IEEE Electrification - September 2022 - 87

MMC based on integrated-gate commutated
thyristors (IGCTs) has been
proposed for HVdc applications to
improve the efficiency, reliability,
voltage rating, and largely reduce
cost. The hybrid MMC using mixed
HB-SMs and FB-SMs can be an alternative
to overhead line-based HVdc
systems. With at least 50% of FB-SMs,
it can achieve dc fault-regulating
capability with reduced cost and
power losses compared to traditional
FB-MMCs. However, due to the high
cost, complexity and immaturity of
those topologies, they are not widely
deployed in practical HVdc projects.
HB-MMCs are currently dominating
the MMC-HVdc market.
Thanks to the intensive R&D and
field application of MMC-HVdc technology,
its voltage and capacity have
reached the same level as its counterpart,
line-commutated converter
(LCC)-based HVdc. Table 1 lists the
products of HVdc converter manufacturers
worldwide. It can be seen
that some suppliers are now able to
produce MMCs for ±800-kV ultraHVdc
(UHVdc) applications. This
makes the hybrid LCC/MMC HVdc a
promising solution, integrating
advantages of the two technologies
in terms of capital cost, power losses,
control flexibility, fault blocking,
and so on.
The world's first hybrid LCC/
MMC HVdc project is the Skagerrak
4 link (500 kV/700 MW), commissioned
in 2015. In this project, the
newly built MMC-HVdc link was tied
together with the existing Skagerrak
3 LCC-HVdc link to form a bipolar
configuration. The MMC link not
only boosts the transmission capacity
but also mitigates the risk of
commutation failures on the nearby
LCC through MMC's fast reactive
power support. MMC's black-start
capability also helps achieve a fast
system restoration. Similar technology
has been used in the HokkaidoHonshu
HVdc link (250 kV/300 MW)
in Japan, which was commissioned
in 2019.
China's Kun-Liu-Long project,
commissioned in December 2020, is
the world's first ±800-kV hybrid LCC/
MMC UHVdc transmission network.
It is a three-terminal system in a
bipolar configuration, wherein the
power-sending end is an LCC station
with 8,000 MW, and the two receiving
ends are MMCs with
3,000 and 5,000 MW.
The MMCs use cascaded
low-voltage (LV) and HV
bridges to build up the
dc voltage to 800 kV, as
illustrated in Figure 4.
Moreover, in each bridge,
the hybrid MMC with
mixed HB-SMs (30%)
and FB-SMs (70%) is used to achieve
the dc-overhead line fault clearance.
The total length of the overhead transmission
line is more than 1,489 km.
The Baihetan-Jiangsu ±800-kV
UHVdc transmission project is
another hybrid LCC/MMC UHVdc
project under construction in China.
Hydropower from the Baihetan station,
the world's second-largest such
station, will be transmitted over
2,100 km to the load center in Jiangsu
province. The configuration of
this project is depicted in Figure 5.
In an FB-MMC, SM
capacitors can be
inserted into the
circuit in either
voltage polarity.
The power- sending end is an LCC
station with 8,000 MW, and the
receiving-end station contains a
cascaded LCC (4,000 MW) and MMCs
(3 × 1,333 MW). A 12-pulse LCC is
used for the HV inverter bridge. The
dc terminal voltage of this LCC is
400 kV. There is a common dc bus of
400 kV at the LCC's LV
terminal, to which the
three parallel-connected
HB-MMCs connect.
The LCC and MMCs will
be integrated into the
500-kV Jiangsu power
grid at different locations.
This UHVdc project
is
planned to be
fully commissioned in late 2022.
There are specific concerns
about this cascaded LCC/MMC
design. One of the most challenging
issues is the low multi-infeed effective
short circuit ratio (MIESCR)
caused by the existing multiple
LCC-HVdc links located in the Jiangsu
power grid. Additionally, several
LCC-HVdc links infeeding to the
same power region are under construction,
which may further exacerbate
the MIESCR and therefore
increase the risk of suffering
TABLE 1. A list of HVdc converter manufacturers
(nonexhaustive).
Manufacturers
LCC
Hitachi Energy
Siemens Energy
GE Grid Solutions
Toshiba
Mitsubishi
NR ELECTRIC
RXHK
XUJI Group
XD Group
Tebian Electric
Apparatus
NARI Group
±1,100 kV, 12 GW
±800 kV, 10 GW
±800 kV, 6.4 GW
±500 kV, 1.2 GW
-
±1,100 kV, 12 GW
-
±1,100 kV, 12 GW
±1,100 kV, 12 GW
±1,100 kV, 12 GW
±1,100 kV, 12 GW
MMC
±640 kV, 3 GW
±525 kV, 2 GW
±525 kV, 2.1 GW
250 kV, 300 MW
±500 kV, 1 GW
±800 kV, 5 GW
±800 kV, 5 GW
±500 kV, 1.5 GW
±800 kV, 5 GW
±800 kV, 5 GW
±535 kV, 3 GW
IEEE Electrification Magazine / SEPTEMBER 2022
87
IEEE Electrification - September 2022

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