Insulating Liquids: Basic Properties, Types and Applications Explained

In liquid filled transformers and other power distribution equipment, the fluid inside is the lifeblood that serves both as a dielectric and a cooling medium. Insulating liquid can be found in many different types of apparatus, including transformers, tap changers, circuit breakers and switches.

Which insulating liquid is chosen for a particular application depends on the required performance and installation of the equipment. For example, an indoor transformer application would call for an insulating liquid less prone to fire hazards whereas equipment located outdoors might call for a liquid with better cooling properties.

A technician will encounter many different types of insulating fluid during the course of their career depending on the age, type, and location of the equipment they may service. Some liquids are no longer available on the market due to environmental hazards and performance issues but still remain in service with older equipment.


Technical Properties of Insulating Liquids

Before we can discuss the specifics of each fluid and how they compare, its important to have a general understanding of the properties that goes into considering the type of insulating liquid to be used. The main considerations are fire hazards, dielectric strength, and environmental impact.

Fire Point: The temperature at which the liquid will continue to burn after ignition for at least 5 seconds. NFPA 70 Article 450-23 requires "less flammable liquids" to have a Fire Point greater than 300°C.

Flash Point: The lowest temperature at which the liquid can form a vapor near its surface that will "flash," or briefly ignite when exposed to an open flame. Flash point is considered to be a general indication of the flammability or combustibility of a petroleum liquid.

Dielectric Strength: The maximum electric field strength that a liquid can naturally withstand without breaking down and becoming electrically conductive. This is the main property that determines its viability as an insulating liquid. Higher dielectric strength means it is has higher resistance to electrical charges.

Insulating Fluid Dielectric Breakdown Voltage TestingInsulating Fluid Dielectric Breakdown Voltage Testing. Photo: Wikimedia.

Power Factor: Also called the "dissipation factor," this property indicates how much energy is dissipated through the fluid as heat. The power factor measures how "efficient" the insulating fluid is and can be a great indication of contamination and deterioration. A lower power factor means its a better insulator.

Viscosity: The thickness of a fluid describes its internal resistance to flow, which can be thought of as a measure of fluid friction. Water has a low viscosity which makes it flow quickly whereas honey has a higher viscosity which makes it flow slowly. A liquid with a lower viscosity allows for better flow and heat transfer through a cooling system.

Specific Gravity: The ratio of the density of the fluid to the density of water. Since water has a specific gravity of 1.0, free water in an insulating fluid will migrate to the top or bottom depending on the specific gravity of the insulating liquid. The dielectric breakdown of most insulating fluid is inversely related to the water content, meaning the dielectric strength of the fluid is lowered as its water content increases.

Electrical Dielectric Fluid Specific Gravity Explained
Electrical Dielectric Fluid Specific Gravity Photo: TestGuy

Interfacial Tension: The force of attraction between the molecules at the interface of two fluids, namely oil and water. Interfacial tension indicates the presence of soluble contaminants and oxidation products in an insulating fluid, a decreasing value indicates an increase in contaminants and/or oxidation products within the fluid.

Pour Point: Indicates the lowest temperature at which the insulating fluid will flow. This value is important in cold climates to ensure that the oil will circulate and serve its purpose as an insulating and cooling medium.

Biodegradability: Describes the ability of an insulating liquid to decay through the action of living organisms. This is a direct indication of how harmful a fluid is to the environment when it spills or cannot be contained.


4 Major Insulating Fluid Types

The common types of insulating liquid used in today's market consist of mineral oil, silicone, hydrocarbon, and natural esters. Each of these fluids have their own unique physical and insulating properties which determine their use.

Mineral Oil: Conventional mineral oil is probably the oldest and most widely used dielectric fluid a technician will encounter. This type of liquid is a top choice for outdoor transformers due to a longstanding record of dielectric strength and thermal performance. The main disadvantage of mineral oil is that its considered to be a flammable liquid with low biodegradability, which places restrictions on its use and containment.

Oil Circuit Breakers with Mineral Oil
HV Oil Circuit Breakers are typically filled with mineral oil. Photo: Wikimedia.

Silicone: When less-flammable liquid is required, Silicone has traditionally been the insulating fluid of choice. It has a relatively high point of ignition, which makes it ideal for use indoors and in vaulted areas. Silicone has disadvantages as well, such as chemical byproducts and high cost associated with its use. When compared to mineral oil, it has a similar dielectric strength and higher specific gravity but is not biodegradable.

Hydrocarbon: Fluids that contain highly refined petroleum oils have fire resistant properties that make them an ideal choice for use where less-flammable liquid is required. These fluids have excellent insulating and cooling qualities but they have a lower point of ignition compared to silicone and are also more expensive than mineral oil. Hydrocarbon fluid has a specific gravity and power factor similar to that of mineral oil and is highly biodegradable.

Natural Ester: When considering environmental impact, natural ester fluid is a top choice because it is derived from non-toxic natural oils (such as soy) and are completely biodegradable. They are self-extinguishing which makes them ideal for indoor installations and can absorb moisture better than other liquids. Natural esters also have the highest dielectric strength when compared to the other fluid types. The major disadvantage of natural esters are higher cost and higher power factor which can lead to higher operating temperatures.

Polychlorinated biphenyls (PCB)

Polychlorinated biphenyls were used in electrical transformers manufactured between 1929 and 1977, with the majority being installed in residential and commercial buildings and industrial facilities prior to 1978. A “PCB transformer” is a transformer that is known, or assumed under TSCA, to contain PCBs at concentrations greater than 500 parts per million (ppm). “PCB-Contaminated Transformers” known, or assumed under TSCA, to contain between 50 and 499 ppm PCBs are also subject to EPA regulations.

PCBs have been used as coolants and lubricants in transformers, capacitors, and other electrical equipment because they don't burn easily and are good insulators. These compunds have been shown to cause a variety of adverse health effects and are associated with acne-like skin conditions in adults and neurobehavioral and immunological changes in children. The EPA has also classified all PCBs as probable human carcinogens.


Insulating Liquid Testing

Regular maintenance of electrical equipment is paramount to ensuring long service life and identifying potential failures before they occur. As stated earlier, the insulating fluid is the lifeblood of a liquid-filled apparatus, and regular sampling and laboratory analysis can give great insight into the condition of the equipment that regular electrical tests may not detect.

Laboratory tests typically measure the physical and electrical properties of an insulating fluid discussed earlier such as dielectric strength, flash point, interfacial tension, water content, power factor, and specific gravity to name a few. Visual examinations such as color and transparency can also give great insight into contaminates and whether electrical arcing has taken place inside the equipment.

Dielectric Fluid Laboratory Testing ExplainedLaboratory tests typically measure the physical and electrical properties of an insulating fluid. Photo: piqsels.com

NETA acceptance and maintenance testing standards specify the analysis that should be performed on electrical power distribution equipment in accordance with ASTM standards. Each apparatus, voltage class, fluid type and service age will require its own unique set of tests and parameters based on these standards.

Related: ASTM Insulating Oil Test Methods

Transformers

  • Dielectric Breakdown
  • Acid Number
  • Specific Gravity
  • Interfacial Tension
  • Color
  • Visual Examination
  • Moisture Content
  • Power Factor
  • Dissolved Gas

Regulators / Tap Changers

  • Dielectric Breakdown
  • Acid Number
  • Specific Gravity
  • Interfacial Tension
  • Color
  • Visual Condition
  • Power Factor
  • Water Content
  • Dissolved Gas

Oil Circuit Breakers

  • Dielectric Breakdown
  • Color
  • Power Factor
  • Interfacial Tension
  • Visual Condition
  • Neutralization Number
  • Water Content

Oil Switches

  • Dielectric Breakdown
  • Color
  • Visual Examination

Capacitors and Reactors

  • Dielectric Breakdown
  • Acid Number
  • Specific Gravity
  • Interfacial Tension
  • Color
  • Visual Condition
  • Water Content
  • Power Factor
  • Dissolved Gas

References