--- title: "Surge impedance loading and significance" subject: "Power System Analysis I" chapter: 5 key: "semester_4/power_system_analysis_i:surge-impedance-loading" source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/surge-impedance-loading site: BCT Engineering --- # Surge impedance loading and significance Power System Analysis I · Chapter 5: Transmission Line Modeling - Asked in: 2077 Chaitra, 4 marks; 2069 Bhadra, 4 marks (asked 2 times) - Video: [Surge Impedance Loading | Explained | Beginner’s Guide | TheElectricalGuy](https://www.youtube.com/watch?v=Xyp0orDBl2c) — Gaurav J - TheElectricalGuy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/surge-impedance-loading - Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-5 ## Question What do you mean by surge impedance loading? Explain its significance in power system. ## Short answer Surge impedance loading (SIL) is the power delivered to a purely resistive load equal to surge impedance Z_c (for lossless line, Z_c=√(L/C)). At SIL, capacitive reactive power equals inductive absorption, yielding a flat voltage profile. It serves as a benchmark: below SIL, voltage rises (Ferranti effect); above SIL, voltage drops; it guides reactive compensation and line loadability. ## Answer **Surge impedance loading (SIL)** is the power delivered by a transmission line to a purely resistive load equal to its surge impedance $Z_c$ (for a lossless line, $Z_c = \sqrt{L/C}$). $$ \text{SIL} = \frac{V_L^2}{Z_c}\ \text{(MW, with } V_L \text{ in kV line-to-line)} $$ Example: a 400 kV line with $Z_c \approx 400\ \Omega$ has SIL $= 400^2/400 = 400$ MW. #### Physical meaning At SIL, the reactive power produced by the line's shunt capacitance equals the reactive power absorbed by its series inductance: $$ \omega C V^2 = \omega L I^2 \;\Rightarrow\; \frac{V}{I} = \sqrt{\frac{L}{C}} = Z_c $$ So the line neither needs nor supplies reactive power. For a lossless line the voltage magnitude is the same all along the line (flat voltage profile) and only the phase angle changes. #### Significance in power systems 1. **Benchmark of loading:** a line loaded at SIL has a flat voltage profile and unity power factor all along it. 2. **Below SIL** (light load): the line generates net reactive power, so the receiving-end voltage rises (Ferranti effect). Shunt reactors are needed. 3. **Above SIL** (heavy load): the line absorbs reactive power, so voltage falls along the line. Shunt capacitors or series compensation are needed. 4. **Loadability:** the practical loading of a line is expressed in multiples of SIL. Short lines can carry several times SIL (thermal limit), but long EHV lines are limited by voltage drop and stability to roughly 1–1.5 × SIL (about 1.3 × SIL for a 300 km line). 5. **Raising transmission capacity:** SIL increases with $V^2$ and with lower $Z_c$. This is why higher voltages, bundled conductors (which lower $L$ and raise $C$) and series capacitors are used. 6. **Planning and compensation:** comparing actual load with SIL tells engineers how much reactive compensation the line needs. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Surge impedance loading is the power delivered by a line to: A. A load whose resistance equals the line's surge impedance Zc B. A short-circuit at the receiving end C. An open-circuit receiving end D. A load that makes the line's series resistance zero Answer: A. SIL is the power delivered to a purely resistive load equal to Zc, giving SIL = V_L^2/Zc. (Key point: Definition of SIL) 2. A 400 kV line has Zc = 400 Ω. What is its SIL? A. 1600 MW B. 160 MW C. 400 MW D. 0.4 MW Answer: C. SIL = V_L^2/Zc = 400^2/400 = 400 MW. (Key point: SIL formula and value) 3. At SIL, which statement about reactive power is correct? A. The line absorbs reactive power from the sending end B. Reactive power generated by shunt capacitance equals that absorbed by series inductance C. The line generates reactive power only D. Reactive power is zero because the capacitance is zero Answer: B. At SIL, wCV^2 = wLI^2, so the line neither needs nor supplies reactive power and the voltage profile is flat. (Key point: Reactive balance at SIL) 4. A line is lightly loaded, below its SIL. What happens? A. Voltage falls along the line and series compensation is needed B. Voltage profile stays exactly flat C. The line generates net reactive power, receiving-end voltage rises, and shunt reactors are needed D. The line absorbs net reactive power and shunt capacitors are needed Answer: C. Below SIL the capacitive reactive generation exceeds inductive absorption, causing a voltage rise (Ferranti effect) that shunt reactors control. 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