On load tap changer

On Load Tap Changer
(OLTC)
•As the name
suggests, permit tap
changing and
hence voltage
regulation with the
transformer during
on-load.

•“On Load Tap Changer” is used
to compensate the drop in output
voltage of high power
transformer, drop in output
voltage from no load to full load
in know as voltage regulation.
•Flow of current in “LV Side”
cause available voltage to drop
due to inherent resistance of the
winding.
Quick Review of Voltage Regulation
On Load Tap Changer
(OLTC)

On Load Tap Changer
(OLTC)
•Load on consumer end of
transformer is never fixed, it
varies at random, its higher value
is matter of concern.

On Load Tap Changer
(OLTC)Quick Review of Voltage Regulation
•Light Load; Load may be low due
to fact that much of the time not
all the connected load remain
“on”.
•Though at low load there is slight
drop in voltage at the load end
but not matter of concern up to
certain level.

On Load Tap Changer
(OLTC)
•High Load; when so many
connected load gets on same
time high current is drown out
from transformer to drive all.
•Heavy magnitude load current
cause considerable voltage drop
in internal resistance of
transformer windings.
•Considerable drop in voltage
affect all the acting load, output
of each load get reduces due to
availability of less voltage then
desired.

1
2
3
•Tap changer is used to
compensate for this, usually
connected on the HV
winding for two reasons:
1. Because the currents are
lower, the tap changer
contacts, leads, etc., can be
smaller.
2. HV winding is wound outside
the LV winding, it is easier to
get the tapping connections.
On Load Tap Changer
(OLTC)

1
2
3
•Drop in voltage on LV side is
compensated by reducing
number of turns of HV side,
selector arm is moved from
tap 1 to tap 2 to reduce
number of turns.
•Turn reduction increase the
per turn voltage of HV side
results in increase of LV
terminals voltage.
On Load Tap Changer
(OLTC)

1
2
3
•Tap changer is used to
compensate for this, usually
connected on the HV
winding for two reasons:
1. Because the currents are
lower, the tap changer
contacts, leads, etc., can be
smaller.
2. HV winding is wound outside
the LV winding, it is easier to
get the tapping connections.
On Load Tap Changer
(OLTC)

Selector
switches
Bypass
switch
Vacuum
switch
Reactor
coil
Detail Insight of OLTC
1. Selector Switches
These switches select the physical tap
position on the transformer winding.
2. Reactors
During each tap change, there is an interval
where two voltage taps are spanned.
Reactors (inductors) are used in the circuit
to increase the impedance of the selector
circuit and limit the amount of current
circulating due to this voltage difference.
3. Vacuum Switch
During each tap change the duty of circuit
breaker is performed by vacuum switch.
4. Bypass Switch
This switch operates during the tap changing
sequence but, at no time, does it make or
break load current.
High Voltage
Winding
On Load Tap Changer
(OLTC)

Selector
switches
Bypass
switch
Vacuum
switch
Reactor
coil
•Change in tap position during on load is not
a quite simple process, proper steps must be
followed to do this without causing sever
transient and damage to the windings
High Voltage
Winding
On Load Tap Changer
(OLTC)

Step 1
•tap changer selected to tap 1, all
switches in home position.
On Load Tap Changer
(OLTC)

Step 2
•“Selector Switch” arm moving from Tap-
1 to Tap-2.
•“Vacuum switch” open, makes right part
of selector arm isolated.
•Due to moment of selector arm the
“Bypass Switch” stay at home position
and get only in contact with left part of
selector arm, isolating right part.
On Load Tap Changer
(OLTC)

Step 3
•Isolated part of “selector arm’ comes in
contact with tap-2 which is relatively at
lower voltage than tap 1.
•“Vacuum switch” remain open till this
position.
•“Bypass switch” remain only in contact
with left part of “selector arm”.
On Load Tap Changer
(OLTC)

Step 4
•“Vacuum Switch” get closed, tap 1 and
tap 2 get short.
•Current circulates through closed loop
formed by “Selector Switch” arms due to
voltage difference, instant rush of
current is counter by reactor coils in both
arm .
•“Bypass switch” remain only in contact
with left part of “selector arm”.
On Load Tap Changer
(OLTC)

Step 5
•“Vacuum Switch” remain closed, voltage
of turns from tap-1 to tap-2 ceased at
line voltage.
•“Bypass Switch” switch now easily slide
from left part to right part.
•No spark/discharge occur as vacuum
switch is closed in parallel.
On Load Tap Changer
(OLTC)

Step 6
•“Bypass Switch” gets completely in
contact with right part of “Selector
Switch”.
On Load Tap Changer
(OLTC)

Step 7
•“Vacuum Switch” get opened, right part
get isolate.
•Current continues through right part in
to the main winding, Main winding’s
turns between tap-1 and Tap-2 gets out
of context.
On Load Tap Changer
(OLTC)

Step 8
•Left part of “selector switch” can left
tap-1 without facing discharge/spark.
On Load Tap Changer
(OLTC)

Step 8
•“Selector Switch” completely rest on
Tap-2 after leaving Tap-1.
This way tap changing happens, following
a right sequence, same sequence must
be followed for selection of next tap i.e
Tap-3, tap. Tap-4 can’t be selected by
jumping over Tap-3.
Reverse sequence must be followed to
when moving in reverse direction i.e.
from 4 to 1
On Load Tap Changer
(OLTC)

On Load Tap Changer
(OLTC)

OLTC Connections
On Load Tap Changer
(OLTC) R Y B
A
B
C
High Voltage Winding connection without OLTC

OLTC Connections
On Load Tap Changer
(OLTC)
R Y B
A
B
C
•High Voltage Winding
connection with OLTC
•Connections of winding A and B
with OLTC and Bushings

OLTC Connections
On Load Tap Changer
(OLTC)
A
B
C
R Y B
•Connections of winding B and C

OLTC Connections
On Load Tap Changer
(OLTC)
A
B
C
R Y B
•Connections of winding C and A

OLTC Connections
On Load Tap Changer
(OLTC)

OLTC Connections
On Load Tap Changer
(OLTC)
Thank You

On load tap changer

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