Breakdown Mechanism of Solid Material: Understanding Degradation Processes and Factors

Prof. Mithun Mondal BITS Pilani, Hyderabad Campus
SECTION 01

Introduction to Solid Dielectrics

Introduction to Solid Dielectrics

  • Solid insulating materials are integral in electrical equipment (e.g., transformers, generators, circuit breakers, cables).

  • Primary Functions:

    • Provide electrical insulation between live parts and grounded structures

    • Offer mechanical support and structural integrity

    • Environmental protection

  • Used in combination with liquid and gaseous insulations in composite systems.

  • Common Materials: Ceramics, polymers, glass, mica, paper, rubber

Critical Breakdown Characteristic

Unlike gases (fast recovery) and liquids (partial recovery), solids suffer permanent damage and lose dielectric strength completely upon breakdown.

Breakdown Voltage vs Time Relationship

  • Breakdown depends on both voltage magnitude and application time

  • Empirical relation:

    \[V_b \ln t_b = \text{constant}\]

  • Longer application times require lower voltages for breakdown

  • Important for insulation design and testing protocols

Breakdown voltage vs time relationship
Breakdown voltage vs time relationship
Factors Affecting Dielectric Strength

Environmental Factors:

  • Ambient temperature (\(\mathrm{T}~ \uparrow~\to~ \mathrm{strength}~\downarrow\) )

  • Humidity and moisture content

  • Atmospheric pressure

Material Factors:

  • Impurities and structural defects

  • Material thickness and geometry

  • Age and degradation history

Mechanism of solid breakdown
Mechanism of solid breakdown

Electrical Factors:

  • Type of voltage (AC, DC, impulse)

  • Frequency of applied voltage

  • Duration of voltage application

  • Electrode configuration and pressure

Classification of Breakdown Mechanisms

Five Primary Breakdown Mechanisms

  1. Intrinsic Breakdown - Electronic avalanche in pure materials

  2. Electromechanical Breakdown - Mechanical compression failure

  3. Treeing and Tracking - Discharge in voids and surface contamination

  4. Thermal Breakdown - Heat-induced conductivity increase

  5. Electrochemical Breakdown - Chemical degradation over time

SECTION 02

Breakdown Mechanisms

Intrinsic Breakdown

Characteristics:

  • Occurs in pure, homogeneous materials

  • Very short application time (\(\sim 10^{-8}\) s)

  • Represents theoretical upper limit

  • Temperature independent (electronic process)

Energy band model
Energy band model

Mechanism:

  • Electrons gain energy from electric field

  • Cross forbidden energy gap to conduction band

  • Electron avalanche multiplication

  • Critical field: \(E_i = 10^7 - 10^8\) V/m

Intrinsic Dielectric Strength

Materials rarely achieve intrinsic strength in practice due to impurities, defects, and non-uniform fields.

Electromechanical Breakdown

Physical Mechanism:

  • Electrostatic forces compress the dielectric

  • Compressive stress: \(F = \frac{1}{2} \varepsilon_0 \varepsilon_r \frac{V^2}{d^2}\)

  • Material resists with elastic force

  • Breakdown when compression exceeds elastic limit

Mathematical Analysis:

  • Mechanical strength: \(Y \ln \frac{d_0}{d}\)

  • Equilibrium: \(\varepsilon_0 \varepsilon_r \frac{V^2}{d^2} = Y \ln \frac{d_0}{d}\)

  • Maximum compression: \(d = 0.6 d_0\)

Material compression breakdown
Material compression breakdown

Apparent Dielectric Strength

\[E_a = \frac{V}{d_0} = 0.6 \left[ \frac{Y}{\varepsilon_0 \varepsilon_r} \right]^{1/2}\]

Treeing and Tracking Breakdown

Treeing:

  • Gas-filled voids/cavities in solid

  • Field enhancement: \(E_{gas} = \varepsilon_r \times E_{solid}\)

  • Partial discharge in cavities

  • Tree-like discharge channels grow

  • Eventually bridges electrodes

Surface tracking in solid dielectrics
Surface tracking in solid dielectrics
Tracking:

  • Surface contamination (moisture, salt, dust)

  • Leakage current flows on surface

  • Carbonization creates conductive path

  • Permanent damage to surface

Impact on Outdoor Insulation

Tracking severely limits the use of organic insulation in outdoor environments due to pollution and moisture.

Thermal Breakdown - Physical Process

Heat Generation Sources:

  • Dielectric losses: \(P = \omega C V^2 \tan \delta\)

  • Conduction current: \(P = I^2 R = \sigma E^2 V\)

  • Partial discharge losses

Thermal Instability:

  • Temperature rise increases conductivity

  • Higher conductivity → more heat generation

  • Positive feedback loop leads to thermal runaway

  • Critical when: Heat Generated > Heat Dissipated

Temperature vs time
Temperature vs time
Thermal circuit
Thermal circuit

Heat Flow Equation

\[C_V \frac{\partial T}{\partial t} + \nabla \cdot (K \nabla T) = \sigma E^2\]
where \(C_V\) = thermal capacity, \(K\) = thermal conductivity, \(\sigma\) = electrical conductivity

Thermal Breakdown: Impulse vs Steady State

Impulse Case (No Heat Dissipation):

\[C_V \frac{dT}{dt} = \sigma E^2\]

With \(\sigma = \sigma_0 e^{-\frac{u}{kT}}\) and linear field rise:

\[\frac{\sigma_0}{C_V} \frac{t_c}{E_c} \int_0^{E_c} E^2 \, dE = \int_{T_0}^{T_c} e^{\frac{u}{kT}} \, dT\]

Result: Critical field \(E_c\) independent of critical temperature.

Steady-State Case:

\[\frac{d}{dx} \left( K \frac{dT}{dx} \right) = \sigma E^2\]

For thick slab with constant surface temperature:

\[\frac{K}{\sigma_0} \int_{T_0}^{T_c} e^{\frac{u}{kT}} \, dT = \int_0^{V/2} V_x \, dV\]

Result: Minimum breakdown voltage exists.

Impulse Vs steady-state breakdown
Impulse Vs steady-state breakdown
Thermal Breakdown Voltages - Experimental Data
Thermal breakdown voltages for thick specimens at \(20^{\circ}C\)
Material DC (MV/cm) AC (MV/cm)
Ceramics
HV Steatite – 9.8
High grade porcelain – 2.8
Polymers
Ebonite – 1.45–2.75
Polythene – 3.5
Polystyrene – 5.0
Polystyrene at 1 MHz – 0.05
Acrylic resins – 0.3–1.0
Natural Materials
Mica (muscovite) 24 7–18
Rock salt 38 1.4
Crystalline
Quartz (\(\perp\) to axis) 1200 –
Quartz (\(\parallel\) to axis) 66 –
Quartz (impure) – 2.2

Key Observations:

  • DC strength generally higher than AC

  • Frequency effect significant (polystyrene example)

  • Crystal orientation matters (quartz)

  • Purity affects breakdown strength significantly

Electrochemical Breakdown

Mechanism:

  • Long-term degradation process

  • Cavities reduce local dielectric strength

  • Partial discharge creates reactive species

  • Chemical bond breaking by energetic electrons

  • Ion bombardment causes local heating

Material degradation process
Material degradation process

Chemical Effects:

  • Formation of O\(_2\), O\(_3\), NO\(_2\) in air-filled voids

  • Oxidation of organic materials

  • Polymer chain scission

  • Formation of carbonized tracks

Empirical Relation:

\[t \cdot E_b^n = \text{constant}\]
where \(n \approx 8-15\) for many materials

Testing Implications

High AC voltage testing avoided for aged equipment. Very low frequency (0.1 Hz) testing preferred to minimize electrochemical effects during testing.

SECTION 03

Practical Considerations

Breakdown Mechanism Dominance
Breakdown mechanism
Breakdown mechanism
Design Guidelines and Prevention

Material Selection:

  • Choose materials based on operating conditions

  • Consider temperature, humidity, contamination

  • Match thermal expansion coefficients

  • Ensure adequate mechanical strength

Design Practices:

  • Minimize field concentrations

  • Proper stress grading

  • Avoid sharp edges and points

  • Adequate creepage distances

  • Prevent void formation during manufacturing

Prevention Strategies:

  • Intrinsic: Use high-purity materials, minimize defects

  • Electromechanical: Control electrode pressure, use flexible materials

  • Treeing: Vacuum impregnation, void-free processing

  • Thermal: Improve heat dissipation, temperature monitoring

  • Electrochemical: Moisture control, protective coatings

Design comparison for electric field analysis
Design comparison for electric field analysis
Testing Standards and Procedures

Standard Test Methods:

  • AC Tests: 50/60 Hz, power frequency

  • DC Tests: For DC applications, slower stress

  • Impulse Tests: Lightning and switching surges

  • Step-stress Tests: Progressive voltage increase

  • Long-term Tests: Life assessment (1000+ hours)

Dielectric breakdown test circuit
Dielectric breakdown test circuit

Test Considerations:

  • Temperature control (±2°C)

  • Humidity conditioning

  • Electrode configuration standardization

  • Sample preparation (thickness, surface finish)

  • Statistical analysis (Weibull distribution)

Key Standards

IEC 60243: Electric strength of insulating materials
ASTM D149: Dielectric breakdown voltage and strength
IEEE Std 4: High-voltage testing techniques

Modern Developments and Challenges

Emerging Materials:

  • Nanocomposites (enhanced properties)

  • Bio-based insulation materials

  • High-temperature ceramics

  • Aerogel insulators

  • Functionally graded materials

Current Challenges:

  • Renewable energy applications

  • HVDC transmission requirements

  • Miniaturization demands

  • Environmental regulations

  • Recycling and sustainability

Advanced Diagnostics:

  • Partial discharge monitoring

  • Dielectric spectroscopy

  • Thermography

  • Acoustic emission

  • AI-based condition assessment

SECTION 04

Summary

Summary and Key Takeaways

Five Breakdown Mechanisms

  1. Intrinsic: Electronic avalanche in pure materials (ns timescale)

  2. Electromechanical: Mechanical failure under electrostatic stress

  3. Treeing/Tracking: Discharge in voids and surface contamination

  4. Thermal: Heat-induced conductivity increase and runaway

  5. Electrochemical: Long-term chemical degradation (years)

Critical Design Factors:

  • Material purity and processing

  • Field distribution uniformity

  • Temperature and humidity control

  • Mechanical stress management

  • Long-term aging considerations

Engineering Applications:

  • Power cable insulation

  • Transformer bushings

  • Circuit breaker chambers

  • Capacitor dielectrics

  • Electronic device encapsulation