Question

Explain the reasons for failure of gaseous and solid dielectric materials used in electrical engineering application.

Answer 1

Introduction to Dielectric Failure

A dielectric material is an electrical insulator that can be polarized by an applied electric field. Its primary function in electrical engineering is to prevent the flow of current between conductors at different potentials.

Dielectric Failure (or Breakdown) occurs when a dielectric material, under the stress of a high electric field, loses its insulating properties and becomes a conductor. This results in a sudden, large current flow, often in the form of a spark or arc, which can lead to equipment damage, power outages, and safety hazards.

The mechanisms of failure are fundamentally different for gaseous, liquid, and solid dielectrics due to their distinct molecular structures.

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Part 1: Failure of Gaseous Dielectrics

Gaseous dielectrics are the most common insulators. Air is used universally, while others like Sulfur Hexafluoride ($SF_6$) and Nitrogen ($N_2$) are used in high-voltage equipment like switchgear and gas-insulated substations.

The failure of a gas is a reversible process. Once the high electric field is removed, the gas typically regains its insulating properties (unless the resulting arc has damaged the surrounding equipment). The primary failure mechanism is electrical breakdown, which can be explained by two main theories.

1. Townsend's Theory of Breakdown (Electron Avalanche)

This theory explains breakdown in gases at relatively low pressures or small gap distances. The process is as follows:

• Initial Electron: A free electron must exist in the gap, typically created by cosmic radiation or photoemission from the cathode.
• Acceleration: This free electron is accelerated by the applied electric field, gaining kinetic energy.
• Ionization by Collision (Primary Ionization): If the electron gains enough energy before colliding with a neutral gas molecule, it can knock an electron out of that molecule upon impact. This creates a new free electron and a positive ion.
• Electron Avalanche: The original electron and the newly freed electron are both accelerated, leading to more collisions and more ionizations. This chain reaction creates an "electron avalanche" that moves towards the anode.
• Secondary Ionization: For a full breakdown (a self-sustaining discharge), the avalanche needs to trigger new avalanches. This is achieved by secondary processes:
  • Positive Ion Bombardment: The positive ions created in the avalanche drift slowly to the cathode. Their impact can release more electrons from the cathode surface.
  • Photo-emission: Excited atoms in the avalanche emit photons, which can strike the cathode and release electrons.
  • Metastable Atoms: Excited, long-lived atoms can diffuse to the cathode and release electrons.

Failure Condition (Townsend's Criterion): Breakdown occurs when the rate of secondary electron creation is high enough to create a new avalanche for every initial avalanche, leading to a self-sustaining current.

2. Streamer Theory of Breakdown (Kanal and Streamer Mechanism)

Townsend's theory is too slow to explain the very rapid breakdown observed in gases at high pressure (like air at atmospheric pressure) or across long gaps. The Streamer Theory provides the explanation:

• Initial Avalanche: The process starts with an electron avalanche, as in Townsend's theory.
• Space Charge Formation: As the avalanche moves towards the anode, the fast-moving electrons leave a trail of slow-moving positive ions behind them. This concentration of positive charge at the head of the avalanche creates a strong local electric field.
• Photoionization: This intense local field is strong enough to cause photoionization in the gas surrounding the avalanche head, creating new electron-ion pairs.
• Streamer Formation: The newly created electrons are drawn towards the positive avalanche head, forming secondary avalanches that merge with the primary one. This creates a highly conductive plasma channel called a "streamer".
• Bridging the Gap: The streamer propagates rapidly towards both the anode and cathode, effectively bridging the gap and creating a low-resistance path for a spark or arc. This process is much faster than relying on ions to drift to the cathode.

Key Factors Influencing Gaseous Breakdown (Paschen's Law)

Paschen's Law states that the breakdown voltage ($V_b$) of a uniform field gap in a gas is a function of the product of the gas pressure ($p$) and the gap distance ($d$).
$V_b = f(p \cdot d)$

• High p·d (High Pressure / Long Gap): An electron undergoes many collisions. It's difficult for it to gain enough energy between collisions to cause ionization. A higher voltage is needed to accelerate it sufficiently.
• Low p·d (Low Pressure / Short Gap): An electron may cross the entire gap without making enough collisions to start a significant avalanche. The probability of ionization is low. A higher voltage is needed to increase the ionization efficiency.
• Paschen Minimum: There is an optimal p·d value where the breakdown voltage is at a minimum, as it represents the most favorable condition for an electron avalanche.

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Part 2: Failure of Solid Dielectrics

Solid dielectrics are used in cables (XLPE, PVC), capacitors, bushings (porcelain, glass), and as structural insulators. Unlike gases, breakdown in solids is a permanent and destructive process, creating an irreversible conductive path. The mechanisms are more complex and varied.

1. Intrinsic or Electronic Breakdown

This is the "purest" form of breakdown, based only on the material's properties. It occurs at extremely high electric fields ($>10^6$ V/cm) and is very rapid ($~10^{-8}$ s).
Mechanism: The electric field is so strong that it directly imparts enough energy to electrons in the valence band to pull them into the conduction band. These electrons are then accelerated, causing an electron avalanche similar to that in gases, but within the solid's crystal lattice.
Practicality: This type of breakdown is rare in practice because other failure mechanisms occur at much lower electric field strengths. It represents the theoretical maximum dielectric strength of a material.

2. Thermal Breakdown

This is a common failure mechanism, especially in applications with AC fields or high ambient temperatures.
* Mechanism:

1.  All dielectrics have small but non-zero conductivity and dielectric losses, which cause a tiny current to flow.
2.  This current generates heat within the material ($P = \sigma E^2$, where $\sigma$ is conductivity and $E$ is the electric field).
3.  If the **rate of heat generation exceeds the rate of heat dissipation** to the surroundings, the temperature of the material begins to rise.
4.  For most dielectrics, conductivity increases with temperature. This creates a positive feedback loop:
    *   Higher Temperature → Higher Conductivity → More Current → More Heat Generation → Higher Temperature...
5.  This process, known as **thermal runaway**, continues until the material melts, chars, or chemically decomposes, creating a permanent, low-resistance conductive path.

3. Electrochemical Breakdown (Chemical Degradation)

This is a slow, long-term degradation process caused by chemical reactions within the dielectric, accelerated by the electric field.
* Causes:

*   **Oxidation:** Oxygen or ozone (often created by partial discharges) attacks the molecular structure of the material, making it brittle and weak.
*   **Hydrolysis:** The presence of moisture can cause chemical reactions with the dielectric material (especially polymers), breaking down the molecular chains.

• Tracking and Treeing: This is a critical form of electrochemical breakdown.
  • Tracking: The formation of a conductive carbonized path on the surface of an insulator, usually due to the combined effect of moisture and contamination.
  • Treeing: The formation of fine, tree-like erosion channels within the bulk of the solid dielectric. It starts at a point of high electrical stress, such as a void, impurity, or sharp conductor point. Partial discharges inside these voids degrade the material, causing the channels to grow slowly over time until they bridge the conductors, leading to complete failure. This is a major cause of failure in high-voltage polymer-insulated cables.

4. Breakdown due to Internal Discharges (Partial Discharges)

This is one of the most significant causes of long-term failure in solid dielectrics.
Mechanism: Manufacturing processes can leave tiny gas-filled voids or cavities within the solid insulation.
Field Concentration: The electric field inside the gas-filled void is much higher than in the surrounding solid dielectric (because the permittivity of gas is much lower).
Internal Breakdown: This concentrated field causes the gas inside the void to break down (a "partial discharge" or mini-spark) long before the solid material itself is stressed to its breakdown limit.
Degradation: These repeated discharges act like tiny hammers, bombarding the walls of the cavity with high-energy electrons and ions. This causes:

*   Localized erosion and pitting.
*   Chemical degradation from byproducts like ozone and nitric acid.
*   Localized heating.

• This process gradually enlarges the void and initiates electrical treeing, which eventually leads to complete failure.

Summary of Failure Reasons

| Dielectric Type | Primary Failure Mechanisms | Key Characteristics |
| :-------------- | :-------------------------------------------------------------------------------------------------------------------- | :-------------------------------------------------------------------------------------------------------------- |
| Gaseous | 1. Townsend Avalanche (low pressure/short gap)
2. Streamer Mechanism (high pressure/long gap) | - Reversible (regains insulating properties)
- Failure is a function of pressure, distance, and gas type (Paschen's Law) |
| Solid | 1. Thermal Breakdown (heat runaway)
2. Electrochemical Breakdown (treeing, tracking)
3. Internal Discharges (voids)
4. Intrinsic Breakdown (theoretical limit) | - Permanent and Destructive
- Often a slow, aging-related process
- Highly sensitive to impurities, voids, and moisture |