Diagnostic methods for fault location in cable insulation have become increasingly vital with the development of long-distance cables. This article presents an improved method for fault location based on the complex reflection coefficient spectrum (CRCS). First, a mathematical model for a long high-voltage cable line with consideration of a distribution-parameter circuit of cable joints is established in the frequency domain, and the CRCS of the cable with joints is analyzed. Next, based on the principle of frequency domain reflectometry (FDR), an algorithm in time-frequency conversion which uses the CRCS as the analysis spectrum is proposed. And the positioning sensitivity of the method is improved by implementing a data process adopting a Chebyshev window function which is controllable and reduces the sidelobe effect effectively. Furthermore, the improved method is evaluated by performing the simulations and two field verification tests. In a field test for a 5.1-km-long 110 kV cable, the multiple cable joints could be located effectively, and the relative error is between 0.14% and 1.49%. Then, the method was conducted for the positioning of a high-resistance fault and multiple joints on a 2.7-km-long 110 kV cable and the relative error is 0.37%. It is worth noting that the cross-bonding links of both cable tests were artificially replaced with temporary links straight across each joint. For the cross-bonded cable system, the test result shows that an obvious reflection is found at the cross-bonding point, which can reduce the reliability and accuracy of the present method.

Several retrofillings have been performed by different utilities in China. However, there is still insufficient understanding of the retrofilling procedures for minimising residual mineral oil and on the effect of the temperature rise of transformers. Following new detailed procedures, an out‐of‐service S11‐M‐315/10 (10 kV 315 kVA) and a new S13‐M‐400/10 (10 kV 400 kVA) distribution transformers were selected to retrofill their mineral oil with FR3, a soybean‐based natural ester. A method was proposed to predict the residual mineral oil content, which uses the kinematic viscosity at 60°C. This method fills a gap in the testing of residual mineral oil content. The mass ratios of residual mineral oil were 1.24% for the S11‐M‐315/10 transformer and 2.05% for the S13‐M‐400/10 transformer, based on the procedures in this study. The fire point of the mixed oil after retrofilling was 352°C, which meets the K ‐class less‐flammable requirements, indicating a proper control of residual mineral oil. After retrofilling, the time of reaching the limits of the 1.5 times rated load was 30% longer than that of the mineral oil transformer. Two retrofilled transformers have been operating in good conditions since November 2019 for the S11‐M‐315/10 transformer and December 2020 for the S13‐M‐400/10 transformer.

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.

Synthetic ester and blended insulating oils have attracted widespread attention owing to their great potential on performance enhancement because of the high capacity with tailorable molecular structure and mixable components. In this work, natural ester insulating oil (FR3) is blended into the synthetic tetra-ester insulating oil (KA) via mixing and oscillating. The physicochemical and electrical property measurements of the ester blends indicate the flash point, fire point, and AC breakdown voltage of KA are capable to be increased, and the dissipation factor of KA is reduceable by the addition of FR3. As expected, a series of physicochemical properties of the ester blends follow a quasi-linear relationship with the variation (in vol%) of FR3. However, the flash point (closed cup) is verified with high nonlinearity according to the results of thermogravimetric-derivative and thermogravimetric-differential scanning calorimetry (TG-DSC) tests in nitrogen. The optimized ester blend (85 vol% (KA):15 vol% (FR3)) is typically manifested by high flash point (260°C), low pour point (-51°C), low dissipation factor (1.44%), high AC breakdown voltage (76.5 kV), and high onset temperature of DSC either in nitrogen (229.99°C) or in the air (272.05°C). The activation energy is analyzed based on the Arrhenius equation, dissipation factor, and resistivity values in order to recognize the optimum ratio of the ester blends.

A high-temperature oil–paper insulation system offers an opportunity to improve the overloading capability of distribution transformers facing seasonal load variation. A high-temperature electrical insulation system (EIS) was chosen due to thermal calculation based on a typical loading curve on the China Southern Power Grid. In order to evaluate candidate high-temperature insulation systems, Nomex^® T910 (aramid-enhanced cellulose) immersed in FR3 (natural ester) was investigated by a dual-temperature thermal aging test compared with a conventional insulation system, Kraft paper impregnated with mineral oil. Throughout the thermal aging test, mechanical, chemical, and dielectric parameters of both paper and insulating oil were investigated in each aging cycle. The thermal aging results determined that the thermal class of the FR3-T910 insulation system meets the request of overloading transformer needs.