Breakdown Mechanisms
- Short term breakdown
- Long term breakdown
Short-Term Breakdown
- If electric field stresses are very high, failure occur in seconds or even faster without any damage to the insulating surface prior to breakdown.
- When applied voltage close to breakdown value, breakdown occurs in one or more discharges.
- When stress exceeds the critical stress at which discharge enters or penetrates from surface and propagates rapidly into volume, breakdown occurs.
- Breakdown can occur after single discharge and more rapidly when electric field in insulation assists the charged particles in the discharge to penetrate into the insulation.
- Breakdown occurs more rapidly when the bombarding particles are electrons, rather than positive ions.
- Under high field conditions local field intensifies due to presence of impurities and variations in thickness plays a very important role; effect depends on the field in insulation before the discharge impinges on it.
Long-Term Breakdown
- Long term breakdown is called ageing of insulation.
The main causes are
- Partial discharge
- Charge accumulation
- Conduction on the surface
Ageing of Insulation Due to Partial Discharge
Process of partial discharge and its effect :
- Partial discharge involves the transfer of electric charge between the two points. The charge on di-electric surface produces a deterioration of insulating properties and depends on the geometry of cavity and nature of dielectric.
- The decrease of ageing depends upon
- Inception without Vi
- Discharge magnitude
Vi depends on :
- Permittivity of dielectrice ∈r
- Thickness of coating
Assumption :
1. Cavity stress Eg = ∈r . E
E = Electric field across dielectric
2. With in cavity stress Eg max / Ey =1
With the assumptions Duster’s law can be applied for small comities Vi; decreases or cavity depth increases following Paschen’s curve of gas breakdown.
- Inspite of corrosion of cavity walls, breakdown will not occur and the life of insulation will be long.
- For applied voltage greater than 2Vi erosion in footer and ageing of insulation to quicker.
- Total capacitance of cavity is not discharged as a single event but as a result of many discharges. Each discharge involving a small area of the cavity wall determined by the conductivity of cavity surface in region of discharge.