transformer trouble , test , drying , parallel operation
TRANSCRIPT
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8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
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Transformer Troubles
a)
Star-siar
transformers.
-
It should
first
be
noted
that
in
the
case
of three-phase
transformers,
ofnecesiit),.
in
ph4ge
in
alllblgg_lh3 9s
at
the same
time,
and
hence
in
the
case ofstar
connected
windings
no third
harmonic
voltage
can
exist
between
lines,
although
they
are
present between
each terminal
and
the
neutral
point.
Similarly,
the
third
harmonic
components
of
current
can
only
exist
if
a ieturn
path
is
providdvia
the
neutral
point. Thus
if a star-star
connected
transformer
has
both
the
primary
and
secondary
neutral
points insulated,
no
third
harmonic
currqntf
san
flaf
L
rr:sultirrg
in
the
presence
of
triple
frequency
voltage
between
the
secondary
terminals
and
the
secondary
neutral
point. The
line voltages
will
be sinusoidal,
but
the
neutral
point
wi11, instead
of
being
at zero
potential, osciilate
by
an amount
equal
to
the
third
harmonic
component.
If,
however,
the
primary neutral
point is connected
to
the
generator
neutral,
a
path
for
the
flow of
the third
harmonic
component
of the
magnetizing
current
will
be
provided
and
the wave
of
flux
and induced
voltages
on
the
secondary
side
will
be
fiee
from
harmonics.
Similarly,
if the
secondary
side is
used
as
a
three-phase
four-wir::
s,rzstem,
a path
for
the
flow of
a
third
harmonic current
will
be
provided
via
the
load,
and
in
this case
both
third
harmonic
currents
and
voltages
exist
on
the seconciary
side.
Because
ofthis
objective
feature
star-star
connected
transformers
are seldom
used.
If,
however,
connected
in
a
tertiary,
or
stabilizin.g
winding
is
provided on
the
delta,
a
local circuit
i
Such an
auxiliary winding
can also be
used
to
supply power
for
local
putposes
at
a
voitage other
than that
of the
primary
or
secondary
windings.
The
minimum
capacity
of the stabilishing
winding,
apart
liom
any
consideration
of
the supply
of
external
load
currents,
is determined
by
the third
harmonic
compQnqnl-oflhe
-qlaglslzi]u
current required.
A stabilishing
winding
also introduces
additional
reactance
eflect
between
the
incclming
and
outgoing
lines,
and serves
as a leactor
on
the
system.
Star-interconnected
star
transformers.
-
This
is a
useful
type
of
winding
as,
due
to
the
interconnection
between
the
two
halves
of
each
phase winding
on
the
secondary
side,
the
third
harmonics
ofthe
voltage
cancel
out,
so
that
no
third
harmonic
components
of
(b)
{"'dgx-:*
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(c)
voltage
can
exist between
the
terminals
and
the
secondary
neutral
point. This
allows
three-phase,
four-wire
unbalanced loads to be supplied
without the
objectionable
presence
of
circulating
currents. In spite
of this, however,
the
possibility
of
neutral
inversion still
exists
with
this type of
comection.
Delta
connected
transformers.
-
Transformers
which
have
a delta connected
winding
on
either the
primary or the secondary side or both,
provide
a
path
for
the
flovv
of
the
requisite
third
harmonic
component of
the magnetizing
current,
thus relieving
the
supply no-load
current of
the
necessity
of supplying
this.
Delta-star
connected
transformers are the
most common,
as
the secondary
will
provide
a three
phase, four-
wire system
free
from third
harmopic
voltages
between
line
and neutral.
Star-delta
transformers
are
only
suitable
for balanced
loads.
The
primary
current
is sinusoidal,
and a third harmonic
circulating current
flows in the secondary
delta
connection
without
appearing
in
the secondary
lines.
The
presence
of
this
third
harmonic
circulating
current
coffects
the flux
curve
to an approximately
sine shape,
and thus
suppresses
the
third
harmonic
induced
e.m.f.
A condition
of
balance
between
third
harmonic magnetizing
current, and an
approximately
sine
shaped flux
wave
is
thus
established.
In the
case
of
a
delta-delta connected
transformer,
the
third
harmonic
component
of
the no-load magnetizing current
is
shared between
the
two
windings
depending
on
their
respective impedances.
If
third
harmonic
currents
are
present
in an
abnormal degree,
they
may
prove
objectionable,
due
to the
increase
in
transformer
or cable
losses. W}'ere
third
harmonic voltage exist
on
the
secondary side,
they may result
in
heavy cable
charging
currents,
due
to
the reduced
triple
frequency
capacitive reactance
of
the
cables,
while
in the
case
of condensers
for
power
factor improvement
excessive
currents
may
be
taken
with
resultant
overheating,
which
has been
known to
cause eventual
breakdown
of
the
insulation.
Some
telephonic
disturbances are also traceable
to
third
harmonic
pressures
in transformers. It
is also interesting
to
note that
third
harmonic
components
will be
prominent in switching in current rushes, where
paths
for their:
flow
are
provided,
due
to
the
saturation
of
the
iron
circuits
during this transient
period.
t.
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Sglg
lntp15[4
rlspects
of
-
:,-:
,,..,
Il44lformers
.
,,,.,.
What
is
transformer
Noise?
'i.he
main
source
of
sound in
a
transformcr
is
a
magneto-strictiorr. Tliis
is thc
properly
of
eiectr.ical sheet steel
whicir oi lrses
it to
elongate and
contract
in
the
preselce
ol
the
n.ragnetic
field. These
periodic
mechanjcal nlovements produce
sound
vibrations
in the
core,
having
a
ftindamental freciuency
equu) to
twice the
flux
frequency.
l)ue
to
a non-linear
relationship
between n.ragneto-striction
and
llux
ciensity a
number 1b harmonically
related
irigher
frequency
are also
produc.':d.
This results
in
a
noise
spectrum,
composed
of fr"tndamental
frequencies
of
120 cycles
per
second and
harmonics
of
240,360,480,600 etc.
The
pulsations
of
the
core also
travel
through other
parts
of
the transformer,
which have a
tendency
to vibrate
at one or other
ofthese
same
frequencies.
These
add
to
the
overall
sound
level
in
the forrr:
of
interlaminar core and
coil
vibrations,
enclosure,
and
sub-asscmbly
vibrations.
The
amount
of
thi'r
rLddition
is dependent
upon the surfaoe
area and resonant
frcquency
of
eac,h componeni
part.
Holir to control
transformer
sound and
minimize
its transmission
-
As
outUned
above, there
are four main sources
of
sound
in
a
transformer:
i.
Core
vibrations
2.
Interlarninar
core anci
gap
vibrati,rns
and coil vibrations
3.
Subassembly
vibrations
4.
Enclosurevibrations
I,Iever
Make Megger
Tests
uuder Vacuum
or Without
Oil
The
great
the vacuum
(r'nless
it's
perfect)
the
Iower the spark-over level. At
tlre vacuum
values
normally
reached in
li:r: fi.:ld,
it
is
very likely the
megger
learlings
.,vi11
L;e
zero;
in
other u,urds,
rreeping
or
flashitg to
gouncl
will occur.
Breakdown strength
under vacuuur is ir
function
of both
pressure
and distance.
It
is not
our intent at
this poirt
to
go
into
a
long discourse
on the
phenomenon
of
voltage
Lreakdown
but
simply to
CAUT-IO|'I
yott
not
Io
megger
a
unit
while
under
vac;uum.
It
is
also
recommended
that you
do
nct
rnegger
a
unit in
air
because
the values
may
lead
to
wrong
conclusions and will not
agree
with
readings
taken
at
a later
date
when they
are
taken
under
oil.
Readings taken
in oil
are
about ll2to
ll4
that of the insulation resistance in
air.
Evaiuation
of
Insulation
Megger
readings,
power
factor
readings,
and
oil dielectric
breakdown values are
inlpofiant
indicators
as to
whether
or not
a
traxsformer
is wet.
The
dielectric
strength o{
oil
for high
voltage
oorver transfonrers musi
be 30kv
or belter before it
is
put
into
a
new
unit
in
the ficid.
The megger
readings on high voltage power
tlansformers,
when corrected
Io
20"C, should be
1
megohm
per
kv
or higher.
.'
Whal is traflsfor
er Noise.doc
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o
On
EHV
transformers
a
minimum value
of 3 megohms
per kv is
required.
Several
one
minutes
readings
should be taken
with
a
motor
driven
megger
and
the
megohm
results
oornpared
rvith factory
data. Power
factor
should be
less than
2% on
oil-filled
transformers
q,itl.r
two
windings
and
less lhan
2-1l2lo
on
units
with load
tap
changers when
corrected
to
20"c
In
sul ation
There
is not
much
that
can be done
in
the
way of
maintenance
of
the solid insulation
of oil-
insulated
transformers
and regulators,
but
correct
load and
temperature
control can
do much
to keep insulation
aging
within normal limits. Aging of insulation
is
affected
by
the operating
temperature
of the
windings and
the
condition
of the oil
in which
it
is submerged.
Oil
of high
acidity
will
accelerate
again
of
the
solid
insulation.
The
aging
rate
of
organic
insulation
doublre.for approximately
cach
8"C increase in its
temperaturejthe
lower
the
operating
temperature,
the longer
the insulation life.
Since
aging
is
so much
more
rapid at
the higher range
of temperature,
excessively
high
temperatures
should
be
avoiJed as
much as
possible.
On
the
other
hand,
little
is
gained
by
having
insulation
life
exceed the obsolescence life ofthe
apparatus.
Maximum
life is assured
only
under
uniform,
normal loading. Therefore, loading
should
be controlled
as much
as
possible to
obtain
that
objcctivc.
1'he Secondary
of
a
current transformer (CT)
should not
be
open-circuited
under:rny
circumstances.
Why?
If
the
CT secondary
becomes
open
circuited,
a
high voltage
will
exit
across the
secondary
because
the large
ratio
of se,,ondary to
primary
turn causes the
transforrter
to act
as a step-up
transformer.
Also,
since thc
counter ampere-tums
ofthe
secondary
no longer
exist,
the flux in
the
core, instead
of
being
due
to the
difference of the
primary and
secondary
ampere-turns,
will
now
be due
to
the total
primary
ampere-turns
acting
alone.
This
causes a large
increase
in
the
flux,
producing
excessive core
loss
and heating,
as well as
a
high
voltage across
the
secondary
termjnals.
Ovei-excitatioir
Capacity
and
Inrush Current
The
extent of
which
the transformer can be overexcited
is
determine
by the highest
permissible
flux
density.
With
the
increase of flux
density
the
no-load
current and
its
harmonics
rise,
and
also
the
losses
in
the
in
the
iron
cor:e,
the
windings
and
the
structural
parts.
The
flux density
is
proporlional to
the Voltage
applied
and
inversely
proportion
to the
frequency
of
that voltage.
The applied Voltage may
deviate
permanently by
J:5%
from
the
rated
voltage
and
the frequency
may deviate permanently by
+2.5'y.
from
the
rated frequency.
When
transformers
are
switched
on, an increased
current of
exponentially
decreasing
amplitude
may
appear.
The magnitude
of
this inrush
current
depends
on the
instantaneous
value
of
the
voltage
and
the
residual
magnetism
of
the
transformer
at
the
moment
of
switching
on.
what is transformer Noise-doc
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*a-
#l
In
the
500
to
1,000
kvA range,
the
amplitudes
of
the
inrush
curents
decay
to
half
their
original
vaiue
after
8
to
10
cycles.
With
units
of
1,000
to 10.000
kVA
this
decay
takes
10 to
60
cycles
and
with
larger
powers 60
to
3,600
cycles
Overload
Capacity
Transformers
can
be
temporarily
or
pelm',nently
overload,
depending
on
the
initial
ioading,
the temperatures
of
the
cooling
media
and
the
permissib:e
maximum
values
of the
winding
temperatures.
If
the
permissible
windings
temperatures
are exceedecl,
the
normal
lile
of
the
transformer
is
shofiened,
unless
the oveiloading
is
compensated
by
periods
of
lower
load.
If
transformers
are
to
be
overload,
it is
important
to
check in
advance
whether
all
parts
of
the
transformer
and
the
equipment
are capable
of
meeting
the
demands.
made
on
them
(for
instance,
bushings,
tap-changers
and
cable end-boxes).
Table
below
contains
representative
values
for
the period
of
overloading
of
oil-immersed
transformers
in relalion
to
the
previous load
and
the
oil temperature
at
the
beginning
of
overloading.
The
averago
winding
temperatures
do not
exceed
105"c
if
the
average
temperaturE
of the
cooling
medium
during
the
period
in
question
is
25oC.
If
transformers
with
F-cooling
are
operat-d without
fans,
the
overloads
and
the
previous
permanent
loads
should
be relatedlo
600/o
ofthe rated
power
ofthe
transformer.
Transformer
Noise
The transformer
noise
is
majnly
genelated by the
magnetostriction
qf
the
iron
core.
It is
dependent
on
flux
density.
changes
of 10%
in
flux
density
cause
of
change
of2_to
3
dB
(A)
in
the noise
level within
the
12
to
16
kGs
range.
The radiated
sound
spectrum
is between
100
and
500H2.
Table:
Duration
of Overloading
of
Oil-immersed
Transformers
Previous
continuous
loading
as
Yo of
rxled
power
Oil t
nr
p
c ralLr re at the
L,.ginning
of the overload
in
'C
for
cooling
medlod
tlrrratirin ofovcrload
lor an overload,as
oZ
of
raied
Po\Yer,
ol
SC, FC, WC
10%
200/. 30%
40%
5OVr
h
h
mtn
mln
mln
50
55
49 3
1.5
60 30
l5
75
68
60 2
1.0
l0
t5
90 78
68 I
0.5
15
8
4
I
VN|L"'
V{% Vt t
Y
{r:t
what
is
transforner Noise.doc
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No
-Load
Losses of
Transformers
The noload
losses
is the
loss measured
by a wattmeter
when rated
voltage at rated
frequen^cy
is applied
to either
winding, the
other winding
being open-circuited.
It includes core
loss, I2R
loss
in
the
winding
due
to exciting current,
and
the dielectric
loss in
the
insulation. TlTis
no-
load loss
appears in
the transflormer,
independent
olthe
load and
for
all
practical
purposes
is
independent of
temperature.
The
core
loss
consists
of hysteresis
loss and
eddy-current
loss.
The hysteresis
loss
is thc
energy
required to
magnetize
the core
first in one
direction
and then
in
the
other.
It increase
directly
with
the
frequency
and
depends on
the maximum
flux
density
and
the
quality
ofthe
core
steel.
The
eddy-current
losses
are
produced by eddy
currents
which are
set-up
in the core
in that
iron
is
a
conductor
ol
current as-well-as
magnetic
flux.
The change offlux
in the core
induces
a
voltage
in
the core
which
causes
eddy current
to flow.
This
loss is
proportional
to
the
square
of
the
eddy current
or
the
square of the
induced
voltage and
is
therefore
proportional
to the
square
of the
flux
density
and
the frequency.
Exciting
Current and
Flux
of
Transformer
The current
which flows
under no-load
conditions
is called
the exciting
current.
'fhe
exciting
current
is
actually
made
up
of
two
components.
One component
supplies the
losses
in
the
core caused
by hysteresis
and
eddy
curents,
and
the other
component
produces
the
magnetizing
force
in
the core.
The
alternating
current voltage
in the
primary winding
of
a
transformer
produces
a
quantity
termed
"FIux"
in
the
core.
This oscillating
llux
is
what
causes the
voltage to
be
produced
on
the secondary
winding
ofthe transformer.
Load
Losses oI
Transformer
'fhe
load losses are
the
losses measured
by
a
wattmeter
when sufhcient
voltage
is
applied to
one
winding to
produce full-load
current
in
the
other winding
which
is short circuited.
The
load loss consists
of
the
I2R
loss
in
the
winding
due
to full-load
current
passing through
the
windings
and a
stray
loss
which
is produced by
stray
flux
inducing
currents in
the winding.
The
stray
losses can
be reduced
by
)aminating
and
transposing
the copper
in the
windings.
The stray loss
may vary
lrom 10 to
20
percent of the
I'R
loss at
full-load
current.
Both losses
are affected
by
changes
in
temperature.
The I2R
loss increases
with
temperature,
while
the
stray
loss decrease
with temperature.
The load
losses of a transl'ormer
ate
guaranteed
on the
basis of
an average
winding
temperature.
All losses
are dissipated
in
the
form of
heat and
must
be
controlled
if
the
transformer
is
to
be efficient
and
practical
in
design.
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8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
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DIELEC
TRIC
ABSO
*--li;
?i
i,.(haoral
A2LI
'fhllo
an
lnsuLatlon
noggor hae
boon
used
for
many
yoars,
thero aro
stlII
nany uE-
bqn
faotore
1n ltg
use
for
moasurlng
the
quallty
of
lnsulation,
espaolally
that
@
h.rgo
genorators.
Thoro
aro
somo
quostlong as to tho
bsst voltage
to
bo usod;
tho
offoot
of
tho
tomperature
of
tho
wlndlngs,
tho
longth
of tlne
roadlnge
should
bo
takon
and
tho froquonoy of
tho
roadlngs,
to what
oitant
duat aBd
dirt
on
the
vladtags
ntght
affoot
tho roadlngs,
and
flnally,
Yhat
should
bs
coasldorod
tho
lcnror ihfts
of usofulness
rond.o .lng
tho replaco:nont
of insulrtlon
advtsablo"
Yftrilo
thors
le Do
cLear-cut
agroomont
on how
to interprot spot
neggior roadlngs,
tho ln-
dratry ic la
egreement
that
perlodlc
roadlngs,
tekon ovsr
a courso
of
yoars
uuder
:
rldlar
couditions,
will
give
a
truo
plcture of tho dotorlorablon
of the lneulatlon
;f;.ud
oau
bs used
to forotell
whsn rewlnding
or
re-lnsulatlon
w111
be noaded.
Ths
t,l*ro
o" loso
aocepted
nsthod
of
rooording
thls
porlodio
Lrformattou
1g
in tho fono
,'lr.o
a dlolectrlc
absorption
curveo
Jlhs rords
rdielectric
aloorptionrt rofor
tq-t-h9,.abq-o1p o3
o
f shargo
ls.
tho.lnsula--
ho rords
"d1e
Ioctric
a_b
s_o_ fllo-n-
_"
ral ar
-l_q_l-_hg,
-aD
q-ofPllofl
-9-r
snar8o I'u.
troe
-
r.nEurn--
193.
bu_t__ sjLeloc.ldq.gb-s-o-qp-t-l.o-n-o-urve
is
u
aually-plcbted..wlth
lnsulatlon
valuea,
ia..Sad
fion
a
mo,ggor,-.p1otto
d
aga{,11g --tlne.-
T-h-l-s-is-.rqq,-b1- ,i1.s--Lrye.r
se-qt-c-{l-
..road
fron
a_ms-Bgorr_
Plo_ttod_aga1:l1s -_ 1nlg.-
:t-t1-1_8_r8-
{oa,r_ly
_ ,n-_Llry9.r
80-ql-q-ql:-
fLt-qqtr:g_a
sof
p-t
io:e.-curvelt-n
th;t it
will
have
a
gradually lacroaslng charaoterlstlo
lOotrlo
abi-orpt
1o]1 curve.r, -n
that it
wlll
have
a
Bradually
lncroaslnB
charaotgrl.s
,
Gh-".rp";f
to
trru r""toh*
3f
u.
gradually
decroaslng.lrrlact3rrltlo
which
would
bo
tho
case
lf
oharging
or'rrabsorbodrr
current
were
plottod
agalnst
tlme"
tAbrgllggl-gg5rglt
lg-y,g
curront
whlch
flows
whon
a constant
poJenttal
ilLtnpr9ls
B9l
u
tho
Instran
or Yolra
Eqr\aPPl
lcallou
at a-varfabG-
ra{e
,
-Adiffi[.fae--on=._
ffd-"66rAEE
eE
o?6F a
c
on
siadtatfe-Ten
ati on
i-varlab1e
factors" Ij_:_q_lr*P_91."d
-
Although,
as
statod
ProviouslY,
roDt
ney
peroist
for
hours,
tho
a
nuoh
ehortar
tims.
Tho sloPe
tbo
roslatanco
at 10
nlnutos
to
fitraffi
ti-mila
c
to
r s
r
-cE-pa
cf tanc e
cur re
nt
.
die
I
o
ot
r I
c
ab
s
orp-
t@apaElEin6i--curro{rstnstrgouirej-t-o-ShaTEo
fEiipliltanoe
of iho
maETno
r,rfnding
to tho
core.
Dloloctrlo
absorptlon
or
polari;lng
current
ls
a curront
caused
-by
tho absorption.of
e]
ectr
lcal.
-
charge
tn
ibo
dinl"it"t",
in
thc
forrns
of
tho oI{igning
of dipolos
or
charged
polar
molocules
throughout
the lnsulatlnS.rnatorial and
the
ptllng of
froe
oloctrons
on
the
lnter-
facosi Loakago
ourront
is oompoood
of
the currant
that
pasies
lnternally
through
tho lnsuLatloi
and
that whioh
passeo
or
loaks
along
the outsldo
.of
ths
insulatLon.
0f
these
dlfforont curronts,
the capacitanco
curront
ls
of
appreoiablo
valus
for
.
oEly
a
fsw
seconds
after
the voltags
ls
appllod,
and
tho Loakago
curreDt
remalns
.oou;tant
as
long
as the voltage
is applled"
Tho
shapo
of
tho
dioloctrlc
absorptlon
./our"g"
thorsfore. ia Indicative
of
tho rats
of flov
of diolectrlo
absorptlon our-
Y
ront.
whloh
hae a
docroasi.ng
oharacterlstic
but with
a much lonor
rats
of
chango
than
has
tlio
capacltancs
current"
In
faot,
in a
good,
dry lnsulatlon
thls
currsnt
nay
docroaso
gradua).Iy
over
a
por.lod
of
ssvsral
hours"
The
rate
of
docreaso
of thls
ourrent
anC,
Joo"uqrr"r,tly,
tho
shapa
of
the
disloctrlc
absorptlon
curvo
ls
usuolly
nors
lndlcatlvo
of
tho
quallty
of the
lnsulatlon
thau
tho
actual valus
of
oirmrg
aoacurod
for
th6 varlous
polnte on
tho ourvo.
For
th18
roason
lt
18
excoedingLy
lnportant
for
the
data
for
tho
curve
to
be obtatnod
under
alnllar
oonditlons
oach
tlno
a
partlcular
pleoe
of
equlprnont
ls tosted.
tho
gradual
decrsaso
of
dloleotrlc
absorptlon
cur-
charactoristic
of
ths curvo
becomeg
eetabllshod
in
of tho
ourve, orprossed
1:r
ths
form
of
a ratlo of
.
..
tho
rasistanco
at oD
rainute,
is
called the
polariza-
tloqlndox
(P.
,-)-
and
ls ueod
ln
comparing
and
aualyzlng
tho
curvos"
-
8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
8/35
fl
I
LZ-2
P"I"
=
lnsulation
rssistanco
8.t I0
minutos
Insulatlon
rosistanco
at
1
ninute
A do-pclarizsrtion
or
diachargo
cur.ve
for
a
liivon
pioco
of
oqutpment
would
rese
tho aLoorptlon curve
in
revorseo so
to
speak"
Consoquontly
an,oqual-
amount
of
1e
roqulrid
to d18char6o
a
glvon
pleco of
equipmont
&ftor
a
voltago
has
booa
appl
a"
*a,
corsu*sd
in
building
up
tho charge.
Thls
is
an
oxooodlEgly
lmPortant
oo[.
sldsratlon
ln
tostlng
oqui-pnent.
llfhan
severaf
tosts
qrs
mado
at
dlf,feront
ture.s
e.nd voltagoo
as requlrod
for
our standard
testa it-ig
aliolutely
osasntlal
that
the
wi.ndln[s
bolng
testod
bo
grounded
botwoon.runs
for
F
at loa
I
Slnoe
ths
dogroo
of
polarizatton
of
the ineul.atlon
of
equipnent
imodiatoly'aftei
ehut-down
and
Just
procedlng
a tost
cannot
bo
known,
it
18
advlsabls
that
to
bo
teetod ohould
be
grounded aa
soon
as
praotloabls
and
tho
ground.
bo
loft
u.
touohod
througilout
the
porlod
of
proparation"
If
thts
porlod
18
ahort,
lt
1o
to
ths timo
through
whioh tho volta6o
wa; apPllsd
in
the
provious ruu'
procautlon
io
tak6n,
the
fo1).owlng
tost
will
havo
no valuo,
beoau6s tho
dls
,rLtt
lo
partlally polarlzod st
ths
boginnlng
of
the toot"
advlgable
to
oheck the
wlndlng
to dstormlne
whsthsr
the
groundlng
timo
hae
bsoa
sufflc 6nt
to drain ths
lnsulatlon
oomplotely.
Th18
cal1 bo
dons
by
oonnocttaS
tho
wlnclLtg
under
to8t to
tho
nogger
Lnstrumont
wlth
the
motor ds-onorglzed
aud
notlng
if
iho
lnetrumont
pointor dsflocte
up-scalo
boyond
lnflnlty"
Another factor
which he,s eonslderablo
effoct
upon
tho
dteloctrlc
aboorptlon
chargo
on
the
windlng,i
to touch
the
wladln6;l
oircuits
whi1o.
tho
to
If
the
gonorator.
or
soon
aa
1t
ls
shut
and
any
spot
mogg;or
readlng
ls
tho
tomperature
rjf
the
dielectric"
Fornulas
havc
boen
dove-loped,
-iowever,
*it},
*tioh
fairlyaccurate
tamperaturo
coofflcients
of
reslstancc
can
be
developod
for
insulations
providing
tost data
aro avallable,
takon
at
two
differeni
tomporatures"
I
.Jtn
matrng
dlElectric
absorptlon
tosts
any
abnormality
tn readln6s
should
bs
ln-
-vestigatid,
for
a
serious
break-down
of
insulation
may
havo
takon
P1aco.
Safoty_
Pi
ocayt]ons
Tho
2,500-volt
moggor
builds
up
a relativoly
high etatic
this
chargs
should
bo dischargod
before
anyono
is
aLlowod
Likowise,
no
one
should
bo
allowod
to
work
oa
tho
excitor
is
in
pr ogr"o i; ri
"
Pre
l ini
rnar-y
Opcrations
Tho
follovring
Proli-r:lina::y
operations
should
be
performed"
othor
eqlripmJ,li
iu
i., sorvico
they
should
bo
porformod
as
1.
Havo
tirc
grounci
switch
closed
to
drain
off
any
-chargo
on
tho.
Btator
wlndlngi
It
is
goo d
prlctico
to
loavo
tho
noutral
OCB
cLosod
throughout.
the
ehut-down
poriod
and
to
maintain
a
Eround
on
all
windings
to
bo tostod
throughout
the
pror
P0r1(Ju
urru
parato.y
poriod
boforo
tefts
aro
mado.
hrrthormoro,.if.thore
t:."1{
P::ilti^-
ivhsther-
tho
dlainage
tj"ms
hus'boen
sufficient,
tho
winding
should
bo
tesl:9
{9tif
rosidual
ehargo.
This
is
dono
by
connecting
iho
windlng
to
tho
mogEor
rrlth
tho
^oiot
a"-or',u. trua.
If
the
inst;mont
polnier
deflocts
up-scalo
boyonil
i:cflnlty'
tho
winding
should
bo grounded
for
additiona]
tirno
'
2"
Remove
gonoro.tor.fieId
slip-ring
bruEhos
the
main
exciter
armatur
and
plIot
exciter
on
the
stator"
.
Jfti
and
ground
this
fleld.
AIeo
and
keop
all
g.roundod
during
ground
tho
to
)
-
8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
9/35
_ _ .
.
,tktzhl/5)+)
r2-1
.
:
t
s,
J
;
f,
).
Ctuo"
tho
stator
and
its connecting
vrindin8s
of all lightning
arrostors and
:
potonti.al
transforners,
and
disco:rnoct
tho pc,ror cable
as near to
tho
generator
as
poso1b1e.
If
ar.y appreciabls omount
of cablo ls lofb
connocted
in
the circuitl
the
nogger
roadlnga
v"l"II
bo
influoncocl
by
thj-s cable,
r"..td thelr
5611
not
be trus
roslstivity
reaclings of
tho
stotor.
Tho
length,
size,
and
type of oable
should
be
rocorded
on
the data
sheets.
Also,
if
tho cablos
termlnato
outslde
ths
bu11d1n8,
rot-I'and
dry-bulb
temperahrres
must
be oltained.
tl.r
[.
flIpe
all
p]aces
n'hore
coblos
ter:rLj-nate
and cond.uctors
go
to open
rviring.
.t
:,i,
lhars
is
ahvays moro
or J,oss loaka5e
ovor the
end.s of
the
cables
to
the
shoaths
at
l,:,il
ruoir
placos,
and
wiping
the dust
cieposi.ts
will
reduce
tho nurnber
of
).oakago
patho.
[]['
Cto*ing
the
wlndings
befcro tho tost is
of
qusstionablo value,
but
it is
deslrablo
ili:
to
iroo. all
porcelain
insulators
wipud off
that aro
on
the circuit
being
moggered.
[it
fi,ft
includes
tho noutral onrl
as
weIl as
the
lino
ond
of
the
winding.
Tho
fact
$Ij:
tU"t tho
windings woro or wore not cleanod
should
be rocord.ed.'
If
the
vri:rdlngs
[l]:
aro
cleanea,
rnake
tests
al'i;or cleaning
only.
),.
i
Test
Procsd.rrro
.,/.
i,.,
---
\l1?-
t..
i...
0onorator Stator
h:'
--
iii'
.&to
negger
reacllngs
at
[O'
C.
or
as
near
this
tomperaturo
as
practicable.
Two
il.
rota of
ieadings
aio
to
bo
rnader
tho first
at
lOO
volts;'(62J
'6]-ta
if
that
is
the
Ni
rrtiug of
tho
megger),
and the
second.sot
ab
2 00 volts. Tho
:notor
on
tho mogger
[r':
rhould be
allowed
to c
on,:,
up
to
fuI]
spoed beforo
tho
ground
on
tho
st&tor
is rs-
li:ll
rcvoa.
Reaclings
should
i;o
rnads
at I -soconrl
intorva-]s
for
tho first
minuts
and
at
fii;
r;:;ir";; iilEi"ui.'r",
tho next r minutos
for
a iotal
tims
of
16
minutes. Thuo
i'
'
ttroro
are
two seriss
of
tests
and two curvos
should
be dra'an.
It
1s
essontial that
thr-,
rvindinns
irnder
test
bo
grounded
).on6
enough
botween
tosts
ii':
6-difrn-olr-TE6-iTdt1c
ti o.
-Sec
tho
f
j-rst
two
paragraphs
on
pag
Az-z
undor
i,r;,
"
rrorrllrtl.r.1r
J
uPtJ
a\"r
.|.,
l.i.
Ilolds
and
Exc
ltor
s
t'i,-
ll,
thko
suro thB.t the
vrindiugs
have
been
groundor).
for
a
len5th
ol
time suff:ciont
to
j
draln
all charp-e fron
t'J
dtelectrli.
Th.r
lcst
flivon
in Proliminary
Oporatic::s
;
(poragraph 1) should
bc rnadc if
there
is
any
rluestion.
llll(e
only
one
sot of
retldings, usin6
tho
625-?o1t
or
lOO-volt
mog6or
at
l4Oo
C'
lho
roading
intorvais
strould be
the
s8.rTro as
on
tho
stator,
but
they
should
oniy
be
ocotlnued. until
tho
curvc
flattens
out
vrhioh
is
usu811y
less
than.ten
minutos.
111
voltnoter
and
instrlmont
wiring
shou]J
be
disoonneoted.
while
megSering
ls
In
Progr88s.
,/ ,,
Y[6lviu
Bridgo
Tests
.-:-
.i
,
trdlo
bridgo
testo aro
not
. "i"'
Stator
or arnature
trind:ing
,,,i
: ftor
tho absorpti"on
tcsts.
part of
the
absorption
tosts,
the rosistanco
of
tho
ihould
be
rnoasured
with
a
Kolvin
bridge
immedlatoly
rations
I'or
.
indic
ati
ons and a
test
1or
prosence
or'&
c
-
8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
10/35
A2-4
CaDacitanco
Bridge
Te
st
Uslng
a
capacitancc
bridge,
rloasure
in66
and
giound.
Alsol
record
t)re
the
capacitance
botive
en
dissipation
factor
vrhich
.
ij
the
stator
wind-
I
i-s
also
read
on
..
tilo
capacitance
bri"d
go
.
*0uardini'.
of
Cablos
'
hiold
of
col'laj'n
Ssrorator'
r
'
It
has
bcen
founct
possiblo
to
Suard
the
tround,:
,,-
^^Lr^d
+n^h
*ha.,..
ih'ili."iirl;;";;.";;"uiiu3t
or
tno
insriration
on
the
cabros
fro4.tho
,,,,
megger
readings
on
the
;;';;-;;;
siator'
"llt'^l:
::::''nl:':::":'"tlllli*""1
Ii '[i"lll'';:li"]tt"""
;;";i';-;;";;;
shio-Iis,
ancr-:I:.::".:'::i.:::::il
*"-
tho,nesser
is
connecte
-
8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
11/35
DRYING
F
TRANSF
.fhere
are
a
number
of
ipprbved
methods
for
dry'in
z
out
the transformer
core
and
coils'
Gencrally
,lr.
i;;;'oitn-.-*-urr.rurer
should
be
foLlorved.
If
the
drying
out
process
is
carelessty
or
***.4,
performei,
great
damage
may
-result.to
the.
transformff
insulation
through
over
hcaring_
Drying
out
of
pcrver
r-.fir
rrr
.tl,ruld
be carried
out
ur,der
the
personal
sup,'nision
;;;:-;;-;ed
officcr.
,re
following
gcnci.lr
insrrucrioos
are
grr'q
for
carr)'ing
out
rli'ing
opcrelic.:n.
.l
hc
transformer
must
b
suppled
during
the
drying
operatron
with
a
quantit]. o1'hcat
equivalenl
tothclatenlh.utofv,porisaionfor-tlremoisrurctot,cremol'cd.Atnormalaln]ospheric
prcssure
watcr
*.ill
onry,rfiir.-,,piary'
it"*
insulation
aI tempsrarurc
aho.c
E00C.
J'ct
the
insulation
is
litely
to
b"
badll:Lmag"4
ii
t"-'''-
part of
it
is
heatcd
to
more
than
100"C'
evcn
for
quite a sltort
timc.
U'the
on
it
o"ttt'"'i,
it
wi
also
bc
damaged'
The
orer-heading
nill
cause
Jrarti""
and
products
of
oxjdation
will
promote
fuiher
dcterioration
of
the
oil
evsn
at
lorvcr
;;il.
fi
.ry
,tro-inlure
thc
jnsulatjon.
i'hc
rarge
of
tempcr,arure
^within
n'hich
a
transformcr
can
be
effccti ,cly
Ji"d
i.
thus
vcrl'
narro \,
the
morc
so,
zs
ferv
of
thE
mcthods
by
*,hich
it
can
be heated
gr.,.
oJo*
distributitn
of
teinperature.
it
is
for
tlds reason
that
the
operation
is
so
critical.
a
.rrying
t
"n.r,rrmr:r
sloul,J
ne'er
be le{1
unattended.
I-l
-
8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
12/35
The
temperature
limits
and
Ioads
should
be
strictly
adhered
in
ordcr
to
obtain
desired
results
*
ithout
dimage
to
translormers'
NOTE:
It
m-ay
be
nqled
that
highcr
the
prescribed
oil.
temperature'
the
smaller
the
load;
which
means
trrat
additional
blrrk.i;*g;f;."oJ
i"
required
for sim
ar
currsnts
th,n
for
higher
currents,
in
order
to
bring
the
;rr
",."rp;;;;p
to
rh.
varues
shown
in
rhc
above
table.
itisalsoessentialthatplentyofr.err(ilationbcprolide
-
8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
13/35
/'z
I
)re
Megohms
v&1
dpproxima
tely
n
,ersl}ry
with
temp"raturc
and
for
a
l00C
change
of
tenrpcrature
the
Megohms
ttt-gt
'i"i"i"
ofipproximately
2
:
I
for
example'
:
Megohnis
at
"at
'"
at
(
.1
850C
750C
650C
-
-o^
))
(-
300
600
1200
2400
r
ror
oir
circur^rri"1i,
d?.:
,?J;ff,JI'l-Jj;TJffi1l;ilT.:"#F'J':T,l^:i
iffi
ti
hottom
of
rhe
ta*
and
wtthi
hcatcrs
llncd
to
filttt
app'oxil'la'iiiit:
"lrtt't"n
wilt
ut-'"'luirtd
fo'
C\tr'
1io0
g'rllons
of
oil. Local
r..rverltceling
or
'r'"""i"i*'u"
^"iita
so
'thrt
loaiing
on
lhc
h:rl'r'
sltoulJ
nut
crcced
3
\\,/Sq.
cm.
no.
rt
oula"fi.
tii""*tt
"rtr"
"lr
"t"tJJ
gi%
"t
the
ltcatcrs
or
750(l
ncar
rlre
rr
indurSs.
Prcc:tutiotrs:
-
i)
Vent Dipc
with
single
diapfuagrn
should
bc
rcmo'u'cd
and
thc
h
-
8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
14/35
Hence
or
y lop
valve
should
bc
'iosed
leaving
the
botlom
damage
the
ratliators'
one
opened.
,'
tiii\
Normallv
the
dclivery
pipe
of the
frlter
should
be
connecled
to
lhe
bofiom
valve
i\r,'/
;r
th;i;
i";.aintaining
"'ironn
tcmperaturc
in
he
tansformer'
xi\)
ttire
extinguishers
mar
be
prorided
for
conibatirg
accidental
fir':s'
sr')
The
tap
should
b
so
set
for
passing
short
circuit
currsnt
tlut
all
the
wirdirg
is
in
':''l
;;,ri,",,
rlrat
the
entte
siilding
mly
bc
dried'
rri)
lf
mcrcwy
thermometers
are
used
special
care
should
be
taken
o
prerent
thc
bulb
l-,rcakhg
oa
.o*.qr*f
'ntt"a'"1
ltlcury
into
the
transformcr
winding'
\Iercury
rt
"r*ottttt]-'iottiJ
intUutt-'
be
used
rvith
thc
meul
shiclding
Ibr
pr.r."iittg
.r.fl
utcidtntt
lt
is normally
ricsirable
to
usc
dial
lwc
therlnometcrs
or
'\lcohol
thermometers
as
far
as
possible
r:rii)
Rciorc
rccording
insulation
rcsistancc
valucs'
thc suppl)
to
the
rrindurgs
slro'uld
,''\rr./
ir"."i.i,.a
"n-La,fr.
i..a.
discorur,.ctctl
sircc
[]rc
leads
mar
c,'ntrihutc
to
]orr
t*0.g.
obtaincd'
duc
to
their
inhcrcnt
characteristic
Drl
ing
undcr
\
acuunl:
:iismcthodcoruistsolheatingupthetranslbrrtsr.bl'shortcircuituntleroilandapplyinga
rcuum
10 removc
the
ntni't*?
;i+'it
^ttttoa
can
be
cnlplol
cd
lbr
translt-rnncrs
ntounted
in
:ong
steel
tanl:s
suirb)y
dtt;gt";';;;li:i
ttl
s:tc))'
xitir-srand
erlcmal
prcssure
rcsulting
cm
a
vacuum
of
27
inches
i' 'ott
lr4oJem
transformers
are
gcncral11'
strong
cnough
to
idrsland
this
Pressure
llowevJ'
i'
J"'U'f
i
cascs'
this
me
thod
shoulcl
r'-ot
bc
emplol
cd rvithout
-*iots
approvat
of
nlunuf"ru'io
;-"'*"
punrp
should
be
connectcd
to
thc
top
of
lilc
anslbrmer
ank.
..r:
should
bc
LrI:en
to
see
tllrt
thc
diaplLr:rgm
on
tl:c
c-rplosion
\cnt
of
thc
tlattslormer
is
mo'cd
and
thc
opsning
tt'rtilp
*itft
u'uf^iltg
platc'
lhc
transformcr
tan-L
sidcs
should
be
.soed
*'ith
some
covcring
"ti
"j'"tp'J"
fttt
oi
iimil-',nalcrial
to
prevent
unnecessary
loss
i
*;
'ii;;
;;;;,
;,;;"ili"g-;;
,adiarors
should
be
closcd
to obtair
the
deslred
tsmpsrature'
hetrarsformershouldbefieldwitlroilsuchthatthecoilsandirsulationarecompletelycovered
v
oil.
Onc
oi
thc
n'indings
ftl'V
lt f-
V
l
should
bc
short
circuited
and
3
phase
supply
ncithcr
rcater
nor
apprcciably
""'Utt'tft*
1fl"
;*fta'ntt
voltage
of
the
transformer applied
to
the'
ther
n inding
to
enable
" "t
f'flf"'i-"'*"nt
as
possible
but not
larger
than
full
load
currenf
obtaurcrj.
'fl)c
tcrnperatuc
;;;;;;t'i;
shoultl
bc
ti"J
tio*ty
t"o
approximat"ly'550C
in
::
lir.
r
8 ho*'s;
ro
o:oc
;n
*',.
:';;J;;
,
;i
u..,
zs'c
i'"t'"
tt'il0
a
noun
'tJl.,
tre
7s0c
imperatlrlc
is
reached
tf"
t"*io"tt'
should
be
disconnected
from
the
source
of
power
and ilie
il
draincd
out
and
rr;u'rm
'rifi
tJ"i"t'^"tf
itte
ll
'lhc.racuum
should
bc
a
mjnimum
of
25" but
,referably
28'.
'1hc
tutu*''
should
be
maintained
until
rempclarurc
drops
to 50'C'
lhc
raruformer
should
again
bt
l'x;;;i;
r.I"t
();r
"
600C
atul
tlte
short
circuit
switchcd
on
and
the
ycle rcpcated.
Temperaure
-JLt'f"i""
resistance
of
the
windings
should
be
laken
evcry
tryo
rou.rs
and
the
drying
process
conthued
Lntil
ihe
mcgger:iadings
risc
ovgr
the
minimum
and
are
,tcadily
rising.
-
8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
15/35
Thc
tabl,r
below
shows
the
relation
behl'een
vacuunl
and
volatilisatio[
tcmperatule
of the
water
contained
in
Lhe
oil:-
.
1''i:"
.Jil:H*
Y#*:*:"
degrees
Centigrade
oo ?a
'99.15
98.60
97.71
9s.85
e?.77
87.89
80.42
69.21
53.31
. ?I)O?'
0
75.54
7
5.35
7 5.01
71.26
i2.85
70.5
I
56.80
61.12
52.60
.10.54
23.50
0
0
5
10
20
30
40
50
60
70
80
90
100
From
the
dbo.",e
it
can
be
clearly
secn
thrt
the
eraporation
temperaturc
of
moisture
dcpends
^.iriv
,p""-,fie
degleo
of
,acuum.
If
l'acuum
is
mai,taincd
the
moisture
in
thc
core
and
lvinding
evaporatcs
even
at
very
low
lemporaturc-s.
the
tcmpcraturc
ol
thc
translbrmcr
-is
raised
t
l,
,Alfo.rour
short
circuit
oi
the
ri'i-ndings
and
oil
circulatign
tlrrough
strcant
linc
llltcr.
'l'hc
oil
medium
is
used
mainly
for
distributing
the
heat
uniforml'
throughout
thc
core-
and
rvinding'
It
l,
""..,'",v
tlrat
tlrc
oi]
should
be
pu,.
-d
hot
while
iI is
bcing
field
in.
For
this
purpose
it is
aduar..,tag"ous
to
usc
a
largc
tank
oi
suitrbl"
capacity
wlcre
the oil
is
circulared
tl,ough
strearn
line filrer
to
naintain
high
temperaturc
bc,forc
lilling
in.
Onl}'aftcr
thc
transfonncr
is
complctcll'
fillcd
n.ith oil
the
shortlircuii
should
bc sn'itchcd
on.
l-lnder
rto
circumstan::s
the
short
circuit
should
be ,*,it.h"d
on i;ither
during
thc
pcriod
rvhen
tlte
vacuum
is
being
appLcd
or
during
fillinginoperation
-
8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
16/35
Raise
the
temperah-Ee
of
thc
core
and
windings
gradually'
Before'applying
vacuum
make
sure
that
the
tank is
designed
for this
purpose'
:
I)recautions:-
i)
ii)
iii)
i1)
Vent
lpe
diaphragrn
rnust
be
removed
and
plate.
the hole
blan-ked
off
with
a
sturdY
Ilrecautions
regarding
simultaneous
short
circuit
and
oi1
circulation
have
already
;;;;
unier
fialheadtng
a:rd
the
sarnc
m-ay
be
complied'
Drling
bY
Hot
Arr
Circulation:
ln
tiris
method
tle
hot
air from
elcctric
healcr
is
blorm
to utc
Lranslbrm0r
rvindings
tluough
art
;;;il;;
towcr
part of
the
tank
and
is
allowed
to
escape
ttubugh
an opel
ng
in
the
top
cover. The
oil from
tlte
core
-a
*inairg.
should
be completely
rcmoved
before
starting
the
a.yng*.toperation.Thtrarsformershoul-ldbeloggedwithtarpauLinorothersirni]armateri3lto
pr'",',:iito*
of
tcat
and
to
assist
in kcepi-ng
t1e
whole
of
the
intcrior
tanl
at s
ch
a
temprafure
;-;;;,;;
;ndensation.
Thermu*.teri
should
be
fixed
in
suitable
places
to i.dicate
the
".pri"*.
,f
ij
the
int.t
,it
iiij
tf"
outgoing
air'
'lhe
hot
air
should
bc
supplied
by
mcans
of
a
nf"*:*
-a
electric
heaters
*hi.'h
rrlry
be"controllcd
so
as lo
regulate
thc
temperaturc
'lhe
mouth
of
the blower
at
rvhich
the
cool
air
cnten
should
bc
cor''erod
rvith
onc
or two
lal
ers
o{
clea:r
.,"",.r".ntopleventdrrstbeingdraw,niltothcrvindings.Itisncccssan,toforccasmuchofthc
;;u.d;;;r;;rsible
through
th"e
ducrs
olrhc
rransformcr
rvinding
lor
accomplishing
d{ing
to
".u""",r-ri.,
LaJIles
should
be
placcd bernecn
core
and
tenk
closing
inlen'ening
spacc
as
Inu ll
as
pcrssible.
.Ihe
remDeraluc
of
thc
tncorning
air should
bc raiscd
gradualll'
from
55cC
to
850C
in
about
24
;il;ffi;;i.,.i
r,
ss'C
ut;r11ct
till
the
end
of
d4'ing
'l
o
ersure
uniform
drying
the
volume
,i
fra"r.a
air
should
be
such
that
the
outlet
temperaturc
an
inlct
temperalure
are
as
near
as
f".riur".
During
drl,ing
out
operation,
the
insulating
material
loses
the
nroisture
contcnt
prescnt
i,,
i1'
*a
Lpr"""".-
iJ
di"l""t
i"
characte
ristio.
Ar
thc
samc
time,
the
mechanical
strcngth
is
corsiderably
decrcase
-
8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
17/35
-
.
.
jjifi;H"dilr;,G]fi'iio'
n**'"*;
of
the
airblast
exceeds
presetvalue'
nsliouldbemadeforfacilitalingautomaticcutoflofheatersincasethc
blorvcr
stoPs.
Suitable
baIflcs
may
be
p'rovided
1o
disperse
the
tsmPcrafure
inside
the
tank'
Trarsformer
may
be
propcrly
lagged
to
rnaintain
unifonn
inlct
and
outlet
temperature'
to
prev 'nt
condensation
undemiath
the
top
cover'
The
temDeranfe
of
the
incoming
air
may
be
gradually
raised
so
that
the
maximum
*d"*.;id
be
reached
in
about
24
hours'
OiI
should
5e
completely
drained
offbefore
commencing
the
drying
opcration'
6,
,,^
4,
'-
_
7
hot blast
lo
maintain
uniform
-
8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
18/35
.'-4.,.,,
h-Y
PARALI,EL
OPERATIOI\I
OF
TRANSFORMERS
General
conditions
for
the
satlsfactory
operation
of
Transformers
in
Parallel.
SINGLE
trarsformcr
may
bc
said
to
opcrate
satisfactorily
if it
carries
its
rated
ioad
without
"it
Be
dr9P, but-where
two oi
more
tsansformers
are to
operate
in
parallel
on
the
prt".ry
id
secondary
bul bars,
several
othsr
conditions
must
also
be futfilled-
Under
this
saiisfactory
opcration
also
implies
equafity
ofload
sharing
in
proportion
b
rhe
rating
each trarsformer,
absence
of circulating
currents
and apgoximately
similar
internal
power
in
each
transilomrcr.
In
order
to
satiiff
these latter
requiremcnts,
the
following
conditions
bc met:
(a)
The
phase sequence
or order
of
voltage
rise
on
the
termirals
of
the
two tansformers
which
are
to operate
in
parallel
must
be the same.
(b)
The
potarity
of
thc two
tansformers
must be
thc
same.
Two
trarsformers
might
havc
'
.
O," r-"-"
pi,..
rrqu*""
voltagc
risc
but on tcsting
out
it
might
bc
fo,nd
that
zcro
voltage was not
obtained
between
similarly
marked
terminals
on
the
to
units
due
to
oe
pota;ty
being
incorrecg
which
would
cause
the- furstantaneous
terminal
PrSssures
to
be 18O
degrees
out
of
phase.
If
zero
voltagc is
obtained
between
ail
pairs
of
similarly
maried
termhals
on
the
two
transformers
then it
is
an indication
that
the
phase
sequencc
as
well
as the
polarity
are
identical
in
each case'
(c)
The
two
tarsformers
must
have
the s:rme
p(essure
ratio
so
that
on
ope,r
ckcuit
the
' '
s"cond".1,
'iottages
are
eqnal.
When
installing
npw
kansformcrs
the
position
of
the
tapping
swilches
should
be examined.
(d)
The
angle
of
phase displacemcnt
between
the
primary
aod
secondary
vectors
mlst
be
'
'
thc
samc
for
cach
transformcr.
This
phase angle
differerice
is
a fixed
inhef
snt
(e)
For satisfactory
sharing
of the
load
cunent
the impedance
voltage
drop
at firll
load
should
be
the same
for
both
transformsrs.
This
:necessitates
the
transformers
having
the same
values
of
pe.rcentage
impedance
voltage drop,
but
not
necessarily
the
same
valuc of
ohmic
impcdan""f
fiir
latter
value dcpending
on
thc
KVA
rating
of
the
tansformer,
(f;
In
addition
to
having
equal
pcrcentage
impedance
voltage
&ops
it
is advisable
that
the
two transformsrs
iaVc-
nearly
the
same
ratios
of
percentage resistance
to
psrcentage
reactance
pressure
drops,
which
necessitates
similar
ratios
of
resistance
to
icactance in
each
casc.
If
this condition
is
not
met
there will
be a
phase displacernent
between
the
tsansformer
and the
total
toad
currents
resulting
in
different
power
factors
for each
bansformer
and necessitatin
E
a
grcatet KVA
output
than
is
acnully
required,
with
accompanying
additional
heating losses
and
Poorer
voltage
regulation.
2.
Loa'd
Sharing
under
Normal
Conditions
The
noltage
chaacteristic
of
a tansforma,
connecling
terminal voltage
with
load
curreng
will
usual-ty
bc
a
&ooping
curvc somewhat
similar
to
that
of a dircct
current
shut-wound
gcnerator'
Howevcf,
the
actual
value of
the
terminal
voltage
at
any
Particular
load
wiil
depend not
only
on
the
value'of
the imlrcdancc
voltage &op
but
the
phase
relation
of
thie
&op
to
the
apptied
voltage,
*ft.t
io
tum
will
iepcnd
on
ttre
power
factor
of
the
load. Two
transformers
which
are
to
operate
irt
prt.U"f
shotld hlve
,i-rtu.f,"
'
s:uLp'd
crment-voltage characteristics,
so
that thc
*op
in
terminal
pressure betweefl no
load
an{
iiU
Ioarl is
the
sa.:ne in
each. |1i
otiier
words,
tiiey
shouici
-
8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
19/35
-
8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
20/35
ft
.i"
a-s
usua
howcver,
for
Ere
traosformer
rr-rne-a1gc
to
b
cxpressed
as
I'crcntage
drops
at
firll
load
-t
,g
*#frai"Itt
the
"qui*r*t
tnnedance
of
the
traosformcr'
l*t
,4:pcrcentage
impedance
of
transformer
No'
1
and
17;
perc*fiageimpedance
of
rransfomra
No'
2
Thenassumingforthcprese,rtthatlrand12arethefullloadcrrrerrtsofthctransformen,Zland
;J;;hti";;pedanLs
maY
bc
exPressed
as
.-
"Y
'r
-
Il'1oo'
Thus
from
which
11
then
bccomes
and
similarlY
--
zzY
.t7-
Ir.lffi'
Zz
-TT
lz=It
z2
VI1
+
z1VI2
zzVIt
lkVA
)
x
zz
r,=r,
@iTT;E6jT;
Theaboveisvalidforbottrsingle.phasearrdthree.phasetransformers.
The
correct
Pairing
of
Transformers
Thecorrectpairingoftraruformersislargely.
-
8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
21/35
:#, .?iffih:
:ffi
I',*"Ty
;:g- r:
bue
bar,ettering
-h
ffi*T,:f**;
;;il
;,Ja;X
i:ffi*t
H,J:,,,ffi.ffi
,r,
Ph
-
8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
22/35
-
8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
23/35
R'
-
R'
=
iz
-
iir
= tt*icc
phase voltage,
Y'
-
Y'
=bz
-
br
=
twice'phasc
voltage,
B'
-
B'
=
cz
-
Gr
=
twice
phasc
voltage,
whilc
readings betwecn other
pairs
of
tcrminals
will
give
indications
of
phase
volts.
Similar results
are
obtained
for delta{elta connected
tansformer, this
case bcing
shown
in
Fig.l
.
The
secondaqr
of
the second transformer
is shown
with
each
phase
.
rwersed
with respect
to
the fst
kansformer.
the
result
is
:
complete
rewrsal
of
the
veotors
of
induced
terminal
e.m.f.s,
and
giving
180
degrees
phase
displaccrnent
between the similarly
te
nuedbus
b3rs-L',.Y', B',
as
in
the case of
ttre
star'star
,.
rl
^
x
r
t
n'
Ftc,
5
-Eff.4t
of intcrchanging
two
cxrcrnal conncctions
on
primary
sidc
of
della-star
transformcr
.
connectcd transformers.
If
voltagc ri,adings
behreefl
the
secondaries
are
to
be
made, one
phase must
first
bc
conncctd
as
thsre
is
no
cornmon neutral point
availablc. If
R',
R'
are
commoned,
thcn reading
between
Y'
-
Y'
and
B' -
B'
will
givc
twicc
normal
lino
volts, as
can
bc
seen
&om
tho vcctor
diagpms'
(c)
Phase
sequence,
and
the
interaction
between
this
and
polarity.
- Phase
scquence
is ody
a
iactor
of
importance
in
ths
parallel
opcration
of threo
phasc
tansformers'
* ofballk"
of
three single_ptnse
units
opsratod
as
a
three-pharc
group-.
It
has
besn shown
in
the
precrding scction
in
the case
of
tansformn
with
eimilarly
connected
primary
ina r""ondrry
windings
such
as star-star
tarsformers,
that
a
rgvcfsal
ofttr.
poturity on one
sidc,
although
it
will
rsnder
parallel-opcration
impossiblg
will
not
alier
ths
phasc sequance
of
thc
se&ndary
induccd
volages,
Conracrscly
a
pair oi star-star
or dclta{clta
transformers
which
haw
thc
sarnc
relative
polarifu,
but
whose
secondaries
have
oppositc
phase sequencel
'ay
F
operated
in
parauet by
rcconnecting
the
cxtamal
connections
oo
cither
thc
6
4
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8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
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primrry or thc
socoodary
sides so
as
to
produce
similar
phase
5
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/\
-,\
8r
R
Y
E
B,
B,
Y,
f
.....-E
4
1;";8,'1,:,"n"i\".11i,.lli
o1';''llEl'X'i"if
lil'
lLil3l""fl$"3i"o'""
this
property
pf
star-delta
and
delta-star
transformers
-also
makes
it
possible
to
cnrrect
rhs
cffect
of
*""*';";;;"[surtine
from
a
rcvcrsal
of
rtre
intcmal
connections
on
the
primary
lti"il'J;
ti;es'
by
a
change
in
the
ext'rnai
conncctions,
and
thus
'"'*";
;Ji;.iiiu"i
"pi'"ii"''
*ithout
alteration
to
the
intemal
connectiors'
Fig'
"
f,;;;t#J'1":'i*
ITj""#IL.I";3,*il'i
H:
#*x:6:l'g*;,15{i;13*ff"1**;e*,;1[
ffffi.j'HIffi-T"ffi'?,"J#;i
**",
"f
d'
X:
".,HlHf,'#:
;;;"
;h*"
,Jqt'*"'
o-f
the
secondary
t*-
IT
"I
I
made
the
sarne
as
o*.
,",
*".
iiitire
additional_
reversal
of
thc
secondary
ext
nal
comection
',
^"J:i"i"
tr"t
**naitr
tY
tff*1|,*
trJ";
ffiil;.",
then
be
paralleled'
Two
other
sets
io*".
t"
*otttd
out
in
a
similar
manner'
rhe
case
of
a
delta-star
transformcr
-*
*U'l
r:
::ffiLH"::"I"#t:l1l
i*.?tr^*
on
the
delta
side
of.one
ol
1"',:nu
..ftom
which
it
will
be
Iffi"i-th*'gi"ing
reverse
pol"rity'
is
shown
T-tl?
""##J'".
.i"'*"""a'"v
Hi#;imift
Poiarity
ancl
phase
sequonce
of
*-
sidc
bus
bars
by
suitabty
rlJi'ittttir'*
t"
"*t"-"r
connections
as
in
tho
Provrous
case
r:::11:1fii'::"'""ff'
T;ffi{;if
H;#i"q,Wrtr,*"n*ffi;.ffi
::
"*Tfi'..1a"^rlri"i6ov in
parallel
unless
m--ca"u
;;r;
-plmary-
ana
[:[r]m'
mex'
;:"[f"'
';r[+&l5'
ffi
'*m;:
tris ohase
displacrrnent
depends
primarily'
on
the
typ^
primary
and
secondary
i"riii"tt'*a
"
tl*
""p'bl"
of
variation
by
alteration
ot
ilti-.v
orth"
wioil[l:f#;.t'.f,,i.;
or
ssoondaf,v
sidss'
In
cadam
di--
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8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
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q.pes
of
tansforf,cr
aho,
the
nhase
rlisnflmcnl-'
rraY
bc
altcred
by
a-
rErrmgcme'[t
of
trc
cxrerral
-c;;cctionr'
which
prowides
a
vahublc
means
ol
suirabty
pairing
trmsform;;ffi;;i
u" ioi-o--to
hrw
differcnt
phasc
disolaccmcnl
In
othcr
t**
;'o.J;;-.,d
Ph*t
displacernent
b"*alt"
"ri"-v
and
secondary
ioat'Ja
"tft'g"t
"'n
otity
bc
changed
by
'reconnechon
or
#;#;;.;;;;
;;
;
io"r
m"in
i*f
F
consider
sfrich
can
be
":*-"rfii
W
a""rr"g;t
"
t1p"
ot
tt*"rormer
applicable
to
each
case'
l
:
2.
Y
,
Y
I
'.''
F,6.
9
-srar.srar
rransrormer,.shorrhjjI?.ili;:,tJ'jl:::mcnt
tcr*ccr rri'r:::)
and s"ooaJr]
9
^1
L\ c,t
Jt"]
,J
al
cl
,'-Lfl
Y'--+_+
l--
rl
^
/Y
,,H,
.,'
R'
FlG. I
-D:a8ram
shoqing
how-
incortc'l
.Polarity
of
delta sidc
of
delta'star
''-'
ir"nsfo.m-cr
m"i'ii
ii'iiita
ui
iiicratibn
of
thc
crrcmal
conncctions
s
having
zero
phase
displacement
berween
primary
and
'-'
s."ond"ry
induced
lerminal
vologes'
Transformer
under
this
headings
consist^
of
slar-star'
delta-deh4
and
delta-
intsrconnected
"t"
""*fttrnt.'"fne
case
for
a
star-star
connected
unit
is
shown
in
Fig.
s
.
and
from
ilU;;t';Garns
it
will
be
noted
that
the
secondarv
vector
of induc'ed
tj'"
;L;;i;
*"t
prt""
does
not
differ
in
angular
position
&om the
.*."p*aiog
ffi""""-
--J
trt'
th:
enmary
and
secondary
line
;J;;";;'f'B,
*i;
-
b2
are
prallel
and
have
the
same
sense'
&
Y
D
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8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
27/35
(2)
Threc-phasc
hansformen
having
lg0 degrees
phase
dilplacemcat
betwecn
pdmary
and
secandry
incladed
tirminol
vitog"s.
-
,
The
tansfomrem
belonging
to
this
class
havc
similar
typcs
of
windings
to
those
considcred
in
the
abo.ne
section,
but
in
cach
case
'
\1;
FtO.
lO
-Sur.rcvcrsc
srar lransforocr-
showing
lg0 d.8rc.s
ph.se
disolaccmcnr
bcrwccn
primary
and
s
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8/11/2019 Transformer Trouble , Test , Drying , Parallel operation
28/35
rcs
and
that
the
PrimarY
and
ffi;#*T.Hry,H"TH#:St#ffi1ffi
ffiI#:'*
Hcpcndent
'
mH :ffi#Hr,
::,fffi;":;x*s
e
phasc
dispracomcnt
bctwc
c'n
m*f:*r:****1
il",il'il".;;;ry::f
;ili::f
f; ,i',H.T:"HXffi
,;;-;-"","."i1a"r'"+
terminal
vollagc
on
eacn
and
ttrat
the
vectors
or
the
ilfi;;;;i;
I
u'
P9 ?
;"*
T*H:1"J.t#"iJi;
*:"#L'":iitrr*.*;i:ffi
. 1'lf"ffl;t""*a
6''r'
Figs
z
and
'I3
show
the
corrcsponoing
iiigr"
t
for
star-delta
and
star-interconnected
star
transformers
which
have
a
similar
phasc
displaccrnent
characteristic.
(4
)'11ree-phase
. IT*Tfi
^rHX?
"
"?,2iif;2""
r,?#;.*;Xff
':
"
phase
displacement
betweerl
['flmary
drrs
Jvvvri*r
z
D
.l
I
I
I
I
tl
l.
v
,
11
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The
Bridsh
Stendard
Specilicetion
for Power
Transformer'
No'
171
(1959)
(a) Need
for
uni{ormity.
-
Transform.ers
which
1nay
have.
to
bo
paralleled
rc
not
;fi;)i-;,fi&ia
w
o"
,"*"
ir*
-a
o"rri"re
it
is
ar
obvious
advantage
if
il#n
'*
.gr*a
-"rt
od
"f-";G
i*'l"als
a1t
rnakine
internal
connections'
which
can
bc
artopted
tty
"U
ffi"f..t**.
B.S.
1?1
(1959)
providcs
the
;;;;;;,-darfiLtior,
;
futd
-q
*Tdi"qT-ections
this
information
bcing
given
in
section
14';;Iht
"tdification
lach
tlpe
of
taruformcr
has
dcsigrratcd
to
it
three
tV
U"f"
*i'i"f
chssiS
it
according
to
its
intcrnal
connections
and
phase
Otpi.."t*t
betwesn
pti*"r'
and
secondary
induced
voltages.
Thephase
di.pl"J;;;;;J;il
bv;"1""]-face
number
so
that
0u
is
indicated
by 0.0
o'clock
aJ;;-t
by;0
"ti*f.
etc'
In
this
chapter
the
wind-ing
and
vector
diagrams
frr""
U"*
ituJted
in
accordancc
with
B'S'
No'
171
(1959)'
For
threc-ptraso
ruosfor-o,
ther"
tt"
fo*
main
groups
arranged
according
to
the
phase
displacem"rrt
U"rw"*-tt'"-ptl"'*y
'1a
*gt"i'*
induced
voltages'
The
reference
Ddo,
for
"-"-pfi'-iiai""tit 'h-"t
F:
transformer'
is
delta-delta
".*""i"4
""ties
uflder
rn"'io'g*p
l,
and
tlut
it
hal
zero
phase
displacernent'
This
tansformer
may
ut-ti't"ti'a'i"
paralleJ
with
a transformo
whose
charactsristics
are
dcfioea
-Uy-ti,
reference'Yyo,
because
although
rhis-is a
star-
star
transformer
it
rru,
"tro
,lnaer
main
group
I
and
has zcro
phase displacernent
Similarly,
a
bansformcr
of
"ft"n"t
yJf
refcrs
to
a star-delta
unit
belonging
to
main
group 3
ancl
has
"
PhJ;;i;;"*ent
corresponding
to I'0
o'clock
or
-
30"'
The
enginecr
who
is
resporsible
for
the
commissioning
or
connecting
up
of
.t
"*i"il*,
ir
well
aalvised
to
obtain
a copy
of
the
abovc
specification'
Marking
of
terminals
and
windings
'
- Capitel
l"I"'=.
-"
used
to
indicatc
the
H'V'
and
small
lettsrs
thc
L.v.
#;;;;
*rutt'
i"
*tit
book
have
besn
refsrred
to
as
thc
pdmary
and
secondary
;;G'
The
corresPonding
primary
and
secondary
winding
of
each
phase
",,
A'o
fr"*
suffx
n''
inberg.as
Ar'
Az'