Managing the DC link neutral potential of the three-phase-four-wire neutral-point-clamped (NPC) inverter in FACTS application

To compensate for the zero sequence current arising from asymmetrical operation of a transmission line, a neutral point clamped (NPC) inverter with a fourth wire must be employed. The most significant challenge arising from controlling of the four-wire NPC inverter consists in stabilizing the neutral point potential at the DC link middle value. Unlike the three-wire case, all the switching states available from the four-wire NPC inverter are attributed to output voltage formation, no freedom remains for neutral potential control. As long as the output voltage is the primary control objective, the neutral potential must be left to an alternative approach for stabilization. A spontaneous neutral line current control loop exists also with the four-wire NPC inverter, which renders the neutral point potential to be self-balancing. However, under asymmetrical operation, the neutral point will be stabilized at some value other than the DC link middle potential. In particular, the response time of the current control loop is strongly load dependent. The proposed three-level buck-boost converter works with high switching frequency due to the nature of zero voltage switching. It offers tight control of the neutral potential with small rating device, fast response and high efficiency. Moreover, it uses the same NPC pole and requires no additional layout consideration.