Due to DC unit fault of converter station

continuous subsynchronous oscillation occurred in an ultra supercritical 1 000 MW unit.In order to evaluate the effect of this subsynchronous oscillation event on security of the unit shafting

mathematical model for this ultra supercritical 1 000 MW unit with subsynchronous oscillation was established

considering the coupling effect between the electric and unit shafting.By using simulation method

a small amplitude sub synchronous frequency voltage was connected in the grid-side of system

and subsynchronous oscillation of the shafting was inspired

to simulate the torque acting on the ultra supercritical 1 000 MW unit shafting during the subsynchronous oscillation.The results show that

the die speed deviation in this ultra supercritical 1 000 MW unit is stable at 0.002 86 p.u.

and that of the machine tail is stable at 0.001 20 p.u.

which is basically the same as the vibration curve(the vibration ratio of the machine head to the end is about 0.4).The main oscillation frequency is 23.8 Hz

which is the natural frequency of the second order torsional vibration of the shaft system.The torque between the low generation wheel is the largest

the state and torsional vibration of the wheel should be closely monitored during the actual operation.