---
title: "Real power/frequency control in interconnected system"
subject: "Power System Analysis II"
chapter: 1
key: "semester_5/power_system_analysis_ii:real-power-frequency-control"
source: https://www.bctengineer.com/q/semester_5/power_system_analysis_ii/real-power-frequency-control
site: BCT Engineering
---

# Real power/frequency control in interconnected system

Power System Analysis II · Chapter 1: Interconnected Power System

- Asked in: 2068 Chaitra, 6 marks
- Video: [What is Automatic Generation Control ? | Real Time Control of Power System Frequency & Interchange](https://www.youtube.com/watch?v=WMSRdriPPZ0) — Power System Operation Slides
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/power_system_analysis_ii/real-power-frequency-control
- Whole chapter: https://www.bctengineer.com/md/semester_5/power_system_analysis_ii/chapter-1

## Question

Explain how real power/frequency control is maintained in a interconnected power system.

## Short answer

A load increase initially draws kinetic energy, lowering frequency. Primary control uses speed governor droop to share the load change among all units, leaving a steady-state error. Secondary control (AGC) integrates ACE = ΔPtie + BΔf to adjust generation and restore nominal frequency and scheduled tie-line flows.

## Answer

In an interconnected system all generators run at one common frequency. Frequency stays constant only while **total generation = total load + losses**. Real power/frequency control (load-frequency control, LFC) keeps this balance.

#### What happens on a load change
1. **Inertial response (first seconds):** when load rises by $\Delta P_L$, the deficit is supplied from the kinetic energy of all rotors. From the swing equation $\frac{2H}{\omega_s}\frac{d^2\delta}{dt^2} = P_m - P_e$, they slow down and frequency falls.
2. **Primary control (governor):** each speed governor senses $\Delta f$ and opens the turbine valve/gate. With speed regulation (droop) $R$:

$$
\Delta P_{m,i} = -\frac{\Delta f}{R_i},\qquad
\Delta f_{ss} = \frac{-\Delta P_L}{\sum_i \dfrac{1}{R_i} + D}
$$

   The load change is shared by all units of all areas, in inverse proportion to their droop. A steady-state frequency error remains, and tie-line flows change from schedule.
3. **Secondary control (AGC):** each area computes its Area Control Error

$$
ACE_i = \Delta P_{tie,i} + B_i\,\Delta f
$$

   An integral controller changes the speed-changer set points until $ACE_i = 0$. Frequency returns to nominal and each area again supplies its own load, with tie flows back at schedule.
4. **Tertiary control:** economic dispatch sets the new outputs at minimum cost.

```text
 dPL -> [Power system] -> df
           ^               |
           |  dPm          v
      [Turbine] <- [Governor 1/R] <- df
           ^
     speed changer <- integral(ACE)
```

Example: in a two-area system, if area 1 load increases, at first both areas raise generation and power flows from area 2 to area 1 over the tie-line; AGC in area 1 then raises its own generation so the tie-line flow returns to schedule.

## Check yourself

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

1. Frequency stays constant only while:
   A. generation exceeds load
   B. voltage is 1 pu
   C. tie-line flow is zero
   D. total generation = total load + losses
   Answer: D. Any imbalance changes rotor speed. (Key point: Frequency constancy)

2. Immediately after a load rise, the deficit is supplied by:
   A. kinetic energy of rotors, so frequency falls
   B. the AGC
   C. tertiary control
   D. shunt capacitors
   Answer: A. Inertial response comes first. (Key point: First response)

3. Governors share load change:
   A. equally regardless of R
   B. in inverse proportion to their droop R
   C. in proportion to R
   D. only in the area where it occurred
   Answer: B. dPm,i = -df/Ri. (Key point: Droop sharing)

4. After AGC acts, finally:
   A. a frequency error stays
   B. tie flows stay changed
   C. frequency is nominal and tie flows return to schedule so each area supplies its own load change
   D. only primary control remains
   Answer: C. The integral of ACE drives ACE to zero. (Key point: AGC result)

## Related in Power System Analysis II

- [Interconnected system load-frequency and Q-V control](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Ainterconnection-control-schematic) — Power System Analysis II, chapter 1
- [Interconnection advantages and real power-frequency balance](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Ainterconnection-advantages-frequency) — Power System Analysis II, chapter 1, asked 3×
- [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×
- [Merits and demerits of interconnected system](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Ainterconnection-merits-demerits) — Power System Analysis II, chapter 1, asked 3×
- [Power angle depends on real power](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Apower-angle-real-power) — Power System Analysis II, chapter 1

## Related in other subjects

- [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
- [Interconnected system SLD, isolated vs interconnected, advantages](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ainterconnected-sld-isolated-advantages) — Power System Analysis I, chapter 1
- [SLD of interconnected system vs isolated system](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ainterconnected-sld-vs-isolated) — Power System Analysis I, chapter 1
- [Interconnected system SLD and functions of each part](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ainterconnected-sld-functions) — Power System Analysis I, chapter 1