E-34

Keywords

Y3

equipment or facility

Z4electricity

E29electrical machinery

Toshiba Energy Systems & Solutions Corporation

Adjustable Speed Pumped Storage SystemsFeatures
S A pumped storage power plant can store a large amount of electric power for a long time, which
contributes to leveling of demand of the power system, and is instrumental in making efficient operation of the overall power system possible.
However, conventional pumps only operate at a fixed revolution speed, so input (power) adjustment
during pumping is not possible.
S By using an adjustable speed pumped storage system in which it is possible to change the revolution
speed of pump, the following advantages for operation can be expected.
1) It is possible to adjust the power during pumping at night or when there is a light load, so conventional
thermal power plants that are operated for frequency adjustment can be stopped, which contributes toeconomical operation of the power system and CO2 reduction.
2) It is possible to absorb fluctuation in output power from renewable energy sources such as wind poweror solar power generation, and thus it is possible to improve the stability of the power system.
⇒ This makes it easier to promote the adoption of renewable energy sources.
3) By operating the turbines at an optimum revolution speed when generating during peak load times, it
is particularly possible to improve the efficiency of partial loads.
4) Instantaneously adjusting power and voltage contributes to stabilizing fluctuation in the power system.
Basic Concept or Summary
N1 = Nm + N2
N1: Rotating Speed of Stator Rotating
Magnetic Field
Nm: Rotating Speed of Rotor
N2: Rotating Speed of Rotor Rotating
Magnetic FieldN2 has a rotating rate equivalent tothe output frequency of thefrequency converter.

Main
Transformer

Configuration of an adjustable
Speed Pumped Storage System

Explanation of the Fundamentals
of Adjustable Speed Operation

1) The rotor of the generator is cylindrical and has three-phase winding.
2) Three-phase AC current flows to the rotor from a frequency converter, generating a rotating magnetic
field, where the revolution speed thereof (N2) is proportional to the converter output frequency.
3) The revolution speed N2 of the rotating magnetic field is added to the revolution speed Nm at which
the rotor rotates, which maintains synchronization with the rotating magnetic field N1 of the stator on
the stationary side, having a relationship N1 = Nm + N2. In other words, even though the unit speed ischanged the difference between the synchronous revolution speed and the rotor revolution speed iscompensated by the rotating magnetic field, so the power output of the generator stator can be maintained at a fixed frequency.
4) When the change of the revolution speed is about ±5 to 8%, the input during pumping can be adjustedby approx. 60% to 100%.

Effects or Remarks
S It becomes possible to stop the thermal power plants that were additionally operated in order to adjust thefrequency of the system during light loads such as at night, so it is possible to reduce the consumption offossil fuels such as petroleum and coal.
Explanation: In order to adjust the system frequency, the generated power output or the consumed power
must be adjusted so that the generated power = consumed power. During hours when the demand for
power is low such at night, many thermal power plants and hydropower plants are stopped so that theadjustment fee for the generated power is reduced. Therefore, there used to be no option but to adopt anenergy inefficient way of operation of maintaining the capacity to cope with the fluctuation of the demand byoperating multiple thermal power generation units with adjustable output at low output. However, since it is
now possible to maintain the capacity to cope with the fluctuation of the demand after the application of theadjustable speed pumped storage system, it is possible to reduce the number of power generation units inthermal power stations operated during the night and, consequently, reduce consumption of fossil fuels.
S When generating operation, the unit can be operated at lower output power by changing the rotating speed
and the adjustment range of output power can be widened.
Explanation: The pumped storage systems are often considered to be a power adjuster, and they can serveas system stabilizers to ensure the necessary amount of power by increasing output in cases where otherpower sources unintentionally drop from the system during minimum output operation. Since theadjustable-speed pumped storage systems can be operated at lower output than the conventional pumped
storage systems and have wider output power adjustment range between its minimum to maximum (rated)
outputs, the number of the operating units in pumped storage systems and power required for next pumping
operation can be reduced, and it is expected to lower fossil fuel costs and CO2 emissions.
S By reducing the fossil fuel consumption, as described above, it is possible to reduce the amount of carbon
dioxide, while benefitting economically.
S The adoption of using renewable energy sources such as wind power and solar power is advancing with thegoal of reducing CO2, however, since natural energy is used, output can be caused to fluctuate and become
unstable due to weather or the like, and as the percentage of renewable energy sources used increases, it
becomes difficult to adjust the frequency of the system.
Such fluctuations in the output of generated power can be absorbed by an adjustable speed pumped
storage system, which can be expected to assist in stabilizing the system, particularly in regions where ahighly excessive amount of power is generated, by solar power during the day, or where there is anexcessive amount of power generated by wind power at night, when the load is low.
S By operating the turbine at an optimal revolution speed during generating, it is possible to improve theefficiency during partial load operation by about 3%.
S It is possible to instantaneously change the output current phase angle of the converter by vector control,and thus it is possible to instantaneously change the active power and reactive power of the generator
motors (during both generating and pumping). As a result, it is possible to suppress power fluctuation in thepower grid and, when there is voltage fluctuation, it is possible to suppress the voltage fluctuation byadjusting the reactive power at speed comparable to the processing speed of the static VAR compensator(SVC).
S As application of adjustable speed pumped storage system, a flywheel generation system for stabilizing thesystem frequency has been put to practical use. In Okinawa island, (Japan), where the power system isindependent, the frequency changed due to power fluctuation that occurred by start and stop of arc furnaces
at a steel plant, so by installing a flywheel generator, the power fluctuation is stored as rotation energy of
the rotor and flywheel, and by discharging that energy, the system frequency could be stabilized.
Installation in Practice or Schedule
Domestic

TEPCO Yagisawa Power Plant 85 MVA 1990 (Started world's first operation)
TEPCO Shiobara Power Plant 360 MVA 1995
J-Power Okukiyotsu Power Plant No. 2 345 MVA 1996
J-Power Yambaru Seawater Pumped Storage Power Plant 31.5 MVA 1999
Okinawa Electric Power Company Chujyowan Sub Station 26.5 MVA flywheel generator 1996
(Application)
TEPCO Kazunogawa Power Plant Unit 4 475 MVA 2014
Hokkaido Electric Power Company Kyogoku Power Plant Unit 1 230 MVA 2014
Hokkaido Electric Power Company Kyogoku Power Plant Unit 2 230 MVA 2015

Contact:

Toshiba Energy Systems & Solutions Corporation,
Power Systems Division
http://www.toshiba.co.jp/thermal-hydro/en/hydro/products/equipment/index.htm

E-34