Twenty-first century distribution transformers

July 27th, 2014, Published in Articles: EE Publishers, Articles: Energize, Featured: EE Publishers

 

William Stanley, working for Westinghouse is credited for building the first refined and commercially used transformer. In 1886 he used his transformers in the electrification of Great Barrington, Massachusetts, USA. This was the first ever implementation of an AC power distribution system using step up and step down transformers.

Today, more than 120 years later, the same methodology is still used for long distance transmission of power, however the 21st century has brought its own demands and expectations and especially for distribution transformers:-

  • Higher efficiencies/lower losses to reduce running costs
  • Simplified infrastructure requirements to reduce implementation costs
  • Reduced maintenance requirements to reduce costs
  • Reduced risk of harm to person or property due to transformer failure
  • Reduced risk of harm to the environment during operation, in event of failure and at end of life
  • Reduced risk of loss of assets or production due to theft

New technology

Although transformers have been with us for a very long time and the basic principal of operation remains unchanged, there have been several design and construction advances in recent years. Several of these afford significant technical and commercial advantage, often together with other benefits. In particular where distribution transformer technology was stagnant over decades, there are now rapid changes taking place.

The universal standard was the oil cooled transformer with cellulosic paper insulated copper windings, hermetically sealed with no radiators at the lower end and free breathing with radiators and core type construction in larger ratings. On a global scale more and more of these stalwarts are being replaced with different designs and construction. We will examine some of the main drivers for change, the new options and trends for the future.

Most development was driven by the need for one or more of:

  • Improved efficiency
  • Increased safety
  • Reduced environmental impact

Recently, a growing need has emerged in developing countries for distribution transformers that are robust, maintenance free and unfriendly toward oil and copper theft. These requirements generally work against the usual competitive motivation of reduced manufacturing cost, but in achieving these it has been found that although transformer costs typically increase, significant and much greater cost benefits arise in the implementation phase and/or in service. In addition to oil cooled there are now several other types of transformer construction. The most common that are employed for electrical distribution are listed below. Each type also has several variants:

  • Oil cooled
  • Dry type – open ventilated (OVT)
  • Dry type – cast resin (CRT)

Each type has strengths and weaknesses which render it better suited than another for particular applications.  The comparison table below illustrates some of the key characteristics. Efficiency, impulse withstand and noise are not listed since they are considered to be design deliverables.

Table 1: Characteristics of various types of transformers.

Table 1: Characteristics of various types of transformers.

In this time of change oil cooled transformers are also receiving attention with high flash point natural ester oils and synthetic replacements for cellulosic insulation, but in terms of new technology suited to a wide variety of distribution applications, the most interesting developments are in cast resin transformers (CRT). This will therefore be the focus of this paper.

Fig.1: Cast resin transformer.

Fig.1: Cast resin transformer.

Efficiency

Perhaps the most interesting developments have been in the drive to reduce losses and improve efficiency. The original cast resin transformer with standard losses was improved with a low loss version and then further improved with the extra low loss.  This has now been completely overshadowed by the new Ecores development which is at least an order of magnitude better and meets the new European Union regulation number 548/2014 of 21 May 2014. The benefits and relative performance are best illustrated by Fig. 2, which compares payback of the standard loss (C0 Bk to EN50541) with the Ecores transformer.

Fig. 2: Comparison of payback between standard loss (C0 Bk) and Ecores technology.

Fig. 2: Comparison of payback between standard loss (C0 Bk) and Ecores technology.

Thermal class

Cast resin transformers have been subject to a limitation in thermal performance of the insulation system (class F 155°C).  This has been a restriction in some applications where space is at a premium, or additional overload capacity or an additional safety margin has been desired. This restriction has been lifted with the introduction of cast resin transformers meeting thermal class H standards (180°C), paving the way for use in more demanding applications such as mining, traction and marine.

Noise

An incidental benefit of the Ecores technology is a huge reduction in noise levels (Fig. 3). While this may be of little interest for oil cooled transformers, it is important for cast resin transformers which are recommended to be located as close as possible to load centres in order to derive installation and operational cost benefits.

Fig. 3: Comparison of noise levels between standard, extra low loss and Ecores technology.

Fig. 3: Comparison of noise levels between standard, extra low loss and Ecores technology.

Technical performance

When there were only oil cooled distribution transformers the design engineer or specifier had no options.  In a way it was easier, but only for transformer selection. How the transformer has to be accommodated can lead to costly complications because of the oil, especially if safety and environmental issues are important in the application. Now, choosing a dry type transformer, and a cast resin transformer in particular, can greatly simplify the overall project as well as significantly reduce both capital and operational costs. This is achieved by positioning the CRT right at the load centre, with short LV cable runs and without the need for oil containment bund walls or elaborate fire protection schemes.

As a result of their inherent construction, CRTs can be expected to deliver high performance in respect of:

  • Resistance to short term overloads
  • Resistance to mechanical shock.
Fig. 4: Resistance to short term overloads make CRTs ideal for passenger and freight rail applications.

Fig. 4: Resistance to short term overloads make CRTs ideal for passenger and freight rail applications.

They can also be designed and constructed to offer outstanding performance in respect of:

  • Efficiency
  • Resistance to thermal shock
  • Vibration tolerance
  • Resistance to surges
  • Impulse voltage withstand
  • Low noise
  • Resistance to corrosive and aggressive chemical environments

As with all things, CRTs are designed and manufactured to many different quality and technical standards.  For trouble free installation and service, performance requirements should be carefully considered and specified according to the needs of the application, operating environment and customer preference. Consultation with reputable suppliers who are abreast of the latest technical developments and standards is usually worthwhile to ensure cost effective matching of needs and specifications.

Maintenance

In recent years maintenance has become a big issue with simply too few skilled and experienced hands to go around. Here the CRT scores on all points because not only is it easier to install and commission, but it requires No maintenance apart from periodic dusting off. No breathers, no oil leaks, no oil sampling, no drying out or regeneration of oil. No topping up or replacement of oil. No oil instrumentation, no valves. No oil pipes and flanges, no Buchholz relays, no oil level gauges. A maintenance engineer’s dream come true.

Savings opportunities

It is normal for a CRT in isolation to be modestly more expensive than its oil cooled counterpart. However when the overall project cost is considered (particularly enclosure, oil containment measures and cabling) it is not unusual to find that the CRT solution saves as much or more than the entire cost of the oil transformer. But it does not stop there. By taking advantage of the safety features of CRTs and locating them close to the load, the shorter lengths of low voltage cabling results in massive reductions in I2R losses. The resultant energy savings benefit the end user throughout the life of the transformer. Further energy and cost savings are of course available by specifying the new Ecores technology previously mentioned.

Old and new considerations

Although it may often no longer be the reason for choosing a CRT, safety was the single most important criterion which accounts for their development in the first instance. Too many unfortunate accidents resulted in loss of life and property due to transformer explosions and fires. Smoke inhalation and smoke induced obscurity also played a major role in the incidents. This is the reason for special test standards for certification of fire behaviour (self-extinguishing properties and smoke characteristics) of CRTs because even though safety is inherently improved by elimination of oil, it is still important to ensure that if a fire is initiated by an electrical fault or other means that it will extinguish as soon as the heat source is removed and will not produce hazardous amounts of smoke.

The obvious environmental benefits of CRTs arise from elimination of oil, but it is possible to enhance this and ensure minimised environmental impact by specifying aluminium winding conductors. Unlike copper which is a limited resource, aluminium is available in relative abundance and requires a fraction of the amount of energy to produce and process. Provided the transformer design is correctly adapted, there is no technical reason apart possibly from dimensions to avoid aluminium.

In some countries, theft of transformer oil and copper leads to a greater number of losses than does functional failure. Cost effective, long term solutions are being sought after. All dry type transformers eliminate the oil factor, while foil type CRT windings are an additional deterrent because the value is much lower and extraction extremely difficult. In some instances reparability can be an issue. All transformers can be repaired, but not always economically. CRTs are particularly easy to repair by removing the top yoke and simply replacing the failed coil(s).

Fig. 5: Aluminium foil used in manufacture of foil discs for the MV windings.

Fig. 5: Aluminium foil used in manufacture of foil discs for the MV windings.

Trends

Whereas oil cooled transformers remain the standard solution with the highest volumes installed and still going into service, there is a growing trend globally toward more widespread use of dry type transformers and especially CRT. Initially the motivator was safety in high rise buildings, but the numerous advantages of CRT came to be realised and this has led to more and more successful applications for CRTs.

The renewable energy sector world-wide, for one, has embraced cast resin transformers in order to benefit from the features such as low environmental impact, low environmental risk and low maintenance together with high efficiency. This is mirrored locally by the adoption of CRTs as a standard for the majority of REIPPP projects awarded.

Fig. 6: PV farms adopt CRTs as a standard.

Fig. 6: PV farms adopt CRTs as a standard.

More recently CRTs are being successfully adapted for use outdoors, usually installed in a suitable enclosure in pad/ground mount or pole mount applications. Reasons for this development are the low maintenance requirements, no risk of adverse effect on the environment due to coolant spillage and the reduced desirability for theft due to elimination of oil and copper.

With increasing energy prices, a continuing trend is greater focus on transformer efficiency to achieve both reduced no-load and load I2R losses in order to capitalise on the significant energy savings that can be achieved over the lifetime of the transformer. On a global scale environmental considerations are becoming increasingly important, but perhaps have been less prominent in South Africa than most first world countries.

Standards

The applicable standard for dry type transformers is SANS 60076-11:2005/IEC 60076-11:2004, and the applicable standard for dry type transformers which covers transformer efficiencies is EN50541-1:2011.

Fig. 7: Testing to SANS 60076-11:2005/IEC 60076-11:2004 requirements.

Fig. 7: Testing to SANS 60076-11:2005/IEC 60076-11:2004 requirements.

Conclusion

After many years of relative stagnation, distribution transformer technology is rapidly evolving, especially in the case of cast resin transformers. Compared to other options they now offer a number of unique features that make them the ideal solution for many demanding applications. Exceptionally efficient and higher thermal class options, safe, robust, minimal maintenance, unattractive for theft and environmentally friendly are plus points. The modestly higher cost is completely offset by even higher savings in situations where the CRT can be placed close to the load.  Where this is not possible or of no benefit and there are no safety, environmental or theft issues then the traditional oil cooled transformer still holds its own as a time proven lower cost option.

References

[1]. The history of the transformer: http://edisontechcenter.org/Transformers.html
[2]. SANS 60076-11:2005:  www.sabs.co.za/Business_Units/Standards_SA/Controls/Published_Standards/PS67E.pdf
[3]. EN 50541-1:2011: www.en-standard.eu/csn-en-50541-1-three-phase-dry-type-distribution-transformers-50-hz-from-100-kva-to-3-150-kva-with-highest-voltage-for-equipment-not-exceeding-36-kv-part-1-general-requirements/?gclid=CO2v24XfqL8CFYQIwwodeSQAIg
[4]. Ecores: TMC Italia Spa: www.tmctransformers.com

Contact Mervyn Low, TMC Africa, Tel 086 141-4777, mervyn.low@tmcafrica.co.za

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