Power Cable Condition Assessment

8/13/2019 Power Cable Condition Assessment

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We Take care of Your Assets

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)



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



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



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



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)

Power Cable Condition Assessment

Documents

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