Optimal scheduling for wind-solar-hydro hybrid generation system with cascade hydropower considering regulation energy storage requirements

Large-scale integration of renewable energy into the grid can lead to significant changes in the net load, peak-to-valley difference, peak and valley occurrence time of the power system. As a result, the power of hydropower plants must take a rapid adjustment response. Aiming at the coordinated operation of multiple energy sources, such as wind power, solar power, cascade hydropower station and energy storage pumping station, a coordinated scheduling model is proposed which can fully improve the consumption capacity of wind and solar power by aiming at the maximum power generation, minimum net load fluctuation and minimum wind and solar abandonment. Through the configuration of three different pumping station capacities, the influence of energy storage pumping station capacity on the complementary power generation system is analyzed. When the pumping station capacity is large enough, the output of the wind and solar can be completely consumed. The studies show that the cascade power station and pump energy storage regulation have a strong net load filling valley effect, which can effectively reduce the impact of wind and solar access on system operation, maintain the efficient and stable operation of the unit, and ensure the absorption rate of renewable energy.


Introduction
With the increasing demand for energy, vigorously developing renewable energy has become an important strategic measure for energy development [1,2].By the end of 2022, the global renewable energy installation capacity has reached 3372 GW, consisting of 1053 GW solar power and 899 GW wind power.Renewable energy represented by wind energy and solar energy has grid-connected power The 17th Asian International Conference on Fluid Machinery (AICFM 17 2023) Journal of Physics: Conference Series 2707 (2024) 012100 IOP Publishing doi:10.1088/1742-6596/2707/1/012100 2 generation, which provides new solutions for solving energy problems.With the continuous increase in the proportion of renewable energy, the output of renewable energy has a significant impact on the safe and stable operation of the power grid.Hydropower has the characteristics of clean, low-carbon, and flexible operation.Scientific and coordinated planning for hydropower energy is a key measure to build a safe and efficient energy system [3][4][5].By utilizing the continuous storage capacity of cascade hydropower and the rapid adjustment ability of hydropower units, the wind and solar output can be smoothed and the safety performance of the power grid can be improved [6,7].
Giving full play to the regulation role of cascade hydropower in the basin and realizing the complementary power generation of cascade wind-solar-hydro hybrid system is an effective way to promote renewable energy consumption, improve the generation profit and avoid risks [8][9][10][11][12].In the new situation of high proportion of new energy access, how to deal with short-term landscape uncertainty has become a hot spot of short-term hydropower dispatch research in recent years.Multiresource power generation technology can achieve complementary advantages between different energy types and become an important means to reduce the impact of wind power and solar large-scale gridconnected [13][14][15].Considering the cascade hydropower, a short-term optimal scheduling model for wind-solar-hydro hybrid generation system is established by Xie et al., and the research results indicate that owning to the integration of wind and solar energy, the spinning reserve and regulation reserve are crucial for maintaining sufficient levels of power supply reliability and safety [10].Zhang et al. explored cooperative game theory to resolves the conflicts among wind, solar, hydro power.The distribution methods were analyzed from the aspects of existence, uniqueness, rationality, and calculation feasibility [16].Jin et al. established a model for long-term typical daily peak-regulating operations of hydro-windsolar energy.The results indicate that the model achieves decoupling of hydro-wind-solar energy on time scale, and carry out effective peaking [17].Lu et al. established a new type of coordination model for large hybrid energy systems consists of wind energy, solar energy, and cascade hydropower stations.Research has shown that optimal scheduling of hybrid systems can smooth output power fluctuations, improve the controllability of wind and solar output, and increase the total expected profits [18].Zhang et al. proposed a short-term optimization model which coupling cascade hydropower plants and renewable energy generation, consists of wind and solar, taking the complementarity of various renewable energy sources and the hydraulic relationship and water flow delay between cascade reservoirs into account [19].In summary, hydropower can make up for the shortage of wind, solar and other new energy generation, so that the dispatch structure is more reasonable.
However, most of the cascade power stations show excess power generation capacity and insufficient power generation water in the dry season.Combined with the cascade power station pattern, a large energy storage pump station is used to pump water from the lower cascade reservoir to the upper cascade reservoir by taking advantage of the not match between the demand and renewable power, and the energy is stored as water potential energy to realize the time shift of renewable energy power.Figure 1 is the cascade energy storage pump station.At the same time, further reuse water resources and effective storage capacity of reservoirs, transform uncontrollable solar and wind power into relatively stable and reliable power sources, improve the utilization of hydropower resources and increase power generation efficiency.Therefore, the combination of cascade hydro power, energy storage pump station, wind and solar power can ensure a more stable output of the hybrid generation system.

Wind-solar-hydro hybrid generation system scheduling model
In order to maximize the use of renewable energy, maintain the safe and stable operation of the power grid, and fully exploit the coordinated optimization potential of cascade hydropower, a short-term optimal scheduling model with cascade pumping station storage was established with the objectives of maximum power generation, minimum net load fluctuation and minimum wind and solar abandonment.

Objective functions (1) Maximizing the total power generation of wind-solar-hydro hybrid generation system
In the benefit evaluation of the complementary power generation system, the power generation is the main factor.Taking the maximum total power output of the system as the objective function, which can be expressed as follows: ( ) where, W t P is the wind farm power output at time t, PV t P is the solar power station power output at time t, H t P is the hydro station power output at time t, t  is the time duration.
(2) Minimizing the net load fluctuation The net load is the actual load of the hydro power unit after deducting the wind-solar combined output in the system.In order to fully compensate the fluctuation of wind-solar output and avoid the frequent and large adjustment of hydro power unit output, to minimize the net load fluctuation of the complementary power generation system.The objective function can be expressed as follows: where The consumption capacity of renewable energy is expressed by the sum of the abandoned wind-solar power in the scheduling period, and the more abandoned wind-solar, the weaker the consumption capacity of renewable energy.The objective function can be expressed as follows: ( )

P
is the upper bounds of the predicted solar power station output at time t.
(3) Cascade hydropower constraints Cascading hydroelectric power plants are required to meet not only the load requirements of the electric grid, ensuring the ecological environmental needs of both upstream and downstream, but also coordinating the water head, the inflow and outflow relationships of hydropower plants, and improving utilization efficiency.These constraint conditions must consider the generation power constraint, the initial and final storage capacities, and the water balance of each hydropower station.I: Hydropower output constraint where, , ht V is the reservoir capacity of hydro station at time t, ,1 ht V − is the reservoir capacity of hydro station at time t-1, , ht q is natural inflow of hydro station.,

P
is the upper limits of pumping station power, P t P is pumping power of pumping station at time t.Considering that within a scheduling period, all the pumping capacity of the pump station is used for the generation of hydropower units in the cascade power station.
(5) System power balance constraint In the optimal scheduling model for wind-solar-hydro hybrid generation system, and the power balance system constraint can be expressed as follows: ( ) where, Load t P is system load at time t.

Case studies
Optimal scheduling for multi-energy systems comprising wind, solar, and hydro is a mixed-integer nonlinear programming problem.CPLEX is an efficient optimization tool for linear programming, mixed-integer programming, and quadratic programming, and as such provides excellent solutions to complex mixed-integer linear programming problems.This paper uses MATLAB platform to call CPLEX to solve the model and determine the economically optimal scheduling strategy.
In order to verify the validity of the optimization model, a system including a wind power plant, a solar power plant, two cascade hydropower stations, and a pumping station was adopted.The installed capacity of wind power, solar power, cascade hydropower station 1 and cascade hydropower station 2 are 17245.6MW, 11457.8MW, 2500 MW and 1800 MW respectively.The scheduling problem for 24 time periods in a day was taken as an example to conduct numerical tests on the above model with different energy storage pumping station capacities (1250 MW、2500 MW、3750MW).
The scenarios of typical day in four seasons of a year were selected for optimization.The forecast power of each typical day is shown in Figure 2. It can be seen the fluctuation of typical daily load in the four seasons is basically the same, the spring load is slightly smaller than the other three seasons, the summer solar energy resources are the most abundant, the spring and the autumn wind resources are richer than others.After wind power and solar energy are connected to the grid, the peak-valley difference of net load increases, and the net load value in spring is the smallest, and the net load difference in autumn is the smallest.It shows that the anti-adjustment characteristics of the solar and wind power make it widen the load peak and valley, and the peak valley difference is transferred, and the greater the proportion of wind power and solar power connected to the grid, the greater the randomness of the impact on the system.In order to explore the effect of pumping station capacity on the optimal operation of wind-solarhydro complementary power generation system, the four seasons coordinated scheduling modes of different pumping station capacities were analyzed.The optimal scheduling results under different pumping station capacities are shown in Table 1.As can be seen from Table 1, the solar abandon rate of the complementary system decreases with the increase of the capacity of the pumping station, but the total power generation increases with the increase of the capacity of the pumping station.When the capacity of the pumping station is 3750 MW, there is almost no wind and solar abandonment.The coordinated dispatch output fluctuation and wind-solar abandonment of the cascade wind-solar-hydro hybrid complementary power generation system in four seasons are shown in Figure 3-6.It is found that with the increase of the capacity of the pumping station, the abandoned solar energy decreases, and there is no wind curtailment phenomenon.
Figure 3 shows the coordinated scheduling of the wind-solar-hydro complementary generation system with different pump station capacities in spring, when the capacity of the pumping station is 1250 MW, in order to consume more wind and solar resources, the pump is operating at the rated working condition for 24 hours, the corresponding amount of solar abandoned is 42.2% in spring.When the capacity of the pump station is 2500 MW, there is no wind abandoning, and the amount of solar abandoning is reduced to 16.4%, corresponding to the pumping power of the pump station is 54040.5 MWh, and the rated operating time of the pump is 10 hours (10:00-20:00).Meanwhile, the cascade power stations all operate at the minimum allowable output, which cannot completely consume the wind power and solar power.In the case of limited regulation capacity of hydropower and pumped storage, some renewable energy generation had to be abandoned in order to ensure the regulation capacity.When the capacity of the pumping station is 3750 MW, there is no abandoning wind and solar, and the pumping power of the corresponding pumping station is as high as 65837.6MWh.The rated operating time of the pump is 8 hours (11:00-18:00).Figure 4 shows the coordinated scheduling of the wind-solar-hydro complementary generation system with different pump station capacities in summer.when the capacity of the pumping station is 1250 MW in summer, the pump is operating at the rated working condition for almost 24 hours, the corresponding amount of solar abandoned is 18.4%, and the pumping power of the pumping station is 29276.5 MWh.When the capacity of the pump station is 2500 MW, the amount of solar abandoning is reduced to 3.2%, corresponding to the pumping power of the pump station is 47401.3MWh, and the rated operating time of the pump is 10 hours (10:00-19:00).Meanwhile, the cascade power stations all operate at the minimum allowable output, which cannot completely consume the wind power and solar power.When the capacity of the pumping station is 3750 MW, the pumping power of the corresponding pumping station is as high as 57251.9MWh.The rated operating time of the pump is 8 hours (11:00-18:00).
Figure 5 shows the optimal scheduling results in autumn.When the capacity of the pumping station is 1250 MW, the pump operates under rated conditions for 24 hours, the corresponding solar abandonment amount is 20.7%.When the capacity of the pumping station is 2500 MW, the amount of abandoned solar is reduced to 0.5%, the corresponding pumping power of the pumping station is 54678.6MWh.When the capacity of the pumping station is 3750 MW, the pumped power of the pumping station is up to 64775.0MWh, and the rated operating time of the pump is 7 hours.Figure 6 shows the optimal scheduling results in winter.With the increase of pumping station capacity, the amount of solar abandoned decreases, and there is no wind abandoned phenomenon, which is the same as in the other three seasons.When the capacity of the pumping station is 1250 MW, the pump operates under rated conditions at 2:00-8:00 and 11:00-19:00, the corresponding solar abandonment amount is 30%, and the pumped electricity of the pumping station is 27744.6MWh.When the capacity of the pumping station is 2500 MW, there is no abandoned wind, and the amount of abandoned solar is reduced to 9.4%, the corresponding pumping power of the pumping station is 38734.3MWh, and the rated operating time of the pump is 8 hours (11:00-18:00).When the capacity of the pumping station is 3750 MW, there is no abandoned wind and solar, the pumped power of the corresponding pumping station is up to 47675.8MWh, and the rated operating time of the pump is 6 hours.The scheduling results of different typical days show that the energy storage pump station exerts flexible adjustment capabilities and follows the fluctuations of wind-solar power and load in real time.

Conclusions
Given the serious situation of environmental pollution and fossil fuel depletion, there is an urgent need to change the energy structure.Wind power and solar power are two sources of renewable energy, clean and environmentally friendly, which can significantly improve resource and environmental issues.Aiming at the change of power structure under the background of energy transition and the limitation of existing multi-energy complementary scheduling methods, a cascade wind-solar-hydro hybrid energy storage pump station cooperative scheduling model considering multi-energy complementary characteristics was established to maximize total power generation, minimize net load fluctuation and minimize wind and solar abandonment.The effect of pumping station capacity configuration on the optimal operation of the complementary system in different seasons is analyzed.It is found that with the increase of pumping station capacity, the amount of abandoned wind and solar decreases, while the power consumption of the pumping station and total power generation increase.When the pumping station capacity is 1250 MW, the wind is not abandoned in the four seasons, and the solar energy is discarded 42.2% in spring, 18.4% in summer, 20.7% in autumn, and 30.1% in winter.When the capacity of the pumping station is increased to 2500 MW, the amount of solar discarded in spring is 16.4%, and the other three seasons are reduced to less than 10%, indicating that the corresponding pumping station capacity can better achieve the absorption of wind and solar output.When the pumping station capacity is further increased to 3750 MW, the complete absorption of wind power and solar output is basically achieved, but at the cost of consuming more pumping station construction costs and pumping power of the pumping station.Therefore, the comprehensive comparison shows that when the pumping station capacity is 2500MW, it can meet the requirements of the complementary system.The results show that the coordinated scheduling of cascade wind power, solar power and hydropower energy storage can make full use of complementary energy, improve operating conditions, achieve efficient consumption of renewable energies and give full play to the goals of flexible power regulation.

Figure 1 .
Figure 1.The cascade energy storage pump station.

Figure 2 .
Forecast power for typical days in four seasons: (a) load and net load, (b) solar and wind.

3 .
(a) With pumping station capacity of 1250 MW (b) With pumping station capacity of 2500 MW (c) With pumping station capacity of 3750 MW Figure Daily complementarity of wind-solar-hydro hybrid energy systems with different pumping station capacities optimal scheduling and wind-solar abandon in spring.

4 .
(a) With pumping station capacity of 1250 MW (b) With pumping station capacity of 2500 MW (c) With pumping station capacity of 3750 MW Figure Daily complementarity of wind-solar-hydro hybrid energy systems with different pumping station capacities optimal scheduling and wind-solar abandon in summer.

6 .
(a) With pumping station capacity of 1250 MW (b) With pumping station capacity of 2500 MW (c) With pumping station capacity of 3750 MW Figure Daily complementarity of wind-solar-hydro hybrid energy systems with different pumping station capacities optimal scheduling and wind-solar abandon in winter.

Table 1 .
Results of optimal scheduling with different pumping station capacities.