GENERAL PHILOSOPHY

1.0 - INTRODUCTION

1.1 In today's competitive market scenario power utilities are under tremendous pressure to cut down their maintenance costs as they form a significant portion of the operation costs. This has led the utilities to adopt condition-based maintenance of the equipments rather than usual preventive maintenance being carried out at a fixed interval of time. Maintenance intervals are normally fixed on the basis of type of equipment and sometimes on the equipment history. However, tests or measurements are also carried out to assess the condition of the equipment.

2.0 TYPES OF MAINTENANCE

Different types of maintenance being done on equipment are:

i) Breakdown maintenance
ii) Preventive maintenance
iii) Condition based monitoring
iv) Reliability centered maintenance

i) Breakdown Maintenance

As the name implies the maintenance is carried out when the equipment fails. This type of maintenance may be appropriate for low value items. However for costly substation equipments, it is not desirable to wait till the breakdown of the equipment, as this cost more to the utility as well as the availability and reliability of power gets affected. The revenue loss due to non-availability of the system shall be much more than the cost of the failed equipment. Therefore identifying the defect before failure, is more appropriate to plan repair / replacement.

ii) Preventive Maintenance

The preventive maintenance of equipment is being mostly adopted by almost all the utilities. In this type of maintenance, the equipments are inspected at a pre-determined period. The frequency determined based on the past experience and also guidance from the manufacturer of the equipment. This type of maintenance would require specific period of shut-down. Maintenance procedure, periodicity of maintenance and formats for maintaining records for various types of sub-station equipments have been discussed separately in detail in a separate section.

 iii) Condition Based Monitoring

This type of maintenance technique is adopted to assess the condition of the equipment. The condition of the equipment is assessed based on different condition monitoring tests. Some of the tests are done on on-line and some are done on off-line. However, this type of maintenance would need sophisticated testing equipments and skills for analyzing the test results.

iv) Reliability Centered Maintenance

This is the recent technique being adopted in maintenance philosophy. The basic objectives of reliability-centered maintenance are:

- Maintenance should keep the equipment at desired level of performance

- Optimizing / minimizing the maintenance / shutdown period so as to enhance the availability of the equipment.

- Deferring / avoiding the replacement of components and major/minor over-hauls till it is absolutely necessary.

Reliability centered maintenance policy is based on the life cycle cost concept and the decision for replacement of the equipment is taken based on techno-economic considerations. From the view point of RCM our objective should be to devise a system, which does not need periodic maintenance and at the same time predict in advance possible failures/problems of the equipment. To meet this aim we have to develop equipment which require either no or very little maintenance and on the other hand the concept of condition based maintenance should be implemented. Realization of this objective will result in enhancing availability, reliability and reduction in manpower for maintenance purposes.

3.0 FAILURES OF EQUIPMENTS

3.1 Failure of any equipment should taken up seriously. Detailed analysis of each failure should be done which will help in reduction/stopping of repeated failures of same nature. It is general experience that in spite of doing regular maintenance, failure of the equipment can't be totally eliminated. Number of EHV equipment failures have been reported practically by all the utilities and some of them have been quite serious resulting in consequential damage to the adjoining equipment. Circuit breakers operating on high pressure when they fail, they explode like a bomb resulting in scattering of insulator pieces to a larger distance and damage to the adjoining equipment. Similar situation have also been faced with the failure of surge arrestors and current transformers. Some of the typical failures of equipment and the remedial measures adopted have been discussed in a separate section.

4.0 NEED FOR CONDITION BASED ASSESSMENT OF EHV EQUIPEMNT

4.1 In the present competitive environment, all utilities are making efforts to reduce the O&M expenditure. This puts lot of pressure on the utilities to minimize the outage period due to failure of equipment. This necessitates adopting of condition based monitoring as the Need of the Hour. This has necessitated all the power utilities to introduce condition based monitoring for EHV class equipment so that actual condition of the equipment and its residual life could determined. Modern techniques are available for condition based monitoring and the concept of residual life assessment is picking up world wide.