Abstract: Aiming at the lack of realtime detection methods of operation status for NPC three-level inverter，an

indirect detection method was studied. In this method，the commutation process of NPC three-level inverter was

analyzed. And according to Kirchhoff's law，the current and voltage models of three-level circuit test were built.

Compared with the direct measurement method which needs large numbers of measurement devises，only three

current detection units and three voltage detection units were used to obtain the operation status of four IGBTs and

two clamping diodes of single-phase bridge arm in this method. The correctness of the method was verified by the

pulse experiment of single-phase bridge arm. The experimental results show that this method can obtain the operation status of power devices for NPC three-level inverter in real time. At the same time，the installation of equipment is more convenient，and the measurement cost is reduced. It has certain reference significance for the design and application and fault prevention of NPC three-level inverter.

      Key words: inverter；power device；online test