The typical failures of cylinder cutting and cylinder merging in double-reheat 1 000 MW coal-fired unit during(extremely) hot start-up are studied. The changes of main parameters during four(extremely) hot start-up processes are analyzed in detail

including 100% load rejection three times and 50% load rejection once. Four failure examples are discussed

namely the(extremely) hot start-up phase of the cylinder cutting

two failures of load rejection caused by cylinder merging

reheater protection trip after cylinder merging and the successful cylinder merging. The results show that

the three-cylinder combined start-up method is likely to cause the ultra-high pressure cylinder cutting during the(extremely) hot start-up. The use of Fastly rate in digital electrohydraulic control system to release the valve limit of ultra-high pressure regulating valve can easily cause load overshoot and sudden increase of the turbine axial displacement

which will cause severe disturbance of the unit parameters

failure of cylinder merging

and even load rejection. After successful cylinder merging

the target load should be adjusted in time to suppress the sudden change of the total flow command. Increasing boiler fuel in time and automatically adjusting the bypass opening to stabilize the main steam pressure is the key to safe operation after cylinders merging. The research results provide an important reference for the safe

stable and fast start-up of the unit in(extremely) hot state.