Deck 2: Insulating Materials

What is an advantage of using gas as a insulating material?

The electric strength of gasses restores immediately after flashovers and discharges, through the recombination of free charge carriers.

What are the advantages of SF6 compared to air?

It has three times higher electric strength, reducing the area and volume needed for switchgears

What distinguishes SF6 from all other gasses with high electric strength?

It has a low temperature of condensation

It is very chemically stable under ambient conditions making it non-toxic

During flashovers and switching arcs SF6 molecules are broken down into their components, which are so reactive that they immediately recombine into the inert form of SF6

High electron affinity which gives it a high dielectric strength

What is the downside of SF6?

It has a 23900-times larger global warming potential in comparison to CO2

Explain which properties of SF6 are responsible for its high electrical performance.

How do they affect the development of electron avalanches and streamers?

Why the inception/breakdown voltages are higher in SF6 compared to atmospheric air?

SF6 has a electronegativity so it captures free electrons and forms negative ions. During flashovers, SF6 molecules recombine back into inert SF6 maintaining the gasses strength.

Free electrons are captured to form negative ions, reducing the effective ionization coefficient. This suppresses electron avalanche multiplication and limits the space charge formation required for streamer development.

Consequently, higher electric fields are needed for streamer inception and breakdown in SF₆ compared to atmospheric air.

What are the desired material properties for insulators?

Should

• withstand high electric stresses

• survive in environments with dust, UV, rain, salt, fog

• have enough mechanical strength to bear tensile, compressive and cantilever forces

• exhibit good surface properties

What are some inorganic solid insulating materials and their overall benefits?

Porcelain, glass, and mica products

High temperature resistance, insensitive to chemical, weather, and radiation stresses

What are porcelain insulators usually used for and their pros and cons?

They are used for external insulators such as suspension insulators (hang conductors to overhead lines)

Pro: Suitable for all thermally highly stressed insulations and glazing prevents chemical damage
Con: Brittle, encounters issues in humid and polluted environments causing surface flashovers

What are toughened glass insulators usually used for and their pros and cons?

Glass insulators are usually used for cap-and-pin insulators. Glass fibers are also used as reinforcement in composite insulators with reinforced epoxy resin rods and silicone sheds 

Pro: Resistance to UV and other environmental effects, high dielectric strength and resistance to electric

puncture

Con: It has lower mechanical stress than hard porcelain, susceptibility to leakage current erosion, which can trigger shatter

What are mica product insulators usually used for and their pros and cons?

Uses: Mica is used for machine insulators, generator insulators, and in dry transformers for taping of conductors

Pro: Mica has good resistance against partial discharge and a high thermal resistance

Con: Mica produces are made of mica combined with a binging agent, and the binging agent usually affect its thermal and electrical properties.

What are polymeric insulators and the main types?

Composite structure consists of a fibre glass core providing mechanical strength and a polymer composite housing as weather-shed protection

EPDM, Silicone rubber (SIR), and Alloy rubber (a blend of EPDM and SIR)

List the benefits of polymeric insulators compared to porcelain or glass

Non-brittle, Low weight, good insulating performance, less maintenance

What are silicone insulators used for and their pros and cons?

Silicone is used for silicone-shed composite insulators and push-on cable fittings with potential grading

Pro: Silicone had excellent surface hydrophobicity where precipitation forms isolated water droplets. This prevents the formation of continuous films restricting pollution flashover.

Con: The mould seam from manufacturing can cause electric field distortion and accumulate pollution

What are the pros and cons of epoxy insulators?

Pro: Can be moulded into many different forms to suit variety of applications, end fittings can be provided, eliminating the need of external fittings

Con: Probability of severe leakage current erosion, weak electrical strength along mould line, originally limited to indoors due to UV degradation.

Draw a sketch showing the structure of a composite insulator and indicates it’s construction elements

Glass fiber reinforced insulating rod core, silicone sheath and weather-shields, corona rings, and end-fittings

What are thermoplastic insulating materials (Polyethylene, XLPE) used for and their pros and cons?

It is largely used in the insulation of high voltage cables

Pro: Excellent dielectric properties giving very high breakdown field strengths

Con: The service life of polyethylene insulators is reduced due to water treeing. Insulation breakdown is caused by diffused moisture.

How is silicone used in controlling the electric field of a high voltage cable?

At cable fittings and joints a silicone stress cone is used as a potential grading electrode

The electric equipotential lines are forced to fan out and separate gradually along an extended profile.

This drops the peak electric field intensity to a safe fraction of its uncorrected value.

How are insulating liquids used and their pros and cons?

Insulated liquids are used to impregnate cavities in all types of mediums to increase the electric strength as high as possible.

Pro: Liquids have a significantly higher electric field strength under normal pressure compared to gasses
Con: The structural design of a device must take into account the thermal expansion of the insulating liquid. Gas bubbles trapped in the insulating liquid are prone to cause partial discharges