Control of a grid connected converter during weak grid conditions

The focus on renewable energy sources have been increasing since the late 80s in Denmark. Especially focus on wind power has been increasing, with implementation of big on- and offshore wind farms. However, when installing the wind farms, these are often placed distant from the utility grid, like the case for offshore wind turbines. This can cause the wind farm grid networks to be weak. A weak network is an extra challenge to inject energy into, since it might exceed grid codes and grid standard values, if not compensated for. To ensure quality of the energy injected to the network, a Voltage Source Converter (VSC) is used. Using these VSCs the quality of the injected energy can be according to the standards requirements. This paper shows the effects of different weak grid conditions on the control system of the inverter, which includes lowered bandwidth and higher response time. Furthermore the controller has been designed to be stable for different Short Circuit Ratio (SCR) values. The designed controller and filter has shown to be operating under various weak grid conditions by simulations and for a 5kW laboratory setup.