HVDC for low-loss electricity transfer

High-voltage direct-current (HVDC) technology enables electricity to be transported over long distances with low losses. This makes it a key technology for adapting transmission grids to deal with the increasing number of renewable energy sources.

Power grids have an essential role to play in achieving carbon-neutrality. In a decentralised, renewable energy system, these grids connect onshore and offshore wind farms. And, producing zero-carbon electricity with private and industrial consumers. Between 2021 and 2030, investment in new transmission grids will run to an estimated EUR 152 billion in EU member states.

Optimising existing grids

In this context, renewing grid infrastructure is not the only factor that will improve its transmission potential. Numerous technologies to boost the performance of existing grids are already available. “Grid optimisation technologies allow us to make more of the grids we already have. They reduce how much we need to spend on new grid infrastructure. They give us more efficient grids which can accommodate more renewables and reduce the amount of wind and solar that we need to curtail,” says Giles Dickson, CEO of industry association WindEurope. He also notes that most grid optimisation technologies are already tried-and-tested and available.

Using real-time-monitoring systems, transmission lines can be pushed closer to their thermal limits in operation. This enables the maximum capacity to be utilised more effectively without compromising safety. One other technology concerns electronic systems that monitor transformers’ operating parameters. Such  as oil temperature, ambient temperature, or load to simplify maintenance and reduce accidents. Beyond mere data acquisition, embedded smart features can be combined with AI at device level to create ageing models or perform hotspot calculations. All with the aim of enabling predictive maintenance. Communication takes place via a secure network or using wireless or mobile communications. New transformer technology improves the grid’s controllability, reduces malfunctions, and opens up connection to direct-current grids.

Long-range energy transmission

DC power grids in particular are becoming more important as the requirement of linking up renewable-energy sources becomes more common. After all, major sources of renewable energy are usually far away from consumers – offshore wind farms being a good example. As it is predominantly equipped with established AC (alternating-current) technology, the existing grid cannot provide the kinds of capacity required to transport these amounts of electricity, however. What’s more, too much energy is lost during transmission over long distances with AC technology.

For this reason, HVDC (high-voltage direct-current) transmission systems are becoming more and more important. Here – at the supply end of the process – the alternating current is initially converted into direct current before transmission. In the receiving station, the current is then converted back to AC for it to be consumed. This technology enables transmission losses to be reduced by 30 to 50 per cent on average. Compared to a three-phase grid.

“HVDC technology is making a major contribution to a carbon-neutral future energy landscape by enabling widespread integration of renewable-energy generation over long distances,” explains Niklas Persson, Managing Director Grid Integration at Hitachi ABB Power Grids. For the NordLink project – the “green cable” for exchanging German wind power with Norwegian hydropower – the company supplied both converter stations, among other things. Without HVDC technology, the losses in energy transmission using the world’s longest power link via undersea cable would simply have been much too high.

Converting electricity more efficiently

Two essential converter technologies are used in converter stations for modern HVDC systems: conventional line-commutated converters (LCCs) and voltage source converters (VSCs). The latter comprise an array of “sub-modules” arranged in series that are made up of IGBT half-bridges with storage capacitors. They facilitate considerably higher grid stability, significantly reduce electrical losses, and minimise the effort involved in filtering. Moreover, the use of power electronics with a silicon carbide (SiC) base promises exceptionally low switching and power losses, not to mention high reliability. According to Mitsubishi Electric, losses in semiconductors can be reduced by up to 50 per cent in this case, meaning that even more valuable “clean” energy from renewable sources will reach consumers.


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