Offshore Low-frequency AC Transmission System Based on Back-to-back Current Source Converters

Low-frequency alternating current (LFAC) is an attractive approach for integrating offshore wind energy. Since actively commutated current source converters (CSCs) do not require a large number of capacitors and have the capability of black start, this paper then proposes an LFAC system for offshore wind farm connection based on fundamental-frequency-modulated CSCs used as onshore back-to-back converter stations. To reveal the operation principle, the CSC mathematical model is first performed, and system parameters are designed. Moreover, a coordinated control strategy of the wind turbine and the low-frequency-side CSC is proposed to stabilize AC voltage amplitude while regulating frequency. Furthermore, a black-start control method is introduced for the LFAC system's start-up. Then, typical operating conditions such as black start, wind speed fluctuations, and typical grid failures are simulated and analyzed in PSCAD. Eventually, the analysis of comparison among different solutions is carried out to verify the economics and feasibility of the proposal.