TRANSFORMER
S. Kar Chowdhury
Substations Department
CESC Limited
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Elementary Transformer
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Elementary Transformer
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Main & Leakage Flux
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Core Construction:
Hollow Core Construction
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E & I Section Laminations
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E & I Section Laminations
Shell Type Core Construction
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Rectangular Cut Mitred Cut
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Advantages of Mitred & Step Lap Construction
• Mitred core joints allow efficient flux transfer
along natural grain lines between yoke & core legs
• Step lap construction further enhances efficiency
by reducing joint fringing-thus reducing core loss
& exciting current
• For rigidity & support top & bottom channels
are clamped with tie plates- arrangement known
as Flitch. Low noise & vibration.
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Core Material – CRGO - Physical Properties
• Density in gms/cc: 7.65
• Silicon Content: 3.1 %
• Resistivity : 48.0 micro-ohm-cm
• Stacking Factor:
M4 – 96 %
M5- 96.5 %
M6- 97 %
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Coil Construction :
• Cross-over Coil- Suitable for current upto 30 A.
Round section conductor is used
• Disc Coil – suitable for current in the range of 40
to 60 A. Rectangular section conductor is used.
• Helical- suitable for current range of 70 to 100 A.
• Spiral- suitable for current range more than 100 A
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Section of Crossover Coil
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Section of Disc Winding
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Spiral winding for 315
KVA Dist. Trafo.
2-layer spiral winding with 2 x 3
rectangular section conductor
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Diagram & Rating Plate:
• Capacity in KVA or MVA
• Voltage for both Primary & Secondary in KV
• Current for both Primary & Secondary in Amps.
• No. of phases ie three phase or single phase
• Frequency in Hz
• Vector Grouping if three phase system
• Type of Cooling ie ONAN / ONAF
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• % age Impedance
• No-load Loss & Load Loss
• Weight of Core & Coil – untanking weight
• Volume of oil in litres
• Type & make of tap changer
• Maker’s serial no. & year of manufacture
• Tap connection diagram
• Max. Temperature rise of oil & winding
• Name of Customer / purchaser
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Voltage Variation & OLTCs:
A tap changer is basically used to change the turn
ratio between two windings.
This ratio determines the voltage ratio and is essential
for voltage stabilisation under variable load conditions.
OLTCs normally have regulation range of +/- 20% in
steps of 1.25%. There may be a max. of 35 steps
Other applications include rectifiers & furnace in
industries, HVDC & phase shifters in Transmission
network.
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OLTCs – Main Components:
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OLTCs – Type of Regulation:
Regulating range
is equal to the
voltage of the
tapped winding
Linear Switching
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The changeover
selector sw. extends
the regulating range
to twice the voltage
of the tapped
winding by
connecting the
regulating wdg. To
different ends of
main wdg.
Changeover (+/-) Switching
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The changeover
selector sw. extends
the regulating range
to twice the voltage
of the tapped
winding by
connecting/disconne
cting the coarse
regulating winding
Coarse / Fine Switching
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OLTCs – Type of Connection:
Single Phase connected 3-Phase Star connected
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3-Phase Delta 3-Phase Auto Transformer
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Cut away section of
resistance type OLTC-
Diverter unit
Courtesy ABB
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Outer view of V type
tap changer – diverter
& selector combined
Courtesy MR
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Cut away section of
conventional
Diverter type OLTC
with Changeover
selector switch
Courtesy ABB
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Inner view of V type
tap changer – diverter
& selector combined
Model: V III 350 Y 76 10 17 3W
Courtesy MR
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OIL TAP - M
Model: M III 350 Y 76
Courtesy MR
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Principle of Operation- switching sequence
Tap –1 : Normal position
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Tap-1 to Tap-2: Step-1
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Tap-1 to Tap-2: Step-2
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Tap-1 to Tap-2: Step-3
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Step-4 Tap-2 Normal Position
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OLTC – Design Philosophy:
The switching mechanism is mounted inside a sealed
cylindrical glass fibre reinforced epoxy enclosure to
separate oil of the Diverter unit from that of Transformer
main tank- ensuring that oil affected by arcing cannot
contaminate the transformer oil.
In diverter-selector philosophy, the tap selector has two
branches. During operation only one tap branch is
energised while the other branch is free to move to the
next tap.
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An input signal to the motor drive unit makes the
selector move to the next higher or lower tap whilst
charging the energy storage device that drives the
diverter switch. The operation is synchronised to ensure
that tap selection is complete before the storage device
releases its energy to activate diverter switch.
The electrical switching sequence takes approximately
50 ms. During this time the current path is changed but
is never broken.
The transition resistances share load current and limit
circulating current during transition of switching
contacts from one tap to another.
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Switching Sequence- From tap-6 to tap-5:
Selector V – on tap 6
Selector H – on tap 7
Main contact x carries
load current
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Selector H has moved
to tap-5 in no-load
state
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Main Contact x has
opened.
Load current diverted
through Resistance &
contact y
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Load current diverted
through both
resistors.
Circulating current
is limited by Ry+Ru.
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Contact y has opened.
Load current carried
by Resistance &
contact u.
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Main Contact v has
closed.
Load current flows
through contact v.
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Improvement in Switching Technology:
Major reasons for improvement in switching technology:
Wear & tear of contact surface due to arcing.
Arcing by-products cause contamination of oil.
Tap changer manufacturers are embracing vacuum
technology as an alternative with distinct advantages:
Minimal wear & tear of contact surface due to
arcing.
Arcing by-product contained within vacuum
bottle.
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Video Show OLTC Operation:
Video Show PRV Operation:
