Imagine a time when harnessed wind energy from windmills in Iowa would be able to provide power to homes in Northern California. This might sound far-fetched, but with high-voltage direct current (HVDC) technology, this may become our reality in the future. For many years, all forms of electricity transmission have used alternating current (AC) systems. Compared to more traditional AC systems, HVDC systems offer superior efficiency, lower energy losses, and exciting applications, thus making the technology very promising.
HVDC is a method of transferring power with high voltage DC instead of AC like we currently do. The main difference between AC and DC is that AC electricity periodically changes direction (think of a sine wave), whereas direct current is straight and…, well, direct! The process of creating HVDC is quite similar to AC in most ways.
The process starts with normal power generation in the form of AC, as that is how most electricity is produced. The produced AC starts at around 10-30kV at generation. This AC voltage is then stepped up to a higher voltage using transformers and is now at around 100-800kV depending on the use case.
The biggest change in the system occurs with this step, the AC to DC conversion. After the electricity is converted from high voltage AC to DC, the power is finally transferred through the power lines and stepped down to a usable voltage in a transformer near peoples’ homes.
After the voltage is converted to DC using one of these two methods, it is transferred to homes in a not-too-dissimilar way from traditional AC power. Finally, once the power arrives near homes, it is converted back to AC and then stepped down using transformers, to provide the 120-240 volts AC to power homes.
Unlike in DC currents, AC currents experience the skin effect. Essentially, this causes more current to flow near the surface than in the center. The skin effect reduces the amount of cross-sectional area that can be used for current flow, which causes power losses and conductor heating.
DC currents do not have to deal with the skin effect because they flow uniformly through the conductor and do not oscillate. In essence, this lessens the amount of conductors needed, as you can rely on the full capacity of each wire. Ultimately, the cabling cost for HVDC systems is much lower than that for AC systems.
But wait, there’s more! While AC electricity has frequency, DC electricity does not (it doesn’t oscillate). Currently, different countries distribute electricity at various frequencies.
For example, while electricity is distributed at 60hz in the US, most countries in Europe and Asia distribute electricity at 50hz. If countries want to connect their power grids with each other, there would be issues because the frequency must be the same.
As mentioned before, DC electricity does not have frequency, therefore making the connection of power grids much easier. Each country can convert the DC power into AC power of whatever frequency works for them. Due to this, HVDC thoroughly supports the globalization of power grids and makes it a whole lot easier!
Though HVDC certainly is exciting, one cannot deny the fact that there exist limitations to this technology. For example, converter stations present at both ends of the transmission lines cost $200 million per gigawatt per converter. Additionally, the process of converting DC to AC or vice versa also results in reactive power consumption, which can lead to power loss or/and reduced efficiency.
Because of this, expensive filter-compensation units and reactive power-compensation units need to be installed in order to improve the overall power quality. These units are also very expensive, making the overall cost of installation higher than the installation cost of AC systems. Although the costs will break even at a certain point, budgeting and planning to spend so much money just at the start is a huge challenge.
As of now, HVDC is only used for industrial applications and long-distance power transfer. Some major examples of this are undersea cables used to transfer power, notably the North Sea Link project that connects Norway’s hydroelectric power with the UK grid and China’s Changji–Guquan line carrying 12,000 megawatts of power for over 3000 km.
These systems are also becoming quite useful for renewable energy integration, integrating offshore wind farms and hydroelectric plants into the national grid. HVDC could make systems like these more common could allow for more renewable energy integration from more remote areas, building a more powerful energy grid overall.