Environmental Engineering Reference
In-Depth Information
When the inverter modulation index,
m
i
, is increased beyond the full PWM
region, the available voltage can be increased to the maximum, or its six step
square wave amplitude given in (6.4) for
U
an1
. Within the boundary marked by the
inner dotted circle and the outer hexagon sides, the available voltage lies in the
range given by (6.6). The outer circle illustrates the maximum voltage vectors that
are obtained when the inverter output voltage is a single state vector. Because the
hexagon sides define the absolute limits of inverter voltage output, it is not possible
to have voltage vectors extend beyond the hexagon sides:
p
p
p
p
U
dc
U
dc
2
<
U
s
<
ð
V
rms
, fund
Þ
ð
6
:
6
Þ
The eight vectors shown in Figure 6.8 are described mathematically by (6.7),
provided the inverter power supply mid-point and load neutral are not connected.
There are schemes in which it is desirable to split the dc link so that a mid-point
voltage is obtained. The mid-point in fact may be connected to the 'wye' connected
load neutral,
n
.
2
3
U
dc
e
jk
ðp=
3
Þ
k
¼
1, 2,
...
,6
U
k
¼
ð
6
:
7
Þ
0
k
¼
0, 7
I
m
-axis
Six step
square wave
region
U
3
U
2
U
1
R
e
-axis
U
4
U
0
,
U
7
Pulse dropping
region
Full PWM
region
U
5
U
6
Figure 6.8 Switching state regions defined
The inverter voltage vector given by (6.7) may be clearer by stepping back to
Table 6.1 and following the process through from gate drive commands (
a
,
b
,
c
)to
inverter mid-point voltages,
U
a
0
,
U
b
0
,
U
c
0
, and finally to the load line-to-neutral
voltages,
U
an
,
U
bn
and
U
cn
. The inverter gate drive commands are
S
x
(0,1) logic
levels where logic high (1), which is upper switch in an inverter pole, is gated ON and
