There are two ways to transmit electrical power in bulk, either by HVAC or by HVDC. When it comes to selecting between these two, there has always been an ambiguity. With the passage of time, it has been observed that HVDC transmission offers numerous advantages over HVAC transmission.
In recent years, HVDC has become increasingly popular due to its high efficiency. The majority of HVDC lines transmit power at voltages ranging from 100 kV to 800 kV. HVDC serves as an asynchronous link, allowing power to be transmitted between asynchronous AC power systems (systems running at different frequencies and voltage levels). The power transmitting through an HVDC line can be controlled, which increases the system’s stability against sudden power disruptions.
WHY HIGH VOLTAGES ?
Transmission cables are good conductors; however, they still have some resistance, especially when the cables are long. This indicates that the power is dissipated because of the current’s heating effect. Low voltage transmission can lead to significant power loss due to high currents. However, if the power is being transmitted at a high voltage, then the power loss is negligible. Thus, power is always transmitted at high voltage to decrease the power dissipation due to the resistance of wires. For a given amount of power transmitted, multiplying the voltage by 2 will convey a similar power at just a large portion of the current. The power dissipated in wires proportionate to the square of the current, the power losses will be reduced by a factor of four if the voltage is multiplied by two. While power loss in transmission can likewise be diminished by expanding the conductor size. However, bigger conductors are hefty and expensive.
WHY DIRECT CURRENT ?
Asynchronously, on the grounds, DC has no recurrence(frequency), it may be utilized well to link two distinct AC power systems without stressing over synchronizing these systems. Moreover, a DC connection is like an open circuit to AC systems. This demonstrates its importance in terms of system stability, behaving as isolation between two distinct systems in the occurrence of an electrical fault, like lightning or equipment failure. The DC circuit restricts the fault’s path by isolating the asynchronous link between faulty and healthy parts of the network to avoid a cascading failure. Furthermore, for significant distances, it is more efficient. DC transmission towers occupy less space due to their simple structure and less height. Since DC only requires two poles at maximum, less wiring is required (at times, one pole is enough with the ground used as a return). DC towers are also more cost-effective because they only carry two wires rather than three separate phase cables.
- DC transmission is economical, since only two conductors are required.
- There are less transmission losses due to absence of reactive power
- Because of high voltage transmission, current flow is less, leading to less I2R losses.
- There is no skin effect in DC transmission. Hence thin cheap conductors can be used.
- HVDC links can be used to interconnect two AC systems with distinct frequencies.
- Initial cost is less due to small towers.
- Protection measures can be taken faster in HVDC due to use of electronic converters.
- Power levels on HVDC system can be controlled exceptionally quickly if an issue arises.
- There are no dielectric heating problems in insulated conductors of HVDC
- HVDC has minimal interference with other signals.
- The voltage levels are balanced in HVDC.
- HVDC is highly efficient due to absence of line charging current.
- Water crossing.
- Asynchronous interconnections.
- Bulk power transmission.
- Transmission to areas with severely restricted ROWs (Right of Ways).
- HVDC network grids.
Authors: Rumela, Zain and Abdul Rehman | Event Management & Logistics