Modeling and Step Response Analysis of Back-to-Back VSC for LFAC Transmission

Low frequency AC (LFAC) is being proposed as an intermediate transmission technology between HVAC and HVDC. It could be attractive as a means for connecting offshore wind farms to the grid, in lieu of VSC-HVDC. Its single converter station could even be located onshore. Because it could operate at a fraction of 50/60 Hz frequency, LFAC could transfer significantly more power and for longer distances, without compensation, than HVAC. Unlike HVDC, LFAC could have the advantage of employing conventional AC circuit breakers for fault current interruption. Thus, it could be readily used for multi-terminal connections. This paper presents state space modeling of an ideal back-to-back VSC (frequency converter). Eigenvalues of the linearized model are used for stability and step response analysis of the converter. PSCAD/EMTDC simulations are then used to verify the accuracy of the derived model. This state space model could be applied in the analysis of LFAC systems.