There are some major reasons why AC power is preferred over DC power lines for transmission:

• AC voltage helps to convert the voltage levels with one transformer only, making long-distance transportation much simpler than DC, which involves converting more complex electronic circuits.                                                

             Power = V x I x PF (P=Power, V=Voltage, I= current and PF=Power factor)

• In case of higher voltage level, the more power can be supplied to customers for the same amount of current. The line current is decreased with an increase in voltage to transfer the same power to the long distance. It is, therefore, necessary to change the voltage level for transmission without interruption. The AC transmission system converts voltage and current with the aid of the transformer without changing the frequency.
• AC power takes advantage of using transformers to step-up the voltage at the power station and can step-down the voltage for the end-users whereas the booster and chopper are used for stepping up and stepping down the voltage in the DC transmission line.
• DC is equipped with the unity power factor at a maximum power transmission range of 500 miles because of the capacitive and inductive reactance losses of the transmission line and the effect of resistance in AC. DC is only affected by the resistance losses of the cables.
• The AC does not need any converters or inverter at the consumer end because all the equipment works on AC. But in some cases like the generation from renewable energy is based on DC, which requires converter or inverter.
• DC systems are very costly, which is one of the reasons why they are not generally used below the transmission voltage.

Historically, we have seen the AC power line for transmission rather than DC power. But nowadays, specific conditions for increasing transmission efficiency and reliability will cause us to challenge the use of alternative current (AC) transmission systems.

Intensive work is being carried out on direct current (DC) systems because of the limitations of AC voltage systems such as power losses, unavoidable transfer of reactive power and the increasing power losses due to skin effect or corona. Due to high transmission power values, it isn’t very easy to use long cables (submarines).

We can use the DC transmission for extra-high voltage and ultra-high voltage transmission for the following reasons:

• The AC transmission line has issues with synchronization and reliability, while the DC transmission line is reliable and synchronized. DC is direct current having zero frequency. It is possible to connect two different systems without the synchronization of the systems.
• This is important in terms of system stability, as this separation helps to distinguish two separate systems in the event of an electrical failure, such as lightning or equipment failure. The DC circuit limits the fault path, effectively protecting the unaffected part of the linked network. The connection of DC is an open circuit compared to the AC system.
• For DC, the power route can be changed more easily. This makes DC useful in places where there are opposite summer loads and winter loads, such as a country that is colder in winter in one area and warmer in summer in another. Thus in result, the DC circuit could be used to export electricity from the warmer area to the colder region in winter and from the cooler region to the warmer region in summer.
• Few DC system losses occur as there is no reactive part present at a DC circuit, so there are no AC system losses, such as capacitive, inductive or skin effects.