Modelling and control of slip power recovery schemes for small hydro power stations

It is well known that small hydropower stations (SHS) are characterised by wide variations in water- flow-speed due to the absence of an elaborate reservoir facility. As a result, conventional synchronous generators which are basically constant-speed constant - frequency systems are not well suited for use in such schemes. The usual problem is low efficiency arising from relatively poor control of active and reactive power flow in the system. This paper investigates the use of a class of doubly fed induction generators (DFIGs) for the energy conversion process in SHS. In contrast to the conventional DFIG employing the so-called back-to-back configuration, the proposed scheme utilises two voltage source inverters (VSI), each connected to the stator and rotor respectively. This gives rise to four degrees of freedom of control, which presents several advantages including field weakening operation and control over twice the rated speed range. Simulation studies of a 600 KW machine are presented to show the efficacy of the proposed scheme.