Megger TTR320 Series, Instruction Manual

Page: 40 / 68

M
AVTMTTR320-EN Rev A Oct 2008
32
5. Reinstall the fuse holder in its receptacle in the input power module. Connect
the power cord to the TTR test set and to an energized power source. If the
electronic circuits still do not function properly, contact the factory for
service.
Calibration
A complete performance and calibration check should be made at least once
every year. This will ensure that the TTR test set is functioning and calibrated
properly over the entire measurement range. The 3-phase TTR calibration is
performed on each new or repaired unit before sending it to a customer. There is
a special 3-phase TTR final calibration procedure which requires a NIST-
traceable test equipment to be used. As a result of such calibration procedure,
each TTR test set may be NIST certified.
To check the 3-phase TTR calibration at a customer site or in the field, the
Megger Calibration Standard Cat.No550055 or equivalent standard should be
used. To perform a quick simplified calibration check, the Megger Calibration
Standard CatNo550555 may be used. The last standard has eleven switchable
turns ratio settings which cover the turns ratio range from 1:1 to 2000:1.
TTR320 – Self check
A customer may want to make sure that the test set is functioning properly
before testing a transformer. Traditionally, for the transformer turns ratio testers,
a customer performed this test by connecting H1-X1 and H2-X2 test leads. The
1:1 turns ratio was expected to be measured. Additionally, the displayed test
results were interpreted as a test set calibration check.
The 3-phase TTR test set is designed and optimized for the transformer turns
ratio testing. It uses a mixed analog-digital technique to provide the high accurate
readings of the tested transformer turns ratio. The test set measurement
approach is based on high impedance input and output transformer voltage
monitoring circuits. The high impedance of the monitoring circuits is optimized
for the accurate transformer turns ratio testing.
When performing a 3-phase TTR functional test, the H and X test leads
interconnect the input and output monitoring circuits without a transformer
being connected. Such connection cause an impedance mismatch and, as a result
of the mismatch, the displayed turns ratio reading is not exactly 1:1 as it may be
expected. A customer should be aware that a functional test is not a real
transformer turns ratio test. Due to the intrinsic and inevitable impedance
mismatching, the 3-phase TTR functional test readings may be from 0.9980 to
1.0020. The TTR functional test readings do not represent its calibration. It
simply shows that the TTR test set circuitry functions properly. To check the 3-
phase TTR calibration, a customer should use the Calibration Standards
mentioned in Calibration section of the manual.