ABSTRACT

An alternative method of electrical governing for small-scale hydro, now made possible by the development of large power static-variable frequency converters, is to allow the turbine to ‘free run’. This is a method adopted for wind-driven generators and considered for some wave-energy systems. The generators operate at variable frequencies according to the load, and the output is converted to direct current controlled at a constant value by a current regulator, and back again to alternating current (Figure 2.13). In effect, the power is transmitted to the a.c. system via a back-to-back d.c. link. The disadvantages are the need to vary a number of units to suit the constant flow as the load varies, the low efficiency at part load and the need to design for fairly frequent runaway conditions.