Voltage Response Measurements for the Diagnosis of Insulation Condition in Power Transformer

A large proportion of existing transformers within electric utilities are approaching the end of their design life. Insulation degradation continues to be a major concern for these transformers. Insulation materials degrade at higher temperatures in the presence of oxygen and moisture. The degradation from thermal stress affects electrical, chemical and mechanical properties. Utility engineers use a number of modern diagnostic techniques to assess the insulation condition of aged transformers. Among them Dissolved Gas Analysis (DGA), Degree of Polymerisation (DP) measurement and Furan analysis by the High Performance Liquid Chromatography (HPLC) are frequently used. In recent years, new diagnostic methods have been promoted to complement the classical insulation resistance, power frequency dissipation factor and polarisation index measurements. These new methods are based on time or frequency domain polarisation measurements. In frequency domain measurement, a sinusoidal voltage is applied and the complex dielectric constant is determined from the amplitude and phase of the current flowing through the sample. On the other hand, time domain measurements are conducted by the application of a step voltage across the insulation sample. Time domain measurements based on polarisation and depolarisation current measurements and return voltage measurements have gained significant attention over the last several years. Particularly, there has been growing interest in the condition assessment of transformer insulation by the Return Voltage Method (RVM). In recent times decay voltage measurement has also been proposed to monitor the moistening process of insulation after polarising the insulation for a long time. Polarisation and depolarisation currents have two components - conductive current and polarisation current. The conductive component provides information about the intensity of the conduction process and the polarisation component provides the intensity of the polarisation processes within a defined time range. It is difficult to separate the two components of currents and the measurement of such small current is also sensitive to electromagnetic disturbances. The principle of the voltage response method is based on the measurement of the discharge and return voltages of the insulation. From the magnitude and shape of the voltage curves the quality of insulation affected by moisture and ageing can be predicted. Moisture content of the insulation can be estimated from the decay voltage measurement, while progress of thermal ageing can be estimated from the return voltage measurement. The two measurements can be conducted independently and hence conduction and polarisation processes can be investigated independently. In this research project we have investigated a number of transformers of different ages with the return voltage and decay voltage techniques. A brief analysis of the decay and return voltage methods will first be outlined in this paper. Then results from the two groups of measurements (one from the decay voltage and the other from the return voltage) will be presented