The transmission of electric power over long distances was developed at the beginning of the twentieth century. Since then, it has made rapid progress in its design and methods of operation, which has resulted in greater reliability and continuity.
The electric power can be transmitted either by means of an AC or DC system. Each system has its own merits and demerits. It is desirable to discuss the advantages and disadvantages of the two methods to make the comparison of AC and DC transmission system.
Advantages of DC Transmission System
1. It requires only two conductors for transmission as compared to three conductors in an AC system. It is also possible to transmit power through only one conductor by using earth as a returning conductor causing much saving in conductor material (copper or aluminum).
2. Inductance, capacitance, phase displacement, and surge problems are eliminated in DC transmission.
3. There is no skin effect in DC. Therefore, the entire cross-section of the line conductor is utilized. Thus, the conductor offers less resistance.
4. Since there is no inductance and capacitance, the voltage drop in a DC transmission line is less than an AC line for the same sending end voltage and load. Therefore, a DC transmission line has better voltage regulation.
5. For the same working voltage, the potential stress on the insulation (in case of DC system is 70% (1/√2times) of that in the AC system. Hence, a DC line requires less insulation.
6. There is less corona loss in a DC line, which reduces the interference with communication circuits.
7. There are no stability problems and synchronizing difficulties in the DC transmission.
Disadvantages of the DC Transmission System
1. Electric power cannot be generated at high voltages in the DC system due to commutating difficulties.
2. For the transmission of power at the high voltages, the DC voltages cannot be stepped up.
3. The DC switchgear (i.e., circuit breakers) have their own limitations.
Nowadays, a high voltage AC transmission system (3-phase, 3-wire) is mostly employed. To make the comparison of AC and DC transmission system following advantages and disadvantages of a high voltage AC transmission system should be kept in mind:
Advantages of High Voltage AC Transmission System
1. The AC power can be generated at high voltages (33 kV or 11 kV).
2. The AC voltage can be stepped up easily and efficiently to the economic limit for transmission by transformers. It can also be stepped down by transformers to any value as per requirement for distribution and utilization.
3. The maintenance of AC switchgear is easy and cheaper.
Disadvantages of High Voltage AC Transmission System
1. It requires more conductor material (copper or aluminum).
2. The construction of an AC transmission line is more complicated than the DC transmission line.
3. Due to skin effect, the effective resistance of the AC line is more, which increases the losses and correspondingly decreases the efficiency of transmission.
4. Due to inductance and capacitance, the voltage drop in the AC line is more than the DC line for the same sending end voltage and load. This makes the voltage regulation poor.
5. The AC transmission lines have more corona loss, which creates interference in communication circuits.
Comparison of AC and DC Transmission System
A 3-phase, 3-wire AC system is employed for transmission of electric power because, in this case, power at high voltage can be generated, and the voltage can be stepped up and stepped down very easily and efficiently by using transformers.
However, in the above discussion, it is clear that high voltage DC transmission is superior to high voltage AC transmission.
Therefore, it is preferred to adopt a high voltage DC transmission. This can be achieved by using rectifiers and inverters to convert AC into DC and vice-versa easily and efficiently. The present-day trend is towards AC for generation and distribution and high voltage DC for transmission.
The 3-phase AC power is generated at the generating station, and voltage is stepped up by transformer T1, located at the sending end. Then AC is converted into high voltage DC by rectifiers, and power is transmitted. Then at the receiving station, DC is converted into AC by inverters, and a transformer T2 steps down the voltage as per requirement for distribution of electric power.
A graph, cost versus transmission distance for AC transmission and DC transmission (HVDC – High Voltage DC Transmission ) is shown in Figure. It is clear from the graph that AC transmission is economical for smaller transmission distances, whereas DC transmission is economical for larger transmission distances.
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