Conclusions
According to the investigations in this paper, the thermal optimization target for 3L-NPC wind power inverter under extreme LVRT is to reduce the junction temperature in the NPC diode, which is the hottest power device of the whole inverter system, and the conduction loss is dominant on this condition.
By the proposed insight generation method for modulation sequence of 3L-NPC inverter, it is possible to develop a desired thermal optimized modulation as well as a neutral point potential control method. The proposed thermal optimized modu- lation sequence, which enables neutral point potential controllability for 3L-NPC wind power inverter, can effectively reduce the conduction time of NPC diodes, and achieve 32 K less steady-state junction temperature in NPC diodes under 0.05 p.u. LVRT compared to the normal modulation method. However, due to the thermal time constants as well as the short duration period of extreme LVRT, the temperature improvement by the optimized modulation method is limited in the dynamic LVRT process. The proposed thermal optimized modulation methods are special feasible during the LVRT operation, where the modulation index is relatively low and more redundant switching states can be utilized.
Nevertheless, the optimized modulation still achieves significant reduction of the maximum junction temperature and temperature fluctuation by 22 and 8 K, respectively, within the 500 ms LVRT duration, and more equal thermal distribu- tion is realized between the different switching devices. It is expected that the proposed optimized modulation can extend the lifetime of 3L-NPC wind power inverter under low voltage ride through and satisfy stricter grid codes in the future which may require longer LVRT time and higher reliability of the converter.