A turbine is a type of rotating equipment designed to remove energy from a fluid. For a given flow rate, turbines remove a specific amount of the fluid's energy head.

In a hydroelectric power plant, turbines convert the moving water’s kinetic energy to mechanical (rotational) energy. Each turbine is mechanically coupled with a generator that converts rotational energy to electrical energy. Each generator's output terminal transmits electricity to the distribution grid. At steady state, the electricity produced by the turbine-generator system is equal to the electrical grid load on the generator.

The figure below is a generalized schematic of a hydroelectric power generation plant. A reservoir (usually elevated) supplies a low pressure tunnel and a penstock. Water flows through the penstock under increasingly higher pressure (and velocity if diameter decreases) as it approaches the turbine. Most of the turbine's rotational energy drives a generator to produce electricity. Water emerges from the turbine through the draft tube and tailrace and flows into the downstream reservoir. Surge tanks can be connected to the penstock and/or tailrace to limit the magnitude of transient pressures, especially if the length of the upstream conduit/penstock or if (rarely) the tailrace is relatively long.

Hydraulic turbines and penstocks often operate under high pressure at steady-state. Rapid changes such as electrical load rejection, load acceptance or other emergency operations can result in very high transient pressures that can damage the penstock or equipment. During load rejection, for example, the wicket gates must close quickly enough to control the rapid rise in rotational speed while keeping pressure variations in the penstock and tailrace within established tolerances. Using Hammer, designers can verify whether the conduits and flow control equipment are likely to withstand transient pressures that may occur during an emergency.

Electrical load varies with time due to gradual variations in electricity demand in the distribution grid. Depending on the type of turbine, different valves are used to control flow and match the electrical load. Turbines can be classified into two broad categories: a) impulse turbine, and b) reaction turbine.