Case Study:

India’s national power grid supplies electricity to millions of homes, industries, and transport systems. Most electrical energy generated at power plants is Alternating Current (AC), which changes direction periodically at a frequency of 50 Hz in India. AC power is transmitted through high-voltage transmission lines and then stepped down for domestic use. 

One major reason AC is widely used is its compatibility with transformers, which can easily step voltage up or down. When electricity is transmitted over long distances, the voltage is increased to hundreds of kilovolts to reduce current. According to the relation (P = VI), transmitting power at higher voltage reduces current, which significantly reduces transmission losses because power loss in wires is proportional to (I^2R).

However, modern power systems also use High Voltage Direct Current (HVDC) technology in certain situations. For example, India operates HVDC links such as long-distance transmission corridors connecting remote hydroelectric or renewable energy sources to major cities. HVDC systems can be more efficient for very long distances (over several hundred kilometres) because they avoid issues such as reactive power losses and skin effect that occur in AC lines. 

Consider a simplified scenario: a power station needs to transmit 500 MW of power to a city located 800 km away. Engineers must decide whether to use high-voltage AC transmission or HVDC transmission. AC systems are easier to integrate into the existing grid and use cheaper equipment like transformers. However, HVDC systems may reduce losses over extremely long distances but require expensive converter stations to convert AC to DC and back again. 

Thus, engineers must evaluate efficiency, cost, grid compatibility, and transmission distance before choosing between AC and DC systems.

Questions

Section A - MCQs

1. Why is high voltage used during long-distance AC power transmission?
A. To increase current in transmission lines
B. To reduce energy loss due to resistance
C. To increase resistance of wires
D. To eliminate transformers

2. If power transmitted is constant, increasing the voltage will:
A. Increase current proportionally
B. Decrease current in the transmission line
C. Keep current unchanged
D. Increase power loss

3. HVDC transmission is often preferred over HVAC for very long distances mainly because:
A. It completely eliminates electrical resistance
B. It avoids reactive power losses and can transmit power efficiently over long distances
C. It requires no conversion equipment
D. It operates without voltage

4. Which device allows AC voltage to be easily stepped up or stepped down in transmission systems?
A. Rectifier
B. Transformer
C. Capacitor
D. Inductor

Section B - Short Answer Questions

1. Explain why high voltage reduces power loss in transmission lines using the relation (P = VI). 

2. Why are transformers suitable for AC but not for DC systems? 

3. State two advantages and one limitation of HVDC transmission systems.

Section C - Long Answer Question

1. A power plant must transmit 200 MW of electrical power to a city.

Case A: Transmission voltage = 20 kV
Case B: Transmission voltage = 200 kV

1. Calculate the current in both cases using (P = VI).
2. Explain how increasing voltage affects power loss in the line.
3. Based on your calculations, justify why power companies prefer high-voltage transmission systems.

Answer Key

MCQ Answers:

1. B
2. B
3. B
4. B

Section B Answers

1. High voltage reduces power loss

Given (P = VI)

For constant power:
If voltage increases --> current decreases.

Since power loss in wires is P_loss = I^2R

Lower current --> much smaller energy loss.

2. Transformers and AC

• Transformers work on electromagnetic induction, which requires a changing magnetic field.
• AC current continuously changes direction, producing a changing magnetic field.
• DC produces a constant current, so it cannot induce changing flux in the transformer core.

3. HVDC advantages and limitation

Advantages:

• Lower losses for extremely long transmission distances.
• No reactive power or frequency synchronization issues.

Limitation:

• Requires expensive converter stations to convert AC to DC and back.

Section C Solution

Given:

Power P = 200 MW = 200 * 10⁶ W

Case A

Voltage V = 20 kV = 2 *10⁴ V

I = P / V
I = 200 * 10⁶ / 2 * 10⁴
I = 10,000 A

Case B

Voltage V = 200 kV = 2 * 10⁵ V

I = 200* 10⁶  / 2*10⁵
I = 1000 A

Interpretation

• Current reduced from 10,000 A --> 1,000 A.
• Since power loss P_loss = I^2R, reducing current drastically reduces heat loss.
• Therefore, high-voltage transmission is more efficient.