Collection and transmission system

Floating offshore wind is still in its early stage of development. So far, no large scale FOWFs have been built. Only a few projects have been realized ranging from single prototypes up to small pre-commercial wind farms of 5 units [27]. However, the electrical system of a large FOWF would be similar to a bottom-fixed offshore wind farm (BOWF). Therefore, in this section different types of configurations for the collection and transmission system are presented that have been developed for BOWF in order to address the challenges that can appear in the electrical system of an offshore wind farm. MVAC configuration

The medium voltage alternating current (MVAC) configuration is applied for connecting all wind turbines and to transmit the energy to the onshore substation. A power cable connects a number of turbines in series and ex- ports the generated power in the medium voltage level of the wind turbine transformer, typically about 24kV to 36kV. No offshore substation is used in this configuration. The onshore substation generally contains a transformer, switchgear, reactive power compensator, harmonic filters, metering and the connection to the point of common coupling (PCC). Figure 2.10 illustrates this configuration.

MVAC was applied in most of the first offshore wind farms that were lo- cated near to the shore and had low rated power capacities. For larger wind farms located remotely offshore, alternative technologies need to be applied since the power losses in alternating current transmission typically increase with the distance [45].

MVAC / HVAC configuration

In the MVAC / HVAC configuration, an offshore substation is considered to step up the medium voltage of the wind turbine collection grid to high voltage. The electric power is then transported in HVAC via one or several submarine export cables to the onshore substation. The high voltage applied in the export cables depends on the maximum power to be transmitted and the ability of the power cable. Typical high voltage values are 132kV and 150kV. However, 220kV is also recently studied for new large offshore wind farms. Figure 2.11 illustrates this configuration [46]. The offshore substa- tion contains the transformer, switch gear and reactive power compensation. The onshore substation is similar to the offshore counterpart and is used to adapt the voltage level, frequency and reactive power to the requirements of the local grid in order to integrate the electric power. This configura- tion is mostly applied today, because due to the high transmission voltage, lower power losses occur along the cables and higher capacities of power are transmittable [45]. However, the construction of the offshore substation involves a large investment and in case of a failure large power losses can occur. Moreover, HVAC seems not technically and economically feasible for very large distances. An exemplary wind farm based on this technology is Walney Extension, which is currently the largest offshore wind farm with a rated power capacity of 659MW [47].

MVAC / HVDC configuration

The transmission of electric power in HVDC is an alternative solution to HVAC for longer distances and larger wind farm capacities. The reason is that in direct current configurations the power transfer capability for long distances is not reduced as much as in HVAC cables, which generate charging currents and reactive power [46]. The break-even distance at which HVDC becomes more feasible than HVAC technology is at about 80km depending on the project circumstances and voltage levels [48]. The drawback of this technology is that large and expensive AC/DC converters are required since the wind farm grid and the onshore distribution grid are operated in AC. In addition, a separate offshore converter station is required besides the AC transformer substation. However, the converters have the benefit of being capable to control both the voltage and the power injected to the main grid in order to fulfill the grid code requirements imposed by the transmission system operator. Figure 2.12 shows a configuration of this technology [49].

The technologies that have been developed for the converters are line com- mutated converters based on thyristors and voltage source converters (VSC) based on switching devices. Line commutated converters have been widely used in onshore transmission applications, but the drawback of this tech- nology is that it needs a minimum reactive power to work is more affected by to potential AC grid faults than VSC technology. VSC is a relatively new technology and has the advantage that the semiconductors switching is decoupled to the grid voltage and therefore are able to adapt reactive power and provide power system stability. Another benefit is that VSC configurations need fewer filters and therefore require less space on the sub- station [5, 46]. However, the cost of this technology is higher. VSC has been the preferred technology for offshore wind farm applications in recent years.

The leading developers are Siemens with the HVDC plus and ABB with the HVDC light solution, both based on VSC configuration [5]. The first HVDC converter station was commissioned in 2009 by ABB and serves to connect the BARD Offshore 1 wind farm with a rated capacity of 400MW to the German national grid [50]. This first project had several technical prob- lems including a shutdown caused by a fire on the platform in 2014 [51]. However, since then, lessons have been learned and several more HVDC transmission links have been built in the North Sea to transfer the electric power generated by the offshore wind farms to the local grid [38].

DC / DC configuration

In light of the increasing power capacities and distances that offshore wind farms are being constructed, researchers aim to find alternative solutions to the conventional transmission configurations in order to reduce power losses and costs. Some of those novel configurations are presented next. Figure 2.13 illustrates a configuration based only on direct current. The DC/DC configuration is an alternative solution that applies direct current both in the turbine collection grid as well as in the export cable. Thus, the internal wind turbine DC/AC converter is replaced by a DC/DC converter. Furthermore, the heavy AC/DC converter and transformer in the offshore substation are replaced by a DC converter that steps up the medium voltage from the wind turbines to the required high voltage transmission voltage. In this way the size of the offshore substation can significantly be reduced and cost saved. Moreover, reactive power compensation is not required [5, 52].

However, drawbacks of this configuration are the complex and cost inten- sive DC/DC converters and a lack of adequate protection methods [5, 52]. LFAC configuration

The LFAC configuration is an alternative solution that suggests using low frequency (LF) in an AC collection and transmission system. Figure 2.14 presents this configuration.

Figure 2.14: LFAC configuration configuration [5].

The LFAC configuration requires a frequency converter onshore to adapt the low frequency of the offshore network to the frequency of the local grid. The lower frequency would also offer the possibility of a simpler, lighter and cheaper design of the wind turbines. Furthermore, the lower frequency reduces significantly the capacity charging current and thus increases the transmission capacity and distance of an AC network. The drawback of this technology is that the transformer size would increase significantly as well as the associated costs. So far this configuration has not been considered by the industry for further development [5].

Multi-terminal HVDC

Most of the current HVDC projects consider only point to point transmis- sion. However, multi-terminal HVDC systems will gain great importance in the near future in order to provide interconnections between countries and increase system stability. In a multi-terminal configuration a link is created between two or more separate transmission systems by increasing redun- dancy and thus improving system performance. An advancement of this concept is the proposal of a European HVDC super grid that would range from Scandinavia to southern Europe and would combine the generation capacities of all regions [5, 41].