Gaseous and Liquid Insulating Materials: Air, Nitrogen, SF6, Mineral Oil and Pyranol

Gaseous and Liquid Insulating Materials: Air, Nitrogen, SF₆, Mineral Oil and Pyranol

Insulators are not only solids. The air between two overhead wires is an insulator. The oil in a transformer is an insulator too, and it also takes heat away. In this note we study the gaseous and liquid insulating materials named in the syllabus.

Before reading this note, see Insulating Materials: Properties and Classification for the properties used here. For the basics of why insulation is needed, read Insulating Material: Properties, Importance and Examples. For dielectric strength, which is used throughout this note, see Dielectric Material, Dielectric Strength, Dielectric Loss and Loss Angle.

1. Gaseous Insulating Materials

Gases are good insulators because their molecules are far apart and there are very few free charges. Gaseous insulation does not wear out. If a gas breaks down, it recovers by itself when the voltage is removed (self-restoring insulation).

1.1 Air

Properties:

• Free of cost and available everywhere.
• Dielectric strength is about 3 kV/mm at normal pressure (approximate).
• Self-restoring: it recovers after a spark.
• Its strength falls when it is damp or at high altitude, where the pressure is low.
• It supports burning (it contains oxygen), and it slowly oxidises metal parts.

Applications:

• Insulation between overhead transmission and distribution lines
• Insulation between bus bars and live parts in open-type substations
• Air-break switches and air-blast circuit breakers
• Dielectric in air capacitors (variable capacitors)

Exam tip: "Air is an example of insulating material (True/False)" has been asked. The answer is True.

1.2 Nitrogen

Properties:

• Colourless, odourless and chemically inert gas.
• Does not burn and does not support burning.
• Does not oxidise or corrode parts, as it contains no oxygen.
• Dielectric strength is similar to or a little higher than air.
• Cheap and easily available (about four-fifths of air is nitrogen).

Applications:

• Filling in gas-filled capacitors and cables under pressure
• Gas cushion above the oil in some sealed transformers, to keep oxygen and moisture away from the oil
• Inert atmosphere in some high-voltage test equipment

1.3 Sulphur Hexafluoride (SF₆)

SF₆ is an artificial gas made from sulphur and fluorine. One molecule has one sulphur atom joined to six fluorine atoms.

Fig. 1: SF₆ molecule

Diagram in words: A grey circle marked S (sulphur) is in the centre. Six white circles marked F (fluorine) are joined to it by lines, one on each side: up, down, left, right, front and back. The six strong bonds make the gas very stable.

Properties:

• Colourless, odourless, non-toxic (in pure form) and non-flammable.
• Very heavy: about five times heavier than air.
• High dielectric strength, about 2.5 times that of air at the same pressure (approximate). The strength increases further as pressure increases.
• Excellent arc-quenching property: its molecules catch free electrons, so the arc dies quickly.
• Chemically stable and inert at normal temperature.
• Good heat-carrying ability.

Applications:

• SF₆ circuit breakers for high-voltage lines
• Gas-insulated substations (GIS), which need much less space than air-insulated ones
• High-voltage cables, transformers and switchgear

Important point: SF₆ is a strong greenhouse gas. It must not be released into the air, and its decomposition products after arcing are poisonous, so it is handled with care.

Exam tip: "Write the properties and applications of SF₆" has been asked in two papers (once together with Bakelite). It is a high-priority 4-mark question.

2. Liquid Insulating Materials

Liquids have higher dielectric strength than gases. A liquid also fills small gaps and carries heat away by circulating. So a liquid insulator does two jobs: insulation and cooling.

2.1 Mineral Oil (Insulating Oil)

Mineral oil is obtained from petroleum by refining. It is the most widely used liquid insulator in transformers, switchgear and cables.

Fig. 2: Oil-filled transformer (simple sketch)

Diagram in words: A big tank holds the core and two windings. The tank is filled with insulating oil up to a marked level. On top, two bushings carry the connections out. A small tank called the conservator is joined by a pipe to the main tank and takes up the expansion of the oil when it heats.

Properties:

• High dielectric strength (about 10–15 kV/mm for clean, dry oil; approximate).
• Low viscosity, so it flows easily and cools the windings by circulation.
• High flash point (it does not catch fire easily) and low pour point (it does not freeze in cold weather).
• Good chemical stability: it does not form much sludge in service.
• Low moisture content. Even a small amount of water lowers its dielectric strength a lot.

Applications:

• Power and distribution transformers (insulation and cooling)
• Oil circuit breakers (insulation and arc quenching)
• Switchgear, oil-filled cables and capacitors

Limitations: It is flammable, it slowly oxidises and forms sludge, and it absorbs moisture. So transformer oil is tested and filtered regularly.

Exam tip: "Enlist the properties of insulating oil" has been asked as part of a 4-mark short question (together with epoxy resin).

2.2 Pyranol (Synthetic Insulating Liquid)

Pyranol is the trade name of a synthetic insulating liquid. It belongs to the chlorinated hydrocarbon group (also called askarel). It was made as a non-burning alternative to mineral oil.

Properties:

• Non-flammable (it does not burn easily).
• High dielectric constant, so it is good for capacitors.
• High dielectric strength and good chemical stability.
• Does not form sludge easily.

Applications:

• Capacitors
• Transformers used in places where fire is a big risk

Important point: Liquids of the Pyranol type are poisonous and do not break down in nature, so their use is now banned or strictly restricted in many countries. Safer synthetic liquids (for example silicone liquids and synthetic esters) are used instead. This safety note is not part of the syllabus text, but it is useful to know.

3. Comparison of Gaseous and Liquid Insulators

Basis	Gaseous insulators	Liquid insulators
Examples	Air, nitrogen, SF6	Mineral oil, Pyranol
Dielectric strength	Lower (air about 3 kV/mm)	Higher (oil about 10–15 kV/mm)
Recovery after breakdown	Self-restoring	Mostly recovers, but oil may form carbon
Cooling	Poor (except SF6 under pressure)	Good, by circulation
Use	Overhead lines, circuit breakers, GIS	Transformers, switchgear, capacitors

Everyday Examples to Remember

• The air gap between two wires on a pole: air as an insulator.
• The oil tank of a transformer on the road: mineral oil for insulation and cooling.
• A 132 kV or 220 kV substation in a closed hall: SF₆ gas-insulated.

MCQ and True/False Facts

• Air is an insulating material (True).
• The full form of SF₆ is sulphur hexafluoride.
• SF₆ is used for arc quenching in circuit breakers.
• Mineral oil is used in transformers for insulation and cooling.
• Pyranol is a synthetic insulating liquid.

Important Questions for Examinations

Short Answer Questions

1. Write the properties and applications of SF₆.
2. Write the properties and applications of insulating (mineral) oil.
3. Write short notes on air and nitrogen as gaseous insulating materials.
4. Write a short note on Pyranol.
5. Compare gaseous and liquid insulating materials.

Long Answer Question

1. Discuss gaseous and liquid insulating materials with their properties and applications.

Quick Revision

• Gaseous insulators: air, nitrogen, SF₆. Self-restoring.
• SF₆: heavy, non-toxic, non-flammable, strength about 2.5 times air, excellent arc quenching. Used in circuit breakers and GIS.
• Mineral oil: insulation plus cooling in transformers and switchgear. Keep it dry.
• Pyranol: synthetic, non-flammable liquid used in capacitors. Now restricted because it is harmful.

Related Topics to Study Next

Before this note: Insulating Materials: Properties and Classification. Related reading: Insulating Material: Properties, Importance and Examples, Dielectric Material, Dielectric Strength, Dielectric Loss and Loss Angle and Classification of Electrical Engineering Materials.

Next: Plastics as Insulating Materials; Mica, Asbestos, Ceramics, Glass and Other Solid Insulators.

Conclusion

Air is the cheapest insulator, nitrogen is a safe inert gas, and SF₆ is the best gas for high-voltage breakers because it is strong and quenches arcs fast. Mineral oil insulates and cools transformers, and Pyranol was used where fire risk was high. Knowing the advantage and limit of each helps you choose the right one.