IEEE Power Electronics Magazine Compendium - March 2018 - 85

transformer compactness (power density,
size, weight, and volume) versus parasitics
LV Terminals
capacitance optimization/minimization,
Y2
maintaining ZVS and low reactive power
flow for higher efficiency, EMI considerD2
ations for smooth current control, flux-satD1
P
uration protection, and midpoint voltage
Y1
balancing of 3L-NPC converter poles are
HV Terminals
reported in detail in [21]. The design considerations and operation of such MV HF
transformers are addressed in [22]-[24].
Oil Fill Hole
Figure 9 shows a photo and the HF
equivalent circuit of the MV transformer
designed for the TIPS DABC. The design
of this transformer and extractions of the
model parameters are presented in [24].
The output LV dc bus has a voltage senHV Insulation
sor for feedback control. There are two
Enclosure
MV voltage sensors connected to 3L-NPC
midpoint voltages on the HF transformer
(a)
primary side. The 3L-NPC midpoint voltages need to be balanced by feedback
Y2
Cpy2
control since the nonidealities and dead
R1
times might make it unbalanced even if
Cpy1
LP
RP
L1
y1
the PWM control signals are balanced
P
D2 1
L
[25]. This is done in control by adjusting
C1
y
Ny
the positive and negative time periods
P
Rm Lm Cm
y2
Np
of a switching period. The primary-side
Css
d
1
D1
transformer currents are also sensed
Ld
Nd
for feedback. A field-programmable gate
P2
array-based control platform using an
d2
ultrafast 12-bit parallel analog-to-digiY1
Cpd2
tal converter with a sampling rate of 2
Cpd1
megasamples/second is implemented. A
(b)
synchronous frame d-q-based inner current control loop enables fast response
and provides another level of overcurrent fig 9 An HF MV DABC transformer rated 22 kV/800 V, 10 kHz, and 35 kVA: (a) the
protection [25], [26]. The midpoint dc bus transformer and (b) an equivalent circuit model of the HF transformer [24].
voltage balancing is achieved through
feedforward control.
first option will increase the size of the HF transformer,
Figure 10 shows the experimental waveforms of
and the second option will increase switching losses. As a
the DABC operation with 6-kV input and 400-V output at 7.4more viable solution, external MV primary-side dv/dt filkW and 10-kHz switching frequency.
ters are used for attenuating the ringFor each DABC pole voltage and curing. Additional tie inductors are used
rent waveform, one phase is shown in
on both the secondary LV sides to
The 3L-NPC converter
Figure 10. The LV dc-output supplies
reduce circulating current between
is operated with 41%
a resistive load. The MV primary-side
star- and delta-connected converters.
current shows considerable ringThis ringing in the currents reduces
duty ratio to generate
ing; this is caused by interaction of
the available DABC ZVS range.
a nearly sinusoidal
transformer parasitics capacitance
For ZVS operation, the MV conprimary current in the
with the high dv/dt (up to 100 kV/ns)
verter current must lag its pole
voltage, and for the secondary LV
of 15-kV SiC IGBT switchings. The
HF transformers.
converter, it must lead. Smaller magringing can be lowered by reduction
netizing inductance helps DABCs
of parasitics capacitance by HF transto
achieve
ZVS
operation,
albeit with efficiency penalty
former design and also by lowering the 15-kV SiC IGBT
as
it
causes
increased
reactive
power-flow resulting
switching dv/dt by increasing gate driver resistance. The
September 2015

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Table of Contents for the Digital Edition of IEEE Power Electronics Magazine Compendium - March 2018