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In the absence of on-line %RS measurements, utilities have historically used moisture-in-oil
laboratory measurements (in ppm) to evaluate the average water content in their transformer oil and
cellulose. Taken in combination with oil quality test results, asset managers were able to qualitatively
assess their risk level based on the minimum and maximum operating temperatures of their
transformers.
This approach can be used with the Calisto/Calisto 2 IED. The advantage of monitoring dissolved
moisture content in ppm is that on-line measurements can be compared directly with laboratory
measurements. The moisture in ppm will vary proportionally with the load. In order to provide
highly accurate moisture readings in ppm, Calisto/Calisto 2 instruments use experimentally
determined moisture solubility functions for common dielectric fluids.
Calisto/Calisto 2 can also report %RS at a standard temperature of 25°C (77°F). For each type of
oil, these readings are proportional to the moisture-in-oil ppm readings, related by the moisture
solubility of the oil at 25°C (77°F). Changes in %RS@25°C can therefore serve the same role as
ppm for transformer condition assessment. Since different oils have different capacities to retain
moisture, the %RS@25°C readings allow easy comparison of relative saturation in different
transformers running with different oil types.
Many transformer experts prefer to monitor the %RS at a temperature that is important to the
transformer. For example, a transformer operating in a desert climate could have high ppm levels of
water, but have a comfortably low %RS@T given its high average operating temperature. To take
another extreme, in cold operating or shutdown conditions, the %RS at the coldest temperature
determines the tendency for water to condense from the oil.
Sometimes the focus is, instead, on the %RS at specific critical locations on the transformer. The
rate of degradation of paper insulation is widely believed to correlate with the moisture level in the
paper. The %RS in the oil, evaluated at the temperature of the paper, provides an estimate of the
working %RS in the paper.
To facilitate accurate %RS@T measurements, the Calisto Precision Oil Temperature Probe is
recommended.
To change the Calisto/Calisto 2 water measurement mode, start Calisto Access, click on the
Measurement tab, and select the desired mode.
Oil characteristic coefficients are used by Calisto/Calisto 2 to calculate the dissolved gas and
moisture. For most accurate readings, it is therefore necessary to configure the monitor’s oil type to
agree with the type of oil in the transformer. Under the Measurements tab, Calisto Access provides
a drop-down list of common oil types: Voltesso 35, Technol US 4000, Nytro 10 GBN, Midel 7131,
Silicone, Envirotemp FR3 and Luminol TRI. By selecting the oil type, the experimentally
determined characteristic coefficients for that oil type are configured in the Calisto/Calisto 2. Clients
with other types of oil may choose the oil type which most closely resembles their oil, or select
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