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Overmodulation

Add third harmonic or triplen harmonic zero-sequence signal to three-phase signal

Library

Control and Measurements/Pulse & Signal Generators

Description

The Overmodulation block increases the linear region of a three-phase PWM generator by adding a third harmonic or triplen harmonic zero-sequence signal V0 to the three-phase original reference signal Uref. This zero-sequence signal does not appear in the line-to-line voltages.

A modulation index of up to 1.1547 (exact value = 2/sqrt(3)) can be used without pulse dropping.

The Overmodulation block implements three overmodulation techniques:

• The Third Harmonic overmodulation technique. In this technique the third-harmonic signal V0 subtracted from the original signal is calculated as

${V}_{0}=\frac{|U|}{6}×\mathrm{sin}\left[3\cdot \left(wt+\angle U\right)\right]$

• The Flat Top overmodulation technique. In this technique the portion of the three-phase input signal exceeding values +/−1 is computed. The three resulting signals are then summed and removed from the original signal Uref. The resulting modified signal Uref* is therefore a flat-top three-phase signal that contains zero-sequence triplen-harmonics. The block outputs a value between −1 and 1.

• The Min-Max overmodulation technique. In this technique the minimum and maximum values of the three components of input signal Uref are summed and divided by two, and then subtracted from the input signal. The resulting modified signal Uref* also contains zero-sequence triplen-harmonics. The block outputs a value between −1 and 1.

Dialog Box and Parameters

Overmodulation type

Select the overmodulation technique you want to apply to the Uref signal: Third harmonic, Flat top, or Min-Max.

Inputs and Outputs

Uref

The three-phase reference signal of three-phase PWM generator.

Uref*

The overmodulated three-phase signal of three-phase PWM generator.

Characteristics

 Sample Time Inherited Scalar Expansion No Dimensionalized Yes

Example

The power_OverModulationpower_OverModulation example compares the three overmodulation techniques implemented in the Overmodulation block. Choose the overmodulation technique (type 1, 2, or 3 on the first input of the Multiport Switch) and run the simulation. Observe the resulting waveforms in Scope 1.

The model sample time is parameterized with variable Ts (default value of 5e-6). To run a continuous simulation, specify Ts = 0 in the MATLAB Command Window and change the Simulation type parameter of the Powergui block to Continuous before starting the simulation.