Comparison Of HVAC and HVDC Transmission Network

What is HVAC vs HVDC

Electrical power needs to transmit a distance from its generation station to transmission and distribution between users. Initially, transmission system born as a DC system, but finally stopped it and grew as an AC system. Now again retaining and developing HVDC for power transmission.

The feature comparison between HVDC and HVAC is as below:

Investment cost:

Considering the power carrying conductors, DC transmission requires fewer conductors than AC transmission - 2 conductors per DC circuit whereas three conductors per 3 phase AC circuit. 

HVDC needs smaller tower for line support thus, HVDC transmission line would cost lesser than an HVAC line. 

But, the terminal converter stations in HVDC are much more expensive which are not required for HVAC transmission. 

Over a certain line length what is called break-even distance, HVDC line becomes cheaper than HVAC. The break-even distance for overhead lines is around 600 km and for submarine lines, it is around 50 km.

Losses:

There are no skin effect losses in the DC system. Corona losses also significantly lower in the case of the DC system comparing the AC system. An HVDC line has considerably lower losses compared to HVAC over longer distances.

Controllability:

There is better voltage regulation in the HVDC system due to the absence of inductance. Also, the HVDC system offers greater controllability compared to HVAC.

Asynchronous interconnection:

We worldwide there are 50 Hz or 60Hz AC system is existed, it is not possible to the interconnection between different frequency 50 Hz and 60Hz AC power grids. But in the DC system, there is no issue about frequency.

Interference with nearby communication lines:

Interference with nearby communication lines is lesser in the case of HVDC overhead line than that for an HVAC line.

Short circuit current:

In longer distance HVAC transmission, short circuit current level in the receiving system is high. An HVDC system does not contribute to the short circuit current of the interconnected AC system.

What are the advantages of HVDC Power system?

The major advantages of the HVDC power transmission system are as below:

i) The use of this system reduces the number of conductors and insulators in the power system. As a result line cost is reduced.

ii) As a result of using this system, the amount of Phase to phase and ground to ground clearance is less.

iii) As a result of using this system pole or tower can be installed very cheaply.

iv) The amount of corona loss in this system is much less.

v) The power loss of the line is greatly reduced.

vi) No reactive power is generated or absorbed in the HVDC system. So no reactive power compensation is required in this system.

vii) HVDC system does not have any frequency of current so skin effect and proximity effect does not occur in the transmission line.

What are the disadvantages of an HVDC transmission system?

The major disadvantages of an HVDC transmission system are as below:

i) The cost of circuit breakers used in HVDC systems is much higher.

ii) No transformer is used to change the voltage level in this system.

iii) The cost of this system is much higher as the converter substation is connected to the sending end and receiving end terminals of the transmission line.

iv) The inverter used in the converter substation has limited capacity.

v) The two terminals of the inverter and rectifier device create harmonic, which is used to reduce the active filter. Using an active filter is very expensive.

vi) A heat loss is created in the converter substation, which is reduced using an active cooling system.

Most powerful underground power transmission cable system- 640 kV extruded HVDC cable

The world recognized cable manufacturer NKT claims that their 640 kV extruded DC cable system is the latest result of the intensive investment in research and development in the field of HVDC transmission technology. It sets a new world record for extruded HVDC cable technology enabling the integration of renewable power and connecting energy markets.

They said by using cables, efficient power delivery can also be made through densely populated or environmentally sensitive areas. A single pair of 640 kV extruded HVDC cables could, for example, transmit enough green power to supply three million households.