Development and Outlook of UHV Transmission Technology

PREFACE

With the continuous consumption of fossil energy, increasingly

severe climate change and worsening environment pollution, the

human being has put forward higher requirements for reliable,

environmentally friendly and sustainable energy, bringing global

energy interconnection into being. The core of sustainable

development is clean development. UHV transmission technology is

critical for the reliable and efficient delivery of clean energy, and it is

of great significance to promote its global allocation.

After a century of development, the transmission technology of

power system has made great progress in terms of voltage level,

transmission capacity and distance. As for UHV AC power

transmission technology, UHV DC transmission technology, and

flexible DC transmission technology, their respective voltage and

transmission capacity have exceeded 1,000kV/5,000MVA,

±1,100kV/12,000MW and ±800kV/5,000MW. Flexible AC

transmission technology has started to be applied in 1,000kV projects.

The development of UHV power transmission technology provides a

strong support for the world’s large-scale energy development and

long-distance transmission, facilitates a large-scale optimization of

energy allocation, and greatly improves the reliability and economic

efficiency of power supply in human society.

I

Development and Outlook of UHV Transmission Technology

UHV power transmission technology will see a broader development

space and application market in the construction of global energy

interconnection (GEI). UHV power transmission technology can

make it possible for thousands of kilometers and 10 GW power

transmission and cross-border and transcontinental grid

interconnection, while flexible DC transmission technology can

enhance the flexibility of system operation, meet the friendly

interconnection of clean energy such as photovoltaic and wind power

and support their flexible configuration. The two technologies are of

great significance to enhancing the system stability, ensuring the wide

access, transmission and consumption of clean energy and improving

the flexibility and reliability of power grid operation.

Since GEI involves the development of different types and scales of

clean energy, the power transmission of different capacities and

distances and the intensive interconnection of power grid with

different characteristics and development levels, it raises higher

requirements for UHV power transmission technology. During the

construction of GEI, it will be characterized by higher clean energy

proportion at the power source side, larger clean energy base scale,

longer transmission distance, more complex grid topology and

environment, and higher requirements for reliable, flexible and

affordable power supply. Therefore, to further enhance the power

transmission capability of UHV AC and flexible power transmission

technologies and extend their scope of application, improve their

II

Development and Outlook of UHV Transmission Technology

stability, security, flexibility and grid-integration strategies and

reduce the power transmission cost is an important development

trend for power transmission technologies.

In view of this, the report summarized the current situation, main

technological innovation and engineering practice of current UHV

AC/DC power transmission technologies and flexible AC power

transmission technology. Combined with the technical demand for

large-capacity and long-distance clean energy transmission and more

reliable and flexible interconnection, the development trends of

various technologies were further prospected, to provide reference

for the industrial development.

The report is composed of three sections, which separately introduce

the development history and current situation, technological

innovation, core equipment, test system, typical projects and

development prospect of UHV AC, UHV DC and flexible power

transmission technologies.

GEI is the optimal scheme for optimizing the allocation of clean

energy and solving such complex problems as global energy dilemma

and climate change, while UHV power transmission technology will

be an important guarantee for achieving this grand goal, with broad

development space and application prospect. The research results will

play a positive role in promoting the research and development of

advanced power transmission technologies and key facilities,

improving the efficiency of clean energy development and delivery,

III

Development and Outlook of UHV Transmission Technology

promoting the progress of power industry and supporting grid

interconnection on a larger scale.

IV

Development and Outlook of UHV Transmission Technology

CONTENTS

1. UHV AC Power Transmission Technolog y .........................................2

1.1 Overvie w .............................................................................................2

1.2 Key Technology and Equipmen t .........................................................4

1.3 Test and standar d .................................................................................5

1.4 Typical Project s ...................................................................................6

1.5 Future outlook .....................................................................................7

2. UHV DC Power Transmission Technology .......................................10

2.1 Overvie w ...........................................................................................10

2.2 Key Technology and Equipmen t .......................................................12

2.3 Test and standar d ...............................................................................13

2.4 Typical Project s .................................................................................15

2.5 Future outlook ...................................................................................16

3. Flexible Transmission Technology .....................................................19

3.1 FACT technolog y ..............................................................................19

3.2 VSC HVDC transmission technology ...............................................23

1

Development and Outlook of UHV Transmission Technology

UHV transmission technology is critical for the cross-continental

and cross-border interconnection, long-distance clean energy

delivery,

transmeridian

complementation,

cross-season

complementation, and global allocation, and in the course of GEI

construction, new requirements including long distance, large

capacity, low loss, high efficiency, good flexibility and high

stability are springing up.

1. UHV AC Power Transmission Technology

1.1 Overview

The UHVAC transmission technology is mature, and has

become the key technology to build large-capacity and large-

scale strong synchronous power grid. UHVAC transmission is

a technology which is for the transmission of AC power of

1,000kV or above and has a single channel transmission capacity

of about 10,000MW and a maximum transmission distance above

1,000km. By the end of 2019, there are 12 UHVAC transmission

projects in service in the world and 3 UHVAC transmission

projects under construction, which are located in China and

constitute a total UHVAC transmission length of more than

20,000km. For a UHVAC transmission project, the total

construction cost required for a substation is about USD 150-200

million with a unit capacity cost of USD 52,000 per MVA and a

single-loop cost of about USD 1,000,000 per km.

 The Soviet Union: The Soviet Union is among the first

2

Development and Outlook of UHV Transmission Technology

countries to conduct research on UHV transmission technology

and also the only one in the world with operating experience in

UHV transmission besides China.

 Italy: After establishing its 1,000kV research plan, ENEL

began the research and technical development of UHV

transformation technology at different test stations and

laboratories in 1971.

 Japan: Japan initiated the research and development plan of

UHV transmission technology in 1972. After the installation of

the complete set of UHV facility at the UHV test site of

Shinharuna Substation in 1995, the live examination was

conducted and the facility operated with the voltage reduction of

500kV.

 United States: American Electric Power (AEP) and General

Electric Company (GE) started to measure the audible noise,

radio interference and other environmental effect at the UHV

Transmission Technology Research Test Station in Pittsfield in

1974.

 Canada: In the HV laboratory of Hydro-Québec, a test was

conducted for the power transmission system with the rated

voltage of 1,500kV.

 China: China leads other countries around the world in the

UHV AC power transmission technology, and has realized large-

scale application of key technologies and core facilities, and built

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Development and Outlook of UHV Transmission Technology

satisfactory test bases and standard system.

1.2 Key Technology and Equipment

For UHV AC power transmission technology, breakthroughs

have been made in a series of technological innovations and

the complete set of core equipment. During the construction of

UHV AC projects, China carried out system analysis and

researches on key UHV AC power transmission technologies,

including overvoltage and external insulation, and

electromagnetic environment. In this way, China has overcome

key technology issues such as system security and stability

control, complex environment external insulation characteristic,

overvoltage suppression, and electromagnetic environment

control, determined the key technical parameters and conditions

of projects, developed a series of standards, regulations and

specifications, developed the complete set of UHVAC equipment

and fully grasped UHV AC power transmission technology,

leading the development direction of global grid construction. In

the equipment research and development, China has fully

overcome the coordinated control problems in terms of multiple

physical fields such as electric field, magnetic field, thermal field,

force field of UHV and large-capacity transformers and reactors

under strict transportation size restrictions, and successfully

developed series of UHV AC core equipment such as UHV

4

Development and Outlook of UHV Transmission Technology

switches, UHV transformers, etc. which adopt a large number of

new designs, new structures, new materials and new processes

and have passed strict tests, inspections and actual operation

examinations, with their excellent performance indicators

reaching the highest HV equipment manufacturing level in the

world.

Fig.1.1 UHV AC transformer

1.3 Test and standard

A complete development and research system has been

established for UHV AC power transmission technology,

which has rich engineering experience. Through technology

research and development and engineering practice, China has

built a complete system research and development platform and

engineering test base, mastered the complete set of technologies

for UHV AC projects, including planning and design, equipment

integration, construction and installation, commissioning and test,

and operation and maintenance, and built a large batch of UHV

AC demonstration and commercial projects. China has completed

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Development and Outlook of UHV Transmission Technology

the system integration of UHV power transmission technology,

accumulated valuable experience and achieved the organic unity

of technology and economic efficiency. The projects are

characterized by safety, reliability, advanced technology and

practicability. The compilation of standards in China’s UHV AC

power transmission technology standard system has been fully

completed, of which 58 standards (29 national standards and 29

power industry standards) have been released and fully applied in

China’s UHV AC projects. Many of China’s technical

achievements have been accepted by the International

Electrotechnical Commission (IEC), the International Council on

Large Electric Systems (CIGRE) and the Institute of Electrical

and Electronics Engineers (IEEE) and other world authoritative

technical organizations.

Fig.1.2 The group conference of IEEE standard set

1.4 Typical Projects

Since 2009, China has built a series of UHV AC transmission

projects, supporting a strong transmission network at all

levels and forming a connection channel for grid

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Development and Outlook of UHV Transmission Technology

interconnection. The Southeast Shanxi-Jingmen UHV AC

Demonstration Project is the first attempt of the global UHV AC

project. The Huainan-Shanghai UHV AC Transmission

Demonstration Project is the world’s first commercial operation

of the same-tower dual-circuit UHVAC transmission project. The

Sutong GIL integrated pipeline gallery project is the pipeline

transmission project with the highest voltage, largest capacity and

longest distance in the world. The Yuheng-Weifang UHV AC

transmission project is the UHV AC project with the largest

investment scale and the longest transmission distance so far.

Fig.1.3 Sutong GIL integrated pipeline gallery project

1.5 Future outlook

For the construction of GEI, UHV AC power transmission

technology boasts broad application future and will be further

improved in terms of key technology, economic efficiency and

flexibility. 1,000kV UHV AC power transmission projects have

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Development and Outlook of UHV Transmission Technology

been applied in a large scale. The technology has a huge advantage

in the aspects of long-distance power transmission for energy base

and cross-regional grid interconnection. To meet GEI’s

requirements for super-large-scale grid interconnection and power

transmission, UHV AC power transmission technology will

develop towards energy-saving corridor, low loss, environmental

friendliness, and intelligence. The development should focus on

researches on core equipment with optimized power transmission

system, high reliability, flexibility and economic efficiency, and

adaptation to various extreme weather conditions.

For Economic aspect, from demonstration projects to

commercialization projects, the cost of major equipment for

UHV AC projects has shown a downward trend. 500 kV UHV

AC technology is currently prevailing, and its future cost will see

limited reduction. 1,000kV UHV project adopts new voltage class.

Since it requires large investment in the initial stage of equipment

development and does not meet conditions for large-scale

production, the equipment price is higher, resulting in higher

project cost. With matured technology and large-scale production,

equipment cost will gradually decline. According to related

research, it is expected that the 1,000kV UHV main transformer,

GIS, and shunt reactor are expected to drop by 24%, 35%, and 15%

respectively in the future. Given the proportion of major

equipment investment, the equipment purchase cost of the whole

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Development and Outlook of UHV Transmission Technology

substation can be reduced by 28% and the total project investment

cost by about 10%.

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Development and Outlook of UHV Transmission Technology

2. UHV DC Power Transmission Technology

2.1 Overview

The continuous breakthrough of UHV DC power transmission

technology makes it a core technology for the long-distance,

super high-capacity and high-efficiency power transmission.

In the UHV DC power transmission, the voltage classes of

±800kV and ±1,100kV are normally expected, the rated

transmission capacity is about 8,000MW to 12,000MW, and the

transmission distance is 2,000km to 6,000km. The total

investment required for the DC substation of voltage level

±800kV and ±1,100kV is USD 670 million and USD 1,180

million respectively, with a unit capacity cost of USD 84 per kW

and USD 98 per kW respectively. China leads other countries and

regions around the world in the key technology, equipment

research and development, test system and project practice of

UHV DC power transmission. By the end of 2019, there are 18

UHV DC power transmission projects in service around the world,

including 14 in China, 2 in India and 2 in Brazil.

 Canada: While conducting UHV AC tests, Canada has also

conducted in-depth research on the UHV DC technology.

 India: India has included the development of the ±800kV

UHV DC power transmission project in its "Twelfth Five-Year"

power grid plan, and planned to construct two ±800kV UHV DC

power transmission projects, Assam-Agra 800kV UHV DC

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Development and Outlook of UHV Transmission Technology

Power Transmission Project and Raigarh-Pugalur ±800kV UHV

DC Power Transmission Project, to transmit hydropower from the

northeast to the west and the east and thermal power from the

central regions to the north, respectively.

Fig.2.1 The converter of Raigarh-Pugalur project

 Brazil: Belo Monte ± 800kV UHV DC Power Transmission

Project in Brazil is one of the first UHV DC power transmission

projects in Brazil, transmits more than one-third of the power

generation of Belo Monte Hydropower Station (with an installed

capacity of 11,000MW), the second largest hydropower station in

Brazil, to the load center in southeastern Brazil.

 China: China's UHV DC power transmission projects account

for about 80% of the world's UHV DC projects, with the highest

voltage and capacity in the world, and thus have promoted the

upgrade of China's power grids and provided important

experience for the world’s power development.

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Development and Outlook of UHV Transmission Technology

2.2 Key Technology and Equipment

With continuous innovation of the UHV DC power

transmission technology, breakthroughs have been made in

the complete set of technologies and core equipment. UHV DC

power transmission projects have many differences in terms of

main connection and operation modes. For key UHV DC

technologies and equipment, pioneering work has been carried

out on overvoltage and insulation coordination, external

insulation, equipment performance, etc., with innovations made.

Breakthroughs are also made in such worldwide technological

problems and key equipment as 800kV and 1,100kV converter

transformer, converter valve and bushing, laying a solid

foundation for the successful operation of projects. In terms of

main connection topology, China’s technology is different from

that of the single 12-pulse converter valve for UHV DC power

transmission, but usually adopts positive and negative poles with

dual 12-pulse (400+400, 550+550) equi-voltage series converter

valves with bypass switches and smoothing reactors between the

pole line and the neutral line, taking into account factors such as

power transmission capacity, equipment manufacturing and

transportation, operational flexibility and reliability.

12

Development and Outlook of UHV Transmission Technology

Fig.2.2 ±800kV、4000A converter valve

Fig.2.3 Converter transformer iron core

2.3 Test and standard

A complete development and research system has been

established for UHV DC power transmission technology,

which has rich engineering experience. Through technology

13

Development and Outlook of UHV Transmission Technology

research and development and engineering practice, China has

built a complete UHV DC research and development system and

engineering test research center, mastered the complete set of

technologies for UHV DC projects, including system research,

system design, engineering design, equipment integration,

construction and installation, commissioning and test, and

operation and maintenance. Based on the complete set of ±800kV

UHV DC standards, China has developed 14 international

standards, 50 national standards, and 73 national electric power

industry standards. This complete set of standards has help SGCC

acquire the right to operate Belo Monte UHV DC Transmission

Project in Brazil and facilitated the establishment of IEC TC115

with the secretariat in China, who has taken the lead in the

formulation of 5 IEC standards and released 4 standards, and is

compiling 7 IEC standards.

Fig.2.4 Outdoor test field

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Development and Outlook of UHV Transmission Technology

Fig.2.5 UHV Test hall

2.4 Typical Projects

China's UHV DC transmission projects account for about 80%

of the world's UHV DC projects, and their voltage and

capacity are the world's largest, which has promoted the

upgrade of China's power grid and provided important

experience for the development of world power. The

Xiangjiaba-Shanghai and Yunnan-Guangdong ±800kV UHV DC

transmission demonstration projects are the first batch of UHV

DC demonstration projects in the world, and the Qinghai-Henan

±800kV UHV DC transmission project is the world's first built

specifically for clean energy delivery. The Zhundong-South

Anhui ±1100kV UHV DC transmission project is the world's

most technologically advanced DC project. Brazil’s Belo Monte

±800kV UHV DC transmission project is the first UHV DC

transmission project in the Americas.

15

Development and Outlook of UHV Transmission Technology

Fig.2.6 Real Map of Fulong Converter Station

Fig.2.7 Brazil’s Belo Monte ±800kV UHV DC transmission project

2.5 Future outlook

Under the background of large-scale clean energy delivery

worldwide and large-scale resource allocation, UHV DC

power transmission technology boasts broad application

future and will be further improved in terms of key

16

Development and Outlook of UHV Transmission Technology

technology, economic efficiency and flexibility. In order to meet

GIE’s requirements for ultra-large capacity and ultra-long-

distance power transmission, the transmission distance, capacity,

topology and key equipment of the UHV DC power transmission

system need to be further upgraded and improved to adapt to the

global large-scale clean energy delivery and global extreme cold,

hot and high altitude conditions, specifically including the

development of complete set of DC power transmission

equipment with higher voltage class and hybrid UHV DC system.

Meanwhile, it’s still necessary to further improve the economic

level of power transmission system and reduce power

transmission cost.

For economic aspect, UHV DC technology is generally

economical in power transmission, and there is still space for

decline in the future. Taking into account factors such as

equipment prices, tariffs, construction conditions, and

environmental climate in different regions of the world, the

construction cost level may fluctuate to a certain extent on the

current basis. From demonstration projects to large-scale

application, the cost of main equipment shows a downtrend. With

the further maturity and scale application, it’s expected that UHV

DC converter transformer, converter valve, smoothing reactor

and others will drop by 24％, 15％ and 29％ respectively.

Combining the investment proportions of each kind of main

17

Development and Outlook of UHV Transmission Technology

equipment, the purchase cost of main equipment for the whole

station is expected to decrease by 24%.

18

Development and Outlook of UHV Transmission Technology

3. Flexible Transmission Technology

The flexible power transmission technology based on high-

power power electronic devices has get great development,

which has played a huge role in improving the flexibility of

power grid operation, energy quality and clean energy access

capability. With the application of large-power power electronic

devices in the electric power system, the control and regulation of

modern electric power system will be more flexible and

convenient. AC/DC power transmission technologies based on

modern power electronic devices, collectively referred to the

flexible power transmission technology, include flexible AC

power transmission technology and flexible DC power

transmission technology. The former improves the flexibility of

system operation through changing system voltage, current

amplitude, phase and nature, etc. The latter is a VSC-based DC

power transmission technology, which adopts self-turn-off power

electronic devices.

3.1 FACT technology

The flexible AC power transmission technology focus on

optimizing power flow, improve system stability and

expanding the traditional application scope, which has been

applied in a large scale. The flexible AC power transmission

technology can change the active and reactive power flow

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Development and Outlook of UHV Transmission Technology

distribution, power level and voltage level, so it has become an

important measure for AC grid to improve the operation

reliability and flexibility. The flexible AC power transmission

equipment includes series, parallel and composite equipment,

such as shunt reactor, series capacitor, static var compensator

(SVC), static synchronous compensator (STATCOM), thyristor

controlled series compensator (TCSC), and unified power flow

controller (UPFC). Different FACTS equipment is able to control

different power grid parameters, so they have different functions.

A brief summary of the power grid parameters controlled by

various FACTS equipment and their applications are shown in

Table 3.1.

Table 3.1 Power Grid Parameters that can be controlled by FACTS Controllers and

their Application Functions

Power grid parameters that

FACTS controller

Application functions

can be controlled

Reactive power

compensation, voltage

control, improvement of

voltage, transient, medium

and long term stability and

damping power oscillation

Voltage control, power flow

control, improvement of

voltage, transient, medium

and long term stability,

damping power oscillation

and short-circuit current

restriction

SVC，STATCOM Injected reactive power, node

SVC, STATCOM

voltage

TCSC

Line impedance

SSSC，IPFC

Line voltage drop

Voltage control, power flow

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Development and Outlook of UHV Transmission Technology

SSSC, IPFC

control, improvement of

voltage, transient, medium

and long term stability,

damping power oscillation,

interline power flow control

(IPFC) and short-circuit

current restriction

Reactive (and active) power

compensation, voltage

control, power flow control,

improvement of voltage,

transient, medium and long

term stability, damping power

oscillation and short-circuit

current restriction

Node voltage, line voltage

drop and its phase

UPFC

FACT technology has achieved breakthroughs in many key

technologies and equipment, and has been widely used

worldwide. Yifeng in China TCSC Project is a series

compensation project with the largest compensation capacity and

the most complex system operation environment in the world.

Ethiopia HOLETA SVC Project in Ethiopia is one of the largest

reactive power compensation projects with the largest adjustable

capacity that have been put into operation in the world at present.

Southern Jiangsu 500kV Unified Power Flow Controller (UPFC)

Project in China is the UPFC project with the highest voltage class

(500kV) and the largest capacity (750MVA) in the world at

present. Southeastern Shanxi-Nanyang-Jingmen UHV Series

Compensation Project in China is the first 1000kV FSC project in

the world.

21

Development and Outlook of UHV Transmission Technology

Fig.3.1 HOLETA SVC Project

Fig. 3.2 Full View of Yifeng 500kV TCSC Project

Because of its fast speed and control flexibility, the flexible AC

power transmission technology will have a broad future. But

it needs to make further improvement in such aspects as

capacity, reliability, coordinated control and economic

efficiency. The flexible AC power transmission technology will

enable to make full use of existing power grid, with minor changes

to existing equipment. However, it can only be used in single

22

Development and Outlook of UHV Transmission Technology

project and EHV projects at present. For large-scale application of

the technology, it still need to regulate multiple flexible AC

devices, solve the problems of coordination and connection, make

further research on core devices in 1,000kV AC voltage and

reduce the engineering cost.

Fig. 3.3 Full View of Southern Jiangsu 500kV UPFC Project

Fig. 3.4 Southeastern Shanxi-Nanyang-Jingmen UHV Series Compensation Project

3.2 VSC HVDC transmission technology

The flexible DC power transmission technology focuses on

23

Development and Outlook of UHV Transmission Technology

solving problems in such aspects as weak system/passive

system access and new energy grid integration. With the

expansion of global wind power and PV development scale

and the emerging of complex grid interconnection

requirements, its advantages and efficiency become

prominent. The flexible DC transmission technology based on

self-turn-off device boasts such capability as resistance against

commutation failure, voltage support, weak-current system

interconnection, passive network power supply and construction

of multi-terminal and DC grid, and is an important means for

large-scale and long-distance clean energy delivery. By the end of

2019, there are about 40 flexible DC transmission projects in

service in the world, and its maximum voltage is ±800kV,

maximum transmission capacity is 5,000MW and longest

transmission distance is over 1,400km. In recent years, China has

made outstanding achievements in the technological innovation

and practice of flexible HV DC transmission technology, catching

up with its world first-class counterparts. As of August 2020, 8

flexible HV DC transmission projects have been put into

operation in China, among which, the Nan’ao Project is the first

multi-terminal flexible HV DC transmission project, the Xiamen

Project is the world’s first true bipolar project, the Zhoushan

Project has the largest number of terminals (5 terminals), the

Zhangbei Project is the world’s first overhead line-based four-

24

Development and Outlook of UHV Transmission Technology

terminal flexible HV DC project with the largest transmission

capacity (3,000MW), and the Wudongde Project is the world’s

first hybrid flexible UHV DC transmission project.

Fig.3.5 Wudongde ±800kV fixed VSC-LCC project

In the VSC HV DC transmission field, technological

innovations have been achieved in converter, polar main

wiring and DC system topology, as well as control and

protection strategies and equipment based on the flexible HV

DC transmission system. The VSC HVDC transmission network

is a converter structure with fully controlled devices (IGBT) and

energy storage capacitors. The converter topology includes voice

source converter (VSC) and modular multilevel converter (MMC).

The DC main wiring of VSC HVDC transmission system includes

two structures, namely symmetrical unipolar (pseudo bipolar)

structure and symmetrical bipolar (true bipolar) structure. The

VSC HVDC transmission system can be divided into dual-

25

Development and Outlook of UHV Transmission Technology

terminal system and multi-terminal system, which can form

different hybrid system topologies with the conventional LCC

commutation mode. According to the power source and the grid

power of the sending end and receiving end, and the number of

placements of the sending end and receiving end, the VSC + LCC

hybrid system structure can be flexibly adopted.

Fig.3.6 UHV VSC DC converter

In worldwide, the flexible HV DC transmission projects are

mainly distributed in Europe, followed by North America, Asia

and Australia. The 80kV Gotland project in Sweden is the world's

first commercial flexible direct transmission project. The France-

Spain ±320kV INELFE project is a typical power interconnection

project in Western Europe. The Zhangbei ±500kV flexible DC

grid test demonstration project in China is the world's first four-

terminal DC power grid project. The ±800kV Wudongde UHV

hybrid three-terminal flexible DC project in China is the world's

26

Development and Outlook of UHV Transmission Technology

first UHV hybrid flexible straight project.

Fig.3.7 The route of Gotland Project System in Sweden

Because of its flexibility and adaptability, the flexible DC

power transmission technology will have a broad future. But

it needs to make further improvement in such aspects as

capacity, reliability and economic efficiency. Restricted by the

performance of basic devices, it’s hard for the flexible DC power

transmission projects to have the same capacity as the LCC ones.

Since a large number of power electronic devices are used, the

reliability is also a huge challenge in the engineering application.

Combined with the new power electronic devices and advanced

large-scale energy storage technology, to develop UHV flexible

DC technology based on IGCT and active flexible DC technology

based on energy storage, increase the capacity, reliability and

economic efficiency of flexible DC system and solve the

decoupling of hybrid AC/DC power grid through making

innovative converter structure and topology is an important

27

Development and Outlook of UHV Transmission Technology

development direction of this technology. In the future, it is

required to further develop the basic components of flexible UHV

DC transmission system, multi-terminal DC power grid, control

and protection and other key technologies, and accelerate the

development of DC circuit breakers, converter valves, power flow

controllers, and fault limiters and other key equipment for ultra-

high voltages of ± 800kV or above to realize the flexible and

stable reception and long-distance and large-scale transmission of

high proportion or 100% of new energy power. The fast-

advancing large-scale energy storage technology is applied to

develop active flexible DC transmission technology based on

energy storage through inverter structure and topology innovation,

and solve the problem of AC and DC grid fault decoupling.

Compared with traditional DC technology, flexible DC

transmission technology has a relatively short development period,

and the overall cost is higher but shows a downward trend. In

terms of converter stations, since the price of the basic component

IGBT of flexible DC converter valves is much higher than that of

LCC thyristors, and the number of components required is also

larger, the cost of flexible DC transmission projects is higher than

that of conventional DC under the same voltage level and capacity.

Generally speaking, the cost of the flexible DC converter station

is about 30% higher than that of the conventional DC converter

station. The cost of converter stations generally accounts for 30%

28

Development and Outlook of UHV Transmission Technology

to 40% of the total project cost. The higher the voltage level, the

higher the cost of the converter station, and the higher the

proportion of the total project cost. The main factor affecting the

total project cost is the converter station cost. The main cost

components of a converter station include investment in core

equipment such as converter transformers, converter valves, DC

field equipment, and control and protection system. For UHV

flexible DC converter station, the cost of converter transformer,

converter valve, DC field equipment and control and protection

system accounts for 25 ~30%, 35%, 5~10% and 2% of the total

investment respectively. At present, the construction cost of

flexible DC converter station is about 30% higher than that of the

LCC station of the same capacity. Along with the wider

application and technological improvement of such devices, the

construction cost is expected to drop to the level of an LCC station.

Fig.3.8 Zhangbei VSC DC Project

29