© abb group november 5, 2015 | slide 1 dry type transformers zaragoza aluminum vs. copper

© ABB Group April 20, 2023 | Slide 1

Dry type transformers ZaragozaAluminum vs. Copper


Index

Historical Background: Why Using Copper?

Next Step: Why Using Aluminum?

Common Reasons for Winding Material Choice

Cu / Al Comparison: Coefficient of Expansion for Conductors and Resin

Cu / Al Comparison: Electrical Conductivity.

Cu / Al Comparison: Calorific Capacity.

Cu / Al Comparison: Mechanical Short Circuit Behaviour.

Cu / Al Comparison: Thermal Short Circuit Behaviour.

Aluminium Foil Disk Winding: Summary

Conclusions


Historical Background: Why using Copper?

During the 2nd World War, Copper (Cu) became scarce and manufacturers started using Aluminium (Al) as an alternative for electrical transmission and distribution.

For transformers, up to the last 15 years, there were no suppliers of Al strip, small foil or wires.

There were no manufacturers for special winding machinery.

Only LV windings were made in Aluminium.

Manufacturers were using Manufacturers were using Cu on HVCu on HV

Because there were Because there were no no other optionother option……


Index





Cu / Al Comparison: Electrical Conductivity.

Cu / Al Comparison: Calorific Capacity.




Conclusions


Next Step: Why using Aluminium?

Aluminium is the second most plentiful metallic element on Earth, so there are no problems with supply.

Second lowest density.

High corrosion resistance thanks to its Alumina protection.

High electrical and thermal conductivity.(Higher conductivity than copper, weight for weight).

Low and non fluctuating price.

……till the option appeared due totill the option appeared due toseveral reasons that led the changeseveral reasons that led the change


Cu / Al Comparison Subscript

Preliminary reasons for winding Preliminary reasons for winding material choicematerial choice

Coefficient of ExpansionCoefficient of Expansion

Electrical ConductivityElectrical Conductivity

Calorific CapacityCalorific Capacity

Mechanical short circuit behaviorMechanical short circuit behavior


Index





Cu / Al Comparison: Electrical Conductivity

Cu / Al Comparison: Calorific Capacity

Cu / Al Comparison: Mechanical Short Circuit Behaviour

Cu / Al Comparison: Thermal Short Circuit Behaviour


Conclusions


Cu / Al Comparison Common Reasons for Winding Material Choice

Choicing Reasons TRUE FALSEAluminum-wound transformer terminations areincompatible with copper line and load cables. Properly terminating line and load connections is more difficult for aluminum-wound transformers. Line and load connections to copper-wound transformers are more relieable than those to aluminum-wound transformers.Aluminum-wound transformers are lighter in weight than their copper-wound equivalents. Aluminum-wound transformers have higher losses because copper is a better conductor. Aluminum-wound transformers have higher hot-spot temperatures because copper is a better thermal conductor than aluminum. Copper-wound transformers can be made smaller than aluminum-wound equivalents.

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Index










Conclusions


Cu / Al ComparisonCoefficient of Expansion for conductors and resin

Coefficient of Aluminium Expansion 23 (per ºCx10-6 at 20ºC) Coefficient of Copper Expansion 16.6 (per ºCx10-6 at 20ºC) Coefficient of Resin Mix Expansion 34 (per ºCx10-6 at 20ºC)

Aluminium expands one third more than copper. Some says it creates problems with bolted connections.

Using proper hardware (split washers) aluminium joints are equal to Copper joints.

Hexagonal Bolt DIN 933

Spring Waser DIN 6796

Busbar

Plain Waser DIN 7349

Hexagonal nut DIN 934

First ConclusionFirst Conclusion::

Proper Solutions Proper Solutions for Al terminals for Al terminals

and Cu cable and Cu cable connectionsconnections

(Cupal Alloy)(Cupal Alloy)


Cu / Al ComparisonCoefficient of Expansion for conductors and resin

Critical point: Thermal expansion due to temperature changes, produces mechanical stresses in systems composed of different material as HV winding.

Due to manufacturing process, these stresses are larger when the transformer is cooling down after casting stage.

As closer are the thermal expansion coefficients lower the mechanical stresses

Second ConclusionSecond Conclusion:: Tensile stress in the resin is lower with Aluminium windings.


Index










Conclusions


Cu / Al ComparisonElectrical conductivity

Aluminium (Al): 36.6 106 -¹m-¹

Copper (Cu): 59.6 106 -¹m-¹

Aluminium has only 61%

of Copper conductivity, so…higher losses???

For two identical transformers (same winding resistance and length (amps/turns), the conductor section will be inverse to the electrical conductivity.

+

-

ConclusionsConclusions:: Section increasing means that result in energy losses is the same. Costwise comparison: Lower load losses are cheaper with Al windings.

The section will be…

RCu = LCu/SCu x Cu ;RAl = LAl /SAl x Al

SAl /SCu = Cu / Al = 59.6/36.6 = 1,63

SAl = 1,63 SCu


Index










Conclusions


Cu / Al ComparisonCalorific Capacity

Aluminium specific heat (cAl): 0.220 cal/gºC Copper specific heat (cCu): 0.092 cal/gºC

Mass of aluminium design is 0.488 times the copper one:

• Calorific capacity Al = MAl x cAl ; Calorific capacity Cu = MCu x cCu

• Calorific capacity Al/Calorific capacity Cu = (MAl / Mcu) x (cAl / cCu) =

= 0,488 x (0,220/0,092) = 1,167

16,7% higher for Al.

ConclusionsConclusions:: Al windings withstand better surge and overloads currents.

Better thermal behaviour to short circuits for Al windings.

Longer working life for insulations materials.


Index










Conclusions


Cu / Al ComparisonMechanical Short Circuit Behaviour

Critical point: Located on Low Voltage

HV windings: No short circuit forces due to stiffening by epoxy cast

Into a beam simulation, considering Aluminium conductivity is 63% of Copper and from inertia moments

Bending stress: = Mxt / 2I

ConclusionConclusion:: Aluminium foil is a very safe alternative to copper concerning mechanical stress

under short circuit conditions.

According to this, bending stress in aluminium foil is 2.54 times lower than in copper foil.

Aluminium Al = 2.34 M / t

Copper Cu = 6M / t


Index










Conclusions


Cu / Al ComparisonThermal Stresses under Short Circuit Conditions

Critical point: Located on Low Voltage

Under short circuit conditions, the current density in LV winding rises up to several times as the inverse of impedance voltage in per unit, assumed network impedance is zero

As per IEC, temperature in short circuit is calculated with the following formula:

ConclusionConclusion::For the same losses the behaviour of aluminium winding is safer than copper ones, against the thermal stresses produced by a short circuit


Index










Conclusions



Simple and safe design.

Insulation distance easy to keep.

Lower voltage stress.

Thermal expansion of Al and resin is very similar, so thermal stresses are kept to the minimum.

At equal conductivity Al design weight lighter. Very useful for transformers located inside buildings or with limited accessibility.

Aluminium winding is more compact: higher bonding strength and improved short circuit behaviour.


Index










Conclusions


Conclusions Al windings have the following advantages

Better thermal and dynamic behaviour to short circuits for Al windings

Al windings withstand better surge and overloads currents

Longer working life for insulations materials

Stronger mechanical design

Lower prices

This is ABB Zaragoza standard…This is ABB Zaragoza standard…push for itpush for it!!!!

© abb group november 5, 2015 | slide 1 dry type transformers zaragoza aluminum vs. copper

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