State failure phenomena observed in insulating material. State four reasons for failure of gaseous and solid dielectric materials.
1 Answer
Failure Phenomena in Insulating Materials
Insulating materials (dielectrics) are designed to prevent the flow of electric current. Failure occurs when the material loses its insulating properties and allows a current to pass through it, a process known as dielectric breakdown. The key failure phenomena are:
Electrical Breakdown: This is a rapid, catastrophic failure that occurs when the applied electric field exceeds the material's dielectric strength. The high field strips electrons from their atoms, creating a conductive path (an arc or spark) through the material. This is often an irreversible process, especially in solids.
Thermal Breakdown: Insulation failure is caused by excessive heat. The heat can be generated by the current in the conductor ($I^2R$ losses), dielectric losses within the insulation itself, or from the external environment. As the temperature rises, the material's chemical structure degrades, it may melt or char, and its insulating capability drops, leading to a runaway effect and eventual electrical breakdown.
Chemical and Electrochemical Degradation: This is a slow, aging-related failure caused by chemical reactions. Common causes include:
Oxidation: Reaction with oxygen, often accelerated by heat and UV light, making the material brittle.
Hydrolysis: Reaction with water/moisture, which can degrade the material and lower its resistivity.
* Contamination: Contact with oils, acids, solvents, or other corrosive substances that attack the insulating material.Mechanical Failure: The insulation fails due to physical stress or damage, which then creates a path for electrical breakdown. This includes:
Cracking due to vibration, thermal expansion/contraction, or aging.
Abrasion or wear from rubbing against other surfaces.
* Puncturing or cutting during installation or maintenance.Tracking and Erosion: This is a surface breakdown phenomenon. Contamination (like dust, salt, and moisture) on the insulator's surface creates a conductive path. Small leakage currents flow through this path, causing localized heating that dries it out. This process creates small sparks that carbonize the insulator's surface, forming a conductive carbon track. Over time, this track can grow (tracking) and eventually bridge the electrodes, causing a flashover. The physical damage caused by the discharges is called erosion.
Four Reasons for Failure of Gaseous Dielectric Materials
Gaseous dielectrics, like air or sulfur hexafluoride (SF₆), fail when a conductive plasma channel forms between electrodes.
High Electric Field Strength (Overvoltage): If the voltage applied across the gas exceeds its dielectric strength, a process of electron avalanche begins. A free electron is accelerated by the field, gains enough energy to ionize a gas molecule upon collision, releasing more electrons. This cascade effect (described by Townsend or Streamer theory) rapidly leads to a conductive arc. This is the primary cause of breakdown from lightning strikes or switching surges.
Presence of Impurities and Contamination: Foreign particles (like dust, metal filings, or fibers) in the gas distort the electric field, causing high local field concentrations. These points of high stress can initiate an electron avalanche at a much lower overall voltage than in a pure gas, leading to premature breakdown. Moisture is also a critical contaminant that reduces the dielectric strength of most gases.
Adverse Pressure and Temperature: The dielectric strength of a gas is highly dependent on its density (and therefore its pressure and temperature). According to Paschen's Law, at very low pressures, the breakdown voltage is high because there are few molecules to ionize. As pressure increases, breakdown voltage drops to a minimum before rising again. Operating a gas-insulated system outside its designed pressure/temperature range can significantly lower its breakdown voltage and lead to failure.
Ionization by External Sources: Radiation (such as UV light, X-rays, or cosmic rays) can pre-ionize the gas by creating initial free electrons. While normally insignificant, in a system already stressed close to its breakdown voltage, this external ionization can provide the "seed" electrons needed to trigger a complete breakdown avalanche.
Four Reasons for Failure of Solid Dielectric Materials
Solid dielectrics, like polymers (XLPE), rubber, porcelain, and glass, are prone to both rapid and long-term degradation.
Partial Discharges (PD) and Electrical Treeing: Many solid insulators contain microscopic voids or cavities filled with gas. The gas inside the void has a much lower dielectric strength than the surrounding solid material. The electric field can be high enough to cause repeated small sparks or partial discharges within these voids. These discharges slowly erode the solid material, creating fine, branching channels called "trees." Over time, these trees grow through the insulation and eventually bridge the electrodes, causing a complete and catastrophic failure.
Thermal Stress and Overheating: Continuous operation at high temperatures (due to current overload, poor ventilation, or high ambient heat) causes the solid material to degrade. Polymers can soften, melt, or become brittle and crack. This thermal aging permanently reduces the material's dielectric and mechanical strength, making it highly susceptible to electrical breakdown.
Moisture Ingress and Chemical Contamination: Most polymeric insulating materials are susceptible to moisture. Water absorption lowers the insulation resistance, increases dielectric losses (which generates more heat), and can facilitate chemical degradation (hydrolysis). This process, sometimes called "water treeing," creates diffuse, water-filled channels that degrade the insulation and can lead to electrical treeing and eventual failure. Contamination by chemicals like oils or solvents can also dissolve or weaken the material.
Mechanical Stress and Physical Damage: Physical damage is a common cause of failure. A sharp bend in a cable can create micro-cracks. A nick or cut from a tool during installation creates a point of high electrical stress. Constant vibration can cause fatigue and cracking. These mechanical defects create weak points in the insulation, concentrating the electric field and providing an easy path for breakdown to occur.