CESC-O&M

O&M OF TRANSFORMERS - CESC’s CURRENT PRACTICES

FOR PRESENTATION AT THE PROPOSED

WORKSHOP ON BEST PRACTICES IN DIFFERENT AREAS CONCERNING DISTRIBUTION SYSTEM ORGANISED BY CENTRAL ELECTRICITY AUTHORITY IN JANUARY 2004.

O&M OF TRANSFORMERS - CESC’s CURRENT PRACTICES D. Nandy Dy. Chief Engineer CESC Limited

S. Bhattacharya Manager CESC Limited

SYNOPSIS : Transformer is a costly and vital equipment in T&D network. Its outage due to failure causes great inconvenience in network management and involves high expenditure on account of repair/replacement. Any Utility, therefore, takes all possible measures to reduce downtime / failure of transformers to a minimum and to extend their lives, at the most economic cost. With the enactment of the Electricity Bill 2003, this is an area of any Utility’s operation, which comes under greater focus. In CESC, it is an age-old practice to monitor the load as well as the health of the transformers. While actions like balancing of load, augmentation of capacity, addition of sources are initiated for overloaded transformers, various life enhancement measures / preventive maintenance practices are followed as a proactive action. Upgrading of specification of transformer and frequent review of O&M practices, based on service experience is a continuous process in CESC. Stringent acceptance tests like short circuit test and temperature rise test have been introduced to ensure quality. In this paper an endeavour has been made to share CESC’s current O&M practices for transformers. Different techniques of health assessment and life extension have also been discussed. 1.0 INTRODUCTION : CESC Ltd., presently a RPG Group Company, has been responsible for generating & distributing electricity in and around the city of Calcutta since 1897. It is still a vertically integrated Company with a generation capacity of 1005 MW (from its 4 Generating Stations) and transmitting & distributing power to 1.84 million consumers (including 1700 HT consumers), meeting a peak demand of around 1300 MW (with requisite support from WBSEB) in 567 sq. km. of licensed area. In the last financial year 5557 MU of Energy was sold. The T&D network has 15 nos. 132 kV and 82 nos. 33 kV Substations with 264 ckt.km of 132 kV and 1054 ckt.km. of 33 kV overhead circuits/ underground cables. The Company presently has 30 nos. 132 kV class and 151 nos. 33 kV class power transformers besides 4592 nos. 6 kV / 11 kV class distribution transformers (DT). In order to remain competitive, Utilities throughout the world are now forced to drastically reduce their expenditure for generation and transmission/distribution of electricity and as a fall out of this the budget for capital expenditure for new plant is often reduced to a bare minimum. It, therefore, becomes essential to extract as much life as possible from the capital-intensive equipments. Transformers being one of the costliest equipment in a T&D network, it is a common practice, amongst the Utilities, to maximize their service lives by taking timely precautionary measures and adopting correct O&M practices. CESC also is no exception to this. In CESC the first 132 kV transformer was inducted in its network way back in 1965 and in the same year five more transformers of the same class were added. Two more were commissioned before ‘70s and all of them are 2

still performing satisfactorily. In 33 kV class, out of 64 transformers commissioned before 35 years from now, 55 transformers (86%) are still in service. Amongst these, 46 are serving for more than 40 years while 25 of them are serving for over 50 years. The average life of the failed nine transformers was 46 years. Life extension as above has been possible through routine preventive maintenance, close monitoring of insulation and related parameters of transformers, adopting corrective measures like on-line oil filtration, oil replacement, drying out etc., in time, and also through continuous up gradation of O&M practices based on failure analysis and performance of equipment. 2.0 PRACTICES FOLLOWED : CESC being an age-old Utility, the transformers in its network are of varied age group and design. There are some very old transformers with high factor of safety while there are others, manufactured in today’s competitive age, with marginal cushion. During this long period, numerous changes have occurred in all spheres like the material technology, the design tools/techniques, the manufacturing processes, the testing methodologies and equipments. To maximize service life of all such transformers in its network and to increase operational reliability vis-à-vis MTBF (mean time between failures), the practices followed in CESC are structured on three main aspects (i) continuous up gradation of specification of transformers and bringing necessary stringency in it, based on service experience, (ii) exercising regular stage inspection for both new and repaired transformers and carrying out type tests like temperature rise test, short circuit test (for DT only) etc. and (iii) carrying out preventive / predictive maintenance and applying life extension measures on regular basis. Design Stringencies / Special Requirements : Salient features of stringent specification, common to both power and distribution transformers, are : 

To keep provision for occasional unpredictable overloading without affecting the service life oil and winding temperature rise limits specified in the specification are made more stringent than relevant standard and limit for winding temperature gradient also introduced.

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Keeping provision for over fluxing in the design.

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Specifying limit for minimum IR / PI value of winding.

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Specifying core material of grade Hi-B for low no-load loss.

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Specifying no. of switching surges per annum.

Salient features in the specification for power transformers are : 

Specifying limit for minimum Tan  value of winding and OIP type terminal bushing.

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Using transformer oil as per IEC 296, class I instead of IS 335 and specifying % Naphthenic carbon content more than 50.

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Specifying step-lap core construction to reduce no-load loss.

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Use of Fibre-glass as the material for OLTC barrier and CT secondary terminal boards in place of cast resin.

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Use of Bellow type conservator in place of conventional free breathing type.

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Use of PRV along with splashguard and oil drain pipeline in place of explosion vent.

Salient features in the specification for distribution transformers are :

3



Use of Copper winding with preference to paper insulation for oil type and with 220 oC class Nomex paper insulation for dry type.

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Use of Tap-link board instead of tap-switch, to avoid repetitive failure of tap switches due to accumulation of sludge, high joints in contacts due to ageing of compression springs etc. (Studies indicated that failure of tapswitches not only warranted repair at site but also at times led to failure of transformers).

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Supply of HV winding in 8 coil segments to ensure better heat dissipation with marginal reduction in short circuit strength.

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Use of 8 nos. (usual 4 nos.) 16mm. (usual 12 mm.) tie rods between top and bottom yoke support channels. (Thereby clamping force on core & winding is increased which in turn enhances the short circuit withstand ability).

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Use of modified terminal arrangement with pad clamps on LV terminal bushings in place of washer and nut arrangement to provide sufficient contact area and thereby reduce the possibility of high resistance joint leading to oil leak.

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Use of HRC fuses with proper discrimination, on both HV and LV sides.

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For oil type transformer, radiator top header is specified to be below the level of terminal bushing to avoid stoppage of oil circulation through radiator in the event of draining of oil by leakage through the terminal bushings.

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Specifying maximum flux density.

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Standardized only one rating, namely 315 kVA. Loss capitalization based on ‘Total Owning Cost’ is carried out during evaluation for procurement and

selection. However other factors like performance record of a vendor, their quality awareness, after sales service, financial standing, concern for customer etc. are also given due consideration. Vendor selection for supply of new and repair of transformer is always associated with one or more visits to the works of the manufacturer / repairer to ascertain (i) infrastructural facilities available for manufacture, testing etc. (ii) quality assurance (iii) attitude and commitment. A comprehensive database is maintained to monitor the performance of the vendors. Subsequently, frequent interaction with the manufacturers / repairers and visit to their works from time to time are done to maintain / improve quality in manufacture / repair. Vendors are treated as a part of CESC’s team for product refinement. Further, CESC does not encourage patch-winding repair during repair of faulty DT. Instead replacement of total winding is carried out to provide fresh life at economical cost considering total cost of repair, replacement and other direct/indirect costs. Inspection / Testing : Frequent stage inspections during processing of both new and repaired transformers are carried out. Necessary infrastructural facilities have been set up for carrying out loss measurement and temperature rise test of distribution transformers and checking tripping/fusing characteristics of fuses / ACBs. Loss measurement and temperature rise test are carried out on randomly selected transformer one from each contract / batch without prior knowledge of the manufacturer / repairer. Provision for carrying out temperature rise test at user end also, has been 4

incorporated in the specification. For power transformer, temperature rise test is witnessed one from each batch of supply. Short circuit tests are carried out at established laboratories with CPRI / ERDA on randomly selected distribution transformers, after they are delivered to stores, without any prior intimation to the manufacturer / repairer. Since this is a costly proposition, such tests are carried for each manufacturer / repairer once in every 3 / 4 years. Photographs of the core coil assembly of the passed transformer are preserved for comparison, from time to time, with other transformers of same capacity and make. Condition Monitoring and O&M Measures : For both power and distribution transformers, endeavour is made to keep the operating parameters, namely voltage and frequency within tolerance band. For power transformer, loading also is restricted below the limit as per relevant loading guide standard. Readings of different parameters like voltage, current, frequency, tap-position, oil/winding temperature etc. are taken every half hourly and checking oil level, condition of silica-gel etc. are carried out once in a 8-hours shift. The distribution transformers, being so large in nos. and scattered all over the places, it is impossible to exercise the same amount of monitoring / care like that for power transformers. However load monitoring of each DT is carried out once a year and load balancing / relieving of overloaded transformers, to the extent possible, is carried out as a follow up action. Pre-festival checks for any oil leakages / low oil level / over-heated joints for DTs at strategic locations are also carried out for taking necessary corrective actions. Oil sampling from power transformers are carried out once in a year for checking electric strength, resistivity, Tan , water content, IFT, flash point (occasional) and DGA monitoring. IR and Tan  monitoring are carried out for transformer winding and OIP type terminal bushing once in 3-years. Exhaustive databases are maintained on DGA / oil test results and Tan  values of winding and bushing to monitor the health of the transformers and initiate action for predictive maintenance. Since induction of DGA monitoring in 1999, quite a few cases of abnormalities were diagnosed and rectified before it could lead to a failure. One such case study of interest is detailed in Annexure-1. Exhaustive database on make, year of purchase, technical details, peak load reading, accessories & fitment, date of commissioning, location, history of maintenance / repairs etc. and performance are maintained. All failed transformers are inspected thoroughly prior to repair / disposal with a view to finding out the probable cause of failure, since such inspection provides vital feedback on design/manufacturing deficiency or any abuse that it might have been subjected to. Depending on the outcome of such inspections, review and amendments of specification are initiated. Operation & Maintenance practices followed are detailed in Annexure-2. 3.0 LIFE EXTENSION : Even though the present day transformers are not manufactured with the same degree of margin in design as those of the earlier times due to stiff market competition, still CESC aims to extract the same amount of life from them. It is indeed a very difficult proposition, yet it is possible to achieve a reasonably good result if the state-of-thetechniques/tools for condition monitoring/assessment of transformer insulation are practiced and corrective measures/refurbishment are carried out in time. 5

In life extension drive broadly there are two areas of operation, one is condition assessment and the other is timely initiation of corrective measures. Strategy : Tests as per IS: 1866 for monitoring of oil condition are carried out once in a year while measurement of IR is done with a periodicity of once in three years for each and every power transformers in the network. Transformers with poor oil condition but with satisfactory / moderate IR values are subjected to on-line oil filtration by using HIVAC oil filtration machine and usually results in satisfactory improvement. However when both the oil condition and IR values are low, such on-line filtration does not usually provide satisfactory result because it only improves condition of liquid insulation but can hardly bring in any improvement of the solid insulation. When IR values are found very low and the condition of oil is found very poor, especially if the acidity is high (0.5 unit or more), total oil replacement followed by on-line filtration with HIVAC oil filter yields reasonably satisfactory results. In situations where the improvement of IR is not satisfactory or the IR improvement is not sustainable, in situ drying out is carried out. Further, if resiting of a power transformer is required to be carried out and if its IR values are found low and/or if it is aged / last refurbishment carried out 20/25 years back, drying out of the transformer is usually carried out. Based on condition assessment, all power transformers, by and large are subjected to life extension measures, as mentioned above, at least once, if not several times in their life span. Techniques : Oil filtration : Mobile HIVAC oil filtration plant is used for carrying out oil filtration of power transformers. This is done on-line, utilizing the heat generated in the transformer to facilitate faster moisture liberation from the winding. As a spin off, less power is consumed in such filtration in comparison to that of off-line filtration. As an added advantage, time requirement is also less. Adequate precaution is, however, required to prevent accidental entry of air in the oil circuit. As stated earlier, in this process condition improvement of liquid insulation is only achieved, without hardly any improvement in the condition of solid insulation. Total oil replacement : In this process entire oil of the main tank and radiator bank is drained out including the sludge. Core and winding and the radiator banks are washed with fresh oil and sludge is removed again. Washing of core and winding is repeated several times, if required. Oil filling is then carried out with new and tested oil through HIVAC filtration machine followed by on-line filtration. In this method moderate improvement of IR is generally achieved. Drying out : At manufacturer / repairer works, drying out is carried out by conventional vacuum drying oven / autoclave and this results in maximum improvement of the condition of solid insulation. Drying out is also carried out at site but in a different way, as infrastructural facilities of shops are not available. Critical aspect of any drying out process is addressing two main issues namely, effectiveness and ability to ensure control on the process. Both these factors arise from the difficulty of assessing the moisture content in insulation. Almost all known processes of in situ drying out have been tried out at different times. The methods are : 6

Drying out by external heating and vacuum pulling : In this method the transformer tank is enclosed in a shed or covered by tarpaulin / asbestos cloth and heaters are fitted, external to the main tank, for indirect heating of core and winding. Vacuum pulling and Nitrogen filling are carried out alternatively till the water extraction is nominal. Subsequently oil filling under vacuum is carried out by new and tested oil through an oil filter machine. Drying out by hot air blowing and impedance heating : This method comprises simultaneous hot air blowing (at 100 to 110 oC air discharge temperature) and impedance heating (by 15 - 20 % of rated current), till the IR attains a steady value. Then vacuum pulling followed by nitrogen filling is carried out and this is repeated till satisfactory IR values are achieved. Subsequently oil filling under vacuum is carried out by new and tested oil through an oil filter machine. This method requires considerable amount of care. There are instances of transformer getting damaged while performing drying out by this method. Drying out by impedance heating and vacuum pulling : This is the latest method adopted and it consists of impedance heating (to raise temperature of winding to nearly 40 oC) under vacuum (much below the vapour pressure of moisture corresponding to the said winding temperature), till IR attains a steady value or till water extraction rate comes down to minimal. Thereafter, vacuum pulling followed by nitrogen filling is carried out and these are repeated till satisfactory IR values are achieved. Subsequently, oil filling under vacuum is carried out by new and tested oil through an oil filter machine. Results : It is generally considered that refurbishment does not extend the life of a transformer, it only ensures that the transformer will provide a service life normally expected from it. Only through refurbishment involving complete replacement of winding and insulation, the transformer can be treated as new both in terms of life expectancy and reliability. However, our experience is somewhat different, as explained below with statistical data. CESC has been carrying out such life extension work since 1962. In last few years, drying out has been taken up at an average of 4/5 transformers per year and so far refurbishment of 58 nos. of transformers have been carried out with satisfactory results, as mentioned below, excepting only one case with unsatisfactory results. Out of the existing 181 transformers, the two oldest transformers still serving are 60 years old. 25 transformers are in the age group 50 years or more and 36 transformers in the age group 35-50 years. All of them are thus running beyond their normal expected lives and records indicate all of them had underwent at least one drying out process and in a few cases, multiple times. Failure rates of transformers in CESC system for last five years are : % Failure rate

2003-04

2002-03

2001-02

2000-01

1999-00

Power Transformer

0.65

0.65

0.66

2.72

1.4

Distribution Transformer

1.81

1.73

2.72

2.01

2.71

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Despite the fact that the old transformers are believed to have been manufactured with high margin in design, quality workmanship, great care and without giving much emphasis on cost, even then the life of such transformer is commonly considered to be around 35 years. Since such transformers in our system, which have undergone refurbishment, have already reached service lives much beyond what is normally expected out of them (as indicated above), there should be no doubt in accepting this as the effect of life extension measures. 4.0 FUTURE PATHWAYS : Although the failure rate of distribution transformers in CESC system is not high (as mentioned earlier), there is scope to increase the average life of distribution transformers and further improving on the failure rate, bench marking with corresponding figures of the advanced countries, through proper O&M practices. Major issues like condition monitoring & life assessment, which are closely associated with our endeavour to increase MTBF, service reliability and service life, need to be addressed in near future. Since life of a transformer is primarily governed by the life of solid insulation of the winding, state-of-the-art techniques/tools need to be applied for precise assessment of moisture in the same. RVM test (Residual Voltage Measurement) to detect moisture content in paper insulation and Furan analysis to detect ageing of solid insulation in service, are some of the latest techniques to be adopted for precise assessment of insulation health and initiation of subsequent corrective measures. 5.0 CONCLUSION : O&M practices of any equipment, though changing continuously with the technological advancements, the basic objectives have remained the same, namely (i) reducing breakdown outages, (ii) increasing operational reliability and (iii) maximizing service life - all at the most economic cost. While the first two objectives can be achieved with close health monitoring by adopting state-of-the-art techniques/tools and taking necessary remedial measures, maximization of service life can be attained by carrying out refurbishment at the right time. Further, manufacturers’ recommendations need to be blended with service experiences to formulate correct O&M practices. This should be followed by periodical review on the basis of experience on performance and in-depth analysis of failures for further refinement. Last but not the least, continuous updating of domain knowledge with technological advancements, at home and abroad, is important for adopting correct O&M practices. Condition based maintenance is gaining much more importance now-a-days over time based periodical maintenance. In CESC, though time based periodical maintenance was in vogue, the requirement for condition-based maintenance was felt and presently it is being followed. There is, however, scope for further improvement in the O&M practices being followed and lot of work need to be done to bring failure rate vis-à-vis operational reliability in our system at par with global standards and also to sustain it, thereafter. 6.0 ACKNOWLEDGEMENT : The authors thank the management of CESC for according permission to publish this paper. The authors thank Mr. D.N. Mozumdar, The Executive Director for his valued guidance and support in writing this paper. The authors also thank their colleagues for their kind co-operation in the matter.

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