Page 38 - Energize August 2021

TECHNICAL

High efficiency distribution transformers

by Mike Rycroft, Now Media

Transformers are significant loss-contributing components in the power grid and distribution transformer
(DT) losses are estimated to account for almost one third of overall transmission and distribution losses.
Improving the efficiency of distribution transformers is a priority in many countries.

he DT is the most common fixed component in the South determined by measuring the losses under no load (i.e. with an open
African grid, with Eskom alone having approximately 300 000 circuit secondary) and full load (short circuit) conditions. The open
Tdistribution transformers in service. If we assume a similar circuit test determines the core losses, which are independent of the
number for municipalities, there could be upwards of 600 000 DTs load, and the short circuit test measures the resistive losses at full
in service in this country. Seemingly small percentage point load current. Efficiency is calculated from these results. Efficiency
improvements in DT efficiency can reduce losses significantly. varies with load as shown in Figure 2.
After transmission and distribution power lines, distribution The long-term efficiency of the DT will depend on the load
transformers represent the next highest source of losses in a profile and how this matches the efficiency curve of the DT. To
utility’s electrical network. Distribution transformers are relatively take the load variation into account, the term “all day efficiency”
easy to replace (in comparison with power lines) and their is used. This is the average efficiency over a typical day for a
efficiency can be easily measured and labelled. Higher efficiency particular transformer. The all-day efficiency is defined as the
is however associated with higher price and the savings in energy ratio of energy (kWh) output over 24 h to the energy input over
need to more than offset the increased cost. Taking life cycle cost the same period. Although this may be informative, it does
into account, the specification and installation of high efficiency not provide a good basis for comparing efficiency of different
transformers can be an economically sound investment despite the products and setting compliance standards. For this reason, most
higher purchase price. 2 efficiency performance standards specify efficiency measured at a
percentage of the nameplate rated load. In most cases this is 50%
Standards but in some cases 35% is used.
Unlike other parts of the world, Africa has no mandatory standard Since the distribution transformer does not supply the rated load
on energy efficiency for the distribution transformers which could for the whole day, the all-day efficiency of such a transformer will
support the realisation of these cost saving potentials. Many be less than ordinary or commercial efficiency. For determination of
countries around the world have established minimum energy all-day efficiency of a transformer, it is necessary to know how the
performance standards (MEPS) for transformers. These include the load varies from hour to hour during the day. Not only the maximum
European Union, China, Mexico and USA. Other countries have efficiency but the shape of the efficiency curve is important for all-
introduced comparative and/or endorsement labels on a voluntary day efficiency.
or mandatory basis. When setting MEPS, countries usually follow Higher all-day energy efficiencies are achieved by designing
IEC 60076 test methods. distribution transformers to yield maximum commercial efficiency at
South Africa follows SANS 780: Distribution transformers, which less than full load (usually 50 to 75% of full load). This is achieved
specifies the no-load and full-load losses for DTs. The standard has
been adapted over the years to raise efficiency requirements.

Efficiency measurement
The efficiency of a transformer is defined as the ratio of the output
power to the input power. As the efficiency varies with the load,
commercial efficiency is normally specified at full load and is

Figure 1: World MEPS for distribution transformers 3 Figure 2: Efficiency variation with load 1

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