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Management of transmission power lines in heavily polluted environments

This white paper is based on the article «Insulator Contamination Assessment and Mitigation for AC and DC Overhead Lines» by J. M. GEORGE present during the 2018 CIGRE US National Committee Grid of the Future Symposium.
This paper will review the basic concepts required for understanding the contamination process, the tools available for their evaluation and the classical mitigation methods.

Airborne dust or coastal salt contamination of overhead line insulators can seriously compromise the performance of a transmission line, either AC or DC, generating potential heavy financial losses and additional maintenance costs.

Understanding basic concepts of the contamination process as well as the tools available for their evaluation can help finding the most appropriate sustainable mitigation methods.

Among the diverse options offered in the relatively large spectrum of possibilities, utilities are often selecting designs which long term performance especially in harsh conditions is known today to be
questionable, imposing additional inspection costs as well as service interruptions. CIGRE and IEC have established guidelines which can be considered as a good starting point in a rational approach for pollution mitigation. Specific creepage distance, profiles and shapes of insulators as
well as surface properties of dielectric materials are taken into consideration on a theoretical level which still must be challenged by reality and actual laboratory testing.

While these parameters will be introduced in this paper, a specific mention for DC is necessary since the polarized effect around the conductors will act like a magnet amplifying the level of pollution compared to an AC line on the same route. Examples will be shown to better demonstrate the specificities of a unidirectional field on the performance of a DC overhead line.

Classical evaluation and mitigation methods require an outage during which either samples are periodically taken down to measure the pollution level or preventive line washing at predefined intervals is performed. Ideally information on the condition of the string of insulators should be more useful if provided on a real time bases. This would allow maintenance action at the proper time without any risk or unnecessary premature spending.

Innovative techniques for real time evaluation of the condition of a string of insulators are now possible thanks to smart insulators capable to communicate in real time their pollution condition. This paper will describe the fundamental aspects of this IoT technology where the insulator itself produces a diagnostic. Instead of measuring the level of contaminants through physical sampling on a string, this development will concentrate directly on the consequence of the environment on the performance
of the string by measuring the actual leakage current. Using wireless communication technologies, the data is transferred to a dedicated server where the information will be analysed and presented to the end user with a diagnostic of risk of pollution related flashover. Such processes imply a detailed knowledge of the signature of each type of insulator in terms of leakage current since threshold values depend upon shape and profile. Actual examples will demonstrate this aspect.

A similar approach is currently being implemented for other parameters impacting service conditions of a transmission line such as vibrations, actual load on a span, ice accumulation…this data will be systematically
analysed and displayed on a universal platform where end users can check the condition of a line, including automatic alerts for maintenance crews.

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