6CP200/100-10
9-1
SECTION 9: CHECKING OIL PURITY
NOTE: Typical test data for the oil used by Phenix Technologies are provided at the end of this section.
1. The most reliable means of determining the purity of the oil in the field is to test the dielectric strength. Test
sets for this purpose available on the market differ in design, particularly in the shape of the electrodes and the
test container. Tests on samples from the same oil carried out with electrodes of different types give different
breakdown values for the same distance between the electrodes. Oil test values should therefore always be
accompanied by information about the type of test gap used. The most common types of test gaps are:
a. Cylinder gap.
b. Spheres, 12.5 m in diameter.
c. Flat discs, 25 mm in diameter.
d. Segments of spheres, 35 mm in diameter, radius of sphere 25 mm.
Phenix employs for field testing Gap C, flat discs, which is per ASTM specification D-877.
2. Regardless of which type of test gap is used, it is important that container and electrodes be well cleaned
and dried before oil filling. Cleaning should be done per the instructions with the oil testing equipment.
3. Sample the oil at the top of the transformer. Phenix recommends a minimum test level of 30 kV when using
the ASTM D-877 test procedure. If the oil tests lower than this, consult the Phenix Technologies Service
Department for recommendations.
If action is required, it is recommended to replace the oil with new oil. Alternately, the oil can be removed and
re-purified, then replaced. Either action then requires that the High Voltage Unit be processed in a vacuum
chamber after refilling to remove any air and moisture.
NOTE: If air has been injected into the oil, it is recommended that a period of two days be allowed after
completion of oil filling until energizing to allow the oil to absorb any occluded air or gas in the insulation.
4. Oil Specifications
Transformer oil is a high quality insulating oil specially refined and formulated to meet the exacting
requirements of major electrical equipment manufacturers and users. It also meets Federal Specification VV-I-
530a.
High quality mineral-based transformer oil is recommended as the insulating and cooling medium for oil-
immersed transformers. It is also recommended for arc-forming apparatus such as switches, circuit breakers,
tap changers, reclosers, and fuses. General Characteristics of transformer oil include:
a. High Dielectric Strength:
Transformer Oil has high electrical insulating values as shown by their dielectric strengths of over 30,000 volts.
b. Excellent Oxidation Stability:
Excellent oxidation stability and high resistance to sludging provide protection, particularly where high
temperatures are likely to be encountered. These products are specially refined to ensure long time service
with a minimum of maintenance.
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