Power transformers are a critical, capital-intensive asset for the utility industry. As an asset manager reviewing the life expectancy of a transformer, or a substation operator responsible for determining the loading capabilities of a transformer, you should be concerned with the water content in your transformer.
One of the most important ageing indicators of transformers is the water content in the solid part of the insulation (paper, pressboard). Accurate diagnostic tools for determining the health of your transformers is critical. The OMICRON DIRANA is a unique and efficient device which determines the water content in the solid insulation.
Moisture entering in to oil-paper insulation can cause three dangerous effects in transformers: it decreases the dielectric withstand strength, accelerates cellulose ageing (de-polymerization) and causes the emission of gas bubbles at high temperatures.
Water in transformers comes from four sources: residual water after drying, water from cellulose and oil ageing, water through leaky seals or repairs, and water due to breathing. Therefore, even in the case of a non-breathing transformer the moisture can reach a critical level.
The DIRANA measures the dielectric response of solid insulation in equipment. The dielectric response is a unique characteristic of the particular insulation system. The increased moisture content of the insulation results in a changed dielectric model and, consequently a changed dielectric response. By measuring the dielectric response of the equipment in a wide frequency range, the moisture content can be assessed and the insulation condition diagnosed. For the dielectric response, the measurement is performed as a traditional ungrounded specimen test (UST), made from the high voltage winding to the low voltage winding (CHL) in a two winding transformer. We are most concerned with the CHL test, as this is the measurement which contains the most cellulose insulation material. The test connections and modes are the same as used in a traditional transformer insulation Tangent Delta (or power fact)test with the difference being that it is performed at a low voltage, up to 200vpp, and the test is performed at frequencies from 1 kHz to 10µHz.
The unit combines the polarization current measurement (PDC) method in time domain with frequency domain spectroscopy (FDS) and thus significantly reduces the testing time compared to existing techniques. Essentially, time domain measurements can be accomplished in a short time period but are limited to low frequencies. The extended measurement range of 5 kHz down to 50 µHz, allows the DIRANA to discriminate between the oil, insulation geometry and paper. The result is independent from the moisture equilibrium.
The patented technique combines the advantages of both principles. It acquires data in the time domain from 10 µHz to 0.1 Hz and in the frequency domain from 0.1 Hz to 5 kHz. This reduces the measuring duration by up to 75% compared to exclusive frequency domain measurements.
DIRANA’s moisture determination is based on a comparison of the transformer’s dielectric response to a modelled dielectric response. A fitted algorithm compares the measured data with the modelled data and calculates the geometry data, the moisture content, as well as the oil conductivity. The moisture assessment is based on IEC 60422. The software is very easy to use, and the user only needs to enter the oil temperature.
Aged transformer oils often have increased values of conductivity due to acids and other ageing byproducts. This can lead to incorrect water content results. The insulation model in the DIRANA’s software compensates for this influence.
Excessive water content can be extremely detrimental to the life expectancy of a transformer. The Dirana provides a simple, non-intrusive method of detecting this moisture and alerting the user of the need to take action to alleviate the problem.
Tel: 01785 251 000
