PLECS 3.6 Online Help

Space Vector PWM

Purpose

Generate PWM signals for 3-phase inverter using space-vector modulation technique

Library

Control / Modulators

Description

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The space vector modulator generates a reference voltage vector, -→
Vs  , at the ac terminals of a three phase voltage source converter shown below. The reference vector is defined in the αβ   coordinate system: -→     *     *
Vs = Vα + jVβ  .

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Operation

The construction of the reference voltage vector, -→Vs  , is graphically depicted below.

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Construction of the reference vector -→
Vs  .

Internally, the space vector modulator consists of a sector detection and vector timing calculation function that is executed at the beginning of the switching cycle. In this function, the operating sector and relative on-times of the switching vectors are calculated. During a switching cycle, a vector generation and sequencing function is called at the switching instants to update the switch output.

The sector detection calculation determines the sector in which the reference voltage vector -→Vs   resides. The relative on-times, τa,τb,τ0  , for the switching vectors -→Va,-→Vb   and -→V0   are then calculated. In each sector, two unique switching vectors named -→V
 a   and -→V
 b   are available. Two zero vectors, named -V→1,-→V2
  0  0   are also available. The relationship between the relative on-times and the reference vector is shown below for an arbitrary sector. The relative on-times are calculated by projecting the reference vector onto the vectors -V→
  a   and -→V
 b .

The vector generation and sequencing function creates a switching cycle by time-averaging the switching vectors according to their on-time values. There are many possible switching sequences that can be implemented since the order in which the vectors -→V ,-→V ,-→V
 a  b  0   are applied during a switching cycle is arbitrary. In addition, one or both of the -→V
 0   vectors can be used. For further information, please read the documentation that accompanies the demo model "Space Vector Control of a Three Phase Rectifier using PLECS". This documentation can be found at www.plexim.com/sites/default/files/plecs_svm.pdf.

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Relationship between relative on times, τa,τb,τ0   , switching vectors, -→ -→  -→
Va,Vb,V0   , and reference vector, -→
Vs  .

Parameters

Modulation strategy
The modulation strategy can be set to 'Alternating zero vector' or 'Symmetrical' using a combo box. With alternating zero vector modulation, only one of the two -→
V0   switching vectors is used during a switching sequence. One switch leg is always clamped to the positive or negative dc bus voltage and only two of the three inverter legs are switched.

With symmetrical modulation, the two -→
V0   switching vectors are used: one at the beginning and one at the middle of a switching sequence. All three inverter legs are switched during a switching sequence.

Switching frequency
The switching frequency in Hz.
Switch output values
The switch output values in the high and low state. The values should be selected to match the inverter's gate control logic so that a high value turns on the upper switch in the leg and the low value turns on the lower switch. The default values are [- 1 1]  .

Inputs and Outputs

DC voltage
The input signal Vdc   is the voltage measured on the dc side of the inverter.
Reference voltage
This input, labeled V*αβ  , is a two-dimensional vector signal comprising the elements [V *α,V*β]  .
Switch output
The output labeled sw   is formed from three switch control signals, [Sa,Sb,Sc]  , which control the inverter legs A, B, and C. Each switch signal controls the upper and lower switches in the respective leg.

Probe Signals

sector
A value in the set of [1..6]    that indicates the sector in which the refererence vector, -→Vs  , is located.
tau
A vector signal comprising the three relative on-time values, [τa,τb,τ0]  .
sw
A vector signal consisting of the three gate signals for the inverter legs, [Sa,Sb,Sc]  .