Transformer Reliability
The last ten years have seen growing interest in the evaluation and monitoring of
power transformers - mainly because a vast number of transformers worldwide are fast approaching the end of their effective operating life. The reliability of the existing units within global distribution networks must thus be assessed to understand if the transformers are fit for use, or need to be replaced or retrofitted.Within power systems, transformers form a substantial part of the asset costs; financial pressures today persuade most utilities to use existing units for as long as possible, as long as overall system reliability is not at risk. Thus, transformer asset management looks set to play a major part in future planning.
Factors Influencing Power Transformer Reliability
This article focuses on the factors influencing power transformer reliability, which are:
1.Operating Environment
The operating environment is the set of conditions which the transformer will be exposed to during its service life. This includes weather conditions, location, electrical loading, operating methods, system parameters and application. Each element that influences the life of the transformer must be recognized and documented.
National standards like ANSI, NEMA and others address many of these elements and provide recommendations or standard values wherever applicable; which may be of benefit to the person or organization responsible for assembling the data.
2.Transformer Specification
The specification of a transformer should describe the function it is to perform and the constraints which must be met. It should describe each of the identified operating conditions which affect the transformer.
3.Transformer Design
Transformer design ensures the operating conditions of the transformer as mentioned in the specifications are converted to concrete design parameters and into usable information by the manufacturing organization.
The design engineer must get all the needed information within the specifications, and any clarification must be obtained from the user or use parameters detailed in the national industry standards. While manufactures continue to advance the state of the art to meet changing operating environments, it is still fundamental to the designer that each known environmental condition be considered and addressed in the design. Any omission could result in decreased service life and premature failure.
4.Manufacturing Process
Here the physical product is created from the paper design. The reliability of a transformer when it completes the manufacturing cycle will differ from the reliability as demonstrated in the design, due to the variations in the manufacturing process of materials, processes, handling, machines and personnel.
Reliability will also differ between similar products. Quality techniques are aimed at manufacturing the product as close as possible to the designed product. To verify the quality and reliability of the manufactured transformer, two types of tests are performed:
Compliance Test: The compliance test ensures that the transformer meets the specified design parameters
5.Installation and Shipment
The time from when the transformer leaves the factory until it is energized by the user is very crucial. Any mishandling during shipment and installation can lead to failure of the product at an early stage.
The transformer is typically shipped either by rail or road; and these conditions can sometimes deteriorate. As a reactive measure, the ride history of the transformer is monitored and recorded. An impact recorder can provide evidence of rough handling so as to investigate if damage has been done to the product. An impact recorder is a recording accelerometer which will track either two or three axis movement. The primary channel records impacts in the horizontal movement, the second records the vertical movement and the third monitors the transverse or side to side movement. If rough handling is suspected, immediate documentation should take place for the benefit of the user and the manufacturer.
Once the transformer is accepted, the unit should be located at its permanent location for installation. The steps involved will be clearly written down by each manufacturer in the form of an instruction book. All items which will come in contact with the oil should be thoroughly cleaned. And people entering the transformer should not carry any loose items. Insulation resistance tests, insulation power factor tests, winding ratio and winding resistance tests are performed. These are the means to ascertain whether changes have occurred during transit.
6.Transformer Maintenance
Transformer maintenance can be separated into two categories:
Repair Maintenance: A reactive mode to repair an in-service failure or malfunction. Its function is to restore the product to its usable condition as fast as possible.
Scheduled or Preventive Maintenance: The purpose of this is to provide lubrication or adjustment to components and also to monitor the state of the transformer and its ability to continue to perform its function. The function of maintenance is to prolong life and thus the reliability of the transformer.
The need to augment reliability of power systems will only increase. Preventive diagnosis and maintenance of transformers is thus a crucial activity. Both manufacturers and users must jointly accept the responsibility of ensuring that transformers are properly specified, designed manufactured, installed and maintained.
As today's markets insist on higher quality energy supplies, utilities face the challenge of uninterrupted delivery. This places the burden on the key components of power distribution networks - transformers. Dedicated solutions are thus necessary to help utilities and power producers to manage equipment more efficiently.
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