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 bubbles to form at elevated temperatures. Additionally, the condition (water content) of the oil is determined. The Dirana is also applicable to the condition assessment of bushings, instrument transformers, cables and rotating machines. Other applications include verification of proper drying for a new transformer at the factory or confirmation that the transformer is properly dried 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 is the combination of the two measurement techniques – Polarization Current Measurement (PDC) and the Frequency Domain Spectroscopy (FDS). This allows for accelerated measurements, even in the very low frequency ranges which are often required for measurements on transformers that are new, cold and dry. The extended measurement range of 5 kHz down to 50 µHz, allows the Dirana to discriminate between moisture in the oil, insulation geometry and paper. The result is independent of moisture equilibrium and insulation temperature. By combining the time and frequency domain methods (PDC and FDS), the Dirana achieves superior speed that halves conventional measurement times. Measurements made with the Dirana are more accurate than traditional oil sampling, as compensation for equilibrium conditions and the use of equilibrium diagrams are no longer required.
The Dirana's assessment is based on a database containing data from measurements of real oil-paper-insulation systems with varying 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 the extremes "dry" and "extremely wet". Additionally, 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 has been gathered and the test can be stopped. Therefore the measurement time can be minimumized. Training videos are now available that discuss moisture analysis and test set operation.