There are a number of field tests that
are considered good predictive maintenance practices and these should
be performed periodically to spot trouble. These tests are also
useful for diagnosing transformer trouble.
A Megger test consists of
applying a high DC voltage, usually 1000 V, to each winding with the
other windings grounded and to all windings connected in parallel.
The Megger readings are in megohms and these must be temperature
corrected for meaningful results.
The megger readings should be compared
to earlier test results to detect any downward trend in resistance
values. The voltage produced by a megger is high enough to cause
insulation breakdown if there are gross faults, but is really not
sensitive enough to detect minor problems in transformers in the
higher voltage classes.
A Doble test is somewhat more
sensitive than the Megger test. An AC voltage, up to 10 kV, is
applied to the winding insulation and leakage current is measured. In
addition to the leakage current, the power factor of the insulation
is computed.
A high power factor indicates lossy
insulation, which can mean imminent trouble. In addition to the
winding insulation, the Doble test is used to measure the power
factor of bushing insulation. When testing condenser type bushings,
the capacitance tap is utilized.
The Doble test set is also used to
measure the excitation current through the winding by applying an AC
voltage across the winding. High power factor readings during this
test can indicate flaws in the turn-to-turn insulation.
A TTR test can be used as a diagnostic
test in the field. Always connect the TTR test set clamp leads to a
secondary winding of the transformer under test. Connect the TTR test
set clip leads to the primary winding that is on the same core leg as
the secondary winding being tested, observing that the polarity of
the red clip test lead matches the polarity of the red clamp test
lead.
Set the ratio dials just above zero and
give the generator wheel a half turn. The galvanometer should deflect
to the left, indicating the ratio dials need to be raised. A
deflection to the right means that the polarity of the test leads is
incorrect.
This can be corrected by swapping the
two clip test leads. After the correct polarity has been verified,
slowly turn the generator and make the appropriate adjustments to the
ratio dials in order to keep the galvanometer needle centered (zero
current in the clip test leads). When the ratio dials are almost set
to the right ratio, the generator can be cranked faster to get the
proper voltage indication on the voltmeter (8 V).
If the voltmeter reads low voltage with
the ammeter reading high current, this is usually an indication of
shorted turns, either in the primary or in the secondary. A zero
deflection on the galvanometer at every ratio settings indicates an
open primary winding because no current can flow in the clip
test leads.
If the galvanometer deflection is
always to the right and cannot be corrected by reversing the test
leads, then this may indicate an open secondary winding and voltage
cannot be generated in the primary winding.
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