Environmental Engineering Reference
In-Depth Information
It consists of an independently-controlled neutral leg, a rectifier leg and two phase legs. The
neutral leg, taking the topology in (Zhong
et al
. 2006), consists of two switches, one inductor
and two split DC-link capacitors. It is controlled to maintain the neutral point close to the
mid-point of the DC link, which is common to the single-phase supply and to the two phases
generated. The provision of the neutral point makes the three-phase voltages independent so
that unbalanced loads can be operated and the operation of one phase does not affect others.
The rectifier leg, together with the neutral leg, form a rectifier and the two phase legs, together
with the neutral leg, form two single-phase inverters to generate two phase voltages that are
±
3
displaced from the single-phase supply. The rectifier leg is to provide a compensation
current so that the current drawn from the supply is clean and in phase with the voltage, to pass
the power for the other two phases and to maintain a stable DC-link voltage. It operates as a
boost converter so that the DC-link voltage can be maintained at a value needed by the two
phase legs without a transformer. The two phase legs generate two independent phase voltages
which, together with the single-phase supply, form independent balanced three phase voltages.
An LC filter is connected to each phase leg to filter out the harmonics caused by the switching.
It is worth noting that the provided three-phase voltages have the same level as the single-phase
supply voltage and, hence, there is no need to use transformers for loads rated at the supply
voltage level, which is normally the case.
With this topology, the load of Phase
a
is fed directly by the single-phase supply and the
loads of Phases
b
and
c
are supplied by the converter. Hence, the converter does not process
the full power, which reduces the cost and improves the efficiency. If balanced loads are
connected, only two-thirds of the power is provided by the converter. If no loads are connected
to Phases
b
and
c
, then the converter operates as an active power filter and a reactive power
compensator for Phase
a
so that the current drawn from the grid is clean and in phase with the
supply voltage. Another important property of the topology is that the rectifier leg, the neutral
leg and the phase legs can be controlled independently, which offers considerable freedom
in designing the controllers. In principle, any control strategies developed for reactive power
compensation and active power filters can be applied to the rectifier leg. Any control strategies
developed for neutral legs can be applied to the neutral leg and any control strategies developed
for inverters can be applied to the phase legs.
2
14.3 Basic Analysis
Assume that the single-phase supply voltage
v
is
V
=
V
0
,
∠
where
V
is its RMS value, and that the generated two phase voltages together with the supply
voltage form balanced three-phase voltages to supply three-phase linear loads with the load
angle (lagging) of
θ
c
, respectively. Moreover, assume that the power loss in the
converter is negligible. Then the (real) power supplied is equal to the (real) power consumed
by the load, i.e.,
θ
,
θ
b
and
VI
=
VI
a
cos
θ
+
V
b
I
b
cos
θ
b
+
V
c
I
c
cos
θ
c
,
=
VI
a
cos
θ
+
VI
b
cos
θ
b
+
VI
c
cos
θ
c
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