JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV HIGH VOLTAGE ENGINEERING Previous Question Paper (July - 2021 - R16)

In high voltage transformers, insulation is categorized into solid, liquid, and composite systems. Mineral oil is the most common liquid dielectric, providing both insulation and cooling. Solid insulation includes cellulose-based materials like kraft paper, pressboard, and cotton tape, which are impregnated with oil to enhance dielectric strength. Synthetic materials such as epoxy resins and Nomex paper are used for high-temperature applications.

In H.V. bushings, the insulation system must withstand high electrical stress and mechanical loads. Common materials include porcelain or polymer (silicone rubber) for the external housing due to their excellent weather resistance and tracking properties. Internally, oil-impregnated paper (OIP) or resin-impregnated paper (RIP) is used in condenser-type bushings to grade the electric field, ensuring uniform stress distribution along the bushing length.

Vacuum is considered an excellent insulator because it contains no particles to facilitate charge transport. In a perfect vacuum, the absence of gas molecules prevents ionization by collision, which is the primary mechanism of breakdown in gases. However, in practical high-voltage applications, vacuum breakdown is governed by electron emission from the electrodes rather than gas ionization.

The breakdown mechanism in vacuum is explained by the following theories:

1. Particle Exchange Mechanism: Microscopic charged particles (ions or dust) are detached from one electrode and accelerated by the electric field to the opposite electrode, causing secondary emission and eventual breakdown.
2. Field Emission (Fowler-Nordheim): At high electric fields, electrons are pulled from the surface of the cathode due to field enhancement at microscopic protrusions (whiskers) on the electrode surface.
3. Clump Theory: Loosely attached particles on the electrode surface are detached by electrostatic forces, gain kinetic energy, and impact the opposite electrode, creating a localized vapor cloud that leads to a discharge.

To maintain vacuum as an effective insulator, electrodes must be polished to a high degree to minimize field enhancement, and the vacuum level must be maintained at a very low pressure (typically below 10^-6 Torr) to prevent any residual gas ionization.