During communication failure between converter stations， if emergency switch off sequence（ESOF） occurs at inverter side due to serious faults， it mainly depends on DC under-voltage protection to block converter at rectifier side. Constant power control is usually employed at rectifier side， therefore， blocking with bypass pair at inverter side frequently leads to low voltage and large current operation mode of rectifier station. This adversely affects the adjacent AC grid and makes the voltage in AC bus of converter station reduce rapidly. As a result， the adjacent generators may start oscillating and even lose synchronism. Based on the analysis of mal-operation of generator out-of-step protection resulting from high voltage direct current（HVDC） under-voltage operation， it is concluded that both out-of-step protection and DC under-voltage protection should coordinate with each other properly， and the principle of setting cooperation is proposed. This method has been successfully applied to HVDC projects and their adjacent AC power plants in China， which improves the performance of protection greatly in AC-DC hybrid grid.