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Abstract

Apparatus for damping oscillations at or near a natural resonant frequency in a power transmission system, in which an oscillator emits a reference signal oscillating at the natural resonant frequency; a multiplication circuit for multiplying an operating signal corresponding to the prevailing operating condition of the HVDC-system with the reference signal; a memory and a clock-pulse generator for serially sampling the output signal from the multiplication circuit by clock-pulses; the memory has the capacity to store sampling values up to a number which at least corresponds to the period of the reference signal; a summation circuit which, via a parallel output from the memory, for summing a number of the latest sampling values which corresponds to at least one period of the reference signal; a diagnostic and determination circuit provided with the output signal from the summation circuit, in which circuit the size, sign, variability and presence of an output signal from the summation circuit is tested according to determined criteria; means for generating a regulating signal provided with an output signal from the summation circuit, and a circuit closer activated by the output signal from the diagnostic and determination circuit when a criterion for oscillation which is to be damped is fulfilled, which circuit closer is placed between the output of the circuit closer for generating a regulating signal and a regulating device in the power transmission system.

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First Claim