Transformers Handbook 2014

18 / 88

1
16 Transformers + Substations Handbook: 2014
As the electrification of Africa continues, choosing the right
component is critical if the result is to be cost effective and
efficient.
Transformers are essential for the transmission, distribution and utili-
sation of electrical energy. They are used in virtually every commercial
and industrial building, from the service transformer that reduces dis-
tribution voltage to a more usable voltage for buildings to step-down
transformers that serve individual floors, to small transformers for in-
dividual equipment. Transformers can be expected to operate for 20
to 30 years or more.
Over such a long life span, the operating cost of a transformer can
greatly exceed its initial price, so selection of the right transformer for
economic performance involves examining the unit’s capacity (size)
and efficiency. In this context, efficiency means looking at the core
steel and the winding material.
Transformer losses
In simplest terms, transformer losses comprise core losses (also called
no-load losses) and coil losses (called load losses).
Core losses originate in the steel core of the transformer and are
caused by the magnetising current needed to energise the core. They
are constant, irrespective of the load on the transformer, hence the
term ‘no-load’. They continue to waste energy as long as the trans-
former is energised. No-load losses vary depending on the size (kVA)
of the transformer and the core steel selected;
hence the emphasis on proper sizing.
Coil losses, or load losses, originate in the
primary and secondary coils of the transformer and
are a result of the resistance of the winding ma-
terial. This is where the selection of copper wind-
ings can make a difference.
Proper sizing
Transformers are sometimes installed in advance
of occupancy, so the engineer does not necessar-
ily know the load that will be placed on the unit.
As the installer is often not the party paying the
electricity bill, there can be a tendency to oversize
the transformer capacity relative to the load it will
see. Since the no-load loss is a function of the kVA
capacity of the transformer, careful selection of
transformer capacity, appropriate to its intended
task, will ensure the lowest core loss.
Energy Star (TP-1) transformers may
not be efficient enough
Energy Star, an international standard for energy
efficient consumer products, originated in the USA
where it was created in 1992 by the Environmen-
tal Protection Agency and the Department of En-
ergy. Since then, Australia, Canada, Japan, New
Zealand, Taiwan and the European Union have
adopted the programme. Devices carrying the
Energy Star service mark generally use 20 to 30%
less energy than required by federal standards.
The Energy Star label is applied to transformers that meet a certain
minimum standard for efficiency, known as the National Electrical
Manufacturers Association (NEMA) TP-1 [1]. This standard is intended
to promote the manufacture and use of energy efficient transformers
by establishing minimum efficiency standards, albeit with certain built-
in assumptions. It contains a simplified method for evaluating the initial
cost of transformers along with the costs of core and load losses. It
also presents tables of minimum transformer efficiencies based on
kVA size, voltages and liquid or dry-type.
Unfortunately, there is nothing particularly efficient or cutting-edge
about transformers that meet TP-1. Yes, they are an improvement on
so-called ‘standard’ transformers, which are still made and sold widely.
However, many transformers are available from various manufacturers
that exceed the efficiency levels of TP-1, and can provide a faster
payback of their purchase price.
Proper transformer sizing and
copper windings
By E Swanepoel, Copper Development Association Africa (CDAA)
Visit
Copper Development
Association Africa
The Copper Development Association Africa
(CDAA) has represented the local copper
industry in southern Africa since 1962. Its
head office is based in Johannesburg and,
on behalf of its members, the organisation is
committed to promoting and expanding the
use of copper and copper alloys throughout
Africa.
We should never forget that copper remains an important and expensive
component of a transformer. However, losses within the machine have an
associated cost and it is useful to understand the trade-off between the
initial cost of the copper versus the cost of the losses over the lifetime
of the machine.