Page 86 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMER COMPONENTS AND MAINTENANCE

TRANSFORMER COMPONENTS AND MAINTENANCE 4.15

TABLE 4.1 Insulating Oil Tests Available
ASTM Used New Aging Fault
test method Test name Units oil oil analysis analysis
D877/D1816 Dielectric breakdown kV ♦ ♦
D974 Neutralization number mg KOH/g ♦ ♦
D924 Power factor @ 100°C % ♦
D924 Power factor @ 25°C % ♦
D971 Interfacial tension dyn/cm ♦ ♦
D1500 Color Scale 0.5 to 8.0 ♦ ♦
D1298 Specific gravity @15°C ♦ ♦
PCB ppm ♦ ♦
Water content ppm ♦ ♦
D97 Pour point °C ♦ ♦
D1935 Steam emulsion s ♦
D92 Flash point °C ♦
D445 Viscosity °C ♦
D2440 Stability test ♦
Carbon content ppm (g/kg) ♦
Metal particles ♦
Oxidation inhibitors ♦
Furans ppb ♦
Degree of polymerization Units P ♦
v

1. Provide insulation
2. Provide efficient cooling
3. Protect the windings and core from chemical attack
4. Prevent the buildup of sludge in the transformer
The condition of the oil in the transformer determines the transformer life. Annual testing
is the minimum requirement to ensure acceptable dielectric strength for the oil. These are
the benefits of annual testing:
1. It indicates the internal condition of the transformer. It detects the presence of sludge.
The sludge must be purged before it can precipitate on the windings and other surfaces
inside the transformer.
2. It indicates any deteriorating trends. The naphthenic insulating oils used in transform-
ers today have been used for more than 50 years. The deteriorating trend of these oils is
well known. Many data are available to permit the comparison between normal and
abnormal oil.
3. It prevents a forced outage. Incipient problems are detected early. Corrective action is
scheduled with minimal disruption. The tests that are required for transformer oil are
listed in Table 4.2. The results of one test only cannot indicate the condition of the oil.
The true condition of the oil is obtained by considering the combined results of the eight
tests together (especially the first four tests).

The ASTM D-877 (flat disk electrode) is the classical test for determining the dielectric
strength of the oil. It will detect contaminants such as free water, dirt, or other conducting
particles. However, it will not detect the presence of dissolved water, acid, or sludge.

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