Deck 6: Electric breakdown in liquids and oil-paper insulation

What is the purpose of using insulating liquids?

Electrical Insulation:

• Fills voids and gaps where solid insulation might leave air pockets → reduces risk of partial discharges

Cooling/heat dissipation:

• Carries away heat from windings, cores, or other energized parts

Arc quenching:

• In circuit breakers insulating liquids help suppress and extinguish electrical arcs

Protection against moisture and cantonments:

• Acts as a barrier to oxygen and water preventing corrosion

Diagnostic medium:

• Liquids can be tested to detect faults like partial discharges, overheating, or arcing inside equipment

Why is purity of insulating liquids very important?

Impurities in liquids are sources of electrons: dust, water particles, ionic particles, dissolved gases

This can lead to electric breakdown

The presence of 0.01% of water in transformer oil reduces its dielectric strength to 20% of the dry oil value

Describe the mechanisms of electrical breakdown in transformer oil

Electronic Breakdown (Intrinsic): At very high fields (>1 MV/cm), electrons can gain enough energy between collisions to ionize liquid molecules directly

Electro-Mechanical Breakdown: High electric field produces electrostatic pressure inside the liquid which can form tiny cavities or voids. The local field becomes very high inside them → discharge occurs

Cavitation / Bubble Theory: Impurities or local heating can form microscopic bubbles. Inside bubbles, the dielectric constant is lower → local field rises sharply. If the field in the bubble exceeds the gaseous breakdown strength, an avalanche discharge occurs

Suspended Particle Mechanism: Conductive particles in the liquid such as dust or metal flakes distort the electric field. Strong local field around the particle causes ionization → initiating discharges

How is the performance of oil affected by pressure, temperature, presence of moisture and contaminants?

Pressure & temperature: Causes the formation of gas bubbles that reduce the oil’s breakdown strength

Moisture & contaminants: Greatly reduces the dielectric strength of insulating oil from increased ionization

What is the effect of effects of oil volume on breakdown in liquids?

Oil breakdown strength generally decreases with an increasing oil volume due to the increased probability of finding defects or contaminants within the larger volume.

What are the benefits of using vegetable oil based transformer fluids?

Effective at voltage levels up to 500 kV

Environmental Benefits: Biodegradable, non-toxic, lower fire hazard

Performance Advantages: High flash point, good dielectric strength, moisture tolerance

What is the purpose of impregnating cellulose with oil?

Oil fills the pores and cavities between cellulose fibers, replacing air and greatly increasing the dielectric strength.

The oil-filled pores have high electrical strength due to their small size, while cellulose fibers provide a barrier effect against discharge development

What are the advantages of oil-impregnated cellulose compared with pure oil insulation?

Cellulose fibers divide the oil into numerous small gaps, inhibiting discharge propagation and increasing dielectric strength.

The cellulose provides mechanical support and allows stable insulation structures to be formed

What are the advantages of oil-impregnated cellulose compared with dry cellulose?

Oil replaces air in the pores, eliminating weak gas-filled cavities where partial discharges could develop.

Impregnation greatly increases dielectric strength, since dry cellulose insulation has unacceptably low electrical strength.

How is hybrid oil-cellulose insulation implemented in HV transformers?

Transformer conductors are wrapped with overlapping layers of cellulose paper, while pressboard provides insulating barriers between components.

The insulation is dried under vacuum and impregnated with dry, degassed oil, filling the pores and eliminating trapped gas bubbles.

What are the mechanisms of electrical breakdown in oil-cellulose insulation?

Partial discharges can initiate in oil-filled pores, larger cavities, or gaps near electrode edges, progressively eroding the cellulose and eventually causing breakdown.

Moisture and aging increase conductivity and dielectric losses, reducing insulation strength and potentially causing thermal breakdown.

Why are multiple overlapping layers of paper wound around transformer conductors?

Individual paper layers contain microscopic defects and holes; overlapping multiple layers prevents defects from aligning and forming a continuous weak path.

Typically 4–5 layers are used to cover defects and joints, improving dielectric strength and preventing breakdown.