A 600-kilometer balancing act

Mark Twain famously said, “climate is what we expect, weather is what we get.” The climate of Northern Europe is predictably wet and windy, but the weather is famously unreliable. Yet one thing that cannot be unreliable is the electricity supply.

In recent history, Germany has heavily invested in offshore wind power in order to reach (and exceed) its target of utilizing more renewables, reducing CO2 emissions and nuclear power generation. But the variability of wind speed, even in the North Sea, results in an intermittent power supply – varying from day-to-day and season-to-season. This results in sporadic excesses and shortages of electricity, at the mercy of the weather. This variation in power flow creates challenges for the grid operators and must be managed by utilizing alternative power sources, such as costly and, often, undesirable thermal generation.

Meanwhile, several hundred kilometers north, in Norway, around 95 percent of electricity generated comes from hydropower. Hydropower generation produces environmentally friendly, sustainable and cheap electricity and, much like those taps at home, can be accessed on-demand. The only limitation is the amount of water in the reservoir. This factor, is also at the mercy of the weather, but the vast volumes of water act like enormous batteries – charged up and ready to use whenever needed.

The concept then is simple; Norway has the dependable hydropower that can quickly and affordably react to a drop in wind speed, and Germany has occasional excess wind power, allowing the Norwegian reservoirs to fill up. Only one small problem remains – around 500 kilometers of North Sea.

Mixing electricity and water
For short distances underwater, it’s no problem to transmit power using traditional alternating current (AC) electricity. But transmitting power via 500 kilometers of subsea cable is a different story.

The back-and-forth, alternating current creates electromagnetic fields that interact with sea water, consuming huge amounts of power. This would mean throwing away so much electricity that, in reality, no one would build such a system.

Fortunately, that’s not the end of the story. The other type of electricity, direct current (DC), has no such limitations and can efficiently transmit power over hundreds or even thousands of kilometers with very low losses.

Because creating and installing 500 kilometers of cable is complicated, the power transmitted must be maximized. This is done by transmitting the electricity at extreme voltages, in this case as high as 525,000 volts; over 2,000 times the voltage in a home.

The resulting high-voltage direct current (HVDC) system will have a capacity to transmit 1,400 mw of electricity; enough power to supply 3.6 million German households and the most powerful system of its kind ever made.

The last piece of the puzzle
Both hydro and wind power generation produce AC electricity but it’s far from the voltage needed for effective transmission. This is where Hitachi Hitachi Energy Power Grid comes in.

Hitachi Energy provides sophisticated HVDC converter stations which take this AC power and multiply the voltage using large power transformers and then “flatten” the electricity flow into DC.

Once the electricity is in this form, Hitachi Energy advanced MACH control system gives the system operator full control over the power. The power can be increased or decreased or, at the click of a button, the flow fully reversed – as changeable as the weather but completely reliable.

Constructing the converter
Building an HVDC converter station is, of course, already a highly specialized project that only a handful of people in the world can complete. Yet this is made even more challenging by the fact that the Norwegian station is over 50 km inland, in a mountainous region close to the hydropower generation plant.

The winding mountain road makes transporting seven, more than 200-ton power transformers a precarious activity. But, with over 120 HVDC projects completed, Hitachi Energy has had a great deal of practice, despite some tight squeezes.

These transformers are manufactured in central-Sweden’s “high-voltage valley”; Hitachi Energy world-leading hub of electrical power systems research and development, systems design, manufacturing and testing. From the factory, these transformers are sent by specialist trains, boats and, in this case, a 74-meter -long, 104-wheeled trailer.

Once the transformers arrive at the station, they must be transferred from the trailer to their final destination. The move to this protective concrete enclosure is only a few meters but it requires some precise and careful maneuvering that can take several hours of tedious sliding, millimeter by millimeter, into position.

This project, named Nordlink, is due to be completed in 2020 and will be an important step in the European Union’s mission to reduce CO2 emissions by 80-95 percent by 2050. And, combined with the many other HVDC transmission systems provided by Hitachi Energy around the world, will help enable a stronger, smarter and greener grid.