Application of electro-optic modulation technique for PD monitoring of power transformers

Partial discharge (PD) detection is an important technique for assessing the health of high voltage power transformers. A partial discharge signal within an oil-filled power transformer may reach a winding first, then travel along the winding to the bushing core bar. The bushing, acting like a capacitor, can transfer the high frequency components of the partial discharge signal to its tap point. Previous published work [1] has shown that these high frequency components at the bushing tap point can be detected using a suitable sensor and transmitted to remote digital equipment for on-line analysis. In a high voltage substation there is often excessive electrical noise which may corrupt any measured PD signal from the bushing tap point as the bushing acts as an antenna. If signal transmission is implemented using standard coaxial cables this can result in further corruption of the PD signal. The use of optical transmission techniques therefore has clear advantages not only through improved noise immunity but also because it realises electrical isolation and improved operator safety. This paper details the application of an electrooptic modulator to generate transmission signals over polarization maintaining optical fibre from the measurement point. The feasibility of this approach has been investigated using PD signals measured at the tap point of a 60kV bushing, two different PD sources have been used and in both cases it was possible to detect PD activity above 40pC.