Abstract: A protection device for high resistance ground faults in a power network according to the invention has a fault detection principle which is based on an indirect study of non-harmonic frequency components of the phase currents. When such a fault has occurred, a considerable change of the energy contents of these frequencies arises. This change can be detected by a device according to the invention. If by comparison (4e) between digitized input signals (I') and a harmonic Fourier model (4d) of the same signals, i.e. generation of the residuals of the system, it is found that a difference exists, and if the corresponding loss function V.sub.N (4f) for a certain time exceeeds a lower limit value - all on condition that a zero sequence current (I.sub.O) exists - then there is a high resistance ground fault on any of the phases of the network.

Abstract: The invention comprises a protective relay with selective phase selection in electric power distribution systems when there are double transmission lines (M1-P1,M2-P2) between two stations (TM, TP) and where the ends of the line are connected to a common busbar system located in each station. The protective device includes a protection unit, terminal, at both ends of each line. All the terminals (M1, M2, P1, P2) include an identical basic design for measurement of phase quantities (17), filtering (18) and A/D conversion (19); furthermore, the identical part includes a parameter estimator (20) for Fourier series expansion of measured phase signals and generation of the respective residuals. The parameter estimator cooperates with a frequency estimator (21) and a sampling regulator (22).

Abstract: The present invention comprises a longitudinal differential protective relay for protection and supervision of power line zones in a distribution system. The protective relays are located at each end of the zones that are desired to be monitored. One of the protective relays operates as a master for the other slave protective relays. A TRIP function indicating a line fault in any protected zone is performed by transmitting locally processed measured signals to the central protective relay which determines if and when any measure is to be taken. Each local protective relay comprises a measuring device, a filter and an A/D converter for phase currents and voltages. The digital signals are used to generate a truncated Fourier model of the measured signals in a parameter estimator which cooperates with a frequency estimator and a sampling regulator which, via a synchronization unit, ensures that sampling is performed simultaneously in all protective relays included in the protective system.