Breakdown in solid Dielectrics• Solid dielectric material are used in all kinds of

electrical apparatus and devices to insulate one current carrying part from another when they operate at different voltages.

• A good dielectrics should have low dielectric loss, high mechanical strength, should be free from gaseous inclusions and moisture and be resistant to thermal and chemical deterioration.

• Solid dielectrics have higher breakdown strength compared to liquids and gases.

Breakdown in solid DielectricsSolid insulating material

Organic materials such as paper, wood & rubber Inorganic materials such as

mica, glass & porcelain ,synthetic polymers such as PVC, epoxy resins

Breakdown in solid Dielectrics• When breakdown occurs , solids get

permanently damaged while gases fully and liquids partly recover their dielectric strength after the applied electric field is removed.

• The various breakdown mechanism can be classified as follows.

a) Intrinsic or ionic breakdown b) Electromechanical breakdown

c) failure due to treeing and trackingd) Thermal breakdown

e) Electrochemical breakdown f) Breakdown due to internal discharges.

Breakdown in solid Dielectrics• Breakdown Vs time characteristics for

various mechanism

Intrinsic Breakdown• When voltage are applied only for short durations ,

intrinsic breakdown takes place.• The maximum electrical strength is 15MV/cm for

polyvinyl-alcohol at -196ºC.• Intrinsic breakdown depends upon the presence of free

electrons which are capable of migration through the lattice of the dielectric

• Usually , a small no. of conduction electrons are present in solid dielectric, these electrons gets multiplied with the application of high field.

• Based on above phenomena, intrinsic mechanism are classified as

(i) Electronic Breakdown(ii) Avalanche or streamer breakdown

Electronic Breakdown• Intrinsic breakdown occurs in time of the order of

10-8 sec and therefore is assumed to be electronic in nature.

• The initial density of conduction (free) electrons is also assumed to be large and electron-electron collisions occur.

• When an electric field is applied , electrons gain energy from the electric field and cross the forbidden energy gap from the valence to conduction band.

• When this process is repeated, more & more electrons are available in the conduction band, eventually leading to breakdown.

Avalanche or Streamer Breakdown• This is the breakdown due to cumulative

ionization.• An electron within the dielectric drift towards the

anode from cathode & during this process electron gains energy from the field & loses it during collisions.

• When the energy gained by an electron exceeds the lattice ionization potential , an additional electron will be liberated due to collision of the first electron

• This process repeats resulting in the formation of an electron avalanche.

• Breakdown will occur when the avalanche critical size.

Electromechanical Breakdown• When solid dielectrics are subjected to high electric

fields, failure occurs due to electrostatic compressive forces strength.

• If the thickness of the specimen is do and is compressed to a thickness d under an applied voltage V, then the electrically developed compressive stress is in equilibrium

This is the breakdown voltage

This equation is approx. as Y depends on mechanical stress

Electromechanical Breakdown

• When the material is subjected to high stress the theory of elasticity does not hold good and plastic deformation has to be considered.

• Recently , this theory has been modified based on the concept of fracture mechanics.

• In this mechanism, filamentary shaped cracks propagate through the dielectric material releasing both the electrostatic energy and the electromechanical strain energy stored in the material due to the applied electric field.

Thermal Breakdown• When an electric field is applied to a dielectric

conduction current (however small it may be), flows through the material.

• The current heats up the specimen and temperature rises.

• Heat generated is transformed to the surrounding medium by conduction through the solid dielectric and by radiation from its outer surfaces.

• Equilibrium is reached when the heat used to raise the temperature of the dielectric and heat radiated out equals heat generated.

Wdc or Wac = WT Heat dissipated

Breakdown occurs when Wdc or Wac > WT

Thermal Breakdown• Heat generated under dc stress E is given as

Wdc = E2 σ W/Cm3

where σ is the conductivity of the specimen

• Under ac fields, the heat generated

Wac =E2 f.εr tanδ/1.8*1012

Where f= frequency in Hz

δ=loss angle of the dielectric

material

E= rms value

Thermal Breakdown• The heat dissipated (WT) is given by

WT =Cv dT/dt + div (k gradT)

where Cv= specific heat of the specimen

T= temperature of the specimen

k= thermal conductivity of the specimen

t= time over which the heat is dissipated

Thermal Breakdown• Equilibrium is reached when the heat generated (Wdc or Wac)

becomes equal to the heat dissipated (WT)

• In actual practice there s always some heat that is radiated out.

• Breakdown occurs when Wdc or Wac exceeds WT

Heat lost is shown by straight line

E1 & E2 for two different Electric field

Temperature at which breakdown occurs

Example A solid specimen of dielectric has a

dielectric constant of 4.2 and tanδ =0.01 at a frequency of 50Hz. If it is subjected to an alternating field of 50KV/cm, calculate the heat generated in the specimen due to the dielectric loss

Use this formula and calculate the result

Wac =E2 f.εr tanδ/1.8*1012

= 50*50*106 * 50*4.2*0.01/1.8*1012

= 0.291 mW/cm3