Depending on the types and severity of grid faults as well as the corresponding LVRT control behaviors, the operation conditions of grid connected power converter like delivered power, phase angles, and modulation index are signiﬁcantly different compared to the normal operation. It should be noted that the power device Tin, Dout, Din, and Dnpc under the LVRT operation of 3L-NPC converter may have even higher junction temperature than the most stressed normal operation condition. This should be taken into account when choosing the power devices and heat sink system for the wind power converter.
According to the investigations, the thermal optimization target for 3L-NPC wind power inverter under extreme LVRT is to reduce the junction temperature in the NPC diode and inner switch. Compared to the normal modulation, the proposed thermal-redistributed modulation sequences, which all enable full neutral point potential control ability, can achieve more equal thermal distribution and reliving the hottest power devices under extreme LVRT operation of 3L-NPC inverter. The proposed thermally optimized modulation methods are especially feasible during the LVRT operation, where the modulation index is relatively low, and more redundant switching states can be utilized.
In the typical three-phase three-wire converter structure, there are four current control freedoms, and it may not be enough to achieve satisfactory performances under unbalanced AC source condition. In the three-phase converter structure with zero-sequence current path, there are six current control freedoms. The extra two control freedoms coming from the zero sequence current can be utilized to extend the controllability of converter and improve the performance under unbalanced AC source condition. By the proposed control strategies, it is possible to totally cancel the oscillation in both the active and reactive powers, or reduced the oscillation amplitude in the reactive power. Meanwhile, the current stress in the faulty phase is also relived compared to the typical three-wire system.