Small-signal Stability Characterization of Interaction Phenomena between HVDC System and Wind Farms
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The main objectives of this research work are to determine the underlying mechanisms leading to instability in a system with dominant presence of tightly regulated power electronics components and to identify the causal relationship between power electronics component control algorithm and the grid stability with the purpose of establishing design guidelines that can guarantee the system stability. The research focuses on the analytical investigation of the smallsignal stability and interactions between components of an offshore DC grid and verification of such investigation by benchmarking with numerical simulations and laboratory tests. The simulations are employed to assess the influence of active and passive parameter uncertainties while the experiments verify their impact. The research results identify the critical configurations leading to unexpected interactions based on the theoretical analysis and the simulation results, and specify general guidelines for avoiding failures and restoring stability. Small-signal stability analysis in the frequency domain based on the state-space modeling and eigenvalue analysis, and the impedance-based analysis are carried out for the VSC-based HVDC transmission system and wind farms integration through the HVDC system. In this scenario, the stability properties of converter controllers widely used today have been identified and compared using the equivalent impedance analysis. Through this analysis, a controller that emulates the synchronous machine (The synchronverter) was found to exhibit an R-L (resistive-inductive) circuit nature on its equivalent impedance, which conferred this controller a better ability in keeping stability compared to PLL based dq-domain control in point of stability and control in integrating offshore wind farm through an HVDC system. A Grey-Box method for extracting critical controllers bandwidth, when limited information is available about the system, has been developed based on the impedance measurements. This method has proven to be effective in identifying critical parameters and in re-shaping the impedance to restore the stability.