The transfer of electric power from generating stations to the consumer's load is known as an electric supply system. The electric supply system mainly comprises transmission and distribution lines.
The transmission lines are necessary for bulk power transfer from a large group of generating stations to the main load centers, for system interconnection, and for the transfer of power from the main transmission network to distribution substations.
Transmission of power can be done in two ways, namely ac system and dc system. Every system has its own advantages and disadvantages let us see them.
Advantages of DC Transmission System :
- It requires only two conductors for the transmission of power. By using earth as a return path, power transmission using a single conductor is also possible. Hence, the cost of the conductor material is reduced.
- Potential stresses developed on the insulation in the dc system are 70% as compared to the ac system. Hence, it requires less insulation.
- In the case of dc transmission, there is no effect of inductance and capacitance, no power factor, and surge problems.
- There is no skin effect, hence the full cross-section of the conductor is utilized.
- Since the power factor remains unity and the voltage drop is very less in the case of dc transmission system for over long distances. This eliminates the use of stabilizers compared to ac transmission.
- The effect of the corona is reduced by using earth as a negative conductor and the lines as a positive conductor.
- Construction of dc lines is easy as compared to ac lines.
- The underground cables can be effectively used in dc power transmission because of less potential stress and negligible dielectric loss.
- For a given load and sending end voltage, the dc transmission decreases the voltage drop in the line which results in better voltage regulation compared to ac transmission.
- Less interference with communication lines.
Disadvantages of DC Transmission System :
- Electrical power cannot be generated at high voltage because of commutation problems.
- It is difficult to step up and step down the voltage in dc transmission.
- It requires costly switchgear, as the low resistance interruption method can not be used for the circuit breakers.
- The cost of converting, inverting stations is very high.
- It requires converting equipment at sending end and inverting equipment at receiving end substations.
Advantages of AC Transmission System :
- It is easy to generate electricity at high voltages (3.3 kV, 6.6 kV, and 11 kV) easily.
- The voltage generated can be easily stepped up by using a step-up transformer and also it can be stepped down by using a step-down transformer.
- The cost of the ac switchgear is less.
- Maintenance of ac substation is simpler.
- The transformer substations are much efficient than the Motor-Generator sets used in dc systems.
Disadvantages of AC Transmission System :
- The construction of transmission line towers is not easy for ac. Also, the cost required is high for ac transmission line towers compared to dc.
- The amount of conductor material required is more as compared to dc transmission lines.
- Due to the presence of skin effect the line resistance will increase.
- To avoid the corona effect, the spacing between line conductors should be kept more, which further increases the cost of towers.
- Alternators must be synchronized before they are made parallel.
- In cables the alternating current causes sheath loss.
