Hydraulic Transients in the Operation of Small Hydro Plantsby H. S. Gill, Acres Int Corp, Amherst, United States,
P. R. Rodrigue, Acres Int Corp, Amherst, United States,
Document Type: Proceeding Paper
Part of: Waterpower '91: A New View of Hydro Resources
Adequate governing stability is a primary consideration for most large hydroelectric generating units. However, almost all small units installed recently in the United States are intended for installation in a very large electrical grid where speed is controlled by the system frequency (60 hz). Because of this, governing is of relatively minor importance. Gate operators rather than speed sensing governors are common control devices for smaller turbines. Furthermore, rapid wicket gate operating time which is usually necessary for good speed regulation is no longer a requirement. In the interest of economy, plants have been constructed with very long penstocks without surge protection. To prevent excessive waterhammer, the wicket gate closure time is lengthened, and the units reach near full runaway speed on load rejection. Such practices are in variance with most published literature on large hydro plant design. The design concepts used for several modern small hydro plants, with regard to governing and considerations for speed rise and hydraulic transients are reviewed. A comparison is made between the design practices for small units with the more traditional criteria which is used for larger turbines or for units which must operate on isolated load. Examples of plants with long penstocks are given. The effect of the hydraulic design (specific speed) of the turbine is reviewed. This includes the throttling effect of low specific speed turbines, as well as flow acceleration with high specific speed Kaplan units. Both factors cause hydraulic transients which are more or less independent of wicket gate time.
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