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Despite attempts to increase the active power capability of vector-controlled voltage source converters (VSCs) connected to very weak grids, the interaction between the control dynamics and physical system is not completely understood. The result is often complex strategies that are difficult to implement. This paper proposes an intuitive modification of the VSC control based on physical considerations and dynamics of existing control. Several physical variables as seen from the point of common coupling (PCC) are found to contribute to the detrimental behavior of the VSC under weak connections. Hence, eliminating the impacts of these variables through feedforward eliminates their influence and significantly improves the active power capability. Notably, the basic structure of the vector controlled VSC is kept and its output-impedance is effectively reshaped. The proposed modification is validated through nonlinear time-domain simulations in MATLAB/Simulink Simscape Power System and results demonstrate the simplicity and intuitiveness of the modified structure.

A progressive interconnection of existing HVDC links to form grids and the development of completely new HVDC grids from different vendors are expected shortly. One of the current challenges of such endeavour is unintended interactions due to independently designed controllers. This article proposes a design methodology for decentralized controllers to mitigate such interactions in multi-vendor voltage source converter (VSC)-HVDC grids. The approach presented relies on the unique standalone input-output impedance transfer function of each VSC, and the global impedance transfer function as seen from each terminal after interconnection with other VSCs. Subsequently, network-level controllers are designed by attempting to match the global responses at selected locations based on a novel interaction analysis, to the unique transfer function model of the vendors at the corresponding location. This approach reduces the entire problem to an impedance matching problem. We demonstrate the efficacy and flexibility of both the methodology and the designed controllers in mitigating interactions due to the independent design of VSC controllers through nonlinear simulations on a four-terminal droop controlled HVDC grid.

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Pervasiveness of power converters in the electric power system is expected in the future. Such large penetration will change the current power system dynamics leading to uncertain, unexpected, and potentially critical responses. This paper investigates the stability and resonance of a VSC-HVDC (Voltage Source Converter High Voltage Direct Current) link within an AC grid, whilst providing insights into resonances having a role on the grid. This is studied through the impedance-based modelling of the entire system (AC and DC grids), including controls of converters. Additionally, the impact of the different parameters of the hybrid AC-DC power system such as control systems and grid components on the system dynamics and stability is investigated. From this study, the impact of the system components and the controls of the converter on overall resonance response and stability is shown, including potential undesired sub-synchronous and harmonic resonances due to AC-DC system interactions. The analytical impedance-based models developed and obtained is validated through time-domain simulations, the physical model of the whole system is built in Simscape ™ Power Systems ™ and control systems in MATLAB/Simulink ® (R2017b). This has demonstrated the validity of the model to deal with and detect such dynamics.