For the ultra HVDC (UHVDC) with the hierarchical connection mode at the inverter side

considering the change of the Thevenin equivalent parameters (TEP) of post-fault AC grid

a coordinated control strategy to the subsequent commutation failure (SCF) at both layers is newly proposed. The originality of this work is manifested in three aspects. 1) The mechanism of the SCF at the fault layer is newly found by deriving the analytical expression of the extinction angle with the TEP

and that at the non-fault layer is newly found by the voltage-time area theory with the DC current coupling. 2) An estimation model for the TEPs of two AC grids at the inverter side is proposed with the post-fault quantities. To address the random noise and inaccurate measurement data

an adaptive robust least squares method based on the median principle is proposed to solve the TEP model. 3) A coordinated control strategy with the estimated TEP is proposed to compensate for the extinction angle at the fault layer and limit the DC current at the non-fault layer

thus suppressing the SCF. The simulation results verify the suppression effect of the proposed control on the SCF under different fault conditions.