GATE Electrical Engineering - Complete Master Library

 

📘 Surge Impedance Loading (SIL)

Surge Impedance Loading is the natural loading of a transmission line. At SIL, reactive power generated equals reactive power absorbed.

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🔹 1️⃣ Characteristic Impedance

For long transmission line:

Zc = √(L/C)

This is called Surge Impedance. Typical value: 300–400 Ω for overhead lines.

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🔹 2️⃣ Surge Impedance Loading (SIL)

SIL is defined as:

SIL = V² / Zc

Where: V = Line voltage Zc = Characteristic impedance Units → Watts or VA

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🔹 3️⃣ Physical Meaning

At SIL:

• No reactive power flow
• Line neither absorbs nor supplies reactive power
• Voltage profile is flat

If Load > SIL → Line absorbs reactive power If Load < SIL → Line supplies reactive power

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🔹 4️⃣ Example Problem

Given: Line voltage = 220 kV Zc = 400 Ω Calculate SIL. SIL = V² / Zc = (220 × 10³)² / 400 = (48.4 × 10¹⁰) / 400 = 1.21 × 10⁸ W

SIL ≈ 121 MW

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🔹 5️⃣ Important Observations

• Higher voltage → higher SIL
• Lower Zc → higher SIL
• Used for line design

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🔹 6️⃣ Concept Question

If a line operates below SIL, what happens?

It supplies reactive power (capacitive nature).

Very common 1-mark GATE question.

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🎯 GATE Focus

• SIL formula must be memorized
• Understand physical meaning
• Know reactive power behavior
• Voltage profile concept

SIL = Natural Loading of Transmission Line