Short-circuit Contributions from Fully-rated Converter Wind Turbines : Modeling and simulation of steady-state short-circuit contributions from FRC wind turbines in offshore wind power plants

Abstract: In recent years there has been an increase in wind power plants installed out at sea. The generated power of wind turbine generators (WTGs) are collected through numerous subsea cables into a single hub, the offshore platform. Subsequently, this platform is interconnected with the onshore main grid through a further stretch of cable. In the event of a fault, a sudden increase in current, so called short-circuit current, will occur somewhere in the system. The short-circuit current will, depending on the duration and location of the fault, potentially harm the power system. In order to accurately determine the dimensions and rating of the equipment installed in the offshore wind power plant (OWPP), the magnitude of this current needs to be studied. Furthermore, depending on the country in which the OWPP is installed, the transmission system operator (TSO) might pose different low-voltage-ride-through (LVRT) requirements on the system. One such requirement is that the installed turbines should provide voltage regulation through injection of reactive current. A type of generator able to achieve this is a so-called fully-rated converter windturbine generator (FRC WTG). Through a power electronic interface, the reactive and active current components of the generator can be freely controlled. With a high level of reactive current injected during a fault in the OWPP, the short-circuit contribution from these FRC WTGs needs to be evaluated. In this master’s thesis, a method has been developed in order to determine the steady-state short-circuit contribution from multiple FRC WTGs. This methodology is based on an iterative algorithm, and has been implemented in the simulation tool PowerFactory. To evaluate the performance of the method, two case studies were performed. In order to improve simulation times, an already existing WTG aggregation model has been implemented to reduce the number of turbines in the test system. From the results, it is concluded that the method obtains the expected FRC WTG short-circuit currents with sufficient accuracy. Furthermore, the deviation from the expected results are evaluated using a numerical tool. This project was initiated and conducted at ABB in Västerås, Sweden.