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TRANSFORMERS
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INTRODUCTION
Background
is tribution trans formers are very efficient, with losses of less than 0.5% in large units .
Smaller units have efficiencies of 97% or above. I t is estimated that transformer losses
in power distribution networks can exceed 3% of the total electrical power generated. I n
I ndia, for an annual electricity consumption of about 500 billion kWh, this would come to
around 15 billion kWh.
Reducing los ses can increase trans former efficiency. T here are two components that
make up trans former los ses . T he fir s t is "core" los s (als o called no- load los s ), which is
the result of the magnetizing and de-magnetizing of the cor e dur ing normal operation.
Core los s occur s whenever the trans former is energized; core los s does not var y with
load. T he second component of los s is called coil or load los s , becaus e the efficiency
los ses occur in the pr imary and secondary coils of the trans former . Coil loss is a
function of the res is tance of the winding mater ials and var ies with the load on the
trans former .
I n selecting equipments , one often conveniently avoid the concept of life cycle cos ting. But
the truth is that even the most efficient energy trans fer equipment like a trans former,
concept of life cycle cos t is very much relevant. The total cost of owning and operating a
trans former must be evaluated, since the unit will be in service for decades. The only
proper method to evaluate alternatives is to reques t the manufacturer or bidder to supply
the load and no-load losses, in watts. Then, simple calculations can reveal anticipated
losses at planned loading levels . Frequently, a small increase in purchase price will secure a
unit with lower operating cos ts .
T he load profile of electronic equipment—from the computer in the office to the
var iable speed dr ive in the factor y—dr ives both additional los ses and unwanted
dis tor tion. S ince trans former manufacturer s tes t only under ideal (linear ) conditions , a
subs tantial gap ex is ts between published loss data and actual los ses incur red after
ins tallation. In fact, test results published in a 1996 I EEE T ransaction paper
documented an almos t tr ipling of trans former los ses when feeding 60kW of computer
load rather than linear load. S lightly different practices ar e followed in US A and UK to
account for harmonics while selecting trans former s .
A guide to this guide
T his Best Practice Manual for Electric T rans formers summarise the approach for energy
conservation measures pertaining to selection, application and operation of electric
dis tribution trans formers .
T he details of des ign methodology and the varied approaches for materials, construction
are not in the scope of this manual. However, some theoretical aspects are discussed where
ever deemed fit.
Dielect r ic L osses
T his los s occur s due to electros tatic s tres s reversals in the insulation. I t is roughly
propor tional to developed high voltage and the type and thicknes s of insulation. I t
var ies with frequency. I t is negligibly small and is roughly cons tant. ( Generally
ignored in medium voltage trans former s while computing efficiency ).
Hys ter is is Loss
A sizeable contribution to no-load losses comes from hysteresis losses. Hysteresis
losses originate from the molecular magnetic domains in the core laminations, resisting
being magnetized and demagnetized by the alternating magnetic field.
Each time the magnetis ing force produced by the pr imary of a trans former changes
becaus e of the applied ac voltage, the domains realign themselves in the direction of
the force. T he energy to accomplis h this realignment of the magnetic domains comes
from the input power and is not trans fer red to the secondar y winding. I t is therefor e a
los s . Becaus e var ious types of core mater ials have different magnetizing abilities , the
selection of cor e mater ial is an impor tant factor in reducing cor e los ses .
Eddy Cur rent Losses in T he Core
T he alternating flux induces an EMF in the bulk of the cor e propor tional to flux dens ity
and frequency. T he resulting circulating cur rents depends inversely upon the res is tivity
of the mater ial and directly upon the thicknes s of the core. T he los ses per unit mas s of
core mater ial, thus vary with square of the flux dens ity, frequency and thicknes s of
the cor e laminations .
By us ing a laminated core, (thin sheets of s ilicon s teel ins tead of a solid core) the
path of the eddy cur rent is broken up without increas ing the reluctance of the
magnetic circuit. Refer fig 2.4 below for a compar ison of solid iron core and a
laminated iron core.
Fig. 2.4B shows a solid core, which is split up by laminations of thicknes s ‘ d1’ and
depth d2 as shown in C. T his is shown pictor ially in 2.4 A.
Eddy Cur rent Losses in conductor s:
Conductors in trans former windings are subjected to alternating leakage fluxes created by
winding currents. Leakage flux paths, which pass through the cross section of the
conductor, induce voltages , which vary over the cross section. These varying linkages are
due to self-linkage as also due to proximity of adjacent current carrying conductors . These
induced voltages , create circulating currents within the conductor causing additional losses.
T hese losses are varying as the square of the frequency.
For an isolated conductor in space, the varying self-linkage over the section, leads to
clus tering of the current near the conductor periphery. This is known as S kin Effect. The
same effect, with the addition of flux from surrounding conductors , (Proximity effect) leads
to extra losses in thick conductors for transformer windings. These losses are termed as
Eddy Current Losses in conductors .
T he T es t Certificate mentions the load losses, which include these eddy losses in conductors
at supply frequency (50 Hertz) as also the eddy losses in tank structure in general at the
same frequency in the case of oil cooled trans formers. For dry type trans formers, tank
losses are absent.
Ext r a Eddy L osses in S t ructural Par t s
Some leakage flux, invariably goes in air paths away from the trans former. S trength of this
s tray flux diminishes and varies inversely with dis tance. I f it links with any conducting
material, it will produce eddy losses in that material. For oil immersed trans formers , some
s tray flux links with some parts of the tank and causes extra eddy current losses in the
s tructure. These losses are absent in dry type trans formers .
S imilarly, extra flux due to outgoing L.T . conductors carrying large currents cause extra
eddy current losses in the s tructural portion surrounding the leads .
Both these losses vary with frequency 0.8 , as stated earlier.
T he above discussion on trans former losses is given only to gain familiarity with the
fundamental principles. The most important losses are core loss and copper loss. The
other losses are described mainly to give a complete picture on losses.