Anand Jain, Hod EE
Govt. Polytechnic College, Hanumangarh
Transformer
Construction
Transformer
Construction
Core-Shell
Transformer
Construction
Core-Shell
Anand Jain,Hod EE
Govt. Polytechnic College, Hanumangarh
A transformer consists of two electrical isolated windings coupled through a
magnetic medium. When one of the winding is connected to ac supply of one
voltage level, it can produce alternating supply of the same frequency but different
voltage depending upon the turns ration. The two major types of construction of
transformers (used in transmission and distribution of electrical energy) are core
type and shell type. Depending on the application, these transformers can be
classified as distribution transformers and power transformers.
The most important function performed by transformers are,
 Changing voltage and current level in an electric system.
 Matching source and load impedances for maximum power transfer in
electronic and control circuitry.
 Electrical isolation.
The standard ratings of oil immersed, naturally cooled 3 phase 11 kV/433 250 V distribution transformers shall be 10, 16, 25, 63, 100, 160, 200, 250,
315, 400, 500, 630, 1000, 1250, 1600, 2000 and 2500 kVA
The transformer either single or 3-phase, usually consists of the following elements:
a)
Magnetic circuit, consisting of limbs (core), yokes and clamping structures
The core support the winding and it offers a low reluctance path to the magnetic flux. The core is a
stack of cold rolled grain oriented annealed silicon steel insulated lamination. Both sides of 0.3 to 0.5
mm thick laminations are coated with hot oil proof insulation to reduce eddy loss. 3 to 4% Si content in
steel increases its resistivity to eddy currents and reduce hysteresis losses.
CRGO laminations are cut at an
angle and core leg and core yoke
laminations are interleaved in
mitred joints. This reduces No
load losses, No load current &
Noise level. The permissible no
load current is only 3-5% of full
load current.
Core is bolted together and to the frames firmly to
prevent vibration or noise.
b) Electric Circuit: A standard transformer will have
two winding sets which are insulated from one
another. In the winding construction single HV coil
is wound over LV coil.
In high voltage winding, large number of turns of
thinner cross section is used. Low voltage winding
has thicker conductors and has fewer turns than its
high voltage counterpart.
HV and LV windings are wound from Super Enamel
covered /Double Paper covered, aluminium /copper
conductor/ foil 100 kVA and below and only copper
conductor/foil above 100KVA. Current density for HV
and LV winding should not be more than 2.8 Ampere
per sq mm for copper and 1.6 Ampere per sq mm for
Aluminium Conductor.
LV winding shall be such that neutral formation will be
at top.
c) dielectric circuit, consisting of
insulation in different form and used at
different places in the transformer,
namely: core to LV, LV to HV, etc.
Transformers normally use
cardboard and insulating paper as a
means of isolating both primary and
secondary winding from each other
as well as the transformer core. Inter
layer insulation is epoxy dotted
Kraft Paper/Nomex and pressboard.
All spacers, axial wedges / runners
used in windings are made of precompressed Pressboard-solid. In
cross-over coil winding of HV all
spacers and all axial wedges /
runners are dovetail in shape.
Heavy cellulose sheets are used as
insulation between two phases.
Another insulating material comes in the form of
transformer oil. Axial & Radial cooling ducts in and
between sections of the windings allow the free flow
of oil around the conductors.
In addition to dissipate heat due to losses in a
transformer, insulating oil provides a medium with
high dielectric strength in which the coils and core
are submerged. This allows the transformers to be
more compact, which reduces costs. Oil cannot retain
high dielectric strength when exposed to air or
moisture. Dielectric strength declines with absorption
of moisture and oxygen. These contaminants also
deteriorate the paper insulation. For this reason,
insulating oil is prevented from contacting air. A
conservator with breather is used to pressure
variations resulting from thermal expansion and
contraction of insulating oil.
The two major construction of transformers are
core type and shell type.
CORE TYPE TRANSFORMER
In core type transformer, the magnetic core is built
of laminations to form a rectangular frame and the
windings are arranged concentrically with each
other around the legs or limbs. The low voltage
winding is wound near the core and high voltage
winding is wound over low voltage winding away
from core in order to reduce the amount of
insulation and increase life of it.
SHELL TYPE TRANSFORMER
In shell type transformers the windings are put
around the central limb and the flux path is
completed through two side limbs. The central
limb carries total mutual flux while the side
limbs forming a part of a parallel magnetic
circuit carry half flux.
CORE TYPE
The top and bottom horizontal
portion of the core are called yoke.
The yokes connect the two limbs and
have a cross sectional area equal to or
greater than that of limbs. In a coretype transformer, half of the
primary winding and half of the
secondary winding are placed
round each limb to reduce the
leakage flux.
.
SHELL TYPE
In shell type transformers the cross
sectional area of the central limb is
twice that of each side limbs.
CORE TYPE
SHELL TYPE
1.
Easy in design and construction.
Comparatively complex.
2.
Low mechanical strength due to non-bracing of windings.
High mechanical strength.
3.
Reduction of leakage reactance is not easily possible.
Reduction of leakage reactance is possible.
4.
The assembly can be easily dismantled for repair work.
It cannot be easily dismantled for repair work.
5.
Better heat dissipation from windings.
Heat is not easily dissipated from windings since it is
surrounded by core.
6.
Best suited for EHV (Extra High Voltage) requirements.
Not suitable for Extra High Voltage requirements.
7.
Economic and popular in use.
Used in welding or high current transformer where more
bracing support to winding is required.
Our electric power transmission and distribution systems can deliver generated a.c.
energy over long distances economically and efficiently only because of extensive use of
transformers. As the transformers are such vital links in the network they must be very
reliable especially in conditions of lightning and overload including occasional short
circuits and switching surges.
Transformer design is optimized to minimize
manufacturing and operating cost of losses for utility.
The two important design part of transformer are
magnetic core frame and windings.
Core is built with Cold Rolled Grain Oriented low loss
silicon steel laminations.
Winding is made up of high conductivity & soft drawn
E.C. Grade copper or aluminum. Enamel conductors are
double paper insulated. Windings are designed to fulfill
mechanical, thermal and electrical requirements.
Insulation is required in a transformer, wherever a difference in potential exists between two points.
Solid cellulose insulating materials and
transformer oil are mostly used in
distribution transformers. Main and most
important insulation between High and low
voltage coils are pressboard, separator or
cooling ducts. A Pressboard represents a
thick insulation paper made of extremely
pure cellulose fiber, suitably treated in the
manufacturing process and then compacted
at very high pressure. All spacers, axial
wedges / runners used in windings are made of
pre-compressed Pressboard-solid.
Diamond Dotted Press-paper (DDP) with
epoxy resin layer being used to insulate
adjacent layers of conductors or foils.
Heavy cellulose sheets are used as insulation
between two phases.
d=diameter of circumscribing circle
D=distance b/w centers of adjacent limbs
H=over all height
W=length of yoke =D+a
Hw=height of window
Ww=width of window
a=width of largest stamping
Hy=height of yoke
D=d+Ww
H=Hw+2Hy
Width over two limbs = D+ outer dia of hv
Width over one limb = outer dia of hv
d=diameter of circumscribing circle
D=distance b/w centers of adjacent limbs
H=over all height
W=length of yoke = 2*D + a
Hw=height of window
Ww=width of window
a=width of largest stamping
Hy=height of yoke
D=d + Ww
H=Hw + 2Hy
Width over two limbs = 2*D + outer dia of hv
Width over one limb = outer dia of hv
Width of core = Dy= b
height of yoke Hy= a
length of yoke = W = 2Ww+ 4a
over all height =H = Hw+ 2a