High voltage transmission lines use _________.

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Suspension insulators Explanation: Suspension insulators are exclusively used for high voltage (HV) transmission lines, typically above 33 kV, because they provide a flexible, modular design that is both economical and mechanically robust. By connecting multiple insulator discs in a series string, the insulation level can be easily increased to match higher voltage requirements. Working Principle: The suspension arrangement allows the conductor to hang below the cross-arm, which helps in improving clearance and reducing the stress on the tower structure. The total voltage across the string is shared by individual discs; however, due to shunt capacitance to the ground, the voltage distribution across the discs is non-uniform, usually resulting in the highest voltage stress across the disc closest to the conductor. Key Formulas: $V_{string} = V_1 + V_2 + ... + V_n$ — Total voltage across an n-disc string; $String Efficiency = \frac{Total Voltage}{n imes Voltage across unit nearest to conductor} imes 100$ Key Points: Used for voltages above 33 kV.; Modular design allows replacement of damaged individual units.; Mechanical strength increases as the number of strings is increased.; Higher factor of safety compared to pin-type insulators. Advantages: Flexible design allowing easy voltage rating adjustments.; Lower cost per unit for high voltages.; Damaged units can be replaced easily.; Mechanical stress on the cross-arm is directed downwards (compressive). Disadvantages: Non-uniform voltage distribution across the string.; Requires more vertical clearance space.; Susceptible to vibration and string swinging under high wind. Applications: Extra High Voltage (EHV) transmission lines.; Ultra High Voltage (UHV) transmission lines.; Long-span river crossings. Additional Info: Pin insulators are unsuitable for HV lines because the size increases exponentially, making them heavy and mechanically weak.. String efficiency is typically less than 100% in suspension strings, often requiring guard rings to improve the voltage distribution. Answer: B is correct — Suspension insulators are used for high voltage transmission lines due to their modular, economical, and mechanically stable configuration. Exam Tip: Remember that for voltages exceeding 33 kV, the mechanical and electrical constraints make Pin insulators economically unviable, making Suspension insulators the standard choice in power systems.

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