Comparison Of Ac And Dc Transmission

ac and dc transmission: Electrical energy generated at power stations can be transmitted to different load centres, using overhead lines or underground cables. The electric power can be transmitted either by using AC transmission system or DC transmission system.The two ways in which the electrical energy is transmitted are: primary transmission and secondary transmission.

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ac and dc transmission

Generally, the electrical energy is transmitted at higher voltages, using overhead
lines in primary transmission and using underground cables in secondary transmission.
The advantages of transmitting electrical energy at higher voltages are:

Less volume of conductor material is required.
The current flowing through the line is reduced, which in turn reduces line losses and helps in increasing the efficiency of transmission
Voltage regulation of the transmission system gets improved.

primary transmission and Secondary transmission

In primary transmission, the electrical energy is transmitted to main-load centres, where the voltage level is 1500 kV or 700 kV or 400 kV or 230 kV.

In secondary transmission, the electrical energy is transmitted to sub-load centres, where the voltage level is 33 kV or 66 kV or 110 kV.

The electrical energy can also be transmitted using DC transmission.

AC Transmission

DC Transmission

The comparison between AC and DC transmissions is listed in Table

Comparison Between AC and DC Transmissions

Difference between ac and dc transmission is given in the table below.

AC Transmission	DC Transmission
Requires three conductors for transmission and hence requires more copper material.	Requires two conductors for transmission and hence requires less copper material.
Inductance, capacitance, phase displacement and surge problems exist.	No such problems exist.
Voltage drop in the transmission line is more due to the presence of inductance.	Since no inductance exists, voltage drop is less.
Poor voltage regulation	Better voltage regulation
Skin effect exists	Absence of skin effect
Requires more insulation, as the potential stress on the conductors is more.	Potential stress on the conductor is less and hence requires less insulation.
Corona loss and interference with communication circuits are more.	Less corona loss and reduced interference with communication circuits.
Dielectric loss exists in the system.	Free from dielectric loss in the system.
Stability problem and synchronizing difficulties exist in the system.	DC transmission system is free from these problems.
AC voltage can be stepped up for transmission of power.	DC voltage cannot be stepped up for transmission.
Electrical energy can be generated at a high AC voltage.	Commutation problem exists in generating electrical energy at a high DC voltage.
Easy and cheaper to maintain an AC system.	Difficult and costlier to maintain a DC system.
Difficult to construct	Easier to construct