S2PC 2016, Nagaoka, Japan

Requirements of Cell Capacitor for High Power-Density Designin Modular Multilevel Converter with H-bridge Cell
Toshiki Nakanishi＊, Jun-ichi Itoh＊
1Nagaoka University of Technology, Japan

This paper presents a volume evaluation of capacitors for high power-density design in a step-down rectifier employing a modular multilevel
converter (MMC) in a power system interconnected to a 6.6 kV AC power grid. This paper focuses on minimizing the capacitor volume byconsidering rated ripple current and the number of cells as evaluation factors. In this design, commercial electrolytic capacitors are utilized
as cell capacitors instead of customized capacitors. Additionally, the conditions for high power density in the MMC are clarified based on thevolume evaluations of the electrolytic capacitor.
Keywords Modular Multilevel Converter, High Power Density Design, Volume of Electrolytic Capacitor, Ripple Current

vAr1

1. Introduction
Recently, step-down rectifiers employing a modular multilevel
converter (MMC) to avoid utilization of a bulky transformer in aDC micro-grid has been proposed [1]. The conventional powersystem connected to a utility grid of 6.6 kV has an isolatedtransformer. However, the volume of the isolated transformer islarge because the isolated transformer operates in commercialfrequency. Hence, a step-down rectifier with an H-bridge cell type
MMC is suitable for DC micro-grid in terms of volume reduction.
In many AC-DC converters employing MMC, the design
methods for achieving high power-density have been considered by
setting specific design criteria [2]. However, the design criteria
focusing on the utilization of the electrolytic capacitors in the Hbridge cell type MMC have not yet been reported in detail, eventhough electrolytic capacitors possess a high voltage rating ofseveral hundred volts and a high capacitance per capacitor volume.
This paper discusses the design criteria with the aim of obtaining
high power density in the MMC which is based on the volume
evaluation of the electrolytic capacitors. The original contributionof this paper is to clarify the number of cells and to select the rated
ripple current associated with the smallest possible overall volume.
Finally, the conditions in order to obtain high power density and therequirements of the electrolytic capacitors are clarified.

2.

Configuration of proposed system

Fig. 1 shows the configuration of an H-bridge cell type MMC. In
practice, multiple cascaded cells are used because the effect of
harmonic distortion reduction is increased by increasing cellnumbers. The MMC is also able to reduce the voltage rating of thedevices on each cell due to the cascade cell connections, and solower voltage switching devices can be utilized with the MMC. The
MMC converts a medium AC voltage to a DC voltage of several
hundred volts. Finally, the DC output voltage of 400 V is obtainedby the isolated DC-DC converter.

3.

Operation of step-down rectifier using MMC

Fig. 2 shows the waveforms for the input phase voltage, the input
current and the DC output voltage in the miniature model. Thea)

Correspondence to: Toshiki Nakanishi
E-mail: nakanishi@stn.nagaokaut.ac.jp,

cell

vcAr1
Ar(n/2) vcAr(n/2)

vAr v

celliAr6.6 kV,
50 Hzir
isitLaiBr

La

DC bus ofMicro-grid
400 VvArs
Vmmc

cellvcBr1

cellvBrvBr1

IsolatedDC-DCconv.

vcBr(n/2)

S1

S3

S2

S4 C

cellvBr(n/2)

H-bridge cellFig. 1. Circuit configuration of step-down rectifier incorporating MMC.

waveforms for the input phase voltage and the input current confirm
that a unity power factor is obtained. The total harmonic distortion(THD) of the input current is 3.2%. It can also be seen that the DCoutput voltage waveform in the lower side of Fig. 2 indicates thatthe step-down rectifier converted the input voltage of 200 V into aconstant output DC voltage of 75 V. Therefore, the MMC achieves
step-down rectification.
Fig. 3 plots the waveforms for all cell capacitor voltages that areconnected to the R-phase leg. The capacitor voltage is controlled bythe capacitor voltage command. As a result, the proposed stepdown rectifier maintains the capacitor voltage of each H-bridge cellto the voltage command level of 130 V. In addition, the maximum
voltage error between the voltage command of the cell capacitor
and the measured voltage is 2.0% or less.

4. Evaluation of capacitor volumeFig. 4 shows the relationship between the ripple current and thecapacitor volume attracted from the database of commercial
electrolytic capacitors. In this graph, the number of capacitors
connected in parallel in one cell increases with the required ripple
current values. The minimum point of each mark indicates theripple current and the volume of the single capacitor. These data
demonstrate that the overall volume of the capacitors is reduced byconnecting capacitors with small rated ripple currents in parallel,
compared to the use of only one capacitor with a large rated ripplecurrent. Consequently, the parallel connection scenario is preferable.S2PC 2016, Nagaoka, Japan

volume with different devices and numbers of cells is remarkably
high. These results demonstrate that, as the first step of the design,
it is necessary to focus on selecting the number of cells at which thecapacitor volume is small based on the voltage rating ratio.

5. Conclusion
This paper detailed the volume evaluation of electrolytic
capacitors based on the rated ripple current and the number of cells.
As the conditions and the requirements for the high power density,
the volume minimization was achieved by selecting the number ofcells so that the voltage rating ratio is close to 1.0. Besides, in theelectrolytic capacitor, the low inside heating characteristic and thevoltage rating of more than 500 V were desired.
2500000
2.50(Only parallel connection of capacitors)

Voltage rating : 400 V

Volume of capacitors [dm3]2250000
2.25

Endurance : 85 °C, 5000h

2.00
2000000
1750000
1.75

1.50
1500000
1.25
1250000
1.00
1000000

5A
10A
15A
20A
Rated ripple current of25A
one capacitor @ 50Hz30A
2530 35 40 45 50 55 60
25 30 35 40 45 50 55 60

0.757500000.50
5000000.25
2500000.00
00055

1015 20
10 15 20
Ripple current (effective value) [A]

Fig. 4. Relationship between ripple current and capacitor volume.
Required voltage rating in each number of cells
Voltage rating of one capacitor × Series connected number

Voltage rating ratio =Volume of capacitors [dm3](Only series connection of capacitors)

Fig. 5 plots the relationship among the number of cells, thecapacitor volume and the voltage rating ratio. Note that the numberof series connected capacitors increases when the required voltage
rating increases above the voltage rating of a single capacitor,
which requires the use of a series connection of capacitors. Thecapacitor volume is determined by the quantity of series connected
capacitors multiplied by the number of cells under the same ripple
current conditions. The voltage rating ratio is defined as the ratio
between the required voltage rating and the actual voltage ratingwhen the capacitors are connected in series. As a result, thecapacitor volume becomes small when the voltage rating ratio at
each point is close to 1.0. Consequently, it is possible to achieve thedesired volume minimization by designing a device so that thevoltage rating ratio is close to 1.0.
From above results, the requirements of electrolytic capacitors in
the cell are that (i) the rated ripple current per unit volume is high,and (ii) the voltage rating per unit volume is high. Additionally, theconditions to achieve further volume reduction are as follow;(a) An equivalent series resistance of the capacitor should be small.
(b) Heat dissipation characteristic of inside heating should be high.
(c) The voltage rating is more than 500 V.
Conditions (a) and (b) are required to suppress the raise of inside
heating. The volume reduction of the capacitor is achieved bydecreasing the raise of inside heating. In addition, Conditions (c) isrequired to eliminate balancing resistances. When 1.7-kV IGBT andthe capacitor with the voltage rating of 400 V are applied, three ormore capacitors are necessary. Moreover, balancing resistances arerequired when more than three capacitors are connected in series.
When the capacitor with the voltage rating of more than 500 V isemployed, the balancing resistances are not required. Besides, thevolume of capacitor with the voltage rating of more than 500 V
should be as small as possible for the volume reduction.
Fig. 6 presents the breakdowns of the overall volume when theMMC output voltage is 1200 V and 1.7 or 1.2-kV IGBTs are used.
Note that the minimum total volume of inductors and heat sink ineach number of cells is chosen by Pareto front optimization. As aresult, it is obvious that the capacitor volume is the largest
component in the overall volume and the variation of the capacitor

Voltage rating of one capacitor : 400V

19Voltage rating ratio

17161615
2000

151413

Input Phase Voltage (R-phase) [250 V/div] 10 msec

504
8008071200

1812004734
800

1153
12002691
2800

8073
8400

8121620242832

Number of cells [1/leg]Fig. 5. Relationship between the number of cells and capacitor volume.

Input Phase Current (R-phase) [5 A/div]
160
160

0140
140

Voltageratingratio

1081
1200

961
1200

865
1200

786
800

721
800

MMC Output Voltage [25 V/div]

Volume [dm3]120120100100

Output power: 1000 W Input line voltage: 200 V Output Voltage: 75 V0 Inductance Lb : 8 mH(%Z=6.1%) Carrier frequency : 8 kHz
Fig. 2. Waveforms of input voltage, input current and DC output voltage.

Capacitor

80806060Inductor

4040130 VCapacitor voltage (R-phase) [50 V/div]
vcAr1, vcAr2, vcBr1, vcBr2,

202000Cell Capacitor : 1300 mF
Number of cell per leg : 4Heat sink
The number
1of cells

1618
24
20222
34
5
6
1.7-kV IGBT
1.2-kV IGBTFig. 6. Breakdown of the overall volume in 1.7 and 1.2 kV-IGBT.

10 msec0Fig. 3. Waveforms of capacitor voltage in R-phase leg.

References[1] T. Nakanishi, J. Itoh, 9th ICPE, No. WeA1-5, pp.815-822 (2015)[2] J. E. Huber, and J. W. Kolar, 2013 ECCE, pp. 359-366, 2013.