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World's First Adjustable-Speed Pumped-Storage Generating System

The technology of the adjustable-speed hydroelectric generator, which had been developed 60 years before, was revived as the world's first adjustable-speed pumped-storage generating system.

World's First Adjustable-Speed Pumped-Storage Generating System

In a conventional hydroelectric power plant, the generators rotate at a constant speed corresponding to the system frequency (50 Hz in eastern Japan and 60 Hz in western Japan). However, it was known that water turbines coupled to the generators could run with higher efficiency if their rotating speed could be adjusted to conform with variations in the operating head and power output.

Toshiba developed Japan's first adjustable-speed generator (rated 750 kVA), which could run a water turbine at its optimal speed to achieve the best operating efficiency. It went into operation in 1930 at the Yoshino No. 2 Power Station of the Kanazawa Municipal Utility Board (later merged into Hokuriku Electric Power Company). Although it was a highly advanced generator in those days, its technology was not implemented thereafter in other power plants.

With the rapid postwar growth of the Japanese economy, power demand increased significantly. In parallel, the imbalance of power demand between day and night became larger and load fluctuations also became greater. A power generation system must be capable of quickly adjusting to load variations in order to maintain stability of the power grid frequency. Hydroelectric power generation can readily satisfy this requirement because its output can be swiftly adjusted by regulating the flow rate. From the 1960s onward, however, very few sites were available in Japan for constructing large-capacity hydroelectric power plants that could cope with large load variations.

Pumped-storage power plants were therefore introduced as an alternative to conventional hydroelectric plants. Conventional hydroelectric plants use the natural flow of a river to generate electric power, whereas pumped-storage power plants use the water in an upper reservoir pumped up from a lower reservoir during the off-peak period at night. For pumping, this system uses surplus power generated by other generating sources such as nuclear power stations. This means that large-capacity pumpedstorage power plants can be built anywhere, regardless of the availability of a large river flow. Since the power output of pumped-storage plants can be adjusted as quickly as that of conventional hydroelectric plants, they are now playing an indispensable role in Japan for the stable supply of power and maintenance of system frequency.

However, the generator-motors of pumped-storage units have so far mostly been synchronous machines that have to run at a constant synchronous speed. Due to the basic characteristics of pump-turbines, the input power in the pumping operation cannot be changed when the system is running at a constant speed. If it were possible to change the rotation speed, it would be possible to change the pump input. This would make it possible to adjust the balance of power supply and demand and to regulate the system frequency even in pumping operations at night.

Focusing on this idea, Toshiba conducted a joint study with Tokyo Electric Power Company and developed the world's first adjustable-speed pumped-storage system using a generator-motor with low-frequency AC secondary excitation. The first commercial unit was commissioned at the Yagisawa Pumped Storage Power Plant in 1990.

The principle of this adjustable-speed generator-motor was the same as that of the generator installed at the Yoshino No. 2 Power Plant 60 years earlier, except for the use of a cycloconverter as the low-frequency AC excitation system in the new unit. The cycloconverter is a static frequency converter composed of a number of large-capacity thyristors. This adjustable-speed pumped-storage unit is controlled by a high-speed, high-performance digital controller, which allows its input or output to be changed much more quickly than in conventional hydroelectric units. Such fast power control realizes more complex and\advanced control than in conventional units, such as the stabilization of large power oscillations in the grid in the event of an emergency.

金沢市電気局 吉野第二発電所

Yoshino No. 2 Power Station of the Kanazawa Municipal Utility Board
750kVA Adjustable-speed generator

電源開発 奥清津第二発電所

Electric power development
Okukiyotsu No. 2 Power Plant
Adjustable-speed generator-motor rotor

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