Modelling VSCs connected to weak grids

This paper addresses the oscillatory stability phenomena studied in the literature and reported by network operators in grid-connected VSC systems. Negative interactions have been observed at different frequencies in these systems, causing the tripping of power converters (e.g. wind turbines or HVDC applications). The interaction between the controller of power converters and poorly damped AC networks are identified causes of these events. This paper analyses two methods to model grid-connected VSC systems applied in the literature to study stability: the state-space and the impedance-based model. Validation has been carried out between the two methodologies by analyzing eigenvalues and singular values of the system. Furthermore, a time-domain simulation has been done to validate the state-space model with a non-linear MATLAB/Simulink model. The stability is studied in a base case system with a grid-connected VSC to the main AC grid via an LC circuit. A sensibility analysis of the strength of the grid and an analysis of subsynchronous and harmonic oscillatory resonances has been carried out for weak grids.Europe’s power system has seen significant changes in recent decades, notably the development of renewable energy sources. However, this transition is far from complete, and further changes are essential to make our energy system ready to play its part in realising the climate goals set at COP21. At present, renewable energy sources are increasing their share of electricity generation. This is particularly the case for offshore wind energy. InnoDC's 14 participants prepare 15 early career researchers to play their role in the energy transition that will take place over the next 20-40 years. The project focusses on the development of the electricity transmission system, targeting the connection of offshore wind, the integration of offshore wind with the existing power system (including the use of HVDC), and the operation of the future power system where large scale wind is connected to a hybrid AC and DC power system. Technological development for offshore wind is ongoing. This research project focusses on the models and methodologies for the integration of these new technologies (e.g. offshore wind turbines, VSC HVDC converters, long AC cables) into the power system. Challenges in these areas will be addressed in this project: firstly, these new devices behave inherently differently to traditional power system components. Secondly, the multi-actor/intersectoral nature of these systems means that they have distinct elements and devices interfacing with each other, each with limited information of the overall system. The project will train the researchers in developing prototype tools to aid the developers and users of these new energy systems. This training network aims to train the researchers of the future in the pivotal sector of renewable energy and grid technology, which is largely led by research and industry. This project prepares the researchers of tomorrow to maintain Europe’s position of leadership in renewable, smart energy and tackling climate change.Postprint (published version