A method for enhancing the reliability of wide area control in a power transmission system, said system comprising a wide area control unit that is provided with actual measurement values of system data for performing wide area control in said power transmission system based on the actual measurement values of system data, the method comprising the steps of: obtaining a set of actual measurement values in the form of time stamped phasors from measurements made in the power transmission system, generating, in parallel with the obtaining of actual measurement values, estimated measurement values (y1'. y2',... yn') corresponding to the actual measurement values of the set based on at least one control signal (CS; CS1, CS2,... CSp) being sent into said system and a model of the system, investigating if one or more deviation condition associated with at least one actual measurement value is fulfilled through comparing time stamps with a time delay threshold, if one or more deviation condition associated with at least one actual measurement value is fulfilled, replacing at least the actual measurement values of the set being associated with said one or more deviation condition with corresponding estimated measurement values (y1') for obtaining a modified set of measurement values, where one deviation condition is fulfilled for an actual measurement value if it has a time delay through the system exceeding the time delay threshold, and sending the modified set of measurement values to the wide area control unit for performing damping of inter-area power oscillations, wherein the step of replacing comprises replacing a group of actual measurement values appearing in the set if one value of the group is delayed, where the group is associated with a certain coherent area of the power transmission system, where a coherent area is an area where a group of electrical machines are moving coherently, i.e. they are oscillating together. obtaining a set of actual measurement values in the form of time stamped phasors from measurements made in the power transmission system, generating, in parallel with the obtaining of actual measurement values, estimated measurement values (y1'. y2',... yn') corresponding to the actual measurement values of the set based on at least one control signal (CS; CS1, CS2,... CSp) being sent into said system and a model of the system, investigating if one or more deviation condition associated with at least one actual measurement value is fulfilled through comparing time stamps with a time delay threshold, if one or more deviation condition associated with at least one actual measurement value is fulfilled, replacing at least the actual measurement values of the set being associated with said one or more deviation condition with corresponding estimated measurement values (y1') for obtaining a modified set of measurement values, where one deviation condition is fulfilled for an actual measurement value if it has a time delay through the system exceeding the time delay threshold, and sending the modified set of measurement values to the wide area control unit for performing damping of inter-area power oscillations, wherein the step of replacing comprises replacing a group of actual measurement values appearing in the set if one value of the group is delayed, where the group is associated with a certain coherent area of the power transmission system, where a coherent area is an area where a group of electrical machines are moving coherently, i.e. they are oscillating together.