The performance degradation mechanism and improving techniques of the last stage of a steam turbine in deep load regulation scenario

Large scale access of new energy power requires thermal power units to be frequently regulated to ultra-low load condition, leading to a significant degradation of the last stage performance of a steam turbine. The affecting mechanisms and improving techniques of performance degradation were investigated through numerical simulation. It is found that the volumetric flow rate of the steam at the inlet of the rotor blade is almost linear to the relative torque of the stage and deemed as a key state parameter of load change. Approaches of reducing the back pressure and cutting half of the LP (low pressure) cylinder both obviously increase the volumetric flow rate at the inlet of the rotor blade, improve the flow field of the last stage, and significantly enhance the overall performance of the last stage at low load. A combination of the two approaches achieves a synergistic effect, increasing the volumetric flow rate at the inlet of the rotor blade by around 183 %, and recovering the blade torque by 182 % of the unit value. Meanwhile the highest temperature at the blade tip is reduced from 360 K to 305 K, resulting in significant improvements in the safety and efficiency performances in the last stage.