Wind energy is a growing energy resource in the current energy transition. Massive new amounts are planned to be built in the face of the climate and energy crisis. The current state-of-the-art type-4 Wind Turbines (WT) uses a Fully Rated Converter (FRC) to decouple the rotor spe
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Wind energy is a growing energy resource in the current energy transition. Massive new amounts are planned to be built in the face of the climate and energy crisis. The current state-of-the-art type-4 Wind Turbines (WT) uses a Fully Rated Converter (FRC) to decouple the rotor speed from the grid frequency. In recent years this type of WT has been involved in so-called sub-synchronous events. The Grid Side Converter (GSC) of the FRC is very similar to other Inverter Based Generation (IBG) such as: Photovoltaics (PV), Battery Energy Storage Systems (BESS) and HVDC-links. As this research is focused on the GSC, it is very relevant to the integration of other Renewable Energy Resources (RES) as well.
This thesis investigates Sub-Synchronous Control Interaction (SSCI), a relatively new phenomenon. The GSC interacts with the grid, which can cause a Sub-Synchronous Oscillation SSO, an oscillation below the system's fundamental frequency. This oscillation can lead to damage, increased components wear or triggering protection systems. After different facets of SSCI are discussed, a new solution is put forward in this thesis. By feeding wide area Phasor Measurement Unit (PMU)-based measurements into the control system of the WT, an algorithm is constructed to mitigate SSCI. This algorithm is tested on the ability to mitigate sub-synchronous event risks.
First, a WT model is selected in the DIgSILENT PowerFactory software to recreate sub-synchronous behaviour. This model is tested in the weak radial grid. The influence of the different control parameters on the sub-synchronous behaviour is investigated and comped to literature studies. Different tools are used for this research. PowerFactory is used for small signal analysis to obtain a linear dynamic model of the system. RMS-time simulations are used to test the oscillatory behaviour of the system. These results are further analysed using eigen-analysis on the linear model and Fourier and Prony analysis on the time simulations.
Then, the same WT-model is implemented in a modified IEEE-39 bus system to see the behaviour of SSO propagation in a transmission network. The results are compared to an analytical method to determine voltage oscillations based on mode shapes. However, as SSCI has a fundamentally different driving force, this method is not valid in this case. In the same transmission network, the influence of grid operations is tested. Different variations are tested: the connection point, power output of the WT, grid loading and disconnection of nearby connections. As all these features can vary during normal operation, it is vital to understand better which circumstances increase the risk of SSO event.
At last, using the knowledge obtained, a new control algorithm is proposed. This algorithm uses wide-area voltage angle measurements. These measurements can be provided by PMUs. A remote signal is fed into the control system. Minimal modifications are made to the control system for this. Additional damping is performed on the local voltage reference angle depending on the movement of the remote signal. This prevents the initialisation of SSCI. This implementation of a WAMC is tested using eigen-analysis and time simulations and shows promising results.
The results of this thesis research are also modelled in the NextGen GridOps Framework of DNV. The framework aims to stimulate a fast deployment of the finding of this thesis research in the field. This is accomplished by sharing knowledge in an accessible, structured environment. This allows taking the findings into consideration for solving the complex problems encountered by transmission and distribution system operators.
