Ageing and degradation of power transformers — how to interpret Return Voltage Measurements

Return Voltage Measurements are an effective tool to characterize the degradation of cellulose-oil-insulations and are widely used for the diagnosis of power cables. For power transformers the use is not so widespread. Nevertheless the application on power transformers is also very promising, especially because the geometry of the insulation system in a power transformer is closer to the conditions described by the Maxwell-model for two dielectrics in series. Return Voltage curves for new or not severely aged transformers fit very good the type and shape of the voltage curve predicted by the Maxwell equivalent circuit. Ageing and degradation phenomena in the oil and the paper lead to a deviation from this behaviour.