---
title: "Interconnection importance, P-f and Q-V balance"
subject: "Power System Analysis II"
chapter: 1
key: "semester_5/power_system_analysis_ii:interconnection-pf-qv-balance"
source: https://www.bctengineer.com/q/semester_5/power_system_analysis_ii/interconnection-pf-qv-balance
site: BCT Engineering
---

# Interconnection importance, P-f and Q-V balance

Power System Analysis II · Chapter 1: Interconnected Power System

- Asked in: 2074 Asoj, 8 marks
- Video: [PF and QV Loops | Real power Frequency | Reactive power Voltage control | Power system operation](https://www.youtube.com/watch?v=ubaTYzK-Tk0) — Simple n Easy Learning
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/power_system_analysis_ii/interconnection-pf-qv-balance
- Whole chapter: https://www.bctengineer.com/md/semester_5/power_system_analysis_ii/chapter-1

## Question

Describe about the importance of interconnection in power system. Explain briefly about the real power - frequency balance and reactive power - voltage balance in power system.

## Short answer

Interconnection enhances reliability, reserve sharing, load diversity, and economic dispatch. Real power mismatch changes frequency; primary governor droop and secondary AGC using ACE restore balance. Reactive power mismatch alters voltage locally; AVR adjusting excitation, shunt capacitors/reactors, and OLTC maintain voltage.

## Answer

**Interconnection** means tying several generating stations and control areas together by transmission lines so that they work as one synchronous system and jointly supply the total load.

#### Importance of interconnection
1. **Reliability:** if a generator or plant trips, others supply its load through tie-lines.
2. **Reduced reserve:** reserve capacity is shared, so less idle plant is needed.
3. **Load diversity:** different peak times mean lower total installed capacity and higher load factor.
4. **Economy:** economic dispatch uses the cheapest sources first; surplus energy can be exchanged (e.g. Nepal exporting wet-season hydro power to India).
5. **Large efficient units** become practical.
6. **Frequency stability:** large total inertia keeps frequency deviations small.
7. **Maintenance** can be planned without load shedding.

#### Real power – frequency balance
At constant frequency, total generation equals total demand plus losses: $\sum P_G = \sum P_D + P_{loss}$.

- A sudden load increase is first met from the **kinetic energy** of all rotating machines. By the swing equation $\frac{2H}{\omega_s}\frac{d^2\delta}{dt^2} = P_m - P_e$, they decelerate and frequency falls.
- **Primary control:** speed governors increase turbine input. With droop $R$ and load damping $D$, the steady-state deviation is $\Delta f = -\Delta P_L/(\sum 1/R_i + D)$. All connected units share the change.
- **Secondary control (AGC):** an integral controller acting on $ACE = \Delta P_{tie} + B\,\Delta f$ adjusts the speed changers until frequency returns to 50 Hz and tie-line flows return to schedule.

So, **real power mismatch → frequency change**, and frequency is a system-wide quantity.

#### Reactive power – voltage balance
For a line of reactance $X$:

$$
Q_{12} \approx \frac{V_1(V_1 - V_2\cos\delta)}{X} \approx \frac{V_1\,\Delta V}{X}
$$

- The voltage difference between two buses depends mainly on the reactive power flowing between them.
- If reactive demand at a bus exceeds local supply, Q must flow in over the line reactance, and the bus **voltage falls**. Surplus Q (light load, line charging) makes the voltage **rise**.
- Voltage is a **local** quantity; Q cannot be transmitted far because of high $I^2X$ losses, so it is balanced locally.
- Means of control: generator excitation with AVR, shunt capacitors and reactors, synchronous condensers, SVC/STATCOM, and on-load tap changers.

So, **reactive power mismatch → voltage change**.

| Balance | Variable | Scope | Controller |
|---|---|---|---|
| P – f | Frequency | Whole system | Governor, AGC |
| Q – V | Voltage | Local bus | AVR, compensators, OLTC |

## Check yourself

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

1. One key importance of interconnection is:
   A. shared reserve and load diversity lowering installed capacity
   B. higher fault level as a benefit
   C. identical loads
   D. no need for control
   Answer: A. Reserve is pooled and non-coincident peaks reduce capacity. (Key point: Importance)

2. At constant frequency:
   A. sum PG = sum QD
   B. sum PG = sum PD + Ploss
   C. sum PG > sum PD always
   D. Ploss = 0
   Answer: B. Generation must equal demand plus losses. (Key point: P balance)

3. Surplus reactive power (light load, line charging) results in:
   A. voltage fall
   B. frequency fall
   C. voltage rise
   D. no change
   Answer: C. Excess Q raises the bus voltage. (Key point: Q-V relation)

4. Compared with frequency, voltage is:
   A. a system-wide quantity
   B. unaffected by Q
   C. controlled only by governors
   D. a local quantity balanced near where Q is needed
   Answer: D. Q cannot be transmitted far because of I^2 X losses. (Key point: Scope)

## Related in Power System Analysis II

- [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×
- [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
- [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
- [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×
- [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

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