6.2.4.2 Gearboxes and other speed increasers

When the turbine and the generator operate at the same speed and can be placed so that their shafts are in line, direct coupling is the right solution; virtually no power losses are incurred and maintenance is minimal. Turbine manufactures will recommend the type of coupling to be used, either rigid or flexible, although a flexible coupling that can tolerate certain misalignment is usually recommended.

In the lowest power range, turbines usually run at less than 400 rpm, requiring a speed increaser to meet the 1000?1500 rpm of standard alternators. The speed increaser can be chosen from the types commercially available in the market:

· Parallel-shaft gearbox,
· Epicyclic gearbox,
· Right angle gearbox with bevel gears,
· Belt drives.

Gearboxes substantially increase the noise level in the powerhouse and require additional maintenance. Moreover the friction losses may amount to 2% of the output power. Flat or V shaped belts constitute the simplest and cheapest solution.

6.2.4.3 Generators

Generators transform mechanical power into electricity. Although most early hydroelectric systems used DC generators to match early commercial electrical systems, nowadays only three-phase AC generators are used in normal practice. Depending on the characteristics of the network supplied, the choice is between:

· Synchronous alternators, equipped with a DC excitation system (rotating or static) associated with a voltage regulator to provide voltage, frequency and phase angle control before the generator is connected to the grid. Synchronous generators can run isolated from the grid and produce power since excitation power is not grid-dependent.

· Asynchronous generators, that are simple electric squirrel-cage induction motors, with no possibility of voltage regulation, which operate at a speed directly related to system frequency. They draw their excitation current from the grid, absorbing reactive energy, so they cannot generate when disconnected from the grid.

Synchronous alternators are more expensive than asynchronous generators, at least up to about 2 MW, and are used in power systems where the output of the generator represents a substantial proportion of the power system load. Asynchronous generators are used in large grids where their output is an insignificant proportion of the power system load. Their efficiency is 2 - 4% lower than that of synchronous generators over the entire operating range.