The integration of large-scale floating wind farms into transmission grids plays a vital role in meeting European energy demands and climate targets. A technology which could potentially accelerate the grid integration of floating wind farms is subsea substations currently being pursued by several OEMs. This technology indicates significant cost reductions as compared to both floating and fixed offshore substations. A typical subsea substation configuration consists of subsea transformers and associated subsea distribution to interface subsea collectors and inter-array cables. The system can be further enhanced by using subsea circuit breakers for each of the array feeders. This paper provides practical insights into the operational challenges of floating wind farms with long subsea cables and subsea substations and evaluates the technical trade-offs between system configurations with and without subsea switchgear, with respect to voltage and reactive power control, grid code compliance, system protection, and overvoltage mitigation. Power system studies are conducted for a 500 MVA floating wind farm module connected to shore via approximately a 120 km 245 kV AC subsea export cable and a subsea substation.

Steady-state load flow simulations were performed using DigSilent PowerFactory to evaluate operating scenarios at various wind generation levels. Due to the large reactive power generated by the long subsea cable, reactive power compensation from wind converters must be utilized to avoid installing offshore reactors. Other benefits of utilizing wind converters include, but not limited to, balancing transmission system voltage profile, maximizing high voltage alternating current (HVAC) transmission transfer capability and reducing the required size of onshore reactive compensation equipment. Three reactive power control strategies for wind plant controllers are investigated: two of them are constant reactive power control and the other constant power factor control.

Protection philosophy and relay coordination settings were assessed using NEPLAN for two system architectures: one relying solely on onshore circuit breakers and one incorporating 66 kV subsea switchgear within the subsea substation. For the configuration without subsea switchgear, protection of the inter-array cables from the onshore side is feasible; however, strict grid code requirements for fast fault disconnection force the onshore breaker to operate with minimal delay, which compromises protection selectivity and the availability of the plant. For the configuration with subsea switchgear, selective and localized fault isolation of faulted inter-array cable sections is achievable while maintaining operation of the remaining healthy sections and meeting fault clearance criterion defined in grid codes.

EMT simulations using PSCAD were carried out to analyze fault clearance scenarios for both configurations. Inter-array fault clearance was identified as one key scenario to study in terms of switching overvoltage. The results show that without subsea switchgear, switching overvoltage poses a risk to violation of standard insulation level of system components unless surge arresters are used. With subsea switchgear, maximum peak of switching overvoltage is reduced at the 220 kV level and at the 66 kV level by approximately 15% - 35%, leaving larger safe margin within equipment insulation capability.