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A Grid as Big as China
BIG PICT URE: This Beij ing dispatch
center con trols most of Chin a’s
ultrahigh-voltage lines and monitors
renewable energy use.
36 | MAR 2019 | SPECTRUM.IEEE.ORG
PO WER SHI FT: This station in Zhejiang
province imports hydropower from
Sichuan p rovince as dire ct current and
convert s it to alternatin g current.
XU YU/XINHUA/REDUX
STATE GRID CORP. OF CHINA
Building the world’s first ultrahigh-voltage ACDC power grid is hard. Operating it is harder By Peter Fairley
SPECTRUM.IEEE.ORG | MAR 2019 | 37
SUPERSIZED: Pushing UHV
technolo gy to 1,100 kilovolts
require s upscaled comp onents like
this 800-metric-ton transformer.
ABB
NIMBY: Coal p lants in Inner M ongolia
feed this station near Shanghai,
reducing the megacity’s air pollution.
IMAGINECHINA/AP
38 | MAR 2019 | SPECTRUM.IEEE.ORG
VCG/GETTY IMAGES
Wind rips across an isolated utility station in northwestern China’s desolate Gansu
Corridor. More than 2,000 years ago, Silk Road traders from Central Asia and Europe
crossed this arid, narrow plain, threading between forbidding mountains to the south and
the Gobi Desert to the north, bearing precious cargo bound for Imperial Beijing. Today
the corridor carries a distinctly modern commodity: gigawatts of electricity destined for
the megacities of eastern China. One waypoint on that journey is this ultrahigh-voltage
(UHV) converter station outside the city of Jiuquan, in Gansu province.
Electricity from the region’s wind tur-
bines, solar farms, and coal-red power
plants arrives at the station as alternat-
ing cur rent. Two dozen 500-metric-ton
transformers feed the AC into a cavern-
ous hall, whe re ACDC converter circ uits
hang from the 28-meter-high ceiling,
emitting a penetrating, incessant buzz.
Within each circu it, solid-state switches
known as t hyristors chew up the AC and
spit it out as DC owing at 800 kilovolts.
From here, the transmission line tra-
verses three more provinces before ter-
minating at a sister station in Hunan
province, more than 2,300 kilometers
away. There, the DC is converted back
to AC, to be fed onto the regiona l power
grid. Since it opened in mid-2017, the
26.2 billion y uan (US $3.9 bill ion) Gansu–
Hunan transmission line has moved
about 24 terawatt-hours.
The sheer scale of t he new line and the
advanced grid technoloy that’s been
developed to support it dwa rf anything
going on in prett y much any other coun-
try. And yet, here in Ch ina, it’s just one of
22 such ult rahigh-voltage megaprojects
that grid operators have built over the
past decade. I n the northwestern reg ion
of Xinjiang, China recent ly switched on
its largest UHV link: a 1,100-kV DC cir-
cuit that co st over 40.7 billion yu an. The
new line’s taller t ransmission towers a nd
beeer wi res parallel the Ga nsu–Hunan
line th rough the Ga nsu Corridor, before
diverting to Anhui province in the ea st.
The result of all t his eort is an emerg-
ing nationw ide supergrid that will inter-
connect China’s six regional grids and
rectify the huge geographic mismatch
between where Ch ina produces its clean-
est power (in the north and west) and
where power is consume d (in the densely
populated eas t). By using hig her voltages
of direct current, which ows through
conductors more uniformly than does
alternating current, the new trans-
mission lines dramatically reduce the
amount of power th at’s lost along the way.
But even as China celebrates the com-
pletion of more than 30,000 km of UHV
lines, power engineers are struggling
to master the resulting hybrid ACDC
transm ission system. They must ensure
that the new long-haul DC lines don’t
destabilize China’s regional AC grids.
For example, if the 8-gigawatt DC line
from Gansu were to u nexpectedly go o
line, the power shoc k could cause wide-
spread blackouts in Hunan and beyond.
To minim ize the threat, the St ate Grid
Corp. of China, a state-owned compa ny
that runs most of China’s transmission
and distribution grids, intentionally
limits the line’s throughput to no more
than 4.5 GW. In practice, t he line has car-
ried less than one-quarter of its design
capacity on average. That’s one reason
why over one-third of Ga nsu province’s
theoretical wind output and one-ft h of
its solar potential went unused in 2017.
Other UH V lines in neighbori ng regions
have simila rly operated below capacity.
And eastern provinces don’t have suf-
cient incentive to import the cleaner
power that the UHV lines oer.
The ultimate solution to both issues,
according to State Grid engineers, is to
double down on UHV. They argue that
the country must move far more enery
via UH V DC to maxim ize the use of renew-
CRUSHING IT: China’s newes t
UHV line fro m Xinjiang to An hui has
set world records for transmission
distance, power, and voltage.
SPECTRUM.IEEE.ORG | MAR 2019 | 39ILLUSTRATION BY Erik Vrielink
Hong Kong
Wuhan
Chengdu
Lhasa
Urumqi
Jiuquan
Chongqing
Guangzhou
Shanghai
Shenyang
Harbin
Beijing
Changsha
Coal
Nuclear
Hydro
Solar
Wind
UHV DC
UHV AC
UHV AC
under construction
UHV DC
under construction
Northwe st
China’s win d
and sola r power
plants a re the
world’s largest.
able enery wh ile slashing reli ance on coal.
State Grid is a lso building a world-leading
set of ultrahigh-voltage AC lines, to help
eastern C hina’s regiona l AC grids ab sorb
the output from those massive lines.
“The UHV AC power grid is like a deep-
water port, and the UHV DC is like a
10,000-ton ship. Only the deep-wate r port
can support the 10,000-ton ship,” says
QinXiaohui, vice di rector of power system
planni ng with State Grid’s China Ele ctric
Power Research Institute, in Beijing.
Meanwhile, power authorities every-
where are watching. Gregory Reed, a
DC transmission expert who runs the
University of Pittsburgh’s Center for
Enery, says China’s UHV grid puts it far
ahead of the rest of the world. “They’re
investin g signicantly, and they’ve gone
right to the highest levels of technoloy
capabilit y from day one. There’s no com-
parison anywhere else in the world. It’s
like we’re all still pedaling our bicycles,
while the Formul a 1 race car goes yi ng by.”
China’s UHV movement was born of
a limo ride. It was late 2004, and Liu
Zhenya, then pre sident of State Grid, was
sharing a car with Kai Ma, minister of
the National Development and Reform
Commis sion (NDRC), the powerful state
body that reg ulates China’s growth and
major investments. As Chinese policy
expert Yi-chong Xu desc ribes in her 2017
book Sinews of Power (Oxford University
Press), Kai complained of the crippling
power shortages of the day. Liu blamed
“weak and fragmented” grids, ones ill-
equipped to excha nge bulk power. And
he proposed a bold solution: massive
cross-country power lines utilizing the
most advanced UHV technologies.
Within a year, Kai’s NDRC had
approved an ambitious and compre-
hensive plan th at embraced Liu’s vision.
It combined UHV DC lines, which excel
at moving bulk power from one spot to
another over long distances, and a U HV
AC backbone to reliably distribute that
power to consumers. State Grid would
lead the engineering and ensure that
domestic suppliers would manufact ure
90 percent of the UHV equipment, thus
building up a new high-tech export sec-
tor for China.
Over the next decade, Liu delivered.
He put some 2,000 State Grid engi-
neers on the project and funded more
than 300 professors and 1,000 grad-
uate students at Chinese universities
to conduct power-grid-related R&D.
State Grid expanded and refocused its
research centers to attack specic UHV
issues, including how to safely handle
the higher electromagnetic elds and
the more potent impul ses during switc h-
ing and faults.
In January 2009, State Grid ener-
gized its rst UHV demonstrationline—
a650-km, 1,000-kV UHV AC tra nsmission
line that linked the North China and
Central China reg ional grids. Ten years
on, State Grid has completed 19 of 30
proposed UHV lines.
That aggressive build-out has helped
fast-growing urban centers such as
Shanghai stave off power shortages
despite delays in the expansion of
China’s nuclea r power capacity and con-
straints on local coal power due to air-
quality concerns. The new UHV grid is
also helping t he country lead the globa l
transit ion to renewable generation, mov-
ing 161.5 terawatt-hours of hydro, wind,
and solar enery in 2017 alone.
ABB, Siemens, a nd other internationa l
power-technoloy compan ies have been
instr umental in developing and validat-
ing key components of the Chinese UHV
grid. But State Grid has insi sted on shar-
ing the intel lectual propert y for the tech-
nologies developed at its behest.
In a 2014 interview, Executive Vice
President Liu Zehong described one
tense episode in 2006 when State Grid
asked international suppliers to help
develop 6-inch-d iameter thyri stors capa-
ble of handli ng more current than 5 -inch
thyri stors could. The suppliers initia lly
balked, sa id Liu, but ultimately relented
because of State Grid’s “determined atti-
China’s Hybrid AC-DC Grids
Ultrahigh-voltage DC lines move coal-fired and renewable
generation thousands of kilometers to China’s megacities.
UHV AC helps distribute the imported electricity.
Greater
Shanghai’s
energy
impor ts
may triple i n
the coming
decade.
China So uthern Power Gr id split its
AC grid in tw o to make it more sta ble.
40 | MAR 2019 | SPECTRUM.IEEE.ORG
PETER FAIRLEY
tude” and the “huge market opportuni-
ties” of the Chinese market. Two years
later, Chinese r ms were manufactur ing
the resulting 6-inch switches.
For all of State Grid’s progre ss, it sU HV
deployment remai ns uneven and incom-
plete. China could end up with just half
of the 89,000 km of UHV lines that its
plans ca lled for by 2020 and none of the
anticipated UHV links to Kazakhstan,
Mongolia, and Russia. Many proposed
projects—particularly for the UHV AC
backbone—have fa iled to gain the NDRC’s
blessing. As a re sult, many areas st ill have
no UHV AC line s, and both types of U HV
are delivering well below expectations.
What has blocked f ull implementation
is an intense debate over the future of
UHV. Some Chinese grid experts ques-
tion the hundreds of billions of yuan
spent on UHV projects and what they
see as State Grid’s monopolization of
grid engineering and manufacturing.
Provincial ocials have chafed at the
central ization of grid plan ning and oper-
ation that UHV requires.
Some exper ts have also critic ized Liu’s
ultimate goal for the UHV AC back bone—
linking up and synchronizing China’s
regional grids—as far too risky. Han
Yingduo, a member of the prestigious
Chinese Academy of Engineering and
a professor at Tsinghua University, in
Beijing, has warne d that unify ing China’s
grid would ma ke it far more vulnerable to
cascad ing blackouts, like the one in 2003
that knoc ked out power in the northeast-
ern United States and Canada.
Because no other country has ever
built a hybrid UHV ACDC grid, State
Grid engineers are having to feel their
way along. In a traditional lower -voltage
network, the grid operator typically
reserves emergency power to cover the
sudden loss of the grid’s largest asset.
That may mean keepin g a gigawatt or two
of extra power generation at the ready.
Now add multiple UHV lines to your
network, each ca rrying 8 to 12 GW, and
your requirements for reserve power
rise dramatically.
Maintaining the ideal voltage on a
UHV grid is also enormously challeng-
ing. Thyristor- based UHV converters
consume what’s known a s reactive power—
found in AC systems i n which the current
and voltage are out of pha se. (By contrast,
active, or real, power is the power that’s
actua lly consumed by the gr id’s loads; its
current and voltage waves are aligned.)
By consuming reactive power, the UHV
converters tend to pu ll down the voltage
of surround ing AC lines, so converter sta-
tions have equipment to supply reactive
power and prop up the AC voltage.
But if an AC line’s voltage sags, nearby
converters w ill consume even more reac-
tive power, pulling voltage dow n further.
A voltage sag can also disrupt the thy-
ristors’ ability to switch from one cur-
rent path to another, a process known
as commutat ion. A severe commutation
failure will cause the converter to shut
down, deepening the AC voltage drop
and starting a potentially destructive
feedback loop that could end in a black-
out. “Successive DC commutation fail-
ures will trig ger a chain reaction,” says
Qin, the sys tem planning expert at State
Grid’s Beijing research institute.
The resulting blackout could travel far
and fast, notes Zha ng Fang, a system oper-
ator in State Grid’s National E lectric Power
Dispatchi ng and Control Center, in Beijing.
When a UHV D C circuit goes o line u nex-
pectedly, it creates a power surge hun-
dreds or thousands of kilometers away,
on the AC grid that feeds it. “The UHV
DC line is actually acting as an amplier.
A small AC disturbance in the receiving
end can bec ome a large AC disturba nce in
the sending-end grid,” says Zhang.
To minimize the risk of multiple con-
verter failures and cascading blackouts,
engineers for State Grid’s East China
regional grid have deployed a iber-
optic control net work that automatic ally
rebalances supply and demand. If nec-
essary, it can boost line voltage within
200 mil liseconds of a voltage drop, using
a set of fault responses that have been
built into the East China grid’s ACDC
converters. As soon as the ber-optic
network ags an outage on a UHV DC
line, the conver ters pull up to 10 percent
more power over the remai ning DC lines
to keep the gr id operationa l. The optical
control sche me can also restore balance
by releasing power f rom pumped hydro
plants, which store enery by pushing
water uphill. And it can trigger small
controlled blackouts, shutting o some
distribution feeders to reduce demand
while spar ing hospitals and ot her essen-
tial loads.
These measures h ave enabled a trio of
UHV DC lines that deliver hydropower
from the Southwest China grid to oper-
ate continuously at their combined
21.6GW design capacity. The result is
SPECTRUM.IEEE.ORG | MAR 2019 | 41
an elect rical trifec ta: Greater Shangha i,
China’s most densely urbanized and
industrialized region, gets more clean
power; the Yangtze River Delta’s mega-
dams spi ll less excess water duri ng ood
season; and State Grid earns more rev-
enue from its UHV investment. Even
so, Shanghai still runs short of power
for several weeks each summer, forc-
ing State Grid to pay big customers to
idle their factories. Keeping pace with
growth may require tripling Shanghai’s
electricity imports within a decade.
At the national control center, in
Beijing, mounting pressure to push
more clean power through State Grid’s
UHV lines is hard to miss. The main
screen displays the statu s of the AC and
DC trunk lines, providing a real-time
view of the entire system. Dominating
the left wa ll are warning lights tracking
renewable ener y curtail ment in each of
25 provinces—a nd who should be xing it.
Green lights mean that all of the poten-
tial solar and wind power is being used.
Blue, yellow, and orange lights indicate
renewable enery waste, which State
Grid’s provincial, regional, or national
controllers, resp ectively, must try to stop.
“We are determined to consume the
renewable enery to the maximum
extent. That’s our job,” says Zhang.
Controllers may reroute power from a
province with low electricity demand
to another where demand is higher. Or
they may steer elec tricity to one of State
Grid’s 21 pumped hydro plants, which
collectively can soak up 19 GW.
In theory, Chinese law has long
required grid operators to prioritize
renewable ener y. But i n practice, each
province ha s its own plans and prior ities,
which tend to favor electricit y generated
locally. For in stance, in Zhejia ng province,
south of Shang hai, signic ant opposition
to importing electricity has hampered
the operation of an 8GW UHV DC line
from Ning xia province, according to a na-
lysts at Bloomber g New Ener y Finance.
On the windy, sunny day when I vis-
ited Gansu’s DC converter station last
year, its UHV li ne was carry ing just 3 GW
of its 8GW capacity. That was the cumu-
lative output from several renewable
plants. But t he province al so has an addi-
tional 15 GW of solar and wind that’s
connected to the new line but not yet
authorized to feed power into it.
Change is coming. Two months after
my visit, power companies in coastal
Jiangsu province struck a deal to buy
power from Gansu’s largest wind farm
via another UHV DC line. And last
November, State Grid began building
a UHV DC line from Qinghai province
to move even more of Gansu’s renew-
able generation. Meanwhile, the NDRC
is stoking demand by mandating mini-
mum rates of renewable enery use by
each region.
State Grid’s long-term goal to inter-
connect its regional grids should also
reduce curt ailment, expert s say. Zhang
Ning, an authority on renewables inte-
gration at Tsinghua University, points
out that the Southwest grid’s hydro-
power can balance the uctuations in
the Northwest’s wind and solar out-
put. “If we interconnect the West, cur-
tailment of wind power there can be
reduced from more than 20 percent
to 5 percent,” he estimates, and both
regions’ use of coal can also be cut.
Even as State Grid irons out the kinks
in its UHV grids, the company is push-
ing its equipment and expertise abroad.
It has led the creation of nine UHV
standards through the International
Electrotechnical Commission and
the IEEE—a move that researchers at
Argonne Nation al Laboratory, in Illinoi s,
warned would help Chinese suppliers
“crowd others out of the global market.”
State Grid is a lready working on its rst
international UHV DC project: a pair of
800-kV lines to move power f rom Brazil’s
Belo Monte megadam. But subsequent
UHV sales have been slow to material-
ize. That may be because most coun-
tries do not yet need, or cannot aord,
a 1,000-kV AC or DC line.
Undaunted, former State Grid chair-
man Liu is now crusading to build trans-
continental and intercontinental UHV
grids. The same technoloy that went
into building the 1,100-kV line from
Xinjiang to A nhui could eciently move
power up to 5,000 kilometers. “If we
just tur n that line around to point west,
we are getting close to Europe. So the
technoloy is available,” says Magnus
Callavik, general manager of A BB
Sifang Power System, a Beijing-based
joint venture between Swiss power-
engineering giant ABB and China’s
Sifang Automation.
Callavik says he is convinced that
continental-scale UHV DC will happen,
sooner or later. In a world that must
decarbonize, iguring out how to bal-
ance variable enery supplies such as
solar and w ind generation with reg ional
loads is a growing concern. “Transmis-
sion is a very cost-ec ient way of doing
that,” says Callavik.
In China the question is how quickly
State Grid will overcome the technical
and polit ical obstacles t hat are holding
back UHV ’s carbon-slash ing potential. If
the countr y continues to rely heavily on
coal power, importing that power over
thousands of kilometers w ill help clear
the air in China’s eastern megacities.
But the countr y’s carbon footprint will
remain u nchanged, and the benet s for
the global climate will be nil. Mobiliz-
ing gig awatts of renewable p ower over a
UHV g rid, on the other hand, prom ises a
real change, for China and t he world. n
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chinauhvgrid0319
MODERN IMPORTS: A trio of
UHV DC line s traces the Silk Ro ad
inChina’s Gansu province.
