As the demand for power is increasing, government policies are providing incentives to utilities for adopting renewable energy sources. Utilities are investing time and money to improve the transmission of renewables from offshore wind and solar.
Today, most power grids use high-voltage alternating current (HVAC) for transporting energy over long distances, but that technology is susceptible to losses during transmission, has limits on power transfer over long distances, and has limited power control capability. A high-voltage direct current (HVDC) system converts the power from alternating current (AC) to direct current (DC) at the sending end, transmits the power using DC, converts the power back from DC to AC at the receiving end, and delivers the power to the receiving end AC grid.
Application of HVDC technology is expanding not only for large bulk power transfer over long distances, but also in the interconnection of renewable energy sources.
Bulk Power Transmission Efficiencies
Transmission cost depends on numerous factors, such as the size and quantity of conductors, equipment needed at the terminal stations, and transmission tower size.
HVDC is particularly well suited for bulk power transmission over long distances for several reasons:
- A bipolar HVDC system consisting of two high-voltage conductors on one tower offers reliability comparable to a double-circuit HVAC line, significantly reducing the transmission line costs and right-of-way requirements.
- Losses in a transmission line depend on the resistance of the line. One factor that impacts the resistance is skin effect, which causes the effective resistance to increase with increasing AC frequency. Use of DC eliminates the skin effect, reducing overall transmission losses.
HVDC transmission systems require converter stations at each end of the line to convert the AC to DC and back. Cost of HVDC converter stations can be substantially more than a conventional AC substation with similar power throughput. That expense may be counterbalanced by reduced transmission line costs and reduced losses. This becomes more evident as the distance and/or power transfer level increases.
Cable Length Advantage
HVAC transmission cable length is limited because as the length of cable increases, the capacitive charging current increases. It can reach a point that the capacitive charging current approaches the total current carrying capacity of the cable. HVDC has no capacitive charging current, and higher levels of power can be delivered over longer distances. HVDC cable length is theoretically only limited by capital cost. Enabled applications include:
- Connection of offshore wind farms: As development of offshore wind generation assets increases, the location of wind turbines generators is moving farther from the shore. The increasing distances between the generators and the onshore point of interconnection means the necessary cable length is increasing. HVDC can be a viable transmission option, unlocking the true potential of renewable energy.
- Transmission into congested areas: Increasing demand, particularly in congested areas, coupled with challenges of accessing rights-of-way has driven the need to maximize power transfer and a push to go underground. HVDC is an excellent option for high-power cable transmission installations, maximizing the amount of power transfer per cable.
Power Controllability
Within an HVAC system, the ability to control power flows in any given parallel path is limited. Power flows are dictated by the relative impedance of various parallel paths from a given source of generation to a given load. HVDC, on the other hand, offers very fast and accurate control of the power flowing within its system. The operator can select the amount of power to be transmitted over the link. If that power is available at the sending end, it is then converted to DC, transmitted to the receiving end, converted back to AC and injected into the receiving AC system. Auxiliary control functions can further enhance AC system’s stability by providing frequency control and damping of power swings within the AC grid.
HVDC systems offer higher transmission capability and lower transmission losses over long distances than AC and provide better ability to control power flows. Additionally, they provide the ability to transmit more power over longer lengths of cables, making them an attractive alternative for the transition to renewable energy sources.
Learn more about HVDC converter technologies, system components and the benefits for utilities.
