Question

HVDC transmission is preferred over HVAC for very long distances because:

• A. DC equipment is cheaper.
• B. There are no charging current losses or skin effect.
• C. It doesn't require transformers.
• D. It is easier to tap off power.

Hint:

HVDC is the only practical solution for long undersea power transmission (cables > 50 km).
This is because the high capacitance of submarine AC cables draws massive charging currents, which can completely overload the cable with reactive current alone.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the technical reason why High Voltage Direct Current (HVDC) transmission is preferred over High Voltage Alternating Current (HVAC) transmission for long-distance power transmission.

Step 2: Detailed Explanation:

• AC transmission lines have continuous line-to-ground capacitance. This capacitance draws a continuous charging current under AC voltage:
\[ I_c = 2\pi f C V \]
• For long lines or underground/undersea cables, this charging current becomes very large, causing significant reactive power flow and line losses even at no-load. This limits the useful active power capacity of the line.

• In DC systems, the steady-state frequency is zero (\(f = 0\)). Therefore, after the initial charging phase, there is zero charging current, eliminating capacitive reactive power losses.

• Furthermore, steady-state DC current distributes itself uniformly across the entire cross-section of the conductor. This means there is no skin effect, resulting in lower effective conductor resistance and reduced \(I^2 R\) thermal losses compared to AC.

• Although converter stations make HVDC systems more expensive at shorter distances, for distances exceeding the "breakeven distance" (typically 600–800 km for overhead lines), the savings from lower line losses and cheaper line construction (2 conductors instead of 3) make HVDC more economical.

Step 3: Final Answer:
HVDC is preferred because there are no charging current losses or skin effect.