Performance Comparison and Selection of Transformer Fluid

Transformer fluid directly affects the working state of the components and the cooling efficiency of transformer. There are three kinds of transformer fluid used for electric locomotive, EMU and suburban rail vehicles: mineral oil, silicone liquid and synthetic ester based insulating oil. In this paper, the three kinds of oil are compared from the fire safety, environmental protection, reliability and low maintenance. It provides a strong basis for the selection of transformer fluid. By comprehensive analysis, synthetic ester based insulating oil can completely replace mineral oil and silicone liquid. With rail transport safety and environmental protection standards improving, synthetic ester based insulating oil will be the best choice for transformer.


Introduction
Traction transformer is an important equipment of electric locomotive and EMU, its cooling mode is by insulating oil. Transformer fluid directly affects the working state of the components and the cooling efficiency of transformer. Therefore, the choice of transformer fluid is essential.
Currently, there are three transformer fluid used in electric locomotive and EMU: mineral oil, silicone liquid and synthetic ester based insulating oil (synthetic ester).
These three kinds of insulating oil can all meet the basic performance requirements of transformer fluid, but with the rapid development of rail transport technology, it puts forward higher requirements on the performance of transformer fluid. Meanwhile, safety and environmental protection must be considered in the selection of transformer fluid.
In the following article, three kinds of transformer fluid are compared from fire safety performance, environmental performance, reliability and low maintenance. The results can provide a strong basis for the selection of transformer fluid.

Fire Safety Performance
It is the most suitable transformer cooling method to use insulating oil (not other cooling medium) radiating heat, but this approach also determines that there is a fire hazard for transformer. In the field of rail transportation, the transformer fire is absolutely not allowed to happen.
The fire will spread with terrifying speed, and result in a huge human and property losses. Fire safety performance of transformer fluid can be measured from the fire danger grade and ignition-resistant properties.

Fire Danger Grade Comparison
Fire performance comparison result of mineral oil, silicone liquid and synthetic ester is shown in Table   1

Ignition-resistant Properties Comparison
Place the oxy-acetylene torch flame (

Biodegradability Comparison
Biodegradation is a procedure that organic matter is decomposed and made harmless, which can be accepted by environment. Synthetic ester is non-toxic and easily biodegradable, its biodegradability has passed the standard test by laboratories officially recognized abroad.
The biodegradation process of synthetic ester is shown in Figure 3.The degradation rate is 10% at the third day, 71% at 10th day, 89% at 28th day. This result meets the OECD biodegradable class requirement and the fully biodegradable requirement in IEC 61039 standard.

Fig 3 Biodegradation Process of Synthetic Ester
The biodegradation process of mineral oil and silicone liquid is shown in Figure 4. It is obvious that the degradation rates of both mineral oil and silicone liquid are less than 10% at the end of the test period 28 days, which makes a sharp contrast with synthetic ester.

Risk of Water Pollution Comparison
Umwelt Bundes Amt UBA assess chemicals and divide levels based on biodegradability and the potential impact on aquatic organisms, synthetic ester is classified as non hazardous to water products. Mineral oil and silicone liquid are classified as hazardous to water classes, and the specific test parameters and reference limits are shown in Table 2. Mineral oil and silicone liquid will damage environment. It was confirmed that synthetic ester leaks into river will not cause harm to aquatic organisms at very high concentrations (1000 mg / l). Once mineral oil or silicone liquid occurs event of leakage it would cause serious environmental pollution, and this effect will not be eliminated in many years. In addition, synthetic ester also passed the following stringent international environmental certification: z OECD Biodegradable Environmental Report z UBA Environmental Report on non water pollution z US government Water Environmental Report Therefore, synthetic ester owns an obvious advantage of environmental performance, environmental pollution will not be worried.

Reliability
The

Moisture Resistance Comparison
Moisture resistance of transformer fluid is reflected by water content. With moisture content increasing, the performance of transformer oil will decline, mainly in the following points: 1 Increased water content will decrease dielectric strength.
The breakdown voltage and the water content of synthetic ester, mineral oil and silicone liquid at 20 is shown in Figure 5, Obviously, the water content of mineral oil and silicone liquid is very low, and the breakdown voltage decreases rapidly with the water content increasing. In contrast, the water content of synthetic ester is much larger than mineral oil and silicone liquid, even in the case of the water content over 600 ppm, synthetic ester also maintains a high breakdown voltage (more than 75kV). 2) Increased water content will cause accelerated aging of the insulation paper.
Various studies show that the insulating paper aging rate has a direct relationship with the water content, the water content of the cellulose every 1% increase the life of insulating paper will be reduced by 10%. With the aging of cellulose, it will release water, which would aggravate the aging process. The good water absorption capacity of synthetic ester will reduce the water content of insulating paper, so it will slow down the speed of aging. 3) Increased water content will form bubbles easily.
Bubble will decline the electrical performance of insulating oil. According to IEC 60076-14 standard, the temperature of bubbles production is directly related to the water content of the cellulose. Under overload conditions, the temperature of iron core wrapped by insulation paper will rise. If the water content of insulating paper is 2.6%, the bubble generation temperature is 130 ° C. If the water content is 1.1%, the bubble generation temperature is 165 ° C. Since the synthetic ester can ensure insulation paper drier, the safety threshold is higher under overload condition.

4)
Increased water content will increase the risk of releasing free water from the oil In the transformer cooling process from working temperature to environment temperature, it is possible to release water from transformer fluid which depends on the water content saturation value. The value of synthetic ester is high, which means it is not easy to achieve its saturation limit and it is difficult to release free water.

Lubrication Property Comparison
Lubrication  Figure 6, the wear value of synthetic ester is the minimum, which means the lubricating effect of synthetic ester is the best.

Low Maintenance
The average life of the transformer is 30 years, the life and replacement cycle of transformer fluid is influenced by oxidation resistance and aging resistance. If transformer fluid and transformer have the same life, they can be replaced simultaneously.

Oxidation stability Comparison
Oxidation

Aging Resistance Comparison
The acidity is a key indicator of transformer fluid aging.
Transformer is experimented seal aging experiment which is made of common materials (such as copper) and using synthetic ester as transformer fluid. After more than a year of experiment period, the acidity of the synthetic ester is shown in Figure 8. Under the conditions of 125 , Meanwhile, viscosity, density and ignition parameters were monitored during the experiment period, and these parameters are not changed a lot. In addition, synthetic ester did not produce corrosive sulfur and other sediments during the experiment period. All these illustrate that the stability of synthetic ester is good, it may extend the transformer fluid replacement cycle. As people pay more attention to safety, environmental protection, synthetic ester will be the preferred transformer fluid of electric locomotives, EMUs and suburban rail transport vehicles.