Symmetrical PWM (3-Level)

Purpose

Generate 3-level PWM signal using symmetrical triangular carriers

Library

Control / Modulators

Description

3-level PWM generator with two symmetrical triangular carriers. The input m is the modulation index. The switching function s outputs either \(1\), \(0\) or \(-1\). If the modulation index is a vector, the switching function is also a vector of the same width.

The block can be used to control the IGBT 3-Level Converter (3ph) or the Ideal 3-Level Converter (3ph). In these cases the modulation index must have a width of \(3\) according to the number of inverter legs.

Fig. 185 illustrates the Natural Sampling method. In the left figure, the negative carrier signal is obtained by flipping the positive carrier vertically around the time axis. In the right figure, the positive carrier is vertically shifted to construct the negative carrier. The latter technique reduces the switching frequency and hence the semiconductor stress in three-phase converters.

../../_images/pwm3levelsymnat.svg — Fig. 185 Natural sampling for 3-level PWM

Fig. 186 illustrates the different Regular Sampling methods offered by this block. With double edge sampling (left figure) the modulation index is updated at the carrier tips and zero-crossings. With single edge sampling (right figure) the modulation index is updated only at the outer tips.

../../_images/pwm3levelsymreg.svg — Fig. 186 Regular sampling for 3-level PWM

Parameters

Sampling: Select a sampling method. If you select Natural Sampling, the carrier signal may begin with \(0\) or \(1\) at simulation start. The Regular Sampling method lets you choose between double edge and single edge sampling.
Carrier frequency: The frequency \(f\) of the triangular carrier signals, in hertz \((\mathrm{Hz})\).
Carrier phase shift: The time offset of the carrier signal, in per unit \((\mathrm{p.u.})\) of the carrier period.
Carrier limits: The range of the triangular carriers. The default is [-1 1].
Negative carrier: Select the phase shift between the negative and positive carrier signals. The negative carrier may be constructed from the positive carrier either by flipping or shifting.