AC as a preferred option
Although the alternating current is the dominant mode for the transmission of electrical energy, in a number of applications the advantages of HVDC technology make it the preferred option over alternating transmission.
Submarine cables where high capacity causes additional AC losses ( eg the 250 km Baltic cable between Sweden and Germany ).
Mass transmission over long distances, point to point, without intermediate socket, for example in remote areas.
Increase the capacity of an existing electrical network in situations where the installation of additional cables is difficult or expensive.
Allowing the transmission of power between unsynchronized AC-distribution systems.
Reduce the profile of cables and pylons for a given power transmission capacity, as the HVDC can carry more power per conductor of a given size
Connect a remote power plant to the distribution network; for example, the bipolar line of the Nelson River in Canada ( IEEE 2005 ).
To stabilize an electrical network mainly without increasing the maximum potential short-circuit current.
Reduce corona loss ( due to higher voltage peaks ) compared to HVAC transmission lines of similar power.
Reduce the cost of the line, because HVDC transmission requires fewer conductors; For example, two for a typical two-pole HVDC line, compared to three for a three-phase HVDC.
HVDC transmission is particularly advantageous in submarine power transmission. Long submarine AC cables have high capacity.
500 MW HVDC light transmission interconnection
ABB has commissioned a 500 megawatt HVDC Light (VSC) transmission interconnection connecting the Irish and UK Grids, enabling cross-border energy flows and improving grid reliability and security of electricity supply.
The East-West interconnector includes a 262 km high voltage cable link, 186 km of which is submarine.
Therefore, the current required to charge and discharge the capacitance of the cable causes additional power losses when the cable is under AC voltage. while this has minimal effect on DC transmission. In addition, alternating current is lost due to dielectric losses.
In general applications, HVDC can carry more power per conductor than AC, because, for a given power, the constant voltage in a DC line is less than the peak voltage in an AC line.
This voltage determines the thickness of the insulation and the spacing of the conductors. This lowers the cost of HVDC transmission lines compared to alternative transmission and allows transmission line corridors to carry higher power density.
An HVDC transmission line would not produce the same type of very low frequency electromagnetic (ELF) field as an equivalent AC line. Although there are concerns about the potential adverse effects of such fields, including the suspicion of increased leukemia rates, the current scientific consensus does not consider the sources of ELF fields and their associated fields to be harmful.
Deployment of HVDC equipment would not completely eliminate electric fields, as there would always be DC electric field gradients between conductors and earth. These fields are not associated with health effects.
Because HVDC allows the transmission of power between unsynchronized AC systems, it can help increase system stability. It does this by preventing cascading outages from spreading from one part of a larger power transmission network to another while allowing the import or export of energy in the event of outages. less important.
This feature has encouraged the wider use of HVDC technology for its stability advantages alone. The energy flow on an HVDC transmission line is defined with the help of converter station control systems. The energy flow does not depend on the mode of operation of the connected power systems.
Thus, unlike HVAC links, intersystem HVDC links can have arbitrarily low transfer capacity, thus eliminating weak tie problems, ”and lines can be designed based on optimal power flows.
Likewise, the difficulties of synchronizing different operational control systems of 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. In addition, power flow regulation can be used to dampen oscillations in power systems or in HVAC lines in parallel.
For example, the rapidly growing Indian power system is being built in the form of several interconnected regional power systems 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 one. This mode is used to transmit large capacities over very long distances from large hydroelectric and thermal bases. Other applications concern long-distance transmission projects with few power connections along the line (Yinbiao 2005).