Correct Answer - Option 3 : Electric power can be generated at high DC voltage without any commutation problems.
DC transmission: Now a day power transmitted by the DC has taken into consideration due to the following advantages over the AC transmission.
- Only two conductors are required as compared to three conductors for AC transmission.
- There is no inductance (L), capacitance (C), phase displacement, and surge problems in DC transmission.
- Due to the absence of inductance, the inductive reactance drop is neglected hence voltage drop in a DC transmission line is less than the AC line for the same load and sending end voltage. Therefore, it 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, electrostatics stress on the insulation is less than that of AC transmission. Hence, DC transmission required 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.
Disadvantages of DC transmission:
- Electric power cannot be generated at a high DC voltage due to commutation problems in the DC generator.
- The DC voltage cannot be stepped up for transmission of power at high voltages.
- There is some limitation in DC protective device i.e, DC Switch, DC Circuit Breaker.
AC transmission: The majority of transmission of power is done by AC transmission due to the following advantages.
- The power can be generated at high voltages i.e, up to 33 kV
- The maintenance of AC sub-stations is easy and cheaper.
- The AC voltage can be stepped up or stepped down easily.
- This permits to transmit power at high voltages and distribute it at safe potentials.
Disadvantages of AC transmission:
- An AC line requires more conductor material than a DC line.
- More complicated construction than a DC transmission line.
- Due to the skin effect in the AC system, the effective resistance of the line is increased which causes a further voltage drop.
- An AC line has capacitance. Therefore, there is a continuous loss of power due to charging current even when the line is open by means of the Ferranti effect.
