JNTU B.Tech IV HIGH VOLTAGE ENGINEERING Previous Question Paper (September - 2022 - R16)

Controlling electric field intensity is critical in high voltage equipment to prevent dielectric breakdown and ensure long-term insulation reliability. The primary objective is to maintain the maximum electric stress below the dielectric strength of the insulating material by managing the distribution of potential and field lines.

The following methods are commonly employed to control and reduce electric field intensity:

• Geometric Shaping (Grading of Electrodes): The electric field intensity is highest at sharp edges or points due to the concentration of flux lines. By rounding off edges, increasing the radius of curvature, or using spherical or toroidal electrodes, the field is spread over a larger area, significantly reducing the local stress.
• Potential Grading: In multi-component insulation systems, such as bushings or cables, the potential is distributed non-uniformly. By using conducting layers or grading shields, the potential distribution is forced to be linear or controlled, preventing localized high-stress regions.
• Capacitive Grading: This involves inserting metallic foils within the insulation (e.g., in condenser-type bushings) to create a series of capacitors. This ensures a uniform voltage distribution across the radial thickness of the insulation, preventing stress concentration near the central conductor.
• Resistive Grading: For insulators exposed to pollution or high humidity, resistive coatings or grading rings are used to maintain a uniform potential distribution along the surface of the insulator, preventing surface flashover.
• Dielectric Constant (Permittivity) Grading: By using layers of materials with different dielectric constants, the electric field can be redistributed. According to the principle of continuity of electric displacement, the stress is inversely proportional to the permittivity of the material. Placing materials with higher permittivity in regions of higher stress helps in reducing the stress in those areas.
• Use of Field Deflectors and Grading Rings: Grading rings are placed at the high-voltage terminals of insulators or transformers to reduce the potential gradient at the ends. These rings act as equipotential surfaces that redistribute the electric field lines away from the critical insulation interfaces.
• Insulation Coordination: This involves selecting the insulation levels of various components such that the equipment can withstand expected transient overvoltages without failure, ensuring that any necessary discharge occurs at a designated protective device rather than within the equipment itself.

By applying these techniques, the peak electric field intensity is minimized, thereby reducing the probability of partial discharges and ensuring the integrity of the high voltage insulation system.

Insulating materials in high-voltage equipment must possess high dielectric strength, low dielectric loss, and excellent thermal stability. The selection depends on the specific requirements of circuit breakers and power capacitors.

Insulating Materials in Circuit Breakers:

1. Sulfur Hexafluoride (SF6) Gas: An electronegative gas with high dielectric strength and excellent arc-quenching capabilities. It is chemically inert, non-toxic, and non-flammable, making it ideal for high-voltage gas-insulated switchgear.
2. Transformer Oil: Used in bulk oil circuit breakers, it acts as both an insulator and an arc-quenching medium. It provides high dielectric strength and helps in heat dissipation through convection.
3. Vacuum: In vacuum circuit breakers, the high dielectric strength of the vacuum gap is utilized to quench arcs. It is highly effective for medium-voltage applications due to the rapid recovery of dielectric strength after current zero.
4. Solid Insulators: Materials like epoxy resins, porcelain, and glass are used for bushings, support insulators, and operating rods to provide mechanical strength and electrical isolation.

Insulating Materials in Power Capacitors:

1. Impregnated Paper: Traditionally, high-quality kraft paper is used as the dielectric. It is impregnated with synthetic fluids to eliminate air voids, which prevents partial discharge and enhances the breakdown voltage.
2. Polypropylene Film: Modern power capacitors often use biaxially oriented polypropylene (BOPP) film. It offers extremely low dielectric loss (tan δ) and high dielectric strength, significantly reducing the physical size of the capacitor.
3. Synthetic Impregnants: Non-PCB fluids like phenyl xylyl ethane (PXE) or monoisopropyl biphenyl (MIPB) are used to impregnate the film/paper dielectric. These fluids are chosen for their high compatibility with plastic films and excellent chemical stability under high electric stress.
4. Ceramic Dielectrics: Used in high-frequency or specialized power capacitors, ceramic materials (like titanium dioxide or barium titanate) provide stable capacitance values and high thermal resistance.