Maintenance of Substation

General Philosophy

In today’s competitive market scenarios, 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 equipment rather than the usual preventive maintenance being carried out at a fixed interval of time. Maintenance intervals are normally fixed based on the 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 Major Substation Types of equipment And Remedial Measures Failures

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

Maintenance Techniques For Switchyard types of equipment (CBs, Instrument Transformers, Surge Arrestors, And ISOLATORS)

This section briefly covers the maintenance practices/techniques being adopted by the power utilities for switchgear pieces of equipment installed in EHV/HV stations. However, competitive market utilities are under tremendous pressure to bring down maintenance costs in the present scenario. This has led to the adoption of condition-based monitoring rather than conventional scheduled maintenance. The frequency of the condition-based maintenance is fixed based on the type of equipment and its previous history. Tests or measurements are carried out to determine the condition of the equipment. It is the general experience of practically all the utilities that most of the equipment doesn’t show any abnormality during scheduled maintenance and there is no guarantee that the equipment shall not fail before the next scheduled maintenance. Thus, the scheduled maintenance efforts are in vain as well as costly because 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 deterioration of the components 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 are 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 the power current, changes in cable insulation became essential. While carrying power currents, cables became hot and heat sensitive. Gutta-percha was, therefore, found unsuitable for insulated power conductors. To suit the new requirements, a superior insulation system using a fibrous material was dried, impregnated under vacuum, and enclosed in a seamless lead sheath. They were the forerunners of paper-insulated lead sheathed cables that came into existence around 1890 and have been used since then.

Safety Electrical Clearances

Most of the equipment in a substation is provided with sufficient insulation from breaking down, yet others like bus bars, terminations of high voltage equipment, the equipment connected to take-off 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 that 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 guidelines from time to time to maintain certain minimum safety clearances depending upon the voltage class from bare conductors and parts of various types of equipment exposed to the atmosphere. The provisions are mandatory and all the concerned like utilities and consumers have to maintain these minimum safety clearances. They are necessary for the safety of the operating personnel and the safety of the equipment.

Earthing for EHV Sub-stations

One of the important aspects of the operation of 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 widespread in the supply network right from the generation to the apparatus on the consumer premises. The requirements of earthing vary at different points based on the fault level, soil resistivity, and the safety considerations

Instantaneously One of the important aspects of 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 widespread in the supply network right from the generation to the apparatus on the consumer premises. The requirements of earthing vary at different points based on fault level, soil resistivity, and safety considerations.

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

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Number of Pages: 129

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