Prospect of new pumped-storage power station

1 Introduction

At present,the transformation of energy structures is being accelerated in China.The proportion of renewable energy in the power grid is continuously increasing as well.In order to promote energy revolution and to achieve energy-sustainable development,the State Grid Corporation of China proposed the construction goal of the first-class energy interconnection [1-3].In the future,energy will be much cleaner and greener.Multi-energy cooperation and joint complementarity are the new characteristics of the energy interconnection era.

With the rapid development of ultrahigh-voltage direct current (UHVDC) transmission,new energy,and nuclear power installation,the power supply structure has changed greatly,and the problems of peak and frequency regulation and active power control are prominent.A pumpedstorage power station is an important guarantee for the safe and stable operation of the main power grid frame.It is irreplaceable for stabilizing the power frequency and ensuring power security.As of January 2019,45 pumpedstorage power stations,a total installed capacity of 55.22 million kilowatts,are operating and being built by the State Grid Corporation of China,whose capacity benefit is considerable.

In recent years,with the development of renewable energy,the technological economy of chemical energy storage technology has been rapidly improved.In 2018,a 100-MW chemical energy storage power station was constructed in the power grid to support peak and frequency modulation in Zhenjiang,Jiangsu.A 60-MW chemical energy storage is being built in Guazhou,Gansu in 2019 to improve the utilization of sufficient local wind power.The construction of two chemical energy storage stations can provide a valuable demonstration of the application of chemical energy storage as an auxiliary to the power grid.

The ideal energy storage system in the future should not only have sufficiently quick response ability,but also enough energy-storage capacity effect.What is certain is that the new pumped-storage power stations with variablespeed pumped-storage technology,chemical energy storage technology,and photovoltaic energy storage system have many advantages compared to the traditional pumpedstorage power station.The benefits and comprehensive utilization level of the original power station can be maximized.Breaking through the inherent limitations of traditional stations,a new generation of power stations can better adapt to the development of smart power grids,and meet the opportunities and challenges in the era of energy interconnection.

2 Conception of a new generation of pumpedstorage power station

2.1 Using flexible pumped-storage technology to build flexible power station

The variable-speed pumped-storage unit is an international frontier technology.As a major technological reform in the category of pumped storage,it can achieve automatic load regulation and frequency modulation with a wide range of pumping conditions.Flexible access to the power grid in the pumping condition can greatly improve the impact of the step response type compared to the traditional fixed-speed unit.The automatic tracking load characteristics greatly enhance the ability to accept new energy for the power grid [4,5].

The new-generation pumped-storage power station with variable-speed pumping technology will greatly enhance the flexible control operation level of traditional pumpedstorage stations,as follows:

(1) Stability is better.The fixed-speed pumped-storage power station has a step-type output.Take one of pumped storage power stations as an example.It takes only about 16 s from -50 MW to -300 MW,and just 14 s from -300 MW to 0 MW.It means a 300 MW unit trips several times in one day,which has a great impact on the Fujian province power grid.Compared to the fixed-speed unit using static frequency converter(SFC) start-up in the pumping condition,the variable-speed unit can achieve self-start via an AC excitation system.As the output frequency of the AC excitation system changes gradually when the unit is started,it can achieve a smooth start-up,which can reduce the impact on the power grid in the states of pump or generator.At the same time,it is better to control the internal phase angle by directly using the inverter to adapt to power system disturbances.

The view and schematic diagram of a typical pumpedstorage power station are shown in Fig.1 and Fig.2,respectively.It generally consists of an upper reservoir,underground reservoir,underground powerhouse,water conveyance pipeline,and switch station.The underground workshop is mainly composed of primary and secondary equipment,such as generator motor,pump turbine,control equipment,and outlet circuit breaker.

Fig.1 Top view of typical pumped-storage power station

Fig.2 Schematic diagram of pumped-storage power station

The concept of an adjustable-speed system is shown in Fig.3.The converter output frequency is equal to the frequency of the rotor magnetic field.For example,the frequencies of the stator magnetic field and rotor mechanical speed are 50 Hz and 48 Hz,respectively.Then the frequency of rotor magnetic field is 2 Hz,which means that the converter output frequency is also 2 Hz.

where N1 is the frequency of the stator magnetic field,Nm is the frequency of the rotor mechanical speed,and N2 is the frequency of the rotor magnetic field.

Fig.3 Simple conceptual diagram of adjustable-speed system

(2) Adjustment performance is better.The variablespeed unit is obviously superior to the fixed-speed unit in terms of response time and adjustment speed.The 400-MW variable-speed unit of the Okawachi Pumped Storage Power Station in Japan can change 32 MW output power or 80 MW input power within 0.2 s [6].The regulation rate of Beijing Shisanling Pumped Storage Power Plant with automatic generation control(AGC) is approximately 100 MW/min.For the start-up time,the variable-speed unit needs ～2.5 min,and the fixed-speed unit needs ～5 min [7-8].When the power system is disturbed,the variable-speed unit can achieve high-speed adjustment of active power,which is beneficial in suppressing the active power fluctuation of the power system [9].

(3) The adjustment range is much wider.The variablespeed unit can improve the operating efficiency in the larger water head (lift) range,and control the frequency automatically according to the power system.Based on the comparisons between Okawachi’s variable-speed units and fixed-speed pumping units,the traditional pumping units are not adjustable in the power range of 60% to 100%under the pumping condition,and the adjusting power loss is more under 60%.The adjustment range of variable-speed units can be expanded from 50% to 100% and from 30%to 100%,and the power loss is much less than that of the fixed-speed unit.

(4) It has good ability to regulate the system’s reactive power and absorb reactive power deeply.According to Russian related research,in the 750 kV power system,the variable-speed unit can stably operate after deeply absorbing reactive power,and can significantly reduce the number of shunt reactors.The variable-speed unit can continuously adjust reactive power,so it can provide important support toward the stability of the voltage level in the various operating conditions of the high-voltage power grid and reduce the power loss.

2.2 Combining electrochemical energy storage to create sensitive pumped-storage station

The combined operation of a pumped-storage power station and electrochemical energy storage can better solve two important problems.One is that the power response speed of the pumping unit cannot reach the second level,and the other is that the safety and reliability of the power station are insufficient.

2.2.1 Development situation of electrochemical energy storage technology

Electrochemical energy storage technology can simultaneously meet the application requirements of power storage and energy storage for the power grid.The battery energy can be released for durations of several seconds to several hours,whose output power can be adjusted in the rated power range.Battery technology has been commercialized or is being commercialized.

Among all types of electrochemical energy storage,lithium-ion battery technology is developing the fastest,with rapid growth and maximum installed capacity ratio in the market.Its charge and discharge rate can reach 1-3C and the working temperature range is wide enough,from -20 °C to 60 °C,with a very long cycle life.After a comprehensive comparison of typical performance indexes such as response time,energy density,power density,energy efficiency,cycle times,and typical applications,it is the best to choose lithium batteries to enhance the grid performance.

In recent times,the cost performance of energy storage batteries in sustainable development has increased year by year.The current invested cost of lithium batteries is 1,500 yuan/kWh,and the comprehensive operating cost is approximately 7,000 yuan/kWh.According to the development speed of lithium battery economy and performance,it is estimated that the invested cost can be reduced to 800 yuan / kWh by 2025,and the comprehensive operation cost will be approximately 4,000 yuan/kWh.The comprehensive cost will be significantly reduced if the automotive energy storage battery can be reused by power grids.

2.2.2 The response speed of pumped-storage power stations increased significantly

Taking a conventional pumped-storage power station as an example,the conversion time is 200 s from static and full-load power generation.A 300 MW pumpedstorage unit requires 150 s from full-load pumping to fullload power generation under emergency conditions.From“load” to “power,” the energy storage battery takes only tens of milliseconds.The incident response speed is greatly increased.Electrochemical energy storage batteries can quickly stabilize new energy fluctuations,such as wind and light,and meet the time response requirements for the connection of the new energy grid.The electrochemical energy storage system with inverters can independently output active and reactive power to meet the compensation requirements of the frequency and voltage of grid power,respectively.The pumped-storage power station working together with the energy storage battery can increase the response speed more quickly,improve the fault ability,achieve multi-time scale coordinated control,and greatly improve the comprehensive performance of pumped-storage power stations.

2.2.3 Key technology of combined operation According to the comprehensive performance of the pumped-storage power station and the characteristics of the energy storage battery,the configuration plan of a combined system including the variable-speed unit,fixed-speed unit,and the energy storage battery needs to be constructed.Regulation-capability improvement technology,such as peak modulation,frequency modulation,and phase modulation,is studied to construct the joint power-control strategy.The model,indicators and methods of reliability evaluation of the comprehensive system should be established and proposed.A comprehensive benefit analysis model of the combined system needs to be established.The comprehensive benefit optimization measures under multiple working conditions should be proposed to establish the pumped-storage power station and the energy storage battery.The application scheme of an energy storage combined system needs to be built.

2.3 Combining photovoltaic energy storage system to create green pumped-storage station

The photovoltaic energy storage system has control flexibility,fast responsiveness,and high reliability.The pumped-storage power station usually has better solar energy and site resources.Therefore,it is a better choice to install the optical storage system in a large-scale pumpedpower station to improve the comprehensive performance.

2.3.1 Advantage of joint operation The optical storage system and diesel engine unit can stand by each other as an emergency reserve or blackstart power supply [10,11].It can improve the reliability of the accident-safety power supply in the pumped-storage station.The joint operation of the optical storage system with sufficient capacity and the pumped-storage power station can improve the response speed of peak modulation,frequency modulation,and phase modulation of the power grid.The power response speed of the new pumpedstorage station can reach the millisecond level,which greatly enhances the safety,reliability,and comprehensive adjustment capability of original large-scale pumped storage-power stations.

Both sunlight and water resources are green and clean energy.If they can be jointly developed in pumped-storage power stations,the site resources of pumped-storage power stations can be fully utilized,and the comprehensive performance,efficiency,and economic benefit of power stations can also be improved to a greater level.

2.3.2 Core technology of joint operation

The core technology of the optical storage system combined with the pumped-storage power station mainly includes the multi-time scale control and operation strategy of the comprehensive adjustment capacity of the combined system,stability and optimization strategy of the joint operation,reliability evaluation method and optimization strategy of large-scale pumped-storage power station,comprehensive benefit and improvement measure of optical storage system accessible to the pumped-storage power station,and the overall application scheme of the largescale pumped-storage power station connected to the optical storage system as backup power.

3 Optimal location of new-generation pumpedstorage station

According to the nature of the power grid,the optimal location of the new-type pumped-storage power station that combines the flexible technology of photovoltaic,electrochemical energy storage,and variable speed units for development and construction should have the following three characteristics.

Variable-speed units also need to comply with the operation status and mode of conventional pumpedstorage power stations.There are three operating states of conventional pumped-storage power stations,namely,resting state,pumping state,and power generation state,as shown in Fig.4.There are six operation modes,viz.,static to power generation mode,static to pumping mode,power generation to static mode,pumping to static mode,power generation to pumping mode,and pumping to power generation mode.Among them,the first four operation modes belong to normal operation mode,and the last two modes belong to emergency operation mode.Only when the pumped-storage power station needs to participate in the disposal of serious system faults quickly,can the last two modes be used.The upper and lower storage tanks of pumped-storage power stations have maximum and minimum storage capacity limits,and any operation mode must operate within the capacity limits.

Fig.4 Sketch of typical operation states and operation modes of pumped-storage station

3.1 New energy-concentration area

The large-scale interconnection of clean renewable energy such as wind and solar power brings a great challenge to the real-time balance and stable operation of the power system.Based on the considerations of improving resource utilization,reducing the impact of new energy,and making system operation stable and the economy better,increasing the response speed and adjustment range of pumped-storage power stations,and enhancing the compatibility between new energy and pumped storage power stations is urgently required.

In the special areas where new energy sources are concentrated,the open space of pumped-storage power stations can be used to build solar energy and wind energy storage systems,and new energy sources can be connected and coupled in pumped-storage power stations to build a new generation of pumped-storage stations.

The new-generation pumped-storage station can automatically track power-grid frequency change and quickly regulate active power.Electrochemical energy storage can improve power-compensation response speed.Variable-speed pumped-storage units can achieve real-time automatic frequency tracking.The pumpedstorage power station can achieve long-term storage of large-capacity power by itself.The multiple-energycombined pumped-storage station can also improve the quantity of new energy connecting to the power grid on the premise of guaranteeing the stability and safety of the power grid.In addition,the start response and regulation rate of the power station are simultaneously improved,which can help better track the output process of random fluctuation in power sources to reduce the impact of new energy grids on the system [12-15].

In order to cope with the impact of the rapid growth of renewable energy on the power grid,the Japan Omarugawa Pumped Storage Power Station installed four variable-speed pumped-storage units.The operating experience shows that the variable-speed pumping unit can compensate for the shortcomings of traditional fixed-speed units.According to the operating experience of the Japanese power grid,a power system equipped with a certain number of variablespeed units and the power-type energy storage battery can fully achieve the power adjustment at full time,reduce the impact of new energy access to the power grid,increase the utilization rate of resources,and improve the consistency between the power grid and renewable energy [16,17].

3.2 UHV receiver concentrated area

For long-distance and large-capacity resource transmission,the questions of power system stability,voltage drop,and reactive compensation are becoming increasingly difficult for the backbone power grid supported by UHV.

The AC excitation of the variable-speed units can effectively solve the problem of the fixed-speed operation of the traditional turbine.It can fundamentally solve the influence of harmonics on the performance of the motor,better adapt to the starting point and terminal point of UHV,and also have better ability to deal with accidents.An electrochemical energy storage system with appropriate capacity can also provide fast reactive power compensation.The inherent phase modulation operation mode of the pumping unit can provide the system with sufficient reactive power demand.The variable-speed pumped-storage unit can decouple the reactive and active power,effectively solve the problem of insufficient or excessive UHV reactive power,and sufficiently aid in system voltage stability [18].

The new generation of pumped-power station with multiple energy sources can provide sufficient reactive power to meet the system reactive power demand during accidental voltage drops in the UHV power grid,and provide large-capacity reactive power compensation to the system together with the synchronous condenser.In the concentrated area of the UHV receiver stations,the building of multi-energy-coupled new-generation pumped-storage power stations can provide large-capacity reactive power support to stabilize the voltage of the power grid.

3.3 Load center areas

Because of the variable-speed unit,optical storage,and chemical energy storage battery,the new generation of pumped-storage power station can provide sufficient independent reactive power and active power to the power grid.If the stations can be selected in the load center areas,the frequency and voltage stability is an important guarantee for the power grid safety on the user side.

The reactive component emitted or absorbed by the unit and the power factor can be adjusted by adjusting the excitation current of the variable-speed pumped-storage unit.It is most important that the variable-speed pumpedstorage unit should operate steadily when absorbing a large amount of reactive power.The electromagnetic regulation of the power generation state can be quickly completed by adjusting the phase of the excitation current in order to ensure the rapid adjustment of the generator voltage or reactive power.The independent adjustment of active and reactive power can significantly improve the static and transient stability of the power system.When the load changes,the AC excitation speed regulating motor can change the speed quickly with the frequency.It can make full use of the rotor kinetic energy to release or absorb the load,which makes the grid disturbance less than that with the conventional motor,to improve the stability of the power grid.Secondly,the variable-speed unit can improve the regulation characteristics of the pumped-storage power station and better ensure the safe and stable operation of the power grid.A wider adjustment range allows for better response to the frequency variation requirements of the power system.Combined with the active and reactive splitting support capability of the chemical energy storage battery,the best result can be achieved for the adjustment speed and accuracy,adjustment range,and response range.The operation of the load-center power grid can be stabilized.

Combined with the current development trend of the power grid,the new energy concentration area,UHV concentrated area,and load center area are all preferred locations for the new generation of pumped-storage stations.

4 Analysis of typical pumped-storage station

Taking one of the provincial power grids in East China as an example,the single power supply structure,the increase in external power supply,and the expansion of wind power scale have become disadvantageous factors for safe and stable operation.It is a typical regional power grid with prominent contradiction between large-scale access of new energy and power grid regulation,as well as a load-concentrated area in East China.Therefore,the pilot application of a new generation of pumped-storage power stations in this region has significant demonstration significance.

Fig.5 Schematic diagram of new pumped-storage station

The construction of a new generation of pumped-storage power station will turn the original fixed load in the power grid into a fast adjustable load,and improve the power flow control during normal operation of the power grid and the stability control ability under emergency conditions.Combined with energy storage batteries,the station can provide good security for the grid.

At present,there are two technical routes for variablespeed pumped storage,viz.,AC excitation and full-power frequency conversion.Considering the comprehensive factors,such as land occupation,technology,and manufacturing,AC excitation is suitable for pumpedstorage units of 300 MW and above.Full-power frequency conversion technology is suitable for medium and small sizes of 200 MW and below.After comprehensive consideration,it is possible to consider the development and upgradation of the original pumped-storage power station.

The original pumped-storage power station project is an important energy construction project during “the Tenth Five-Year Plan” of this Province.It is located in the southwestern suburb of the city,about 3 km away from the urban area.It is a daily regulated pure pumped-storage power station.The power station is a second-class-type project including West Reservoir in the city as the lower reservoir,the new-built upper reservoir,water conveyance system,underground powerhouse,ground switch station,and central control room system.The power station is equipped with four 150 MW single-stage reversible pumped-storage units with a total installed capacity of 600 MW.The 220 kV two-circuit transmission lines on the same tower are connected to the East China Power Grid via the 500 kV clear current substation.In the power grid,it is responsible for many tasks such as peak shaving,valley filling,frequency modulation,phase modulation,accident standby,and black start.

If this pumped-storage power-station represents a new generation of pumped-storage power stations,the installation of four 50 MW full-power variable speed units,a set of 100 MW energy storage battery system,and the appropriate photovoltaic energy storage in the power station empty space,combined with the conventional fixedspeed units can be considered.Joint operation will enhance the comprehensive performance of conventional pumpedstorage stations and provide guarantee to the grid to access local wind energy safely and steadily,as shown in Fig.5.

After the completion of the new generation of pumpedstorage power stations,the following aspects of the original pumped-storage power station can be greatly improved:

(1) The full-power variable-frequency small and medium-sized unit and chemical energy storage working together have better complementarity in function positioning and technical economy.The joint application in regional power grids can fully utilize their respective advantages,improve the level of regional power grid regulation,improve system hot standby and rotating standby,and participate in power grid peaking and frequency modulation at different time scales while quickly stabilizing new energy power fluctuations for safe and stable operation of power grids.It is a good technical solution.After the completion of the project,China can fully grasp the variable-speed technology of pumped storage and the coordinated control technology with large-capacity energy storage batteries,and enhance the level and strength of comprehensive development and utilization of pumped storage in China.

(2) Because of the limitation of the working water head of the pump turbine,the traditional pumped-storage power station can only be selected in high mountain areas,far away from the load center.The variable-speed pumped-storage unit can adapt to a wider range of water heads,improve the stability of the unit,reduce the difficulty in manufacturing the pump turbine,re-establish the construction conditions for many stations that had not qualified before,and build a new energy-storage station with photovoltaic and chemical energy storage systems,which can play a greater role in serving regional power grids.

(3) The variable-speed unit technology of the new generation of pumped-storage power stations can achieve the smooth start of the pump condition and flexible grid connection,which greatly improves the impact of the traditional step-type output of the fixed-speed unit on the grid and enhances the ability of the grid to accept new energy.It can achieve high-speed adjustment of active power.The 400-MW variable-speed unit can change the output power by 32 MW or input power by 80 MW within 0.2 s.The stronger excitation control freedom greatly improves the unit’s phase-in operation capability.

5 Conclusion

The new generation of pumped-storage power stations combines the electrochemical energy storage system,variable-speed pumped storage technology,and energy storage systems,such as wind and light and other new energy sources,which can achieve multiple energy interaction,synergistic complementarity,multi-time window response,and can achieve the double support of active and reactive power.The power grid provides various functions such as peak shaving,frequency modulation,and accident reserve.It can also improve the ability of the grid to accept new energy and quickly stabilize fluctuations.After the completion of the demonstration project of the new generation of pumped-storage power stations,the power grid frequency modulation and load adjustment is more flexible,which can not only improve the emergency response speed of the pumping unit,but also improve the standby capacity of the power station accident,achieve multi-time scale coordinated control of the grid power,and strengthen the ability of the power grid to deal with sudden accidents,effectively improving the capacity of pumpedstorage power stations to serve the power grid.