Transformer, Liquid-Filled
ASTM D924 measures the dielectric loss of an insulating liquid as dissipation factor (the tangent of the loss angle) or power factor (its sine), together with relative permittivity when it is asked for. A guarded test cell filled with the liquid is energized at power frequency, and the loss is reported in percent at a stated temperature, most often 25 C and 100 C. Below 5 percent the two quantities agree to about one part in a thousand, so they are used interchangeably for liquid in good condition. Power factor is part of the insulating-liquid battery in NETA ATS and MTS, alongside dielectric breakdown, acid number, water content, interfacial tension, color and visual condition.
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 sample is drawn per ASTM D923, preferably through a closed system, in enough volume for at least three fillings. The method has two procedures. The referee procedure uses a three-terminal guarded cell in a thermostatted oven, holds the inner electrode within 0.5 C of the target temperature, normally applies an average stress of 200 to 1200 V/mm (the method sets 200 V/mm as the referee minimum and no maximum) at 45 to 65 Hz, and measures two separate fillings, which must agree within 0.0001 plus 10 percent of the higher value; otherwise a third filling is measured. The routine procedure allows a two-terminal cell, a 2.0 C temperature band, frequencies from 25 to 1000 Hz and a single filling, and it has no minimum stress; the 200 V/mm minimum belongs to the referee procedure. Cell cleanliness controls the result, and the time the liquid spends hot is kept short because heat changes it.
The dissipation factor or power factor at 25 C and at 100 C in percent, which of the two quantities the instrument reported, the procedure (referee or routine), the test frequency, the electrode stress or test voltage, the measured cell temperature for each reading, the second filling and its agreement check when two fillings are run, and relative permittivity when required, with the liquid type and the companion insulating-liquid results from the same sample.
ASTM D924 sets no limit. The 25 C reading, and the 100 C reading where a table carries one, are graded against ASTM D3487 for new mineral oil, ASTM D6871 for new natural ester, ASTM D4652 for new silicone, and the NETA ATS and MTS tables for liquid received in new equipment and in service. The 100 C value is the more sensitive screen for contamination and aging. The precision statement covers mineral oil only: at 0.1 percent and 25 C, repeatability is about 0.013 and reproducibility about 0.086 percent, and at 1.0 percent and 100 C about 0.24 and 0.47 percent. The standard sets no rounding rule; the form reports three decimals in percent.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
UltraDb references ASTM, ANSI, IEEE, NETA, NFPA, NEMA, FAA, TIA, and IEC standards descriptively, to identify the published methods its forms are built from. UltraDb and Verlecta are not affiliated with, endorsed by, certified by, or licensed by NETA, ASTM, ANSI, IEEE, NFPA, NEMA, FAA, TIA, IEC, or Megger. Standard names and section numbers are the property of their respective organizations. Always test to the edition your contract specifies. Where a standard is sold through the ANSI webstore, Verlecta may earn a commission on a purchase made through our link, at no extra cost to you. A commission never decides which standards we cite or how a form is built.