IEEE Power Electronics Magazine - June 2020 - 61

Leg A
Upper SBR
Voltages (kV)

Leg A
Upper SBR
Currents (A)

110

ipA,1

inA,1

ipA,2

inA,2

0
-40
(a)

1.1
1
0.9
0.78

0.79

0.8

0.81

2.24

2.25
2.26
Time (s)
(b)

2.79

2.8

2.81

2.82

FIG 14 The (a) leg A upper and lower SBR currents and the (b) leg A upper branch voltages. In spite of the SBR currents being balanced, the SBR voltages tend to diverge in case the SBR energy-balancing controller is deactivated (middle part of the plot). The
number of parallel SBRs per branch was set as M = 2.

Leg A
Upper SBR
Voltages (kV)

Leg A
Upper SBR
Currents (A)

110

ipA,1

inA,1

ipA,2
ipA,3

inA,2
inA,3

-40
(a)

1.1
1
0.9
0.78

0.79

0.8

0.81

2.24

2.25
2.26
Time (s)

2.79

2.8

2.81

2.82

(b)
FIG 15 The (a) leg A upper and lower SBR currents and the (b) leg A upper branch voltages. In spite of the SBR currents being balanced, the SBR voltages tend to diverge in case the SBR energy-balancing controller is deactivated (middle part of the plot). The
number of parallel SBRs per branch was set as M = 3.

controller. It can be observed that, before this controller
was disabled, the SBR voltages remained perfectly balanced (the left-most zooms in on the bottom-most plots).
However, upon its deactivation at t = 1.5 s , the SM voltages
of all of the SBRs start to diverge (the middle zoomedin images). At the time instant t = 2.5 s , the SBR energybalancing controller was reactivated, resulting in the balanced SBR voltages in both of the presented cases (the
right-most zoomed images).
Figure 14 presents the leg A lower and upper branch
currents, along with the upper branch voltages, during the
zoomed-in subintervals in Figure 12. SBR currents remain
balanced, owing to the actions of the SBR currents-balancing controller presented in Figure 9. However, notwithstanding the SBR currents balance, SBR voltage divergence can
be observed in the middle plot, which corresponds to the
period at which the SBR energy-balancing controller was
still deactivated.

Similar to Figure 14, the importance of the SBR energiesbalancing controller, in case M = 3, can be observed from
Figure 15. Conclusions identical to the ones already provided in the previous paragraph can be made.

Conclusions
This article presents the method of extending the power
capacity of the MMC by paralleling its SBRs. The proposed control method enables the balancing of energies
among the converter SBRs while keeping their currents
partially balanced at all times. For an available SM design,
the rated power of the converter can be effortlessly multiplied without the need for a major redesign of the existing
converter parts. Consequently, the realization of cost-effective solutions that require minimal engineering efforts is
enabled. Also, the proposed SBR energy-balancing method
does not depend on the number of parallel SBRs. Hence,
the scalability of the MMC depends exclusively on the
June 2020

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IEEE Power Electronics Magazine - June 2020

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