EHV AC transmission IMPORTANT QUESTIONS

EHV AC transmission IMPORTANT QUESTIONS

Ql) (a) List levels of transmission voltages which are used all over the world.

What levels of above voltages are regarded as EHV AC transmission-Voltages-discuss.

(b) Give configuration of EHV- towers and discuss one of the most popular tower used in EHV AC transmission.

Q2) (a) Discuss temperature rise of conductors and current carrying capacity in EHV AC system.

(b) Discuss how will you select appropriate cable for EHV AC transmission system.

Q3) (a) Define voltage gradient of conductors in an EHV AC system. How is it possible to calculate for maximum surface voltage gradient - discuss.

(b) A 735 kV line has the following parameters :

N = 4, r = 0.0176m, distance B = 0.4570m for the bundled conductor of each phase. The line height and phase spacing in horizontal configuration are H = 15m & S = 15m. Calculate the maximum surface voltage gradients on the centre phase and the outer phases.

Q4) (a) What are Corona-Loss formulae? Write expressions for Corona-Loss relations based on voltages.

(b) For the conductor radius (r) = 1 cm, height (H) = 5m, frequency (f) = 50 Hz, calculate Corona Loss (Pc) for E = 1.1 Eo and air density (5) =1. Also derive the formula used for PC.

Q5) (a) What are audible noise (AN) and radio interference (RI)? Indicate their limits in EHV AC lines.

(b) What is meant by Day-Night equivalent noise level? The L₅₀ level of a line is 55 dB (A). The day-night hours are 15 and night time is 9 hours in duration. Calculate the day-night equivalent and decibel adder (dB(A)) to the day time AN-level.

Q6) Discuss electrostatic field of EHV AC lines. Also discuss the effect of high electrostatic fields on human beings under a line. Why does a normal human being not experience a shock when walking underneath an EHV - line? Why do birds survive even though they come into contact with EHV - transmission lines?

Q 7) (a) What is lightning strokes to EHV lines? Also elaborate Lightning - stroke mechanism.

(b) Discuss general principles of the Lightning protection problem.

Q8) (a) What is meant by compensation of a long EHV - line? What are different methods of compensation - discuss.

(b) Consider the simple two-machine transmission model in which VAR -compensator is shunt connected at the midpoint of the line. Show that relationship between Active power (P), Reactive power (Q) and Load angle of the generating unit (5) for ideal shunt compensation are

V² . 5 P 2 sinX 2VY Si1 - cosQ-42X\ /Where symbols used represent usual meanings.

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