This document provides an overview of the high voltage direct current (HVDC) power transmission and the advantages of using HVDC compared to high voltage alternating current (HVAC). This application report focuses on the control and protection requirements of HVDC station, products and solutions that can be leveraged during control and protection system design.

Electricity is produced by large power plants and is then carried over long distances (> 100 km) at high voltages (110 kV or more) by the transmission lines, which is stepped down to the level of the distribution network (11 kV or 415 V), bringing electric power to the consumer. Most power plants generate electricity as AC and the entire system uses AC afterwards since the voltage can be stepped up or down easily by the use of transformers. Bulk power from the generation plant is transported to consumers using transmission lines operating at high voltages. For shorter distance transmission of power (< 100 km), AC transmissions are widely preferred. When the transmission distances are longer (> 500 km), limitations are seen in using AC transmission.

The electrical supply demand is increasing globally. Power plants are often located near to energy sources, to minimize costs and environmental effects. These power plants often are located away from heavily populated areas or cities, therefore, transporting the electricity generated economically and efficiently is important. This is accomplished by transmitting the generated power at a high voltage. High-voltage (HVAC) is preferred for transmission purposes mainly because higher voltages are easily achievable by means of a transformer (stepping up at the power plant and stepping down at the substation).

To meet the growing demand, utilities are looking at improving the system performance by interconnecting the grid to balance the load and looking at using newer technologies (HVDC or Flexible AC Transmission Systems FACTS) for improving efficiency. The advantages of HVAC is simpler voltage transformation and easier current interruption. In some cases, it is not possible to use HVAC transmission technology, when long transmission lines (> 500 km) are involved due to voltage instability and higher transmission losses. The disadvantage of HVAC is limitations on long distance transmission, current carrying capacity, reactive power (need reactive power compensation at different locations along the transmission lines) loss, skin effect (non-uniform current distribution in conductors carrying, where most of the current is found in the conductor’s outer layers increasing effective resistance) and the Ferranti (received voltage higher than the transmitted voltage) effect. Using HVDC for transmitting power over long distances is the solution.