Transformer
When choosing a transformer, there are two primary concerns:
the load and the application. Several factors must be evaluated carefully while
making the choice, to ensure that the needs of both primary concerns are met.
To use a cliché, it is typically a 'no-brainer' to choose
smaller transformers. A unit with a kVA rating that is larger from the
anticipated load can quickly be picked up. But if you are selecting a large
unit for an electrical utility system, to be part of a large distribution
network, you are typically making a much larger investment; thus the evaluation
process is much more detailed and elaborate. With over 90 years of experience
in this industry, Pacific Crest Transformers has put together a quick checklist
to help you make your choice judiciously.
There are three major questions that influence your choice:
Does the chosen unit have enough capacity to handle the
expected load, as well as a certain amount of overload?
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| Transformer |
Can the capacity of the unit be augmented to keep up with
possible increase in load?
What is the life expectancy of the unit? What are the initial,
installation, operational, and maintenance costs?
Evaluation Factors
The cost and capacity of the transformer typically relate to
a set of evaluation factors:
1. Application of
the Unit
Transformer requirements clearly change based on the
application.
For example: in the steel industry, a large amount of
uninterrupted power is required for the functioning of metallurgical and other
processes. Thus, load losses should be minimized - which means a particular
type of transformer construction that minimizes copper losses is better suited.
In wind energy applications, output power varies a great extent at different
instances; transformers used here should be able to withstand surges without
failure. In smelting, power transformers that can supply constant, correct
energy are vital; in the automotive industry, good short-term overload capacity
is a necessary attribute. Textile industries, using motors of various voltage
specifications, will need intermittent or tap-changing transformers; the
horticulture industry requires high-performance units that suit variable
loading applications with accurate voltage.
These examples serve to underline that type of load
(amplitude, duration, and the extent of non-linear and linear loads) and
placement are key considerations. If standard parameters do not serve your
specific application, then working with a manufacturer that can customize the
operating characteristics, size and other attributes to your needs will be
necessary. Pacific Crest regularly builds custom transformers for unique
applications.
2. Insulation Type
(Liquid-Filled or Dry Type)
While there is still debate on the relative advantages of
the available types of transformers, there are some performance characteristics
that have been accepted:
o Liquid-filled
transformers are more efficient, have greater overload capability and longer
life expectancy.
o Liquid-filled
units are better at reducing hot-spot coil temperatures, but have higher risk
of flammability than dry types.
o Unlike dry type
units, liquid-filled transformers sometimes require containment troughs to
guard against fluid leaks.
Dry type units are usually used for lower ratings (the
changeover point being 500kVA to 2.5MVA). Placement is also a crucial
consideration here; will the unit be indoors serving an office
building/apartment, or outdoors serving an industrial load? Higher-capacity
transformers, used outdoors, are almost always liquid-filled; lower capacity,
indoor units are typically dry types. Dry types typically come in enclosures
with louvers, or sealed; varnish, vacuum pressure impregnated (VPI) varnish,
epoxy resin or cast resin are the different types of insulation used.
Liquid-filled types: choice of filler material
The choice of filler material is usually based on factors
that include temperature rating of the transformer, mechanical strength of the
coils, dielectric strength of the insulation, expansion rate of the conductors
under various loads, and resistance to thermal shock of the insulation system.
Liquid-filled types: temperature considerations
Using fluid both as an insulating and a cooling medium,
liquid-filled transformers have rectangular or cylindrical forms when
constructing the windings. Spacers are utilized between the layers of windings
to allow the fluid to flow and cool the windings and core.
Within the sealed tank that holds both core and coils, the
fluid flows through ducts and around coil ends, with the main heat exchange
taking place in external elliptical tubes. For transformers rated over 5 MVA,
radiators (headers on the top and bottom) are used for additional heat
transfer. Modern paper insulation in liquid-filled units allows a 65ºC average
winding temperature rise.
Dry type: temperature considerations
Dry type insulation provides dielectric strength and ability
to withstand thermal limits. Temperature rise ratings are typically 150 ºC, 115
ºC, and 80 ºC, based on the class of insulation used (see box).
3. Choice of
Winding Material
Transformers use copper or aluminum for winding, with
aluminum-wound units typically being more cost-effective. Copper-wound
transformers, however, are smaller - copper is a better conductor - and copper
contributes to greater mechanical strength of the coil. It is important to work
with a manufacturer that has the capability and experience to work with either
material to suit your specific requirement.
4. Use of Low-Loss
Core Material
Core choice is a crucial consideration, and core losses
should be determined properly. Losses that occur in the core are due to
hysteresis and eddy currents. High quality magnetic steel should be used so
that hysteresis losses are reduced; laminated cores are chosen to minimize eddy
current losses.
5. Protection from Harsh
Conditions
It is very important that transformer core, coils, leads and
accessories are properly protected, especially when used in harsh environments.
Liquid-filled transformers should be of sealed-type construction, automatically
providing protection for the internal components. For highly corrosive
conditions, stainless steel tanks can be employed. Some approaches to building
dry-type transformers for harsh environments include cast coil units, cast
resin units, and vacuum pressure encapsulated (VPE) units, sometimes using a
silicone varnish. Unless the dry-type units are completely sealed, the
core/coil and lead assemblies should be periodically cleaned, even in non-harsh
environments, to prevent dust and other contaminant buildup over time.
s, thus augmenting the load capability of the transformer as
well.
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