Traction load has a characteristic of transient step impact and will set off weak oscillations in which the generators nearby participate in the accessed grid. Based on the characteristics of wide geographical distribution and fixed disturbance time of traction load

the online monitoring of the system damping corresponding to the particular easily stimulated mode is realized. The characteristic of the disturbing traction load is first analyzed based on the data measured from the supervisory control and data acquisition(SCADA)system. Then the mechanism of oscillations caused by step impact of traction load and the main influences are analyzed on a single-machine infinite-bus system and multi-machine power system

and the online monitoring steps are introduced. Finally

the monitoring units are selected and the correspondence between SCADA measured data of traction load and phasor measurement unit(PMU)measured data of observation units is analyzed in Henan power grid. The online damping monitoring of the system is realized based on the response to the traction load impact.This work is supported by National Key Basic Research Program of China(973 Program)(No. 2013CB228204)and National Natural Science Foundation of China(No. 51207046).