Yang Xu

Ester liquids have become viable alternatives to conventional mineral oils for power transformer applications, mainly driven by increasing demands for enhanced fire safety and environmental sustainability. This paper reviews the historical development and functional properties of synthetic and natural ester liquids, with particular emphasis on their implications for transformer design and operation. Key physical and chemical characteristics of ester liquids are introduced, followed by a comprehensive review of their dielectric performances under alternating current and lightning impulse stresses, including pre‐breakdown, breakdown and partial discharge behaviours under different electric field conditions. The thermal performance of ester liquids is examined through their intrinsic thermal properties, results from laboratory‐scale experiments, computational fluid dynamics studies and full‐scale transformer temperature rise tests under forced and natural cooling modes. Ageing processes of ester liquids and ester‐impregnated cellulose insulation are critically reviewed, highlighting the different degradation mechanisms compared with mineral oil insulation systems and their impact on insulation lifetime and thermal class. Practical application aspects such as moisture characteristics, dissolved gas analysis, compatibility, retrofilling and low‐temperature applications are also discussed. The paper consolidates recent advances in understanding ester liquids, identifies remaining technical challenges to support the wider adoption of ester liquids in high‐voltage power transformers.

Retrofilling mineral-oil-filled transformers with ester liquids has been widely applied by users around the world to improve fire safety and environmental performance of their power grids. However, the impregnation state of retrofilled insulation systems has not been sufficiently clarified, and the effective thermal class achieved through this retrofilling practice remains unclear. In this study, a FTIR-based quantitative method is used to determine the residual mineral oil (MO) content in cellulose insulation materials, and is shown to be insensitive to variations in moisture content and cellulose aging. Using a laboratory prototype incorporating actual transformer conductors and pressboard, the evolution of impregnation state in oil–paper insulation systems retrofilled with synthetic ester (SE) was experimentally investigated under a temperature of 98 ℃ representative of full-load transformer operation. The results show that most of the initially retained MO in wrapped insulation paper is effectively replaced by SE, even from the innermost layers adjacent to the conductor, resulting in a state approaching ester-dominated impregnation. MO migration from wrapped insulation paper occurs predominantly during the early stage after retrofilling, whereas the pressboard governs the long-term tail of the migration process, manifested as a continuous increase of MO content in the insulating liquid. These findings demonstrate that the oil–paper insulation system can transition toward an ester-dominated impregnation after retrofilling, and provide a reference for assessing the potential enhancement of the effective thermal class of retrofilled transformers.

With increasing demands for fire safety and environmental friendliness, retrofilling transformers with alternative insulating liquids has become a growing trend. However, retrofilling power transformers remains uncertain, as more residual mineral oil compromises K‐class flame retardancy, and its impact on environmental performance has yet to be clarified. Meanwhile, there is a lack of evaluation methodology and retrofilling procedure for power transformers. This paper presents a synthetic ester retrofilling conducted on a 20 MVA/110 kV transformer. Kerosene vapour‐phase drying was implemented to supplement traditional oil draining, and the insulating liquid achieved a fire point of 304°C, meeting K‐class flame retardancy after retrofilling. The estimated residual mineral oil content was less than 0.8 wt%, significantly lower than the typically reported value (e.g., 3 wt%–5 wt%). Standardised testing confirmed that the synthetic ester containing 3.5% mineral oil maintained its environmental performance, achieving 82.6% biodegradability. Additionally, the evaluation methodology and recommended procedure for retrofilling power transformers are proposed in this study. The performance changes of the retrofilled transformer were discussed in detail. A significant decrease in lightning impulse insulation margin and insulation resistance was observed, which should be given particular attention.

The high-voltage direct current (HVDC) system based on the voltage source converters (VSC) has been increasingly commissioned for its advantages of providing more controllability and flexibility for network operations. In a VSC-HVDC system, the cross-linked polyethylene (XLPE) cable is one of the main assets. The cable is exposed to transient overvoltage and overcurrent, which can cause its insulation degradation leading to faults and even failures of the cable. It is thus essential to investigate the transient propagation process along the XLPE cable in the VSC-HVDC system and subsequently determine the proper insulation coordination. This systematic study should be achieved by adopting an appropriate cable transient model. In this paper, with consideration of cable’s multilayer structure, frequency-dependent and temperature-dependent material parameters, and cable joints, an improved transient model of the XLPE cable operated in a practical bipolar VSC-HVDC system is proposed. The sensitivity analysis of the cable transient model is performed. The accuracy of the model is verified through a field test with a 1.2/50 μs surge impulse on a 10.9 km long ±320 kV XLPE cable operating in the bipolar VSC-HVDC system. It is found that high temperature can increase the damping of wave propagation, whereas frequency-dependent permittivity of the semiconducting layer can affect the wave propagation in the inter-sheath modes. Cable joints have an insignificant effect on wave attenuation but can contribute to the waveform distortion for the surge impulse. The effect of the semiconducting layers and corrugated sheath on the attenuation and wave velocity may cause severe damping along the cable, leading to varying waveforms with lower oscillated amplitudes and shorter durations in the system. The findings in the paper can pave a way for implementing an accurate transient analysis for the XLPE cable.

The compatibility of construction materials with insulating liquids not only affects the selection of materials but also the reliability of fluid-filled transformers after refilling with alternative insulating liquids. With the application of alternative insulating liquids for environmentally friendly and high-temperature insulation systems, the industry requires compatibility testing standards. Thus, in this study, the compatibility between different coatings and gasket materials with synthetic esters in fluid-filled transformers was investigated. To approximate the operation status, the testing conditions were upgraded by raising the temperature to the thermal class of insulation systems as suggested by IEC 63177 (committee draft as FDIS stage). Moreover, several tests were conducted to assess the performance of insulating liquids and construction materials for different durations, including 168, 336, 672 and 1008 hours. The results revealed the necessity of extending the test duration to 672 h and evaluating the performance of the insulating liquid and construction material in the compatibility test. In particular, the compatibility of polyesterimide, two-component epoxy resin, nitrile rubber, acrylic rubber, fluorosilicone rubber, and fluorine rubber with synthetic esters was examined. The results showed potential risks of incompatibility between the synthetic ester and the nitrile rubber, acrylic rubber and two-component epoxy resin selected in the experiment.

Synthetic ester insulating oils have been more widely researched owing to their tailorable performance to satisfy the more rigorous oil requirements for power transformers. In this paper, three biodegradable diesters (NPOE, NPCE, and NCOE) based high performance electrical insulating oils were prepared by the esterification of neopentyl glycol (NPG) with C18, blends of C8 and C10, and blends of C8, C10, and C18, respectively. The high conversion yield of neopentyl glycol (NPG) is achieved by optimum conditions: e.g. upon NPCE the fatty acids to NPG molar ratio of 2:1, the initial water content of 1 wt%, water absorbent’s adding at onset moment, 3 batches of adding NPG, the catalyst (IM 100) dosage of 1 wt%, and temperature of 50°C. As compared to chemical catalyzation, enzymatic catalyzation displays milder reaction conditions, no toxicity, and no formation of side products. After the purification of vacuum distillation, adsorption, and filtration, the physicochemical properties of the three synthetic diesters were characterized. As a result, the optimized synthetic diester (NCOE) is recognized by the lowest pour point (-57°C), sufficiently high flash point (closed cup ~ 200°C, open cup ~ 214°C), fire point (234°C), and initial oxidative temperature (196.6°C). The high performance of the insulating oil (NCOE) is also identified by the excellent AC breakdown voltage (67.3 kV) and moisture resistance. This work offers a green and low energy consumption approach for producing a high-performance synthetic insulating oil, which is expected for scale-up production and application in power transformers.

Contact method is an important method to measure the complex permittivity of polymer films. There are some possible factors affecting its accurate measurement for thin films by direct contact method. To solve the problem, this paper analyzes the reasons of errors induced by flatness tolerance and edge effect from the view of three-electrode structure, and that induced by the state of film surface as well. An equivalent electric circuit model is proposed based on direct contact method and the state of film surfaces. The correction formulas used to measure complex permittivity of films with coarse surface are derived according to the model, and then verified by experimenting different materials. Experimental results show that errors of relative permittivity are under 5% after calculation by the correction formula. In conclusion, the proposed measurement technique is theoretically and practically applicable for the complex permittivity measurement of low-thickness polymer films about 10~100 μm.

Natural ester insulating oil is widely used in transformers due to its good fire resistance and biodegradability. It is mainly composed of triglycerides, and there are some differences in fatty acid composition and content among natural esters. Therefore, the fault diagnosis methods based on dissolved gas analysis are different. Three kinds of natural ester insulating oils are used to study the dissolved gas in oil under typical electrical and thermal faults of power transformers. Thermal faults from 200°C to 800°C and electrical faults with different discharge energies, which include partial discharge, breakdown discharge, and arc discharge, are simulated. The characteristic gases and their variation trends of natural esters under thermal and electrical faults are obtained through the experimental results. The gas generation characteristics of the three natural ester insulating oils under typical electrical and thermal faults are almost similar, but the relative percentages of characteristic gases have certain differences. Natural ester insulating oil with a higher unsaturation degree tends to produce more H ₂ and less C ₂ H ₄ . Finally, the fault diagnosis methods are used to determine the fault gas data, and the Duval pentagon is modified and improved according to the gas generation characteristics of natural esters.