--- title: "Need and methods of reactive power compensation" subject: "Power System Analysis I" chapter: 6 key: "semester_4/power_system_analysis_i:reactive-compensation-need-methods" source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/reactive-compensation-need-methods site: BCT Engineering --- # Need and methods of reactive power compensation Power System Analysis I · Chapter 6: Performance Analysis - Asked in: 2081 Chaitra (new course), 4+4 marks; 2075 Baisakh, 5 marks (asked 2 times) - Video: [Lecture-13 Reactive Power Compensation of Transmission Line](https://www.youtube.com/watch?v=iuE7XwpTdBk) — Hiren Kherala - 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/reactive-compensation-need-methods - Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-6 ## Question Why is reactive power compensation required in a transmission line? Explain the methods used for reactive power compensation. ## Short answer Reactive power compensation is required for voltage control, loss reduction, increased power transfer, power factor improvement, and voltage stability. Methods include shunt capacitors/reactors, series capacitors, synchronous condensers, SVCs, STATCOMs, and tap-changing transformers. ## Answer **Reactive power compensation** means supplying or absorbing reactive power at suitable points of the system, so that voltage stays within limits and transmission capacity is used well. #### Why compensation is required 1. **Voltage control:** the voltage drop in a line is mainly $\Delta V \approx \frac{QX}{V}$. Heavy lagging loads lower the receiving voltage; light loads raise it (Ferranti effect). Compensation keeps voltage within ±5–10 %. 2. **Line behaviour changes with load:** below SIL a line generates Mvar (which must be absorbed by reactors); above SIL it absorbs Mvar (which needs capacitive support). 3. **Reduce losses:** reactive current increases line current and $I^2R$ loss. Supplying Q locally reduces it. 4. **Increase power transfer:** in $P = \frac{V_SV_R}{X}\sin\delta$, series capacitors reduce $X$ and shunt support holds up $V$, raising the stability limit. 5. **Improve power factor** and release capacity of generators, transformers and lines. 6. **Voltage stability:** prevents voltage collapse in heavily loaded systems. #### Methods **1. Shunt compensation** - **Shunt capacitors:** connected at load buses or substations to supply lagging Mvar during heavy load; they raise voltage and p.f. They are cheap, but their output falls with $V^2$. - **Shunt reactors:** connected at the ends of long EHV lines to absorb the line's charging Mvar at light load; they control the Ferranti rise and switching overvoltages. **2. Series compensation** - **Series capacitors** in the line cancel part of the line inductance ($X_{eff} = X_L - X_C$, typically 25–70 % compensation). They increase power transfer and stability and reduce voltage drop. Risk: subsynchronous resonance. **3. Synchronous condensers** - Synchronous motors running without mechanical load: when over-excited they supply Mvar, and when under-excited they absorb Mvar. They give smooth, continuous control but are costly and need high maintenance. **4. Static VAR compensators (SVC) and FACTS** - SVC: a thyristor-controlled reactor (TCR) plus thyristor-switched or fixed capacitors give fast, variable Q. - STATCOM (voltage-source converter), TCSC (thyristor-controlled series capacitor) and UPFC give faster and wider control. **5. Other means:** tap-changing transformers (redistribute Q and adjust voltage) and generator excitation control. | Method | Supplies/absorbs | Main use | |---|---|---| | Shunt capacitor | Supplies | Heavy load, p.f. | | Shunt reactor | Absorbs | Light load, long lines | | Series capacitor | Reduces X | Long lines, stability | | Synchronous condenser | Both | Continuous control | | SVC / STATCOM | Both, fast | Dynamic voltage support | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does reactive power affect line voltage? A. Voltage drop is approximately QX/V, so heavy lagging loads lower the receiving voltage B. Voltage drop is approximately PR/V only, independent of Q C. Q raises voltage at heavy load always D. Q has no effect on voltage, only on frequency Answer: A. In a mostly inductive line, delta V is approximately QX/V, so large Q gives large voltage drop. (Key point: Voltage drop and Q) 2. How does a line behave with respect to Mvar below and above SIL? A. Below SIL it absorbs Mvar; above SIL it generates Mvar B. Below SIL it generates Mvar; above SIL it absorbs Mvar C. It always absorbs Mvar D. It always generates Mvar Answer: B. Charging Mvar (fixed with voltage) exceeds I^2X absorption at light load; the reverse at heavy load. (Key point: Line below and above SIL) 3. What is the purpose of a series capacitor in a long line? A. Absorb charging Mvar at light load B. Increase the line reactance C. Cancel part of the line inductance (Xeff = XL - XC) to raise power transfer and stability D. Supply Mvar in proportion to V^2 at the load bus Answer: C. Reducing X in P = VsVr sin(delta)/X increases transfer capability. (Key point: Series capacitor) 4. Which device is used at the ends of long EHV lines to absorb charging Mvar at light load? A. Shunt capacitor B. Series capacitor C. Tap-changing only D. Shunt reactor Answer: D. Shunt reactors absorb excess charging Mvar and limit Ferranti rise and switching overvoltages. (Key point: Shunt reactor use) ## Related in Power System Analysis I - [Reactive power compensation and its benefits](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Areactive-compensation-meaning-benefits) — Power System Analysis I, chapter 6, asked 2× - [Compensation does not reduce load reactive demand](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Acompensation-does-not-reduce-load-q) — Power System Analysis I, chapter 6 - [Line as source and sink of reactive power](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aline-source-sink-reactive) — Power System Analysis I, chapter 6 - [Line as source and sink of reactive power, effects](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aline-source-sink-reactive-effects) — Power System Analysis I, chapter 6 - [Shunt compensation: need and two methods](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ashunt-compensation-two-methods) — Power System Analysis I, chapter 6, asked 2× - [Reactive power proportional to voltage drop](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Areactive-power-voltage-drop) — Power System Analysis I, chapter 6 ## Related in other subjects - [Charging reactive power of line at no load](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Acharging-reactive-power-no-load) — High Voltage Engineering, chapter 3, asked 2× - [T/F: HVDC converter reactive power need](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-hvdc-reactive-power) — Transmission and Distribution System Design, chapter 1 - [Power angle and series compensation](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Apower-angle-series-compensation) — Power System Analysis II, chapter 6 - [Effects of real and reactive power mismatch](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Apf-qv-mismatch-effects) — Power System Analysis II, chapter 1, asked 4×