Method and arrangement for detecting and damping transcient oscillations at or near a natural resonant frequency in a power transmission system

Method for damping oscillations at or near a natural resonant frequency in a power transmission system, comprising the steps of obtaining a reference signal (R) from an oscillator (Osc) oscillating at the natural resonant frequency;

• multiplying the reference signal by an analogue operating signal (f(t)) corresponding to the prevailing operating condition at some point in the power transmission system;
• sampling the analogue signal resulting from the multiplication with a sampling frequency (.omega..sub.s) which is at least twice as high as the frequency of the reference signal;
• storing the digital sampling values successively in a memory to a number which at least corresponds to the period of the reference signal;
• thereafter calculating at each sampling instance a sum of a number (n) of the latest sampling values which correspond to one or more entire periods of the reference signal, whereby the sum represents the phase angle and amplitude of an oscillation in the operating signal;
• testing the sum in at least one diagnostic and determination circuit (Diag) for detection of oscillations according to predetermined criteria and determination of activation of regulating action;
• calculating a regulating signal (Reg) for each sampling instance dependent on the characteristic sum for the oscillation and dependent on the reference signal R, and transmitting the regulating signal (Reg) to a regulating arrangement in the power transmission system when the oscillation meets any one of the predetermined criteria so that the oscillation is dampened to predetermined tolerances.