P/0.15 current relay has been used. Methods to determine a trigger threshold and sensitivity of the protection to interwinding failure as well as to ironwork failure have been suggested.     

多用途500kV备用变压器的研究

冀北电力公司500 kV变电站运行的500 kV/400 MVA的单相自耦变压器,往往因参数差异而无法统一配置备用变压器.通过仿真计算并联运行环流、零序电压,研究了多用途备用变压器的参数选取方案,可替换任意该类变压器,同时满足电力安全可靠运行的要求,目前已成功应用于电力生产中.     

浅析变压器油色谱分析的灵敏性

对两台发生放电故障的变压器的油品进行试验分析,讨论了不同的变压器油色谱分析的灵敏性.     

Novel modeling for the prediction of aged transformer oil characteristics

The effect of aging on transformer oil physical, chemical and electrical properties has been studied using the international testing methods for the evaluation of transformer oil quality. The study has been carried out on twelve transformers in the field and for monitoring periods up to 8 years. The properties which are strongly time dependent have been specified and those which have a great impact on the transformer oil breakdown voltage have been defined. Mathematical models for the breakdown voltage, total acidity and water content as a function of service periods have been given. The validity and applicability of these models for future prediction of these properties have been verified by the good agreement between the measured end predicted values. A multiple linear regression model for each transformer oil breakdown voltage as a function of its water content, total acidity and service period has been introduced and its adequacy has been illustrated by statistical analysis. Another multiple linear regression model has been developed by combining the results of a group of transformers into that of a single equivalent transformer. This model has been validated by predicting the properties of some other transformers and comparing them with the measured values. The comparison showed a good agreement for the results of transformers which have either been used or not in the derivation of the model.     

变压器油的正确选择与使用

介绍了变压器油的主要性质与使用的关系，并根据变压器油在变压器中所起的作用．以及变压器所处的环境来正确选择和使用变压器油。     

Differential Protection for Special Industrial Transformers

Power transformer differential protection has been used for decades on standard three-phase power transformers. However, special industrial transformers, such as 24-pulse converter transformers, could not typically be protected easily with the standard power transformer differential relays. The main reason is the nonstandard phase angle shift of 24-pulse converter transformers. Such 24-pulse converter transformers are often used in industrial and railway applications. This paper will show that it is possible to provide differential protection for such special transformers by using standard numerical transformer differential protection relays and external interposing current transformers (CTs). However, the external interposing CTs can be designed in a standardized way. This approach will significantly simplify the application of differential protection for special industrial transformers.     

S11型配电变压器的技术经济分析及应用

阐述我国S11型配电变压器的发展过程，分析比较了卷铁芯和叠铁芯2种结构的优缺点，并对S11和S9型配电变压器的损耗参数进行比较。建立了S11型配电变压器技术经济分析的数学模型，对更换S9型和S7型配电变压器的投资回报以及使用S11型配电变压器对广电集团公司电网线损的贡献分别进行了分析和测算。计算表明S11型配电变压器具有非常可观的经济效益。S11型配电变压器的技术已趋成熟，生产已具规模，具备大批量采购的条件，建议新采购时应以S11型配电变压器为主，对于运行12a及以上的老旧S7型（或更早期的）配电变压器应考虑逐步更换。     

A Comparison of the Impregnation of Cellulose Insulation by Ester and Mineral oil

Ester oils have been successfully used in distribution transformers for many years as satisfactory substitutes for mineral oil. In high voltage transformers however their application has been much slower partly due to the fact that the high viscosity of ester oil and hence uncertainties in efficient impregnation of cellulose insulation. In this paper the characteristics of the impregnation of cellulose insulation by ester oils are assessed and compared with mineral oil. Studies are carried out on thick laminated pressboard blocks, typical of support insulation structures in high voltage transformers. The degree of oil impregnation with time at various temperatures is examined. It is shown that the factors governing the impregnation processes with ester and mineral oils are very similar, i.e. mainly viscosities and surface tensions, and if a higher impregnation temperature is used for ester oils the same degree of oil impregnation can be achieved as with mineral oils.     

Relative properties of the new combustion-resistvegetable-oil-based dielectric coolants for distribution and powertransformers

Electrical distribution transformer insulation systems are being reevaluated based on their total life-cycle cost from economical, safety, and environmental perspectives. Due to the inherently higher efficiency design offered by liquid-cooled transformers, development has focused on fluids having improved environmental and health properties while maintaining the fire-resistant properties of “less-flammable” fluids. This paper reports our findings from several years of research and development on alternative dielectric systems using both natural and synthetic esters. Because esters have lower inherent resistance to oxidation than conventional mineral oil, a novel insulation system was developed to overcome this potential handicap. Single- and three-phase prototype field installations using these new dielectric coolants are discussed     

温度对典型光学电压互感器误差特性影响的研究

光学电压互感器容易受到温度影响,其误差特性是实际工程应用关注的焦点。文中首先介绍了几种典型原理的光学电压互感器,建立了光学电压互感器试验平台及试验方案,并针对已经实用化的2种原理的光学电压互感器开展了温度影响试验,提出了电容分压原理的光学电压互感器温度性能较好的观点。