Field Experiments on a Magnetic-Coupling-Based Online Frequency Response Curve Measurement System for the Online Monitoring of Power Transformers

Online frequency response analysis (FRA) is expected to be capable of diagnosing winding deformation defects in operating power transformers. However, there is a lack of available experimental field research on online detection methods of frequency response curves. In this paper, we establish a magnetic coupled online frequency response curve measurement system to execute FRA for operating transformers and apply this system to two working power transformers. The principle, technical process, key requirements, structure, performance, and results of field experiments on the online system are presented. The feasibility of online FRA is analyzed through comparisons with frequency response curves from offline measurements. Our online system does not affect transformer safety and can be installed on some transformers without powering them off. The interference generated by a 500 A load current can be compressed by a factor of 100 to measure an FRA current with an amplitude of tens of μV to several mV. The measured online frequency response curve has approximately the same resonance frequency as the offline measured curve and can thus be used to diagnose winding deformation defects.