IEEE Electrification Magazine - September 2016 - 12

Current (A)
Current (A)

Voltage (kv)

Current (A)

Voltage (kv)

a strong effect on the required passive filters. The effect of the voltage
waveform on the current quality is
shown in Figure 14. For these simulations, the device's switching frequency and load inductance are
0.005
0.01
0.015
0.02
0.025
0.03
the same. The waveforms are for
(a)
converters using 3.25-kV nominal
dc-link voltages and 6.5-kV IGBTs.
40
1,000
The MMC has the best voltage
30
750
20
500
quality, although it requires the
10
250
0
0
largest number of modules. The
-250
-10
-500
-20
multiple-dc-link converter would
-750
-30
-1000
-40
have the same quality of the out0
0.005
0.01
0.015
0.02
0.025
0.03 0.035
put with half the number of mod(b)
ules equal to that of an MMC. It is
important to note that these con1000
40
750
30
siderations are valid with the cur500
20
250
10
rent availability of semiconductor
0
0
-250
-10
devices. However, the new genera-500
-20
tion of wide-bandgap devices
-750
-30
-1000
-40
might substantially change the
0
0.005
0.01
0.015
0.02
0.025
0.03 0.035
present state of the art, enabling a
Time (s)
simpler design of medium-voltage
(c)
converters without transformers
Figure 14. A comparison of different converter types feeding a passive load with a 0.05 per-unit
and, hence, with higher efficieninductance: (a) an MMC, 11 submodules, and two-level bridges; (b) a back-to-back converter, two
cies and lower costs.
interleaved modules, and three-level bridges; and (c) a multiple-dc-link converter, two-series
submodules, and three-level bridges. Only the MMC does not use an output transformer.
The waveform shapes of the
three-phase side current for the
difference frequency is higher than the catenary freback-to-back and multiple-dc-link converters are identiquency, which reduces filter requirements.
cal to those shown in Figure 14(b), whereas those for the
The indirect ac/dc/ac converter is formed by two casMMC are identical to those shown in Figure 14(a). Table 1
caded MMCs. They use half-bridge modules and are concompares the three converter topologies, where n is the
nected by a dc link, but this dc link does not have any
number of submodules of the converter.
capacitor. This topology has no limitation on frequency
Cophase and Advanced Cophase Systems
separation, so it can be used also when the input and outAn alternative electrification system called a cophase
put frequencies are the same. This converter is the only
power supply is a hybrid solution between a STATCOM
choice for 50-Hz railway supplies. For a 16.67-Hz supply, it
and a full-power static converter. It uses a static singlewould require larger floating capacitors and higher-curphase-to-single-phase power converter and an impedrent-rating devices for its single phase compared to the
ance-matching YNvd transformer, which acts as a
direct MMC converter.
three-phase-to-two-phase converter. The winding ratio is
One important difference between the three converter
chosen such that the three-phase side load is balanced
topologies is the output voltage waveform because it has
1,000
750
500
250
0
-250
-500
-750
-1,000
0.035

40
30
20
10
0
-10
-20
-30
-40
0

TABLE 1. A comparison of different converter topologies for static frequency converters.
Number of
Submodules
per Unit

Number of
Capacitors
per Unit

Three-Phase
Side Waveform Levels

Single-Phase
Side Waveform Levels

Number of
Transformers
Required

Effective
Switching
Frequency

Direct

6×n

n+1

2 or 1 for high n

n × fdevice

Indirect

10 × n

Back-to-Back Converter

2+n

2 × fdevice

Multiple-dc-Link Converter

2×n

3×n

n + 1 or n for high n

2 × n × fdevice

Converter
MMC

I E E E E l e c t r i f i cati o n M agaz ine / SEPTEMBER 2016

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2016