The high development costs of power systems combined with customer demand for high quality make the maintenance management of the power transmission grid a key element in the continuity and reliability of the electrical supply. This is the result of both the need to minimize failure and the increased limitations imposed on grid operators by asset maintenance.

CHALLENGES FACING UTILITIES

In recent years, the development of high-voltage transmission systems in many countries has not been able to keep pace with the increasing demand for energy. Utilities must face the challenges of how to manage maintenance at reasonable cost, how to improve the quality of service to customers and how to reduce asset-scheduled unavailability. As transmission lines are subject to weather and environmental factors, failure is inevitable. The consequences of a failure should be fixed by adequately studying the grid configuration, protections, generation dispatch and so forth.

Success is not ensured solely by the application of an ordered management system, it must be complemented by the correct selection of maintenance strategies. This is achieved by studying assets in their operational context and failure mode to determine what should be done in regards to maintenance. TRANSBA S.A. (Buenos Aires, Argentina) implemented a maintenance-management strategy that has delivered improved results:

• The forced unavailability of outages has decreased from 3.72 to 1.98 failures every 100 km (5.90 to 3.14 failures every 100 miles) in transmission lines per year in the last eight years.

• The ratio of a lack of energy supplied versus energy supplied to customers has decreased from 0.00020 to 0.00008.

• The correct performance of 132-kV protection and breakers has increased from 88% to 96%.

• The disconnection of transformers has fallen from 1.04 to 0.44.

All these are important improvements in the quality of service TRANSBA renders to its customers, and they also provide the benefit of a reduction in the fines levied on the utility.

MAINTENANCE MANAGEMENT SYSTEM

To outline the principles of maintenance management, TRANSBA defined a management system structure and maintenance strategies to determine the operating structure by which it will undertake operation and maintenance tasks:

• Determination of clear and measurable goals
• Proactive planning of all critical activities
• Precise assignment of responsibilities
• Statistics to identify problems and allot resources
• Personnel training and teamwork
• Use of own resources and outsourcing
• Management audit
• Result controls.

TRANSBA has been consolidating its system of quality, the utility's main management tool, in accordance with the ISO's 9001:2000 quality management standards for all activities performed by the utility. The management system structure comprises:

• General maintenance procedures that indicate what is done in maintenance to avoid failures

• Programming and planning procedures that indicate when maintenance should be done

• A work-order system that registers or serves as objective evidence that what had to be done was actually thoroughly done, so that the utility can track maintenance completely

• Maintenance instructions that regulate work methods and indicate how maintenance should be done.

MAINTENANCE STRATEGIES

Maintenance requires skilled labor, spare parts and tools, as well as a plan to carry out work and software to administer management. Above all, there should be an adequate selection of maintenance strategies, that is to say, what should be done in maintenance. At TRANSBA, the general procedures define strategies and allot responsibilities, indicating what maintenance is done and who is responsible for it.

In each activity, the maintenance strategies adopted by the utility are defined. They consist of different kinds of interventions according to the operational contexts, periods or conditions for the intervention, types of materials used and responsibilities. At present, the utility is incorporating Reliability Centered Maintenance II (RCM2) techniques for the revision of those strategies.

There are also general procedures of planning, maintenance, programming, work orders, statistics control, instrumental control and training. For example, in the case of overhead transmission lines, the following are defined: type and number of lines, control and follow-up of automatic recloser failures, measurements to be made, type of insulation used and lines that must be patrolled by teams with thermography equipment.

Critical and regular behavioral limits are established as well as what should be done in each case. The other maintenance activities are handled similarly. From these general procedures, the set of instructions for each maintenance task is derived, for example, climbing poles, insulator replacement, live work, maintenance of accessories for transformers, physical-chemical tests and breaker maintenance.

Strategies imply corrective, preventive, predictive or detectable maintenance, as required. Most assets require one or more of these maintenance modes: in lines, it is basically corrective and preventive; in power transformers, it is preventive and predictive; in measurement transformers and breakers, it is predictive; and in protection, it is mainly detectable.

STATISTICS CONTROL

The volume of equipment involved in the reliability of a transmission system is very large, and failures that are likely to occur may be complex. In a transmission system, like in many others, zero failure has infinite cost. That is the reason why it is not possible to eliminate failure, but we have to keep it under control and try to eliminate failure that has serious consequences (such as stated in RCM2).

We must take into consideration that, because of the characteristics of our activity, it is not always possible to eliminate the consequences of failure. According to our regulation, investment in redundancy (lines and transformers) is dependent on the customers. However, it is possible to reduce or eliminate the consequences of failure that are the company's responsibility (such as protection asset, selectivity, insulation, maintenance of operation asset, transformers and auxiliary services).

Planning must be proactive and considered with statistics control, a tool that facilitates the measurement of maintenance effectiveness and asset reliability. In this way, the utility can detect problems to make a better distribution of resources. A good selection of management ratios is essential to receive feedback about the improvement process and to assess investment. Everything that is controlled and has an impact on the quality of the service and costs must be measured.

For the 132-kV level, and in accordance with the design and operational context, when considering transmission lines, it is acceptable to have between 6 and 15 failures with automatic reclosers every 100 km (9.5 to 23.8 failures per 100 miles) per year. The number of probable failures depends on the insulation and isoceraunic levels, and equally on environmental conditions such as birds, contamination and vandalism that are not easily controlled.

Thus, in a system like TRANSBA's, which has 6000 km (37,300 miles) of transmission lines, between 350 and 900 failures with automatic reclosers are expected within a year. At present, TRANSBA's average number of failures is 470. Each of these failures means the use of a chain of protection, auxiliary and operation assets. Something similar happens with the almost 500 medium-voltage connections (exposed to a much larger average number of failures) and the 146 power transformers (whose reliability depends on the maintenance of transformer accessories and the effectiveness of the protection and medium-voltage feeder breakers).

Each incident provides useful information about the asset condition. By choosing ratios that detect problems inherent to adequate maintenance, it is possible to exert maintenance control accurately using statistics as feedback in the improvement process.

TRANSBA's most relevant technical ratios of maintenance management uses are:

• Failure every 100 km in transmission lines
• Forced unavailability of outages every 100 km of lines
• Protection and breaker effectiveness
• Lack of energy supply to customers because of transmission failures
• The disconnection of transformers.

Each of these ratios is grouped according to causes, with trend and bar charts that provide information and management feedback for improvement. Additionally, standards of maintenance costs per operative unit and type of installation and fines are kept. This is the case for the standard on forced unavailability of outages on transmission lines (Fig. 1), which has a direct relation to fines and the quality of service that customers receive.

According to Fig. 1, an annual remedial activity is started to analyze each particular transmission line, the causes that led to disconnections and the steps to be taken to reduce or eliminate them. Protection and breaker failure are subdivided, with a set of different steps to be followed in each case. Figure 2 shows the forced unavailability of outages that are attributable to protection.

There are also annual plans of protection and breakers where all the data detected through the digital and analog analysis of protection relays and RTUs' records are registered. This is always done after each failure in the network to check the correct functioning of all trigger chain, opening and closing times of the breakers, time of reclose, delivery and reception of tele-protections, trigger block, backup startup and failure distance measurement.

When there is an abnormality in one of the records, a maintenance plan is incorporated and, in this way, loss of information is avoided while traceability of the asset behavior is retained. The follow-up is done so as to check whether the detected problem is repaired; a record of the defects is kept. This is very important when maintenance strategies through RCM techniques or investment assessment have to be analyzed

The previous analysis is also carried out with the rest of the indicators, such as the transformer disconnection ratio, which is directly related to the quality of service that the customer receives due to the fact that, in most cases, the disconnection leads to power being cut. The follow-up to the reasons that cause these disconnections also provides valuable information that determines steps to be taken, areas in which to train and where investments need to be made (Fig. 3).

LINE FAILURE

Unlike the previous examples, the line failure standard records the line performance, but it does not provide conclusive evidence on maintenance. In fact, a high-failure standard may mean that there is a problem in insulation or the maintenance of a line, but generally this is not so because the line may be affected by environmental factors.

Therefore, the line failure standard must be analyzed together with the forced unavailability of outages. The cause of each particular failure must be studied so as to determine a course of action and whether corrective actions are necessary. In TRANSBA, the quality of service customers receive has reached very good levels, while the line failure standard has not decreased. This is related to weather or environmental factors, which are independent of maintenance, and it reinforces the idea that the goal should not be to eliminate failures but to have a system that responds adequately to them, successful automatic reclosers, and selective clear fault by analyzing system configurations, operating units and protection (protection effectiveness standard and breakers). Also, an adequate analysis of the causes of line failure permits, in some cases, the implementation of line repairs that modify some of those causes (for example, anti-perching devices).

In all the cases, skilled working teams perform corrective or preventive actions annually at TRANSBA.

THE BOTTOM LINE

At TRANSBA, the implementation of management strategies, together with an ambitious investment plan, have resulted in the positive evolution of all the utility's reliability standards and quality of service. Figure 4 shows the evolution of the most important indicators over the last six years.

Apart from TRANSBA's viewpoint and result measurement, the utility annually invites customers to provide their opinions of the quality of service TRANSBA offers. The results of the 2006 survey revealed that 99% of the customers surveyed described the utility's management as very good or good (Fig. 5).

Finally, a further important aspect of TRANSBA's maintenance management strategy is the management system internal audit, which is done from the quality management, an area that is independent of direct operative responsibility (Fig. 6). The fulfillment of the established requisites (internal-external) and management system effectiveness are periodically audited, following a fixed schedule, in each of the electric utility sectors to verify the fulfillment of the established policies (corporate and particular of each area), the procedures and the corresponding planning to achieve the objectives.

Maintenance management assures the validity of procedures and instructions involved in this activity through a checklist that comprises all the requisites that should be undertaken. This auditing should not be considered bureaucratic as it helps keep the system active, detect failure and improve management. When the procedures seem to be an obstacle to the correct management, they should be updated.

The internal auditor is seen as an ally to the operative sectors, which are in search of permanent management improvement and effectiveness. The implementation of an audited and certified management system like ISO 9001:2000 has allowed TRANSBA to reach standards of quality of service that are comparable to similar ones in developed countries. These standards have been complemented with clearly defined management strategies, maintenance strategies and investment plans that have produced cost reductions and improvement in quality of service; an organization of human resources that facilitates the assignment of responsibilities and decision making; and statistics that show maintenance effectiveness in giving feedback to management, thus allowing them to make informed decisions.

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Roberto José Ferrelli is a senior engineer in maintenance management and has been the maintenance manager of TRANSBA S.A. since 1994. He has more than 20 years experience in high-voltage transmission maintenance and was part of one of the specialist teams responsible for ISO 9001 and RCM implementation in TRANSBA S.A. Ferrelli earned his degree in electrical power system engineering from BahÍa Blanca University in Argentina. He has published several papers for CIER and CIGRÉ on the maintenance management of transmission systems. roberto.ferrelli@transx.com.ar

TRANSBA S.A.

TRANSBA S.A., a subsidiary of TRANSENER S.A., is the largest distribution transmission utility in Argentina with assets that include:

• 83 grid substations
• 108 transmission lines operating at 220 kV, 132 kV and 66 kV, some 6000 km (3730 miles) in length
• 146 transformers with a transforming capacity of 4700 MVA
• 500 high-voltage circuit-breakers, approximately 1000-MV circuit breakers and more than 3000 switches
• 100 dc and ac auxiliary service assets, more than 2000 protection relays, and approximately 3000 measurement transformers, SCADA system and communications.