Modelling and control of grid-tied NPC inverters

Abstract

Grid-tied inverters used for interfacing renewable energy sources and energy storage systems to the grid are set to be essential components in the future smart grid. This thesis focuses on the modelling and control of grid-tied neutral-point-clamped (NPC) inverters, including experimental implementation and validation.

Switched-state-space models of both single-phase and three-phase NPC inverters are developed. Output voltage and capacitor balancing is governed by the transistors of the inverter, which are the only control inputs available. The models allow for the switching nature of the inverter and its non-linear dynamics.

A single-phase NPC switched-state-space model is used to develop and assess the performance of an output voltage regulator, including a space-vector pulse-width modulation (SVPWM) technique that maintains the dc link capacitors balanced. The simulation results demonstrate the efficacy of the controller and the simulation technique.

Similarly, simulation studies were carried out to examine the performance of a three-phase NPC inverter using proportional-integral (PI) decoupled control of active and reactive power. The results of this work were used to provide a bench mark to assess the performance of a novel direct power control (DPC) scheme, including capacitor balancing. Particular attention was given to power and vector analysis in both schemes. The new DPC was evaluated experimentally using a three-phase NPC inverter driven by a field-programmable gate array (FPGA). The results show good agreement with the theoretical predictions and demonstrate the proposed DPC's performance.

Technical documentation of the entire experimental set up is provided and hardware limitations such as resolution of voltage sensors, signal filtering, and switching speed of transistors are observed.

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Identifiers

ORCID for Suleiman Sharkh: orcid.org/0000-0001-7335-8503

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