On-site Testing of Instrument Transformers

Alarming fault rate with explosions and fires recently occurred in some Substations in Mexico. Urgent measures to detect incipient fault were implemented in a large population of over 800 instrument transformers. An excellent combination of methods resulted the correlation between ultrasonic emission and ultra wide band partial discharge detection. Those highly sensitive methods combined provide crucial information about instrument transformer´s performance and allows a fast scanning of over 60 instrument transformers in a single shift. The premises were that all faults with explosions were originated by slow oil degradation that allowed partial discharges activity. That oil degradation distorted field distribution produced acoustic emissions in the ultrasonic region that could be located by a parabolic ultrasonic detector. Since other origin of acoustic emissions were possible like broken sheds or contaminated porcelain, after locating a high reading, UWB partial discharge techniques confirmed or denied our results. Ultrasonic readings ranged from 0-8 dB´s for those instrument transformers with acceptable performance to above 15 dB´s for those instrument transformers with incipient problems. That reading represented 800 pC in Ultra Wide Band PD detection system. In the paper, results show that 8 instrument transformers out of the population of 200 for a single substation required further analysis. All together from 800 Instrument transformer evaluated, 46 required replacement after confirming its bad state by off-line 10 kV tan delta test.