Cigre published the latest technical brochure on moisture determination in transformers in August 2018. The brochure is called TB741, Moisture Measurement and Assessment in Transformer Insulation – Evaluation of Chemical methods and Moisture Capacitive Sensors. As a member of that working group behind the brochure, I can highly encourage you to have a deeper look at this valuable document. It contains a lot of new information on the topic.
Additionally, the TB741 emphasizes some capacitive thin-film polymer sensor characteristics, which are important for the transformer application. Those are, for example, sensitivity and proper calibration at dry conditions, no temperature dependency and tolerance to all compounds present in insulation fluids, to mention a few. According to the brochure, the user must be aware of the importance of all the properties for reliable sensor behavior.
Vaisala invented the capacitive polymer sensors in early 1970’s and named the innovation HUMICAP®. During the last 20 years, we have used a special version of the sensor developed to measure moisture in insulation fluids. Our focus has always been on the measurement reliability: sensitivity for low moisture, linearity, no cross-sensitivity to oil contaminants, good tolerance for insulation fluids and long-term measurement stability as well as full temperature compensation over its complete operating range.
We have tested sensors from field after 10 years of operation in transformers. The factory calibration showed, that all of the tested sensors were within initial accuracy conditions with no need for adjustment – That’s stability indeed! Vaisala is utilizing a world glass calibration for all moisture sensors including 0%RH calibration point and every moisture transmitter is delivered from factory with a traceable calibration certificate. This is something we are very proud of.
An advice for the future: Please challenge your sensor suppliers to make the sensor information and calibration certificates available for you, it will work for your benefit.
Please join a webinar to hear more about the TB741 presented by Ivanka Atanasova-Hoehlein, former convener of CIGRE Working Group D1.52. The webinar is held on Feb 6, 2019 at 13:00-14:00 EET.
To join the webinar, please register at Cigre website.