---
title: "Compensation does not reduce load reactive demand"
subject: "Power System Analysis I"
chapter: 6
key: "semester_4/power_system_analysis_i:compensation-does-not-reduce-load-q"
source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/compensation-does-not-reduce-load-q
site: BCT Engineering
---

# Compensation does not reduce load reactive demand

Power System Analysis I · Chapter 6: Performance Analysis

- Asked in: 2075 Bhadra, 8 marks
- 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/compensation-does-not-reduce-load-q
- Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-6

## Question

What do you mean by reactive power compensation of a transmission line? How can we do that? Also with necessary example justify that compensation of a line does not reduce the reactive power demand of the load.

## Short answer

Reactive power compensation means placing local sources (e.g., shunt capacitors) to supply reactive power, so the line need not carry it. The load's own demand Q = P tanφ is fixed by the load; compensation only changes the supply point, not the amount the load consumes.

## Answer

**Reactive power compensation** of a transmission line means supplying or absorbing reactive power at suitable points (usually at the receiving end or along the line) so that the line itself does not have to carry it. Its aims are to keep voltages within limits, reduce line current and losses, and raise the power transfer capability.

#### Methods of compensation
1. **Shunt capacitors** at the receiving end/substations: supply lagging VAr to inductive loads; raise voltage at heavy load.
2. **Shunt reactors**: absorb the charging VAr of long EHV lines at light load; control Ferranti rise.
3. **Series capacitors**: cancel part of the line reactance $X_L$; reduce voltage drop and raise $P_{max} = V_SV_R/X$.
4. **Synchronous condensers**: over- or under-excited synchronous motors on no load; continuously variable Q.
5. **Static VAr compensators (SVC, TCR/TSC) and STATCOM**: fast, thyristor/IGBT-controlled variable Q.

#### Compensation does not reduce the load's own Q demand
The reactive power a load needs is fixed by the load itself: $Q_L = P\tan\phi_L$. A compensator only changes **where that Q comes from**.

**Example:** a load of 10 MW at 0.8 pf lagging at 11 kV, fed by a short line with $X = 5\ \Omega$.

- Load demand: $Q_L = 10\tan(36.87^\circ) = 7.5$ MVAr.

| Quantity | No compensation | 5 MVAr capacitor at load |
|---|---|---|
| Load P | 10 MW | 10 MW |
| Load Q | 7.5 MVAr | 7.5 MVAr |
| Q from capacitor | 0 | 5 MVAr |
| Q through line | 7.5 MVAr | 2.5 MVAr |
| Line current | 656 A | 541 A |
| pf seen by line | 0.80 | 0.97 |

The motor/load still receives 7.5 MVAr in both cases. What changes is that 5 MVAr is now produced locally, so the line carries only 2.5 MVAr. Line current falls from 656 A to 541 A, so $I^2R$ loss falls by about 32 % and the drop $IX$ falls.

```text
 Source ===== line =====+------> Load (10 MW, 7.5 MVAr)
         P=10, Q=2.5    |
                       ===  Capacitor supplies 5 MVAr
```

Hence compensation relieves the **line and source** of reactive power; it does not reduce the load's reactive power demand.

## Check yourself

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

1. What is reactive power compensation of a line?
   A. Reducing the load's real power demand
   B. Supplying or absorbing reactive power at suitable points so the line need not carry it
   C. Increasing line resistance to damp oscillations
   D. Changing the frequency of supply
   Answer: B. Compensation provides or absorbs VAr locally, keeping voltage within limits, cutting current and losses and raising transfer capability. (Key point: Meaning)

2. What does a series capacitor do for a line?
   A. Absorbs charging VAr at light load
   B. Supplies VAr to the load only
   C. Increases line resistance
   D. Cancels part of X_L, reducing voltage drop and raising P_max = V_S V_R/X
   Answer: D. Series compensation reduces effective reactance, so P_max = V_S V_R/X rises and the drop falls. (Key point: Series capacitor)

3. Which device is used to absorb charging VAr of long EHV lines at light load and limit Ferranti rise?
   A. Shunt capacitor
   B. Series capacitor
   C. Shunt reactor
   D. Synchronous motor at full load
   Answer: C. Shunt reactors absorb surplus charging VAr at light load, controlling voltage rise. (Key point: Shunt reactor)

4. In the example (10 MW, 0.8 pf load with a 5 MVAr capacitor), what happens?
   A. Load Q falls to 2.5 MVAr
   B. Load still needs 7.5 MVAr but line carries only 2.5 MVAr, current falls from 656 A to 541 A
   C. Load P falls to 8 MW
   D. Line current rises
   Answer: B. The load's Q = P tan(phi) is fixed at 7.5 MVAr; the capacitor merely supplies 5 MVAr locally so the line and source are relieved. (Key point: Load Q unchanged)

## 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×
- [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×
- [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
- [Vs from P+jQ and reactive power flow direction](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Avs-from-pq-reactive-flow) — Power System Analysis I, chapter 6
- [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

- [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×
- [Justify not self starting; two starting methods](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-starting-two-methods) — Electric Machines II, chapter 2
- [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
- [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