1.1 Guide for Transformer Maintenance
Maintenance, availability and reliability are closely related and the transformer user has to specify a level of maintenance that will ensure an acceptable level of transformer reliability in the particular context. This guide has been prepared to help transformer users to define and apply best practice for transformer maintenance. Although the document was prepared with specific consideration of liquid immersed transformers rated 69 kV and above and larger than 25 MVA, its principles may be applied to a wider range of transformers. Subjects including best practice, checking and testing in order to evaluate transformer condition, time intervals for the various actions, advanced maintenance activities, and human and material aspects of transformer maintenance are covered.
This guide is not intended in any way to set the level of maintenance that a user must perform on a transformer in order to
maintain a manufacturer’s warranty. The required maintenance should be set out clearly in the documentation supplied with
the transformer.
1.2 Transformer Operation and Maintenance Cycle
A transformer is usually a robust apparatus with very good reliability requiring relatively low maintenance. During the life
of a transformer, the user has to establish a maintenance strategy that will ensure the appropriate level of reliability and an
optimized operational life.
The operational life of a transformer begins with commissioning after it is installed in a substation. Once in operation, a
maintenance strategy starts to be applied. An optimized maintenance strategy will provide the required availability and
reliability of the transformer over its lifetime at minimum cost. It is the goal of good maintenance to detect any
abnormalities before they cause unnecessary damage.
Once an abnormality is detected, then some or all of the available diagnostic techniques set out in this brochure can be
applied to evaluate the severity of the problem, localize it, and determine if the transformer can return to service with or without a restriction on operation. If necessary the appropriate corrective action can then be performed, or depending on the transformer condition, it may be appropriate to invoke a more intensive intervention on the transformer. Ultimately, it may be decided that it is time to refurbish or repair the transformer or even to replace it, depending on the results of an evaluation that will include consideration of the safety (both to utility staff and the general public), the potential environmental consequences and the system reliability aspects of continued operation.
Figure 1 represents the Transformer Operation and Maintenance Cycle, from the time of commissioning to end of life.
The different terms used in the Transformer Operation and Maintenance Cycle are described below.
Commissioning
When a new or repaired transformer is put into service, baseline measurements and tests are made so that the results are available for use as a reference if a problem is suspected in the future. The proper operation of the transformer and all its components are verified.
Transformer Operation
The transformer is connected to the electrical system and a fixed or variable load is applied. The transformer is exposed to the various system and service stresses such as ambient temperature variations, load variations, frequency and voltage deviations, lightning impulses, switching over-voltages, short-circuit.
Time Based Maintenance (TBM)
This maintenance is carried out at predetermined intervals to reduce the likelihood of an item of equipment failing in service. It includes maintenance actions to improve the condition (oil change, lubrication, preventive replacement of parts). The term "systematic preventive maintenance" is also used. A TBM action is given a fixed time interval and the action is carried out irrespective of condition, i.e. the planner defines what should be done and how often it is done. This method can
offer a high degree of risk coverage if the original equipment manufacturer’s (OEM) recommendations for maintenance
(which are traditionally based on regular intervals) are followed. TBM is often considered as the easiest but not the most
cost effective way of maintaining assets. It has the significant advantage of being easily planned and this is particularly important for maintenance that requires an outage.
Time Based Condition Monitoring (TBCM)
These are actions to evaluate the condition of the equipment (for example visual checks, measurement and tests) carried out
at regular and pre-planned intervals. These are most often carried out in conjunction with maintenance particularly for tasks that require an outage. The results of TBCM are often used to decide on the extent of maintenance required at the time or in the future. But the information gained is limited to ‘snap-shots’ at a particular time.
Condition Based Maintenance (CBM)
This maintenance is carried out depending on equipment condition to reduce the likelihood of an item of equipment failing in service. The term "conditional preventive maintenance" is also used. CBM is based on assessing the actual physical condition of the asset and takes into account its usage, occurrence of events, possible wear of moving or current switching parts, and the performance of similar equipment. In order to use this maintenance philosophy it is necessary to assess the asset condition by methods such as TBCM, OLCM and continuous on-line monitoring. CBM applies in cases where
technical condition can be measured and assessed against criteria for invoking action. Incorporating CBM in a maintenance
strategy seeks to reduce costs by performing maintenance only when a change in equipment condition warrants taking action. CBM however requires a more complicated planning process. CBM is often used within a time-based outage plan to defer maintenance to the next available outage.
On-line Condition Monitoring (OLCM)
This is a technique, method or measurement that is, or can be, performed or made with the transformer in operation that provides information about the condition of the transformer. This might include oil sampling for dissolved gas analysis using a laboratory, performing infra-red thermal scanning, or making simple observations such as oil levels in condenser bushings and conservators.
Continuous On-line Monitoring
This is a refinement of the OLCM technique, where a measurement or measurements are continuously tracked or supervised, normally by means of an Intelligent Electronic Device (IED). This device will immediately communicate, either by means of an alarm or message, any significant deterioration in condition to alert staff to take appropriate action. To be effective, the Continuous On-Line Monitor should announce the change of transformer condition in advance of failure.
Continuous On-line Monitoring can form the basis for Condition Based Maintenance and can effectively reduce the risk of
unexpected catastrophic failure.
Maintenance Strategy
The Maintenance Strategy is the combination of the different maintenance philosophies used to achieve the required system reliability. The strategy may include different maintenance philosophies for different components of the transformer. For example, tap changers and bushings. TBM is usually considered to be an elementary strategy, whereas CBM is usually
more cost effective than TBM. A combination of TBM, TBCM, CBM and OLCM is often used to maintain large complex assets such as power transformers. Time based inspections or checks are used for overall condition assessment (for example oil leaks) or the usual diagnostic measurements (for example DGA), while CBM methods are used for wearing parts (for example OLTC diverter contacts). Results of CBM provide knowledge of the average or actual asset condition and this may be used to influence future TBM intervals.
Reliability Centred Maintenance (RCM)
Reliability Centred Maintenance is an optimised strategy that takes into account not only the operation time and/or the technical condition of an asset, but also its position in the network, its operational importance, any potential safety or environmental risk arising from its failure and any likely consequence of its potential outage. In order to apply this maintenance strategy, each transformer has its safety, environmental and operational criticality factors assessed and combined and the asset can then be assigned a value (criticality index) indicating the required reliability. This index is used to influence the future maintenance tasks, their intervals (which may also be condition based) and their priority ranking within a limited resource environment. This leads to assets in risky or important positions being maintained in a different (more intensive) manner to assets in a position where reliability can be allowed to be lower. In practice, the criticality index
is usually combined with a health index to prioritise maintenance activity. RCM may be applied to components either together or in isolation.
Condition Assessment
This is the process by which the condition of a transformer is assessed taking into account all the aspects that could affect future performance. The inputs to this process will be the test and measurement results, observations, operating history,knowledge of the failure mechanisms and processes, previous experience with similar or comparable equipment and any other relevant knowledge and information. The normal output can range from a simple normal or abnormal assessment to a sophisticated ‘asset health index’ which is a ranking or scoring system on a single or multiple scale to allow decisions on
future maintenance or replacement prioritized over a fleet of units.
Interpretation – Special Tests and/or Intensive Monitoring
When a transformer problem is suspected or indicated (for example by routine condition monitoring), all the available information is collected and then evaluated to decide the correct course of action. To facilitate this, a wide range of special off-line diagnostic tests are available and may be used to evaluate the conditions of different parts of the transformer (for example the core, windings, bushings, OLTC and accessories). In some cases, the application of intensive monitoring, for
example continuous on-line monitoring, may be required in order to gather additional data or to operate the transformer safely. The purpose of these tests is to evaluate whether the transformer could be put back into service with or without corrective actions. Continuous on-line monitoring is often useful to gain a deeper understanding of the fault condition and
its dependencies on operating conditions such as load, tap-position and temperature.
Corrective Maintenance - Minor Work
Corrective Maintenance is an operation carried out to restore any part of the transformer which has failed or degraded to the
point where it needs corrective action to avoid loss of performance or a major failure. The need for Corrective Maintenance follows the identification of an abnormal condition and excludes routine maintenance (TBM, CBM). Examples might include oil processing, cooling fan replacement, leak repairs.
Major Work
Major Work ranges from replacement or refurbishment of major components such as bushings, tap changers or the complete
cooling system to the return of the transformer to works for replacement of the windings. Any work that involves the removal of oil from the transformer may be considered to be major.
Technical and Economic Evaluation
The cycle of transformer operation and routine and corrective maintenance is not perpetual. When a transformer suffers severe damage or when the transformer reliability is no longer satisfactory, a technical and economic evaluation has to be made to decide the best option between scrap and replace, repair or refurbish and if the work is to be done on-site or in a workshop. When evaluating the best option considerations such as outage time, spare availability, outage cost, transport
and general equipment condition will be taken into account.
End of Life
The service life of a transformer should end when its condition is such that it cannot be kept in service, nor be put back into
service, primarily because a technical and economic evaluation determines that its return to a serviceable condition is not
economical.
Download Timer
