Signal injection by magnetic coupling for the online FRA of transformer winding deformation diagnosis

The Frequency Response Analysis (FRA) method is used widely in the windings deformation diagnostics of power transformers. But until recently, the FRA method can only be used off line, because it need injecting signal into the windings. For operating transformers, it is almost impossible to inject directly the signal into the windings which are under high potential. Therefore, before applying FRA on operating transformer, the signal injection problem should be solved at first. A new signal injection method is put forward to conduct online FRA. The sweep frequency signal is injected into the neutral pinot grounding line of the windings, by the magnetic coupling of a Rogowski coil. In our laboratory, on an offline 220 kV transformer, the sweep frequency signal was injected into the bushing tap or the neutral point, and the response voltage signal and current signal on the tap or the neutral point were measured. The transfer function curves were calculated and contrasted. The results of the experiments show that injecting signal into the neutral point grounding line has the same effect as injecting voltage signal into the bushing tap. So, injecting signal from the neutral grounding line through a Rogowski coil is feasible, and the online FRA method can be used without breaking the grounding line of the bushing tap or the neutral point.