Electric power can be conveyed through either an AC and DC system. Every transmission system possesses its unique set of merits and demerits. Thus, in order to select the appropriate electric power transmission system, it is necessary to conduct a comparative analysis of the two systems.
Comparison between AC and DC System
Transmission of electric power by high voltage dc system is superior to that by high voltage ac system in many respects.
- DC system requires only two conductors for transmission and it is also possible to transmit the power through only one conductor by using earth as returning conductor,hence there is saving in copper and the cost is reduced.
- There is no inductance, capacitance, phase displacement, or surge problem in dc transmission.
- There is no Skin Effect in Transmission Lines in dc, cross section of line conductor is fully utilized.
- In the case of dc system the potential stress on the insulation is 1/√2 times that in ac system for the same working voltage.Hence,less insulation is required for same working voltage.
- There is no charging current.Hence the loss on no load is eliminated.
- A dc line has less Corona in the transmission line and reduced interference with communication circuits.
- Due to less potential stress and negligible dielectric loss underground cables can be used.
- Since there is no inductance,the voltage drop in a dc transmission line due to inductive reactance does not exist.Hence for the same load and sending-end voltage,voltage regulation for the dc transmission line is better.
- There is no stability problems and synchronizing difficulties in dc transmission system.
There is difficulty in obtaining the necessary high voltage required for transmission as neither electrical power can be generated at high voltage because of commutating difficulties nor voltage can be stepped up.
Advantages of AC system
For the ac system, the main advantage is that the voltage can be stepped up at generating end using a step-up Transformer to the required value for transmission and then stepped down at distributing end using step-down transformers for distribution. Electric power can be generated at high voltage easily and the maintenance of ac substations is easier and cheaper in the case of ac systems.
The disadvantage of ac systems are as follows:
- In case of overhead lines spacing between the conductor is to be kept more in order to provide adequate insulation and to avoid corona loss.
- More copper is required. Hence,the more cost is required.
- Greater the length of the transmission line more is the capacitance of the line.There is a continuous loss on account of charging current even though the line is open.
- Construction of transmission lines for ac system is difficulty than in dc system.
- In case of ac system,the alternators are to be synchronized before putting them into parallel.
- The variation in speeds of alternators are to be controlled within very low limits.
Distribution by ac system is superior to that by dc system as in ac system voltage control is easy utilizing transformers. Ac system can be employed for generation and distribution and dc system for transmission.
Transmission of electric power by dc system became possible because of introduction of mercury-arc rectifiers and thyratrons*,which can convert ac into dc and vice versa efficiently and at reasonable cost.
A single-line diagram of an hv dc transmission system is shown in the above figure. The electric power is generated as ac and stepped up to high voltage by a step-up transformer and converted into dc by rectifying device. The transmission of electric power is carried at a high dc voltage. Dc power is converted back into 3-phase ac using suitable converters and then stepped down to low voltage for distribution by suitable step-down transformers.