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
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 reference frame.
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 and
the space-vector diagram is graphically depicted
below.
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 to be located in
zone 2 of sector 1. In order to generate the reference voltage
on the ac terminals,
the adjacent vectors
,
and
are selected and weighted by time. The
on-time of each vector with respect to the switching period is calculated
as:
This block implements a symmetrical sequence to achieve minimum total
harmonic distortion (THD). The short vectors have redundant switch states, e.g. 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:
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
is the sum of the two dc link voltages
and
.
- Reference voltage
- This input, labeled
, is a two-dimensional vector signal comprising the elements
.
- Switch output
- The output labeled
is formed from the three switch signals
, 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
that indicates the sector in which the reference vector,
, is located.
- zone
- A value in the set of
that indicates the zone in which the reference vector,
, is located.
- tau
- A vector signal comprising the three relative on-time values,
.
- sw
- A vector signal consisting of the three gate signals for the inverter legs,
.