Overmodulation

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

Library

Simscape / Electrical / Specialized Power Systems / Power Electronics / Power Electronics Control

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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 U_ref. 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

  $$V_0=\frac{\left|U\right|}{6}\times \sin \left[3\cdot (wt+\angle U)\right]$$

• 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 U_ref. The resulting modified signal U_ref* 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 U_ref are summed and divided by two, and then subtracted from the input signal. The resulting modified signal U_ref* also contains zero-sequence triplen-harmonics. The block outputs a value between −1 and 1.

Parameters

Overmodulation type

Select the overmodulation technique you want to apply to the Uref signal: Third harmonic (default), 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

Examples

The power_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, at the MATLAB^® command prompt, enter

Ts = 0

and set the Simulation type parameter of the Powergui block to Continuous before starting the simulation.

Version History

Introduced in R2013a