Transformer, Liquid-Filled
ASTM D924 is a laboratory test that measures the dielectric loss, or dissipation, within an insulating liquid such as mineral oil, silicone, or a less-flammable hydrocarbon by applying a known AC voltage across a parallel-plate test capacitor filled with the sample and measuring the power absorbed by the liquid as the capacitor charges and discharges. The test reports the ratio of power loss to apparent power - called the power factor, or dissipation factor, and expressed as a percentage at a reference temperature (typically 25 degrees Celsius) and often also at 100 degrees Celsius to reflect in-service conditions. NETA specifications include power factor in the suite of insulating-liquid acceptance and condition tests alongside dielectric breakdown voltage, acid number, moisture content, color, and interfacial tension.
Insulating liquid deteriorates with age, thermal stress, and oxidation, and the first detectable sign of that chemical decay is a rise in power factor - meaning the liquid is absorbing more energy as heat and less is being stored as electrical potential. A liquid with elevated power factor loses more energy to heat in the transformer windings and core, reduces the unit's efficiency, and signals that oxidation products, water, and other polar contaminants are accumulating faster than the fluid's oxidation inhibitors and water-removal systems can keep pace. Monitoring power factor on a periodic basis tells the operator whether reconditioning (filtration, drying, or degassing) is approaching necessity, or whether the unit's thermal management or breathing protection needs attention before the fluid degrades further.
Acceptance: the liquid is screened before a new or reconditioned unit is first energized, confirming it meets new-liquid limits and that no degradation occurred during shipping or storage. Maintenance: the same test is repeated on a periodic interval - often annually or biennially for critical units, or less frequently for standby equipment - and is typically drawn alongside the rest of the insulating-liquid battery and a dissolved-gas sample so trends can be tracked and interventions planned before failure risk rises.
A representative sample is drawn per ASTM D923, ordinarily from a live sampling valve after the valve has been flushed of standing liquid. The sample goes into a clean, dry, light-protected container and is shipped and stored to minimize exposure to moisture and light before it reaches the lab. There, the liquid is brought to the test temperature (most commonly 25 degrees Celsius, sometimes 100 degrees Celsius for aging assessment), poured into a calibrated D924 test cell with parallel-plate electrodes, and the cell is energized at a specified voltage (most often 2 kV per NETA practice) while the test set measures the in-phase and quadrature components of the capacitive current. The power factor is calculated from those components and reported as a percentage.
The power factor (dissipation factor) in percent, the test voltage applied in kV, the sample temperature in degrees Celsius at the time of test, and the liquid type (mineral oil, silicone, or less-flammable hydrocarbon), captured alongside the companion insulating-liquid screen results drawn from the same sample: dielectric breakdown voltage, acid neutralization number, specific gravity, interfacial tension, color, visual condition, and water content.
The maximum acceptable power factor depends on liquid type, temperature, and service condition, and is set out table-by-table rather than as one universal number. For new mineral oil at 25 degrees Celsius, NETA ATS-2025 Table 100.4.1 sets a maximum of 0.05 percent; at 100 degrees Celsius the limit rises to account for the liquid's thermal state. Silicone liquid and less-flammable hydrocarbon are evaluated against their own limits in Tables 100.4.2 and 100.4.3 respectively. A result above the applicable maximum indicates oxidation and contamination that warrant investigation - typically reconditioning of the fluid or replacement if reconditioning cannot restore acceptable values - before the unit is returned to, or left in, service. See the purchased standard for the complete table and any temperature-specific or equipment-class breakpoints.
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