In order to solve the problem that commutation failure prevention and control of UHVDC hierarchical connection to AC grid system cannot perfectly suppress the converter valve commutation failure on a non-fault level

we proposed an improved control strategy.Several parameters of commutation failure prevention and control were optimized.Meanwhile

we revised the following main parameters in the control logic:amplitude coefficient

voltage change control gain

self maintaining link time constant and prevention and control synergy degree of two voltage levels.By using the ADPSS electromechanical-electromagnetic hybrid simulation software

we simulated the actual power grid data.The results show that the improved control strategy can effectively suppress the UHVDC system inverter’s commutation failure on both voltage levels

for one non-fault level is caused by another with fault of AC network

which has great value in engineering application.