AC and DC Transmission – Concepts (SSC JE + GATE)

 

AC and DC Transmission – Concepts (SSC JE Level)

Transmission of electrical power can be done using:

• AC Transmission
• DC Transmission

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1. AC Transmission

Definition

Transmission of electrical power using Alternating Current (AC) is called AC transmission.

Most commonly used system worldwide.

India uses 3-phase AC transmission.

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Basic Equation

$$𝑃=𝑉𝐼\cos 𝜙$$

Where:
P = Power (W)
V = Voltage (V)
I = Current (A)
cosφ = Power factor

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Features of AC Transmission

• Uses 3 conductors (3-phase)
• Voltage easily increased or decreased using transformer
• Generates reactive power
• Has inductance and capacitance effects

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Advantages of AC Transmission

• Easy voltage transformation
• Easy generation
• Simple switching
• Low cost substations

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Disadvantages of AC Transmission

• Skin effect present
• Corona loss present
• Reactive power loss
• Charging current present
• Higher losses over long distance

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2. DC Transmission

Definition

Transmission of electrical power using Direct Current (DC) is called DC transmission.

Uses constant voltage and current.

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Basic Equation

$$𝑃=𝑉𝐼$$

(No power factor)

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Features of DC Transmission

• Uses 2 conductors
• No reactive power
• No skin effect
• No charging current
• More efficient for long distance

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Advantages of DC Transmission

• No reactive power loss
• No skin effect
• Lower losses
• Better for long distance
• Suitable for underground and submarine cables

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Disadvantages of DC Transmission

• Cannot use transformer directly
• Requires converter stations
• High initial cost
• Complex system

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3. Key Difference Between AC and DC Transmission

Feature	AC Transmission	DC Transmission
Current type	Alternating	Direct
Conductors	3	2
Power factor	Present	Not present
Reactive power	Present	Absent
Skin effect	Present	Absent
Charging current	Present	Absent
Losses	More	Less
Cost	Low initial cost	High initial cost
Long distance	Less suitable	More suitable

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4. Skin Effect (Important Concept)

Definition:
Current flows more on surface of conductor in AC.

Effects:

• Increases resistance
• Increases losses

Skin effect present in AC only
Skin effect absent in DC

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5. Corona Effect

Definition:
Loss due to ionization of air around conductor.

Occurs in AC transmission lines.

Effects:

• Power loss
• Noise
• Interference

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6. Charging Current

Definition:
Current due to capacitance between line and ground.

Present in AC
Absent in DC

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7. HVDC Transmission (High Voltage DC)

Used for:

• Very long distance transmission
• Submarine cables
• Underground cables

Example distances:

More than 600 km → DC preferred

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8. Why AC is Mostly Used?

Because:

• Easy voltage conversion using transformer
• Easy generation
• Lower equipment cost

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9. Why DC is Used for Long Distance?

Because:

• No reactive power
• No charging current
• Lower losses
• Higher efficiency

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10. SSC JE Exam Important Points (Must Remember)

• AC transmission most commonly used
• DC transmission best for long distance
• Skin effect present only in AC
• Charging current present only in AC
• DC has no reactive power
• AC uses 3 conductors, DC uses 2
• DC transmission has lower losses

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AC and DC Transmission – GATE Level Notes (Electrical Engineering)

These notes focus on mathematical models, performance, and comparison—important for GATE.

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1. Basic Power Equations

AC Transmission (3-phase)

Real Power:

$$P = \sqrt{3} V_L I_L \cos \phi$$

Reactive Power:

$$Q = \sqrt{3} V_L I_L \sin \phi$$

Apparent Power:

$$S = \sqrt{3} V_L I_L$$

Complex Power:

$$S = P + jQ$$

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DC Transmission

Power:

$$P = VI$$

No reactive power:

$$Q = 0$$

No power factor:

$$\cos \phi = 1$$

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2. Transmission Line Model (AC)

AC transmission line has four parameters:

• Resistance (R)
• Inductance (L)
• Capacitance (C)
• Conductance (G)

Equivalent circuit per phase:

Series impedance:

$$Z = R + j\omega L$$

Shunt admittance:

$$Y = G + j\omega C$$

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3. Transmission Efficiency

Efficiency:

$$\eta = \frac{Receiving\ End\ Power}{Sending\ End\ Power} \times 100$$

Loss:

$$P_{loss} = I^2 R$$

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4. Power Flow Equation (AC Transmission)

For lossless line:

$$P = \frac{V_S V_R}{X} \sin \delta$$

Where:

Vs = Sending voltage
Vr = Receiving voltage
X = Line reactance
δ = Power angle

Maximum power:

$$P_{max} = \frac{V_S V_R}{X}$$

Occurs at:

$$\delta = 90^\circ$$

This is called Steady State Stability Limit

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5. Ferranti Effect (Important GATE Concept)

Definition:

Receiving end voltage becomes greater than sending end voltage.

Occurs when:

• Line is lightly loaded
• Long transmission lines

Cause:

Line capacitance

Present only in AC transmission

Absent in DC transmission

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6. Charging Current in AC Line

Charging current:

$$I_c = \omega C V$$

Where:

ω = 2πf
C = capacitance
V = voltage

Present in AC
Absent in DC

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7. Skin Effect

Definition:

Current concentrates on surface of conductor in AC.

Skin depth:

$$\delta = \sqrt{ \frac{2\rho}{\omega \mu} }$$

Where:

ρ = resistivity
μ = permeability
ω = angular frequency

Skin effect:

Present in AC
Absent in DC

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8. Corona Loss

Occurs due to ionization of air.

Corona loss formula (Peek’s formula):

$$P_c \propto (V - V_c)^2$$

Where:

Vc = critical voltage

Effects:

• Power loss
• Radio interference

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9. Comparison: AC vs DC Transmission (GATE Level)

Parameter	AC Transmission	DC Transmission
Power B	( P = sqrt{3} V I Cos phi )	( P = V I )
Reactive power	Present	Absent
Power factor	Less than 1 ( <1 )	Equal to 1 (=1)
Skin effect	Present	Absent
Charging current	Present	Absent
Ferranti effect	Present	Absent
Stability limit	Limited	No stability limit
Losses	Higher	Lower
Conductors	3 conductors	2 conductors
Efficiency	Lower	Higher

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10. HVDC Transmission (Very Important)

HVDC = High Voltage Direct Current

Typical voltage:

±500 kV
±800 kV

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11. HVDC Power Equation

$$P = \frac{V_d^2}{R}$$

Where:

Vd = DC voltage
R = line resistance

Loss:

$$Loss = I^2 R$$

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12. Types of HVDC Links

(1) Monopolar

One conductor + ground return

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(2) Bipolar (Most common)

Two conductors:

+V and –V

Example:

+500 kV and –500 kV

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(3) Homopolar

Two conductors same polarity

Rarely used

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13. Advantages of HVDC

• No reactive power
• No charging current
• No Ferranti effect
• No stability limit
• Lower losses
• Suitable for long distance

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14. Disadvantages of HVDC

• Expensive converter stations
• Complex control
• Harmonics generation

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15. Converter Stations

Required to convert:

AC → DC (Rectifier)

DC → AC (Inverter)

Devices used:

Thyristors
IGBTs

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16. Break-even Distance (Very Important GATE Concept)

Definition:

Distance where DC transmission becomes economical compared to AC.

Typical values:

Overhead line:
500–800 km

Underground cable:
50–100 km

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17. Stability in AC vs DC

AC transmission power limited by:

$$P = \frac{V_S V_R}{X} \sin \delta$$

DC transmission power limited by thermal limit only.

No stability limit in DC.

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18. Most Important GATE Exam Points

Must remember:

• Power flow equation
• Ferranti effect
• Charging current equation
• Skin depth formula
• HVDC advantages
• Break-even distance
• Stability limit
• AC line parameters (R, L, C, G)