The 19th century battle between Nikolai Tesla’s alternating current (AC) and Thomas Edison’s direct current (DC) technologies had an undisputed winner. The vast majority of the world’s transmission and distribution grids work now on AC. But DC—and high-voltage direct current (HVDC)—is attracting renewed interest.

New Applications Emerge for HVDC

The benefits of HVDC include its ability to transmit power over long distances without technical limitations and interconnect networks that are asynchronous or that operate at different frequencies. As power grids are adapted to incorporate energy from renewable sources, the flexibility that HVDC connections offer to control power and improve AC system stability are attracting attention. However, HVDC’s integration within an AC system still has to be carefully considered and requires power providers to rethink the role of AC voltage control.

Historically, one of the challenges with HVDC has been the difficulty, expense, and losses associated with stepping up or down voltages, compared with the ease with which this can be achieved with transformers on AC networks. Some of this limitation has been overcome technically, but controlling voltages is still an issue with DC.

Until recently, HVDC applications have predominantly harnessed the technology’s advantages in transmitting large amounts of power over long distances, typically international interconnects. Examples of this are the 2,000-MW England–France interconnector linking the British and French transmission systems, and the 1,000-MW BritNed interconnector between Britain and the Netherlands.

However, there is increasing interest in using DC tactically, at utility and distribution scale, even down to medium-voltage (MV) or low-voltage (LV) levels. New applications include connecting localised grid “islands,” as well as integrating the growing number of solar farms and industrial-scale batteries, which naturally run on DC.

A Transformational Tool?

One of HVDC’s key strengths is its ability to connect networks that are asynchronous or that operate at different frequencies. As an additional plus, HVDC achieves separation between different AC networks, so that any problems on one of them may be isolated from the other.

This is important, as the UK grid transforms from the 1950s model of centralised generation and a passive distribution network to a far more dynamic future of distributed renewables generation and active management of relationships with customers on smart networks. HVDC has the potential to become an increasingly important tool for evolving the grid into something far more modular, hybridized, and flexible than previously envisaged.

The principle of connecting an HVDC link into an AC grid is simple enough: a length of overhead line or cable, or both, with a converter station at each end. The technologies to achieve this have advanced dramatically over recent decades to reduce the losses inherent in the process, but they are still expensive in terms of capital cost and maintenance, compared with conventional AC substations and transformers.

Why HVDC Still Needs Voltage Control

The ability of HVDC links to transmit large and variable amounts of power has profound implications for the AC networks they are connected to, especially in terms of voltages. When power is flowing along the HVDC connection, the “sending” network will experience voltage drops, while the “receiving” end will experience voltage rises. Controlling voltage changes on each of the AC networks connected by HVDC is clearly a key issue.

AC networks increasingly deploy automatic voltage control (AVC) solutions. To be effective, an AVC system must continuously monitor and automatically control the voltages, keeping them within set parameters and preventing runaway voltage levels. It does this by managing the relationship between real power, reactive power, and voltages, which are closely linked and will be influenced directly by HVDC power flows.

AVC solutions should be designed to work with voltages on virtually any type of system, including complex schemes involving distributed energy resources (DERs). The connection to an HVDC terminal can be treated as a “virtual transformer.” The ability to compensate for the fluctuating power factors, and highly variable and flexible loads produced by flows across the HVDC link, ensures that voltages remain under control and in harmony in the AC networks at either end.

AC Integration

Clearly one of the key considerations when designing and implementing an HVDC link is that the converter electronics at each end of the connection should be set up to be compatible with existing and future AVC equipment. HVDC connection equipment should focus on managing power flows and not specifically on trying to control voltages. That is a task best done by specialised AVC technologies designed for AC networks. With that condition, the HVDC link should work effectively, without any detrimental effect on voltages across the AC networks to which it connects.

On the question of where AC/DC converters should be located when connecting substantial sources of DC power to the grid, such as solar farms and industrial-scale power storage facilities, they should be placed as close as possible to the sources. The backbone of our transmission and distribution system is, and will remain, AC for the foreseeable future. So once again, it makes sense for HVDC links to focus on managing power flows, while AC grid equipment remains responsible for voltage management.

HVDC has an important and potentially growing role to play in the evolution of the grid. It should help to enable the transformation of our largely monolithic transmission and distribution system into one which is simultaneously more modular and separated, while being smarter and more integrated—a hybrid grid that is more localised, efficient, resilient, flexible, and better equipped to embrace more diverse sources of generation and storage.

Vincent Thornley, EngD is managing director of Fundamentals Ltd.