Megger DELTA4000, Instruction Manual, Page: 66 / 74

M
AVTMDELTA4000-EN Rev 6 April 2018
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Bushing Hot Collar Tests
Hot collar tests are generally performed on HV bushings that do not have test
taps. However, these tests may also be done as an additional test if the bushings
have test taps
Tests 37 to 46 - These tests measure the outer shell insulation of the bushings,
including surface leakage and leakage current from the bushing surface through
the insulating material to the center conductor. A conductive strap is placed
around the bushing, below one of the skirts, normally the top skirt, and
connected to the HV lead. The Red lead is connected to the bushing conductor.
Test mode GST-GND is automatically programmed by the test form.
Multiple hot collar tests can be performed on each bushing. For each test, the
location of the strap should be clearly indicated on the form in the Designation
column.
Ratio Tests
Measurements of transformer winding ratio can be made with a high voltage
power factor test set when a TTR capacitor is available. Depending on the
voltage rating of the winding being energized, tests can be made up to 10kV.
The significance of this is that higher volts per turn can be applied to the
winding, stressing the winding turn-to-turn insulation more than low voltage
ratio test sets. This may be able to detect high resistance turn faults that may be
undetectable at lower voltage. The tests are performed one phase at a time on
three-phase transformers.
To enable this portion of the test form, select the Show TTR Tests checkbox at
the top of Page 1. A table for ratio test results will become visible near the end