With the development of HVDC transmission

ETT and LTT thyristor valves have some problems and defects at the protection of the thyristor reverse recovery period in the operation expreince of HVDC project

even results in the forced outage of converter. We analyzed and researched the protection tactics of the thyristor valve at the reverse recovery period

and analyzed the defect of dead zone caused by misjudge

malfunction or delay in the protection during operation. For ETT valves

the main reason is that direct du/dt detection is not used. For LTT valves

they are non dual-configuration without any feedback detection. Therefore

du/dt protection is adopted directly in ETT valves

and a double design and feedback detection are adopted for RPU of LTT valves as optimization. Furthermore

we put forward a new idea for domestic production with a more accurate and effective way for the key protection.