On-line testing transformer oil using microwaves

The paper describes preliminary testing transformer oil of the two kinds - "fresh" (unused) and "used" for a long period in real-world conditions. The experimental setup assembled for testing is based on the cut-off rectangular waveguide with plastic tube filled with the tested oil. The insertion losses of the sensor are measured by Vector Network Analyzer Agilent PNA-L N5230C. There is observed a shift of resonance curve of the "used" oil respectively the "fresh" one toward low frequency. It can be interpreted as increasing a real part of dielectric constant of the "used" oil in comparison with its "fresh" counterpart. Also, the "used" oil has higher insertion loss that is caused by increasing its loss tangent compared with the "fresh" oil.