Abstract: A method and a device in connection with digital distance relays for obtaining a backup function in the event of faults in the digital signal processing and/or in the event of faults detected during automatic testing. The method comprises testing the distance relay continuously, in case of a faultless network, with respect to the digital signal processing by means of a central control unit. In case of a characteristic fault on the network, the distance relay is switched over from testing to protective function involving measurement by means of measurement signal units included in the distance relay. In case of a fault on the network and a simultaneous fault in the digital signal processing, fault signals from the measurement signal units, via time delay circuits and logical elements, bring about tripping of the line section subjected to the fault. When a fault is detected in the digital signal processing, an alarm function is triggered.

Abstract: A method for phase-locked sampling measurement of a periodically varying quantity. The samples are designated g(I) . . . g(I-n) and may comprise one complete period or a half of one period. The method involves adapting the sampling frequency and the sample position in relation to the quantity to be measured such that phase locking is attained, and this is done by controlling the time interval between sampling. Whether the sampling frequency needs to be increased, left unchanged or be decreased, depends on a number of decisions depending on the values of samples g(I) and g(I)-g(I-n). A device for carrying out the method is also described.

Abstract: A network impedance protection device operates in dependence on the quotient of voltage and current at a measuring point and is set to initiate a protective action when the measured impedance is lower than an impedance level set in the device. Known designs comprise analog components and functions both as regards quotient formation, comparison and compensation etc. An impedance relay constructed according to the invention utilizes the same principles as the analog design, but all units and functions are digitized. Incoming voltage and current signals are converted in an A/D converter, filtered in tuned filters, rectified in rectifiers, and so on, and deliver an output signal, via summators, when the measured impedance is lower than a set impedance value.

Abstract: The present invention relates to a protection device for detecting at least one faulty phase in an electrical network. The device comprises a band exclusion filter for each phase for filtering away the signals which have the same frequency as the nominal frequency of the network, amplifiers, rectifiers and peak value forming means for each phase. In addition, the different phase signals are fed to a function element which delivers a signal to a level and comparison device for each phase, where the signal is compared with the signals in the respective phase.