World’s first unmanned HVDC offshore platform installed

The construction of Dogger Bank, which will become the world’s largest offshore wind farm, has reached another milestone with the installation of the world’s first unmanned high-voltage direct current (HVDC) offshore substation.

The substation is the first of three platforms supplied by Aibel to be installed at Dogger Bank, which is being built in three 1.2GW phases, A, B, and C.

The Dogger Bank A platform is 65 x 36 meters wide and 39 meters high and sits on a four-legged steel jacketed foundation structure anchored to the seabed at a water depth of approximately 28 meters.

Located approximately 146 km from the nearest land, Flamborough Head, off Bridlington in east Yorkshire, the installation of the platform was delivered by the main contractor Sapiem.

This innovative offshore platform is of lean design and is the first unmanned HVDC platform that will be operated from shore, with access only by a service operation vessel.

The platform will receive 1.2GW of AC power from Dogger Bank A’s 95 offshore wind turbines, convert it to DC and deliver it to an onshore converter station near Beverley in the East Riding of Yorkshire.

Equipped with Hitachi Energy’s latest generation high-voltage DC converter technology, Dogger Bank will be the first offshore wind project in the UK to use this technology to transmit the electricity produced back to shore, ensuring that the power is transmitted efficiently over long distances while minimizing losses.

Olly Cass, project director for the Dogger Bank wind farm, said: “The safe and successful installation of the offshore platform marks an important milestone for the Dogger Bank project. As the UK’s first HVDC platform for an offshore wind farm, it also marks an important milestone for the UK industry more generally.

“The platform will be controlled from shore and removing the need for personnel to remain on the platform means that elements such as living quarters, helicopter decks, and sewage systems can be removed, resulting in a 70% reduction in topside weight (per MW) compared to previous installations and hundreds of millions of pounds in cost savings.

“The team continues to make good progress in installing the turbine foundations and later in the second quarter, specialist cable installation and support vessels will begin installing the network of inter-array cables that will connect the turbines to the offshore platforms to transmit the first power in the summer.”

The main advantages of HVDC transmission over high-voltage AC transmission are:

(1) DC lines are less expensive when the power delivered is the same:

Three-phase AC power requires three wires to transmit, each wire to ground insulation to be designed under its peak 2 times the RMS value, while DC power requires only two wires and the DC peak value is equal to the RMS value if the monopoly is only one.

Therefore, DC transmission can save a lot of transmission materials, and also reduce a lot of transportation and installation costs.

Secondly, the overhead line tower structure of DC transmission is simpler, the line corridor is narrower, and the cable with the same insulation level can run at a higher voltage.

(2) DC transmission of power consumption is small:

Because DC overhead lines use only one or two wires, the active loss is small, and has a “space charge” effect, its corona loss and radio interference are smaller than the AC overhead lines.

(3) Suitable for submarine power transmission:

General AC power transmission is mostly overhead, the line and the formation of a capacitance of the earth, but because the composition of the air capacitance is very small, the impact on the circuit transmission is very small.

But for underground cables and submarine cables, because the cable and the land between the sea will form a larger capacitance, the capacitive resistance is smaller than the high-altitude erection, equivalent to the formation of additional branches, which in turn causes additional line loss.

(4) The problem of reactive angle stability:

In the AC transmission system, all synchronous generators connected to the power system must be kept in synchronous operation.

If a DC line is used to connect two AC systems, the above stability problem does not exist because the DC line has no reactance, which means that DC transmission is not limited by the transmission distance.

(5) Can limit the short-circuit current of the system:

When connecting two AC systems with AC transmission lines, the short-circuit current will increase due to the increased capacity of the system. It is possible to exceed the original circuit breaker blocking capacity, which requires the replacement of numerous pieces of equipment, increasing the amount of investment. When the DC transmission is, there is no such problem.

(6) Fast regulation speed and reliable operation:

DC transmission through the thermistor converter can easily and quickly adjust the active power and realize the tide flip.

If a bipolar line is used, when one pole fails, the other pole can still use earth or water as a circuit to continue to transmit half of the power, which also improves the reliability of operation.

The Dogger Bank Wind Farm, a joint venture between SSE Renewables, Equinor, and Vårgrønn, will be the world’s largest offshore wind farm, consisting of 277 wind turbines. The wind farm will be able to generate 3.6 GW of renewable energy, which is enough to provide clean and sustainable electricity to more than 6 million households.

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