IEEE Power Electronics Magazine - June 2018 - 64

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Fig 11 (a) The arm current and (b) the cell capacitor
voltage having the optimum magnitude of the circulating
current injected.

z	June 2018

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IEEE POwEr ElECtrOnICs MAGAzInE

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inductors (1.5 mH each) and six arm
advantage of the naturally flowing
inductors (1 mH each).
circulating current. For the last perUsing the proposed
formed case study, Figure  12 shows
method, the optimum
the arm current and cell voltage for
Conclusions
the same converter while the ac comThis article proposed an optimum
design point for the
ponents of the circulating current are
design procedure for an MMC AFE
arm inductors of the
eliminated. In this case, the ripple
converter resulting in reduction in
terms resulting from the circulating
the size, weight, and losses of the
MMC AFE will be
current [terms depending on I 2 in (9)
passive elements installed in the consomewhere between
verter. By taking advantage of an
and (10)] are removed, eliminating
optimum design point for the MMC
the positive impact from the circuthe resonance point of
AFE converter, it was shown through
lating current on the capacitor voltthe second and fourth
case studies that the amount of the
age ripple. As seen in Figure 12(b), in
capacitor voltage reduction achieved
this case the capacitor voltage ripple
harmonics.
can be almost the same as the circuincreases to around 57 V, showing an
lating current injection methods
approximately 40% difference com(equal to approximately 40%). Also,
pared with the previous cases.
by moving the majority of the installed inductance from
The shown case studies revealed the capability of the
arms to the phases, a significant reduction in the total
proposed method in naturally downsizing the capaciinductor volume/size can be achieved.
tor voltage ripple across the cells. Using traditional
The case study shown revealed an approximate
design guidelines, the converters would have a total of six
65% reduction in copper usage and an approximate 35%
inductor with approximately 9-mH inductance values for
reduction in copper losses for the inductors used in the
each. However, the proposed design realized the operaconverter structure. The achieved reductions come at
tion of the sa me converter with a total of three ph a se

Time (s)
(b)
Fig 12 (a) The arm current and (b) the cell capacitor
voltage while the ac components of the circulating current
are suppressed.
Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - June 2018
