Energy Gain from Tehri PSP Due to Adoption of Variable Speed Technology

Abstract

The use of variable speed machines with voltage source power electronic converters has gained prominence globally in the field of hydro power plant application. The doubly fed asynchronous machine (DFAM) for pump storage plant offers stability over complete range of head variation and load variation. This technological advantage over conventional fixed speed synchronous machines has led to recent development of large capacity doubly fed induction machines (DFIM). One such project employing four DFIMs, each having a capacity of 250 MW and ±6% of speed variation with respect to synchronous speed (500 rpm) is already under operation in Linthal, Switzerland. The maximum efficiency of synchronous hydro-generating units is achieved under rated head and discharge. While operating farther to the rated head, the efficiency of hydro turbine goes down. Therefore, when a wide variation in the water head is available, the machine rotating at fixed speed over the entire range would result in reduction in turbine performance. Variable speed machine provides the solution by adjusting the speed of the turbine over the available water head range to get maximum efficiency. The design of the stator of these machines is identical to that of synchronous machines. The rotor design of the DFAM, however, differs remarkably from conventional (synchronous) machines. The rotor is similar to the stator and equipped with a three-phase winding embedded in slots of a stacked core instead of salient poles. This paper is aimed at computation of annual energy generation from Tehri PSP with a 250 MW machine under the rated head of 188 m and turbine speed of 230.77 rpm for machine having (a) variable speed and (b) fixed speed. The power generation shall be computed for the various head levels taking into account the efficiencies from Hill curves of reversible Francis turbine for fixed as well as variable speed.