A method of determining relative extents of contribution of plural grid subsystems to oscillation in angle or grid oscillation in an electrical grid, the method being carried out by a computer and comprising in respect of each of the plural grid subsystems: receiving a first quantity which corresponds to oscillation in voltage angle or rotor angle at the grid subsystem; receiving a second quantity which corresponds to oscillation in voltage angle or rotor angle at another one of the plural grid subsystems, the first and second quantities being based on a same measured property; and determining a contribution of the grid subsystem to oscillation in angle or grid oscillation, the contribution being determined in dependence on a phase relationship between the received first and second quantities, determining the contribution comprises calculating the contribution in dependence on a phase difference between the first and second quantities, the method further comprising comparing the thus calculated contributions for the plural grid subsystems to determine relative extents of contribution of the plural grid subsystems; wherein the contribution of a grid subsystem of the plural grid subsystems is calculated by: determining an average oscillation phase angle for the plural grid subsystems; and calculating a sine of a difference between the average oscillation phase angle and the phase angle of oscillations at the grid subsystem, characterized in that the method further comprises: dividing the plural grid subsystems into plural groups in dependence on the phase relationships of oscillations of the plural grid subsystems; determining a group phase for each of the plural groups; and calculating a contribution to oscillation of at least one group of the plural groups in dependence on relationships between the group phases. receiving a first quantity which corresponds to oscillation in voltage angle or rotor angle at the grid subsystem; receiving a second quantity which corresponds to oscillation in voltage angle or rotor angle at another one of the plural grid subsystems, the first and second quantities being based on a same measured property; and determining a contribution of the grid subsystem to oscillation in angle or grid oscillation, the contribution being determined in dependence on a phase relationship between the received first and second quantities, determining the contribution comprises calculating the contribution in dependence on a phase difference between the first and second quantities, the method further comprising comparing the thus calculated contributions for the plural grid subsystems to determine relative extents of contribution of the plural grid subsystems; wherein the contribution of a grid subsystem of the plural grid subsystems is calculated by: determining an average oscillation phase angle for the plural grid subsystems; and calculating a sine of a difference between the average oscillation phase angle and the phase angle of oscillations at the grid subsystem, characterized in that the method further comprises: dividing the plural grid subsystems into plural groups in dependence on the phase relationships of oscillations of the plural grid subsystems; determining a group phase for each of the plural groups; and calculating a contribution to oscillation of at least one group of the plural groups in dependence on relationships between the group phases. dividing the plural grid subsystems into plural groups in dependence on the phase relationships of oscillations of the plural grid subsystems; determining a group phase for each of the plural groups; and calculating a contribution to oscillation of at least one group of the plural groups in dependence on relationships between the group phases.