AC as preferred option
Despite alternating current being the dominant mode for electric power transmission, in a number of applications, the advantages of HVDC makes it the preferred option over AC transmission.
- Undersea cables where high capacitance causes additional AC losses (e.g., the 250-km Baltic Cable between Sweden and Germany).
- Endpoint-to-endpoint long-haul bulk powertransmission without intermediate taps, for example, in remote areas.
- Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install.
- Allowing power transmission between unsynchronized AC distribution systems.
- Reducing the profile of wiring and pylons for a given power transmission capacity, as HVDC can carry more power per conductor of a given size.
- Connecting a remote generating plant to the distribution grid; for example, the Nelson River Bipole line in Canada (IEEE 2005).
- Stabilizing a predominantly AC power grid without increasing the maximum prospective short-circuit current.
- Reducing corona losses (due to highervoltage peaks) compared to HVAC transmission lines of similar power.
- Reducing line cost, since HVDC transmission requires fewer conductors; for example, two for a typical bipolar HVDC line compared to three for three-phase HVAC.
HVDC transmission is particularly advantageous in undersea power transmission. Long undersea AC cables have a high capacitance.
500 MW HVDC Light transmission interconnection
ABB has commissioned a 500-megawatt HVDC Light (VSC) transmission interconnection that links the Irish and U.K. grids, enabling cross-border power flows and enhancing grid reliability and security of electricity supplies.
The East West Interconnector includes a 262 km high voltage cable link of which 186 km runs subsea.
Consequently, the current required to charge and discharge the capacitance of the cable causes additional power losses when the cable is carrying AC, while this has minimal effect for DC transmission. In addition, AC poweris lost to dielectric losses.
This voltage determines the insulation thickness and conductor spacing. This reduces the cost of HVDC transmission lines as compared to AC transmission and allows transmission line corridors to carry a higher power density.
A HVDC transmission line would not produce the same sort of extremely low frequency (ELF) electromagnetic field as would an equivalent AC line. While there has been some concern in the past regarding possible harmful effects of such fields, including the suspicion of increasing leukemia rates, the current scientific consensus does not consider ELF sources and their associated fields to be harmful.
Deployment of HVDC equipment would not completely eliminate electric fields, as there would still be DC electric field gradients between the conductors and ground. Such fields are not associated with health effects.
This feature has encouraged wider use of HVDC technology for its stability benefits alone. Power flow on an HVDC transmission line is set using the control systems of converter stations. Power flow does not depend on the operating mode of connected power systems.
Thus, unlike HVAC ties, HVDC intersystem ties can be of arbitrarily low transfer capacity, eliminating the “weak tie problem,” and lines can be designed on the basis of optimal power flows.
Similarly, the difficulties of synchronizing different operational control systems at different power systems are eliminated. Fast-acting emergency control systems on HVDC transmission lines can further increase the stability and reliability of the power system as a whole. Further, power flow regulation can be used for damping oscillations in powersystems or in parallel HVAC lines.
The advantages described above encourage the use of DC links for separating large power systems into several nonsynchronous parts.
For example, the rapidly growing Indian power system is being constructed as several regional power systems interconnected with HVDC transmission lines and back-to-back converters with centralized control of these HVDC elements (Koshcheev 2001).
Likewise, in China, ±800-kV HVDC will be the main mode used to transmit large capacity over very long distances from large hydropower and thermal power bases. Other applications involve long-distance transmission projects with few tie-ins of power supplies along the line (Yinbiao 2005).
Reference: Argonne National Laboratory – The design, construction and operation of long-distance high voltage electricity transmission technologies