Winding technique

• Just like oil filled transformers , there are varieties of windings in cast resin transformers too.

• Reliability & ease of manufacturing is aslo to be considered while deciding on any winding method.

• Major types in cast resin transformer windings methods are..• Low voltage winding

• Foil winding using thin strip• Layer winding by using Rectangular wires.

• High Voltage winding • Foil disc winding • Layer winding by using Round/Rectangular wire

Winding technique

LV Foil winding:• It uses a single piece of thin strip as conductor. Width of the foil is equal to length

of the winding.• This conductor strip (foil) wound along with a pre-impregnated interlayer

insulation. During curing process foil and insulation are bonded together to form a rigid block. The LV coil becomes self supporting against forces.

• Later the coil is impregnated with resin for environmental protection.• Eddy losses in the thin foil are very less compared to thick rectangular wires,so

windings made with foil are cooler than other types.• Foil winding minimizes the axial forces during short circuit , since currents in the

winding are adjusted to the ampere turns in the high voltage winding.• Is simple introduce cooling duct without decreasing the mechanical stregnt.• It is easy and quick to manufacture this type of winding.

Winding technique

Winding technique

Al cooling duct

Terminals complitely welded on all conductur height

Bushings fixed to clamps

Line terminal

LV layer winding:• Multiple strands of rectangular wire is used in this method.• Conductor thickness is generally high to limit number of strands. So eddy

& stray losses are high in this construction.• Transposition of conductors is mandatory to avoid the circulating currents

in the windings. • Transposition is a tedious and sensitive process. If not done carefully inter-

strand faults cause dielectric failures and hot spots in the winding.• Manufacturing the multistrand layer winding is complicated.• Winding is not balanced very well with the High voltage winding. Axial

forces present during short circuit conditions. • It is not a self standing construction, requires support by cylinders.

sometimes it is casted with moulds.

Winding technique

Layer Winding

Winding technique

Foil Winding

HV Foil disc winding:

• Even distribution of voltage along the height of the winding is important for High Voltage winding.

• Foil disc winding is a winding technique which achives this requirement.• Thin foils of required width are wound with interlayer insulation between

them. Each layer is a turn , so voltage stress is minimized.• Total turns are devided between many discs , so voltage gradient between

discs is also low.• This can be made on automated machines , which gives accuracy &

speed.

Winding technique

HV Foil disc winding:

Winding technique

HV wire winding:• Several winding methods are followed in wire winding • Multilayer winding , random winding , wire discs winding etc. are some

methods.• Generally round wire/rectangular wire is used for winding based on the

rating.• Whatever the method followed , uniform voltage distribution is not

possible in wire type windings. • This affects the impulse withstand strength.• Voltage stress increases progressively towards the end of layer.

Winding technique

Winding technique

Random Wire discs Foil discs Multilayer

Foil disc winding Homogeneous distribution

Wire disc winding Non Homogeneous distribution

Winding technique

What TMC Follows: • TMC follows Foil disc winding technique conisdering its uniform voltage distribution.

Winding technique

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Impulse distribution

Series1Series2

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• Example of 1600kVA transformer 36kV class.

• Impulse level 170kV

• Maximum voltage between discs , in case of impulse , 17kV with 14mm of resin.

• Breakdown voltage of TMCRES resin 40kV/mm at 20°C

Unfilled System:• Unfilled system , as the name suggests, does not use any type of fillers. Resin and

Hardner are the main components.• Unfilled system is used where very low viscosity is a requirement .

Ex: very small coils used in electrionics • Unfilled system can have smaller gaps in the coils , since the resin has low

viscosity.• Casted block in the unfilled system is manufacture with glass Fibres. • Fibres needs to be completely impregnated for achieving the desirable dielectric &

mechanical strength. • Fibres , if not impregnated properly , causes voids in the coil that results in dielectric

failures.• This system can be used with good results up 12 kV class. When the voltage stress

is not very high.

Filled Vs Unfilled system

Filled System:• Filled system means , resin & hardner are added with fillers like

quartz/silica/aluminium hydroxide etc.• Fillers provide

• Flame retardency, self extinguishing & arc resistance• Increases Thermal conductivity • Increases Thermal shock resistance• Reduce the shrinkage• Improve adhesion to the conductor • Brings the coefficient of thermal expansion more close to conductor

material.• Adding fillers increases the viscosity. But viscosity can be reduced by

warming the resin mixture.


What TMC Follows:

• Filled system (or) Unfilled system is to be decided based on the product requirements.

• TMC uses Filled resin system to fulfill the product requirements as mentioned above.

• TMC products has C2-E2-F1 certificated.• TMC supplies transformers suitable for storage upto -50° C , which proves

the strength of filled system.• Insulation material used in the filled system are self sufficient for dielectric

strength. Impregantion by resin is not required.


Impregnated windings :• Other names for this construction are Openwound/Openventilated

windings/VPI/VPE• Does not require any moulds for winding. Flexibility in design.• The winding construction is simillar to disc windings of oil filled

transformers. • Windings are immersed in resin and then cured. Resin coating on

conductors is less than a milli meter.• provides clear view of the windings. Fault location is easy.• Heat dissipation directly from windings. • Strength aganist short circuits is poor. • Interspace between winding discs is air. Limited dielectric strength due to

presence of air between discs.

Impregnated Vs Cast Windings

....contd• Since windings are exposed to atmosphere around them , dust collection

on surface of conductors is possible.• Mositure accumaltes on windings , if transformers are stored. • Transformers can not be switched on immedialtely after long shut downs.• Partial discharges possibility as air is the insulating medium.• Open wound transformers are not conisdered as maintenance free.



Impregnated windings Filled Cast windings

Unfilled Cast windings

Filled Cast windings :• Unique feature of cast resin transformer is conductors of HV winding are

completely encapsulated in a cast resin block which has a smooth surface. • They require moulds to encapsule the windings.• Uses round/rectangular wire (or) Foil as the winding conductors.• Can use both Aluminium/Copper conductors.• Foil disc is most widely used option for HV winding.• Windings are made on mandrel ,moulds are fitted, Resin is filled and then

cured.• Cast resin coils are non hygroscopic.Transformer can be switched on after

long shut downs.


...contd• Cast Coils are perfectly mositure proof , dust proof & has very high

reliability.• Very strong against short circuits as windings are encapsulated. • Heat dissipation surface area increases due to the encapsulation of the

windings.• Low Partial discharges since gap between disc is filled with resin.• Cast resin transformers are practically maintenance free.• Cast winding conductors are not visible to naked eyes. • Repairability of windings is not an issue , if a transformer fails it is never

repaired by an user.


What TMC Follows:• TMC manufactures HV windings using mould • LV winding is made with AL/CU Foil. Mould are not used for casting. They

are Impregnated.• Being low voltage , LV winding does not require casting • Insulation used in LV winding is pre impregnated with resins. During

curing of LV winding insulation and conductor forms a rigid block. Later LV winding is impreganted with resin for environmental protection.

• Such LV windings are suitable for installation under tropical or saline environmnets.

• If customer requires cast LV winding , TMC supplies.


• Both are used as conductors of electricity & long history of being used as winding conductors for transformes.

• Copper is the preferred conductor during initial days of transformer manufacturing, due to it’s ready availability.

• AL usage became prominent during world war2 due to non availability of Copper & improvement in metullargical processes of Aluminium.

• We can achieve same performance parameters by using Allumninum (or) Copper windigs.

• We can compare both conductors by• Electrical Properties• Thermal Properties• Mechanical Properties

Copper Vs Aluminium

Copper Vs Aluminium

Electrical Properties:• Copper has lesser resistivity. To achive same ohmic losses 1.5 -1.8 times

higher cross section of Aluminium to be used.• Core cross section to be increased to offset the volume increase due to

Aluminium conductor.• Specific weight of Aluminium is much lesser than copper. So weight of

Aluminium is 50-55% of weight of copper.• Increase in core weight is compesated by reduction in conductor weight.• AL transforer will have almost equal weight as the copper transformer.• Volume is slightly higher than the copper transformer. (5-10%)• We can achieve same performance paramenters like noload/load losses,

Impedance , temperature rise of windings using AL conductor.

Copper Vs Aluminium

Thermal Properties:• Melting point of Aluminium (665°C) is considerably lower than of Copper

(1085°C). • The service tempearature of cast resin transformer is 155°C for class F

and 180° C for class H transformers.So,Melting point is not of importance.• Thermal conductivity of Aluminium is lower than copper.But while

calculating thermal gradients between winding and ambient air whole coil is take as a block. So heat dissipation does not depend solely on the Thermal conductivity.

• Surface area of Aluminium is higher than Copper for same losses. Moreover, specific heat of Aluminium is more than copper.

• Higher surface area and higher specific heat makes Aluminium coils cooler than copper coils for same losses.

Copper Vs Aluminium

....contd• This is an advantage for longer life of insulation and also during short time

overloads. • Incase of resin encapsulated transformers , it is very important that the

co-efficient of thermal expansion of conductor and resin block needs to be close. If they differ too much, cracks develop during varying load cycles.

• Aluminium co-efficient is close to the resin mixture, which makes this conductor more suitable for cast coils.

Copper Vs Aluminium

Mechanical Properties :• Conductors in the transformers are stressed during the fault conditions.• The forces generated during faults are expressed as compression , tesion

and bending between coil supports.• Breaking strength of Aluminium is lesser than the copper. It is

compensated by the higher cross section of Aluminium used for same current.

• Further High voltage windings are encapsulated in the resin block. Hence , the mechanical strength is provided by the resin.

• For Low voltage Foil windings, using preimpregnated insulation and curing it subsequently makes the coil as a rigid block to withstand forces.

Copper Vs Aluminium

.... contd • Aluminium windings can be designed to have same mechanical integrity

as of copper windings.Other Considerations:

• Larger surface area of the Aluminum windings improves the series capacitance , which is favourable for Impulse voltage withstand.

• Improvements in jointing techniques has made Aluminum to easily and reliably join with copper (or ) other Aluminium conductors.

• Price of Copper is very much volatile and ~ 3,5 times of the price of Aluminium.

Copper Vs Aluminium

What TMC Follows:• It can be concluded that medium power transformers upto 20 MVA can

be manufactured with Aluminium windings without any difficulty.• Aluminium windings are default for TMC.• Aluminium winding transformers are cheaper by 50 - 60 % for same

performance.• Copper winding transformers will be supplied by TMC , if customer

requires them.

Copper Vs Aluminium

Thanks to everybody

Winding technique

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