10-08-2012, 12:47 PM
SPACE VECTOR MODULATION FOR THREE-LEG VOLTAGE SOURCE INVERTERS
SPACE VECTOR MODULATION FOR THREE-LEG VOLTAGE.pdf (Size: 244.56 KB / Downloads: 334)
THREE-LEG VOLTAGE SOURCE INVERTER
The topology of a three-leg voltage source inverter is Because
of the constraint that the input lines must never be shorted and the output current must
always be continuous a voltage source inverter can assume only eight distinct topologies.
These topologies are shown on Fig. 2.2. Six out of these eight topologies produce a nonzero
output voltage and are known as non-zero switching states and the remaining two
topologies produce zero output voltage and are known as zero switching states.
SPACE VECTOR MODULATION
The desired three phase voltages at the output of the inverter could be represented
by an equivalent vector V rotating in the counter clock wise direction as shown in Fig.
2.6(a). The magnitude of this vector is related to the magnitude of the output voltage (Fig.
2.6(b)) and the time this vector takes to complete one revolution is the same as the
fundamental time period of the output voltage.
RIGHT ALIGNED SEQUENCE (SVM1)
A simple way to synthesize the output voltage vector is to turn-on all the bottom
(or top) switches at the beginning of switching cycle and then to turn them off
sequentially so that the zero vector is split between V7(ppp) and V8(nnn) equally. This
switching scheme is shown in Fig. 2.8 for two sampling periods. The signals in the figure
represent the gating signals to the upper legs of the inverter. The scheme has three switch
turn-on’s and three switch turn-off’s within a switching cycle. The performance of the left
aligned sequence, where the sequence of vectors is exactly opposite to the right aligned
sequence, is expected to be similar to the right aligned sequence.
Alternating Zero Vector Sequence (SVM3)
In this scheme, known as DI sequence in literature [6], the zero vectors V7(ppp)
and V8(nnn) are used alternatively in adjacent cycles so that the effective switching
frequency is halved