Introduction & Background
First and foremost, all shielded cables and their terminations or splices should
be partial discharge free, just as the cable was when it was manufactured and
tested at the cable manufacturer’s plant. This is especially true for acceptance
testing and as a basis for trending those existing service-aged facilities, which
may have some partial discharge activity starting.
Partial discharge is the partial failure of medium- and high-voltage insulation.
As the insulation fails, it produces signals that can be detected using advanced
technologies. These signals are a symptom, or Side Effect, that is produced by the
partial failure of the insulation. Therefore, the detection of partial discharge in
cable and other insulation provides an early warning of eventual or impending
failure. Partial discharge testing can be performed on-line without requiring
an outage and can be economically applied on all facility equipment in a survey
fashion. By doing so, failures will be reduced, and reliability will be greatly
enhanced.
In addition to the obvious economic and convenience advantages of on-line
partial discharge testing, research has indicated that on-line partial discharge
measurements better represent the true condition of the insulation, since measurements
are taken under actual operating conditions. This is due to a composite effect
of several complex physical interactions within the operating cable system that
are not present in its off-line state. The prime effects include service factors such
as operating temperature and its effect on partial discharge via thermal expansion
and contraction characteristics, material properties, mechanical loading influences,
current-based power factor, and harmonic frequency effects. Additionally,
long-term energization affects insulation characteristics, as the insulation may
respond differently because it has been energized for a long contiguous time, as
compared to initial short-term energization during an off-line test.
Partial Discharge
in Cable Systems
When evaluating the integrity
of a typical cable installation, it is
important to consider all elements
of the cable system – terminations,
splices, and the cable itself. If any
of these components fail, power is
lost and service is interrupted until
corrective actions can be carried
out. Reliability statistics indicate
that approximately 90 percent of
cable system failures occur at splices
and terminations. This is likely due
to workmanship defects developed
from the inability to create flawless
insulation specimens in the
field under varying conditions by
craftsmen of varying skill levels
as compared to the repeatability
of producing cable insulation in a
controlled manufacturing environment
that utilizes laboratory partial
discharge testing for quality assurance
purposes.
Imperfections in cable system
insulation can be caused by defects,
contamination, poor workmanship,
faulty installation, and many
other problems. These imperfections
create localized stresses in
the electrical field which in turn
create localized breakdowns that
lead to eventual complete failure.
These localized breakdowns create high-frequency pulses
that can be detected and evaluated with specialized sensors
and instruments. This technology continues to advance, and
presently a great deal of information can be gathered by
processing these pulses including the discharge magnitude,
discharge power, number of pulses per cycle, pulse phase
angle, and specific information related to each individual
pulse including pulse width, risetime, frequency and other
detailed information. This information can then be filtered,
sorted, and classified to determine the existence of partial
discharge activity, the level of danger that the discharge
represents, the type of discharge that is occurring, and the
location of the discharge.
Locating Partial Discharge
in Cable Systems
It is very valuable to the facility manager to know if partial
discharge activity exists on a specific feeder cable and the
threat level that this activity represents. Additionally, it is
also important to know where the activity exists so that corrective
actions can be planned (prior to a complete failure)
and taken in a nonpanic emergency mode.
In order to effectively assess cable system integrity, partial
discharge measurements are typically taken at locations
where the cable shields are grounded. These locations usually
exist at each termination and splice, but certain installation
practices may not include shield grounding at the splice
points. When shield grounding does occur at the splices,
relative partial discharge location can be determined by the
partial discharge pulse magnitude and frequency since pulse
magnitude and frequency decrease as the distance from the
partial discharge source increases. Therefore, locations along
the cable system where the highest magnitude and highest
frequency pulses are found indicate the most likely location
of the partial discharge activity.
Different approaches must be taken to locate partial
discharge in cable systems that do not have grounded splice
shield points, and these approaches can also be applied to
confirm partial discharge location in cable systems that do
have grounded splice shield points. Depending on noise
levels, partial discharge pulses can be detected a thousand
feet or more away from the partial discharge source. By applying
special noise reduction filters to the partial discharge
measurement system, it is usually possible to detect partial
discharge in the longest runs of cable just from the terminations.
Even though the length of the cable run and the level
of background noise may adversely influence the accuracy
of the test results, it is still worth attempting the test since,
at a minimum, termination flaws can be identified.
When partial discharge occurs in a cable system, two
pulses of similar size and characteristics propagate away
from the partial discharge site towards the terminations.
Depending on the cable insulation type, shield construction,
and other factors, the speed in which the pulses travel
is relatively consistent. For instance, partial discharge pulses
travel at a speed of approximately 468 feet per microsecond
(142.65 meters / u-sec.) in XLPE insulation.
As can be seen in Figure 1, the time difference between
the first two pulses (the initial pulse and the reflected pulse)
is noted as delta T1-T2. The two pulses will continue to
travel up and down the cable. These two pulses are reflected
at exactly the cable return time (delta T1-T3) from the original
pulse set, creating two sets of pulses, each spaced at the
cable return time, delta T1-T3. Then the partial discharge
location can be calculated from the following formula:
Location from Measurement End (in % Cable Length) =
100*(1- ΔT1-T2/ΔT1-T3)
Under certain circumstances, such as an extremely long
cable or noisy background conditions, the reflected pulse
may fall below the noise threshold as it attenuates along the
cable and may not be distinguishable at the measurement
end. In those cases, it is possible to apply a remote pulse
transponder to the far cable end. The transponder detects
the reflected pulse as it reaches the far end and then injects
a large pulse back on the cable shield that will easily be
distinguished at the measurement end.
In addition to performing hand calculations to determine
partial discharge location, special software is available that
calculates and plots partial discharge location as seen in
Figure 2. This cable partial discharge mapping software can
provide partial discharge location with more accuracy and
more efficiently than hand calculations.
Conclusion
Partial discharge testing is a vital tool for determining
the health of cable systems. This technology can be applied
economically in an on-line survey fashion to provide excellent
cable system condition assessments. This information
can then be used to channel maintenance resources to the
areas that require the most attention. Several advanced
methods can be used to identify, quantify, and locate partial
discharge, depending on the cable system construction and
the partial discharge location accuracy requirements of the
facility.
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