Nonlinear Control of Three-Phase Three-Level Four-Wire NPC Converter

This paper presents a new approach of input-output feedback linearization control applied to the three-phase three-level four-wire neutral point clamped (NPC) converter. Under the proposed control strategy, the NPC converter demonstrates the capability to achieve simultaneously two tasks. The first task is to operate as a nonpolluting rectifier by regulating and nearly equalizing the two dc capacitor voltages while providing power to the dc loads. It is worthy of note that the voltage imbalance between the upper and lower capacitors and the neutral current represent a well-known problem in the three-level NPC converters. As a second task the NPC converter will behave as a shunt active power filter (SAPF) to compensate current harmonics, unbalances and reactive power produced by a combination of single-phase and three-phase nonlinear loads connected at the point of common coupling on the grid. The multivariable state space model of the NPC converter in the dq0 synchronous reference frame is used to design the nonlinear controller which is based on the input-output feedback linearization. It is shown that the model zero-dynamics is asymptotically stable ensuring the local stability of the internal dynamics. The proposed control strategy applied to the NPC converter is verified by computer simulations. The system stability is confirmed under severe dynamic changes in the operation conditions. The results of conducted simulations validate the viability and effectiveness of the NPC converter to maintain grid side currents balanced and near sinusoidal with unity displacement power factor. The simulation results also confirm the capability of the NPC converter to stabilize the dc voltages while supplying unbalanced dc loads.