Trap Parameters Analysis of Oil-paper Insulation by Thermally Stimulated Depolarization Current

As the core equipment of the DC transmission system, the converter transformers have been widely used in power systems. The space charge accumulation, transport and dissipation in oil-paper insulation are more complex than in polymeric insulating materials. Insulation damage caused by space charge is one of the main reasons for transformer fault, and has become a direct threat to the safe operation of the power system. Thermally stimulated depolarization current (TSDC) test is one of the important methods to analyze trap energy levels and charge transport properties of dielectric. TSDC test with different bias voltages (-25V, -12.5V, + 25V and + 50V) in temperature range of -100 degrees C similar to 160 degrees C is used to distinguish dipole TSDC peaks from space charge ones in oil-paper insulation structure. The experimental results show that the -75 degrees C, 50 degrees C TSDC current peaks are not caused by space charge and the 150 degrees C peak meet the space charge characteristics. Activation energy, polarization intensity and relaxation time of each TSDC peak have been calculated correspondingly. The test has significance for better understanding of space charge accumulation, distribution and charge transport properties in oil-paper insulation, and is of great value for mechanism analysis of breakdown in oil-paper insulation.