TECHNOLOGY INSIGHTS by Corné Dames
In her new article, Corné Dames focuses on environmental concerns, fire safety, physiochemical properties, and thermal and dielectric performance of natural esters.
Due to the depletion of the mineral oil source and environmental impacts, the transformer industry has shifted its focus from petroleum-based mineral oil to natural and synthetic esters. Ester fluids have been present on the scene for quite some time now, and the environmental advantages and availability have the industry moving towards this avenue more strongly as time passes. The gradual move to the use of natural and synthetic esters is additionally supported by more data that is forthcoming from industry-based studies that are focused on using these fluids in electrical equipment.
This article will focus on environmental concerns, fire safety, physiochemical properties, and thermal and dielectric performance of natural esters.
Finally, we will discuss the fundamental properties of natural esters and material compatibility related to electrical design.
Introduction
In a transformer, the liquid-cellulose system forms the main component of insulation. This system has a trifold role as the dielectric barrier, mechanical support and heat dissipation route. The insulating liquid is the backbone of the entire system, as this part will have a tremendous impact on the transformer's achieved lifetime [1]. The insulating fluid in a transformer has three functions — to electrically insulate the active parts, to transfer heat from the conductors to the radiators, and to provide a diagnostic medium which enables engineers to assess the equipment's health through regular monitoring and analytical means. Additionally, the insulating liquid is used for arc quenching (in tap changers, for instance) and as an acoustic dampener. Both arc quenching and acoustic dampening are essential for lifetime optimization.
One of the significant concerns related to the use of mineral oil is its flammability. A dramatic escalation of power demand over the past few years has led to overloaded electrical grids, resulting in failures, fires, and consequent oil spills, for which government regulatory agents are imposing stiff penalties due to their environmental impact.
Due to the depletion of the mineral oil source and environmental impacts, the transformer industry has shifted its focus from petroleum-based mineral oil to natural and synthetic esters.
Mineral oils may have a toxic effect on the environment that will impact that area for many years to come. Having poor biodegradable properties, serious mineral oil spills can contaminate the soil and our waterways. Additionally, fire hazards are a real threat when using mineral oils, especially in densely populated areas.
As petroleum sources are depleting, the threat of shortages is becoming a reality. To ensure that ester fluids are used continuously and across many applications, we need to guarantee the reliability and safety of electrical equipment.
Chemistry and the Emergence of Insulating Fluids
Natural esters are extracted from crops like soybean, sunflower, rapeseed (canola), flax, olive, poppy, etc.
International standards
There is an increase in the use of natural esters in the transformer industry. They are currently used primarily in distribution and medium power transformers, in new units, and for retro-fill units. The first large power transformer of 420 kV voltage class was filled with natural ester fluid back in 2013, in the south of Germany, nearby Stuttgart. Currently, there are around 30-40 units in this voltage class that are already energized or in the process of production, including projects in Spain and Italy.
Table 1. Basic properties of insulating liquids: mineral oil and natural ester [2-6]
Physio-chemical Properties
Acidity accelerates the oxidation process in mineral oil and in cellulose systems. In the presence of moisture, the acid can cause rust on the iron components in the transformer. The acids in mineral oil are mostly short-chain carboxylic acids that are formed during the degradation process of mineral oil. These acids are more aggressive, having an essential impact on paper lifetime.
Natural esters have a higher inherent acid content, but they are long-chain free fatty acids that are very mild for other materials and soluble in the natural ester. For instance, the formed acids include the well-known “omega-3”, which is used as a medicine for reducing cholesterol levels.
The interfacial tension (IFT) of a liquid is defined as the measure of the force required to break through an interface between water and oil, thus related to the molecular attraction/repulsion forces between them. Interfacial tension is a significant indicator of polar contaminants and oil decay products in mineral oil. New mineral oils generally have a relatively higher interfacial tension than new natural ester fluids. The lower variation of the IFT in natural esters does not affect the fluid performance, but it limits the use of IFT as an indicator of fluid degradation.
The gradual move to the use of natural and synthetic esters is additionally supported by more data that is forthcoming from industry-based studies that are focused on using these fluids in electrical equipment.
Firepoint and Flashpoint
The fire point and flashpoint are the measures of the resistance of the liquid to catching fire.
The flashpoint is defined as the lowest temperature at which the vapor pressure is sufficient to form an ignitable mixture with air near that liquid's surface.
The fire point is the lowest temperature at which a liquid in an open container will attain a vapor pressure sufficient to continue to burn once ignited.
Mineral oil has a much lower flashpoint and fire point than ester fluids. A fire point of more than 300°C is required to classify a liquid as “less-flammable”.
Up to date, no cases of transformer fires have been reported involving natural ester liquid.
Environmental Impacts
The threat of depletion of crude oil sources is an ever-increasing reality. Exhaustion of mineral oil would lead to severe problems in many areas of the petroleum industry. That said, petroleum products are poorly biodegradable, and they pollute the soil and waterways when spills occur. In densely populated areas, mineral oil filled transformers pose a risk due to the fire hazard and potential environmental issues.
Water Saturation
Water has a detrimental effect on the transformer's overall life and on the electrical properties of the insulating liquid which needs to act as a barrier to ensure there is no electrical flashover. A high moisture concentration in mineral oil leads to visible effects as the oil will seem to be "murky", not clear.
Table 2. List of available international standards on different insulating oils [4, 5, 7, 8, 9, 10]