Technological modifications to power transformers have been slowly developing for several years, mainly to improve the electromagnetic performances and design. Conversely, the development of transformer components and accessories, involving the use of new materials and technologies, has not been subjected to the same adoption and diffusion. This is probably due to the reluctance of power transformer experts to support new electronic technologies and because utility procurement often focuses on minimum purchase cost.

The wide-scale availability of low-maintenance components for primary substations has resulted in utilities seeking new ways to optimize investment in transformers and manage in-service maintenance operations while maintaining satisfactory levels of network reliability and customer service.

The Enel Distribuzione network has a large transformer population, with about 3,500 units installed, and comprises some 1,700 high-voltage/medium-voltage (HV/MV) primary substations. The transformers have standard specifications in terms of design, ratings and test procedures. They are typically loaded to a maximum 70% to 80% of the nameplate rating, so they are not thermally stressed, except under emergency load conditions that can occasionally occur for a short period (a few days).

The use of surge arresters to protect the HV and MV windings is a usual design practice in Europe and limits the dielectric stresses. The short-circuit withstand capability of the windings is limited by several factors:

• Yyn0 connection with HV, not accessible neutral

• Absence of delta-connected tertiary

• High short-circuit impedance to limit the MV short-circuit current at 12.5 kA

• Neutral of MV system insulated or earthed by Petersen coil.

The transformer design features result in a relatively low fault rate if problems such as oil leaks, oxidation of metallic parts, porcelain damage and the occasional bushing faults are disregarded. Nevertheless, with such a large transformer population, maintenance procedures are still regarded as strategically important.

Vacuum-Type OLTCs

Traditionally, power transformers were equipped with resistor-type on-load tap changers (OLTCs) with oil diverter switches — a technology that has been proven reliable and given satisfactory operation. The specified maintenance for such components includes replacing diverter switch contacts, insulating oil treatment and changing represents a significant cost and requires planning for a single-day outage of the transformer. This maintenance is required at intervals of approximately 8 to 10 years, or 100,000 tap-change operations. Over the course of its service life, a transformer will require two or three maintenance outages.

A new technology based on vacuum switching with diverter switches equipped with vacuum-type breakers has been available on the market for a few years; therefore, the number of units in service has increased. The reported performance data on the vacuum-type OLTC confirms the technology is proven and the units are reliable. Vacuum interruption technology has been well tested operationally on Enel's MV network, where a large percentage of the circuit breakers in commission are vacuum units.

Despite the higher purchase cost of vacuum OLTCs compared to traditional OLTCs, there is a significant reduction in the maintenance frequency as the vacuum type requires maintenance of the oil and contacts after approximately 300,000 operations, which should mean the unit remains maintenance-free for the life of the transformer. Also, the operating current range for the new OLTCs is wider than that of traditional ones, and it allows the use of the same OLTC for all transformers in the range of 16 MVA to 63 MVA. Without the need for oil changes and treatment, the vacuum-type OLTC offers safety and environmental benefits.

High-Voltage Polymeric Insulators

The availability of polymeric insulator technology has effectively removed concerns about the consumption of organic material. The lower diffusion characteristics of the polymeric insulators make them more expensive than traditional porcelain insulators. Although limited to experience with gas-insulated switchgear, Enel Distribuzione considers polymeric insulators to have satisfactory in-service performance.

Traditional porcelain insulators require specific maintenance for cleaning operations to prevent electrical discharges on the external surface. If cleaning is required, a transformer outage is necessary. Polymeric insulators are much better at withstanding the effects of pollution. So by accepting their usage cleaning, operations should not be required in lightly polluted areas, and the need for cleaning operations should be reduced significantly in areas with high pollution levels.

Additionally, the problems related to the porcelain damage during transport and handling, as well as the risk of in-service rupture in the form of an explosion, are largely avoided, or at the very least considerably reduced.

Plug-In Medium-Voltage Terminals

The plug-in MV terminals allow direct connection of the MV cables on the transformer, replacing the traditional solution, which requires a heavy, permanent support frame to sustain the busbar connection system between transformer terminals and cable terminals at the prescribed minimum safety height. Although more costly, the new solution simplifies the assembling operation for the connecting cables; the support for the cable can be lighter and an insulating system for the copper busbar is not necessary.

Moreover, the risk of faults caused by animals or large birds making contact with the exposed busbars is avoided. This new solution also offers improved system reliability. A further advantage of the plug-in MV cable terminals is the ease with which transformer parameters can be measured or, in the event of a single-phase cable fault, easy to repair. Currently, demand for the product exceeds the manufacturing capability, so more manufacturers are needed to reduce the cost, which is deemed to be overestimated.

Maintenance-Free Dehydrating Breathers

Each primary transformer is equipped with two silica gel breathers, one for transformer oil and a second for the OLTC diverter switch oil. The maintenance operation for these traditional dehydrating breathers in Enel Distribuzione is a visual check at three-month intervals and the annual replacement of the silica gel.

As the new type of dehydrating breather is expected to be maintenance-free for several years, the payback period for these units, which are more expensive, is estimated to be a few years. Further benefits will accrue as it will not be necessary to recycle or purchase new silica gel; in addition, the utility will no longer have to contend with the environmental issues associated with disposal of silica gel.

Anchor/Fall Arrester Devices

To comply with the personnel safety targets established by Enel to minimize the risk of on-site staff injuries, facilities for anchor and fall arrester devices will be specified for all new transformers. This should provide safety during maintenance or exceptional operations that require working on top of the transformer. Enel's safety procedures are still in the experimental phase. The alternatives under consideration include a rigid anchor guideline fixed on the conservator, the use of flexible anchor devices or fences (fixed or removable) on top of transformers.

Planned Future Developments

To simplify maintenance operations, Enel plans to introduce further improvements:

• Cooling radiators will be optionally specified with a galvanized finish to avoid oxidation, the probability of oil leakages, and having to repaint or replace after only a few years operation in polluted areas.

• Oil dissolved gas analysis (DGA) will be implemented to monitor the condition of transformers, predicting their expected life and, if necessary, identifying the appropriate corrective actions.

• A further optional planned modification concerns the reduction of the audible noise emission of transformers. Typical noise emissions from an existing 40-MVA transformer is 70 dB. Design changes have been made to reduce this level to limit the noise disturbance for occupants of private properties sited close to primary substations. This will reduce the necessity for remedial works to reduce noise levels to within the legal limits.

Reduced Life-Cycle Costs

The technical and economical evaluations being conducted by Enel Distribuzione to minimize the maintenance operations on transformers installed in primary substations are designed to reduce the life-cycle cost of maintenance and maintain or improve system reliability. The new solutions already installed are subject to constant monitoring and evaluation. The results and overall benefits of the new solutions, in terms of financial savings and environmental benefits, will provide a more accurate comparison with the old solutions as the number of new units in service increases.

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Flavio Mauri (flavio.mauri@enel.com) joined the Enel Electrical Centre of Research in Milan, Italy, in 1990 and subsequently moved to Enel Distribuzione headquarters. He is currently working in the engineering and normalization field of power transformers and other power-system components.

Stefano Cheli (stefano.cheli@enel.com) joined the Enel generation department in 1970, and after several years of experience on operation and substation maintenance, he moved to the Enel Distribuzione headquarters in Rome, Italy. Cheli has continued using his considerable experience on operational and maintenance activities.

Company mentioned:

Enel Distribuzione www.enel.it