Difference between HVAC and HVDC Transmission Systems

Differences Digital Electronics Electronics & Electrical

Electrical power is produced at the generating plants, from where it is transmitted over the long distances for utilization at the load points. Since, the transmission of electrical power is performed with the help of transmission conductor. Hence, due to the line parameter, some power loss occurs during the transmission.

Hence, in order to reduce the power loss during the transmission, we need to take some necessary actions. One major of them is to increase the transmission voltage to higher values.

The line voltage plays an important role in reducing the losses in transmission line. The increase in the line voltage reduces the line current and hence the power losses.

As we have two types of electric supplies namely AC (Alternating Current) and DC (Direct Current). Hence, based in that the high voltage electrical transmission systems are classified into two types as:

HVAC (High Voltage Alternating Current) Transmission System
HVDC (High Voltage Direct Current) Transmission System

Read through this article to understand the major differences between HVAC (High Voltage Alternating Current) and HVDC (High Voltage Direct Current) transmission systems. Let's start with a basic overview of what HVAC and HVDC systems are.

What is HVAC Transmission?

HVAC stands for High Voltage Alternating Current. When the supply voltage of the transmission system is ranging from 33 kV AC to 230 kV AC, it is called high voltage alternating current (HVAC) transmission.

In the HVAC transmission, the power generated is stepped up to the high voltages and then transmitted by the transmission lines. It requires at least three line conductors for transmitting the three phase electrical power. However, the HVAC voltage transformation and transmission is simple and inexpensive.

What is HVDC Transmission?

HVDC stands for High Voltage Direct Current. The type of high voltage transmission system when the power is transmitted in the form of DC at a voltage between 100 kV to 800 kV is called HVDC (High Voltage Direct Current) transmission system.

In this system, the electrical power produced in the form of AC is converted into DC with the help of rectifiers and then transmitted through the HVDC lines, and at the utilization end it is again converted into AC. The major advantage of the HVDC is that it requires only two conductors for transmission and has comparatively low power losses over the long distances.

Difference between HVAC and HVDC Transmission

Both HVAC and HVDC are extensively being used in power transmission. However, there are several differences between HVAC and HVDC transmission that are highlighted in the following table −

Basis of Difference	HVAC Transmission	HVDC Transmission
Full form	HVAC stands for High Voltage Alternating Current.	HVDC stands for High Voltage Direct Current.
Description	A transmission system that transmits AC power at voltage about 33 kV to 230 kV is known as HVAC transmission system.	A transmission system that transmits DC power at a voltage about 100 kV to 800 kV is known as HVDC transmission system.
Number of conductors	HVAC transmission system requires at least three line conductors.	HVDC system requires two conductors in bipolar system and one conductor in a monopolar system.
Transformer	In HVAC, transformer is used for voltage transformation.	In HVDC, transformer cannot be used, because the transformer does not work on DC.
Rectifier and inverter	HVAC does not require rectifiers and inverters.	The rectifier and inverter are the crucial components of an HVDC system.
Voltage transformation complexity	HVAC involves simple voltage transformation.	The voltage transformation is complex in case of HVDC.
Suitability	HVAC transmission systems are suitable for distances less than 600 km.	HVDC transmission is suitable for high power transmission at long distances, typically more than 600 km.
Corona loss	In HVAC, the corona loss is more.	HVDC has comparatively less corona loss.
Skin effect	Due to uneven current density, there is skin effect in HVAC system.	In HVDC, current density is uniform in the conductor, there is no skin effect.
Conductor diameter	HVAC requires a conductor of large diameter due to skin effect.	HVDC requires a conductor of relatively smaller diameter.
Right of way	The right of way is broader for HVAC. Right of way is the strip of land required for installation of transmission lines.	The right of way is narrower for HVDC.
Tower size	The tower of HVAC system are tall.	HVDC requires small sized towers.
Submarine power transmission	HVAC is not preferred for submarine power transmission because of stray capacitance of cables.	HVDC is preferably used for submarine power transmission.
Interference	HVAC causes interference with the neighboring communication lines.	HVDC does not cause interference with the nearby communication lines.
Circuit breakers	The HVAC circuit breakers are less expensive and have simple design.	The HVDC circuit breakers are comparatively expensive and have complex design.
Utilization of conductor capacity	In HVAC system, almost 30% of conductor capacity is wasted due to AC peak ratings during the delivery of average power.	HVDC system utilizes full conductor capacity. It is because in case of DC, the peak and average ratings are same.
Cost	HVAC is a less expensive system for power transmission.	HVDC system is little expensive than HVAC.

Conclusion

The most significant difference between HVAC and HVDC is that HVAC requires a minimum of three line conductors, while HVDC requires only two conductors.