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Three-phase Three-level NPC Converter - 1. Introduction

This courseware focuses on the real-time simulation of the Three-Phase Three-Level Neutral-Point Clamped (NPC) converter that operates in (i) inverter mode or (ii) rectifier mode.

It consists of providing the students theoretical background and real-time interactive tools to understand the converter, followed by exercises to put the acquired knowledge into practice.
It is noteworthy that this document is a level-one courseware in which the focus is on (i) the analysis of the circuit and (ii) the open-loop control.
At the end of this courseware, the student will be capable of:

  • Understanding the functionalities of a Three-Phase Three-Level NPC inverter and rectifier.
  • Designing the PWM generation for multi-level converters.
  • Designing passive elements, namely capacitors, for filtering and observing the impact of filters on currents and voltages waveforms.
  • Computing the voltage levels at the output of the inverter as functions of input DC voltage.
  • Observing the impact of changing PWM frequency and quantifying power losses.
  • Understanding the pulsations in an AC-motor, the impact of inductive load and capacitive load on currents, voltages; reversing the power flow.

Figure 1 below shows the circuit in inverter mode.
The power flows from the DC side to the AC side.
The load is a combination in series of resistance, inductance, and a three-phase AC-source.
Notice that the AC-source can be interpreted as a back-electromotive force (BEMF) for motors applications or an electric generator for power grid applications.

Additionally, a three-phase delta-connected set of capacitors can be inserted at the output of the inverter, preceded by three-line inductances, leading thus to a LC-filter.
The student can control (i) the amplitude of the DC source, (ii) the inverter parameters, namely, the PWM frequency and the reference amplitude, (iii) the AC-source parameters, namely, the amplitude, the frequency, and the phase shift.

Figure 1: Inverter Mode

The rectifier mode is depicted in Figure 2 below where the AC-source on the load side becomes the generator while the DC source is disconnected, the PWM generator is disabled, and a resistive load can be added at the DC side.
The power flows from the AC side to the DC side.
The control parameters in this case reduce to the AC-source amplitude, frequency, and phase shift.

Figure 2: Rectifier Mode

On courseware organization:

  • Section 2 describes the circuit block by block, namely, (i) the converter topology, (ii) the control of the switches and the PWM generation, (iii) the voltages levels and waveforms, (iv) the load and filter in inverter and rectifier modes, (v) the measurement stations and (vi) the circuit parameters.
  • Section 3 is dedicated to the description of the panel.
    The three tabs associated with the three scenarios, namely, (i) Inverter Mode, (ii) LC Filter and (iii) Rectifier Mode are covered in detail.
    The goal of this section is to explain to the students the functionalities of the control parameters, the control buttons, the scopes, the displays, and the help menu, making them capable of monitoring the panels and solving the exercises.
  • Section 4 comprises all the exercises. It is composed of four categories, covering the (i) design aspects, the exercises associated to the (ii) inverter and (iii) rectifier modes, as well as the (iv) LC filtering.
    It is worth noticing that, for this level-one courseware
    , most of the exercises will consist of changing the control parameters/buttons and observing/interpreting results.
    Consequently, advanced material such as spectral analysis, dead-time insertion, d-q representation and analysis, synthesis and design of closed-loop controller, integration of external controller will be the major components of the level-two courseware.

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