In the paper load rejection test, performed at a hydro power plant in Sweden, equipped with asynchronous generators, is described and analysed from the electromechanical transient processes and signal distortions points of view. During the test voltages and currents at shunt capacitor and generator terminals are digitised, recorded and thereafter off-line processed. Two adaptive methods are applied for signal processing: the Newton-Type Algorithm (NTA) and the Adaptive Fourier Filter (AFF) Scheme. The features of both methods during nominal frequency conditions and severe frequency changes and signal distortions are investigated. It is proved that the adaptive approach proposed is well suited for voltage and current measurements when the signal frequency changes in a wide range.