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Corrosion management in power lines

altPower transmission lines are a sensitive point of the system, as they are exposed to the environment and inadequate maintenance has dire consequences, especially for users.

by: Juan Guillermo Maya Montoya*

It is common to read in the maintenance reports of companies in the electricity sector with assets installed in corrosive areas, that some of the metal components of the towers are "rusty"; what is not common is that they describe the criticality of the corrosive attack and the time in which it is estimated that the failure of the reported element could occur.

Possibly this situation is presented by the little knowledge of the technicians about the phenomenon of corrosion- In fact, some companies accept or understand as "normal" the untimely disconnection of their circuits due to the failure of an element attacked by corrosion.

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It was precisely the difference in criteria to deal with this problem, which led the ISA Group to promote the development of the "Integral Corrosion Management" methodology, which it has been implementing in its subsidiaries with excellent operational and economic results.

This article describes the activities that make up the methodology developed:

1. Evaluation of maintenance activities and procedures
The real and effective need of all activities and procedures that have been defined to face corrosion must be evaluated. Some companies allocate significant resources to activities that are unnecessary or do not offer effective corrosion protection.

2. Definition of condition qualification criteria for tower components
It is necessary to define corrosion condition qualification criteria for all metal components of towers; these must indicate the remaining service life expectancy of the element in order to timely schedule the replacement. The criteria defined in the ISA Group are based on the observation and analysis of many failed components.

2.1. Tower components for corrosion condition assessment
When implementing the methodology, the critical elements in each type of tower must be identified. The following figure shows some of the critical elements in red.

2.2. Qualification criteria
A good practice is the use of a brief description of the physical appearance of the corrosive attack accompanied by photographic patterns, the following is the qualification criterion defined by the ISA Group to evaluate the condition of the pins of the insulators.

2.2.1. Corrosion grade 1.
In coastal atmospheres, it is characterized by the fact that the pins have a beige color throughout the surface, they can also present iron oxides in the form of small reddish-brown dots, on the opposite side of the prevailing winds it can present very little affectation of the zinc coating.

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In industrial atmospheres it is characterized by a dark brown color on the entire surface of the pin. The main feature of this degree of corrosion is that even when the Zinc coating has been partially or totally lost, the wear of the steel is minimal.

2.2.2. Corrosion grade 2.
These elements have a generalized dark brown color on the surfaces of the most affected faces and in some cases, oxides of laminar shape (exfoliation), with a wear of up to 20% in the original diameter of the pin. The opposite face of the prevailing winds may present very little affectation of the zinc coating or a beige color, with small points of iron oxides.

2.2.3. Corrosion grade 3.
This is the most critical degree of corrosion, it presents severe wear of the steel and localized or circular corrosion products, in some cases the most affected face presents a lateral wear greater than 20% in the original diameter. These elements have largely lost their original mechanical strength characteristics and must be replaced as soon as possible. The criterion of three degrees of affectation can be applied in other components as can be seen in the following figure.

In elements such as information plates, the criterion used is that of the functionality of the element; for example, the "Grade 1" corrosion indicates that even when corrosion is observed anywhere on the surface, the message can be read perfectly; the "Grade 2" of corrosion, indicates that the message can be read with some degree of difficulty, and the "Grade 3" of corrosion, indicates that the message cannot be read due to the corrosive attack.

When defining the degrees of corrosion, it should be borne in mind that they will not only indicate the physical condition of the elements, but will also indicate the approximate time in which the company must schedule the technical and economic resources for maintenance.

3. Staff training
Staff training should be done both theoretically and practically. Special emphasis should be placed on the photographic patterns defined by the company.

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It is recommended to train staff in the following specific topics:

* Basic principles of corrosion.
* Condition qualification criteria for tower components affected by corrosion.
* Procedure for evaluating the metal components of the towers.
* Best maintenance practices.

4. Evaluation of power transmission lines
The evaluation of the corrosion condition of the tower components can be carried out within the periodic inspection routines defined in each company, including in the report sheets the names of the components to be evaluated.

5. Analysis of information collected in the field
It is to be expected that when implementing this methodology for the first time, activities will result in activities that must be carried out urgently or in the short term. The normal thing is that after the analysis of the information, activities such as the following are derived:

* Emergency care for critical condition.
* Painting of towers.
* Change of tower profiles.
* Change of screws.
* Change of hardware.
* Change of shock absorbers.
* Change of information plates.
* Change of insulators.
* Change of guard cables.
* Change of conductive cables.

6. Scheduling of maintenance activities
The critical conditions detected in the transmission lines considered strategic for the company must be scheduled as an emergency; the other cases must be scheduled according to the remaining life expectancy of the questioned element and the availability of the circuits.

7. Medium- and long-term investment programmes
Since in general conditions the components of the towers deteriorate in a characteristic sequence and in predictable times, it is possible to project the maintenance expenses and investments for corrosion in the medium and long term, which allows companies to project their cash flow over time.

As deterioration is a relatively slow process, scheduling expenses for maintenance can be done for up to thirty years without putting assets at risk of failure.

* Energy TransportAtion Management. ISA-Colombia. [email protected]


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