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Transient amplitude or phase changes in current or voltage waveforms may seriously influence synchrophasor estimators accuracy and responsiveness. The recently released IEEE standard C37.118.1-2011 specifies test conditions as well as accuracy and response delay limits for different types of disturbances. In this paper the performances of three state-of-the-art phasor estimators are analyzed and compared in depth under the effect of magnitude step changes, phase step changes and linear frequency variations. Several simulation results show that in all cases the total vector error (TVE) tends to increase linearly with the magnitude of the step (either in amplitude or in phase). This trend is confirmed even when the waveform frequency is off-nominal, provided that the step is large enough. Moreover, the estimators including the phasor derivatives are generally characterized by longer response time than the basic DFT since the step perturbation affects two or three consecutive one-cycle observation intervals. When linear frequency increments occur, the TVE values of all the considered estimators lie within the same boundaries determined for static off-nominal frequency offsets. Therefore, the considerations on estimator accuracy are the same as in the static case.