High voltage direct current (HVDC) transmission lines are more efficient for transferring power over long distances, as they incur less power loss when compared with their equivalent high voltage alternating current (HVAC) transmission systems. There is no requirement for compensating reactive power along the transmission line, and higher efficiency translates into reduced transmission expenses to bolster the economic competitiveness of green energy sources in the electricity market. HVDC transmission systems also increase the stability of the power system, permit utilities to completely regulate power flow and enable wind power resources integration.

In general, HVDC transmission lines utilize less land area compared with HVAC transmission lines. Unlike AC voltage, DC voltage does not change track several times per second and the current flows through the whole conductor, not just through its surface. However, it may be noted that current transmission systems are mostly AC systems, and efforts are underway to expand and upgrade them to DC systems.

HVDC transmission systems are particularly suitable if the power system network is spread over long distances. Currently, HVDC transmission systems are popular in submarine applications, where they connect land to offshore wind farms or transmit power to areas where overhead transmission line systems are not feasible. HVDC cables are also being considered for transferring power over land to satisfy increasing power demand.

Consider the merits and demerits of HVDC transmission systems over HVAC transmission systems.

Merits of HVDC transmission systems

The major benefits of HVDC transmission systems are lower capital costs and the capacity to transfer a significant amount of power when considering large distances. Power loss is only about 3% for every 1,000 km depending on system construction and voltage level. The energy sources that are producing power at remote locations can also benefit from HVDC transmission systems.

The major areas where HVDC transmission systems have proved to be more efficient than HVAC transmission systems include:

• Endpoint-to-endpoint long-distance transmission of power, for instance in remote areas, with no intermediate ‘taps.'

• Marine transmission cables, which have large capacitance values that result in extra AC power losses. For example, the 600 km Nor Ned cable connecting the Netherlands and Norway, and the 250 km Baltic cable connecting Germany and Sweden.

• Enhancing the transmission network of the current power grid in circumstances where it is hard to add more wires. Or, it may be costly to build new transmission systems and stabilize such unsynchronized AC distribution systems.

• Making a connection between remote power plants and the load centers or the distribution power grid, for example, the Nelson River Bipole.

HVDC transmission systems can stabilize a largely AC grid system without enhancing the short circuit current, and they do not have multiple phases like HVAC systems and require fewer conductors. The skin effect phenomenon that plagues HVAC networks, in which current is distributed within a conductor such that current density is largest near the surface of the conductor and decreases exponentially with greater depths in the conductor, does not occur in HVDC systems. As a result, the latter can deploy thinner conductors and can support power transmission among various countries operating at different frequencies and voltages.

The length of submarine HVAC lines is limited because the whole current-carrying conductor capacity can be utilized to supply the charging current. However, there are no such restrictions with DC transmission cables, which can also transfer more power per line as at a certain power rating, the DC line constant voltage is less than the AC line maximum voltage. DC voltage has a constant higher voltage value, permitting transmission cables to have the same sized conductors. The insulation can transfer 100% more power than AC voltage in the areas consuming large amounts of power, which can also reduce transmission costs.

HVDC transmission systems boost power system stability by preventing cascading failure spread from one area of a large power grid to another, and support power transmission among unsynchronized AC distribution power systems. Another significant merit of the HVDC system is that load changes do not affect synchronization. The pattern and scale of power flow via HVDC link systems can be managed directly and modified to benefit AC systems on both ends of such link systems. Many power companies have considered the broad use of HVDC systems alone for its stability advantages.

Demerits of HVDC transmission systems

The shortcomings of HVDC transmission systems are mainly related to their control, switching, conversion, overall maintenance and conversion. They need static inverters that are costly and have restricted overload capacity. For short distances power transmission, power losses in such inverters can be larger than HVAC systems. The cost of these inverters cannot be offset by lower power loss and decreases in transmission line design expenses.

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