VOLTAGE AND FREQUENCY CONTROL – GATE LEVEL NOTES

 

VOLTAGE AND FREQUENCY CONTROL 

Power systems must maintain:

• Voltage → constant

• Frequency → constant (50 Hz in India)

Control is done using:

• Voltage control → Reactive power control (Q)

• Frequency control → Active power control (P)

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1. FREQUENCY CONTROL (ACTIVE POWER CONTROL)

Basic Principle

Frequency depends on balance between:

$$P_{generation} = P_{load}$$

If:

$$P_{gen} > P_{load} \Rightarrow \text{Frequency increases}$$

If:

$$P_{gen} < P_{load} \Rightarrow \text{Frequency decreases}$$

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Swing Equation (VERY IMPORTANT FOR GATE)

$$\frac{2H}{\omega_s} \frac{d^2 \delta}{dt^2} = P_m - P_e$$

Where:

• H = inertia constant

• Pm = mechanical power

• Pe = electrical power

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LOAD FREQUENCY CONTROL (LFC)

Maintains constant frequency by controlling generator input.

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Levels of Frequency Control

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1. Primary Frequency Control

Done using Governor

Governor senses frequency change and adjusts turbine input.

Characteristic:

$$\Delta P = -\frac{1}{R} \Delta f$$

Where:

R = speed regulation

This is called Droop characteristic

Typical droop:

$$R = 4\% \text{ to } 5\%$$

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Droop Characteristic (VERY IMPORTANT)

Graph:

Power vs Frequency → Negative slope

Meaning:

If frequency decreases → Power increases

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2. Secondary Frequency Control

Done using Automatic Generation Control (AGC)

Purpose:

Restore frequency to exactly 50 Hz

Eliminates steady-state error.

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Area Control Error (ACE)

$$ACE = \Delta P_{tie} + B \Delta f$$

Where:

B = frequency bias constant

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3. Tertiary Control

Manual control by operator.

Used for economic dispatch.

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FREQUENCY CONTROL SUMMARY

Control	Device	Speed
Primary	Governor	Fast
Secondary	AGC	Medium
Tertiary	Operator	Slow

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VOLTAGE CONTROL (REACTIVE POWER CONTROL)

Voltage depends on reactive power balance.

$$Q_{gen} = Q_{load}$$

If:

$$Q_{gen} > Q_{load} \Rightarrow \text{Voltage increases}$$

If:

$$Q_{gen} < Q_{load} \Rightarrow \text{Voltage decreases}$$

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METHODS OF VOLTAGE CONTROL

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1. Generator Excitation Control (MOST IMPORTANT)

Uses Automatic Voltage Regulator (AVR)

Controls field current.

Voltage relation:

$$V \propto I_f$$

Where:

If = field current

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Automatic Voltage Regulator (AVR)

Function:

Maintains constant generator voltage.

Block diagram components:

• Voltage sensor

• Comparator

• Amplifier

• Exciter

• Generator

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2. Shunt Capacitors

Supply reactive power.

Used in distribution systems.

Effect:

Increase voltage

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3. Shunt Reactors

Absorb reactive power.

Effect:

Reduce voltage

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4. Tap Changing Transformer

Changes voltage magnitude.

Formula:

$$V_2 = \frac{N_2}{N_1} V_1$$

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5. Synchronous Condenser

Over-excited synchronous motor.

Supplies reactive power.

Used for voltage support.

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VOLTAGE CONTROL DEVICES SUMMARY

Device	Function
AVR	Controls generator voltage
Capacitor	Increases voltage
Reactor	Decreases voltage
Tap changer	Adjusts voltage
Synchronous condenser	Supplies reactive power

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DIFFERENCE BETWEEN VOLTAGE AND FREQUENCY CONTROL

Parameter	Frequency Control	Voltage Control
Controls	Active power (P)	Reactive power (Q)
Device	Governor	AVR
Depends on	Real power balance	Reactive power balance
Purpose	Maintain 50 Hz	Maintain voltage level

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IMPORTANT GATE CONCEPTS

Frequency controlled by:

Governor

Voltage controlled by:

AVR

Frequency depends on:

Active power

Voltage depends on:

Reactive power

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DROOP FORMULA (VERY IMPORTANT)

$$R = \frac{\Delta f / f}{\Delta P / P}$$