Overview of Power Transformer life cycle-Insurers/Auditors expectations after failure-Determination of Power Transformer maintenance frequency

Power transformers, especially GSUs, usually represent one of the most important and most costly single items in power plant. Furthermore, particularly for large transformers (GSUs/UATs) and/or critical small transformers (excitation transformers), their failure usually results in lengthy outages or downgrading of service. For these reasons, a high degree of care is required to properly install and maintain them.

Generally IEEE/IEC/ANSI/NETA MTS/CIGRE standards/guidelines are 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 normally set out clearly in the documentation supplied with the transformer.

This article will briefly describe answers to below question:

1. What are/could be the basis of insurer requirement for Electrical testing of Big Power transformers?

2.     Is there any requirement for periodic/routine Electrical testing of power transformer mentioned in transformer relevant standards (IEEE/IEC/ANSI/NETA MTS/CIGRE)?

3.     What is the brief overview of Transformer Operation and Maintenance Cycle, from the time of commissioning to end of life?

4.     What are/could be the post hazards/dangers associated with periodic electrical testing of healthy Generator Step up transformer fitted with Extra High Voltage Cable sealing end chambers (Oil filled)?

Maintenance, availability and reliability are closely related and the transformer OEM normally specifies a level of maintenance that will ensure an acceptable level of transformer reliability in the particular context.

1.     What are/could be the basis of insurer requirement for Electrical testing of Big Power transformers?

In case of failure of large transformers in a power plant, insurers normally ask about historical record of maintenance to do gap analysis by comparing with OEM recommendations along with onsite investigations and root cause analysis of failure as insures may have to pay huge cost of availability loss in addition to cost for repair/replacement of transformer.

Generally OEM recommended frequency for maintenance and electrical testing of transformers are very stringent and sometime not practical to apply due to mismatch between scheduled/planned outages and OEM recommended frequency. Insurers may not always aware of all (dis)advantages and risks of frequent electrical testing/maintenance. It is correct that some faults can be seen in an early stage by some tests but hazards associated with post maintenance activity cannot be compromised.

Insurers point of view may have influence of information by suppliers of services/measurement devices (for example Doble) who have financial benefits from this kind of service. For the OEM of the transformer, electrical testing recommendations may also be due to interest of their local business.

So it is recommended for power plant electrical department management team to initiate discussion with insures to agree on some practical scope and frequency of maintenance/testing of large transformers which can be agreed. The maintenance practices vary significantly between transformer users. The possible factors that may influence maintenance practices are:

- Transformer characteristics and specifications

- The quality of the components installed on the transformer

- The required duty of the transformer (load, OLTC operation)

- The transformer environment (temperature, humidity)

- Historical transformer failure rate and failure types

- The level of transformer redundancy and the consequences of unavailability

- The failure mode and its effects on power plant safety

- Company culture and focus based on maintenance

- The availability and costs of labour

- The degree of implementation of modern technologies

- The presence of a maintenance optimization program

- The availability of resources of repair/replacement.

- Ambient conditions of power plant site

- Hazards associated with post maintenance/testing activity.

2.      Is there any requirement for periodic/routine Electrical testing of power transformer mentioned in transformer relevant standards (IEEE/IEC/ANSI/NETA MTS/CIGRE)?

Normally, It is the OEM who recommends maintenance scope and frequency in transformer OEM Manual. But there are no IEC standards where these intervals / tests are noted.

Some electrical and maintenance scope has been recommended in below mentioned documents:

·      IEEE C57.93 - IEEE Guide for Installation of Liquid lmmersed Power Transformers

·      CIGRE 445 - Guide for Transformer Maintenance

·      For medium voltage equipment,  ANSI/NETA MTS can be used as reference document

3.     What is the brief overview of Transformer Operation and Maintenance Cycle, from the time of commissioning to end of life?

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 proper  diagnostic techniques 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.

Below Figure represents the Transformer Operation and Maintenance Cycle, from the time of commissioning to end of life as per CIGRE 445 - Guide for Transformer Maintenance

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 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. 

4.     What are/could be the pre and post dangers associated with periodic electrical testing of healthy Generator Step up transformer fitted with Extra High Voltage Cable sealing end chambers (Oil filled)?

Sometimes it is recommended not to adopt periodic regime of electrical testing for power transformers (especially SUTs/BATs fitted with oil filled EHV cable sealing end chambers) due to associated post activity dangers.

Its really a laborious job involving post activity dangers (listed below) and cost to prepare for electrical testing of SUTs/BATs with oil filled EHV cable sealing end chambers.

It will be good choice to use oil analysis as a primary monitoring and to go for electrical testing as an event based on basis of oil analysis results.

EHV links and low tension links of transformers has to be disconnected to perform electrical testing so the main risk is actually related with oil filled EHV cable sealing end chambers which is a complete sealed compartment with insulation material inside. It should be handled just like the active part of the transformer. It is always  recommend to keep it closed to reduce the risk of moisture / particles ingress as much as possible.Opening of these boxes  is advisable only when fault gasses are found or when you get an alarm such as a buchholz alarm. Some of risks associated with EHV cable sealing end parts are as follows:

·      Introduction of foreign objects (nuts / bolts)

·      Introduction of dust / sand, which can stick to the oil layer on bushing / link / cable termination. This can cause a flash over when taken into service.

·      Moisture ingress inside the cable box. Following objects can be hygroscopic:

o  Link between cable termination and oil-oil bushing (This is in most cases made from a copper conductor surrounded with paper insulation which is hygroscopic)

o  Cable termination (the insulator of the cable termination - OEM should be contacted to get proper recommendation of how this should be protected during maintenance / measurement campaign.

·      Improper oil filling:

o  Air bubbles trapped in the cable box

o  Improper oil filtration => did not circulate well => places with high amounts of particles

·      Bad connections:

o  Bad connection of the electrical contacts after assembly of the cable link => causes hot spots

·      Oil leakages when the compartment is not closed in a proper way or when the gasket is damaged.

·      Oil replacement => trending of fault gasses inside this compartment is not possible anymore.

e Check the recommendations of the transformer OEM (responsible for cable box / oil-oil bushing / link

e Check the recommendations of the cable termination OEM (responsible of the termination)

e Write the correct procedure to keep the risk as low as possible.

Disconnection of IPB on LV side

·      Be careful that the porcelain does not get damaged during disconnection of the links between LV bushing and IPB

·      Check that all bolts are torqued again with a torque key with the correct force.

·      All bolts should be marked in order to be sure the correct torque is applied

·      Air leaks when you have an IPB under overpressure.

Following electrical tests are less dangerous:

·      Transformer turn ratio: not dangerous due very low voltage

·      Tan delta of windings: not dangerous due limited voltage far below insulation level of the windings

·      Tan delta of the bushings:

o  Check that measuring tap of the bushing is closed in a proper way after the measurement. A bushing can explode when this tap is not grounded in a proper way.

o  It is not dangerous due to limited voltage of the measurement device.

o  Check max allowed voltage because the insulation level of the testplug is rather low (for example 1 kV but check it for your bushings).

·      Insulation resistance:

§ Windings: no risk due to limited test voltage

§ Core – Yoke:

·       Check insulation level of these connections (FAT report).

·      Check that core and yoke are proper grounded before the transformer is taken into service. Incorrect grounding can damage the insulation.

·      Winding resistance: test is not dangerous the only disadvantage is the magnetization of the core during the measurement which can cause higher inrush current during start up.