Recurrent Surge Oscillation is done on the windings of large generators such as turbo alternators. The Recurrent Surge Oscillation Test (RSO) helps identify shorts in the winding.
Shorts in the winding occur as the insulation between turns deteriorates and fails. Shorts can cause localised heating and arcing which can further damage the alternators. Shorts can also become earth faults in course of time. Multi-turn shorts can also result in a drop in the voltage.
The Recurrent surge oscillation tests is done by sending voltages of low voltage and high frequency through the winding and checking the waveform at the other terminal. If the waveform has suffered any distortion, it may indicate an abnormality. The waveform can give information such as the location of the fault and its severity.
Some short circuits may not be obvious when the rotor is at rest. The conductors will come in contact with each other only during the running condition, due to the centrifugal force. To identify such faults, the rotor is made to rotate and the test is conducted.