Patent ID: 8381008

Claim:
A method to align measuring times of a first and a second measurement of an electric quantity, where the first measurement is taken at a local end of a line of a power network and is transmitted by a first protection device together with a corresponding first measuring time expressed by a first internal clock via a telecommunication network to a remote end of the line and where the second measurement is taken at the remote end of the line and is transmitted by a second protection device together with a corresponding second measuring time expressed by a second internal clock via the telecommunication network to the local end of the line after the first measurement was received by the second protection device, the method comprising the steps of determining a send transmission time from the local to the remote end of the line, determining a receive transmission time from the remote to the local end of the line, aligning the measuring times of the first and the second measurements by expressing the measuring time of the second measurement in terms of the first internal clock using the send and receive transmission times, characterized in that as long as the first and the second internal clock are synchronized by a global time reference a transmission difference is determined as difference between the send and the receive transmission times and the transmission difference is stored, after losing the global time reference, time signals generated by the first and the second internal clock are used to determine the send and receive transmission times, a clock drift is determined between the first and the second internal clock from the transmission difference stored just before losing the global time reference and from the send and receive transmission times, the measuring times are aligned by additionally taking into account the clock drift, a sudden change in the clock drift is determined and the stored transmission difference is corrected using values of the clock drift determined just before and just after the sudden change.