HVDC has been in existence for a long time but has become so popular these days with weighing advantages for the traditional HVAC transmission technology. HVDC system uses direct current for the bulk transmission. There are different configuration, types and topologies to include HVDC back to back, classical HVDC, etc.
The advantages lies in the fact HVDC links are able to connect to two asynchronous network, a typical example is a 60Hz and 50Hz power system network. For long distance transmission the HVDC links are less expensive and do not suffer a lot of electrical losses. For underground transmission system HVDC is much better because it avoids the heavy currents that are required to charge and discharge the cable due the capacitance effect of the cable. Because the power flow can be controlled independently of the phase angle between the source and the load, it has the capability to stabilize a network if there is any disturbance due to any changes in power demand. Also it has the capability of connecting remote generation to a distribution grid.
Other advantages are the ability to transmit large amount of power over long distance. There is lower capital cost since the number of lines is fewer and a reduction in the profile configuration in the number of pylons required and fewer conductors.
Come with the great advantages, there are also disadvantages and that include a high capital cost for the converters and the converters generate a lot of harmonics and characteristics harmonics and also requires a lot of reactive power consumptions which warrants the installation of harmonic filters to eliminate the harmonics and also to provide the reactive power requirement at fundamental frequency. For a multi-terminal HVDC, it is so complex and requires complex and costly communication system. They also have an overload capability limitation. Also for certain type of HVDC configuration, a close in AC fault on the AC side can affect the DC side. Also the radio noise generated can affect communication system that is close by.
HVDC is, today, a niche market, a very specialized application:
- interconnection of systems of different AC frequencies; or
- transmission of bulk chunks of power over long distances (in parallel to AC systems, like the Pacific inter-tie in WECC); or
- submarine cables (from Sweden to Denmark, of UK to France and several other examples)
As of today we are using the best of both worlds. HVAC obviously is where the majority of power transmission lines (transformers, synchronous machines, distribution systems) but HVDC has come a long ways & continues to grow with advanced technology. We have come from mercury tubes to thyristors to the latest (VSC - voltage source converters that can create or synthesis a sine wave from DC). HVDC transmission has some unique advantages over HVAC & losses are lower than AC on long transmission paths due to the skin effect. As for now, they will continue to coexist - the future will tell the complete story.