This article addresses an advanced vector current control for a Voltage Source Converter (VSC) connected to a weak grid. The proposed control methodology permits high-performance regulation of the active power and the voltage for the feasible VSC range of operation. First, the steady state characteristics for a power converter connected to a very weak system with a Short Circuit Ratio (SCR) of 1 are discussed in order to identify the system limits. Then, the conventional vector control (inner loop) and the conventional power/voltage control (outer loop) stability and frequency responses are analysed. From the analysis of the classic structure, an enhanced outer loop based on a decoupled and gain-scheduling controller is presented and its stability is analysed. The proposed control is validated by means of dynamic simulations and it is compared with classic vector current control. Simulation results illustrate that the proposed control system could provide a promising approach to tackle the challenging problem of VSC in connection with weak AC grids.

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This article is focused on the droop-based DC voltage control design for multi-terminal VSC-HVDC grid systems, considering the AC and the DC system dynamics. The droop control design relies on detailed linearized models of the complete multi-terminal grid, including the different system dynamics, such as the DC grid, the AC grid, the AC connection filters and the converter inner controllers. Based on the derived linear models, classical and modern control techniques are applied to design the different controllers, including a multi-variable frequency analysis to design the grid voltage droop control. In combination with the droop control, a DC oscillation damping scheme is proposed, in order to improve the system performance. The control design is validated through simulations of a threeterminal system.

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This article introduces a hierarchical power control structure for Multi-terminal High Voltage Direct Current (MT-HVDC) systems. The presented hierarchy is similar to the control structure used in classical AC transmission systems and is divided in primary, secondary and tertiary control actions. The voltage control in the MT-HVDC scheme acts in a way similar to the primary control action of generators in AC systems, while the secondary control action is performed by an outer power control loop. The design of the individual controllers and the interaction between these control loops is discussed in detail.Furthermore, the operational characteristics are described and the main operating points are identified. Scenarios including a power reference change and a grid side converter disconnection have been simulated in order to test and verify the proposed method. The main contribution of the paper is the development of a control methodology, providing a separation of the different control actions in different time domains, similar to what is already in use (and therefore known) within traditional power systems.

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