TAN DELTA VLF CABLE TESTING
RESULTS INTERPRETATION
Tan Delta, or Dissipation Factor, or Loss Angle testing, all the same thing, is becoming very routine. Using a
sine wave producing VLF AC Hipot, voltage is applied to the cable while Tan Delta readings are made and
recorded. This paper will provide a brief guideline into the testing procedure and will serve as a starting point
for the interpretation of the results.
When TD testing, the intent is to determine the level of insulation degradation in a cable system. It is an
excellent test method for determining the water tree content in a cable’s insulation. Since different insulation
types (XLPE, EPR, PILC) exhibit different TD characteristics, it is strongly recommended that TD testing only
be performed on singular insulation types. Cable runs of mixed insulations will provide unclear test results.
Also, accessories can affect the TD numbers. For instance, there are some splices and elbows that use stress
grading materials with non linear voltage characteristics that will exhibit increasing TD numbers with
increasing voltage: see curve 2 on page 3. When interpreting TD results, two main criteria are observed: the
absolute TD numbers measured and the change in TD numbers with increasing voltage applied to the cable.
The below table is taken from IEEE400-2001 Standard. The Tan Delta numbers shown are for new XLPE
insulation. Once a cable is installed, the expected numbers will rise. They will also rise once the aging process
begins. Thus, the number shown below are rarely encountered when Tan Delta testing. Typical XLPE numbers
are can be 4 – 5 times greater than what is shown in the chart. For EPR type cable, then numbers can be 5 - 10
times greater and for PILC, 30 - 50 times greater. Also, the test voltage is often not taken up to 2Vo as shown. If
the cable insulation appears weak, the voltage is not raised beyond 1.5Vo to prevent possible failure.
While the absolute numbers are important, they are less important than the change in numbers with increasing
voltage. Whether the TD reading for a cable is 1 or 10, the cable is considered “good” so long as the numbers
do not increase significantly when the applied voltage is raised. A quick “acid test” is to compare the values at
1.5 Vo versus .5 Vo. The higher the difference the change, the greater the degradation. An increase of perhaps
50% is acceptable while a doubling of the base number is not.
This is not an exact science. The typical intent of TD testing is to test a large population of cables to rate them
and compare them. It is largely a comparative test, with the results helping users to prioritize cable replacement
and/or cable injection or to help in deciding what other test may be useful, like VLF Withstand or Partial
Discharge. Performing TD on new installations for a baseline reading is also useful for future testing.
Below are several sample graphs to show various test situations and results.
All three phases look good with
consistent and acceptable rises in TD
numbers from start to finish. Low
numbers consistent with new XLPE.
Test stopped to prevent
cable failure. Phase B is
"Highly Degraded".
Phases A & C - "good" with
little rise in TD numbers with
increasing voltage. TD
numbers in the 8 - 10 range
are typical for new EPR.
Sharp and consistent bend and
constant slope to curve indicate
arrestors still connected or
voltage stress graded splices on
system. Cable may be fine but
results masked by accessories.
Phase A: immediate and
consistent rise may indicate
cable end tracking possibly
due to poor end preparation
Phase C: starts and finishes in same
place as phase A but curve shape is
more indicative of moderately
degraded cable, not surface tracking.