Overview of China's hydropower absorption: evolution, problems, and suggested solutions

Abstract

China has experienced an unprecedented increase in hydropower development with the implementation of the ‘West–East Electricity Transfer’ project. Its total hydropower capacity has reached 350 GW, of which nearly one-third is transmitted to the load centre through an ultra-high-voltage power network. However, the absorption of abundant hydropower in southwest China is still a challenge, with increasing hydropower curtailment each year. This study provides an overview of the evolution of hydropower absorption, analyses the major problems and possible causes, and suggests several technical solutions. It is suggested to optimise the generation operations with power network limitations and transmission schedules to make full use of transmission channels and improve operational flexibility. Meanwhile, receiving power grids should coordinate the operations between southwestern hydropower and their local plants, and hydropower allocation among subordinate power grids. The differences in operation characteristics, regulation ability, and load demands will be helpful for efficiently absorbing large-scale outer hydropower. Reasonable economic incentives and compensation mechanisms are considered as another method to alleviate hydropower curtailment. Ancillary service market and discrepant electricity prices during different receivers and different periods are suggested. The overall analysis results indicate that there is great space for promoting hydropower absorption under existing transmission channel conditions.

1 Introduction

China has tremendous hydropower potential. According to the latest general investigation report of hydropower resources, the national technical and economic hydropower potentials are ∼542 and 402 GW, respectively [1]. Over the last two decades, rich hydropower resources have been extensively exploited to urgently meet increasing needs for electricity and reduce severe greenhouse gas emissions. By the end of 2018, the nation's installed hydropower capacity reached 350 GW, remaining as the largest source of renewable electricity generation and occupying ∼18.4% of the total generation capacity [2]. However, due to the uneven distribution of hydropower resources (see Fig. 1), the majority of hydropower capacity is concentrated in several western provinces, especially in southwest China (e.g. Sichuan, Yunnan, Hubei, Guizhou, and Guangxi provinces) [3, 4]. These provinces are main hydropower producers and also exporters of hydropower because the increasing hydropower capacity substantially exceeds their local electricity demands [5]. The ‘West–East Electricity Transfer Project’ (WEETP) proposed by the central government aims to transmit large-scale electricity in these provinces eastward via high/ultra-high-voltage (UHV) power networks.

Fig. 1

Distribution of provincial hydropower capacity in China, 2018

Currently, there are 16 UHVDC lines related to hydropower projects, whose maximum transmission capacity exceeds 64 GW. Nonetheless, the planned construction of UHV transmission projects still lags far behind the development of hydropower plants. The limited transmission capacity, in part, has led to large-scale hydropower curtailment in recent years. The real data show that hydropower curtailment is becoming part of the new normal in hydro-dominated provinces in southwest China [6]. For example, Yunnan province has been suffering from continuous hydropower curtailment since 2012. Moreover, curtailed hydropower has surged from 2.4 TWh in 2012 to about 29 TWh in 2017 [6, 7]; the amount during these years is approximately equal to the annual electricity production from the Three Gorges Hydropower Project. The lack of adequate coordination between hydropower producers and main receivers is one of the major reasons for hydropower curtailment. Specifically, developing schedules for hydropower transmission via UHV lines heavily depends on the operation requirements of hydropower plants or the power surplus of the aforementioned hydropower producers, which largely ignores the power demands of receivers. As a result, the high penetration of hydropower leads to new operational challenges regarding coal-dependent power grids in receiving areas. These receivers often complain and express great unwillingness to accept unreasonable hydropower schedules [8, 9]. This situation has made a serious impact on hydropower integration.

The export and consumption of hydropower from southwest China have attracted increasing attention from governments, power grid companies, electricity generation corporations, and research institutions. This topic is widely called hydropower absorption in this country. In recent years, there have been a number of studies on hydropower development and absorption in China [10-12]. A few studies have focused on hydropower curtailment [3, 6, 13, 14]. These works provide a good basis for promoting hydropower absorption and reducing curtailment but the real technical solutions to hydropower absorption, especially in receiving power grids, are still lacking. Hence, this paper presents a comprehensive review of hydropower absorption in China, considering its evolution, problems, and solutions. Four major problems of the hydropower are presented, i.e. rising hydropower curtailment, lack of operational flexibility for transmitted hydropower, great difficulty in hydropower integration during off-peak periods, and unreasonable power allocation among multiple power grids. The first problem is related to the hydropower system operation at the generating side, while the other problems focus on the hydropower absorption of receiving power grids. The detailed analysis about these problems and suggested solutions will be given in the later sections.

The remainder of this paper is structured as follows. The next section describes the evolution of hydropower absorption in China from the perspective of transmission range and scale, and the main characteristics of hydropower transmission via the UHV electric network. Section 3 summarises the problems with large-scale hydropower absorption and, correspondingly, the technical solutions for these problems are proposed in Section 4. Section 5 presents a case study and the last section shows the conclusions.

2 Evolution of hydropower absorption in China

2.1 Evolution of hydropower transmission range

China has experienced the sustainable growth of electricity consumption along with the implementation of reform and opening up. During the early stage, the constructed hydropower plants that were generally installed with small generating capacity mainly provided electricity for local consumers. In the developed eastern and coastal areas, however, the increasing needs for electricity were so rapid that scarce hydropower resources were insufficient to support them. Thus, power shortages often occurred and became increasingly serious, though most hydropower resources in these areas have been exploited. In contrast, the West, especially the southwest region, is endowed with rich hydropower resources but has a relatively backward economy. In this situation, interprovincial and interregional power transmission from West to East is extremely necessary and important. In 1972, the first 1 GW-level hydropower plant in China (named Liujiaxia in the Yellow River) was put into operation to offer power for Shanxi province via a 330 kV transmission line [15]. In 1988, the largest UHV (±500 kV) DC transmission project in Asia was commissioned to connect the Gezhouba hydropower plant in the Yangtze River with Shanghai in eastern China [16]. In 1993, Yunnan and Guizhou provinces, which had large hydropower capacities, started to supply power to Guangdong province through an interprovincial power network in the China Southern Power Grid (CSG) [17]. These projects are important symbols of interprovincial and regional hydropower transmissions and operations in China.

With rapid economic growth in recent two decades, this country hasexperienced an increase in the construction of power grids, especially UHVtransmission and transformation projects. As one of two main hydropowerreceivers, the CSG has instituted 8 AC and 10 DC transmission lines, totalling18 channels in the West-to-East Power Transmission Project, with a maximumtransmission capacity of 50 GW [18].Through the UHV network, Guangdong, Guangxi, Yunnan, Guizhou, and Hainan, aswell as Hong Kong, Macao, and several southeast Asian countries are connected.The State China Grid (SCG) also placed seven interregional DC transmission linesinto operation to transmit hydropower of Sichuan and Central China into the Eastof the country, mainly offering power for Shanghai, Jiangsu, Zhejiang, and Anhui[19]. Moreover, the CSG and SCG areconnected by the Three Gorges and Xiluodu hydropower projects that are,respectively, operated by these two power grids simultaneously. Currently, theinterconnected UHV power network is capable of supporting large-scale andlong-distance power transmission nationwide. Fig. 2 shows the UHVDC power network for transmittinghydropower in 2017 and, correspondingly, Table 1 lists the details of each DC transmission line.

Table 1. Main characteristics of UHVDC lines for hydropower transmission inChina

Line	Hydropower plant	Beginning place	Ending place	Voltage, kV	Transmissioncapacity, GW
Tianguang	Tianshengqiao, Longtan	Guizhou	Guangdong	±500	1.8
Sanguang	Three Gorges	Hubei	Guangdong	±500	3
Jinzhong	Liyuan, Ahai	Yunnan	Guangxi	±500	3.2
Xingan	Cascade plants on Wujiang River	Guizhou	Guangdong	±500	3
Niucong	Right bank of Xiluodu plant	Yunnan	Guangdong	±500	6.4
Chusui	Xiaowan, Jinanqiao	Yunnan	Guangdong	±800	5
Puqiao	Nuozhadu	Yunnan	Guangdong	±800	5
Xindong	Cascade plants on the upstream ofLancang River	Yunnan	Guangdong	±800	5
Longzheng	Three Gorges	Hubei	Jiangsu	±500	3
Yihua	Three Gorges	Hubei	Shanghai	±500	3
Linfeng	Three Gorges	Hubei	Shanghai	±500	3
Genan	Gezhouba	Hubei	Shanghai	±500	1.1
Jinsu	Jinping	Sichuan	Jiangsu	±800	7.2
Fufeng	Xiangjiaba	Sichuan	Shanghai	±800	6.4
Binjin	Left bank of Xiluodu plant	Sichuan	Zhejiang	±800	8

Fig. 2

UHVDC power network for hydropower transmission inChina

2.2 Evolution of the hydropower transmission scale

The growth rate of interregional and interprovincial power transmission in China is globally unprecedented. As shown in Fig. 3, the interregional transmission of electricity energy increases from 81.6 TWh in 2006 to 423.6 TWh in 2017 [20]. Among six regions, the southwest, northwest, and central China account for 75.4% of the total exported electricity in 2017. By contrast, the East China received 45% of the total interregional transmission of electricity energy. The annual growth rate is up to 15%. The most outstanding growth occurred in 2013, with 33% or 66.2 TWh. The pace seems to slowdown in recent years but the newly added transmission scale of electricity energy still keeps relatively large, as shown in Fig. 3b.

Fig. 3

Interregional transmission of electricity energy in China. Data source [20]

(a) Total transmission scale of electricity energy every year, (b) Newly added transmission scale of electricity energy every year

A rapid increase can also be observed from several main hydropower suppliers and receivers. For CSG with rich hydropower resources, the interprovincial transmission of electricity energy rose from 67.8 TWh in 2006 to nearly 202.8 TWh in 2017 [18]. Fig. 4 shows a trend of transmission of annual electricity energy. There is a significant increase of 200% in the past decade because a batch of large hydropower transmission projects listed in Table 1 were completed during this period to offer transmission channels for the newly commissioned large hydropower plants (LHPs) on the main branches of the Lancang River, Jinsha River, and Wujiang River. Most of these plants are located in Yunnan Province and operated by subordinate Yunnan Power Grid (YNPG). Therefore, Yunnan is the most important hydropower exporter in CSG.

Fig. 4

Interprovincial transmission of electricity energy in CSG from 2006 to 2017. Data source [18]

Fig. 5 describes the exported electricity from Yunnan since 2006 [21]. It is obvious that the electricity exports have seen a larger increase. The total in 2017 was 124.22 TWh, about 10.5 times of the one in 2006. The maximum transmission scale has reached about 31 GW, showing an annual growth of 30%. Fig. 6 depicts the evolution of the imported electricity into the East China Grid (ECG) in the past several years [20, 21]. The average annual growth rate also achieves 30%, which is far higher than the national average. In 2017, ECG received 191.3 TWh from other provinces, accounting for more than 10% of its total electricity consumption. By the end of 2017, the imported power has reached up to 61.1 GW that occupied 22% of the maximum load of ECG. External hydropower has become an important electricity source for the ECG, especially in Shanghai, Jiangsu, and Zhejiang. According to the ‘13th Five-Year Plan’ of hydropower development, interregional and interprovincial power transmission will continue to retain a rapid growth trend with the additional exploitation of main hydropower bases in southwest China.

Fig. 5

Exported electricity in YNPG from 2006 to 2017. Data source [21]

Fig. 6

Imported electricity in ECG from 2012 to 2017. Data source [20, 21]

2.3 Characteristics of hydropower transmission via the UHV network

• (i) Large transmission capacity: As mentioned above, the commissioned DC lines in China have allowed 70 GW of hydropower transmission from West to East. This indicator is expected to increase to nearly 100 GW by 2020. The UHV transmission project from Xiluodu hydropower plant to Zhejiang province has a maximum transmission capacity of 8 GW [22], leading DC transmission lines.
• (ii) Long transmission distance: Most of China's LHPs are located far away from the load centres, resulting in long-distance power transmission. The overall length of the UHV lines with 500 kV and higher voltages has exceeded 236,600 km by the end of 2017 [23]. Among the working UHVDC transmission projects related to hydropower, 11 transmission lines are more than 1000 km long. The longest DC transmission distance is 2059 km for the Jinsu UHV project.
• (iii) Complex power network connecting hydropower plants: The massive hydropower plants in southwest China are connected to receiving power grids via various complex power networks. For a single hydropower project, taking the Three Gorges hydropower plant as an example, different generating units are able to access several provincial and regional power grids via four UHVDC lines. The upstream and downstream hydropower plants on the same river serve different provincial power grids such as the cascading hydropower plants on the Hongshui River. In contrast, multiple hydropower plants located on different rivers may also be integrated into the same power grid through an UHV transmission line. For instance, the Xiaowan plant in the Lancang River and the Jinanqiao plant in the Jinsha River are integrated with the Chusui DC line to supply power for Guangdong province. Such a complex power network means substantially different electricity demands, high security requirements, strict electrical limitations, many relevant companies with different stakeholders, and the scheduling of hydropower transmission. These factors pose great challenges to the operation of China's large-scale hydropower system.
• (iv) Unreasonable energy structure in receiving power grids: Presently, coal-fired thermal power dominates China's energy system (see Fig. 7) [2, 20], which is greatly different from the energy structures of the United States and Brazil (the top two hydropower capacity countries, except China). In 2018, the thermal power supplied 70% of the nation's total electricity (see Fig. 8). In particular, the proportion is higher in the recipient power grids of hydropower such as 76% in Guangdong, 90% in Shanghai, and 95% in Jiangsu. Such type of energy structure lacks the required flexibility for following the load demands buffering intermittent renewable power, and especially coordinating with southwestern hydropower.
• (v) Inadequate coordination between hydropower suppliers and receivers: There is a common lack of coordination between hydropower suppliers and receivers. Currently, determining transmission schedules depends primarily on the electricity surplus of the hydropower supplier. In most cases, this approach leads to poor power transmission schedules that are inconsistent (or even opposite) with the load curves of the receiving power grids. Fig. 9 shows these situations for three UHVDC lines, named Yihua, Longzheng, and Genan. The hydropower receivers have to passively absorb a large amount of power during off-peak hours and, thus, face additional pressure for peak regulation. On the other hand, the imported hydropower needs to be further allocated into multiple provincial power grids by a regional dispatching authority to meet their peak demands simultaneously. However, the conventional method, which allocates power generation for each period using the fixed ratios specified in the electricity contracts, fails to give efficient quarter-hourly operational schedules. This method may result in a prominent contradiction between responding to peak demands and absorbing external hydropower.
• (vi) Incomplete electricity market: China has a vertically integrated monopolistic power system, though a new reform of the electricity market was launched in 2015 [24]. Generally, governments, national/regional/provincial power grid corporations, and generation corporations negotiate the electricity contracts and schedules for power transmission via UHV lines. This approach effectively addresses the trade-offs between multiple different stakeholders but easily leads to inefficient operation schemes and transmission schedules. Thus, new operational challenges such as peak shaving for multiple power grids and hydropower curtailment are posed.

Fig. 7

Power structure of China by the end of 2018 (unit: GW). Data sources [2, 20]

Fig. 8

Electricity generation from different power sources of China in 2018 (unit: TWh). Data sources [2, 20]

Fig. 9

Power transmission schedules and load curve in ECG

3 Problems facing large-scale hydropower absorption

Renewable energy generation rejection is a common problem facing hydropower, wind power, and solar power in China. There are some similarities between hydropower absorption and wind/solar integration such as inconsistency between power source development and the power network construction, slowdown of the economic growth in resource-rich areas etc. Meanwhile, there also exist differences for these power sources. Wind/solar power generation is characterised as randomness, intermittence, and poor regulation ability, which has seriously restricted large-scale integration into power grid. Different from wind/solar power, hydropower has good regulation performance, and thus is capable of providing peak shaving, frequency modulation, and other ancillary services. However, while large-scale hydropower is transmitted to the eastern power grids via UHVDC lines, hydropower absorption suffers from new problems and difficulties. The transmission schedules do not give full play to the regulation flexibility of hydropower. Moreover, there is no effective response to load demands of power grids, and the power distribution among multiple receiving power grids is usually inflexible even unreasonable. These problems lead to the difficulty of hydropower absorption especially in low-valley period. The following sections present a detailed analysis about four major problems.

3.1 Rising hydropower curtailment

Along with the rapid development of power sources, hydropower curtailment in southwest China has continued to rise in recent years. According to the China Electricity Council, hydropower in Yunnan and Sichuan (two of the largest hydropower provinces) was curtailed by more than 29 TWh in 2017, accounting for nearly 10% of the total hydropower production. Hydropower curtailment increased by 11 times since 2012, shown in Fig. 10. It is foreseeable that more curtailment may occur with massive hydropower projects under construction such as the Wudongde and Baihetan projects, commissioned for the coming years. One of the main reasons is the construction of planned UHV transmission projects that are lagging far behind hydropower development. In addition to the objective conditions, the lack of coordinated operations between LHPs and small-sized hydropower plants (SHPs) and other types of renewable power sources such as wind and solar power are other major technical reasons.

Fig. 10

Hydropower curtailment in recent years in Yunnan. Data sources [25, 26]

Many SHPs exist in hydropower-rich provinces such as Yunnan [27, 28]. During the flood season, these SHPs face great water spill pressure due to very small-regulated storage. These plants must generate their respective maximum installed capacities when floods occur. However, the amount of electricity generated from SHPs is difficult to externally transmit and integrate into the main provincial electricity power network, as finite transmission channels are occupied by large-scale SHPs and LHPs [29]. The key problem regarding the congestion of electricity transmission is attributed to inadequate coordination among LHPs and SHPs, which is an important operational challenge that has always been ignored. On the other hand, large-scale renewable energies such as wind and solar powers have experienced rapid development in southwestern provinces [25, 26], which is a top priority according to electricity use policies in China, inevitably affecting the utilisation hours of hydropower systems. Moreover, since wind and solar powers are highly intermittent and unpredictable, dispatchable generation, especially hydropower, is required to support their high penetration. In this case, large-scale hydropower capacity is usually retained for system reserves to ensure electric grid reliability, which severely restricts hydropower generation and easily leads to hydropower curtailment. The coordinated operation between hydropower and other renewable energies is necessary and significant in addressing large-scale hydropower curtailment.

3.2 Lack of operational flexibility for transmitted hydropower

Hydropower-receiving provinces in China commonly lack generation flexibility because of the coal-dominated energy structure. These provinces are always under tremendous pressure to respond to the peak demands of power grids. Dispatchable resources such as hydropower are, therefore, needed to alleviate this pressure. However, as mentioned above, southwestern hydropower generation often fails to provide the required operational flexibility for recipient power grids due to inadequate coordination between hydropower suppliers and receivers. More seriously, in some cases, the transmission schedule curves via UHVDC lines show an opposite trend with the load demands of the receiving power grids, aggravating the peak-shaving pressure.

In fact, the current way of developing transmission schedules is devoted to utilising the capacity of UHVDC lines as much as possible but it neglects the operational flexibility of hydropower. For example, the Shanghai Power Grid (SHPG), which is a main hydropower receiver, accepted 66.1 TWh via UHV lines in 2017 [20]. This external electricity accounted for 43.3% of the total electricity consumption but contributed only a very small amount of dispatchable power (no more than 15% of the maximum transmission capacity). The main reason is that the generation schedules and operations of hydropower plants in the southwest region have no regard for the real electrical demands of receiving power grids, which means that the optimisation models currently used for developing generation and transmission schedules are unreasonable or inefficient. It is necessary to consider the load demands and security constraints of receiving power grids in the optimisation model, as it is conducive to improve transmission schedules and alleviating hydropower curtailment.

3.3 Great difficulty in hydropower integration during off-peak periods

The provincial electrical load curves in China exhibit obvious changes in 1 day. Generally, light loads occur at night-time, whereas the high loads occur during work and recreational times. In most provinces, the high load is far larger than the light load and may be several times larger. Such large load differences between the peak and off-peak periods have compelled many coal-fired units to follow load changes in the coal-dominant power grids. In this case, the coal-fired units are required to generate with the minimum generation capacity during off-peak hours and gradually increase their output with the increasing loads, which leads to great difficulty in the operation and unit commitment of power plants in hydropower-recipient regions. The reason is that large-scale southwestern hydropower combined with local coal-fired generation largely exceeds their load demands during off-peak hours. Absorbing the external hydropower means that parts of the coal-fired units have to be shut down during these periods. However, this approach easily leads to insufficient peak power during high load hours since restarting coal-fired units is extremely unscientific and uneconomical [30]. Such a situation has been a bottleneck for large-scale power transmission and further hydropower development.

For example, the maximum load difference within 1 day in the SHPG reached 11.9 GW in 2018, ∼40% of the maximum load. As one main recipient of southwestern hydropower, the SHPG receives large-scale southwestern hydropower to satisfy peak demands but struggles to absorb it during off-peak hours. As a result, many large coal-fired units have been asked to work with a specified minimum rate (48%) of the installed capacity or to be shut down during off-peak hours. As another example, the Guangdong Power Grid (GDPG) encounters similar difficulties with the SHPG. This power grid received 202.8 TWh from external plants and provinces in 2017 [18]. To accomplish the task of hydropower absorption, the GDPG has to compel local coal-fired units to greatly curtail the output during light load hours. Technically, this is a complex coordination operation between large-scale UHVDC hydropower and local power plants in receiving regions. The unit commitment and loading of such a hybrid power system needs to be reasonably described and efficiently optimised subject to complex operational conditions and constraints of power plants, power grids, and transmission lines, which is a great operational challenge.

3.4 Unreasonable hydropower allocation among multiple power grids

As mentioned before, southwestern hydropower is simultaneously transmitted to multiple regions or provinces. The allocation ratios among these receivers are usually specified in multilateral electricity contracts. Using the fixed ratios, the central dispatching authority allocates the hydropower generation during each period to determine the quarter-hourly transmission schedule for each receiving power grid. This technique is a conventional method that ensures contract requirements but easily leads to the same power transmission profiles for all receivers. Such operational schedules are irrational or inefficient because of greatly different load demands among power grids [31, 32]. More specifically, the differences in the daily load curves of these power grids are reflected in the magnitude, peak value, peak number, and times of the peak and light loads, which require various hydropower generations to respond to their respective electrical demands. The ECG, which is the main receiver of middle channel of the WEETP in China, is a typical example.

In practise, the ECG is in charge of allocating the input hydropower among its provincial power grids including the SHPG, Jiangsu Power Grid, Zhejiang Power Grid, and Anhui Power Grid. The ECG attempts to use hydroelectricity to satisfy the different peak demands of the provincial power grids because coal-fired thermal units with low operational flexibility dominate the four provincial energy systems. As we see from Fig. 11, however, large load differences make the conventional allocation method impossible to obtain rational and efficient hydropower schedules for all of the power grids, which is a complicated problem within hydropower configuration. In the problem, the differences in the total received electricity specified in the contracts impose additional complexities in solving this problem. In fact, this problem involves complex peak-shaving needs from multiple power grids, with various spatiotemporal operational conditions and constraints over the entire horizon. The focus is on the allocation of peak power, which is greatly different from the peak operation of a single provincial power grid and is mathematically more difficult to describe and solve.

Fig. 11

Different load demands from multiple provincial power grids in ECG

4 Solutions for boosting hydropower absorption

Comprehensive measures are needed to boost hydropower absorption and eliminate or alleviate hydropower curtailment. On the basis of the aforementioned analysis, five technical solutions and policies are provided below.

4.1 Optimise generation operations with power network limitations

In southwest China, hydropower resources and renewable energies such as wind power are usually far from the load centres and need to connect to the main power network through multiple voltage levels. Various transmission congestions frequently occur when power from multiple types of plants in low-voltage grids is bundled into higher-voltage power grids. Thus, optimising generation operations with power network limitations [such as (1)–(3)] to upgrade the utilisation of transmission channels will be beneficial for increasing hydropower absorption. Two methods are available.

• Transmission power limit
  (1)
  where and are the upper bound and lower bound of UHVDC transmission power of line l in period t, respectively; denotes power generation of plant m; m is the plant index; and is the total number related to line l.
• Maximum variation of transmission power
  (2)
  where is the maximum variation of transmission power via UHVDC line l.
• Stability requirement of transmission power during multiple successive operating periods
  (3)
  where v is the minimum number of time periods, where the transmission power is local extremum.

The first method is to coordinate LHPs and SHPs by regulating the storage of large hydropower reservoirs. The coordinated operation is helpful to determine efficient generation schemes to shift the surplus of hydroelectricity from the flood season to the dry season, thereby vacating some transmission capacity for SHPs, which will be beneficial for the alleviation of curtailment and spilled water. In this method, the formulation of a transmission network shown in (1)–(3) is vital to develop a practical coordination model. Maximising the total electricity absorption, which implies balancing high average water heads with reservoir spills, can be selected as the objective of the model. Here, the electricity absorption is equal to total electricity that will be consumed or transmitted.

The other method is to coordinate hydropower plants with wind and solar power sources [33, 34]. The complementary characteristics of inflow, wind speed, and sunlight in southwestern hydropower bases can help to reduce volatility of the combined power. This method has been considered in some real-world projects such as Yunnan [35] and Yalong River [36]. Specifically, electricity production coordination should be carried out in long- and medium-term operations to determine the total scale of electricity transfer and reasonable reservoir level range at different key periods, with the consideration of wind and solar electricity prediction. In short-term operations, the regulation capacity and rapid adjustment ability of hydropower can be used to compensate the fluctuation of wind and solar power generation. The wind and solar power forecast errors can be described as proper operation constraints of hydropower plants to ensure that the optimised hydropower schedules can effectively smooth the renewable power fluctuation. The reserve constraints are commonly used, as shown in (4) and (5).

• Positive reserve constraint
  (4)
• Negative reserve constraint
  (5)
  where and are the required positive and negative reserves of the wind and solar power forecast errors, respectively; and are the minimum and maximum generation limitations of hydropower plant i during period t, respectively; is the power generation of plant i during period t; and is the total number of hydropower plants.

4.2 Optimise transmission schedules to enhance operational flexibility

SCG and regional power grids are currently responsible for determining the daily transmission schedules of UHV lines, which often pay much attention to the security needs of power grids so that the operational flexibility of hydropower may be underutilised. Optimising transmission schedules to enhance operational flexibility will increase the willingness of receiving power grids to absorb more hydropower [8, 37], as they can use the dispatchable hydropower to meet peak demands. This paper considers two situations for the optimisation of transmission schedules.

While the total power transmission profile over 1 day is fixed, coordinating the allocation of hydropower among multiple UHVDC lines will be beneficial to generate efficient and practical transmission schedules. For instance, hydropower plants along the main streams of the Lancang River and Jinsha River bear the main responsibility of electricity export in Yunnan province. They are connected to three different UHVDC lines and AC power network. In practise, the total amount of interprovincial transmitted electricity and the daily transmission profile for each UHVDC line are made in advance by the CSG before the YNPG makes day-ahead generation schedules. As a result, the YNPG has to passively change their operational policies to match with the specified transmission schedules, which largely restricts the operational flexibility of LHPs. In fact, these hydropower plants and transmission lines show significant differences in reservoir storage, inflow, installed capacity, and transmission capability, which provide opportunities for coordinating operational schemes and determining flexible transmission schedules. Certainly, it should be noted that the optimal coordination must guarantee the power balance during each period. In other words, the summation of the transmitted power in all lines during each period should be equal to the given value of the total power transmission curve. The following equation presents a necessary power balance constraint for coordinating the allocation of hydropower among multiple UHVDC lines. It should be mentioned that this optimisation needs to consider the generating ability of hydropower plants related to each UHV line in order to ensure enough power supply

(6)

where denotes the specified total transmission power during period t; is the transmitted power through line l during period t; L is the total number of UHV transmission lines; and T denotes the last time interval under consideration.

In the second situation, controlling the total transmission electricity within the whole time horizon is used to replace the power transmission profile considered above. In this case, the power balance requirement for each period is relaxed but the total electricity target transmitted by UHV lines should be satisfied [see (7)]. Correspondingly, more efficient transmission schedules may be available by using optimal operations. During optimisation, the maximum transmission capacity of each line and the electrical demands of the receiving power grid should be incorporated into the optimal operation of the hydropower plants to promote the acceptance and practicability of hydropower transmission schedules. Instead of coordinating a single period separately in the above method, the present method suggests overall coordination within the time horizon to further improve the operational flexibility of a receiving power grid

(7)

where EL is the specified electricity target transmitted by all UHV lines and is the transmitted electricity via line l.

4.3 Coordinate operations between southwestern hydropower and local plants in receiving regions

A receiving power grid commonly operates multi-type power sources such as hydropower, thermal, and wind and solar sources. These grids have many characteristics different from those of the southwestern hydropower plants, providing an opportunity to coordinate operational policies with transmission schedules to promote hydropower absorption. First, due to differences in geography, weather, hydrology, and other factors, different run-off characteristics exist between southwestern hydropower plants and local plants in receiving regions. A typical example is the Xiluodu Plant located on the Jinsha River in southwest China and the Xinanjiang Plant on the Qiantang River in eastern China. These plants differ greatly in terms of run-off volume and flood season. According to historical records, the average inflow of Xiluodu is 20 times larger than that of Xinanjiang. The main flood period in Xiluodu begins in July and ends in September, with monthly inflows much higher than the annual average inflow. In contrast, the main flood season for Xinanjiang ranges from April to June. Hydrological compensation can be utilised for the coordinated operation of the interprovincial hydropower system consisting of southwestern and local hydropower plants to improve power transmission schedules. The coordination between hydropower and other energy sources has also been demonstrated to contribute to power integration in the grid network [38, 39]. In long-term operations, maximising the generation production in dry season [see (8)] can be selected as an optimisation objective to enhance the level of power supply for receiving power grids. Here, transmission capacity constraints related to UHV lines must be satisfied in the coordinated operations. Short-term operations should consider a peak-shaving objective [see (9)] to meet strong peak demands of receiving power grids with big peak-valley difference. In the optimisation, this objective is usually constrained by the given daily energy target [see (10)] and security network limitation [see (1)–(3)]

(8)

(9)

(10)

where Δt is the time period duration; C_t is load demand of receiving power grid in period t; is the total number of other plants except hydropower plants; and is power generation of plant i during period t. and are the specified energy target and the calculated energy production for hydropower plant m during the entire planning horizon, respectively.

The above coordination method is strongly related to the power source types of hybrid power system. Generally, hydrothermal scheduling can utilise the generation distribution and operational flexibility of southwestern hydropower to optimise the unit commitment and loading of thermal units in receiving regions. As the coal-fired generation capacity dominates the receiving power grid, this approach will have a significant impact on integrating large-scale southwestern hydropower. Another method is to take advantage of the complementary nature of hydropower and intermittent energy sources such as wind. Wind power in the receiving area usually has seasonal characteristics opposite those of southwestern hydropower, which is conducive to receive external hydropower during the flood season. Although they encounter similar generation trends in some situations, the regulating storage of large reservoirs can be used to transfer hydropower from the flood season to other low-wind seasons. Overall, the coordinated operations among various power sources can potentially promote the utilisation of southwestern hydropower.

4.4 Coordinate hydropower allocation among multiple power grids

After receiving the bundled southwestern hydropower through UHVDC lines, a regional power grid such as the ECG needs to allocate the received hydropower among its provincial power grids. Owing to substantially different electric demands, especially peak demands, the power curve allocated for each receiver is expected to be consistent with the respective load curve. Thus, using load differences to coordinate power allocation among multiple power grids will be useful for enhancing their acceptance potentials for southwestern hydropower [40, 41].

Two methods are suggested to solve the hydropower allocation problem. The first method is thatthe regional power grid can utilise the quarter-hourly load profile of anyprovincial power grid to generate the initial ideal power curve, while theelectricity received throughout the entire temporal extent is fixed. Then,according to the peak demands of different provincial power grids, the initialsolutions are iteratively adjusted until the power balance constraint of eachperiod is satisfied. This method is closely related to the daily load curves ofmultiple recipients; therefore, the allocation scheme obtained may be efficientto track power demands at every moment. To obtain an optimal allocation scheme,an optimisation model for peak operations of UHVDC hydropower and local powerplants can be utilised including the objective function [see (11)] and important systemconstraints [see (12) and (13)]. Another approach emphasisesthe trade-off of homogenous electricity among multiple power grids. Usually, themore peak power a power grid receives, the less the peak-shaving pressure. Toensure fairness among all power grids in the case of widespread peak powerdeficiency, the amount of electricity allocated to each recipient within high,moderate, or low load periods should be proportional to the ratio of the totalelectricity over the operational horizon, which means that power exchanges amongmultiple recipients are allowed within a specified period. It should bementioned that both methods indicate the importance of considering power balanceat each period and total electricity demands during hydropower allocation. Theseconstraints are used to design a search algorithm for allocating the generationamong multiple power grids. This algorithm first generates an initial solutionby a load shedding-based method, where the total energy demand [see (13)] and provincial load curve overthe entire time horizon are utilised. The initial solution is then iterativelyadjusted to meet the power balance constraint [see (12)] in each time period and toshave peak loads for multiple power grids. During the optimisation process,multiple objectives of the considered power grids are combined into anequivalent scalar objective using a weighted sum to reduce the complexity ofmulti-objective optimisation. The main principle of this algorithm is toincrease the generation at the highest load period and reduce the generation atthe lowest load period of every iteration. More detail can refer to the previouswork [8].

In equations

(11)

(12)

(13)

where and denote the original and residual load ofpower grid g in period t, respectively; , , and are generation of hydropower planti, thermal plant j, and other plantm allocated to power grid g in periodt; G is total number of receiving powergrids; and , , and are total number of hydropower plants,thermal plants, and other-type plants, respectively. is a general form of , , , and . denotes the specified proportion of energyproduction from one plant or UHVDC line m transmitted to gridg in period t.

4.5 Introduce economic incentives and improve compensation mechanisms

Absorbing large-scale hydropower from southwest China will inevitably reduce the thermal electricity of eastern power grids and, thus, affect the benefits of local power enterprises. The absence of a reasonable economic incentive largely restrains these recipients from taking full advantage of external hydropower. Although China started a new market-oriented reform for the electricity industry in 2015, the export of hydropower still primarily depends on the prices and policies set by the government. Therefore, there is a need to introduce efficient economic incentives and improve compensation mechanisms [42-44]. The following policies are proposed.

First, marginal compensation criteria should be established to compensate local power enterprises that cut down generation production or provide additional auxiliary services for hydropower absorption. The magnitude of reduced electricity is an important factor in establishing the compensation criteria. For different auxiliary services such as peak regulation, automatic generation control, and system reserve, detailed compensation rules and regulations should be available. Another similar way is to establish ancillary service market, where the outer hydropower can use its regulation ability to provide some ancillary services to solve the low electricity price and improve the level of power integration. Currently, some power grids such as CSG are trying to implement this work. Second, a differential price mechanism can be introduced over the operational horizon. Specifically, a higher price is generally set for high load hours, while a lower price is set for low load hours. In this case, to increase their gain as much as possible, hydropower exporters should actively adjust their transmission schedules to respond to the electricity demands of the receiving power grids. In fact, hydropower transmission and allocation call for a trade-off among multiple stakeholders including power enterprises, power grid companies, and other competing sectors. Quantifying the roles of hydropower for different receivers and during different operational periods using electricity prices is helpful to precisely judge the impartiality of operation schemes, and it is also vital to carry out compensation policies in the preliminary stage of the Chinese electricity market.

5 Case study

YNPG is taken as an example to make a brief analysis for the hydropower absorption problems and possible solutions. Yunnan is one of hydropower-dominated provinces in southwest China, with a hydropower capacity of 66.7 GW, ranking second in the country, and occupying 71.7% of provincial total generation capacity. There are 170 large and medium hydropower plants, more than 200 wind and solar plants, 11 thermal plants, and about 1700 SHPs. Owing to low local electricity demands, more than hundreds of TWh renewable electricity especially hydropower are required to transmit to other provinces and southeast Asian countries via UHV power network.

However, Yunnan has been suffering from continuous hydropower curtailment in recent years. The curtailed hydropower has surged from 2.4 TWh in 2012 to about 29 TWh in 2017. This problem is caused by complex objective conditions and subjective factors. For example, the increasing hydropower capacity substantially exceeds local electricity demand but the planned construction of UHV transmission systems lags far behind the hydropower development. The inconsistency between power grid company and local government, inconsistency between hydropower producers and main receivers, and inconsistency among different power enterprises for dispatching same river may restrict full absorption of abundant hydropower. The generation scheduling with random precipitation, wind speed, sunshine, coordinated operations among hydropower, wind, solar, and thermal power, coordination among multiple UHVDC transmission lines, coordination between the transmitted hydropower and local plants in receiving power grids, also have a direct effect on hydropower absorption.

To solve the hydropower curtailment, this paper gives a brief analysis using the above technical solutions. Three aspects are considered. The first is to optimise generation operations of cascaded hydropower plants, coordinated operations among different cascaded hydropower plants in several rivers, coordinated operations among hydropower, and other types of power sources, with power network limitations. The differences in storage, inflow, regulation ability, generation characteristics, and transmission capacity should be fully used. In particular, due to the special climate and geographical location in Yunnan, wind speed is high in winter and spring and low in summer and autumn, showing seasonal characteristics opposite those of inflow. Solar power is also smaller in the flood season than in the dry season because rain reduces sunlight time. Such obvious relations provide great potential for improving the temporal and spatial configurations of these power resources while considering the power network limitations at different voltage levels. The optimised operation schemes can determine reasonable reservoir levels and efficient generation and transmission schedules to reduce network congestion and promote renewable power integration. The second is to coordinate operations between hydropower plants in Yunnan and local plants in receiving regions, and coordinating hydropower allocation among multiple receivers. Currently, the hydropower plants on the Lancang River and Jinsha River in Yunnan provide electricity for Guangdong, Guangxi, and Zhejiang provinces through UHVDC power network. These plants and local plants in the receiving regions show great differences in hydrology, weather, regulating ability, and generation capacity, which can be employed to efficiently schedule electricity generation in spatial and temporal scales. On the other hand, the transmitted hydropower can be further allocated between Guangdong and Guangxi provincial power grids on the interprovincial platform of SCG and Zhejiang and other provincial power grids on the ECG platform. It should be noted that quarter-hourly load curve of each provincial power grid needs to be considered in the hydropower allocation. The third is to establish a reasonable electricity market. China is carrying out a new round of market-oriented reforms to the electric industry. Yunnan, Guangdong, and other provinces are also striving to design and develop sound long- and medium-term, as well as spot market trading rules and mechanisms. Yunnan can fully utilise this chance to break the interprovincial even interregional electricity trading barriers. It may be good choices to sign long- and medium- term electricity contracts, and participate in ancillary services market.

6 Conclusion

Hydropower plays an extremely important role in establishing a sustainable energy system and realising carbon reduction targets in China. The country has made substantial effort in hydropower development in past decades, making remarkable achievements and leading to significant changes in the hydropower market from local consumption to large-scale export via a nationwide UHV network. In the new stage, a great challenge facing the world's largest hydropower system is how to promote the absorption of abundant hydropower and alleviate the severe curtailment of hydro-dominated provinces in southwest regions. According to the detailed analysis of hydropower absorption in this paper, hydropower absorption is closely related to the operations and transmission schedules of current transmission channels. Efforts are encouraged to strengthen coordination among stakeholders such as multi-type power sources, hydropower exporters and importers, and multiple hydropower-receiving power grids. This work will help to make full use of the complementarity among the hydrology, storage capacity, and electrical demands of such complex interconnected systems so that the scale and flexibility of hydropower transmission can be significantly improved. Transmission power curves with high operational flexibility will make hydropower integration easier compared with the commonly used method with fixed power. Considering the electricity demands of receiving regions, coordinating southwestern hydropower, and other energy sources in local and receiving regions and optimising hydropower allocation among multiple power grids will be of great use for improving transmission schedules. It should also be emphasised that economic incentives are necessary in hydropower system operations, as they can motivate power generation enterprises and power grids to efficiently utilise hydropower resources. Using this method may rely on an elaborate design and applicable rules in the reform of the country's ongoing electricity market. Such effort is indeed required for the operations and further hydropower development of China's hydropower system.

In addition, we should note the willing of hydropower-rich provinces for the hydropower transmission. In fact, the willing for the hydropower transmission is closely related to the local electricity demands in southwestern China. It can be predicted that the fast development in Sichuan, Yunnan or other hydropower-rich provinces may gradually weaken their willing of hydropower transmission. Meanwhile, we should also note that, in the next few years, trans-provincial hydropower transmission via UHV power network may be one of important and effective solutions that avoid or alleviate large amounts of curtailed hydropower in flood season.

7 Acknowledgments

The National Natural Science Foundation of China (Nos. 51579029 and 91547201), the open research fund of Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education (No. LOEC-201806), and Fundamental Research Funds for the Central Universities (DUT19JC43). The authors are very grateful to the anonymous reviewers and editors for constructive comments and suggestions.