The working principle of multilevel converter arm bridge in ±500kV flexible HVDC is analyzed

and the basic features of valve section steady-state operating conditions and the DC-side short circuit operating conditions are summarized. Based on the above analysis and the equivalent working principle of voltage stress

current stress and thermal stress of valve section in the steady-state and the short circuit operation conditions

a kind of equivalent test device circuit of power module is designed

which satisfies the requirement of the test with at least one power module of different power levels in steady-state and the short circuit operation conditions. In view of the equivalent test circuit

its control strategy is designed based on the analysis of model

and the protection and control system of the valve section based on a three-layer structure is designed. Two bridge arms composed of a five-module two-valve section system are built

and relevant tests are carried out. The test results show that the proposed equivalent test circuit of the power module test device and protection and control system of the valve section meet the test requirements

and the control method of the multi-valve section system is effective.