Energy Performance Standards of Distribution Transformers

One third of network losses occur in transformers, and of these transformer losses, 70 per cent occur in distribution transformers
 Transformers are static electrical devices that are used in electrical power systems to transfer electrical power between circuits through the use of electromagnetic induction. Transformers convert electrical energy from one voltage level to another. They are an essential part of the electricity network. After generation in power stations, electrical energy needs to be transported to the areas where it is consumed. This transport is more efficient at higher voltage, which is why power generated at 10-30 kV is converted by transformers into typical voltages of 220 kV up to 400 kV, or even higher.
Since the majority of electrical installations operate at lower voltages, the high voltage needs to be converted back close to the point of use. The first step down is transformation to 33-150 kV. It is often the level at which power is supplied to major industrial customers. Distribution companies then transform power further down to the consumer mains voltage.Transformers can be grouped into four broad categories according to their high voltage winding and their function in the network:

Large power
Medium power
Medium voltage distribution; and
Low voltage distribution.
Transformers with their highest voltage above 36kV are generally referred to as large power transformers or medium power transformers, depending on the voltage. These transformers are often used in the transmission of electricity. Medium power transformers are generally considered as those with power ratings greater than 2500 kVA and less than or equal to 60 MVA three phase with voltage ratings > 36 kV to = 230 kV. Large power transformers are generally viewed as those with base self-cooled power ratings exceeding 60 MVA and always including all high voltage ratings of 230 kV as well as all extra high voltage (EHV) ratings of 245 kV or more. Large power transformers can be found at generating power stations and electrical substations to convert electrical power to high voltages for transmission and then back down again at the other end to a medium power transformer for transferring power to a sub-transmission circuit. From medium power transformers, the voltage is further reduced by medium voltage distribution transformers into circuits where the electricity is distributed to end users.
The Transformers installed in the distribution circuit of electricity networks servicing residential areas and commercial and industrial customers are named as distribution transformers. Distribution transformers (DT’s) are mostly involved in stepping voltage down.
As per Leonardo Energy – Transformers report, 2005, the global transmission and distribution network losses will lead to global economic loss of more than $61 billion annually and annual greenhouse gas emissions of more than 700 million tonnes.  In general, it is estimated that one third of network losses occur in transformers, and of these transformer losses, 70 per cent occur in distribution transformers. The report estimates that total electricity lost on utility networks around the world in 2005 was approximately 1,279 TWh, and of that, distribution transformers consumed 298.4 TWh.
High efficiency transformers create economic benefits for society in addition to the reduced greenhouse gas emissions, improved reliability and potentially longer service life if lower temperature rises are experienced through the energy-efficiency improvements. With these benefits in mind, many countries has taken policy initiatives to establish mandatory and voluntary programmes to conserve energy and to help the domestic markets by adopting energy-efficient transformers.
Losses in transformersThe losses are categorised into following three types:No-load loss (also called iron loss or core loss): Caused by the hysteresis and eddy currents in the core. It is present whenever the transformer is connected, and independent of the load. It represents a constant, and therefore significant, energy drain.
Load loss (or copper loss or short circuit loss): Caused by the resistive losses in the windings and leads, and by eddy currents in the structural steelwork and the windings. It varies with the square of the load current.
Cooling loss (only in transformers with fan cooling): Caused by the energy consumption of a fan. The bigger the other losses, the more cooling is needed and the higher the cooling loss. These losses can be avoided if operational temperature is kept low by different loss reduction measures.
An estimation of the total energy loss can be calculated from:Eloss [kW] = (P0 + Pk *I2)*8760P0 is the no-load loss [kW].Pk is the load loss [kW].I is the rms-average load of the transformer.8,760 is the number of hours in a year.
Improving efficiency Improving efficiency at manufacturers end is to reduce losses in transformers during the design and focus is mainly in two elements: core and windings. Transformer design is complex, with many of the characteristics of distribution transformers specified in national or international standards.
The no-load losses can be reduced by selecting a high performance steel for the core. Next to the choice of the steel, the way in which distribution transformer cores are designed, cut, fabricated and assembled, plays an important role in energy efficiency. Increasing the size of the core reduces the density of the magnetic field, and in this way improves energy efficiency.
Load losses are proportional to the square of the load current, so one should always consider how the unit will be loaded over time. Load losses can be reduced by increasing the cross section of the windings. This reduces the current density and consequently the loss, although at a higher construction cost.
The process of winding the conductor coils and then fitting them into the assembled core has a very large influence on the energy efficiency of a transformer. It is a labour-intensive process that requires skilled workers.
Transformer replacementsTransformer replacement before failure can be motivated by several reasons. These include environmental and fire safety regulations, changes in the load or the voltage level, an increased risk of failure due to transformer ageing, or the aim to improve the energy efficiency.
Distribution transformers rarely catch the attention of the Operation and Maintenance department. They do not have any moving parts. They do what they have to do, day after day, year after year, with a remarkably high level of energy efficiency and reliability. Transformers provide an almost constant quality of service. Their decrease in energy efficiency and reliability is at a very slow rate and generally remains unnoticed. Until, that is, they fail and have to be replaced.
Possible reasons for replacements To improve energy efficiencyTo improve the reliability of supplyBecause of a change in load profileBecause of a change in voltage levelTo comply with environmental and fire safety regulations.
A transformer failure occurs when the quality of the internal insulation system fails and a short-circuit results. The electrical insulation of transformer windings consists of a particular type of paper, immersed in oil. The physical properties of this paper are largely dependent on the degree of polymerisation of its molecules, which degrades over time, albeit very slowly and not always at the same pace. An insulation failure typically happens when this degree of polymerisation of the insulating paper drops below a threshold value. In such cases, the paper becomes brittle and the breakdown voltage is reduced. A surge in the voltage level, caused by a lightning strike or a fault on the line, can be enough to cause an internal arc. In the worst case, an internal arc can occur without an external trigger.
In theory, distribution transformers don’t have an age limit. If they are constructed, operated, and maintained well, the insulation paper can preserve its quality for a very long time. However, even newly purchased transformers can fail when circumstances are bad. Consequently, if you want to replace a transformer before it fails, age is a poor criterion to use in selecting the most opportune moment.
If reliability is the only criterion, a rewinding or other type of thorough repair action can be a good alternative to an entire replacement of the transformer. This is especially the case for relatively new transformers for which maintenance measurements have shown that risk of failure has risen substantially above the average. However, in the sense of economic and environmental best practice, other criteria should be considered as well. Energy efficiency is the most important of these considerations.
The life cycle cost has to be calculated considering financial point of view, taking into account the cost of energy losses, failure risk and maintenance into account, as well as the investment cost and the residual value of the transformer at the moment of retirement.
An accurate estimation of the load losses is critical in this assessment. This requires a good prediction of the loading pattern. A sound evaluation of the risk of failure, depending on the ageing state of the transformer, is also crucial. This will require the correct interpretation of maintenance measurements. The replacement issue mainly comes down to the question whether the energy efficiency can be improved sufficiently to reduce the life-cycle cost of the transformer. As the cost of the energy losses mount up to a multiple of the investment cost of the transformer, a minor energy efficiency gain can already be enough to justify replacement.
Standards & RegulationsThe transformer converts power from one system voltage to another & for a distribution transformer, this voltage relationship, or voltage ratio, is determined by the ratio of the number of turns on the high voltage winding to the number of turns on the low voltage winding. As the alternating current in the high voltage winding changes polarity 50 or 60 times a second (i.e. frequency in “Hertz”), it induces a current in the low voltage winding that is proportional to the voltage of the high voltage winding divided by the turns ratio. As the transformer works, it incurs power (and hence energy) losses in the high voltage winding, the low voltage winding, the core steel and in the surrounding transformer tank or housing and fittings. These losses in the surrounding tank/ housing and fittings are called stray losses. The magnitude of the total losses of the transformer relative to the power throughput determines its efficiency.
There are many aspects of a distribution transformer that can be measured through the test methods as per the national or international standards adopted by different countries. Hence the need is felt to harmonise the test Standards.
Testing standards support all product standards and labelling programmes because they are the means by which product energy performance is measured and compared. Harmonisation of energy performance test procedures is a means of facilitating technology diffusion and trade objectives. Harmonised test methods can encourage trade, conformity assessment, comparison of performance levels, technology transfer and the accelerated adoption of best practice policy. For example if energy efficiencies are to used internationally in performance schemes and if transformers are to be imported/exported, it is necessary to specify the measurement accuracies (or uncertainty levels) of test methods to ensure that the manufacturer, the user and the Energy Regulator all get the same result when testing energy efficiencies of transformers. Both governments and manufacturers stand to gain from the harmonisation of testing methods.
Benefits to governments include:

Advertising