Coordinated Voltage and Frequency Control in Hybrid AC/MT-HVDC Power Grids for Stability Improvement

In this paper, an adaptive coordinated voltage and frequency control scheme is proposed for hybrid AC/ multi-terminal high voltage direct current (MT-HVDC) power networks for stability improvement. The coordinated control strategy is realised using a multi-dimensional droop approach, which can also be implemented as a distributed control strategy in hybrid AC/MT-HVDC networks without a communication link between voltage source converters (VSCs). Moreover, the controller parameters are adaptive to the fault condition to improve the stability and suppress post-fault oscillations. A state-space model of the VSC is built while explicitly representing the phase-locked loop (PLL) dynamics. An oscillation index is proposed to quantitatively assess the oscillation severity of the AC voltage, frequency and the DC voltage. The proposed strategy is implemented in the CIGRE test system developed in DIgSILENT Power Factory. According to the small-signal stability study, the proposed multi-dimensional control scheme improves the frequency stability and maintains the AC voltage stability of the hybrid AC/MT-HVDC power network. Under severe fault conditions, the adaptive multi-dimensional control scheme improves the system stability while adjusting the controller parameters based on the severity of the fault. Therefore, the proposed scheme can improve and optimise the overall stability of the hybrid AC/DC power grid.