Solid State Transformers: When being less gives you more

You may have ever used a transformer, or at least you may have seen a box as shown in Figure 1. In our utility grid, the voltages of the power generators (e.g. power plants, wind farms, solar farms) and loads (e.g. buildings, households, factories) are usually low (e.g. 230 V, 690 V) for safety reasons, while when the power is transmitted from the generator to the loads, the voltage is usually increased to very high (e.g. 220 kV) to enable a long-distance transmission (e.g. hundreds of km). Transformers are used to step up/down voltages, as seen in Figure 1.

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Figure 1 Transformers are widely used in our utility grid (the voltages are only examples)

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Figure 2: Transformer (with the same power capacity) size will reduce as the frequency increases. (source: Borgaonkar, Aumkar. (2015). Solid State Transformers: A Review of Technology and Applications. 10.13140/RG.2.1.1491.1443.)

Transformers are mainly composed of iron and copper. Their weight and volume, in general, are reversely proportional to the frequency of the voltage, so if we increase the frequency (Hz) of the voltage, then with the same amount of power capacity, the transformer size can be reduced, as well as the needed iron and copper to make the transformer, as depicted in Fig. 2-2. In this way, the transformer becomes more sustainable.

Changing frequencies

However, our legacy utility grid has a fixed frequency at 50 Hz, called line frequency. The grid is such a huge and complex system that changing its frequency will lead to an unrealistic cost. Thanks to the development of power electronics technology, nowadays, it is possible to make a power electronics converter to convert the grid voltage from 50 Hz to another frequency, e.g. 500 Hz.

Then a medium frequency transformer much smaller than the line frequency transformer can be used to step up/down voltage. As illustrated in Fig. 2-3, compared with a line frequency transformer, a solid-state transformer needs power electronics as an extra to converter frequency of voltages.

Nonetheless, the primary material of power electronics is silicon, which is essentially sand, and the cost is much lower than copper and iron.

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Figure 3: A solid state transformer is composed of a medium frequency transformer to step up/down voltages and power electronics to converter frequencies.

The concept of FlexH2 is to use offshore wind power for onshore hydrogen electrolyzers. To transmit the power over a long distance, the voltage must be stepped up at the wind farm side and down at the electrolyzer side. Solid-state transformers will be designed as one of the innovations to replace the line frequency transformer to power the electrolyzers in a more sustainable way.

Longer electrolyzer lifetime and lower cost

Meanwhile, thanks to the controllability introduced by the power electronics in solid-state transformers, the electrolyzers will have a power supply with higher quality, which leads to a longer lifetime of the electrolyzer and thereby lower the cost of hydrogen.