br>C
ontrolled

Swi
tching
Possible

B
e
nefits for T
r
a
nsforme
r
s

Application
s S
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2
Transform
e
rs
Transform
e
rs
Controlled
Controlled
S
w
itching
S
w
itching
: Programme
: Programme
1.
Introd
uctio
n
2.
Tran
sformer energization
and resulti
ng T
e
mporar
y

Overvoltages
(TOVs)
3.
Mitig
a
ti
on mean
s
4.
Reason
s for usi
ng CSS on tran
sformers
5.
Exa
m
pl
es of
c
a
s
es
where
CSS c
a
n ad
v
a
nt
a
g
eo
u
s
ly

be
used
:
a)
Integ
r
atio
n of
powe
r
plan
t
b)
System re
sto
r
ing
c)
Energization
of t
r
an
sfo
r
me
r n
e
a
r
sen
s
ible equip
m
ent
d)
Replaceme
n
t
or
co
mplete
refu
rbish
m
en
t
of CB
6.
Optimal cl
osing strategy
a)
Impo
rta
n
ce of
the
re
sidual f
l
ux
b)
Switching alg
o
rith
m
c)
Simulations
with Cont
rolle
d Switching
solution
d)
Mean
s f
o
r me
asu
r
ing t
he
re
sidual flux
7.
Me
an
s for me
as
uring th
e
re
sidu
a
l

flux
8.
Con
c
lu
sio
n
s
9
.
R
e
fe
rences S
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3
1: Introduction
1: Introduction
Transformer energization
produces
an inrush
current
whose
m
agnitude depends
on the
transformer characteristics, t
h
e
point on wave
closing
t
ime
and
residual
flux.
Inrush
current
has
a rich
harmonic
c
ontent,
among
w
hich
2
nd
, 3
rd
, 4
th
harmonics
are the
m
ost
important.
Inrush
current
harmonics
may
c
ause temporary
overvoltages
on the
system
due to resonance
prevailing
under
some
operating
c
onditions. S
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4
2:
2:
Transformer
Transformer
energization
energization
and
and
resulting
resulting
TOVs
TOVs
Because generated
temporary-overvoltages
on
the
system
a
t
t
ransformer energization
depend
on:
Network configuration
Operating
c
ondition
Transformer characteristics
Circuit breaker
characteristics
Etc.
Each
case has
to be
studied
closely
(simulations performed, etc.
) in order
to select

the
r
ight
solution and
to specify
t
he
right
characteristics
f
or associated
equipments S
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5
2:
2:
Transformer
Transformer
energization
energization
and
and
resulting
resulting
TOVs
TOVs
example
example
-
-
Net
w
o
r
k
TOV
produ
ced
a
t
t
he
en
ergizati
on
of
a 31
5
-
1
61 kV
tra
n
sf
orme
r f
r
om 3
15
kV
w
i
thou
t
c
on
tr
olled s
w
i
t
chi
n
g S
t
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6
2:
2:
Transformer
Transformer
energization
energization
and
and
resulting
resulting
TOVs
TOVs
example
example
-
-
Inrush
curren
t
produ
ce
d
a
t
t
he
e
n
e
r
giza
tion
of
a 31
5
-
1
61 kV
tra
n
sf
orme
r f
r
om 3
15
kV
w
i
thou
t
c
on
tr
olled s
w
i
t
chi
n
g S
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7
3: Mitigation means
3: Mitigation means
-
-
Reasons for using them
to reduce the overvoltages
(TOV) resulting
from the energization
of large power
transformers [1], [2], [3].
to reduce the mechanical and electrical
stresses on XFOS in order to increase their
remaining life [4], [5] S
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8
3: M
i
tigation
m
e
ans
3: M
i
tigation
m
e
ans
list
o
f
p
o
ss
ib
le
s
o
lu
tio
n
s
list
o
f
p
o
ss
ib
le
s
o
lu
tio
n
s
Use of pre-insertion resistors
Use of controlled switching system (CSS)
Generator
voltage ramping
r
estoration
procedure;
Energization
of
transformers
under
load
on the
sub-transmission side
(if possible)
.
Apply restrictions on the operating conditions
(less
flexibi
lit
y
for th
e operators, m
a
y
b
e
critical fo
r s
y
stem restor
ing scen
a
rios) S
t
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9
3: M
i
tigation
m
e
ans
3: M
i
tigation
m
e
ans
pr
e
pr
e
-
-
insertion resistors
insertion resistors
-
-
Insere
r
le
s r
esulta
t
s
d
e
s
i
mulation av
ec 8
et
14 m
s S
t
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10
4: Use
4: Use
of
of
CSS
CSS
with
with
optimal
optimal
closing
closing
strategy
strategy
A.
Importance of the residual flux
B.
Switching algorithm
C.
Simulations with Controlled Switching solution
D.
Means for measuring the residual flux S
t
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11
4: a) Importance of
4: a) Importance of
the
the
re
sidual
re
sidual
flux
flux
Optimal closing strategy for 3-phase transformers
equipped with a secondary or
tertiary delta winding was
found to be first closing t
he phase having the highest
residual flux, and then closi
ng the two remaining phases
simultaneously:
For the first phase to close,
the optimal clos
ing time is
determined by the residual
flux
relative to the
instantaneous voltage;
Clos
ing time of the two other
phases is optimal upon zero
crossing of the voltage induc
ed onto these phases. S
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12
4: a) Importance
4: a) Importance
of
of
the
the
residual
residual
flux
flux
Accurate
knowledge
of
the
r
esidual
flux
(magnitude and
polarity
on each
phase) of
the
transformers
t
o be
switched-in
is
a key
parameter
allowing
to determine
the
best
s
trategy
f
or
minimizing
the
i
nrush
c
urrent
and
thus
limit
system
overvoltages
and
transformer stresses. S
t
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13
4: b) Switching Algorithm
4: b) Switching Algorithm
The first closing inst
ant is set when the
instantaneous voltage is equal to:
() sin
*
3
2
*
=
V
v
= *
2
*
3
*
arccos
V
r S
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14
4:
4:
b)
b)
Switching Algorithm
Switching Algorithm
Graphical example of the
algorithm application
Graphical example of the
algorithm application

Pha
s
e
B


Pha
s
e
A


Pha
s
e
C
S
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15
5.
5.
Prac
tic
a
l
cases
wh
ere
CSS can advantageousl
y
be used.
Prac
tic
a
l
cases
wh
ere
CSS can advantageousl
y
be used.
5.1
I
ntegration of po
w
e
r plant
5.1.1 Ene
r
gizing at po
w
e
r
plant S/S
5.1.2 Ene
r
gizing at
integr
ating S/S
5.2
Sy
stem res
t
oring
5.3
Energization
of tr
ansfor
mer
nea
r

eq
uipment sen
sible to
sys
tem di
stu
r
bances S
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16
5.1 Integration of power plant
5.1 Integration of power plant
Powe
r pla
n
t substa
tion
I
n
t
e
gra
t
in
g
substa
tion S
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17
5.1.1
5.1.1
Energizing of transf
ormer at
Energizing of transf
ormer at
powe
r
powe
r
plant s/s
plant s/s
Po
w
e
r
p
l
ant
su
b
s
tatio
n
I
n
t
e
gr
a
t
i
n
g
su
b
s
tatio
n S
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18
5.1.1
5.1.1
Energizing of transformer at
Energizing of transformer at
power
power
plant s/s
plant s/s
(
(
Next
Next
)
)
Temporar
y overvoltages
(tov)
w
e
re observed w
h
en
energizing the step-up tr
ansformers located at the
pow
e
r plant substation (refer to fig. 1)
Protection systems may
ope
rate under such voltage
disturbances, possible dam
ages for the equipment
For the cases w
here important
TOVs
are observed, it
is necessar
y
to reduce overvol
tages
using controlled
sw
itching or pre-
insertion resistors. S
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19
Example of
Example of
TOVs
TOVs
obse
r
ved in
the p
o
wer
plan
t
obse
r
ved in
the p
o
wer
plan
t
s/
s
s/
s
during
during
energization
energization
of

of

tr
ans
f
orm
e
r
at
the
Powe
r plan
t S/S w
i
thou
t any m
i
tiga
tion m
e
ans
tr
ans
f
orm
e
r
at
the
Powe
r plan
t S/S w
i
thou
t any m
i
tiga
tion m
e
ans
Fig
1 -
T
O
V
s
a
nd
in
rush cu
rren
t
s o
b
serv
ed when

ener
g
i
z
a
t
i
o
n
t
akes p
l
ace at powe
r

pl
an
t
S
/
S S
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20
P
P
ractica
l
ractica
l
case
1:
case
1:
En
e
r
gi
zi