As a non-renewable resource, mineral oil has been re-evaluated for its value and suitability as the coolant and insulator in transformers because of its environmental unfriendliness, fire hazards, and operating efficiency dissatisfaction. As a result, alternative dielectric liquids meeting the demands for more reliable, safer, and cleaner energy has been a critical need. Plant-based insulating fluids, non-toxic to the environment and ultimately biodegradable, have been flowing into the mainstream for achieving cost efficiencies, performance advantages, and enhancing safety. With more than 20 years of field experience, plant-based oils are now employed in more than 600,000 transformers worldwide, and have been established an enviable performance track record. Far from being just a mineral oil replacement, but more significantly, plant-based oils have filled a gap in some specific application scenarios where mineral oils fail to satisfy the fire-safety and environmental standards. This article reviews the research status of plant-based insulating fluids for transformer, including both challenges and outlook circa 2020. Plant-based oils have many inherent advantages over mineral oil, but also have numerous and unique challenges.

A critical review of plant-based insulating fluids for transformer: 30-year development

Experimental investigations were carried out to study the tracking process on pressboard in esters under ac voltages. The evolutions of creepage discharges were analyzed by correlating the visual records of surface tracking, the phase resolved PD patterns, the discharge current signals and the high speed shadowgraph images. It is found that the impregnated pressboard is susceptive to discharge erosion occurring on and inside the pressboard, which is characterized by "tree-shaped white and carbonized marks". The gaseous "white mark" channels would attract the subsequent discharges to follow the same routes, and the accumulative energy dissipation in these channels would then result in the carbonization of the channels and the emergence of a "carbonized mark". Once formed, the tree-shaped mark can continue to grow even under a reduced voltage or after a re-impregnation, until it bridges the gap and causes a final flashover. For a mineral-oilimpregnated pressboard, high moisture content (>3.5% by weight) is a necessity to initiate a "white mark"; whereas the "white mark" appears at a lower voltage and develops more easily on a dry ester-impregnated pressboard.

Surface tracking on pressboard in natural and synthetic transformer liquids under AC stress

In view of the contemporary scenario and importance of alternative liquid dielectrics, IEEE DEIS Technical Committee on Liquid Dielectrics laid emphasis on pre-breakdown phenomena of ester dielectric fluids. The present article reports the background knowledge on the breakdown theories and behavior of streamers in insulating oils. The major influencing properties of the streamers aroused in liquid insulating systems are discussed keeping in view of the ester based dielectric fluids. Importantly, modelling of streamers, testing approaches, and review of pre-breakdown phenomena in ester dielectric fluids has been presented.

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A review on pre-breakdown phenomena in ester fluids: Prepared by the international study group of IEEE DEIS liquid dielectrics technical committee

The presence of metallic particles in oil insulation degrades the performance of power transformers. At present, a large amount of research has focused on PD and breakdown characteristics initiated by particles in stationary transformer oil. However, in actual use, transformer oil flows which affect the movement of particles, and hence PD and breakdown characteristics change. In order to clarify these, PD parameters and breakdown voltage were measured in moving transformer oil in the presence of metallic particles, and the effects of flow velocity, particle size and concentration were investigated. It is found that as the oil flow velocity increased, PDIV became higher, whereas the discharge magnitude and frequency decreased. On the contrary, the discharge magnitude and frequency increased with particle size and concentration. Besides, the breakdown voltage increased with the circulation of oil, but it became lower with increasing particle size and concentration. A simulation model including electric field and fluid field was employed to study the moving trajectory of particles in flowing transformer oil. When the oil flow velocity increased, the collision number between particles and electrodes decreased and it became larger with particle size and number. In terms of the simulation results, the variation of PD characteristics could be interpreted. The breakdown voltage, as affected by oil flow velocity, particle size and concentration were discussed.

Partial discharge and breakdown characteristics of moving transformer oil contaminated by metallic particles

As vegetable insulating liquids have different chemical compositions from mineral oils, dissolved gas characteristics under thermal and electrical faults have differences; thus fault diagnostic methods based on dissolved gas analysis (DGA) used for mineral oil-filled transformers may not be suitable to vegetable oil-filled transformers. This paper presents experimental results of dissolved gases in Camellia insulating liquid under typical thermal and electrical faults in transformers. Thermal fault simulation experiments include three parts, which are low temperature, medium temperature and high temperature faults, and the electrical fault simulation experiments also include three parts, which are partial discharge, breakdown discharge and arc discharge. Experimental results show that apart from CO and CO 2 , H 2 and C 2 H 6 are the main characteristic gases under thermal faults, and the percentage of H 2 decreases while CH 4 , C 2 H 4 and C 2 H 6 increases with temperature. In addition, H 2 and CH 4 are the main characteristic gases under partial discharge faults; C2H2 is the main characteristic gas under breakdown faults, and C2H2 and C2H4 are the main characteristic gases under arc discharge faults. Furthermore, graphic fault diagnostic methods based on DGA of Camellia insulating liquid including Triangle and Pentagon diagnostic methods are proposed. Compared with the current diagnostic method, the proposed methods in this paper have higher diagnostic accuracy.

Graphic approaches for faults diagnosis for Camellia insulating liquid filled transformers based on dissolved gas analysis

Mineral oil has been widely used in liquid insulation of power transformers. However, it is poorly biodegradable and could cause serious contamination to the environment if a spill occurs. With increasingly strict environmental rules and regulations, there is considerable interest from the Utilities to apply esters in power transformers as substitutions to mineral oil. In order to use esters in large power transformers, their dielectric properties should be thoroughly investigated. This PhD thesis covers the experimental studies on the dielectric properties of a type of synthetic ester (Midel 7131) and a type of natural ester (FR3) in both uniform and divergent electric fields, using a mineral oil (Gemini X) as the benchmark.The dielectric properties of transformer liquids in uniform fields were investigated using breakdown voltage tests. The breakdown voltages of esters should be at least similar to that of mineral oil to allow a replacement in transformers. To obtain a fair comparison, the AC breakdown voltages of well-processed transformer liquids were tested, and their distributions were statistically analyzed. Since the breakdowns of transformer liquids in uniform fields are caused by the weakest-links, conditions representative of in-service transformer liquids were also considered by testing the effects of extraneous factors, such as particles, water and electrode area.The divergent fields were produced by sharp needle electrodes with tip curvatures of a few micrometers. The dielectric properties of transformer liquids in such fields were studied using several methods. A traditional PD detector was used to study the partial discharge characteristics of insulating liquids, such as the inception voltages and the repetition rates. A high speed camera was utilized to identify the streamer generation, propagation and breakdown phenomena. An oscilloscope was used to investigate the current signals associated with these phenomena. The breakdown voltages of transformer liquids were also determined in the divergent field at various gaps. Furthermore, the fault gases in transformer liquids under partial discharge faults were determined and analyzed.The following findings and conclusions can be made from the research in this thesis:*	The AC dielectric strengths of esters in uniform fields are similar to that of mineral oil when they are in a well-processed condition. When practical liquid conditions are considered, the AC dielectric strengths of esters are higher than that of mineral oil.*	The partial discharge behaviours at overstressed voltages can be used to differentiate various transformer liquids. Esters are relatively inferior to mineral oil in terms of higher discharge amplitude, higher discharge repetition rate and more negative partial discharges.*	Mineral oil possesses a higher ability to suppress the propagation of negative streamers. Thus, the AC dielectric strength of mineral oil in the divergent field is relatively higher than those of esters.*	Esters generate the same types of fault gases due to electrical discharge as mineral oil, but in relatively larger amounts.

Partial Discharge Behaviours and Breakdown Mechanisms of Ester Transformer Liquids under ac Stress

SF6/CF4 mixture is considered a potential substitution for SF6 in gas-insulated equipment owing to the considerable insulation and low temperature characteristics of CF4. This paper concerns the substitutability of the mixture on the surface flashover property. Epoxy resin composites for solid insulators are applied since flashover at the insulator interface often occurs and results in severe damage to the safe operation of the equipment. Two kinds of electrode systems are designed to study DC flashover properties with different electric field uniformities. The obtained result shows that the flashover voltage in the 20% SF6/80% CF4 mixture achieved more than 70% of the voltage in SF6 at the same pressure. The mixture shows considerable synergy in the surface flashover process and performed different tendencies with varying gas pressures and contents under different electrode systems due to the influence of surface accumulated charge. Moreover, it is considered that surface charge due to ionization may result in a reversal phenomenon for voltage polarity with increasing pressure. The results indicate the possibility of SF6/CF4 functioning as an efficient substitution of SF6 with a considerable insulation property and improved environmental protection characteristics.

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Xin Wang

Papers

Partial Discharge Behaviours and Breakdown Mechanisms of Ester Transformer Liquids under ac Stress

Surface Flashover Characteristics of Epoxy Resin Composites in SF6/CF4 Gas Mixture with DC Voltage