Question

Which of the following is NOT a disadvantage of HVDC transmission system?
1. The short circuit current required is high when compared to AC systems.
2. Ripples and harmonics are generated when AC is converted into DC.
3. Reactive power cannot be transmitted through a DC line.
4. Voltage cannot be stepped up.

Answer 1

Correct Answer - Option 1 : The short circuit current required is high when compared to AC systems.

Advantages: The high voltage DC transmission has the following advantages over high voltage AC transmission :

• It requires only two conductors as compared to three for AC transmission.
• There is no inductance, capacitance, phase displacement, or surge problems in DC transmission.
• Due to the absence of inductance, the voltage drop in a DC transmission line is less than the AC line for the same load and sending end voltage. For this reason, a DC transmission line has better voltage regulation.
• There is no skin effect in a DC system. Therefore, the entire cross-section of the line conductor is utilized.
• For the same working voltage, the potential stress on the insulation is less in the case of a DC system than that in an AC system. Therefore, a DC line requires less insulation.
• A DC line has less corona loss and reduced interference with communication circuits.
• The high voltage DC transmission is free from the dielectric losses, particularly in the case of cables.
• In DC transmission, there are no stability problems and synchronizing difficulties.
   

Disadvantages:

• Electric power cannot be generated at high DC voltage due to commutation problems.
• During conversion between AC to DC or vice-versa, there are always possibilities of ripple and harmonics components in the system.
• The DC voltage cannot be stepped up for transmission of power at high voltages.
• The DC switches and circuit breakers have their own limitations.
• There is always the requirement of reactive power to the alternator to generate electric power by means of the magnetic field, which requirement cannot be done by the HVDC System.