GENERAL PHILOSOPHY

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.

Different types of maintenance being done on equipment are:

  • Breakdown maintenance
  • Preventive maintenance
  • Condition based monitoring
  • Reliability centered maintenance


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CAUSES OF FAILURES OF MAJOR SUB-STATION EQUIPMENTS AND REMEDIAL MEASURES

This section briefly describes about the probable causes of failure of major substation equipment viz. Power Transformers, Reactors, Circuit Breaker, Instrument Transformers, Surge Arrestors etc. and remedial measures taken to prevent such failures.

MAINTENANCE TECHNIQUES FOR SWITCHYARD EQUIPMENTS (CB’s, INSTRUMENT TRANSFORMERS, SURGE ARRESTORS AND ISOLATORS)

This section briefly covers the maintenance practices / techniques being adopted by the power utilities for switch gear equipments installed in EHV/HV stations. However, in the present scenario competitive market utilities are under tremendous pressure to bring down the maintenance costs. This has led to the adoption of condition based monitoring rather than conventional based scheduled maintenance. The frequency of the condition-based maintenance is fixed based on the type of equipment and its previous history. Test or measurements are carried to determine the condition of the equipment. It is general experience of practically all the utilities that most of the equipment don’t show any abnormality during scheduled maintenance and there is no guarantee that the equipment shall not fail before the next schedule maintenance. Thus, the scheduled maintenance efforts are in vain as well as costly in view of shut-down time and manpower costs. Hence modern condition based techniques are being adopted which are designed to continuously monitor the function parameters and also assess the components deterioration before a mal-operation occurs.

Maintenance of Transformers and Reactors

The techniques / procedures being adopted for both preventive maintenance and condition based monitoring in respect of EHV transformers and reactors is briefly discussed in this section.

UNDERGROUND CABLES

Over a hundred years ago, Gautapercha was used as an insulating material for cables, which worked satisfactorily for telegraphic lines. However with the development of power current, changes in cable insulation became essential. While carrying power currents, cables became hot and heat sensitive. Gauttapercha was, therefore, found unsuitable for insulated power conductor. To suit the new requirements, a superior insulation system using a fibrous material, which was dried, impregnated under vacuum and enclosed in a seamless lead sheath. They were the fore runners of paper insulated lead sheathed cables which came to existence by around 1890, and has been in use since then.

SAFETY ELECTRICAL CLEARANCES

Most of the equipment in a sub station is provided with sufficient insulation from breaking down, yet others like bus bars, terminations of high voltage equipment, the equipment connected to take-of structures etc; are bare unlike in gas insulated substations. There is a need for assurance that the breakdown or flashover will not occur to the operating personnel and some safe distance is to be maintained. As per the studies and experience gained over a period various bodies like CEA in India and elsewhere are issuing guide lines from time to time to maintain certain minimum safety clearances depending upon the voltage class from bare conductors and parts of various equipments exposed to atmosphere. The provisions are mandatory and all the concerned like utilities and consumers have to maintain these minimum safety clearances. They are necessary from the safety of the operating personnel and safety of the equipment.

Earthing for EHV Sub-stations

One of the important aspects in the operation of the protective equipment is proper earthing. By earthing, it means making a connection to the general mass of the earth. Earthing also increases the reliability of the supply service as it helps to provide stability of voltage conditions, prevent excessive voltage peaks during disturbances and also as a means of providing a measure of protection against lightning. Its use is wide spread in the supply network right from the generation to the apparatus on the consumer premises. The requirements of earthing varies at different points based on the fault level, soil resistivity and the safety considerations

Instantaneously One of the important aspects in the operation of the protective equipment is proper earthing. By earthing, it means making a connection to the general mass of the earth. Earthing also increases the reliability of the supply service as it helps to provide stability of voltage conditions, prevent excessive voltage peaks during disturbances and also as a means of providing a measure of protection against lightning. Its use is wide spread in the supply network right from the generation to the apparatus on the consumer premises. The requirements of earthing varies at different points based on the fault level, soil resistivity and the safety considerations

Instantaneously overcurrent relay operates when current exceeds its Pick-up value. The operation of this relay is based on current magnitude and it is without any time delay.


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19 Replies to “Maintenance of Substation”

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