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Pollution Monitoring for Better Selection of Insulators in Contaminated Service Conditions

August 5, 2017 • ARTICLE ARCHIVE, Insulators
PPC Insulators

Failures of insulators due to pollution flashover can prove very costly, causing potentially long outages and requiring expensive and time-consuming maintenance. By most accounts, one of the leading causes of such failures is improper specification of insulators in the first place – to the extent that the designs selected are unable to cope with all the stresses imposed by pollution in the environment. Therefore, it is vital for engineers at power supply companies to correctly understand the real pollution characteristics of the service environment into which insulators will be placed into service. This second of a two-part INMR article from 2012, was contributed by Igor Gutman of STRI and Wallace Vosloo of Eskom. It introduced practical field and laboratory experience with different pollution monitoring techniques as well as presented principles to convert from one parameter to another. It also discussed how to use these parameters for insulator dimensioning according to the requirements of IEC 60815.


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Measuring Leakage Current

Leakage current flowing on an insulator depends mainly on the characteristics of the pollution on its surface as well as its geometry and material. For this reason, measuring and analyzing leakage current can be useful to estimate pollution severity and risk of flashover. Leakage current is usually measured by collecting the current at the ground end of energized insulators. The three most common methods for this purpose are:

1. Surge counting

The number of leakage current pulses (or surges) exceeding some threshold level can be recorded during a period of time. This is important since the numbers of pulses as well as their amplitudes increase when approaching the last stage of the pollution flashover process.

2. Measuring peak current

The number of leakage current pulses (or surges) exceeding some threshold level can be recorded during a period of time. This is important since the numbers of pulses as well as their amplitudes increase when approaching the last stage of the pollution flashover process.

3. Measuring accumulated charge.

Accumulated charge measurement is performed in the same way as leakage current measurement. However, instead of focusing on the values of the highest peaks, the signal is integrated to represent accumulated charge – a parameter more related to the ageing process on the insulator’s surface.

Breaker housings at left were not well specified for service at a 110 kV substation near seacoast and subsequently had to be coated with RTV to prevent flashover. Newer breakers at same substation (top) feature porcelain housings with alternating sheds and higher specific creepage and do not have coatings. pollution monitoring Pollution Monitoring for Better Selection of Insulators in Contaminated Service Conditions p3

Newer breakers at same substation feature porcelain housings with alternating sheds and higher specific creepage and do not have coatings.
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Newer breakers at same substation feature porcelain housings with alternating sheds and higher specific creepage and do not have coatings. pollution monitoring Pollution Monitoring for Better Selection of Insulators in Contaminated Service Conditions p2

Breaker housings at left were not well specified for service at a 110 kV substation near seacoast and subsequently had to be coated with RTV to prevent flashover.
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Over the years, a number of different pollution monitors to measure leakage current have been developed. Products have ranged from simple surge counters to advanced multi-channel devices that even incoporate integrated meteorological stations. A standard such system typically includes an integrated weather station to record wind speeds and direction, humidity, rainfall, temperature and UV-B radiation. All sampled values are then saved at some user-defined interval, typically every 30 minutes. Recorded data is retrieved using a serial port, which also serves to configure the instrument. Using such advanced devices, insulator researchers have been able to obtain a large volumes of leakage current and corresponding weather data. These have helped to better understand the physics of the pollution flashover process on different types of insulators.

Leakage current device installed in South Africa (left) and Norway (right) allows continuous sampling of leakage currents on 9 energized insulators and can record peak currents (positive and negative), average currents (positive and negative), RMS currents, accumulated charge and energy loss. pollution monitoring Pollution Monitoring for Better Selection of Insulators in Contaminated Service Conditions p4

Leakage current device installed in South Africa (left) and Norway (right) allows continuous sampling of leakage currents on 9 energized insulators and can record peak currents (positive and negative), average currents (positive and negative), RMS currents, accumulated charge and energy loss.
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pollution pollution monitoring Pollution Monitoring for Better Selection of Insulators in Contaminated Service Conditions p1

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Sophisticated mathematical approaches have also sometimes been necessary. For example, multivariate analysis (MVA) methods has been used since they are appropriate to study and analyze data structured on many interrelated variables. MVA software was successfully tested in an ageing and pollution performance study performed on seven insulators exposed to pollution at the Kelso Test Station (on the Indian Ocean coast of South Africa) and at a former Test Station on the southern coast of the United Kingdom. An example of applying MVA is shown in Fig. 1.

Fig. 1: Example of application of MVA method: (top) Matrix of factors showing that pollution event consist of wind plus humidity plus light rain. (bottom) Same illustrated by one pollution event. pollution monitoring Pollution Monitoring for Better Selection of Insulators in Contaminated Service Conditions p6

Fig. 1: Example of application of MVA method: Same illustrated by one pollution event.
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pollution pollution monitoring Pollution Monitoring for Better Selection of Insulators in Contaminated Service Conditions Pages from INMR 96s insolator 3

Matrix of factors showing that pollution event consist of wind plus humidity plus light rain.
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