1. Dry type transformers Zaragoza Aluminum vs. Copper
© ABB Group
April 24, 2017 | Slide 1

2. 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
© ABB Group
April 24, 2017 | Slide 2

3. Historical Background: Why using Copper?
Manufacturers were using Cu on HV
Because there were no other option…
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.
© ABB Group
April 24, 2017 | Slide 3

4. 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
© ABB Group
April 24, 2017 | Slide 4

5. Next Step: Why using Aluminium?
…till the option appeared due to
several reasons that led the change
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.
© ABB Group
April 24, 2017 | Slide 5

6. Cu / Al Comparison Subscript
Preliminary reasons for winding material choice
Coefficient of Expansion
Electrical Conductivity
Calorific Capacity
Mechanical short circuit behavior
© ABB Group
April 24, 2017 | Slide 6

7. 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
© ABB Group
April 24, 2017 | Slide 7

8. Cu / Al Comparison Common Reasons for Winding Material Choice1 2 3 4 5 6 7
© ABB Group
April 24, 2017 | Slide 8

9. 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
© ABB Group
April 24, 2017 | Slide 9

10. Cu / Al Comparison Coefficient of Expansion for conductors and resin
Proper Solutions for Al terminals and Cu cable connections
(Cupal Alloy)
Hexagonal Bolt DIN 933
Spring Waser DIN 6796
Busbar
Plain Waser DIN 7349
Hexagonal nut DIN 934
Coefficient of Aluminium Expansion (per ºCx10-6 at 20ºC)
Coefficient of Copper Expansion (per ºCx10-6 at 20ºC)
Coefficient of Resin Mix Expansion (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.
First Conclusion:
© ABB Group
April 24, 2017 | Slide 10

11. Cu / Al Comparison Coefficient 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 Conclusion:
Tensile stress in the resin is lower with Aluminium windings.
© ABB Group
April 24, 2017 | Slide 11

12. 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
© ABB Group
April 24, 2017 | Slide 12

13. Cu / Al Comparison Electrical conductivity
Aluminium has only 61% of Copper conductivity,
so…higher losses???
Aluminium (Al): W-¹m-¹
Copper (Cu): W-¹m-¹
For two identical transformers (same winding resistance and length (amps/turns), the conductor section will be inverse to the electrical conductivity. + -
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
Conclusions:
Section increasing means that result in energy losses is the same.
Costwise comparison: Lower load losses are cheaper with Al windings.
© ABB Group
April 24, 2017 | Slide 13

14. 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
© ABB Group
April 24, 2017 | Slide 14

15. Cu / Al Comparison Calorific Capacity
Aluminium specific heat (cAl): cal/gºC
Copper specific heat (cCu): cal/gºC
Mass of aluminium design is 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, ,7% higher for Al.
Conclusions:
Al windings withstand better surge and overloads currents.
Better thermal behaviour to short circuits for Al windings.
Longer working life for insulations materials.
© ABB Group
April 24, 2017 | Slide 15

16. 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
© ABB Group
April 24, 2017 | Slide 16

17. Cu / Al Comparison Mechanical 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
Aluminium Al = 2.34 M / t
Copper Cu = 6M / t
According to this, bending stress in aluminium foil is 2.54 times lower than in copper foil.
Conclusion:
Aluminium foil is a very safe alternative to copper concerning mechanical stress under short circuit conditions.
© ABB Group
April 24, 2017 | Slide 17

18. 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
© ABB Group
April 24, 2017 | Slide 18

19. Cu / Al Comparison Thermal 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:
Conclusion:
For the same losses the behaviour of aluminium winding is safer than copper ones, against the thermal stresses produced by a short circuit
© ABB Group
April 24, 2017 | Slide 19

20. 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
© ABB Group
April 24, 2017 | Slide 20

21. Aluminium Foil Disk Winding: Summary
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.
© ABB Group
April 24, 2017 | Slide 21

22. 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
© ABB Group
April 24, 2017 | Slide 22

23. Conclusions Al windings have the following advantages
This is ABB Zaragoza standard…push for it!!
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
© ABB Group
April 24, 2017 | Slide 23