Chapter 1　The Development History and Management Experience of Pumped Storage Power Stations

1.1　An Overview of Pumped Storage Power Stations

1.1.1　The Fundamentals

A pumped storage power station is a special type of hydropower station.It makes use of the excessive electric energy in the valley time of power grid load by pumping water from lower place to higher place to storage potential energy.In the peak time of power grid load，it discharges the water from higher place to lower place by turbines.The stored energy potential is then turned into electric energy and feed back the power grid.Pumped storage power stations therefore play the role of peak load modulation and valley load filling and support the power grid.A pumped storage power station is composed of upper reservoir，lower reservoir，water conveyance tunnels，powerhouses，switchyard，etc.As shown in Figure 1.1.

The workflow of pumped storage power station includes storing，transferring and releasing electricity.Pumped storage power station improves the energy efficiency and utilization rate in combined operation of multiple energy types in the power system，such as coal-fired power，nuclear power，wind power，solar energy and tidal power.It helps keep a dynamic balance among production，supply and consumption of power in the power system.

1.1.2　The Functions

A pumped storage power station features quick start/stop，flexible and reliable operation，and quick response to volatile load，and has the functions of peak shaving，valley filling，frequency regulation，phase modulation，load reserve，emergency reserve and black start.

(1)Peak shaving and valley filling.In the valley load time，pumped storage power station uses excessive power and energy of the power grid to pump and store water.In the peak time，it generates power by the pumped water.These are called peaking shaving and valley filling.It can significantly reduce the peak-valley difference of power grid，improve the operation conditions of coal-fired power unit，nuclear power unit and other units in the power grid，and cut down the operation costs and prolong the equipment longevity.The peak shaving and valley filling activity of pumped storage power stations can reduce the loss of water releasing for peak demand supply in flood season，make full use of valuable water resources，cut down the operation cost of power grid in peak time，and alleviate the difficulty of peak shaving of power grid.It can also improve the operation conditions of coal-fired power units by avoiding intermittent output and frequent start/stop of the unit within one day and keep stable output and operation.It helps overcome the difficulties，such as deep peak regulation，frequent load adjustment，and start/stop，confronting coal-fired power units in valley and peak time，extend the longevity of units，reduce the fuel consumption and operation cost of coal-fired power plant，and thus yield considerable system benefits.

The economic benefits to coal-fired operation brought forth by the pumped storage power units are mainly reflected as follows.

1)Its participation in the operation of power grid can reduce the number of operating coalfired power units，improve their operating mode and power generation load rate，maintain an economical load range and thus reduce the coal consumption for power generation;

2)Since the pumped storage power units change the combination of coal-fired power units，using electric units of lower cost to pump water in valley time and the units of higher cost to generate power in peak time，more units of higher cost can be put standby and thus the fuel cost of power grid can be reduced;

3)The peak shaving and valley filling of pumped storage power stations reduces the start/stop of coal-fired units for peak shaving and thus cuts down the start/stop cost of the power grid.

(2)Power supply support.As one of the power sources，pumped storage power stations can provide effective capacity in peak time and thus support the power supply by replacing or postponing the investment in other power resources.

(3)Energy storage.A pumped storage power station is a huge energy storage tool and energy convertor.It uses water to redistribute electric energy in the power system in different time，coordinates the time difference between electric power output and power consumption，and ensure safe and economical operation of the power system.

(4)Load tracking.The load tracking service refers to tracking the change of user's load at different hours of a day and time within an hour by using online power plant，electric energy storage or load equipment.A pumped storage power station is able to change output within a few minutes to track the change of load including upward and downward directions.

(5)Frequency regulation.To ensure stable operation，the power grid should be capable of regulating load at any time to fit the user's load change.However，the range of the entire power grid should be maintained stable.For example，the State Grid requires to control the frequency range between 50 Hz±0.2 Hz.To this end，the power grid should be equipped with a certain number of frequency regulation units capable of quick and sensitive reaction and the possess the capability to accommodate to instant load change.Pumped storage power stations usually can start from static status to full load within two minutes.Its capability to quickly follow the load change helps maintain the frequency of the power grid and improves the quality of power supply，safety and stability of power grid.

(6)Phase modulation.The power grid in operation includes active power and reactive power.Inadequate reactive power may lead to voltage drop of the power system and inversely cause voltage rise of the system.The unstable voltage may directly threaten the quality of supply and safe operation of power grid.Therefore，in addition to the balance of active power in the system balance of power grid，the balance of reactive power is also needed.

Under the four operating modes namely power generation，pumping，power generating phase modulation and pumping phase modulation，pumped storage power stations are able to transmit to the power grid or absorb reactive power and maintain the stability of voltage.In the meantime，they can also improve the performance of power transmission and distribution system，and compensate any abnormality or volatility of electric energy，such as voltage drop，unstable voltage，sub-synchronous resonance，etc.

(7)Standby power source.Black start means that after breakdown of the whole power system，without assistance of other networks，the units with black start capability can trigger the units with no self-starting capability，extends to the scope of system and finally rehabilitates the whole system.

With the development of power grid，the lack of stability and reliability of the system may cause serious system accidents，due to the impact brought by local problems to the adjacent areas.Black start therefore has become one of the most important measures to rehabilitate the system operation after occurrence of accidents.The key to the black start service is the start power source，such as the unit with black start capability.Pumped storage power stations are capable of quick self-start without external assistance.They transmit start power via transmission lines to trigger other units to ensure that the power system can rehabilitate its power supply as quickly as possible.Most pumped storage power stations in China have taken black start tests and have been proven with the capability of black start.

(8)Other functions.In addition to the aforesaid functions，pumped storage power stations can undertake load rejection test during commissioning of high nuclear power and coal-fired power units as well as the special load effect in full-load vibration experiment.On some special occasions they may ensure the reliability of power supply，mitigate blockage of power grid，cut down investment in transmission and distribution systems，etc.

1.1.3　The Benefits

The benefits of pumped storage power stations include static benefits，dynamic benefits and social benefits.The static benefits include power generation benefits，capacity benefits and coal-saving benefits，while the dynamic benefits include auxiliary service benefits，such as frequency regulation，phase modulation，standby and black start.The social benefits include higher power output quality，safer power grid，less environment pollution，more scenic spots，etc.

Thanks to its operation characteristics in the power grid，the pumped storage power station plays a positive role in environment protection and energy conservation.The ecological landscaping of pumped storage power station also impels local environment and social development.In particular，the ecologically friendly utilization mode of pumped storage power station and the beautiful local natural views are in harmony with each other.The water in the reservoir also improves the surrounding aquatic conditions，improving the growth，habitat and reproduction of animals and plants as well as the conservation of the local water sources.The construction of pumped storage power station facilitate the construction of local infrastructure and further improves the quality of natural views，which then effectively boost the development of the related sectors such as tourism and spurs local economic growth.