THE INSULATING QUALITY of transformer oil can provide insight into the reliability and lifetime of transformers. To check that oil quality, most methods periodically draw a sample of transformer oil and perform qualitative analysis in the laboratory. Using a microwave approach, however, it is possible to estimate the dielectric properties of transformer oil without need of a laboratory setting. At the Ariel University of Samaria in Israel, a cutoff resonator designed for the testing of transformer oil has been proposed by Boris Kapilevich, Anatoly Lipsky, and Boris Litvak.
The researchers created an experimental setup based on the cutoff resonator with a plastic tube containing the tested oil. To suppress the degenerated and higher-order resonance modes, they used a resonator based on a cutoff rectangular waveguide. The tube was filled with the liquid under test and placed in the maximum electric field. With the correct choice of tube diameter and waveguide cross-section, the cutoff resonator's principal mode was moved quite a distance from higher modes.
The researchers compared "fresh" (unused) and "old" (used) transformer oil over a long period and in real-world conditions. In doing so, they noted a shift in the resonance curves of the measured insertion losses between both samples of transformer oil. The old oil also exhibited higher insertion loss, which was caused by the increase in its loss tangent compared to the fresh oil. To measure insertion loss, the researchers used the PNA-L N5230C vector network analyzer (VNA) from Agilent Technologies (www.agilent. com). The microwave approach complements existing methods of transformer oil control. See "Application of the Cut-Off Resonator for Microwave Monitoring of Transformer Oil," Microwave And Optical Technology Letters, Jan. 2011, p. 66.