In view of frequent multi-machine power system oscillations in power grid which may cause traditional oscillation blocking and re-opening schemes to be completely invalid, a distance protection oscillation blocking and re-opening scheme in multi-machine power system oscillation scenes is proposed in this paper. First, the ride-through trajectory of measured impedance in multimachine power system oscillation cases and its effect on the operation characteristic of distance protection is analyzed, and the oscillation blocking scheme is formed by combining the operation status information of protection components on 2 line ends. And then, in the oscillation blocking duration, by calculating the estimated error of the dynamic phasor of measured current at the relaying point, the protection re-opening scheme for symmetrical fault during oscillation is established. Finally, simulation tests on PSCAD in 3-machine system and IEEE 39-bus system verify that the proposed scheme could reliably block the protection when oscillation occurs in multi-machine power system, and re-open the protection accurately and reliably when symmetrical fault occurs during oscillation. KEYWORDS distance protection, dynamic phasor, multi-machine power system oscillation, oscillation blocking 1 | INTRODUCTION When power system is disturbed by factors such as transmission line fault and heavy-load line switch on/off, etc., oscillation will easily result. 1 The incorrect operation of distance protection on transmission line which identifies whether fault occurs by calculating the measured impedance during oscillation will enlarge the influence range of oscillation, greatly endangering the safe and stable operation of power system. 2 In regional inter-connected power grid that is becoming ever more complicated, the probability of multi-machine power system oscillation is also increasing rapidly, 3 in which case the oscillation center may move dynamically along different routes, causing traditional double-source analysis model to be invalid. 4,5 Therefore, it is urgent to study in depth the effect of multi-machine power system oscillation on existing protection policy and put forward effective oscillation blocking and protection re-opening scheme.Currently, research on oscillation blocking is mostly based on 2-generator model and is mainly focused on the following aspects. First, identification criteria based on swing center voltage (SCV) could realize re-opening for symmetrical fault during oscillation, with certain time delay. 6,7 Second, identification criteria based on the compensation principle could determine whether it is oscillation or short circuit fault that occurs within the protection range, but it is difficult to set the compensation value when the line is heavily loaded. 8,9 Third, identification criteria based on the apparent