Comparison of Variable Modes of the Last Stages in Half- and Full-Speed Turbines for NPPs

Abstract

In the modern power grids in the European part of the Russian Federation, there is a significant shortage of maneuverable capacity. According to estimates done by the leadingEnergoset’proekt Institute,the base load mode of nuclear power plants (NPPs), which is the most profitable mode of their operation, can be ensured if the total percentage of NPPs does not exceed 22--24% of the total generating capacity. With the further development of nuclear power plants and increasing the share of electricity generated at NPPs, the latter will have to assume the functions of controlling the power system load. Heating up of the exhaust hood at low steam flow rates due to the occurrence of flow separation phenomena in the root section of the last stages is the factor determining the technical minimum for steam turbineload. Engineers who design large-capacity wet steam turbines (over 500 MW) face thefollowing alternative: constructingfull-speed (3000 rpm) units for driving a two-pole generator or half-speed (1500 rpm) units for driving a four-pole generator. Half-speed turbines have certain advantages: they have a fewer number of low-pressure cylinders (LPCs) due to a larger height of the last-stage blades; it is easier to optimize the profiles of their last stages due toa smaller pressure gradient in the bucket clearance, due to a smaller optimal heat drop in the design mode, and due to significantly smaller relative velocities at the root downstream of nozzle vanes and at the periphery downstream of the rotor blades. To compare the performance of half- and full-speed turbines at partial loads, the last stages of these turbines were calculated in the range of operating conditions from the rated design parameters to the idle running mode. The calculations were performed using the inviscid axisymmetric flow method taking into account energy losses according to the procedure developedat the Moscow Power Engineering Institute National Research University’s Department of Steam and Gas Turbines. The calculation results have shown that the power units that can participate in the control of load within the range from 0.3 to 1.0 of the nominal power shall be fitted withfull-speed turbines in which the flow separation mode at the root of rotor blades occurs at smaller relative volume flowratesthan in half-speed turbines.