7
High Voltage Instrument Transformers | DDB/DFK/DDN/DFN
User manual
Type
Code
Test
Observations
Ph
y
sical inspection
A1
Packaging
Look for signs of rough handling.
A2
Insulator
Check that is not damaged and is clean. Check that it is not loose, bent or broken.
A3
Primary
Terminals
Check that they are not loose, bent or broken.
A4
Secondary
Terminal
Box
Check that it is not damaged and that it is securely fastened to the tank or transformer holder.
Check that no water enters the terminal box due to rain.
Check that the ventilation openings are not blocked.
A5
A6
Metal parts
Check for dents, scratches or signs of corrosion.
A7
Unusual
behavior
Check for unusual noises, vibrations or odors.
B1
Primary
connections
Check that there is good contact, that there are no signs of corrosion and that all nuts and bolts are
properly tightened. The use of conductive grease such as Penetrox A13 is recommended to improve
contact.
B3
Secondary
circuits
Check that all screws are properly tightened, and that there are no signs of corrosion.
In the case of secondary windings with intermediate taps, check that the connection matches the real
transformer ratio.
Check that the unused secondary windings are in open circuit.
B4
Grounding
Check that the grounding terminal is correctly connected to the substation grounding network, that
the screws are correctly tightened and that there are no signs of corrosion.
B6
Voltage
transformer
neutral
connection
Check that the neutral terminal (N) of the primary winding is grounded.
Leaving it open when the transformer is energized will cause surges and damage to the
transformer.
B7
Readings
Check that the reading values of the secondary match the values expected.
B8
AF connection
Verify that the high frequency (HF) terminal is connected to ground or to the accessory block. While
this equipment is not in use, the carrier wave groundings witch must remain closed.
C1
Oil level
indicator
Check the oil level in the tank through the sight glass.
C2
Oil stains
or leaks
Check the following areas for visible oil traces:
› Insulator fl anges
› Junctures between parts of the metal enclosure
› Junctures between the secondary terminal box and the tank or base
› Primary terminals’ surrounding area
Dielectric t
e
st
s
D1
PF/DDF/tg
δ
The limit values of this parameter depend on the transformer’s voltage level. The recommended values
are given with the order. It is also important to control this parameter’s change over time. A change
in the value of this parameter indicates a change in the properties of insulation, but it is not defi nitive
proof of faulty insulation. For this reason, if the value of the delta tangent is above the recommended
value or has increased, a careful investigation should be carried out and further testing may be required.
In this case, contact Arteche for assistance.
For details on the test procedure, follow Annex III.
D2
Measurement
of the
Insulation
resistance
For fi eld tests, the use of the megohmmeter (Megger) is very common, and the tests that are usually
done are:
›
N/P2 against secondary and Ground
›
Each secondary against ground
›
Among the secondaries
In the case of new equipment, the value obtained does not provide any information, unless a short circuit
is detected. Generally, values above 200 M
Ω
should be expected. However, the comparison of long-term
values - such as in the case of tan
δ
, or of power factor - can help determine if a device is still in good working
conditions.
Do not test at voltages above 3 kV AC or 1 kV DC.
Do not test for more than one minute. It is not recommended to repeat this test often as the
internal insulation could be damaged.
For details on the test procedure, follow Annex III.
D3
Ratio test
The Portable Transformer Ratio Meter is commonly used to measure the ratio, excitation current and
polarity of windings. The values obtained are a reference that the transformer is in good condition
and can be energized. They should not be compared with the results of factory tests carried out in the
manufacturing plant laboratory.
Due to the high precision required for this measurement, the actual uncertainty of the test must be
taken into account when testing on site.
For details on the test procedure, follow Annex III.
Thermogr
aphic
te
st
s
E1
Thermographic
analysis
Using thermographic cameras, hot spots can be detected during the operation of transformers. In
most cases, hot spots on instrument transformers occur at the primary terminals’ connection point to
the line. If present, check the integrity of the connectors, as well as their correct position and whether
they are correctly tightened.
