The impact of flexible operation on Francis runners

Although hydropower plants have traditionally been operated close to best efficiency point as base load providers, they have always been very well suited to contribute to grid stability because of their high operating flexibility. They can contribute to grid stability by providing considerable output and, at the same time, be regulated from part load to full load to deliver primary control at very short notice. The recent increase in grid instabilities because of the arrival of new energies on the market, such as solar and wind power, coupled with market deregulation, increases the demand for flexible operation of hydraulic turbines. It is therefore not uncommon to see these machines being operated over the whole range, with many start-stops, extensive low load operation, synchronous condenser mode and power/frequency regulation. This paper demonstrates how flexible operation affects the life expectancy of Francis runners.