T&D World Magazine

Dielectric Response Analyzer Accurately Determines Insulation Moisture Content

DIRANA, the dielectric response analyzer from OMICRON, determines in a simple and efficient way, the moisture content of liquid-filled transformers. Accurate knowledge of the moisture content is a crucial factor in the condition assessment of power transformers. High moisture levels accelerate insulation decomposition, decrease dielectric strength and may cause bubble evolution at elevated temperatures. Additionally, the condition of the oil is determined.

The DIRANA is also applicable to the condition assessment of bushings, VT/CTs, cables and rotating machines. Other applications include verification of proper drying at the factory for a new transformer or confirming proper dry out after field assembly, repairs or oil processing.

The DIRANA determines the moisture content on the basis of Dielectric Frequency Response (DFR). Unique to DIRANA are the two measurement techniques (FDS and PDC), which allow for accelerated measurements, even in the very low frequency ranges often required for measuring transformers that are new, cold and dry. The extended measurement range of 5 kHz down to 50 µHz, allows the DIRANA to discriminate among the oil, insulation geometry and paper. The result is independent from moisture equilibrium and insulation temperature. By combining the time and frequency domain methods (PDC and FDS), the DIRANA achieves speed that halves conventional measurement times. Measurements made with the DIRANA are more accurate than traditional oil samples as oil sampling, equilibrium conditions and diagrams are no longer required.

The DIRANA's assessment engine consists of a database containing data from measurements of real oil-paper-insulation systems, under various states of moisture content and oil / paper aging. On this basis, the measurement results are automatically analyzed and the moisture content is then classified according to IEC 60 422 between "dry" and "extremely wet." The software compensates for conductive aging byproducts in the oil and is therefore well suited for older transformers.

A new "Pre Measurement Check" feature analyzes the capacitance, signal-to-noise ratio and noise current to ensure a successful test. The software also recognizes when sufficient data have been gathered and the test can be stopped. Therefore the measurement time can be reduced to a necessary minimum. Training videos are now available that discuss moisture analysis and test set operation.

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