Impact of Test Voltage Magnitude on FDS Characteristics of Oil-Paper Insulation with Different Aging Conditions
Frequency Domain Spectroscopy (FDS) is a method to know the transformer oil-paper insulation condition by measuring the dielectric parameters in a frequency range. As expressed by numerous research results, moisture and aging condition has a significant influence. In addition, the FDS result is also affected by electric field through electrode polarization or Garton effect in low frequency, which may mislead the judgment of the aging condition. However, this influence is seldom considered in previous research. In this paper, study on the impact of voltage level during the oil-paper insulation aging process is shown. The FDS test of power transformer oil-paper insulation system in different aging conditions (accelerated thermal aging from new sample to aged samples of several tens of days), and also every aging condition will be tested in three different voltage level (10, 50 and 200 V). The experiment uses mineral oil and KRAFT paper for the test. Several small sealed containers for oil-paper aging samples are used to reduce the impact of moisture from the air on the FDS test results for every oil-paper insulation samples. Nitrogen is used to remove air from the containers to reduce the impact of oxidation and to better simulate real working condition inside a power transformer. Paper samples are tested to know the degree of polymerization (DP) value of the KRAFT papers in every aging condition. The results show that impact of voltage magnitude on the dielectric response is closely related with the oil-paper insulation aging condition. The voltage level related branch-starting frequency of complex capacitance increases with the aging process and oil conductance. Lower dissipation factor and imaginary capacitance are found in higher test voltage. However, the real capacitance in low frequency is enlarged. These phenomena could be caused by ions and impurities migration in oil-paper insulation. A low electric field less than 100V/cm is recommended for FDS in test laboratory and field.