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Space Vector PWM (3-Level)

Purpose

Generate PWM signals for a 3-phase 3-level neutral-point clamped inverter using space-vector modulation technique

Library

Control / Modulators

Description

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The 3-level space-vector modulator generates a voltage vector on the ac terminals of a neutral-point clamped 3-phase inverter according to a reference signal provided in the stationary αβ   reference frame.

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By controlling the semiconductor gate signals, each ac terminal can be connected either to the high (+), low (-) or neutral (o) point of the dc link. This results in 27 vectors including 12 short vectors, 6 medium vectors and 6 long vectors, as well as 3 zero vectors. Under the assumption of balanced voltages on the capacitors Vdc+   and Vdc-  the space-vector diagram is graphically depicted below.

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The hexagon area can be divided in to six sectors (1 to 6), each of which has four zones (1 to 4). As an example, consider the reference voltage V⃗*  to be located in zone 2 of sector 1. In order to generate the reference voltage ⃗V*  on the ac terminals, the adjacent vectors ⃗V
 1  , ⃗V
 3   and ⃗V
 5   are selected and weighted by time. The on-time of each vector with respect to the switching period is calculated as:

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τa = 1 - 2k sin (θ)

         (π    )
τb = 2k sin-+ θ  - 1
           3

            (     )
τc = 1- 2k sin π-- θ
              3

This block implements a symmetrical sequence to achieve minimum total harmonic distortion (THD). The short vectors have redundant switch states, e.g. V⃗1   can be either generated by the combination (+oo) or (o--). In order to keep the dc link voltages balanced, both switch states must be applied for the same duration during one switching period. The resulting switch pattern is illustrated below:

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Parameters

Switching frequency
The switching frequency in Hz.
Output values
The switch output values in the high, neutral and low state. The default values are [-1 0 1].

Inputs and Outputs

DC voltage
The input signal Vdc   is the sum of the two dc link voltages Vdc+   and Vdc- .
Reference voltage
This input, labeled V*
 αβ  , is a two-dimensional vector signal comprising the elements [V *,V*]
  α  β  .
Switch output
The output labeled sw   is formed from the three switch signals [Sa,Sb,Sc]  , which control the converter legs A, B and C. Each switch signal determines if the corresponding ac terminal shall be connected to the positive, neutral or negative side of the dc link.

Probe Signals

sector
A value in the set of [1..6]   that indicates the sector in which the reference vector, V⃗* , is located.
zone
A value in the set of [1..4]   that indicates the zone in which the reference vector, V⃗* , is located.
tau
A vector signal comprising the three relative on-time values, [τa,τb,τc]  .
sw
A vector signal consisting of the three gate signals for the inverter legs, [Sa,Sb,Sc]  .