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8/13/2019 Power Cable Condition Assessment
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8/13/2019 Power Cable Condition Assessment
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8/13/2019 Power Cable Condition Assessment
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Defects in Cables
Manufacturing Imperfections - Tend to increase
the local stress to either initial failure or higher
rates of aging:
Voids
Contaminants in insulations
Poor application of shield material Protrusion on the shields
Poor application of jackets
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Defects in Cables
Poor Workmanship - Tend to increase the local
stress leading to either early failure or higher
rates of aging:
Cuts
Contamination
Missing applied components or connections Misalignment of accessories
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Defects in Cables
Aggressive Environment - Tends to reduce the
dielectric strength. The impact can be local if the
environment influence is local:
Chemical attack
Transformer oil leaks
Floods Petrochemical spills
Corrosion
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Defects in Cables
Overheating - Tends to reduce the dielectric
strength. The impact can be restricted to short
lengths (local) if the adverse thermal
environment is localized:
Excessive conductor current for a given
environment and operating condition (global).
Proximity to other cable circuits for short
distances (local).
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Defects in Cables
MechanicalTends to reduce the dielectric
strength. The impact can be restricted to short
lengths if the mechanical stress is localized:
Damage during transportation (usually
localized)
Excessive pulling tensions or sidewall bearing
pressures (can be localized or global)
Damage from dig ins (local)
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Defects in Cables
Water IngressTends to reduce the dielectric
strength and increase the stress in the area
surrounding the moisture:
Normal migration through polymeric materials
Breaks in seals or metallic sheaths
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Techniques to assess condition
There is a wide range of techniques available forcondition assessment of High Voltage cables. Beforewe proceed further, it is important to keep thefollowing questions in mind:
EffectivenessDoes the technique do what isintended?
MaturityHas the technique been employed longenough to assure its effectiveness.
AccuracyDoes the technique deliver the correctassessment
ClarityDoes the technique provide actionable andclear assessment
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Standards for Cable Installation,
Testing and Accessories IEC 62067, Power cables with extruded insulation and the
accessories for voltages above 150kV
IEEE 400, Guide for Field Testing and Evaluation of theInsulation of Shielded Power Cable Systems Rated 5 kV and
Above IEEE 400.1, Guide for Field Testing of Laminated Dielectric,
Shielded Power Cable Systems Rated 5 kV and Above WithHigh Direct Current Voltage
IEEE 400.2, Guide for Field Testing of Shielded Power Cable
Systems Using Very Low Frequency (VLF)(less than 1 Hz) IEEE 400.3, Guide for Partial Discharge Testing of Shielded
Power Cable Systems in a Field Environment
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Available Condition Assessment
Technologies and Brief Introduction
Insulation Resistance and PI
IR/PI are the oldest techniques available to assess the bulkinsulation level for presence of contamination.
IEEE 422 standard specifies a min limit for R as= (rated voltage in KV + 1) x 1000 / Length of cable in feet
IR is temperature dependent and PI being a ratio of IR 10 min toIR 1 min becomes temperature independent.
For longer and high voltage cables, more charging current isrequired and hence ordinary meggers cannot be utilized.
Moreover, DC test gives little useful information on condition ofthe cable.
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TDR
TDRTime Domain Reflectometry is used to identify cable faults andis used primarily for fault location identification when a cable has
already failed.
Using TDR distance to failure spot can be measured when signals
injected are returned from open and/or short turns.
A TDR works like a radar. An impulse signal is injected in the cable
from one end and it travels down the cable. Any discontinuity or
imperfection will cause some of the incident energy to come back
to source. Initial pulse and reflected are compared against each
other to locate problems Very little information is provided by TDR for a fault free cable,
hence it is used for fault localization not for condition monitoring.
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Partial Discharge
Partial Discharges refer to the partial breakdown of insulation which occurs only
for a short period of time and does not result in full arc or short.
PD is just one of the many failure accelerating parameter of a cable.
It can be done both online and offline.
Offline PD requires an AC source which for cables become too large due tocapacitance and votlage rating. Online is done using HFCT and UHF sensors. Difficult
to isolate noise of source and actual PD.
Leakage current requirement is calculated as: 2*pi*f*C*V
PD can be done with VLF (Very Low Frequency) source.PD measurement help in localizing minor and major faults involving voids, cavities.
For XLPE cables, PD tend to set in at very last stages of the life of the cable and does
not give any useful information for trending in case of Water Treeing or Ingress.
Its a quality test for factory acceptance.
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Acceptable PD Levels in XLPE
Cables
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Tan Delta
When a steady-state ac test voltage is applied to an insulated cable, the resulting
apparent total current that flows consists of a charging current due to the
capacitance of the cable insulation and a leakage current. The phase angle between
the applied test voltage and the total current is known as the dielectric phase angle.
The complement of the phase angle is called the dielectric loss angle.
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Tan Delta
Tan delta is helpful in assessing entire length of cable and givesgood information about bulk insulation quality. Detects water
trees.
However, as explained for PD, a high leakage current source(tends to be heavy and bulky) is needed esp at power frequency
voltages. Recently in last decade or so, use of VLF source has
become a common practice esp in Americas. IEEE 400.2
standards specifies the application in detail.
At very low power factors (
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LIRA
Line Impedance Resonance Analysis (since early 2000s)
LIRA Equipment injects a white noise signal (containing all frequencies)
into the cable at one end and measures the transfer function of the
cable i.e., Z(f) impedance in frequency domain.
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Cable Model
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LIRA
LIRA Portable
LIRA Portable is the proven test equipment used by Wirescan inservice missions in the harshest environments. Its features are:
Sensitive to small degradations and cable defects
Accurate location of cable degradations / defects
Applicable to a wide range of cables and wires
Applicable on multisegment systems with different cable types
Non-destructive and non-intrusive
Only one access point needed for measuring
Fast and easy connection to test object
Recording and playback for post analysis
Detection of multiple faults / damages / failures, and their severity
Pre-study / simulation of cable data and defects
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LIRA BLOCK DIAGRAM
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Identification of Localized Thermal Aging of
EPR Insulation (216 hr @ 150C)