Analysis of annual power and electricity balance of hydro-thermal system in Yunnan province based on rolling dispatch with leading reservoir

Renewable energy penetration presents challenges for power systems, including unit reconfiguration, resource allocation, and potential electricity shortage. In Yunnan Province, where hydropower is a dominant energy source, annual power and electricity imbalance is a serious issue due to insufficient hydropower regulation capacity and less scheduling flexibility. To address these problems, a rolling dispatch method is proposed to connect medium- and long-term scheduling with short-term scheduling, providing hourly start-stop statuses and output plans for units throughout the year and effectively solving power imbalance issues. Additionally, an economic benefit and electricity shortage analysis of the Longpan leading reservoir in the Jinsha River Basin is conducted, resulting in a 13 billion kWh annual power generation increase and approximately 4 billion yuan in direct economic benefits, greatly improving Yunnan Province’s adjustment ability from dry seasons to wet seasons.


Introduction
The energy structure has been reconfigured gradually in power grids with the growing penetration of renewables [1].Clean energy, mainly hydropower, will undertake the important task of power balance as the proportion of thermal units continues to decline.However, hydropower usually has long dispatch periods and complex cascade relationships, resulting in potential electricity shortages and imbalances.Therefore, in provinces with abundant hydropower resources such as Yunnan, an effective solution is required to confront such challenges [2].
However, power and electricity balance still experience shortfalls in Yunnan Province.The hydropower station must release more water to ensure the power balance when the incoming water is not enough, causing difficulty to return to the planned water level at the end of the year [3].It will also be hard to fully accommodate the increasing renewables in the future with the existing cascade reservoir resource [4].Two main reasons cause the problem.Firstly, it lacks an effective medium-and long-term scheduling model, which can make detailed dispatch for various units on an annual scale.Secondly, the regulation capacity of cascade hydropower is insufficient, causing a large difference between the generation capacity in the dry season and wet season [5].
Several researches have been conducted on medium-and long-term scheduling models.Deterministic models are preferable for long-term hydrothermal scheduling, but their use is restricted to only a few reservoir systems [6].Hydrothermal scheduling can be formulated as a two-stage stochastic linear programming problem, as demonstrated in Reference [7] using a Nordic system with a lengthy scheduling horizon.Nonetheless, this model is time-consuming due to the significant computational burden.To address this issue, References [8], [9], [10] proposed a fast unit commitment (FUC) model that utilizes unit grouping techniques to solve annual generation scheduling problems more quickly.However, such a method may result in potential power imbalances as it cannot accurately represent the hourly status of each unit throughout 8760 hours.
Several researches have also been carried out on the analysis of hydropower systems in Yunnan province.Reference [11] analyzes the output characteristics of three power stations within the system.References [12], [13] focus on optimizing reservoir operation in the Jinsha River by accounting for economic and ecological factors.Reference [14] provides an overview of hydropower resources in Yunnan and the constraints limiting hydropower generation.Reference [15] examines the impact of large-scale small hydropower on the safety and stability of the Yunnan power grid.However, such analysis can not effectively solve the potential electricity shortage problem with less practicality.
This paper proposes a rolling dispatch method for the connection between medium and long-term and short-term scheduling based on the fast unit commitment (FUC) and security-constrained unit commitment (SCUC).Then the economic benefits of constructing Longpan leading reservoir in the medium stream of the Jinsha River and the changes in the province's power balance have been analysed.In the context of the previous research, our paper provides the following contributions: 1) The rolling dispatch method based on fast unit commitment and security-constrained unit commitment is proposed to solve the power imbalance in Yunnan province.It connects medium-tolong-term scheduling and short-term scheduling, and can effectively calculate the hourly on/off status and generation output of various units for 8,760 hours. 2) The economic benefits of building Longpan leading reservoir in the medium reaches of the Jinsha River and the improvement of the province's electricity shortage are analyzed in the context of the 14th Five-Year Plan in China.This solution can greatly improve the hydropower regulation capacity of Yunnan Province and provide an additional output of about 14 billion kWh.
3) Based on realistic data from the Yunnan Power Grid, the solution is successfully implemented in a hydro-dominated regional power system, which can serve as a reference for power grid systems or electricity spot markets facing similar circumstances.
The paper is structured as follows.The rolling dispatch method connecting medium-to-long-term scheduling and short-term scheduling based on the FUC and SCUC is formulated in Section 2. The analysis of the annual power and electricity balance in Yunnan province under the construction of the Longpan leading reservoir is in Section 3. Conclusions are summarized in Section 4.

Rolling dispatch method
Here, we formulate the rolling dispatch method connecting medium-to-long-term scheduling and short-term scheduling.The mathematical model is derived from FUC and SCUC, which can provide the annual power and electricity balance with less amount of calculation.
Firstly, the mathematical model based on fast unit commitment is proposed to solve the medium and long-term power balance during the 8760 hours, which is shown as equations ( 1) to (4).
. ., s t Ay d  (2) EPATS-2023 Journal of Physics: Conference Series 2636 (2023) 012035 Equation ( 1) includes coal cost and switching cost, where T b is the coal cost coefficient, x is the output of cluster units, T c is the switching cost coefficient and y is the switching status of cluster units, respectively.Equation ( 2) is the start-stop switching constraints of cluster units, where A is the switching constraints coefficient and d is the switching demand.Equation (3) represents the inequality constraints of the electricity and power balance of the power system, where G is the cluster units output coefficient, d is the fixed balance demand factor, u is the uncertain unit output mainly based on new energy, and M is the uncertain unit output coefficient.Equation ( 4) is the positive variables constraints including x and y.
According to the model described above, the medium and long-term power balance results can be preliminarily solved.The optimized water level and storage capacity of cascade hydropower, the output of runoff hydropower, the optimal utilization hours of thermal power, and the consumption of new energy provide the monthly boundary conditions for medium and long-term dispatch.
The next step is to divide the annual scheduling into 12 months and conduct individual unit commitment analysis.This process takes into account the monthly boundary conditions provided by the medium and long-term scheduling and takes a comprehensive approach to factor in unit maintenance plans, equipment maintenance plans, start-up and shutdown schemes, and the output of various units.The purpose of this analysis is to compensate for any inaccuracies in the results of cluster units from the medium-and long-term scheduling based on FUC.Once the detailed power balance of each month is calculated, the data for the end of the month should match the data for the beginning of the next month, including the final storage capacity of the cascade hydropower stations.This cycle continues until all solutions are completed in December, resulting in the final annual electric power balance.
Similarly, daily scheduling results are used as operation boundary conditions.Then, short-term scheduling based on security-constrained unit commitment (SCUC) is conducted according to boundary conditions.It comprehensively considers network topology and day-ahead market constraints and improves scheduling accuracy to quarter-hour intervals to obtain precise and detailed plans for each unit.
By following the aforementioned steps, the rolling dispatch of various units can be completed for 8760 hours.This scheduling method is particularly effective in achieving the balance of electric power throughout the year, especially in Yunnan Province, where there are abundant large-scale cascade hydropower resources.The 8760-hour start-stop plan and output arrangement obtained from the rolling dispatch solution provide a reference for the distribution and scheduling of clean energy throughout the year.Additionally, it can aid in refining research on typical days and timely adjusting section transmission and unit maintenance arrangements.The detailed steps are illustrated in Fig 1.

Analysis of the Longpan leading reservoir construction
Here, we analyse the economic benefit and electricity shortage in Yunnan province after the construction of the Longpan leading reservoir.Meanwhile, the analysis is based on the actual data and network topology of Yunnan Power Grid, considering the 14th Five-Year Plan in China.
Considering that the capacity of wind and solar in the existing system is relatively small, and in the case of dual-carbon targets, future dispatch will inevitably face the problem of a high proportion of renewable energy connected to the grid.Based on the new energy projects and load growth expectations in Yunnan Province, we will be based on installed capacity, the installed capacity of wind power and photovoltaics shall be further increased from 15% to 51%, and the load shall increase correspondingly by a certain proportion.The specific parameters are shown in Fig. 2.

Fig.2 14th Five-Year Plan electricity consumption forecast
The medium reaches of the Jinsha River have large hydropower installed capacity, small storage capacity, and weak regulation capacity.Power generation usually depends on the natural upstream water.The construction of leading reservoirs will significantly improve the cascade regulation and storage capacity of the basin and the utilization rate of water resources.Therefore, this part will focus on the construction of Longpan Power Station, and analyze the output in the peak and dry seasons of 6 hydropower stations in the medium reaches of the Jinsha River with a total installed capacity of 13,760MW.In addition, the monthly utilization hours of thermal power refer to the actual situation in 2019.The specific construction parameters of the Longpan leading reservoir are shown in Table 1.

Rated installed capacity(MW)
Rated head(m) Rated storage capacity(billion m³) 4200 171 13.9 After the construction of the Longpan leading reservoir, we analyze the output of six hydropower stations in the medium reaches of the Jinsha River, which are GuanYinYan, RuDiLa, LongKaiKou, JinAnQiao, AHai, and LiYuan, respectively.First, we choose April, a typical month in the dry season, to compare the output of the six hydropower stations before and after the construction of the Longpan leading reservoir.As shown in Fig. 3 and Fig. 4, through the comparison, the output of each power station in the medium reaches of the Jinsha River in April was relatively poor under the current conditions.However, after the construction of the Longpan Power Station, the hourly output of each power station in April has been improved, which is more abundant to fit the power demand.
Compared with the dry season, the power output during the wet season also changes.As shown in Fig. 5 and Fig. 6, we selected August during the wet season and conducted hourly power output analysis.Before the construction of Longpan Reservoir during the wet season, the overall output was larger, and it could reach around 14,000 MW.After the construction of Longpan Reservoir, the output of the six cascade hydropower stations changed.In cases of low electricity demand, it can reduce the output to store electricity generation capacity and regulate the dry season.The detailed power generation parameters of Longpan leading reservoir construction are shown in Table 2.After the construction, power generation increased significantly in the dry and flat water periods, by 43.9% and 39.3% respectively, and only decreased by 1.4% in the wet season.The total annual power generation increased by 19.5%, effectively addressing the issue of power imbalance and improving the regulation capacity of cascade hydropower in the Jinsha River Basin.
We further analyzed the annual power generation and the hourly power imbalance after the construction of leading reservoirs in the Jinsha River Basin.As shown in Fig. 7, there is a certain deviation in the annual daily power generation of the hydropower station before and after the construction of the Longtou Reservoir.After the construction, the power generation in the dry season has increased significantly, which can effectively make up for the large power gap caused by insufficient hydropower generation in the dry season.However, the power generation during the wet season has slightly decreased, which does not affect the overall power balance during this period.Fig. 8 further illustrates this point.Before construction, during the dry season, especially in March, April and December, due to the sharp increase in the proportion of wind and solar power, the overall hourly power gap will be very large, and there will be serious grid security problems.However, after the construction of leading reservoirs on the Jinsha River Basin, the above-mentioned problems have been greatly alleviated, and the overall power imbalance has dropped significantly, almost completely eliminated, and only a small part of the regulation dead zone existed in December.As shown in Table 3, we further analyze economic benefits and electricity shortage parameters in detail on the impact of the leading reservoirs construction in the Jinsha River Basin.After the construction, the cost of coal consumption dropped by about 1.1 billion yuan, and the cost of start-up and shutdown decreased by about 0.6 billion yuan.Moreover, the overall electricity shortage has dropped significantly, from 13.27 billion kWh to 0.07 billion kWh, effectively improving the electricity and power balance.This method leads to direct economic benefits of about 4 billion yuan in consideration of coal prices, which can be further boosted by the potential high-efficiency annual peak-shaving capabilities.

Conclusion
In this article, a rolling dispatch method connecting medium-and long-term scheduling and short-term scheduling based on the FUC and SCUC is proposed.Monthly dispatch uses the monthly results obtained from FUC as boundary conditions, followed by rolling solutions to address the scheduling inaccuracy caused by cluster units.SCUC is then solved for a typical day, taking into account various constraints, including network topology.Through the entire rolling process, the annual power balance can be effectively solved with specific on/off statuses and output plans.This paper also addresses the potential power shortage issue in Yunnan Province by the Longpan leading reservoir construction.The study effectively resolves the problem of inadequate regulation capacity of large-scale cascade hydropower during dry and wet seasons and directly increases the annual power generation by about 13 billion kWh, which fills the power shortage and directly increased economic benefits by approximately 4 billion yuan.

Fig. 7
Fig.7 Comparison of annual hydropower output Fig.8 Comparison of annual electricity shortage

Table 2 .
Power generation parameters of Longpan leading reservoir construction

Table 3 .
Economic benefits and electricity shortage parameters