Insulator sets areamong the most common components in transmission networks and their quality andperformance play a vital role in ensuring a reliable electricity supply. A keypart of that is their behaviour when exposed to a power arc. Poor design withrespect to power arc behaviour can significantly reduce the lifetime of theinsulator set and increase the likelihood of supply interruptions.

There are a widevariety of insulating material options and environments in which insulator setsare used. As such, insulator set designs vary considerably and may even need tobe tailored for specific applications. Even the smallest differences in designcan affect the power arc behaviour of a string. Physical testing is the mostreliable way to assess a string’s power arc behavior and therefore utilitiesincreasingly demand that insulator sets undergo power arc testing before theycommit to installing them on their networks.

Over the lastseveral years, KEMA Laboratories, Prague carried out over 160 power arc testson insulator sets of various types. These tests were performed accordingto IEC 61467:2008. In addition to revealing the most common failure modes fordifferent types of insulator sets, this overview contributed by Robert Jechhighlights a potential issue with how IEC 61467:2008 is currently defined. Thisis an issue that could impact the value utilities derive from today’s tests.The insight gained from this will provide insulator set manufacturers withuseful input into how best to design strings to increase the chances offirst-time success in testing and thereby reduce the time and money spent onredesigns. It may also stimulate discussion around the specification of thestandard to ensure it meets the needs of manufacturers and utilities equally.

Power arc testing ofinsulator sets at KEMA is performed according to the IEC 61467:2008 standard.This standard applies to insulator strings or sets comprised of ceramic,glass or composite materials that will be mounted on metallic poles ortowers and used in AC overhead lines with a nominal voltage above 1000 V.

The standard definesmethods, parameters, circuits etc. for power arc tests on both insulator setsand short strings. Different test arrangements are allowed within thestandard and choice depends on the final application of the insulator sets,according to customer requirements. The test circuit and series should bechosen based on factors such as geometry and type of insulator, its position onthe line and type of tower.

Test Arrangements

A set’s intendedposition on a line determines whether it should be tested with a balanced orunbalanced supply circuit and, depending on line parameters, short-circuitcurrent to be used during testing. If the insulator set will be positioned inthe first or last 5% of a line, it should be tested with an unbalanced supplycircuit while sets to be located between the 5% and 95% points of the linerequire a balanced supply circuit for testing. Similarly, sets for the ‘middle’section (i.e. from 24% to 76% of the line length) require only 20% of the ratedshort-circuit current of the network. Sets for use at locations between the 5%and 24% points – or 76% and 95% points –require 50% and sets in the first orlast 5% require the full short-circuit current of the network.

Meanwhile, type of towergoverns choice of a balanced or unbalanced return circuit during testing. Setsto be used in the center phase window of a tower require testing with abalanced return circuit (referred to as test series X). For outer positions onthe tower or where there is no center phase window, an unbalanced returncircuit is used (referred to as test series Y). Furthermore, the number oftests to be performed depends on whether the customer intends the sametype of set to be used throughout the line (in which case the complete testseries X or Y is performed) or in just along part of the line. Fig. 1 shows atypical test arrangement for a V-string set with composite insulator Thistest arrangement comprises balanced supply and return circuits. The arrangementpictured simulates an insulator set position between the 5% and 95% points ofthe line in the center phase window of the tower.

Depending on type ofinsulator set, IEC 61467:2008 standard may also require that it undergoverification tests in addition to the power arc test, the main being themechanical failing load test (MFLT). This is performed on the insulator unitsto ensure they can withstand mechanical forces after a power arc is applied.Insulator sets may also be required to undergo a dry power frequency flashover(DPFF) test to ensure the insulator does not suffer punctures at voltages belowthe flashover voltage. Additional electrical tests may be performed on thefittings and conductors within the insulator set to verify withstandcapabilities.

Statistical Overview of Tests

Analysis of results oftests carried out on 162 insulator sets over a five-year period is shown below:

Types of Insulator Sets& Components

Insulator sets come in anumber of different design types and insulating materials, depending on theintended application. The insulator sets tested during the five-year periodcomprised:

• 92 suspension sets
• 49 tension sets
• 17 “V” strings
• 4 cross-arm insulators

In terms of the insulatingmaterial used and insulator design, insulator sets tested included:

• 81 composite
• 34 glass, cap & pin
• 12 glass, cap & pin (short string)
• 27 porcelain, long rod
• 8 porcelain, cap & pin

Critical Components

The figures above givean indication of the quality of insulator sets available today and thesustained need for testing before installation. However, perhaps moreilluminating is the insight into the principal failure modes for insulatorsets. This insight highlights areas that manufacturers should pay particularattention to when designing new insulator sets.

Long-rod Insulator Sets

For long-rod sets, threeparticular potential issues were identified. The metallic parts of theinsulator unit can be prone to melt if not well designed. Furthermore,poor design can lead to the sheds near these metallic parts breaking. Finally,the arc can cause metal to evaporate from the set’s protective fitting, causing‘puddling’.

Composite Insulator Sets

The silicone rubber usedin composite insulators exhibits high resistance to power arc testing. However,the critical point for these sets is the where the fiberglass core connectswith the metal end fittings.

Cap & Pin InsulatorSets (Glass or Porcelain)

Cap & pin insulatorsexhibit high mechanical resistance after power arc tests. The main areas ofconcern for these sets are the possibility of sheds breaking and, in the caseof glass sets, cooling of the cap & pin units

Protective Fittings:Load-Bearing Fitting Protection & Arc Direction Fittings

Power arc tests put muchgreater mechanical and thermal strain on these fittings than short-circuittests. This must be taken into account when designing the protective fittingsof insulator sets of all kinds.

There is also risk ofmaterial from these fittings melting onto the insulator units. Care should alsobe taken to ensure that protective fittings do not move during arcing, whichcan lead to the arc root sitting on the load-bearing fittings.

Protective Fittings:Corona Rings, Grading Rings

These fittings are notprimarily designated for arc currents but influence the position of the arcroot and how it moves. As with other protective fittings, risk factors hereinclude material melting onto the insulator units and movement of the fittingscausing the arc root to move away from its intended position. In addition,changes to the contours of these fittings can lead to excessive coronadischarge and radio noise.

Power Arc BehaviourDuring Tests

Carrying out the testsreviewed above provide an opportunity to closely study the behavior of powerarcs during testing. Two distinct basic behaviors were observed. In the firstcase, the power arc runs between the protective fittings designed to define itspath (i.e. the arcing horns, arcing rings etc.). This is the intended behaviorof the power arc and means that most of the stress from the arc is exerted onthe protective fittings and insulator sets with only limited impact on theinsulator material itself. Crucially, this kind of behavior has little impact onthe tower and conductors.

However, a secondbehavior can occur where the arc travels along the line conductor or the tower.In this case, the impact on the insulator set is much less with much lowerstresses on the protective fittings and insulator units. However, there is amuch greater negative impact on the tower or conductor that is carrying thearc.

Currently, IEC61467:2008 does not specify the behavior of the arc during testing. It is alsonot typically specified in the tender documents for grid installation orupgrade projects. This could present a real issue for utilities andmanufacturers of insulator sets. If the design of the insulator set leads tothe arc behaving in the second of the two ways described above, it is likelythat the insulator set will pass the power arc test as the arc is not close tothe insulator and therefore cannot damage it. However, if such a set is used inthe field, any arcs experienced would travel along the tower or line conductorcausing damage. Moreover, damage to these components would be more expensive torepair and lead to longer outages.

Summary

Statistical study ofresults of power arc tests allowed the critical points of various types ofinsulator sets to be identified. This offers valuable insight for manufacturerswhen designing new insulator sets.

In addition, theexperience gained from these tests highlighted a potential issue with theIEC 61467:2008 standard as it is specified today. As the standard does notspecify the behavior of the arc during testing, it could allow potentiallydangerous insulator sets to pass testing. This could give utilities afalse sense of security in the insulator sets they are purchasing. To ensurethe standard meets the needs of utilities, insulator material providers and insulatorfittings manufacturers, this issue should be addressed. To do this, arcbehavior during testing should be specified either through an update to IEC61467:2008 or routinely in tender documents from utilities. The latter coursewould require all utilities to become more aware of this issue.

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