Head first
Designers of a pure pumped-storage plant typically seek to minimize its capital and operating costs by employing either of two techniques: making its hydro turbines more efficient, or boosting the overall plant's rated capacity by increasing the available head (the difference between the two reservoirs' elevation). Two side benefits result if a plant is designed for a higher head: The facility can be made smaller and still produce the same output, and the size of the turbines' discharge can be reduced. For those reasons, TEPCO chose to increase KHP's available head. This decision necessitated the company's development of its "high-efficiency/high-head pump-turbine" as a breakthrough technology.
Historically, TEPCO's large pumped-storage plants have had extremely high effective-head requirements, and Kannagawa continued that tradition by pushing turbine manufacturing techniques to the limit. As Figure 3 shows, the utility's Kazunogawa Power Plant, whose first unit started up in 1999, set the bar high with a maximum effective head of 2,343 ft (1,015 psi) and a planned output of 1,600 MW. Just below it is Kannagawa, with a maximum effective head of 2,142 ft (938 psi) and a planned output of 2,820 MW. The units at both plants comprise vertical-shaft Francis turbines rated at 482 MW (max) and three-phase generator-motors rated at 525 MVA.
3. Bigger is better. Unit capacity and pumping head of pump turbines. Source: Tokyo Electric Power Company Inc.
At the time of Kazunogawa's development, existing pump-turbine technology required two-stage pumped-storage power generation, using three reservoirs (upper, middle, and lower). When Kannagawa hit the drawing board, the construction cost premium for elevating water using state-of-the-art hydro pump technology was 20%, compared with a two-reservoir design.
TEPCO responded to this challenge by developing a pump-turbine with an effective head of over 2,300 ft and a unit output greater than 400 MW. To meet those specs, TEPCO's designers had to solve some very tough problems. They included the typical decrease in the operating efficiency of pump-turbines when subjected to higher effective head, the turbines' limited load adjustment range, and the limited range of dam-water level variation during real-world operation. Adding to TEPCO's difficulty, the industry's experience had been that standard pump-turbine runners capable of handing 1,000-ft effective heads would break if pushed to 1,600 ft—rendering conventional runner design obsolete.
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