Winter 2009-2010 NETA WORLD
1
Answers
1. c. When full, the gauge will indicate at 250C when the top
oil temperature is 250C. Insulating oil will expand and contract
with temperature changes, so nameplates will often state how
much change in liquid level there will be due to temperature.
The best rule-of-thumb is to not add oil unless the level is below
the LOW mark.
2. a. We used to call this a no-load tap-changer, but that really
was not accurate and misled some people to try to change taps
by just opening the secondary of the transformer. A transformer
always draws current from the power system (excitation current) even with the secondary open. This current establishes
and maintains the flux within the core. Always isolate both the
primary and secondary of a transformer before operating this
type of tap-changer.
3. d. Dissolved-gas analysis or gas-in-oil analysis provides
the earliest detection of internal problems in a liquid-insulated
transformer. Most transformer problems originate under the
oil and will usually create some type of combustible gas. DGA
takes a sample of oil and injects it into a gas chromatograph.
Combustible gases are detected in parts per million and are compared to standards to evaluate the condition of the transformer;
however, DGA does not provide indication of remaining life
in a transformer.
4. b. At one time it was one quart, but that may not be enough
for larger transformers or transformers with a lot of gunk (that’s
a technical term) inside the sample valve, so now it is two quarts.
If the transformer is already low on oil, do not take a sample
until it has been topped off and allowed to sit for at least a few
days so the oil can mix.
5. b. Mineral oil has a relatively low power factor when it is
new. It will increase with age, but a higher power factor does
not necessarily mean the oil is bad. Bushing compound, sealants,
and gasketing may increase power factor without harming the
oil’s insulating properties. The cause of an increased power factor
should be determined.
www.netaworld.org Jim White is the Training director for Shermco Industries and the
principal Shermco representative on the NFPA 70B committee. Jim is
the alternate NETA representative on the NFPA 70E committee and
serves as the NETA representative on the IEEE/NFPA Arc-Flash
Hazard Work Group (RTPC) Ad Hoc Committee. He served as the
Chairman of the 2008 IEEE Electrical Safety Workshop. Jim is a NETA
Certified Level IV Electrical Testing Technician and a member of the
NETA Safety Committee.
5. The power factor of new mineral oil should be
__________ % or less.
a. 0.01
c. 0.1
b. 0.05
d. 0.5
4. Sample valves should be flushed with _____ of waste
liquid before collecting a transformer liquid sample?
a. One quart
b. Two quarts
c. Three quarts
d. Four quarts
3. An internal fault can be discovered earliest using which
gas test method?
a. Total combustible gases detector
b. Gas collector relay
c. Nitrogen blanket analysis
d. Dissolved-gas analysis
2. To change voltage levels for seasonal changes, a
__________ is used.
a. deenergized load tap-changer
b. tapped transformer
c. load tap transformer
d. autotransformer
1. The liquid level indicator is calibrated to__________
at normal operating conditions.
c. Temperature at 25°C
a. Center line of gauge
d. Type of insulating liquid
b. High or low gauge
by Jim White
Shermco Industries
Transformer Accessories
and Condition Assessment
Tech Quiz #90