---
title: "Interconnected system load-frequency and Q-V control"
subject: "Power System Analysis II"
chapter: 1
key: "semester_5/power_system_analysis_ii:interconnection-control-schematic"
source: https://www.bctengineer.com/q/semester_5/power_system_analysis_ii/interconnection-control-schematic
site: BCT Engineering
---

# Interconnected system load-frequency and Q-V control

Power System Analysis II · Chapter 1: Interconnected Power System

- Asked in: 2081 Bhadra, 2+4 marks
- Video: [Module 4 - 1. Automatic generation and Voltage Control](https://www.youtube.com/watch?v=qBZXhkBB2Cs) — Nisha Prakash
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/power_system_analysis_ii/interconnection-control-schematic
- Whole chapter: https://www.bctengineer.com/md/semester_5/power_system_analysis_ii/chapter-1

## Question

What are the advantages and disadvantages of interconnected power system? With the schematic diagram, explain how load/frequency and reactive power/voltage are controlled in the power system.

## Short answer

Interconnection improves reliability, reserve sharing, load diversity, and enables economic dispatch, but raises fault levels and risk of cascading outages. Load/frequency is controlled by the P-f loop: a governor reduces speed error and AGC adjusts setpoint using ACE to restore nominal frequency. Reactive/voltage control uses the Q-V loop: an AVR and exciter adjust field current to regulate terminal voltage.

## Answer

An interconnected power system links many generating stations and areas by tie-lines so that they operate in synchronism. Its two main control loops are the **load-frequency (P–f) loop** and the **reactive power–voltage (Q–V) loop**.

#### Advantages
- Higher reliability; a failed unit's load is picked up by others.
- Less reserve capacity, as reserve is shared.
- Load diversity gives lower installed capacity and better load factor.
- Economic dispatch of the cheapest units; power exchange between areas.
- Larger combined inertia gives better frequency regulation.

#### Disadvantages
- Higher fault levels, so higher-rated breakers are needed.
- A fault can cascade and black out a large area.
- Stability and protection coordination become more complex.
- Costly tie-lines and communication/SCADA systems.

#### Control schematic
```text
 P-f loop (frequency)
                   Speed changer (AGC set point)
                            |
 Speed/frequency -> [Governor] -> [Valve] -> [Turbine] --Pm--> [Generator]
        ^                                                          |
        +------------- speed (frequency) feedback ----------------+

 Q-V loop (voltage)
 Vref --> (+/-) --> [AVR] --> [Exciter] --If--> [Generator field]
            ^                                          |
            +-------- measured terminal voltage V -----+
```

#### Load / frequency control (P–f loop)
1. A load increase makes $P_e > P_m$. The deficit is taken from rotor kinetic energy, so speed and frequency fall.
2. The **speed governor** senses $\Delta f$ and opens the steam/water valve, increasing $P_m$. With droop $R$, the steady-state change is $\Delta f = -\Delta P_L / (\sum 1/R + D)$; a small error remains.
3. **Automatic Generation Control (AGC)** adds an integral of the area control error $ACE = \Delta P_{tie} + B\Delta f$ to the speed-changer set point, bringing $\Delta f$ and tie-line deviation to zero.

#### Reactive power / voltage control (Q–V loop)
1. The terminal voltage is measured, rectified and compared with $V_{ref}$.
2. The error is amplified by the **AVR** and drives the **exciter**, which changes the field current $I_f$ and hence the internal emf $E_f$.
3. Higher $E_f$ makes the generator supply more Q and raises the bus voltage; lower $E_f$ absorbs Q and lowers it.
4. In the network, voltage is also held by shunt capacitors/reactors, synchronous condensers, SVC/STATCOM and tap-changing transformers.

Because $X \gg R$, P depends mainly on $\delta$ and Q mainly on $|V|$, so the two loops can be designed and operated independently.

## Check yourself

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

1. In the P-f loop, the speed governor responds to a frequency fall by:
   A. closing the valve
   B. opening the steam or water valve to raise Pm
   C. changing the field current
   D. switching capacitors
   Answer: B. More mechanical input counters the deficit. (Key point: Governor role)

2. In the Q-V loop, a terminal voltage lower than Vref causes the AVR to:
   A. lower field current
   B. open the turbine valve
   C. raise exciter output and field current, increasing Ef and Q
   D. change tie-line schedule
   Answer: C. Higher Ef makes the generator supply more Q and lifts voltage. (Key point: AVR role)

3. The two loops can be designed independently because:
   A. R >> X
   B. frequency equals voltage
   C. Q is system-wide
   D. X >> R, so P depends mainly on delta and Q mainly on |V|
   Answer: D. Weak P-V and Q-delta coupling decouples the loops. (Key point: Independent loops)

## Related in Power System Analysis II

- [Real power/frequency control in interconnected system](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Areal-power-frequency-control) — Power System Analysis II, chapter 1
- [Role of interconnection and reactive power control](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Ainterconnection-reactive-control) — Power System Analysis II, chapter 1
- [Reactive power and voltage balance](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Areactive-power-voltage-balance) — Power System Analysis II, chapter 1
- [Generator reactive power at constant terminal voltage](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Agenerator-reactive-power-infinite-bus) — Power System Analysis II, chapter 1
- [Advantages of interconnected power system](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Ainterconnection-advantages) — Power System Analysis II, chapter 1, asked 3×
- [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×

## Related in other subjects

- [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
- [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×
- [Reactive power and power phasor diagram](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Areactive-power-phasor-diagram) — Basic Electrical Engineering, chapter 7
- [Interconnected power system and advantages](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ainterconnected-system-advantages) — Power System Analysis I, chapter 1