TRANSFORMER FAILURE MODES AND PLANNED REPLACEMENT

John Lapworth and Tony McGrail

The National Grid Company plc

Introduction

Transformers are usually very reliable and durable items of electrical equipment, so much so that their performance is often taken for granted, especially in a power station environment where there are often many more immediate claims on the attention of maintenance engineers. Unfortunately, when a fault occurs in a transformer, it can develop catastrophically and failures are usually very expensive if not uneconomic to repair, often resulting in the loss of what is the most expensive plant item in a substation. Costs of resulting loss of generation or transmission restraints until a replacement can be effected can also be severe.

In recent years there has been considerable interest in the subject of life management of transformers, prompted no doubt by factors such as: 0 equipment getting older and approaching expected design lives

fewer experienced people being available pressure to economise by reducing maintenance organisational changes focussing attention on asset lives

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and a CIGRE Working Group has been set up on the subject.

Failure Modes

Transformers can fail in a variety of ways and for a variety of reasons. Important factors are: I. design weaknesses 11. abnormal system conditions 111. aged condition / service loading IV. pre-existing faults V. timescales for fault development

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A useful model of the failure process is shown in Figure 1. When new a transformer has sufficient dielectric and mechanical strength, with some ‘spare margin’, to withstand expected maximum operational stresses, but ageing processes will degrade this margin until this is no longer the case and the transformer is prone to a failure. Service loading has an influence on failure processes through the thermal degradation of the main winding paper insulation, which results in loss of mechanical strength and insulation shrinkage, and through the ageing of the insulating oil. Some designs will be affected more by loading than others, In addition to ‘normal’ ageing, a transformer may develop a ‘fault’ which results in faster than normal ageing, with consequently a higher probability of failure. System events often play a very important role in failures, either by initiating a fault, or providing the ‘trigger’ for the final failure, and are also responsible for introducing a considerable degree of unpredictability into failure processes.

A typical illustration of the involvement of the above factors is provided by the failure of a 400/132 kV 240 MVA auto-transformer a couple of years ago. The transformer was tripped out of service by protection a few minutes after the overhead line the transformer was feeding was struck by lightning just outside the substation. Diagnostic tests showed that the LV winding of one phase had collapsed as a result of the electromagnetic forces arising during the ensuing phase to earth fault, and this was confirmed by a subsequent internal inspection. The transformer was suspected to have suffered damage from a previous LV fault on the same phase. The design in question was thought to be susceptible to short-circuit failures because of a previous failure of a similar transformer. Lastly, the fact that the transformer had been fairly heavily loaded during its life may have had some impact on the probability of failure because winding shrinkage with age may have affected the winding clamping.

Replacement Strategies

One low risk but high cost replacement strategy would be to ‘replace on age’, the age being chosen to ensure that no reduction in reliability occurred.

At the other extreme, a low capital cost strategy would be to leave all units in service until failure. A ‘replace on failure’ strategy has the apparent attraction of maximising the utilisation of capital assets. However, it may not be compatible with the reliability requirements for a system, and there may be economic penalties for forced outages required until the asset can be replaced, particularly in a competitive generation and supply industry with many commercial constraints. There are also safety and planning implications of living with the increasing probability of random failures to be taken into account. Much work is underway at present to develop monitoring systems to give the earliest possible warning of imminent failure. While these may prevent some catastrophic failures that develop relatively slowly, it is not clear how they can predict the random system events that trigger many failures, and they are unlikely to give sufficient warning to assist in replacement planning. A further factor mitigating against a general replace on failure strategy is the possibility of linked failures and consequential loss of supply. This would occur if the transformers remaining in service after the first failure could no longer withstand the increased post fault loading, or if none of the transformers at a particular site was able to withstand a particular fault condition, e.g. a lightning impulse or short circuit, affecting them all.

A third strategy, the one that perhaps represents the ideal fi-om a life management point of view, is to ‘replace on condition’, when the transformer is such that it no longer meets system reliability requirements. In addition to being able to manage system reliability, compensations for replacing transformers before eventual failure with such a strategy come from the ability to plan the replacement programme, to smooth any ‘asset wall’ and take advantage of lower prices for planned purchases.

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Planned Replacement

In order to implement a ‘replace on condition’ policy, the key parameters to establish are the age at whxh reliability begins to be reduced, together with an estimate of mean life, to produce the probability of failure distribution functions shown in Figure 2. In parallel with this it is necessary to develop models to handle the statistics of failures so that strategic replacement can be tailored to achieve the required system reliability. Within NGC a Monte Carlo simulation is used, allowing the required asset replacement profile to be determined, which obviously must be compatible with capital expenditure plans.

Clearly, such a methodology is only as reliable as the assignment by plant specialist engineers of the basic input data - onset of unreliability and mean life. For this the engineer relies upon I. operational experience, 11. design knowledge, 111. IV. condition assessment tests, V. VI. engineering judgement.

basic research into ageing processes,

evidence from failures and strip downs, and

Inspection of failed and redundant transformers during the scrapping process is an extremely valuable source of information on design, actual condition and likely failure modes, particularly for older units where documentation and personal experience may have been lost. Basic research into ageing processes in insulation systems provides information on the effects of agents of deterioration such as temperature, acidity and moisture. Lastly, condition assessment tests on representative and critical units &om the transformer population provide a means of obtaining information on equipment in service, to enable the population to be ranked for replacement.

Condition assessment

The condition assessment strategy employed by NGC applies tests in two distinct ways in an effort to acquire information in the most cost effective way. Routine tests are canied out on all units on a periodic basis for screening to detect incipient failure and indicate general condition. Special tests are applied only as required for diagnosis and detailed assessment in response to one of a set of triggering circumstances including the following:

To investigate a poor routine test result Following a protection operation indicating an internal fault Following a system event that might have caused damage As part of a commercial asset review To decide whether a redundant asset is worth retaining Before and after moving Before uprating Before and after oil processing to determine the effectiveness of the treatment

& 6 6 Condition assessment tests are the subject of continuous research and development, both in the techniques employed and the interpretation of results. A summary of the currently applied techniques, their application and strengths is given in Table 1.

Before scrapping (to correlate test results with the observed condition ) When new (to provide reference results) To establish the results expected from a ‘normal’ unit

The two key dimensions of a condition assessment are the detection of any latent faults likely to affect reliability and an assessment of aged state. In order to develop a consistent and quantitative approach to the evaluation of test results, ideally a scoring scheme is required to generate an index that provides a overall expression of asset health which can then be used in any asset prioritisation process.

Conclusions

Planned replacement is the ideal from the point of view of maintaining system reliability in the face of an ageing population of transformers. To implement such a policy, information on failure modes and asset health is required, which necessitates a careful programme of inspections of redundant units, basic research and condition assessments.

Acknowledgement

The work was carried out by National Grid Engineering & Technology and the paper is published with the permission of The National Grid Company plc.

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TRANSFORMER CONDITION ASSESSMENT TESTS

Winding resistance

rest I Faults detected

Winding joint and tap-changer selector contact problems

Location of fault

None (integrates over time) None (integrates over time) None or partial

Good if discharge not in winding Good

Good ( can identify phase and winding with fault ) Partial ( depends on how windings can be separated for test ) None ( overall measurement ), although may indicate presence of ‘wetspots ’ . Partial ( can indicate phase of fault )

Partial ( can indicate phase and winding of fault > Partial

Cost & Convenience

Cheap & easy On-line Cheap & easy On-line Cheap & easy on-line Moderate cost on-line Moderate cost on-line Expensive Outage & disconnection required Expensive outage & disconnection required Expensive - outage & limited disconnection required

Expensive - outage & disconnection required Expensive - outage & disconnection required Expensive - outage & disconnection required

Use

Routine

Routine

Routine

Investigation

Routine? Investigation Investigation

Investigation

Investigation Routine at major maintenance?

Investigation

Investigation

Investigation

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0 1998 The Institution of Electrical Engineers. Printed and published by the IEE, Savoy Place, London WC2R OBL, UK.

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