Hydraulic head is the elevation difference the water falls in passing through the plant. Gross head of a hydropower facility is the difference between headwater elevation and tailwater elevation.
(Headwater is the water in the forebay or impoundment supplying the turbine; tailwater is the water issuing from the draft tube exit.)
Net head is the effective head on the turbine and is equal to the gross head minus the hydraulic losses before entrance to the turbine and outlet losses. Doland (1954) defines design head as the effective head for which the turbine is designed for best speed and efficiency.
Rated head is the lowest head at which the full-gate discharge of thz turbine will produce the rated capacity of the generator. It is normally referred to as the rated net head in the guarantee of the manufacturer.
The term is sometimes used interchangeably with the term effective head. Another term used is critical head. Engineering Monograph No. 20 of the U.S. Department of the Interior (1976) defines critical head as the net head or effective head at which full-gate output of the turbine produces the permissible overload on the generator at unit power factor.
This head will produce maximum discharge through the turbine. "Critical head" is used in studies of cavitation and turbine setting. which will be discussed later. Sheldon and Russell (1982) present a composite reference of the various head definitions and terms.
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