77TRACKING ON INSULATOR SURFACES IN PRESENCE OF CONDUCTIVE DEFECTS... Q2 2009 INMR®

Background on Surface Tracking Tests

Leakage currents and arcing that

appear on insulator surfaces under

contaminated conditions can

degrade and decompose polymeric

housing materials. This process

might even develop into tracking,

which is the formation of conductive

carbonaceous paths. Once such

tracking starts, insulation properties

in the area around it are greatly

reduced and proper function of the

insulator is at risk.

The Inclined Plane Test or IPT

(described in IEC 60587 and ASTM

2303) specifies the experimental

conditions to test tracking resistance

of electrical insulation materials by

simulating severe surface

contamination. In this test, a

rectangular-shaped material sample

is mounted at an inclination of 45°

between specially shaped electrodes

separated by a distance of 5 cm and

energized from a high voltage

source. In addition, a layer of filter

papers is attached under the upper

electrode and wetted by a

conductive electrolyte.

The electrolyte is made to flow

constantly between the electrodes

along the sample’s surface and its

rate is regulated depending on

specified test voltage (e.g. at 4.5 kV

the flow rate is 0.6 ml/min).

Duration of the test is limited to

maximum of 6 hours for cases where

tracking does not occur. There are a

number of different failure criteria in

the standards. Among these is a

maximum level of surface leakage

current (60 mA) or a specified

length of the tracking path (e.g. 2.5

mm depending on standard) which,

when reached, terminates the test.

Modern IPT arrangements are

equipped with data acquisition

systems (DAQ) and temperature

sensors to record both leakage

currents and surface temperature

changes. Moreover, the tests are

usually performed in a sealed and

ventilated chamber to control

humidity. The particular chamber used

in this study is shown in Figure 1.

The DAQ system used in this study

allowed surface leakage currents to

be monitored continuously and also

the temperature of a selected small

spot on the sample to be measured

every second. Three samples could

be tested simultaneously.

To properly investigate tracking

behaviour in the presence of

conductive interfacial defects, the

test samples had to fully simulate

the real structure of a composite

insulator, containing both the core

and housing material. This was

achieved by joining rectangular

pieces of FRP epoxy substrate with

an ATH-filled HTV silicone rubber

housing of 6 mm thickness. The

interfacial defect (in the form of a

circular piece of copper foil of 12

mm diameter and 70 µm thickness)

was glued on the epoxy before

bonding with the HTV SIR.

INMR®

Q2 2009 76

This article is the third in a series looking at the development of certain types of ageing phenomenathat could arise in composite insulators in the presence of interfacial defects. The research so far hasconcentrated on defects that might appear at the housing-core interface within large hollow corecomposite insulators, either as a result of improper handling or through poor manufacturing practice.In such cases, the cost involved in insulator replacement would probably be very high. However, inregard to the physico-chemical processes involved, the considerations presented up to now apply aswell for all types of composite insulators, including those used on overhead lines.

An article published in INMR (Number 3, 2008) described past investigation of adhesion defects,where the presence of partial discharges (PDs) and moisture ingress could initiate degradationprocesses affecting long-term behaviour. These defects, for example, could grow in size over time. Atthe same time, development of PD activity as well as the appearance of water and degradation by-products could lead to a localized increase in the material’s conductivity. Such conductive spots canalso be introduced during manufacturing.

Independent of their origin, semi-conductive or conductive spots within interfacial areas in compositeinsulators will locally distort electric field distribution and also enhance it. The aim of the projectdescribed in this article was therefore to establish whether the presence of conductive defects at thehousing-core interface might adversely affect the external properties of insulators, especially theresistance of their housing to tracking. This was to be assessed by testing resistance to surfacetracking on specially-prepared samples of silicone rubber (SIR) bonded onto fibre reinforced (FRP)epoxy substrates containing metallic defects. Tracking performance of these samples was thencompared with the performance of identical samples without defects.

This work was overseen by INMR Columnist, Professor Stanislaw Gubanski and performed jointly byChalmers University of Technology in Gothenburg, the ABB Corporate Research centre in Västerås,Sweden and the Instituto de Investigaciones Eléctricas (IIE) in Cuernavaca, Mexico. PhD student JohanAndersson conducted the experiments while Dr. Henrik Hillborg of ABB assisted in the preparation oftest objects and Dr. Ruben Saldivar at IIE helped to organize the tracking tests. Schneider Electric’sMemorial Fund to honor Erik Feuk (Sweden) provided additional financial support.

IIE is one of Mexico’s main research centers dedicated to the electrical and oil industries. In thisregard, it performs a range of standardized electrical tests on dielectric materials for differentpolymeric insulation systems used in suspension insulators, surge arresters, bushings andencapsulated components. The IIE also has a specialist group working on new polymer compositesand nano-composites.

TESTING

Tracking on Insulator Surfaces in Presence ofConductive Defects within Housing to Core Interface

Figure 1: Test chamber used in the study.

Figure 2: View of test samplecontaining conductive defect.

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79TRACKING ON INSULATOR SURFACES IN PRESENCE OF CONDUCTIVE DEFECTS... Q2 2009 INMR®

the defect edges (from ~ 2 kV/mm

to ~ 5 kV/mm).

Yet another interesting finding of the

simulations was that the presence of

the defect introduced an overall

increased electric field stress not

only localized in its vicinity but also

in the larger surrounding volume.

Figure 6 illustrates a field solution

inside the SIR layer for the model

including both the defect and the

restive electrolyte layer.

Since the solution was obtained for

an applied voltage of 1 V, the

numerical value of 100 V/m in the

colour bar legend of Figure 6

corresponds to field strength of 0.45

kV/mm for 4.5 kV test voltage. The

electric field inside the silicone

rubber housing material between the

defect and the contamination layer

varied between 0.3-0.5 kV/mm

which was almost ten times higher

than for the reference model. This

field strength continued to also be

greater closer to the lower electrode

than in the reference model. Such

an increased electric field strength

could hypothetically have a strong

effect on initiation of surface arcing

and thereby could also accelerate

the degradation of the material.

The photo in Figure 7 illustrates a

sample when tracking has just

started close to the lower electrode.

While this tracking is located

initially on the surface, it penetrates

into the material over time until the

6 mm thick layer of silicone become

damaged and the epoxy-SIR

interface is exposed. In total, 5

reference samples and 6 samples

with defects started to track within

6 hours of testing (see photos in

Figures 8 and 9).

On 5 of the 6 tracked samples with

defects and on 4 of the 5 tracked

reference samples, the tracking

paths were longer than 2.5 cm.

When the samples from these

Figures were compared to each

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Q2 2009 78

The centre of the defect was placed

exactly in the middle between the

lower and upper electrodes (2.5 cm

above the lower electrode, as in

Figure 3).

In this study, 9 test samples were

prepared with internal defects and 9

without defects. Both types were

then tested at 4.5 kV as shown in

Figure 4. The pyrometers seen in

Figure 1 were directed to measure

the surface temperature 2.5 cm

above the lower electrode, exactly

over the centre of the defects.

Findings & Results

To explain how such conductive

defects distort local electric field

distribution, finite element

numerical simulations were

performed. Geometrical models were

thus obtained for different cases,

e.g. with and without the defect.

Solutions were also obtained for a

case when a conductive layer of

electrolyte was added on the

housing surface to imitate the

constant flow of the electrolyte

during the IPT.

These solutions revealed that the

electric field distribution became

significantly distorted by the

presence of the defect when

comparing it to the reference case.

Even stronger field distortion

appeared for the model having both

the defect and the electrolyte layer.

The additional interaction between

the resistive layer on the sample

surface and the defect at the epoxy-

SIR interface more than doubled the

value of local field strength around

Figure 4: Simulated electricfield intensity in the middleof the SIR housing (i.e. 3 mmabove the defect and 3 mmbelow the SIR surface) withpresent contamination layer.

Figure 3: Sample under test that started to track.Figure 5: Initiation of tracking process.

Figure 6: Appearance of tracking on samples without defects.

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81TRACKING ON INSULATOR SURFACES IN PRESENCE OF CONDUCTIVE DEFECTS... Q2 2009 INMR®

On average, the time to track the

distance of 2.5 cm from the lower

electrode appeared earlier on the

samples with defects than on the

reference samples, i.e. it decreased

by 93 minutes, from 316 min

(reference samples) to 223 min

(samples with defect). Details are

presented in Table 1.

Discussion & Conclusions

The results of this study effectively

demonstrate that the tracking

behaviour of an insulator’s housing

material can be seriously affected by

the presence of interfacial

conductive defects.

The simulations carried out as part

of the research generated results

which reveal that the presence of a

conductive defect at the housing-

core interface in combination with a

layer of surface contamination lead

to increased electric field strength in

the vicinity of the defective

interface. This then increases its

detrimental effect on the resistance

of the material to tracking by

reducing the time necessary for its

development.

The question still to be investigated

is the exact mechanism behind such

observed behaviour. One possible

explanation is that an enhanced

localized electric field provides

favourable conditions for

concentrating the partial arcing at

one spot on the housing and also

maintaining this for a longer period

of time. In any case, the results of

this work clearly indicate that any

conductive defects at the housing-

core interface will adversely affect

performance of a composite

insulator. ⌧

INMR®

Q2 2009 80

other, there were clear differences in

the appearance of the tracking

paths. Instead of track development

along the line between the

electrodes, as on the reference

samples, the tracking direction

made a turn around the inserted

copper foil on the defective samples.

This changed tracking direction

could also be followed when the

samples were inspected from below,

where heat released during arcing

darkened the epoxy substrate, as

illustrated in Figure 10.

Visual inspection of the inserted

defects after tracking revealed that

the copper foils remained relatively

intact since most of the tracking

occurred around them. The heat also

separated the epoxy and silicone

materials from each other, close to

the tracking paths. The reason

behind the changed direction of

tracking could be related to the

different electric field distribution

close to the defect.

A change in tracking direction was

not the only difference found

between the two types of samples

tested. When analyzing the data

recorded by the pyrometers (which

measure surface temperature at the

spots 2.5 cm above the lower

electrodes), a significant increase in

temperatures was registered. Figure

11 presents typical current and

temperature graphs for both types of

samples.

Before the tracking started, the

surface temperatures and leakage

currents remained quite stable, at ~

60-80°C and ~ 5 mA respectively.

But after reaching a tracking length

of 2.5 cm, the local temperature

jumped to ~ 700°C and most likely

the temperature at the centre of the

arc reached several thousand

degrees. The current increased

steadily as tracking proceeded and,

at the same time as tracking

reached the target areas for the

pyrometers, rose to ~ 10-20 mA.

Figure 8: Appearance of back side of tracked samples indicatingvisible changes on surface of epoxy substrates.

Figure 7: Appearance of tracking on samples with defects (defect position indicated).

Figure 9: Variation of temperature and current during IPT on a reference sample.

Figure 10: Temperature and current graphs of a sample with defect.

TABLE 1: COMPARISON OF TIME TO

TRACK FOR TESTED SAMPLES

Reference With defect

Time to track 316 ± 55 min 223 ± 44 min

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