Which of the following statements is correct?

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Option B: Surge impedance and characteristic impedance for a transmission line are the same

Explanation & Concepts:
Surge impedance ($Z_s$) and characteristic impedance ($Z_c$) are defined as the square root of the ratio of the series impedance ($z$) to the shunt admittance ($y$) of a transmission line. For a lossless transmission line where $R=0$ and $G=0$, the surge impedance becomes a purely resistive value independent of frequency.

Working Principle: When a surge travels along a line, the ratio of voltage to current at any point is determined by the line's distributed parameters $L$ and $C$. Since the surge propagates at a speed where the line appears as a resistive load, the impedance is expressed as $Z_s = \sqrt{\frac{L}{C}}$. For overhead lines, this value typically ranges between 400 $\Omega$ and 600 $\Omega$.

Key Formulas: $Z_s = \sqrt{\frac{L}{C}}$ — Surge impedance of a lossless transmission line; $Z_c = \sqrt{\frac{R + j\omega L}{G + j\omega C}}$ — General characteristic impedance

Key Points: Surge impedance is purely resistive for a lossless line.; It determines the magnitude of the natural load, also known as Surge Impedance Loading (SIL).; SIL = $\frac{V_{line}^2}{Z_s}$.

Advantages: Simplifies calculation of traveling waves during transients.; Essential for calculating surge protection requirements.

Disadvantages: Assumes line is lossless, which is an idealization.; Does not account for attenuation or distortion in real-world scenarios.

Applications: Design of high voltage transmission lines.; Analysis of lightning and switching surges in power systems.

Additional Info: Surge impedance is also known as the natural impedance of the line.. Option A is incorrect as surge impedance is a property of the line, not a state of breakdown.. Option C is incorrect because corona causes non-linear impedance changes; surge impedance is defined for line parameters.

Answer: B is correct — Surge impedance and characteristic impedance are equivalent terms for the impedance presented by a transmission line to a traveling wave, mathematically defined as $\sqrt{\frac{L}{C}}$ for lossless lines.

Exam Tip: Remember that SIL is directly proportional to the square of the operating voltage and inversely proportional to the surge impedance.

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Option A: Surge impedance is the impedance at the time of breakdown of voltage

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Option C: Surge impedance is the impedance of transmission line when corona takes place

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Option D: None of above

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