M-SRR-Based Microwave Sensor Design and Modeling for Evaluating Transformer Oil Quality and Lifespan Employing Relative Permittivity

Transformer oil serves as an essential insulating medium for high-power electrical equipment such as transformers. Its degradation can result in transformer malfunction and potential disruptions in the power distribution network. Therefore, real-time monitoring of transformer oil quality is crucial to prevent equipment failure and maintain network reliability. In this study, a Microstripline-Split Ring Resonator (M-SRR)-based microwave sensor is designed and fabricated to assess the quality of transformer oil through variations in its dielectric constant. Unlike previously reported passive sensor structures, the proposed configuration enables continuous and non-invasive monitoring of transformer oil aging, providing a viable alternative to conventional chemical analysis techniques. The simulated and experimental results of the forward transmission coefficient (S21) for the Oil Under Test (OUT) exhibit a strong correlation, confirming the validity of the proposed design. The sensor demonstrates a normalized sensitivity range of approximately -38.02 to -5.48% for oil Sample 1 and -22.47 to -5.77% for oil Sample 2, indicating its effectiveness in detecting subtle changes in the oil’s electrical properties during the degradation process. Furthermore, the loss tangent of transformer oil can be evaluated by analyzing the magnitude of the S21parameter at the resonance frequencies, providing additional insight into the dielectric losses and overall condition of the oil.