Comparison of D.C. and A.C. Transmission

Comparison of D.C. and A.C. Transmission

The electric power can be transmitted either by means of dc or ac. Each system has its own merits and demerits.

1. D.C. transmission. For some years past, the transmission of electric power by dc, has been receiving the active consideration of engineers due to its numerous advantages.

Advantages

(i)    It requires only two conductors as compared to three for a.c. transmission.

(ii)   There is no inductance, capacitance, phase displacement and surge problems.

(iii)  Due to the absence of inductance, the voltage drop in a d.c. transmission line is less. For this reason, a d.c. transmission line has better voltage regulation.

(iv) There is no skin effect in a d.c. system. Therefore, entire cross-section of the conductor is utilized.

(v)  For the same working voltage d.c. line requires less insulation.

(vi) A dc, line has less corona loss and reduced interference with communication.

(vii)In d.c. transmission, there are no stability problems and synchronizing difficulties.

 

Disadvantages

(i)    Electric power cannot be generated at high d.c. voltage due to commutation problems.

(ii)   The d.c. voltage cannot be stepped up for transmission of power at high voltages.

(iii)  The d.c. switches and circuit breakers have their own limitations. 

 

2. A.C. transmission.

Now-a-days, electrical energy is almost exclusively generated, transmitted and distributed in the form of a.c.

Advantages

(i)    The power can be generated at high voltages.

(ii)   The maintenance of a.c. sub-stations is easy and cheaper.

(iii)  The a.c. voltage can be stepped up or stepped down by transformers with ease and efficiency.

Disadvantages

(i)    An a.c. line requires more copper than a d.c. line.

(ii)   The construction of a.c. transmission line is more complicated than a dc. Transmission line.

(iii)  Due to skin effect in the a.c. system, the effective resistance of the line is increased.

(iv) An a.c. line has capacitance. Therefore, there is a continuous loss of power due to charging current even when the line is open.