A High-Voltage, Direct Current power transmission system uses direct current to transmit the bulk of the electrical power, in stark contrast to Alternating Current (AC) systems that are more common. HVDC transmission systems are ideally suited for long distance transmission as they suffer less electrical losses and are cheaper. In the case of underwater power cables, an HVDC system avoids heavy currents needed to charge and discharge cable capacitance in every cycle. Even for a shorter distance, an HVDC system could be justified because of the benefits of direct current links. HVDC enables the transmission of power through unsynchronised AC transmission systems. As the power flow through an HVDC system can be regulated independently of the phase angle between load and source, the network can be stabilised from any rapid power change disturbances. The transfer of power between grid systems functioning at different frequencies is also possible with HVDC. This plays a role in improving the economy and stability of each grid by allowing power to be exchanged between two incompatible networks.
HVDC has proven its mettle by enabling the bulk transfer of power over long distances with a higher efficiency and much lower electrical losses. This can even be for distances greater than 1000 km. HVDC allows a stable, secure asynchronous power network connection that works on otherwise incompatible frequencies. HDVC systems also provide precise and instantaneous power flow control. Once they are installed, HVDC systems become a critical cog in any electrical power system, improving overall reliability, transmission capacity and stability.
- Asynchronous grid – Many HVDC links connect two AC systems that may not be synchronous. AC systems need to be synchronised if they are to be connected. Thus, they have to operate at the same frequency and voltage and this is easier said than done. As HVDC is asynchronous, it can readily adjust to any frequency or voltage received. Thus, HVDC is deployed to connect AC systems across the world. For e.g.- The power system in the Eastern part of the United States is not synchronous with that of the western part.
- Long distance crossing – There is no technical limit on the length of an HVDC cable. In a long AC cable transmission, reactive power flow as a result of large cable capacitance limits the transmission distance. No such limitation exists in HVDC and that is why it is the only viable option for very long cable links. The 580 km long NorNed is the longest submarine high-voltage cable in the world. It links southern Norway to the Netherlands. Many more HVDC links are under consideration today, particularly in Europe.
- Controllability – One of the main advantages of HVDC is the ease with which active power can be controlled in the link. In many HVDC links, the main controls are based on the constant transfer of power. This HVDC property has become critical recently because the profit margins of power networks have been shrinking as a result of deregulating electricity markets worldwide. HVDC links can provide additional control facilities and improve AC power system performance. Automatic HVDC control consists of AC network power flow redistribution, constant frequency control, and AC network power swing damping. In a lot of cases, additional control functions make it more likely to safely increase power transmission capability in those situations where stability is a limiting factor.
- Low short circuit current –An HVDC transmission system does not play a role in the interconnected AC system short circuit current. When a high-power AC transmission is built to a load centre, a greater level of short circuit current will be seen in the receiving system. Large cities are increasingly facing the problem of high short circuit currents, leading to the need to replace current circuit breakers if the rating is low.
- Low energy loss – When a cable link needs to be longer than 80 km, it is only possible with HVDC transmission systems. This is because minimal electricity can be delivered in underground cables when there is an AC line. For the HVDC line between England and Scotland that will be through submarine cable, an energy loss of less than 3% is anticipated.
- More energy transmission – Usually, 30%-40% more energy transmission is achieved with HVDC systems as opposed to conventional overhead power lines that carry alternating current.