IEEE Power Electronics Magazine - June 2022 - 20

battery, respectively. Ls1~Ls3 are the leakage inductances of
the three windings of the transformer, respectively. L01 and
L02 are negatively coupled inductors for HV output filter.
The same design applies for L03 and L04 at the LV output. Chv
and Clv are clamping capacitors and used to maintain a high
dc voltage to reduce current stress on both transformer
secondary sides. An exemplary prototype is also shown
in Figure 7(b). The rationale behind this design is that the
current-fed port boosts the LV side voltage to a much higher
value, which significantly reduces the turns ratio thereby
facilitating the transformer design. In addition, duty cycles
are introduced as additional control freedoms, enabling
the possibility of further optimization, e.g., realizing zerovoltage-switching
(ZVS) in most of the power range. Even
though the debate of pros and cons of voltage source and
current source converter continues, and the current-fed
topology is nothing new, such integration attempt actually
brings two types of converter technologies together by
maximizing their potential at the same time.
Using Battery and OBC for V2L
When the grid loses power during a blackout, one mission
of the EV battery and OBC is to form the local grid and provide
minimum energy usage. It then requires the energy to
flow from vehicle to the grid created by the vehicle power
supply; this operation can be called vehicle to load (V2L). A
typical topology is shown in Figure 8(a). At the ac side, a
conventional three-phase four-wire inverter is a promising
candidate. At the dc-bus side, there are split dc-link capacitors
CN1 and CN2, where the midpoint of the split dc-link
capacitors serves as the neutral point. The fourth leg is
formed by Sn1 and Sn2, should a two-level topology be preferred.
LN aids in the regulation of the neutral-point voltage
caused by the load imbalance. Lf and Cf are the grid-side filter
inductor and capacitor, respectively. Note the fourth leg
might not be necessary such as in the case where the dc-link
capacitor tank is large enough to cope with a large neutral
current. With the fourth leg stabilizing the neutral point,
each phase can be essentially equivalent to a buck converter
and controlled independently with conventional SPWM,
with the equivalent circuit of each phase shown in Figure 8(b).
In this way three independent phases, A, B and C are
formed. Each phase can then undertake balanced or unbalanced
load, should the neutral point be controlled well.
Potentially, when
RL " 3 i.e., no-load, the overall grid
,
inverter part of the OBC is supplying just the LC circuit,
which has a natural resonant frequency. If the OBC output
at the ac side happens to have a voltage component around
such resonant frequency,
large voltage and current harmonics
are expected on the phase output, deteriorating the
grid power quality. Note such operational mode is not V2G,
which has the normal utility power grid connected. V2L
function requires the battery and charger to form its own
microgrid, therefore from the control point of view it can
be more challenging than V2G. One approach is controlling
the effective output impedance of the converter, which
Solar Power Generator
+
uC1
-
EV
Isolated
dc/dc
Converter
iC1
iC2
iCN
O
+
uC2
-
iLN
iN
Sa2
Sn2
(a)
iL
L
iC
dc
Bus
-
uspwm
C uC
+
io
RL Rv
Sb2
Sc2
LN
Sn1
Sa1
Sb1
Sc1
Lf
Lf
Lf
ia
ib
ic
Cf Cf Cf
HEMS
Distribution
Board
(b)
FIG 8 (a) A three-phase four-wire inverter used in a V2L EV charger. (b) Equivalent circuit of each phase with virtual resistor.
20 IEEE POWER ELECTRONICS MAGAZINE z June 2022
CN2
CN1

IEEE Power Electronics Magazine - June 2022

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