A multiport coordinated control strategy is proposed for multi-island grid converter station to participate in the main grid frequency stability regulation to heighten the frequency stability of voltage source converter based multi-terminal direct current(VSC-MTDC)system. According to the mathematical model of the interconnected system

this strategy has to design a control law for the direct current(DC)power regulatory amount of each island grid converter using the feedback linearization H∞ method in nonlinear robust control. Secondly

in consideration of the frequency fluctuations of each island grid

this strategy uses the simplex method to optimize the DC power distribution coefficient for the control rate of DC power adjustment amount

thereby obtaining the DC power adjustment amount of optimal allocation and ensuring coordinated participation by all island grids in the main grid frequency stability adjustment process with optimal DC voltage droop control. Finally

a three-terminal VSC-MTDC system is set up on PSCAD/EMTDC. Comparison of simulation results with those of conventional control strategy shows the correctness and superiority of the proposed control strategy. This work is supported by Post-doctoral Science Foundation of China(No. 2015M582543).