Comparison between AC & DC Transmission Lines with their Advantages & Disadvantages
Electric power can be transmitted in both AC and DC for short and long transmission and distribution systems.
There are some advantages and disadvantages of both systems. Let’s discuss the technical advantages and disadvantages of both AC and DC power transmission lines systems.
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In earlier time, the electric power transmission was done in DC due to the following advantages over AC.
Advantages of DC Transmission
- There are two conductors are used in DC transmission while three conductors are required in AC transmission.
- There are no inductance and surges (High Voltage waves for very short time) in DC transmission.
- Due to the absence of inductance, there are very low voltage drop in DC transmission lines as compare to the AC (if both Load and Sending end Voltage is same)
- There is no concept of Skin effect in DC transmission lines. Therefore, conductor having small cross sectional area is required in DC transmission line.
- 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 other communication lines and systems.
- In DC Line, Corona losses are very low as compared to the AC transmission lines.
- In High Voltage DC (HVDC) Transmission lines, there are no dielectric losses.
- In DC Transmission system, there are no difficulties in synchronizing and related 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 needed in the 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, therefore, the length of the DC Transmission lines is greater than AC lines.
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Disadvantages of DC Transmission:
- Due to commutation problem, electric power can’t be produced at High (DC) Voltage.
- In High Voltage transmission, we cant step-up the level of DC Voltage (As Transformer won’t work on DC).
- There is a limitation of DC switches and circuit breakers (and they are costly too).
- The motor generator set is used to step down the level of DC voltage and the efficiency of Motor-generator set is lower than a transformer.
- DC transmission system is more complex and costly as compared to the AC transmission system..
- The level of DC Voltage can not be changed (step-up or step-down) 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 the transmission and distribution lines.
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Nowadays, the generation, transmission and distribution of electric power has mostly been in AC.
Advantages of AC Transmission System
- AC Circuit breakers are cheaper than DC Circuit breakers.
- The repairing and maintenance of the AC sub station is easier and inexpensive than DC Substation.
- The Level of AC voltage may be increased or decreased by using step up and Step down transformers.
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Disadvantages of AC System
- In AC line, the size of the conductor is grater than the DC Line.
- The cost of AC transmission lines is greater than DC Transmission lines.
- Due to skin effect, the losses in AC system are more.
- Due to the capacitance in AC transmission lines, a continuous power loss occurs when there is no load on the power lines or line is open at all.
- There are some additional line losses due to inductance.
- More insulation are required in AC transmission system.
- The corona losses occur in an AC transmission line system.
- AC transmission lines interfere with other communication lines.
- There are stability and synchronizing problems in AC System.
- AC transmission system is less efficient than the DC transmission System.
- There are difficulties in controlling the reactive power.
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The above comparison shows that DC transmission system is better than the AC transmission system but still, the majority of power transmission is done in AC power lines due to cost and uses of transformers for changing the level of voltages at different levels for different purposes.
Although, mercury arc rectifier, thyratron, diodes and semiconductors can be used to easily convert AC into DC and DC into AC. Therefore, some countries transmit the electric power through DC power lines. The range of these DC power transmission is up to 100kV to 800kV+.
Good to know:
- Recently in China, a 12GW power was transmitted at 11kV DC over a distance of 3300km.
- In addition, the longest HVDC power transmission line in the world is 2385km in Madeira, Brazil.
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- Why Does A Capacitor Block DC But Pass AC?
- Is Lightning AC or DC?
- FACTS – Flexible AC Transmission System – Types of FACTS Controllers & Devices
- What is HVDC? – High Voltage Direct Current Power Transmission
- Types of HVDC Systems and MTDC Configurations
- Differences Between HVAC and HVDC – Power Transmission
- Advantages of HVDC over HVAC Power Transmission