By John Chan, P.Eng.
Microscopic examination is an effective cable evaluation tool for assessing the condition of new and service-aged polymeric cables. This test is particularly suited for identifying defects which can affect cable performance and reliability.
Cable dissection coupled with microscopic examination is invariably carried out in accordance with the AEIC (Association of Edison Illuminating Companies) Specification CS5, CS6 or CS7, depending on the cable voltage class and insulation type. These AEIC Specifications are the first known industry specifications to set requirements limiting the size and density of insulation defects such as voids, contaminants and shield protrusions.
Typical Defects of Polymeric Cable
Typical defects in a medium voltage XLPE cable are shown in Figure 1. Defects such as voids, contaminants, shield protrusions and loose semiconductive shields in the insulation structure lead to "treeing" which is a major cause of electrical failure in polymeric power cables.
The various insulation defects are briefly described and discussed below.
Void
A spherical or irregularly shaped cavity in polymeric insulation. If the void is sufficiently large in size (eg. > 10 mils), it can lead to electrical treeing due to high partial discharge magnitude (corona). Voids and the presence of water can lead to water treeing (bow-tie trees), resulting in cable failure in the long term.
Contaminant
Any solid or liquid material that is not an intended ingredient. Contaminants include ambers, fibers, metallic and black particles. Depending on their size, nature and geometry, contaminants can lead to electrical treeing and subsequent cable failure. In the presence of water, they can develop into bow-tie trees.
Shield Protrusion
A needle-shaped projection at the conductor or insulation shield interface. If the protrusion is sufficiently sharp or large in size, it can develop into an electrical tree due to stress enhancement In the presence of water, it can develop into a vented tree.
Loose Shield
A partial or complete separation of the shield layer from the insulation. Poor shield bonding results in high partial discharge magnitude (corona), leading to electrical treeing and cable failure. In the presence of water, large vented trees can develop.
Water Tree
Microchannels in the insulation which develop at defect sites in the presence of water and voltage stress. A bow-tie tree is a water tree which originates within the insulation at a void or cantaminant.
A vented tree is a water tree which originates at the conductor shield or insulation shield, possibly due to contaminants in the shield material.
Electrical Tree
Dark, branch-like structures caused by severe ionization or tracking.
Cable Evaluation Techniques
Cable evaluation involving microscopic examination can be divided into the following specific areas: Cable Dissection, Cable integrity testing and Cable failure analysis.
Cable Dissection
The reasons for cable dissection are as follows:
- To record details of cable construction and design
- To check cable dimensions for compliance with specifications
- To record any irregularities in material or constrution and perform appropriate tests on cable components.
Cable Integrity Testing
The key benefits of cable integrity testing are as follows:
To verify product quality (i.e. to ensure that in- coming cables meet utility specifications). Tests include: cable dissection, dimensional analysis, microscopic examination for insulation defects, volume resistivity of semiconductive shields, insulation shield bond strength, hot oil examination for internal irregularities, etc. These tests are normally performed in accordance with AEIC CS5, CS6 or CS7 requirements.
To allow utilities to compare and rank the quality of cables from different suppliers and target those suppliers with poor quality records.
Cable Failure Analysis -- Medium Voltage Cable
The reasons for cable failure analysis are as follows:
- To determine the cause of failure
- To identify defect types contributing to cable failure (especially those defects of a widespread nature).
- To assist utilities in identitying problem areas in their installed cable circuits.
- To determine the density and size distribution of water trees (i.e. tree count).
- To establish criteria for judging if cable replacement is required. This is accomplished by analyzing and comparing the water treeing characteristics of the failed cable with those of other faulted cables of similar vintage, voltage class and service history from a database of over 200 faulted cables.
John Chan is with J.C.C. Cable Engineering Lab located in Toronto, Ontario, Canada. He can be reached at (416)536-4361.