Modelling and control of the three-level neutral point diode clamped (NPC) converter for high-power applications

This thesis presents systematic modelling and control of the three-level N&barbelow;eutral P&barbelow;oint diode C&barbelow;lamped (NPC) converter and the three-level NPC-based Back-to-Back (BtB) system for power system applications. The concepts of "Generalized State-space Averaging Method" and "Instantaneous Symmetrical Components" are used to model the NPC converter. Based on the developed model, appropriate controllers for (i) the AC-side current control, (ii) the DC-side voltage equalization, and (iii) the DC-bus voltage regulation are proposed and designed. The models and associated control schemes are developed for operation under balanced as well as unbalanced conditions. Specifically, single-phase to ground faults are considered as the main source of imbalance.; This thesis also proposes two new applications of the three-level NPC converter, i.e. for (i) the high-power, high-current (2.5 MW, 2.5 kA) rectifier system, and (ii) the direct-drive, synchronous machine-based wind-power system. Based on the developed models and controls, performance of the three-level NPC converter and the BtB NPC-based conversion system, in the context of (i) high-power rectifier, (ii) wind-power, and (iii) HVDC system, are investigated. The study results are validated based on digital time-domain simulation studies in the PSCAD/EMTDC software environment.