Impact of temperature on the frequency domain dielectric spectroscopy for the diagnosis of power transformer insulation

Dielectric testing techniques, in both time and frequency domains are currently being widely used by the power utilities for the assessment of the condition of transformer oil-paper insulation systems. However, it has been reported that the results of these tests are highly influenced by the operating temperature during the measurement. The distribution, migration and equilibrium of moisture between oil and paper in a complicated insulation system are highly temperature dependent. It requires adequate experience and proper understanding to interpret the dielectric response results in the presence of temperature variations and thermal instability. Proper analysis of the dielectric test result is only possible with a thorough understanding of the physical behaviour of the insulation system in response to temperature. Frequency domain dielectric spectroscopy (FDS) has been investigated for a model transformer at different operating temperatures. FDS data was then used to simulate polarization and depolarization currents. A correlation has been investigated between the operating temperature and FDS measured data, which is then extended to simulated PDC data. Finally the problems of a clear and transparent interpretation of the dielectric test results have been explained