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Symmetrical PWM (3-Level)

Purpose

Generate 3-level PWM signal using symmetrical triangular carriers

Library

Control / Modulators

Description

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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 3-Level IGBT Converter or the ideal 3-Level Converter. In these cases the modulation index must have a width of 3 according to the number of inverter legs.

The figures below illustrate 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.

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The figures below illustrate 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.

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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 Hz.
Carrier offset
The time offset of the carrier signal, in p.u. of the carrier period.
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.
Input limits
The range of the modulation index. The default is [-1 1].