Longitudinal differential protection

Claim 1

Longitudinal differential protective relays for protection of line zones included in power distribution systems, which protective relays, arranged at terminals at all the ends of each line zone, are provided with a measuring device for measuring the current and voltage of the phase quantities, a filter for high-frequency filtering of the measured signals and an A/D converter for conversion from analog to digital representation of the measured and filtered signals, said digital signals being adapted to be supplied to a Fourier parameter estimator which is adapted to cooperate with a frequency estimator and a sampling regulator, said Fourier parameter estimator being adapted to generate the Fourier components for a truncated model (M=a 1 sin ω 0 t+a 2 cos ω 0 t) of the measured phase quantities and the respective residuals, wherein said Fourier parameters and residuals, generated by the parameter estimator, are adapted to be supplied to a local logic unit which is adapted to determine whether an abrupt event has occurred in the power distribution system by comparison of the loss functions V Lx Ck (t) and V Lx Ek (t) with the respective threshold values η LC and η LE according to ##EQU10## where L indicates local logic processing, xε(R, S, T) indicates that both V-functions and corresponding residual functions ε are based on current values, E indicates that the same functions are based on voltage values, k indicates that the equations are valid for each terminal, λ is a forgetting factor within the region 0,5-1.0, and, if the threshold values have been exceeded, the local logic unit is adapted to initiate a restart of the parameter estimator and determine whether the abrupt event derives from a conventional fault in the power distribution system by comparing a function f=√a 1 2 +a 2 2 with two threshold values η 0 and η I according to η 0 >√a 1 2 +a 2 2 >.sub.ηI, and, if the function is within the threshold region, the local unit is further adapted to determine the current direction and the signs of the current residuals, whereby the fourier components, the residuals and the information that the abrupt event derives from a conventional fault and information about the current direction are code-compressed in a signal compression unit to be transmitted to a central logic unit located at one of the terminals and adapted to compare loss functions with a threshold value according to ##EQU11## where G indicates that the loss function belongs to the central logic unit and where σ x (t) is the sum of the current residuals of all the terminals and that, if the threshold value is exceeded, the longitudinal differential protective system is adapted to deliver a TRIP signal and to indicate a faulty line zone and a faulty phase.