Constructing a power system predominantly based on renewable energy sources imposes increasingly stringent demands on deep peak shaving capability and ultra-low-load operation of coal-fired power generating units

thereby presents more severe challenges to the safe operation of steam turbine units under low-load conditions. This paper employs numerical simulation methods

focusing on an in-depth analysis of the operational performance of the last stage of a steam turbine under low-load conditions

and explores various solutions for their working mechanisms and optimization effects under ultra-low-load conditions. It is found that

when the unit transitions from medium-low load to ultra-low load

vortex clusters such as gap vortices

backflow vortices

and separation vortices emerge near the last stage blades

with their extent gradually expanding as the load decreases. Reducing the back pressure of the unit and operating the low-pressure cylinder with cylinder-cutting are effective strategies to attenuate steam turbine vortex flow and enhance the last stage's performance

with a combined application of these strategies yielding better results. For instance

under 20% turbine heat acceptance(THA) conditions

reducing the back pressure from 4.9 k Pa to 2.5 k Pa significantly diminishes the influence range of the last stage vortex cluster

increasing the rotor blade torque from-38 N·m to 73 N·m

thereby markedly improves the last stage performance. Under 10% THA conditions

employing a combination of reduced back pressure and low-pressure cylinder-cutting can completely eliminate the tip clearance vortex

with the radial lengths of the backflow vortices and separation vortices reducing by more than 50%. The optimized rotor blade torque increases by approximately 130 N·m

significantly enhancing the last stage performance.