--- title: "Shunt compensation: need and two methods" subject: "Power System Analysis I" chapter: 6 key: "semester_4/power_system_analysis_i:shunt-compensation-two-methods" source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/shunt-compensation-two-methods site: BCT Engineering --- # Shunt compensation: need and two methods Power System Analysis I · Chapter 6: Performance Analysis - Asked in: 2079 Chaitra, 8 marks; 2071 Bhadra, 4 marks (asked 2 times) - Video: [Short Discussion on Compensation Technique | Short Bites | Power System](https://www.youtube.com/watch?v=ccjfR5Oek5c) — TechInsight - 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/shunt-compensation-two-methods - Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-6 ## Question Why is shunt compensation necessary in a transmission line? Explain any two methods of shunt compensation applied in transmission systems. ## Short answer Shunt compensation is necessary for voltage control, loss reduction, better power factor, higher transfer capacity, and voltage stability. Two methods: (1) Fixed/switched shunt capacitors supply lagging Mvar at heavy load, reactors absorb charging Mvar at light load. (2) SVCs combine thyristor-controlled reactors and switched capacitors for fast continuous reactive power control. ## Answer **Shunt compensation** means connecting reactive-power devices (capacitors, reactors, or controlled devices) in parallel with the line or at the buses, to supply or absorb reactive power. #### Why shunt compensation is necessary - **Voltage control at heavy load:** large lagging loads draw reactive current, which causes a large voltage drop ($\Delta V \approx QX/V$). Shunt capacitors supply this Q locally and hold up the voltage. - **Voltage control at light load:** below SIL the line's charging Mvar exceeds its inductive absorption, so voltage rises (Ferranti effect). Shunt reactors absorb this excess. - **Reduced losses and current:** supplying Q near the load reduces line current, $I^2R$ loss and heating. - **Better power factor** and released capacity of generators, transformers and lines. - **Higher power transfer and voltage stability:** holding the mid-point or receiving voltage up raises the transfer limit $P = \frac{V_SV_R}{X}\sin\delta$ and helps prevent voltage collapse. #### Method 1: Shunt capacitors and shunt reactors (fixed or switched) ```text Bus ---+-------------+----- line | | === C (((L))) | | --- --- capacitor reactor ``` - **Shunt capacitor banks** at substations and load centres supply lagging Mvar; they are switched in steps as load rises. They are cheap, low-loss and simple. - **Shunt reactors** at the ends of long EHV lines absorb charging Mvar at light load and limit the Ferranti rise and switching overvoltages. - Limitation: control is in steps, and capacitor output falls as $V^2$, just when support is most needed. #### Method 2: Static VAR compensator (SVC) ```text Bus ----+-----------+-----------+ | | | [TCR] [TSC] [Filter / thyristor- thyristor- fixed C] controlled switched reactor capacitor ``` - A **thyristor-controlled reactor (TCR)** varies the absorbed Q continuously by controlling the firing angle. - **Thyristor-switched capacitors (TSC)** or fixed capacitors provide the capacitive range. - Together they act as a fast, continuously variable susceptance, from capacitive to inductive, responding within a few cycles. - Used for dynamic voltage control, damping oscillations and improving transient stability. Other shunt methods: **synchronous condensers** (over- or under-excited synchronous machines giving smooth Q control) and **STATCOM** (a voltage-source converter; it is faster than an SVC and keeps up its reactive current output at low voltage). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why are shunt capacitors used at heavy load? A. They supply lagging Mvar locally, reducing voltage drop and line current B. They absorb charging Mvar C. They reduce line reactance D. They lower the generator frequency Answer: A. Supplying Q near the load reduces delta V = QX/V and the I^2R loss. (Key point: Heavy-load role) 2. Why are shunt reactors used at light load on long EHV lines? A. Reactors supply Mvar for the lagging load B. Line charging Mvar exceeds inductive absorption below SIL, so reactors absorb the excess and limit the Ferranti rise C. Reactors cancel line inductance D. Reactors increase power factor at heavy load Answer: B. Below SIL the line generates Mvar and voltage rises; reactors absorb it. (Key point: Light-load role) 3. What is a limitation of fixed or switched shunt capacitors? A. Output rises as V^2 when voltage falls B. They cannot supply Mvar C. Output falls as V^2, just when voltage support is most needed, and control is in steps D. They have very high losses Answer: C. Q = wCV^2, so at low voltage they supply less. (Key point: Capacitor limitation) 4. How does an SVC give fast variable reactive power? A. A mechanical tap changer changes turns ratio B. An over-excited synchronous machine is rotated without load C. A series capacitor is bypassed by a breaker D. A thyristor-controlled reactor varies absorbed Q via firing angle, combined with TSC or fixed capacitors Answer: D. TCR plus TSC/fixed C acts as a continuously variable susceptance, responding within a few cycles. (Key point: SVC operation) ## Related in Power System Analysis I - [Need and methods of reactive power compensation](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Areactive-compensation-need-methods) — Power System Analysis I, chapter 6, asked 2× - [150 km line with mid-point shunt capacitor](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3A150km-mid-shunt-capacitor) — Power System Analysis I, chapter 6 - [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 - [ABCD with shunt capacitor at line end](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aabcd-with-shunt-capacitor) — Power System Analysis I, chapter 5 - [Inductance of a single-phase two-wire line](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Asingle-phase-inductance-derivation) — Power System Analysis I, chapter 4 - [Compensation capacity for 15 MW, 33 kV load](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Acompensation-capacity-15mw-33kv) — Power System Analysis I, chapter 6 ## Related in other subjects - [Expression for receiving-end shunt compensation](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Ashunt-compensation-xp-expression) — High Voltage Engineering, chapter 3 - [Shunt reactors for 750 kV line ABCD](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Ashunt-reactor-abcd-750kv) — High Voltage Engineering, chapter 3 - [Principle applied in two-way linear measurement](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atwo-way-linear-measurement-principle) — Surveying I, chapter 1 - [Waveguide vs transmission line](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Awaveguide-advantages-over-transmission-line) — Electromagnetism, chapter 6, asked 3×