Non-linear stability of inverter-dominated power grids

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The continuous drive towards the decarbonisation of existing power systems is directly connected with the increasing installation of renewable energy resources in them. These renewable resources require power electronic inverters in order to interface them to the existing power networks. The power electronic inverters are dominated by control algorithms and despite the fact that they can offer increased flexibility in operation and control of the network compared to the traditional synchronous generators, they also introduce new and unprecedented dynamics to the network that are threatening the stability of the power systems. It is of vital importance for the smooth operation of future power networks to be able to guarantee the stability of the networks and avoid in this way interruption of services to the public. My PhD research focuses on investigating the stability of the power networks under high penetration of power electronics. More specifically it concentrates on studying and analysing their transient stability, which is the stability of the system after large disturbances, such as short circuit faults, line disconnections and phase jumps, take place in it. The power electronic inverters that are used are distinguished into two main types of inverter depending on their method of synchronisation with the grid and their operation. These two types are the Grid-Following inverters, which inject power to the grid by forming the current and following the grid's voltage and the Grid-Forming inverters which form a voltage source from which power can be drawn. These types of inverters utilize different synchronisation controllers to sustain their synchronisation with the network and therefore their interaction with the network under different disturbances from the grid differs. My research aims to provide insights into their individual behaviours after disturbances take place in the network and deduce valuable conclusions on how they should be configured and used in the network to reduce as much as possible potential instabilities to the system. The approach includes analytical derivations of differential equations describing the behaviour of the inverters under different circumstances and in a range of network configurations. These configurations will include different number of inverters connected to several example grids, covering grids of different strengths and structures aiming to cover a large range of possible scenarios and derive comprehensive results on how these inverters affect the system's stability. The derived analytical equations will be used to perform Phase Plane analysis of the systems and gain visual insights on their behaviours as well as employ other techniques and methods that are used for investigating the stability of non-linear systems. This analysis will be followed by time-domain simulations of the examined systems in simulation software for validation of theoretical results obtained. It is anticipated that the analysis will be useful to grid system operators in choosing how to plan and configure their power grids.