--- title: "Ferranti effect and measures to limit it" subject: "Power System Analysis I" chapter: 6 key: "semester_4/power_system_analysis_i:ferranti-effect-measures" source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/ferranti-effect-measures site: BCT Engineering --- # Ferranti effect and measures to limit it Power System Analysis I · Chapter 6: Performance Analysis - Asked in: 2080 Chaitra, 3+3 marks; 2073 Bhadra, 7 marks (asked 2 times) - Video: [Ferranti Effect | Why Receiving End Voltage Rises | TheElectricalGuy](https://www.youtube.com/watch?v=ZLGeNu7iR_E) — 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/ferranti-effect-measures - Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-6 ## Question What is Ferranti effect? Explain the measures to be considered to tackle the Ferranti effect. ## Short answer The Ferranti effect is the rise in receiving-end voltage above sending-end voltage on light load due to line charging current flowing through series inductance. It is mitigated by using shunt reactors (most common), SVCs, STATCOMs, under-excited synchronous condensers, generator under-excitation, and on-load tap changers. ## Answer The **Ferranti effect** is the rise of voltage at the receiving end of a long transmission line (or cable) above the sending-end voltage when the line is on open circuit or very light load. #### Cause At light load, the line's shunt capacitance draws a charging current $I_C$ that leads $V_R$ by 90°. This current flows through the series inductance, and the drop $jI_CX$ is 180° out of phase with $V_R$ (since $j\cdot j = -1$). So $V_S = V_R + jXI_C = V_R - XI_C$, which is smaller than $V_R$: the receiving voltage is higher than the sending voltage. ```text IC ^ (leads VR by 90 deg) | O---------------------->-----> VR O-------------------> VS | VS = VR - IC.X (VS < VR) ``` Using the nominal-Π model at no load: $$ V_S = \left(1 + \frac{ZY}{2}\right)V_R \approx \left(1 - \frac{\omega^2LCl^2}{2}\right)V_R $$ $$ V_R - V_S \approx \frac{\omega^2LCl^2}{2}V_R $$ So the rise grows with the **square of line length** and the square of frequency. It is significant for EHV lines above about 200–300 km and for cables (high C). #### Effects - Overvoltage stresses the insulation of the line, transformers and equipment. - Problems when energising long lines or after sudden load rejection. #### Measures to tackle the Ferranti effect 1. **Shunt reactors** at the receiving end (and often both ends) of long EHV lines. They absorb the charging Mvar and are the most common method. They may be switched in at light load and out at heavy load. 2. **Controlled reactive devices:** SVCs (thyristor-controlled reactors) and STATCOMs absorb variable reactive power quickly. 3. **Synchronous condensers** run under-excited at the receiving end to absorb Mvar. 4. **Generator under-excitation** at the sending end and coordinated voltage control. 5. **Switching out shunt capacitors**, and charging the line from the stronger end with a lower initial voltage. 6. **Keeping a minimum load** on the line, or splitting very long lines with intermediate substations and reactors. 7. **On-load tap changers** on transformers to bring the receiving voltage back to normal. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the Ferranti effect? A. The receiving-end voltage rises above the sending-end voltage at no load or light load B. The receiving voltage falls well below the sending voltage at heavy load C. Current rises at the receiving end at heavy load D. The conductor current crowds toward the surface Answer: A. It occurs on long lines or cables at light load. (Key point: Definition) 2. What causes the Ferranti effect? A. Lagging load current through the line resistance B. Leading charging current through the series inductance gives a drop in phase with V_R that adds to it C. Corona loss at light load D. Skin effect at high frequency Answer: B. The capacitive charging current I_C leads by 90 deg; the drop across X is such that V_S = V_R - I_C X, so V_R > V_S. (Key point: Cause) 3. How does the Ferranti rise vary with line length l? A. Linearly with l B. Inversely with l C. Approximately with l^2, so it matters for long EHV lines and cables D. It is independent of length Answer: C. V_R - V_S is approximately (w^2 L C l^2 / 2) V_R. (Key point: Dependence on length) 4. What is the most common measure to tackle the Ferranti effect? A. Series capacitors in the line B. Shunt capacitors at the receiving end C. Raising the generator excitation D. Shunt reactors at the receiving end (and often both ends) Answer: D. Shunt reactors absorb the line charging Mvar; adding capacitors or raising excitation worsens the overvoltage. (Key point: Main remedy) ## Related in Power System Analysis I - [Ferranti effect and skin effect](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aferranti-and-skin-effect) — Power System Analysis I, chapter 6 ## Related in other subjects - [Reducing Ferranti effect mathematically](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aferranti-effect-reduction) — High Voltage Engineering, chapter 3 - [Limit cycle effect in recursive systems](https://www.bctengineer.com/question.md?key=semester_7%2Fdigital_signal_analysis_and_processing%3Alimit-cycle-recursive) — Digital Signal Analysis and Processing, chapter 4, asked 2× - [Water pollutants, harmful effects, control measures](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awater-pollutants-effects-measures) — Engineering Chemistry, chapter 3 - [Limit states of strength and serviceability](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Alimit-states-of-strength-and-serviceability) — Design of RCC Structure, chapter 3, asked 3×