IEEE Electrification - September 2022 - 89

Motor Drives and MediumVoltage
dc Systems
Due to salient performance in efficiency
and controllability, MMCs
have also gained wide interest for
medium-voltage (MV) applications,
such as MV motor drives and MVdc
systems. Compared to traditional
MV converters for drive systems,
MMC's excellent output voltage
control allows it to run with almost
any motor. It also does not necessarily
need a transformer and output
harmonic filters. The modular
and redundant configuration of the
MMC improves the system's reliability
and availability. Therefore,
the MMC is suitable for achieving
electrification of the shipboard and
aircraft drive systems wherein reliability
is a critical concern. Figure 6
displays a typical MMC-based motor
drive system. The MMC drives the
ac machine without using a transformer.
The front rectifier can be a
diode-based multipulse rectifier or
a VSC.
An MMC can also be used as static
frequency converters (SFCs) for
traction power supply in electrified
railway systems. Figure 6 shows two
MMC-based SFCs. A three-phase FBMMC
can perform ac-ac conversion
for railway power supply systems in
a low frequency, e.g., 16.7 Hz. Compared
with other VSC-based railway
power supply systems, a direct ac-ac
FB-MMC does not need an expensive
and bulky 16.7-Hz transformer and
33-Hz filter. However, this topology
needs a frequency separation accompanied
by a complex control. An
indirect ac-dc-ac MMC consists of a
three-phase ac-dc HB-MMC and a
single-phase dc-ac HB-MMC. This
topology is suitable for 50-Hz railway
systems because it will need larger
SM capacitors and higher current
rating devices on the single-phase
part compared to the direct MMC if
it is used for 16.7-Hz systems.
Due to application of the MMC in
HVdc, the technology-readiness level
of MMCs for MVdc systems is in
place. MVdc systems
can be viewed as acting
in the same way
as HVdc ones, just
on a smaller scale
and over comparatively
shorter distances
or at a specific site. An MVdc system
can also be a layer that connects
HV and LV systems. Such MVdc systems
allow much more flexible ways
of grid operation beyond the scope of
conventional ac systems by flexible
power flow control and hence a more
efficient use of renewable energy
resources.
Unlike the MMC in VSC-HVdc
applications, a consensus on using
an MMC in MVdc hasn't been built.
Some initial work has been carried
out to select the optimal converters
MMC's black-start
capability also helps
achieve a fast system
restoration.
for MVdc systems,
considering different
converters' reliability,
capital cost, power
losses, and so forth.
Two- and three-level
VSCs are competitive
at a low voltage range, e.g.,
fewer than 10 kV. MMCs start to
show benefits for a voltage range
higher than 10 kV. There have been
some pilot projects in the field to
test the feasibility of MMCs for
MVdc distribution networks, as
presented in Table 2. However, the
selection of MVdc converters still
needs to be case-by-case, based on
requirements such as size, weight,
capacity, voltage level, fault protection,
and so on. There are also
other alternative converters for
800 kV
LCC
500 kV
400 kV
LCC
MMC
2,100 km
LCC
800 kV
500 kV
Figure 5. The Baihetan-Jiangsu hybrid LCC/MMC UHVdc project (positive pole).
TABLE 2. Applications of MMCs in MVdc systems.
Projects
Year
Baolong Industrial
District, China
Guizhou University,
China
Tangjia Bay, China
Zhangbei Flexible
Substation and
ac-dc Distribution
Network, China
Suzhou Industrial
Park, China
2018
2018
2018
2018
DC Voltage/
kV
Converters
±10, ±0.375 HB-MMCs and a two-level VSC
±10, ±0.375 Hybrid MMCs with an HB-SM
(50%) and an FB-SM (50%)
±10,
±0.375,
±0.11
HB-MMCs and an HB-MMC with
IGCT clamp modules
±10 kV, 0.75 MMCs with a clamp-double-SM
in the power electronic transformers
2018
±20
kV
HB-MMCs
IEEE Electrification Magazine / SEPTEMBER 2022
89
IEEE Electrification - September 2022

Table of Contents for the Digital Edition of IEEE Electrification - September 2022
