Wind power seems easy. Put a turbine at sea, lay a cable to land and connect it to the grid. But without power electronics the energy transition would lose steam quickly. Finding smart brains that know how to put it all together to the most optimum setting are hard to find. Like for Shell, which is building offshore wind farms to make green hydrogen to help industries and heavy-duty transport to become sustainable.
Figuring out power electronics is very much like planning your holiday to, let’s say, Spain. Which plane or train to take, which bus connection brings you downtown and how to get from the bus stop to your hostel or hotel. Yin Sun, Senior Researcher Offshore Wind with Shell, says: “How to transform aerodynamic/mechanical torque from a wind turbine into electricity and then provide it to the grid with the end-user in mind. That is the general challenge we are facing.”
From the blades of the wind turbine to you watching your favourite Netflix or HBO series at home, electricity needs to do a lot of “translations”. The wind turbine transformer will elevate the voltage from low voltage (690 V) at the output of a wind turbine, to 66 kV interfacing the inter-array cable that extends further under the seabed to the first offshore power transformer. From there it goes via a more efficient 220 kV high voltage cable to land, where the onshore substation will increase it further into 380 kV high-voltage power to facilitate longer distance power transmission with minimal electrical losses. The 380 kV electrical network is commonly used as the electricity “high-way” in Europe for both domestic and cross-border power transmission. For countries like the US, China and Brazil, 500 kV and 750 kV are frequently used for efficiency power transmission that spans even longer geographical distances. .
“The trick is to have as little loss on the way as possible,” Sun says, “and to choose which works the best: Thomas Edison’s direct current or Nikola Tesla’s alternating current. There are different pros and cons for both, depending on the distance from the offshore wind farm to the shore.”
Further away from the shore
The basic principle: alternating current (AC) is the starting point and normally works the best. It offers the most cost-effective solution for offshore wind project near land. However, as the offshore wind resource near the shoreline tend to deplete quickly, because of the limited surface and public resistance against wind turbines near the beach, continuously new projects are proposed further away from the shore. High-voltage direct current (HVDC) then tends to offer an appealing solution, where the transmission infrastructure cost is very close to that of high-voltage AC (HVAC).
Yet the system integration and stability risks are greatly reduced thanks to advanced control of the Modular Multilevel Converter-HVDC. Those risks cannot be ignored, as a faulty set-up at the Hornsea One wind farm 120 kilometres off the Yorkshire coast of the UK led to a system blackout on 9 August 2019.
“DC is most cost-effective when you are much further from the shoreline and can be programmed to meet various grid integration requirements set by the transmission system operator. The initial CAPEX for HVDC is high compared to HVAC, because with HVDC you need an additional AC/DC converter station. So, for short distances high-voltage direct current is not a good economical choice.”
That is where electrical engineers come in
As with life, calculating what best to use is not a matter of black or white. That is where electrical engineers come in. “In order for wind power to function properly, one needs to build a very complex control system. You’ve got to construct it with the technology of today, but it still needs to be the most feasible solution in 10 to 20 years from now.”
The basic message here: if you don’t do it right from the start, you have a problem. Sun sums it up: “It’s not abstract science. Choosing the best system is a case-by-case determination. On short distances like 20 to 30 kilometres, yes to AC! But on anything above 70 to 80 kilometres you – as the power electronics expert – need to prove what works the best. Even for the HVDC solution, the interaction between a wind turbine converter controller and an offshore HVDC converter controller has to be looked at in detail to ensure inter-operability for all the operational scenarios, beforehand.”
[...] your employer might even lose his license to operate, which is certainly beyond the economic loss of a few hundred million euros.
The key lies with the electrical engineer thinking up the solution. Sun: “No matter how brilliant the equipment itself is, as the engineer you’ll need to understand the system and feel which thrives best in the transition. If a wrong concept is deployed, your employer might even lose his license to operate, which is certainly beyond the economic loss of a few hundred million euros.”
New generation of wizards
“To jump ahead in the energy transition we need a new generation of “power electronics wizards”. Their task will be to convert renewable resources into power efficiently and to connect them to power production plants.” Like the Holland Hydrogen I project. With 200 MW of installed capacity, when it starts operations in 2026, this new facility in the Maasvlakte area near Rotterdam will be Europe’s biggest green hydrogen facility.
Martijn Lunshof is Electrical Researcher within the “Green Hydrogen team”: “The hydrogen plant consists of electrolysers, which are key in consuming renewable electricity generated by offshore wind and solar farms to make fully sustainable hydrogen, meaning produced by wind or solar power and with the CO2 captured and preferably reused. This is needed to supply renewable fuel to industries, the heavy mobility sector and maritime sector. For these, other solutions like batteries won’t work for some time to come, because their capacity and/or weight will not give these sectors what they need to run their operations.”
Downside of the thyristor
Electrolysers have a specific voltage/current curve meaning at a certain current (H2 production) there is a corresponding voltage. This could mean that if the electrolyser is running at 20%, the voltage needs to be controlled to e.g. 400 volts of direct current (Vdc). Operating the electrolyser at 100%, could mean the voltage needs to be controlled to e.g. 450 Vdc. When connected to the power grid, the power (current) control can be done via thyristor-based rectifiers by regulating the voltage accordingly. A downside of the thyristor is that it influences the power quality, which requires correction by a harmonic filter and static compensation. Another option is using different power electronics, like a voltage source converter.
“Electrolyser plants need much more electrical equipment than conventional oil and gas plants; the difference being about 40% versus 15% respectively,” Lunshof says. For companies like Shell having substantial knowledge of power electronics inhouse is not a nice-to-have, it’s a must have.
“In conventional oil and gas, we largely own our distribution infrastructure. But for an electrolyser plant we are becoming a large electricity consumer and need to use the electrical connection of the transmission system operator or build large substations ourselves to deliver the required renewable power to our plants. Making use of the electrical grid can be quite expensive, and the connection cost will probably rise every year.”
If you are depending on expansion of the electricity grid of the region or the country, you may face delays in the energy transition, especially when it comes to the roll-out of large-scale electrolysis in some areas. It simply takes too much time to expand the grid, experts feel.
Moreover, due to the intermittent behaviour of renewable sources, particularly solar power, the connected electrolyser(s) require power control to prevent situation like load rejection when the plant is directly connected, instead of via the electrical grid.
What else is needed?
Becoming an electrical engineer in the energy business means you’d become kind of a detective in figuring out what works best. Not only for the power electronics that manage the flow of electricity, but also for what else is needed. Lunshof: “Does it help to add batteries for power storage, or is it better to take them away? How do we best operate at night, when there is no sun?”
[..] creative thinkers that figure out what is best will make or break the energy transition and thereby the future.
He continues: “Our future electrical engineers need to make a choice between using existing technologies and materials or describe and design new technologies and materials. They need to figure out if hardware or software needs adaptions, and how to best deploy the best installation with the most minimal size and highest efficiency. And how to make it profitable, meaning that the levelized cost of hydrogen from green hydrogen is competitive with fossil-based hydrogen.”
As easy as wind power seems to be, there is much more to it than just connecting a wind turbine with the grid. Power electronics and – most of all – smart, creative thinkers that figure out what is best will make or break the energy transition and thereby the future.
© Copyright cover image: Stuart Conway, Shell