Modelling of VSC-HVDC for Slow Dynamic Studies

With recent advances in technology and the controllability it offers, Voltage Source Converter based HVDC (VSC-HVDC) is expected to be the preferred choice for integration of renewable energy sources and when building large HVDC grids. As it is a relatively new technology when compared to the more mature Classic HVDC, many studies are aimed at operation and stability of VSC-HVDC interconnecting AC grids. When performing simulations on large systems interconnecting AC grids with VSC-HVDC, the computational load will be heavy due to the complexity of the models associated with VSC-HVDC. It is therefore desirable to develop simplified models suitable for studies where the main interest is in slow dynamics, where such complexity might have a negligible impact on the results. In this thesis the goal was to implement a simplified model of a VSC-HVDC in the software tool PowerFactory to be used in transient stability studies. First, a detailed model was implemented in PSCAD and PowerFactory for EMT simulations. In order to use the model for simplified RMS simulations in PowerFactory, it was found that several adjustments had to be made in order to get a more similar performance to the detailed models, when considering the slow dynamics of the system. After the simplified model had been implemented it was verified by simulating different cases and comparing the results to the detailed models in PowerFactory and PSCAD. In most cases the simplified model was a good representation of the detailed models. The verified model was then included in a larger multimachine system in order to evaluate the impact of VSC-HVDC on AC systems. It was found that the computational time could be reduced drastically using the simplified model, with negligible differences in the results. From these simulations it was concluded that due to the high controllability of VSC-HVDC and the ability to produce or consume reactive power as needed, it could be used to improve the transient stability of the AC system