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Large-scale Ultra High Voltage Alternate/Direct Current Hydropower Absorption Problems
Chun-Tian Cheng1,* , Jian-jian Shen1, Xiong Cheng1, Kwok-wing Chau2
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1
Institute of Hydropower and Hydroinformatics, Dalian University of Technology, Dalian 116024
China. E-mail: ctcheng@dlut.edu.cn
2
Department of Civil & Structural Engineering, Hong Kong Polytechnic University, Hong Kong,
China.
ABSTRACT
With huge hydropower plants, such as Xiluodu, Xiangjiba, Nuozhadu and Jingping located in
the watershed of Jinshajiang, Yalongjiang and Lancangjiang being completed and put into
commission, hydropower transmission capacity in China is expanded rapidly. Large-scale
hydropower transmission has a direct impact on the rational allocation of electricity resources in
national scale, especially on the compromise between absorbing hydropower for sending end power
grids and shaving peak load for receiving end power grids. However, owing to the limited
regulation and absorption capacity of sending end hydropower plants, existing transmission
schedules of hydropower in China are based on own operating requirements or surplus electricity of
the sending end power grids. Thus it is vulnerable to “straight line” or “opposite peak shaving”
transmission schedules which in turn aggravate pressure to peak shaving load in receiving end
power grids. This is in opposite to the absorption and quality peak regulation capability of
large-scale hydropower. A new challenge for the coordinating operations is how to utilize load peak
and valley difference and the characteristics of different power sources to absorb surplus
hydropower from supplying power grids and to shave peak load for receiving power grids, so as to
exhibit the complementary roles of inter-basin cascade hydropower stations. The problem involves
optimization scheduling methods, compensation mechanism, peak thresholds and coordination
strategy among regional power grids, province power grids and plants. The key of the problem is to
solve the existing large-scale Ultra High Voltage Alternate Current (UHVAC)/ Ultra High Voltage
Direct Current (UHVDC) hydropower absorption in China. The purposes are to allocate power
resources more rationally, alleviate the pressure of the receiving end power grid from peak shaving,
improve the power source structures of Yangtze Delta and Pearl River Delta, reduce the haze
pressure in these areas, and effectively safeguard the safety, economy and environmental protection,
reliable operation of power grids in China
Keywords: Ultra High Voltage Direct Current; Hydropower; Peak shaving; Power transmission
from West to East; Large power grid
1 Introduction
After the implementation of the national strategy “Power transmission from West to East” for
15 years, China hydropower attains fast construction and development. Large hydropower stations
represented by Xiluodu, Xiangjiba, Nuozhadu and JingpingI&II have entered into fully completion
and commission period (Cheng, 2012; Zhao Xingang, 2012; Tang, W., 2013). Meanwhile, with the
successive commission of Ultra High Voltage Direct Current engineering from Yunnan to
Guangdong, from Xiangjiba to Shanghai, from Xiluodu to Western Zhejiang, from Nuozhadu to
Guangdong, and from Jinping to Sunan, China hydropower has entered into new era of large
capacity, long distance, trans-region, and trans-province mass transmission (Zhou, X., 2010; Huang,
D., 2009; Hennig, T., 2013; Chen, Q., 2014). As of 2013, the total hydropower transmission
capacity of the middle channel under “Power transmission from West to East” has reached 34.67
GW, accounting for about 17% of the maximum load of the receiving end East China power grid in
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2013. Amongst them, the maximum receiving electric scale of Shanghai power grid during summer
peak has exceeded 11.2 GW, accounting for about 47% of the maximum load on that day; the
receiving scale of Zhejiang power grid has continued around 15 GW, accounting for about 27% of
the maximum load of Zhejiang grid in 2013. The total transmission capacity of the south channel
has reached 34 GW, accounting for about 26% of the maximum load of China Southern power grid
in 2013. Amongst them, the maximum receiving electric scale of Guangdong power grid has
exceeded 27.7 GW, accounting for about 33% of the maximum load of the province in 2013.
Sichuan power grid and Yunnan power grid are the main sending-end grids of “Power transmission
from West to East”. The electricity transmission capacity of Sichuan power grid has reached 21.6
GW, accounting for about 30% of its total hydropower installed capacity. The electricity
transmission capacity of Yunnan power grid has reached 21 GW, accounting for about 31% of its
total hydropower installed capacity. With the continuous mass commission of hydropower bases in
Western China and the continuous expansion of the network of “Power transmission from West to
East”, the scale of clean hydropower transmission from Western China to the areas along the
southeast coastline will be gradually increased. Although it will greatly relieve the pressure of
energy shortage and environmental pollution in these areas, it will also bring many unprecedented
challenges on dispatching operation and management for sending-end and receiving-end grids.
In order to understand more clearly the background and problems of Ultra High Voltage
Alternate Current (UHVAC)/ Ultra High Voltage Direct Current (UHVDC) hydropower
transmission in China, this paper will briefly introduce the current status and future development of
hydropower in China, and the pertinent construction of ultra-high voltage grids. It will then
summarize the key problems and challenges arising from the mass transmission of
UHVAC/UHVDC hydropower in China and propose several ways to break the dilemma.
2 Hydropower development and construction of ultra-high voltage power grids in China
2.1 Hydropower development
After more than 20 years and especially over the past decade of rapid development of
hydropower in China, many milestone achievements had been made. Their scale of installed
capacity and rate of growth both set new records in the world hydropower history, which are
reflected in the following aspects:
1) Total hydropower installed capacity ranks first in the world. Since it exceeded 100 GW in
2004, exceeded 200 GW in 2010, and reached 280 GW at the end of 2013, its value is 2.75 times
more than the second rank in the world, i.e., the United States (102 GW for the United States in
2013). It is expected that, in 2020, it will reach 420 GW and that the newly added hydropower will
mainly be non-locally absorbed.
2) The hydropower development is highly concentrated. Hydropower resources in China are
mainly focused on large rivers. In recent years, the total installed capacity of hydropower stations
under construction and already built in Jinshajiang, Lancangjiang, Nujiang, Yalongjiang, Dadu river,
Wujiang, the middle and upper reaches of the Yangtze River, Nanpanjiang, Hongshuihe River, river
reaches along the mainstream of Yellow River accounted for 50% of the national total installed
hydropower capacity (Zhao, J., 2014).
3) The hydropower was developed at an astonishing pace. Only in 2013, the increment
amounted to 1.3 times the installed capacity of the Three Gorges, which is close to the scale of the
seventh rank of hydropower in the world, i.e., Norway. During the past decade, the added
hydropower capacity is 185.1 GW, with an average annual growth rate of 11.2%. Compared with
1949, the hydropower installed capacity was increased 778 folds by the end of 2013.
4) The scheduling scale was highly centralized and the number of power stations was increased
rapidly. By the end of 2013, the total hydropower installed capacity of the Southern power grid, a
single regional power grid, has exceeded 78 GW, beyond the scale of the fourth rank of hydropower
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in the world, i.e., Canada. By 2015, it will reach 100 GW, beyond the scale of the second rank of
hydropower in the world, i.e., the United States. It is the largest regional electricity system in the
world. The number of centrally-dispatched hydropower stations of Yunnan power grid, a single
provincial grid, hydropower was 17 in 2004, became 61 in 2008, increased to 109 in 2013, with
increment of 6.4 times in less than a decade. The hydroelectric capacity was 4.335 GW in 2003 and
was developed to 36.199 GW in 2013, beyond the scale of the seventh rank of hydropower in the
world, i.e., Norway, with increment of 8.3 times within a decade. The development rate of
hydropower system under the Sichuan power grid is even faster, i.e., from 12 GW in 2003 to 52.66
GW in 2013, with increment of 3.4 times within a decade.
5) Installed capacities of both single unit and power station were reaching new heights. It
began in 1960 when the 95 MW hydro-generating unit of the first large-scale Xinanjiang
hydropower station was self-designed and built in China. The then world's largest hydro-units of
700 MW were installed in various hydropower stations including the Three Gorges commissioned
in 2003, Longtan in 2007, and Xiaowan in 2008. The unit capacities of Xiangjiaba and Xiluodu
hydropower stations, commissioned in 2012 and 2013, have attained 800 MW and 770 MW,
respectively. It is expected that the unit capacity of Baihetan hydropower station currently under
construction will attain 1000 MW. After the mass and concentrated development of hydropower
during the past one or two decades, currently amongst the list of the top ten hydropower stations in
the world in term of installed capacity in commission or under construction, four stations come
from China. They are the Three Gorges, Xiluodu, Xiangjiaba, and Longtan hydropower stations,
with ranking the first, the third, the sixth, and the ninth, respectively (Wikipedia, 2014), as shown in
Table 1.
Table 1. List of largest ten hydroelectric plants
Yearly production
Rank
Plant
Country Installed capacity (GW)
(TWh)
1
Three Gorges
China
22.5
82.83
2
Itaip
Brazil
14
98.63
3
Xiluodu
China
13.86
28.55
Guri
10.235
16.2
4
Venezuela
5
Tucurui
Brazil
8.37
41.43
6
Xiangjiaba
China
7.75
18.38
7
Grand Coulee
USA
6.809
24.8
6.4
26.8
8 Sayano-Shushinskaya Russia
9
Longtan
China
6.3
18.7
10
Krasnoyarsk
Russia
6
20.4
6) The scale of long-range hydropower transmission increases rapidly. With the successive
completion and commission of giant hydropower station groups in Xiluodu, Xiangjiaba, Nuozhadu
and Jingping, and the operation of ultra-high voltage main power transmission projects including
±500 kV and ±800 kV Xiluodu and Nuozhadu UHVDC under the Southern power grid, and ±800
kV Fu-Feng UHVDC under the national grid, the maximum power transmission capacity of the
south and middle channels under “Power transmission from West to East” exceeds 68.67 GW. It is
expected that transmission scale will reach 100 GW in 2015, exceeding the total installed
hydropower capacity of United States which ranks the second in the world.
2.2 Construction of UHV power grid
The key initiatives to develop ultra-high voltage (UHV) in China are to alleviate energy supply
in local areas with shortage problem, reduce fossil fuel emissions, and realize long distance
transmission, large capacity, large-scale absorption of clean energy (Chen, Q., 2014;Yuan, J.,2012).
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It is an important safeguard to undertake emission reduction commitment on "non-fossil energy
consumption to attain 15% of primary energy consumption in 2020". According to "The Twelfth
Five Year Plan in China", in 2015, North China UHV power grid, Central China UHV power grid
and East China UHV power grid will form the main framework with "three longitudinal and three
lateral channels". Energy bases in Xilinguole League, West Inner Mongolia, Hebei, and Northern
Shanxi will supply electricity to areas in North China, East China, and Central China via three
longitudinal UHVAC channels. North coal electricity and Southwest hydropower will supply
electricity to UHV loop networks in North China, Central China, and Yangtze delta region via three
lateral UHVDC channels. Among them, the total length of ±500 kV and higher level DC
transmission line is about 46,762 km and the total transmission capacity is about 136 GW.
Figure 1. The cross-regional and cross-provincial extra-high voltage tie line map by 2014 in
China
3 Problems and challenges on UHVDC electricity transmission
3.1 Unbalanced power source structure
The power source structure in China is dominated by hydro-thermal power, where coal-fired
thermal power is in an overwhelmingly dominant position in the grid power source structure. By
2013, China's coal-fired electricity installed capacity is 820 GW, accounting for 65.8% of the
national total power installed capacity; oil-gas electricity is 42 GW, accounting for about 3.3%,
hydropower (including pumped storage) is 280 GW, accounting for about 22.5%; nuclear power is
15 GW, accounting for about 1.2%; wind power is 76 GW, accounting for about 6%; others such as
photovoltaic, accounting for about 1.2%. In comparison, the grid power source structure in the
United States (EIA, 2014) is more balanced. In 2013, coal-fired electricity installed capacity in the
United States is 298 GW, accounting for 29% of the national total power installed capacity; oil-gas
electricity is 424 GW, accounting for about 41.2%; hydropower is 102 GW, accounting for about
9.9%; nuclear power is 99 GW, accounting for about 9.6%; wind power is 61 GW, accounting for
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about 5.9%; photovoltaic is 14 GW, accounting for about for 1.3%; others accounting for about 3%.
It can be observed that, in the United States grid power source structure, quality peak shaving power
sources such as water, oil and gas contribute quite a large portion, accounting for about 51.1% of
the total installed capacity. However, in China, coal-fired electricity is still the main contributor in
grid power source structure, and the share of quality peak shaving power sources is relatively small,
only accounting for 25.8% of total installed capacity. This power source structure with poor
flexibility and complement can hardly cope with complex and varying load demands, especially
peak electricity demand, resulting in huge peak shaving pressure to be prevalently faced by power
grids in China.
With the gradual increase of the share of high quality Southwest hydropower for power grids
in areas along the Southeastern coastline, how to fully utilize the regulating performance of this
high quality hydropower with a view to better mitigating pressures on peak shaving and
environmental issues prevalent in Southeast coastal area becomes the core problem of large-scale
optimal allocation of hydropower in China.
3.2 Peak shaving pressure of power grids
East China power grid and Southern power grid are the major receiving end power grids on the
middle and South channels, respectively under “Power transmission from West to East”. In recent
years, along with the sustained and rapid economic development in the Yangtze Delta and Pearl
River Delta, power load is growing very rapidly, peak load record is scaling new height, load
difference between peak and valley is increasing in multiple, and power grids are facing very
serious peak shaving pressure. In 2013, the peak electricity load in East China power grid reached
209 GW and the maximum load difference between peak and valley exceeded 62.4 GW. When
compared to 2005, the increments are 140% and 118%, respectively. The electricity loads of four
provinces and one city under its jurisdiction also recorded historical high values in 2013, i.e., 29.36
GW in Shanghai, 77.48 GW in Jiangsu, 54.52 GW in Zhejiang, 26.58 GW in Anhui, and 27.98 GW
in Fujian. When compared to 2005, the growth rates are 99.5%, 167.4%, 167.3%, 205.6% and
157.4%, respectively. The peak electricity load in Southern power grid electricity load has reached
129 GW. The centrally-dispatched daily maximum peak-to-valley difference increases from 15.06
GW in 2003 to 49.11 GW in 2013, with a growth of about 2.3 times within a decade. Despite the
continuous increase of the share of hydropower within the grid, they are mostly in Yunnan and
Guangxi provinces and the peak shaving problem in Guangdong power grid is still very acute.
3.2 Hydropower absorption issues
Sichuan power grid and Yunnan power grid are the major sending end power grids on the
middle and South channels under “Power transmission from West to East”. With the vigorous
promotion of the exploitation on the Southern region in China and energy saving and emission
reduction strategies in recent years, large river basins including Jinshajiang, Lancangjiang,
Yalongjiang, middle to downstream of the Yangtze River, Dadu River, Hongshuihe River and small
river basins including Jialingjiang and Minjiang within its jurisdiction are undergoing basin-wide
development. The scale of hydropower is growing rapidly. As of 2013, centrally-dispatched total
hydropower installed capacities in Sichuan power grid and Yunnan power grid reached 52.66 GW
and 36.2 GW, respectively, accounting for 76.7% and 71.1% of centrally-dispatched total installed
capacities of the entire grid, and covering 31.7% of the national total hydropower installed capacity.
Future 5-10 years will be the peak production period of hydropower projects. It is expected that, in
2015, hydropower installed scales of Sichuan power grid and Yunnan power grid will reach 75 GW
and 64.62 GW, respectively. However, such scales of hydropower installed capacities far exceed
their own provincial electricity demands. Large-scale hydropower absorption will become the focal
problem to be concerned by Sichuan, Yunnan as well as by the nation. The key solution to this
problem facing Sichuan and Yunnan power grids will be the construction of large capacity, highly
efficient, and long distance advanced ultra-high voltage electricity transmission.
With the smooth commissioning of ±800 kV UHVDC from Xiluodu to Western Zhejiang,
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Sichuan grid and other provinces constituted "four AC and four DC" networking pattern (De-Bao
UHVDC, Fu-Feng UHVDC, Jin-Su UHVDC, Xi-Zhe UHVDC and four-circuit Sichuan and
Chongqing UHVAC channels). The hydropower transmission capacity attains a new record of 21.6
GW, accounting for 63.3% of the total transmission capacity of the middle channel under “Power
transmission from West to East”. Simultaneously, with the completion and commissioning of
Xiluodu UHVDC and Nuozhadu UHVDC, Yunnan power grid and other provinces constituted "six
AC and four DC" networking pattern. This ten-circuit transmission channel has maximum
transmission capacity of 21 GW and DC-channel transmission capacity of 16.4 GW, accounting for
61.8% of the total transmission capacity of the south channel under “Power transmission from West
to East”. Although the smooth commissioning of these UHV electricity transmission projects can
mitigate effectively the current pressure for delivery of Sichuan and Yunnan hydropower, it still
cannot meet the needs on mass development of clean energy such as hydropower. One of the
reasons is the serious time lag between the construction of the transmission channels and the
commissioning of hydropower stations, as well as the discrepancy between the transmission
capacity and the scale of hydropower stations. Owing to this, Sichuan power grid is fully promoting
the 1000 kV Ya’an-Wuhan UHVAC and ±1100 kV East Junggar-Sichuan UHVDC projects, and
Yunnan power grid is also vigorously constructing middle Jinshajiang-Liunan ±500 kV EHVDC
project. It is expected that they will be put into operation in 2017, when channel capacity and
restricted delivery of hydropower and other issues would then be greatly alleviated.
3.4 Current hydropower transmission problems
With mass and concentrated production in Southwest hydropower base and the constant
expansion of the scale of framework under “Power transmission from West to East”, trans-regional
and trans-provincial UHVDC hydropower will mass feed into load centers such as Eastern China
and Guangdong. However existing transmission schedules of trans-regional and trans-provincial
DC hydropower are mostly based on own operating requirements or surplus electricity of the
sending end power grids, and rarely considering electricity needs of receiving end power grids.
Thus it is vulnerable to “straight line” or “opposite peak shaving” transmission schedules. The DC
hydropower transmission did not alleviate peak shaving pressure of receiving end power grids.
Instead, it resulted in passive elimination of large amount of valley power and aggravated valley
regulation dilemma of receiving end power grids. It cannot undertake the role of quality
hydropower to realize peak regulation, is not conducive to security, economy and efficient operation
of receiving end power grids, and seriously constrains large-scale optimal allocation of Southwest
quality hydropower in China.
4 Ways to break the dilemma on UHVDC hydropower transmission in China
Large power grid platform refers to the formation of a huge power system via the
interconnection of several adjacent regional grids with UHVAC and/or UHVDC transmission lines.
There are large variations on load change pattern, power source structure, and basin-wide natural
water quantity for regional and provincial grids within its jurisdiction. These objectively determine
that large power grid platform has the functions of optimal allocation of power resources including
complementation of hydroelectric and thermal power resources, abundant and low electricity
replacement, trans-basin compensation regulation, peak-alternation regulation, and mutual spare. As
such, the utilization of load differences between sending and receiving end power grids, multiple
power source coordination at receiving end power grids, and price leverage for rational allocation of
power resources under large power grid platform are effective ways to break the dilemma on
UHVDC hydropower transmission in China.
4.1 Using load differences between power grids to coordinate hydropower transmission
Currently, there are mainly two transmission modes for UHVDC hydropower in China, namely,
single station delivery and bundled delivery. Single station delivery refers to the direct transmission
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of all power of the station via UHVDC transmission line to multiple receiving end power grids,
such as Xiluodu power station, and this transmission mode only involves multiple receiving end
power grids. Bundled delivery refers to the mode that, after having collected hydropower from
multiple sources, retains a portion of power at local grids, and transmits the surplus or agreed power
through concentrated UHV transmission line. Power stations such as Xiaowan and The Three
Gorges are typical examples. This mode of transmission involves a sending end power grid and
multiple receiving end power grids. From the viewpoint of geographical location, sending end
power grids and receiving end power grids are often located several thousand km apart. Affecting
by factors such as economic growth, temperature level, time lag on electricity usage, the difference
between total load demand and peak valley periods for each receiving end power grid under
different seasons is quite large. As such, rational hydropower transmission plans can be developed
by utilizing this load differences, resulting in a win-win effect on improving the hydropower
absorption scale of sending end power grids during flood season as well as on alleviating pressure
on peak regulation of receiving end power grids. Xiluodu hydropower station, which is put into full
operation recently, is used as a case study. Under the agreement, 9 units located at the left and right
banks of the station are operated by the State power grid and Southern power grid, respectively.
During flood season, State power grid (mainly transmits to East China power grid) and Southern
power grid (mainly transmits to Guangdong power grid) each accounts for 50% of electricity
amount. During dry season, State power grid accounts for 43% while Southern power grid accounts
for 57% of electricity amount. On the other hand, receiving end power grids often comprise
multiple provincial and municipal power grids under its jurisdiction, which often have large
variations in total load demands, occurrence times of difference between peak and valley loads,
number of peaks and valleys, and the share of transmission power, and hence also have large
complementary power capability. If this load difference can be fully taken into account, it will have
significant effect to improve the scale of hydropower export absorption and alleviate pressure on
peak regulation in multiple provincial power grids.
4.2 Using multiple power source coordination at receiving end power grids for hydropower
absorption
At present, many researchers have carried out extensive research in coordination of
hydroelectric and thermal power resources (Zambon, R., 2011; Yunan, Z., 2013; Rubiales, A, 2013;
Zhang, H., 2013; Nezhad, A., 2014; Martins, L., 2014). Both theory and practice confirm that a
multitude of power resources can significantly improve the load regulation level. Large power grid
platform often comprises various types of power sources with different characteristics on control
needs and constraint conditions, including general hydropower, coal power, oil gas power, nuclear
power, pumping storage, wind power, etc. Another effective way to break the dilemma on UHVDC
hydropower absorption problem is, through optimization of the operation of these power sources,
utilizing the different characteristics amongst these power sources for coordination of hydropower
absorption and hence improving the peak regulation capacity of receiving end power grids.
4.3 Using price leverage for improving the scale of hydropower transmission
Under the electricity market environment, electricity price is the main driving force on power
plant planning. An elevation of the price can stimulate more power plants to generate peak
electricity more frequently. The establishment of a reasonable peak-valley time-sharing electricity
price and peaking compensation mechanism can promote more quality power resources to attain
rational allocation. At present, China's electric power system is undergoing a deepened reform,
basically realizing the isolation of plants and grids, gradually opening into business mode of power
generation side, and forming preliminary electric power marketization. The development of a
rational pricing mechanism can arouse the enthusiasm of external hydropower peak regulation,
improve the scale of external hydropower absorption and alleviate peak shaving pressures of
receiving-end power grids.
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5 Key research theories and technical problems
From the perspective of power system operation, large-scale hydropower trans-regional and
trans-provincial coordinated optimization is actually a very complex coupled multiple power grids
and multiple power sources optimal scheduling problem, involving key technical problems such as
optimization of groups of large-scale hydropower stations, coordinated optimization of multiple
power sources in receiving end power grids, power distribution amongst provincial power grids at
the receiving end, etc. At present, researchers are more focused on building models on coordination
of hydro and thermal powers within a single provincial power grid (Farhat, I., 2011; Ferreira, V.,
2011) or within a single regional power grid (Hernandez, H., 1991; Cheng, C., 2011; Chung, K.,
2011; Chung, K., 2012). Literature on trans-regional compensation scheduling is very rare. Very
few research studies have been undertaken coupling the current and future problems on large-scale
hydropower transmission in China. As such, there is an urgent need for research on theory and
practice in this area.
5.1 Research on load characteristic analysis and coordinated optimization of multiple power
grids
Current large-scale hydropower transmission is determined according to water inflow of the
power generation end or electricity surplus situation of sending end power grids. DC hydropower
transmission plan is compiled without considering the actual load demands of receiving end power
grids. It not only does not carry out the peak regulation function of Southwest high quality
hydropower, but also further aggravates pressure on peak regulation of valley power for receiving
end power grids. From the perspective of large power grids, the critical tasks of the research study
will be load characteristic analysis and coordinated optimization among different power grids, in
order to avoid the occurrence of “straight line” or “opposite peak shaving” transmission schedules.
On the other hand, large-scale external hydropower transmission to multiple provincial receiving
end power grids will be optimally allocated. At the same time, under the premise of fulfilling the
prescribed proportion of electricity quantity in the agreement, consideration should be made to the
differences in load demands among provincial power grids. Focus should also be made on building
coordinated optimal operation models and solutions among regional and provincial grids
considering network security constraints, which is the key to absorb large-scale hydropower during
flood season as well as to perform the function of peak regulation of hydropower.
5.2 Research on coordinated optimization methodology of multiple power sources
Coordinated operation of multiple power sources in large power grid platform is one of the
outstanding operation problems facing power grids in China with UHVAC and UHVDC coupled
hybrid conditions. With the rapid increase of the scale of UHVAC and UHVDC power transmission
and the ever expansion of the scale of power load, this problem becomes more tricky and urgent.
This has become common scheduling and management problems facing regional, provincial and
municipal level power grids including East China power grid, Southern power grid, Zhejiang power
grid, Shanghai power grid, Jiangsu power grid, and Guangdong power grid. However, owing to the
involvement of numerous power sources, the differences of operating characteristics, constraints
and control needs of various types of power sources are quite large. Even for the same type of
power, its regulation performance can also be very different. For instance, cascaded hydropower
systems often comprise different types of power plants such as annual regulation, seasonal
regulation, daily regulation, runoff type, etc. Focus should be made on building coordinated optimal
operation models and solutions of multiple power sources considering differences of power source
characteristics, network security constraints and operational control constraints, which is the key to
absorb large-scale hydropower during flood season, to perform the function of peak regulation of
hydropower, as well as to realize energy-saving emission reduction of power grids.
5.3 Research on price compensation mechanism
Large-scale hydropower transmission involves not only technical issues, but also management
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issues and economic interests. On one hand, large-scale external hydropower absorption during
flood season is bound to increase the depth of peak shaving pressure of thermal power at receiving
end power grids and damage the interest of thermal power plants. As such, establishing a win-win
compensation mechanism is extremely important. Marginal compensation pricing, analysis methods,
and strategies of power transmission are the key technical difficulties in solving the problem. On the
other hand, the shifting of power transmission between AC and DC modes under the UHV large
power grid platform with more locations undertaking peak regulation role must bring about
conflicts among sending end and receiving end power grids, and among multiple receiving end
power grids. This will inevitably increase the pressure on peak regulation of these power grids
during peak period. As such, another difficult problem needed to be solved in this research is to
determine a reasonable peak regulation threshold value. This directly relates to whether or not the
power distribution among sending end and receiving end power grids during peak period is
reasonable, and whether or not their interests are balanced.
6 Conclusions
With the full commissioning of Three Gorges, Xiluodu, Xiangjiaba, and Nuozhadu as the key
backbone projects under the national “Power transmission from West to East” strategy, and the
construction of the associated UHVAC/ UHVDC power grids, China’s hydropower and grids have
entered into UHV grid interconnection era. The two main tasks of UHVAC/ UHVDC hydropower
transmission are long distance, trans-regional, trans-provincial, trans-basin large scale hydropower
absorption and peak regulation. However, constrained by various factors including regulation
capacity of hydropower stations, grid security constraints, power source structures of sending end
power grids and/or receiving end power grids, conflicts of interests, mechanisms and institutional
factors, existing UHVAC and UHVDC power transmissions are more dependent on experience and
administrative measures. It does not fully realize the quality peak regulation role of hydropower and
its effective absorption. There are needs, from the national strategic and practical perspectives, to
advance theoretical and technological research on large-scale UHVAC and UHVDC hydropower
transmission issues and to break the current dilemma. Theoretical and practical experiences in the
coordination of power grids in regional and provincial levels under East China power grid have
demonstrated that, utilizing differences on load characteristics among grids, multiple power sources
characteristics, water inflow process in basin, and regulation performance among groups of
hydropower stations, the benefits brought about by rational optimization of hydropower
transmission are enormous. Considering the prospective huge scale of hydropower transmission in
China, in-depth research studies addressing theoretical and practical large-scale UHVAC and
UHVDC hydropower transmission problems become important and urgent. Results of the research
studies will be able to facilitate more rational allocation of power resources, alleviate the pressure of
the receiving end power grid from peak shaving, improve the power source structures of Yangtze
Delta and Pearl River Delta, reduce the haze pressure in these areas, and effectively safeguard the
safety, economy and environmental protection, reliable operation of power grids in China.
Acknowledgements
This research was supported by National Science Fund for Distinguished Young Scholars (No.
51025934) and National Natural Science Foundation of major international cooperation (No.
51210014).
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