BREAKDOWN IN GASES

BREAKDOWN IN GASES

• Electrons released at cathode = n₀
• _{Electrons at distance x = n(x)}
• _{No. of electrons at distance dx = n(x)adx}
• _{dn(x) = dn+ = dn- = n(x)adx}
• _{dn(x) / n(x)= adx}
• _{Integrate both sides, n(x) = n0exp(ax)}
• _{At anode, n(d) = n0exp(ad)}
•  _{At steady state, I = I0exp(ad)} 
• _{I – average current in gap}
• _{Io – initial current at cathode}
• _{a - 1^st Townsend   Ionisation coef.}
• _{d – gap distance}

CURRENT-VOLTAGE CHARACTERISTIC

Plot of ln(I/I₀) as a function of d will give the value of a

I/I₀ = exp(ad)

TOWNSEND’S SECONDARY COEFFICIENT( g )

• Discharge is not self-sustaining with a alone
• Current falls to zero if initial electrons source is removed
• Additional current produced by secondary emission processes
• g is the no. of secondary electrons produced at cathode per electron produced in the gap
• Causes of secondary electrons;
• +ve ions, g_I - release electrons on colliding with cathode surface
• Photons, g_p - gas molecules excited through collision and release electrons by photoemission
• Metastables, g_m – diffuse to cathode & release electrons
• g = g_I  + g_p + g_m

• Effect of g-process on current;
  
  
  
  
  

• Breakdown Criteria;
            1 - g[exp(ad) – 1] = 0
                          gexp(ad) = 1

• Effect of attachment coefficient, h on current;
• Without secondary effects, g = 0

• With secondary effect, g ¹ 0

• Thus, breakdown criteria given by;

EVALUATION OF h

• Region A1 – effect of h/(a - h)
• Region A2 – straight line slope
• Region A3 – secondary processes are significant

As pd increases, E_s/p decreases

PASCHEN'S LAW

Vs = y(pd)

• Neglecting attachment, breakdown criterion is g(e^ad – 1) = 1
• Since a/p = f(E/p) and g = g (E/p)
• Then at breakdown;
• ad_s = pd_s.f(E_s/p) = pd_s. f(V_s/pd)
• \ g(V_s/pd_s)[exp{pd_s.f(V_s/pd_s)} – 1] = 1
• pd_s is constant, V_s is fixed

TEMPORAL GROWTH STUDIES

• Statistical time lag, t_s – prior to the appearance of electron to initiate primary avalancHe
• Formative time lag, t_f – current build up by secondary process

STREAMER BREAKDOWN

• To explain the observation of formative time lags of ≤ 50 ns
• Depends on primary avalanche reaching a critical size
• Local fields high enough to generate rapidly-moving regions or ‘streamers’ which propagate toward electrodes
• Avalanche developed at point x
• Cloud of electrons at tip and positive ions at tail
• Space charge field E_s cause field enhancement
• Electron produced behind ‘head’ generate avalanches feeding the main avalanche and caused increased ionization
• Thus, breakdown criteria given by;

BREAKDOWN IN NON-UNIFORM FIELDS

• Sphere-sphere gaps, coaxial cylinders, point-plane gaps
• Field strength is a function of position
• Breakdown criteria given by;

BREAKDOWN IN HIGHLY NON-UNIFORM FIELDS

• Strongly divergent fields such as point-plane gaps
• Streamer discharges do not lead to breakdown
• This incomplete breakdown phenomena is called ‘corona’ discharges

VOLTAGE MEASUREMENT BY SPARK GAP

Uniform-Field Gaps

• Specially profiled electrodes with flat central portion and curved outer portion
• Use at spacing up to the flat diameter
• Field does not vary more than 1% in the central uniform field region
• Electrodes overall diameter of three times the flat diameter
• Breakdown voltage is extremely consistent
• Corrected to STP with less than 1% error
• Breakdown voltage follows Paschen’s curve;  V = A(pd) + B(Öpd)
• Calibration of spark gaps in room air;

VOLTAGE MEASUREMENT BY SPHERE GAP

• Not as consistent as uniform-field gap
• Much easier to set uo and maintain in laboratories
• Most frequently used arrangement for voltage measurement
• Reference standard – BS 358 with 3% error

VOLTAGE MEASUREMENT BY ROD-ROD GAPS

• Wide scatter in breakdown voltage
• Strong polarity & proximity effects due to presence of earthed objects in laboratory
• Voltage calibration to within 8%
• Strong humidity effect
• Cheap and easy to set up
• Reference standard IEC 32 (1962)

BREAKDOWN OF HIGHLY NON-UNIFORM FIELD GAPS

Field strength in one or both electrodes is high compared to average stress in gap

Non-attaching gases: dc and ac stress - argon

Voltage-spacing curve

Effect of pressure: dc or ac stress

Surge Breakdown: effect of wave shape

Probability of breakdown voltage

Electric discharge in gases into Crookes, Geissler and Cathode Rays Tubes shows several effects:lightning, fluorescent, deflected rays by magnetic fields and so on.

Old video of a 500,000 volt high tension line switch being opened up.

BREAKDOWN IN SOLID

In solid dielectrics, highly purified and free of imperfections, the breakdown strength is high, of the order of 10 MV/cm. 

The highest breakdown strength obtained under carefully controlled conditions is known as the "intrinsic strength" of the dielectric.  Dielectrics usually fail at stresses well below the intrinsic strength due usually to one of the following causes. 

   
   (a)  electro-mechanical breakdown 
   (b)  breakdown due to internal discharges 
   (c)  surface breakdown (tracking and erosion) 
   (d)  thermal breakdown 
   (e)  chemical deterioration

• Electro-mechanical breakdown

When an electric field is applied to a dielectric between two electrodes, a mechanical force will be exerted on the dielectric due to the force of attraction between the surface charges.  This compression decreases the dielectric thickness thus increasing the effective stress.

Process of breakdown

• Breakdown due to internal discharges

Solid insulating materials sometimes contain voids or cavities in the medium or boundaries between the dielectric and the electrodes.  These voids have a dielectric constant of unity and a lower dielectric strength.  Hence the electric field strength in the voids is higher than that across the dielectric.  Thus even under normal working voltages, the field in the 

voids may exceed their breakdown value and breakdown may occur.

Equivalent circuit of dielectric with void

When the voltage Vv across the void exceeds the critical voltage Vc, a discharge is initiated and the voltage collapses.  The discharge extinguishes very rapidly (say 0.1 s).  The voltage across the void again builds up and the discharges recur.  The number and frequency of the discharges will depend on the applied voltage.

The voltage and current waveforms (exaggerated for clarity)

• Surface Breakdown

Surface flashover is a breakdown of the medium in which the solid is immersed.  The role of the solid dielectric is only to distort the field so that the electric strength of the gas is exceeded. If a piece of solid insulation is inserted in a gas so that the solid surface is perpendicular to the equipotentials at all points, then the voltage gradient is not affected by the solid insulation.  An example of this is a cylindrical insulator placed in the direction of a uniform field.  Field intensification results if solid insulation departs even in detail from the cylindrical shape.  In particular if the edges are chipped, or if the ends of the cylinder are not quite perpendicular to the axis, then an air gap exists next to the electrode, and the stress can reach up to 0r times the mean stress in the gap. [0r is the dielectric constant of the cylinder].  Discharge may therefore occur at a voltage approaching 1/0r times the breakdown voltage in the absence of the cylinder, and these discharges can precipitate a breakdown.

The three essential components of the surface flashover phenomena are;

1. The presence of a conducting film across the surface of the insulation
2. A mechanism whereby the leakage current through the conducting film is interrupted with the production of sparks
3. Degradation of the insulation must be caused by the sparks.

• Thermal Breakdown

Heat is generated continuously in electrically stressed insulation by dielectric losses, which is transferred to the surrounding medium by conduction through the solid dielectric and by radiation from its outer surfaces.  If the heat generated exceeds the heat lost to the surroundings, the temperature of the insulation increases.

Equilibrium will be reached at a temperature 1 where the heat generated is equal to the heat lost to the surroundings

Variation of heat generated by a device for 2 different applied fields and the heat lost from the device with temperature

• Chemical Deterioration

Progressive chemical degradation of insulating materials can occur in the absence of electric stress from a number of causes;

1. Chemical Instability
2. Oxidation
3. Hydrolysis
4. Other processes

Dependence of life of paper on temperature

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