Large-scale Ultra High Voltage Alternate/Direct Current Hydropower Absorption Problems

World Environmental and Water Resources Congress 2015: Floods, Droughts, and Ecosystems © ASCE 2015 Large-scale Ultra High Voltage Alternate/Direct Current Hydropower Absorption Problems Chun-Tian Cheng1,* , Jian-jian Shen1, Xiong Cheng1, Kwok-wing Chau2 Downloaded from ascelibrary.org by HONG KONG POLYTECHNIC UNIV on 11/13/16. Copyright ASCE. For personal use only; all rights reserved. 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 1 World Environmental and Water Resources Congress 2015 1904 Downloaded from ascelibrary.org by HONG KONG POLYTECHNIC UNIV on 11/13/16. Copyright ASCE. For personal use only; all rights reserved. World Environmental and Water Resources Congress 2015: Floods, Droughts, and Ecosystems © ASCE 2015 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 2 World Environmental and Water Resources Congress 2015 1905 Downloaded from ascelibrary.org by HONG KONG POLYTECHNIC UNIV on 11/13/16. Copyright ASCE. For personal use only; all rights reserved. World Environmental and Water Resources Congress 2015: Floods, Droughts, and Ecosystems © ASCE 2015 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). 3 World Environmental and Water Resources Congress 2015 1906 Downloaded from ascelibrary.org by HONG KONG POLYTECHNIC UNIV on 11/13/16. Copyright ASCE. For personal use only; all rights reserved. World Environmental and Water Resources Congress 2015: Floods, Droughts, and Ecosystems © ASCE 2015 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 4 World Environmental and Water Resources Congress 2015 1907 Downloaded from ascelibrary.org by HONG KONG POLYTECHNIC UNIV on 11/13/16. Copyright ASCE. For personal use only; all rights reserved. World Environmental and Water Resources Congress 2015: Floods, Droughts, and Ecosystems © ASCE 2015 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, 5 World Environmental and Water Resources Congress 2015 1908 Downloaded from ascelibrary.org by HONG KONG POLYTECHNIC UNIV on 11/13/16. Copyright ASCE. For personal use only; all rights reserved. World Environmental and Water Resources Congress 2015: Floods, Droughts, and Ecosystems © ASCE 2015 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 6 World Environmental and Water Resources Congress 2015 1909 Downloaded from ascelibrary.org by HONG KONG POLYTECHNIC UNIV on 11/13/16. Copyright ASCE. For personal use only; all rights reserved. World Environmental and Water Resources Congress 2015: Floods, Droughts, and Ecosystems © ASCE 2015 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. 7 World Environmental and Water Resources Congress 2015 1910 World Environmental and Water Resources Congress 2015: Floods, Droughts, and Ecosystems © ASCE 2015 Downloaded from ascelibrary.org by HONG KONG POLYTECHNIC UNIV on 11/13/16. Copyright ASCE. For personal use only; all rights reserved. 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 8 World Environmental and Water Resources Congress 2015 1911 Downloaded from ascelibrary.org by HONG KONG POLYTECHNIC UNIV on 11/13/16. Copyright ASCE. For personal use only; all rights reserved. World Environmental and Water Resources Congress 2015: Floods, Droughts, and Ecosystems © ASCE 2015 1912 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). References 9 World Environmental and Water Resources Congress 2015 World Environmental and Water Resources Congress 2015: Floods, Droughts, and Ecosystems © ASCE 2015 1913 Cheng, C.-T., J.-J. Shen, X.-Y. Wu,K.-w. Chau (2012). "Operation challenges for fast-growing China's hydropower systems and respondence to energy saving and emission reduction." 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