Low Frequency Alternating Current (LFAC) Transmission Systems for Offshore Wind Farms - Case Studies on the Use of LFAC, Based on Horns Rev 3 Offshore Wind Farm
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- Institutt for elkraftteknikk 
This thesis investigates the use of low frequency alternating current (LFAC) for the longdistancetransmission of the power extracted from offshore wind farms. It is based on themeasurement of power flow in different nodes in the system simulating an offshore windfarm being built outside the coast of Denmark, Horns Rev 3 (HR3) . The results forscenarios simulating different lengths of the offshore cable and the operating frequencyof the system are compared. Reactive power compensation is used to improve the powerflow of the system and to counteract the reactive current from the capacitance in the longoffshore cable.Power flow, voltage- and current limitations of the offshore cable, and varying the reactivepower compensation in the different nodes are used to assess if the different scenariosdescribed are feasible or not. Considerations related to size and operation at reducedfrequency on the different power system components are mentioned and discussed briefly.The thesis does not include any detailed analysis with regards to a possible increase incost for the different components operating at reduced frequency.The thesis concludes that the use of LFAC gives reductions regarding power losses, improvingthe power system efficiency. For the real case of HR3, using LFAC excludesthe need for reactive power compensation. The thesis also proves that it is possible totransmit substantial amounts of power over longer distances than any other AC-cable inoperation today. It is proved that using LFAC and reactive power compensation fromshore, it is possible to transmit over 370 MW to the grid with an overall efficiency ofmore than 93 %, at a distance of 200 km using a 220 kV offshore cable. Including reactivepower compensation from the offshore end of the cable, the distance can be increasedto 300 km, delivering over 370 MW of power to the grid with an overall efficiency of 91 %.However, the weight and size of LFAC transformers are considerably higher than theequivalent 50-Hz components, with up to 2.7 times the weight, this would be a challengefor the construction of the offshore substation transformers.Beyond the scope of this thesis further investigations are needed regarding costs for thedifferent components of the system, as this is not included.