AC as preferred optionDespite alternating current being the dominant mode for electric power transmission, in a number of applications, the advantages ofHVDCmakes it the preferred option over AC transmission.Examples include:1. Undersea cables where high capacitance causesadditional AC losses(e.g., the 250-km Baltic Cable between Sweden and Germany).2. Endpoint-to-endpoint long-haul bulk powertransmission without intermediate taps, for example, in remote areas.3. Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install.4. Allowing power transmission betweenunsynchronized AC distribution systems.5. 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.6. Connecting a remote generating plant to the distribution grid;for example, the Nelson River Bipole line in Canada(IEEE 2005).7. Stabilizing a predominantly AC power grid without increasing the maximum prospective short-circuit current.8. Reducing corona losses(due to highervoltage peaks) compared to HVAC transmission lines of similar power.9. 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 inundersea power transmission. Long undersea AC cables have a high capacitance.

Example (VIDEO)500 MW HVDC Light transmission interconnectionABB has commissioned a 500-megawatt HVDC Light (VSC) transmission interconnection that links theIrish 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.Cant see this video? Clickhereto watch it on Youtube.Consequently, the current required to charge anddischarge the capacitance of the cable causes additional power losses when the cable is carryingAC,while this has minimal effect for DC transmission. In addition, AC poweris lost to dielectric losses.In general applications,HVDC can carry more power per conductorthan AC, because fora given power rating, the constant voltage in a DC line is lower than the peak voltage in an ACline.This voltage determines the insulation thickness and conductor spacing. This reducesthecost of HVDC transmission linesas compared to AC transmission and allows transmission linecorridors 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 concernin the past regarding possible harmful effects of such fields, including the suspicion of increasingleukemia rates, the current scientific consensus does not consider ELF sources and theirassociated fields to be harmful.Deployment of HVDC equipmentwould not completelyeliminate electric fields, as there would still be DC electric field gradients between theconductors and ground. Such fields are not associated with health effects.Because HVDC allows power transmission between unsynchronized AC systems, it canhelp increase system stability. It does so by preventing cascading failures from propagating fromone part of a wider power transmission grid to another, while still allowing power to be importedor exported in the event of smaller failures.This feature has encouraged wider use of HVDCtechnology for its stability benefits alone.Power flow on an HVDC transmission line is set using the control systems of converterstations. 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 differentpower systems are eliminated.Fast-acting emergency control systems on HVDC transmission lines can further increasethe stability and reliability of the power system as a whole. Further, power flow regulation can beused for damping oscillations in powersystems or in parallel HVAC lines.The advantages described above encourage the use of DC links for separating largepower systems into several nonsynchronous parts.

Direct-Current (HVDC) Transmission Lines

For example, the rapidly growing Indianpower system is being constructed as several regional power systems interconnected with HVDCtransmission 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 largecapacity over very long distances from large hydropower and thermal power bases. Otherapplications involve long-distance transmission projects with few tie-ins of power supplies alongthe line (Yinbiao 2005).