Comparison between AC and DC Transmission
Electric Power can be transmit in both AC and DC
But there are some advantages and disadvantages of both systems.
So it is important that we discuss technical advantages and disadvantages of both AC and DC Systems.
Some times ago. The Electric power transmission was in DC due to the following advantages.
Advantages of DC Transmission
There are two conductors used in DC transmission while three conductors required in AC transmission.
There are no Inductance and Surges (High Voltage waves for very short time) in DC transmission.
Due to absence of inductance, there are very low voltage drop in DC transmission lines comparing with AC (if both Load and sending end voltage is same)
There is no concept of Skin effect in DC transmission. Therefore, small cross sectional area conductor required.
A DC System has a less potential stress over AC system for same Voltage level. Therefore, a DC line requires less insulation.
In DC System, There is no interference with communication system.
In DC Line, Corona losses are very low.
In High Voltage DC Transmission lines, there are no Dielectric losses.
In DC Transmission system, there are no difficulties in synchronizing and stability problems.
DC system is more efficient than AC, therefore, the rate of price of Towers, Poles, Insulators, and conductor are low so the system is economical.
In DC System, the speed control range is greater than AC System.
There is low insulation required in DC system (about 70%).
The price of DC cables is low (Due to Low insulation)
In DC Supply System, the Sheath losses in underground cables are low.
DC system is suitable for High Power Transmission based on High Current transmission.
In DC System, The Value of charging current is quite low, there fore, the length DC Transmission lines is greater than AC lines.
Disadvantages of DC Transmission:
Due to commutation problem, Electric power can’t be produce at High (DC) Voltage.
For High Voltage transmission, we can not step the level of DC Voltage (As Transformer can not work on DC)
There is a limit of DC Switches and Circuit breakers (and costly too)
Motor generator set is used for step down the level of DC voltage and the efficiency of Motor-generator set is low than transformer.
so the system makes complex and costly.
The level of DC Voltage can not be change easily. So we can not get desire voltage for Electrical and electronics appliances (such as 5 Volts, 9 Volts 15 Volts, 20 and 22 Volts etc) directly from Transmission system.
Nowadays, the generation, transmission and distribution mostly is in AC.
Advantages of AC Transmission System
AC Circuit breakers is cheap than DC Circuit breakers.
The repairing and maintenance of AC sub station is easy and inexpensive than DC Substation.
The Level of AC voltage may be increased or decreased step up and Step down transformers.
Disadvantages of AC System
In AC line, the size of conductor is grater than DC Line.
The Cost of AC Transmission lines are greater than DC Transmission lines.
Due to Skin effect, the losses in AC system are more.
In AC Lines, there is Capacitance, so continuously power loss when no load on lines or Line is open.
Other line losses are due to inductance.
More insulation required in AC System
Also corona Losses occur In AC System,
There is telecommunication interference in AC System.
There are stability and synchronizing problems in AC System.
DC System is more efficient than AC System.
There are also re-active power controlling problems in AC System.
Note: Nowadays, it is possible to step up or step down the level of DC Voltages by Choppers and Boosters. Also there is no commutation problem and we can use rectifier units for that purpose.