What are the advantages and disadvantages of DC and AC Transmission?

Electric power can be transmitted either by AC transmission system (i.e. the voltage and current are alternating) or DC transmission system (i.e. the voltage and current are direct or unidirectional). Each transmission system has its own advantages and disadvantages. Therefore, we need to discuss the technical advantages and disadvantages of the AC and DC transmission systems so that we can select the right system for the electric power transmission.

AC Transmission System

The AC transmission system is the one in which the alternating current is employed for the transmission of electric power. Nowadays, electric power is almost generated, transmitted and distributed in the form of AC supply.

Advantages of AC Transmission

The AC transmission system has the following primary advantages −

  • In an AC system, the electric power can be generated at high voltages (such as 3.3 kV, 6.6 kV, 11 kV).

  • The AC voltage can be increased with the help of a step-up transformer or can be decreased using the step-down transformer easily and efficiently. Therefore, the AC transmission permits to transmit the electric power at high voltages and to distribute it at lower voltages.

  • The repair and maintenance of AC substation and transmission lines is easy and less expensive.

  • The AC switchgears such as circuit breakers are cheaper than DC switchgear.

Disadvantages of AC Transmission

The following are the disadvantages of AC transmission system −

  • The construction of AC transmission lines is more complicated than the DC transmission lines.

  • AC transmission lines require more conductor material than the DC transmission lines as three wire are required for AC transmission.

  • The effective resistance of the AC transmission line is higher than DC transmission line. It is because the skin effect takes place in AC transmission line.

  • An AC transmission line has line capacitance. Therefore, there is a continuous power loss in the AC transmission line due to line charging current even when the line is open.

DC Transmission System

The DC transmission is the transmission of electric power when the direct current system is employed for the power transmission.

Advantages of DC Transmission

The high voltage DC transmission system has the following advantages −

  • DC transmission requires less conductor material than AC transmission as only two wire are required for the power transmission through DC system.

  • DC transmission lines are free from the skin effect. Therefore, the entire cross-section of the line conductor is utilised, hence the effect resistance of the line is small.

  • There is no capacitance in the DC transmission. Therefore, there is no power loss due to the charging current.

  • There is no inductance, phase displacement, and surge problems in the DC transmission.

  • For the same sending end voltage and load conditions, the voltage drop in the DC transmission line is less than the AC transmission line. It is because of the absence of inductance in DC transmission line.

  • A DC transmission line has better voltage regulation than an AC transmission line.

  • For the same voltage, A DC transmission system requires less insulation material because the potential stress on the insulation is less in case of DC transmission system than that in AC transmission system.

  • DC transmission does not suffer from stability and synchronizing problems.

  • In high-voltage DC transmission, there is no dielectric losses.

  • A DC transmission line has less corona loss and reduced interference with the communication circuits.

Disadvantages of DC Transmission

The main disadvantages of DC transmission are as follows −

  • Electric power cannot be generated at high DC voltage because of commutation problems.

  • DC switchgears have their own limitations and they are also expensive than the AC one.

  • DC voltage cannot be directly step-up or step-down for transmitting the power at high voltages and for distributing it at low voltages.

  • It requires extra equipment such rectifier and inverter, etc. which increases the cost of transmission.