ASSET OPTIMISATION STRATEGIES FOR LARGE TRANSFORMERSD A Wilson r Technology and Science Division, NGC For a transmission company such as NGC the transformer stock represents a major capital asset. There are some 650 units with ratings 100-1000MVA and high voltage windings of 400 and 275kV. Their replacement value would approach E1B. Whilst much debate within the asset management area concentrates on replacement policies, the issues are much wider. This paper will attempt to describe some of the technical aspects of these issues which are addressed within NGC’s Technology and Science Division (TSD). The overall strategy is shown in Fig 1. It is through three main thrusts,facilitating new assets, working plant harder and life assessment that asset optimisation is achieved. Each must be given the appropriate balance. Exceptionally long times can be achieved before replacement provided the design has been sound, the transformer correctly rated and operated. The key questions are how to determine the operating regime to work the plant d i c i e n t l y hard to achieve an acceptable lifetime, how best to assess lifetime before failures, how best to specify new plant and how to determine a replacement policy acceptable to the City. Life Assessment The major factor influencing the technical life of a transformer or shunt reactor is the deterioration of the insulation in the main tank. Fortunately other components with shorter lives can be replaced - the tap changer, pumps, fans and radiators. It may be reasonable to contemplate a mid-life oil replacement - where the winding itself is assessed to have a longer than normal life. Research is ongoing at the Leatherhead laboratories of NGC to identify methods of measuring oil ageing and to determine the oil ageing rates. The seminal work on ageing of winding insulation is that undertaken at Leatherhead in CEGB days by Shroff and Stannett’. This showed a linear relationship between log life and log DP, dependent upon operating temperature, a r and water content. The loss in i mechanical strength, quantified by a measurement of degree of polymerisation (DP) on a paper sample, is an indicator of the risk of failure in a short circuit. This earlier CEGB research is continuing in order to identify ageing rates appropriate to the actual environment of NGC transformers, and to identify the most suitable tests on oil and paper to indicate ageing. The work is undertaken at Surrey University in through a five year project sponsored by NGC and NP’. Laboratory studies are supplemented by tests on paper and other insulating materials removed from scrap transformers. Samples are taken from defined positions in the windings and assessed in techniques-such as those being developed in the Surrey investigation. Intemational collaboration is appropriate (although their age vs life curves will differ due to different materials and operating conditions). A CIGRE task force of SC1501 is looking at comparability between test techniques using common samples from scrap transformers, and this is co-ordinated by NGC. There are other aspects of deterioration. Pressboard and paper will shrink, leading to slack windings and risk of failure in external short circuit. Experience has shown some designs to be more prone to this problem and the identification of the type weakness is important. One of the techniques developed by TSD has been the expertise to use fibrescopes, under 1/1 The future will see two new developments. Both uses will however only be possible through collaboration of the owner of the design and the utility. In this actual ambient and oil temperatures are used to calculate the winding hot spot temperature for a projected load condition. A research programme is in place at the Caledonian University to understand the propagation of PD pulses in windings. Site location is attempted using current technology of point sensors on the tank wall. This allows a wide variety of designs to be assessed easily in terms of oil flow. A further improvement over ratings based upon off line calculations will be achieved through the development of the Transformer Thermal Monitor. Within the year this system will cover seventeen transformers. Whilst the initial interest has been in distributed optic fibre systems. Each of the above problems can be assessed during any stripdown during a fault investigation or scrapping. For the future there is the possibility of using fibre optic distributed sensors which are sensitive to vibrations fiom PD within the 1/2 . This avoids pessimistic assumptions and allows N utilisation of the rating4. Other parts prone to deterioration follow failure of glued joints in the pressboard and breaking of linen tapes. a thorough investigation made at all opportunities. This is based upon works test data and a simple computer program to relate the data to operational loads. Some transformers are prone to loosening of the paper wrapping on conductors. It is on the basis of materials research. plant Derformance assessment and failure investigations that life assessment can be made. The code runs on a 486PC with a Windows Graphical User Interface. TSD has recently issued a loading guide for NGC use in ascribing circuit ratings. therefore. again risking overheating. frequency response. A key role to the assessment of the transformer stock is. together w t ih international comparisons possible through CIGRE and DOBLE that realistic life estimates can be made. 1 Where there has been a malfunction it is important to be able to make a condition assessment. to record on video the condition of the clamping. and return voltage. Practical systems can be implemented now using point sensors and NGC has paid for these Luxtron probes to be fitted to two new transformers4. A new code TEFLOW2 has been developed to provide calculations for the complete transformer circuit under the transient loads experienced in normal service and also in faults. this technology remains in the future. any slackness. ambient temperatures and estimated winding temperatures. and this can lead to partial discharge. or cracking in clamping materials3. and so lead to a measurement strategy appropriate for site or factory use. These can lead to local loss of cooling oil flow and overheating of the insulation. Once validated TEFLOW2 will provide rapid assessment of new designs and also estimates of service temperatures fiom service loading conditions . This. A prototype was developed in the 1980s and an improved version has now been commissioned to provide grid control engineers with real time assessments.oil. Working Plant Harder The key to determining the most cost effective operating regime arises through validated ratings. Techniques are being developed in areas of partial discharge (PD).and fiom this an estimated life expended on the paper. This can lead to the identification of type problems on the assessment of life of others in the family . mean and hotspot temperatures4.and to give generic information. Validation has been through rig measurements and operational transformers fitted with fibre optic probes. Vibration can lead to loosening of metal parts. IEE Proc C 132 (6) pp 312-9.either in terms of watching briefs or by direct involvement. Conclusions Since privatisation NGC has made a substantial effort to assess the condition of its transformer assets and to optimise its use of these. To meet this a new suite of specifications for all NGC plant has been produced. only part of the condition assessment of transformers possible through oil testing. Traditionally transformer specifications were very prescriptive and open to complaints of non-compliance with EC purchasing requirements. The future scenario is more likely to be. technique used in NGC to detect this uses a swept frequency sine wave to establish the winding response. It is also as a result of learning more about operational features determining life that policies and specifications for new purchases can be drafted. D. New Assets Most of the transformer stock was purchased between 1960-1970. interpreting the information using Rogers codes and the trend data held in a laboratory information systems. The ak. 1985. A. For the future there is also a need to ensure that new technologies are followed . and Stannett. The Leatherhead laboratories now test some 4000 oil samples per year. . etc. Shroff. the development of this new data base and the new automated test systems has created a world class oil laboratory at Leatherhead'. one where much of the routine fault indication is done in the field. Each unit is tested once per year. alternatives to oil. The normal practice has been to rely upon dissolved gas analysis (DGA) on oil samples interpreting the data using systems developed in the CEGB by Rogers. References 1. and the expertise has been built up to detect movement with some reliability6. Other complementary techniques to assess ageing and malfunction include recovery voltage and Doble testing. This is being achieved through a substantial and wide ranging research programme with the scientists working in close collaboration with plant design specialists and routine test chemists and combining in house research with that sponsored in universities. Most ageing and breakdown processes are relatively slow in developing into a major fault. but City finance. and superconducting transformers.. An appropriate rate of replacement needs to be identified and one which is far less than the initial commissioning rate.H. however. and the laboratory test is a more comprehensive range of oil assessments to indicate life expended in oil and solid components. SF. "A Review of Paper Ageing in Power Transformers". New techniques are under development which will provide a complete assessment of the oils and solid insulation. In those National Grid Technical Specifications (NGTS) it has been particularly important to identify the correct operational conditions and test regime to check compliance.W.tn' Once there has been a short circuit there is a risk of winding movement. These would include alternatives to paper. solid state tapchangers. Within the 199112 year this technology identified eleven super grid transformer units as having significant operational problems requiring investigation. DGA is. It is on the basis of the two theories addressed earlier that the rate and identification of sequence is derived. Replacement will not be fiom Treasury resource. therefore. 7. Printed and published by the IEE.A. 1992. Brighton. Cooper. S. London WCSR OBL. et al. Fitton. Cosgrave. 5.A. 1994. Seventh BEAMA Conference.A.. J. Seventh BEAMA Conference.. and Stevens. J. 6.. 1994. "Thermal Rating of Transformers".A. 3. and Dum.and Lapworth. Funnell. B. G.. M. 1994 "Development of Techniques for Analysis of Materials". Lapworth.P. IEE Proc A 140 (5). Manchester.W. Savoy Place. J. and McCann. "Acoustic Monitoring of PD in GIs Using Optical Sensors".. . "Dissolved G s Analysis of Transformer Oil" a ibid. 4. S.M. "A Reassessment of Load Temperature Thermal Degradation of Cellulose".. A. Emsley.C.WORKING PLANT HARDER NEW ASSET SPECIFICATION Asset Optiknisation LIFE ASSESSMENT This paper is published with the permission of NGC 0 1994 The Institutionof Electrical Engineers.M. E. and Simonson..M.J. P.2.R. I. 1994. Brighton.. UK. J. 114 . IEE Sixth DMMA Conference. Cox. Seventh BEAMA Conference. 1993. G. Brighton. "Transformer Winding Movement and Fault Detection by Frequency Response Analysis". Ali.N. Jannan. pp369-374.