---
title: "T/F: series compensation in EHV lines"
subject: "Transmission and Distribution System Design"
chapter: 2
key: "semester_8/transmission_and_distribution_system_design:tf-series-compensation-ehv"
source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-series-compensation-ehv
site: BCT Engineering
---

# T/F: series compensation in EHV lines

Transmission and Distribution System Design · Chapter 2: Transmission Voltage Level and Number of Circuits

- Asked in: 2078 Kartik, 4 marks
- 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_8/transmission_and_distribution_system_design/tf-series-compensation-ehv
- Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-2

## Question

State whether the following statement is TRUE or FALSE. Justify your answer with a brief explanation: Series compensation may be used in EHV transmission line to increase the power handling capacity of the line.

## Short answer

TRUE. Series capacitors reduce net line reactance, increasing steady-state power limit Pmax = VsVr/(XL(1–k)). This boosts transfer capability on stability-limited long EHV lines.

## Answer

**TRUE.**

- Steady-state power transfer: $P = \frac{V_s V_r}{X_L}\sin\delta$.
- A series capacitor of reactance $X_C$ reduces the net reactance to $X_L - X_C = X_L(1-k)$, where $k = X_C/X_L$ is the degree of compensation (normally 25–70%).
- Hence
$$
P_{max} = \frac{V_s V_r}{X_L(1-k)}
$$
  e.g. 50% compensation doubles the steady-state limit.
- It also improves transient stability, reduces voltage drop and allows better load sharing between parallel lines.

Limits: risk of sub-synchronous resonance with turbine-generator shafts, and need for over-voltage protection (MOV, spark gap) across capacitors. Series compensation does not raise the thermal limit, so it is useful mainly on long EHV lines that are stability-limited.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Series compensation can raise power capacity of an EHV line:
   A. False
   B. True
   C. Only thermally
   D. Only for DC
   Answer: B. It reduces net reactance, raising stability-limited power. (Key point: Verdict)

2. With degree of compensation k, P_max becomes:
   A. VsVr/(X_L(1+k))
   B. VsVr/(X_L(1-k))
   C. VsVr x k/X_L
   D. VsVr/(X_L k)
   Answer: B. Net reactance is X_L(1-k), so 50% compensation doubles the limit. (Key point: Formula)

3. Series compensation does NOT raise:
   A. Stability limit
   B. Thermal limit
   C. Transient stability
   D. Load sharing
   Answer: B. Thermal limit depends on conductor, so it is useful on stability-limited long lines. (Key point: Limit)

4. A risk of series capacitors is:
   A. Sub-synchronous resonance with turbine-generator shafts
   B. Lower stability
   C. Higher voltage drop
   D. Zero fault current
   Answer: A. SSR needs attention along with over-voltage protection across capacitors. (Key point: Risk)

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