Insulator sets are among the most common components in transmission networks and their quality and performance play a vital role in ensuring a reliable electricity supply. A key part of that is their behaviour when exposed to a power arc. Poor design with respect to power arc behaviour can significantly reduce the lifetime of the insulator set and increase the likelihood of supply interruptions.
There are a wide variety of insulating material options and environments in which insulator sets are used. As such, insulator set designs vary considerably and may even need to be tailored for specific applications. Even the smallest differences in design can affect the power arc behaviour of a string. Physical testing is the most reliable way to assess a string’s power arc behavior and therefore utilities increasingly demand that insulator sets undergo power arc testing before they commit to installing them on their networks.
Over the last several years, KEMA Laboratories, Prague carried out over 160 power arc tests on insulator sets of various types. These tests were performed according to IEC 61467:2008. In addition to revealing the most common failure modes for different types of insulator sets, this overview contributed by Robert Jech highlights a potential issue with how IEC 61467:2008 is currently defined. This is an issue that could impact the value utilities derive from today’s tests. The insight gained from this will provide insulator set manufacturers with useful input into how best to design strings to increase the chances of first-time success in testing and thereby reduce the time and money spent on redesigns. It may also stimulate discussion around the specification of the standard to ensure it meets the needs of manufacturers and utilities equally.
Description of Power Arc Tests
Power arc testing of insulator 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 or towers and used in AC overhead lines with a nominal voltage above 1000 V.
The standard defines methods, parameters, circuits etc. for power arc tests on both insulator sets and short strings. Different test arrangements are allowed within the standard and choice depends on the final application of the insulator sets, according to customer requirements. The test circuit and series should be chosen based on factors such as geometry and type of insulator, its position on the line and type of tower.
A set’s intended position on a line determines whether it should be tested with a balanced or unbalanced supply circuit and, depending on line parameters, short-circuit current to be used during testing. If the insulator set will be positioned in the first or last 5% of a line, it should be tested with an unbalanced supply circuit while sets to be located between the 5% and 95% points of the line require 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 rated short-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 or last 5% require the full short-circuit current of the network.
Meanwhile, type of tower governs choice of a balanced or unbalanced return circuit during testing. Sets to be used in the center phase window of a tower require testing with a balanced return circuit (referred to as test series X). For outer positions on the tower or where there is no center phase window, an unbalanced return circuit is used (referred to as test series Y). Furthermore, the number of tests to be performed depends on whether the customer intends the same type of set to be used throughout the line (in which case the complete test series X or Y is performed) or in just along part of the line. Fig. 1 shows a typical test arrangement for a V-string set with composite insulator This test arrangement comprises balanced supply and return circuits. The arrangement pictured simulates an insulator set position between the 5% and 95% points of the line in the center phase window of the tower.
Depending on type of insulator set, IEC 61467:2008 standard may also require that it undergo verification tests in addition to the power arc test, the main being the mechanical failing load test (MFLT). This is performed on the insulator units to 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 below the flashover voltage. Additional electrical tests may be performed on the fittings and conductors within the insulator set to verify withstand capabilities.