High-voltage direct current (HVDC) transmission enables the low-loss transmission of large amounts of energy over thousands of kilometers or through submarine cables. Specially designed converter transformers form the technical link between the classic three-phase grid and the DC link – a key technology for the global energy transition. As renewable energy continues to expand and the need grows to transport electricity over ever-greater distances, HVDC technology is gaining massive importance worldwide – both in the European interconnected grid and in intercontinental links. As renewable energy continues to expand and the need grows to transport electricity over ever-greater distances, HVDC technology is gaining massive importance worldwide.
How does high-voltage direct current transmission work?
At the endpoints of an HVDC link are converter stations that convert alternating current into direct current and vice versa. Converter transformers transmit the electrical power from the AC grid to the converters and are exposed to extreme stresses in doing so – from high DC voltage components, harmonics from the power converters, and particular thermal load profiles. Transmission voltages are typically in the range of ±500 kV to ±800 kV; some projects already achieve ±1,100 kV.
Key Characteristics at a Glance
- Low-loss transmission of large amounts of power (up to several GW)
- Special converter transformers up to 850 MVA per unit
- Used primarily for submarine cables and distances from around 600 km
- Connects asynchronous grids of different frequencies
- Key technology for connecting offshore wind farms
- VSC technology (Voltage Source Converter) for flexible grid coupling
- LCC technology (Line Commutated Converter) for very high power levels
Frequently Asked Questions
Why are converter transformers technically so demanding?
They must simultaneously transmit alternating voltage while safely insulating high DC voltage components. They are also subjected to unusual thermal stress from harmonics generated by the power converters. Insulation coordination is therefore extremely complex, as are mechanical stability and the cooling concept.
What advantages does HVDC offer over classic three-phase transmission?
Lower transmission losses over long distances, no reactive power issues, narrower corridor widths for underground cables, and the ability to couple asynchronous grids. For submarine cables, HVDC is virtually unrivaled from a distance of around 70 kilometers onward.
Which HVDC projects are currently important in Europe?
Major projects such as SuedLink, SuedOstLink, and Ultranet in Germany aim to transport wind power from the north to load centers in the south. Trans-European links between Scandinavia and Central Europe also use HVDC technology for higher transmission capacities.
What are the differences between VSC and LCC HVDC?
LCC systems are based on thyristors and are particularly robust and efficient at very high power levels. VSC systems use IGBT modules and offer more flexible control of active and reactive power, which makes connecting to weak grids and offshore wind farms easier.
What trends are shaping the development of HVDC technology?
Rising transmission voltages up to ±1,100 kV, multi-terminal HVDC grids with multiple feed-in and take-off points, and hybrid AC/DC grids are the most important development trends. Integration with offshore wind farms is also driving innovation in converter transformers.
For demanding projects in the field of high-voltage direct current transmission, our specialists are your point of contact. Talk to us early on about converter transformers and grid connection.