9 Onshore and Offshore Wind Farms

onshore ~ 600 GW

offshore ~ 30 GW

Europe has 285 GW of wind power capacity in 2024, covering almost 20 % of EU energy demand

EU’s designated target is 425 GW

Radial topologies

▪ #WT depends on rated cable current and rated WT power

▪ Most common, simple and cheap

▪ No redundancy within a string

Ring topologies

▪ Improved redundancy (two sides)

▪ Larger costs due to (a) additional or (b) oversized cables

Star topologies

▪ Reduce cable ratings required

▪ Very reliable, since independent connections

▪ Large costs and higher losse

High voltage power cables

Capacitive behavior dominates

▪ The longer the cable, the higher the capacitance C = C’ ∙ l

Switch independent of phase angle

But: vulnerable to high (fault) currents

Variable current flow direction

Active and reactive power can be controlled independently

High Controllability: Allows grid forming control → Offshore AC grid voltage and frequency can be set via the MMC

With this converter type, less AC-side filters are required, which saves spaces and money - especially offshore.

To operate the offshore AC network with wind turbines, a so-called grid forming capability of the HVDC converter is required. Only the chosen converter type can provide this capability.

The reason why wind turbines are required to ride through temporary grid faults is that a large-scale disconnection of WTs caused by a fault could endanger the AC system stability.

If the fault is not cleared within a couple of 100 ms, the WTs are allowed to disconnect from the grid and shut down.

Until the voltage has recovered, the power generated by the turbine is dissipated in breaking resistors (also called “choppers”) located within the DC bus of fully converter-coupled WTs

Wind turbine / rotors cannot be slowed down within a few ms

→ Power is generated during the AC-FRT period, but cannot be transmitted to the AC system since the voltage is (approx..) zero

▪ The energy is dissipated in breaking resistors (also called “choppers”) located at the DC bus of a converter coupled WT

▪ Note: More complex in case of DFIG setup!