BREAKDOWN IN LIQUID

Liquid has higher strength than gas because of it’s density (10 MV/cm). Commonly used as impregnants in cable, capacitor, transformer and circuit breaker. In transformer petroleum oil widely used as heat transfer agent and as an arc quenching in circuit breaker.

The oil helps cool the transformer. Because it also provides part of the electrical insulation between internal live parts, transformer oil must remain stable at high temperatures for an extended period. To improve cooling of large power transformers, the oil-filled tank may have external radiators through which the oil circulates by natural convection. Very large or high-power transformers (with capacities of thousands of KVA) may also have cooling fans, oil pumps, and even oil-to-water heat exchangers.

For very high temperature, silicon oil and fluorinated hydrocarbon are commonly used. It is essential that liquid dielectric is free from moisture, oxidation products and contamination to ensure proper functionality. Factor that affect liquid dielectric strength is the presence of fine water droplets suspended in oil. The presence of 0.01% water in transformer oil reduces it’s dielectric strength by 20%

Time integrated image of a streamer discharge generated in a water-filled coaxial reactor of 44 mm diameter, 100 mm length, and a tungsten-wire center electrode of 75 μm diameter.

Breakdown development in water for a 400-μm gap. With positive biased pin, a single streamer bridges the gap in less than 10 ns.

Transformer oil that is exposed to high temperature which will speed up the aging process. Physical and chemical characteristics such as viscocity, heat stabilization and specific gravity are important in the process of liquid breakdown.

In the latest development, High Temperature Hydrocarbons (HTH) Oils and Tetrachloroethylene (C2Cl4);

Electrical properties of liquid dielectric comprise of:

1. Relative permitivity between 2.0 to 2.6
2. High resistivity from 10¹⁶ W-m
3. Low power factor
4. High dielectric strength to withstand high stress

Characteristics of liquids:

• Pure Liquid – do not contain any impurities, eg. C6H14 (n-hexane), C7H16 (n-heptane), paraffin hydrocarbons
• Commercial Liquid – consists of complex organic molecules such as gas bubbles & suspended particles, eg. oil

The main impurities present are dust, moisture, dissolved gas & ionic impurities.

Purification Process

1. Filtration – Dust particles when present becomes charged and reduce breakdown strength, eg.mechanical filters, spray filters, electrostatic filters
2. Centrifuging -
3.  Degassing & distillation – to control the amount of oxygen and C02 
4. Chemical treatment – increase ion exchange 

Breakdown tests of liquid are conducted using test cell. The test cell includes Electrodes made of spheres of 0.5 to 1 cm diameter (up to 100 kV). Electrodes separation, surface smoothness & present of oxide films are critical in measurement of the test.

Breakdown in Pure Liquids

• Low field (< 1 kV/cm) : conductivities of 10^-18 – 10^-20 obtained due to impurities remaining after purification (ions dissociation). 
•  Intermediate field : current reaches saturation value
• High field ( >100 kV/cm) : current increase rapidly nearer to breakdown (field-aided electron emission)

A partial discharge, or PD, is a localized dielectric breakdown of a small portion of the electrical insulation. PD can be initiated by voids, cracks, or inclusions within a solid dielectric, at interfaces within solid or liquid dielectrics, in bubbles within liquid dielectrics, or along the boundary between different insulation materials. (Source OMICRONenergy)

Conduction and Breakdown in Commercial Liquid

1. Not chemically pure & have impurities
2. Breakdown mechanisms depend on nature & condition of electrodes, liquid properties, present of gases and impurities
3. 4 mechanisms – Suspended Particle, Cavitation & Bubble, Themal Mechanism & Stressed Oil Volume Mechanism.

Suspended Particle Mechanism

• Impurities present as fibres or dispersed solid particles 
• Electrostatic force acting on impurities
• Solid impurities – force directed towards maximum stress
• Gas impurities – force directed towards areas of lower stress
• Form a stable chain bridging the gap.

Cavitation & Bubble Mechanism

The applied hydrostatic pressure determine the breakdown strength. Formation of vapour bubble responsible for breakdown are due to;

• Gas pockets at electrodes surface
• Electrostatic repulsive forces
• Gases products by electron collision
• Vapourization of liquid by corona at sharp points and surface irregularities

Thermal Mechanism

Can be describe as breakdown under pulse condition. High density current pulses give rise to localised heating and formed bubbles (eg. Kettle). Breakdown occurs due to elongation of bubbles to critical size and bridge the gap. The breakdown strength depends on pressure and liquid molecular structure of material.

Stressed Oil Volume Mechanism

Breakdown strength is determined by largest possible impurity or weak link. Breakdown strength is inversely proportional to the stressed oil volume (reduce breakdown). Breakdown voltage influenced by gas content in the oil, viscocity and the presence of impurities