A state-space approach for control of NPC type 3-level sine wave inverter used in FOC PMSM drive

This paper presents the design and analysis process of state feedback controllers for NPC type 3-level sine wave inverter. In order to achieve good dynamic features in a state feedback control an internal model of reference input and a feedforward path are introduced. During synthesis process of the state feedback controllers maximum permissible dynamics of voltage control in the linear range of modulation is taken into account. It is shown that initially gains of the controller and feedforward path are non-stationary and depend on the angular velocity. Stationary approximation of a non-stationary state feedback controller is presented in details. Proposed state feedback control structures for continuous voltage shaping NPC type 3-level voltage source inverter are examined in FOC PMSM drive. The novelty of the presented controller lays in a stationary approximated state feedback control structure designed in terms of maximum permissible dynamics of a voltage control system. Simulation and experimental results (at the level of 3 kW) of the designed control algorithms are included. A sinusoidal output voltage waveform of the 3-level in- verter with an output LC filter can be formed in a wide range of the drive operation with the help of the additional voltage control system. Deadbeat controllers (6) or a cascade con- trol structure with classical PI controllers (11) can be applied to shape a sinusoidal output voltage waveform of the invert- er. The mathematical model of an inverter with an output LC filter is non-linear and non-stationary (12). Properties of the model described above cause that, thanks to full vector component decoupling ability, state feedback control is an at- tractive approach to control the NPC inverter with an output LC filter. Non-stationary gain matrices of the state feedback voltage controller can be obtained using pole placement tech- nique (13) or linear-quadratic optimization method (12). In both cases coefficients of the designed state feedback con- troller will be non-stationary (i.e. it depend on the angular velocity). In order to control space vector components of the filter output voltages without steady state error (in a case of step variations of the reference voltages), an internal model of the reference input (14) is added. Further improvement of the dynamic features in state feedback control is possible by introducing a feedforward path into control algorithm (13). Our primary goal is to design and examine a state-space control system for continuous voltage shaping NPC type 3- level voltage source inverter in terms of minimization PMSM torque ripple caused by a non-sinusoidal output voltage of the inverter. There are two case studies presented here: state feed- back with internal model control (SFC1), state feedback with: internal model control, an additional feedforward path and feedback from measurable disturbance (SFC2). During the synthesis process of the state feedback controllers maximum permissible dynamics of voltage control in the linear range of modulation is taken into account. Stationary approxima- tion of a non-stationary state feedback controller is presented in details. Finally, simulation and experimental results of the designed control algorithms are shown.