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Miljenko Hrkac, SPT Global Technology Manager , custonmer day, Guatemala october 16th/2012 Aluminium Al i i T Transformers f Technical and Economical Analysis Topics History of Aluminium Material selection in transformer design Technical economical and environmental aspects Technical, K-factors and Aluminium ABB experience Summary History of Aluminium Unknown about known Aluminium is the third most abundant element of the Earth’s Earth s crust, behind that of oxygen and silicon and the first of the metallic elements consisting 7.3% mass of the total crust. Due to Aluminium’s Aluminium s high affinity to bind with oxygen, oxygen it is never found free in nature, but only in combined forms such as oxides or silicates. The ancient Greeks and Romans used alumen (alum, potassium aluminium sulfate K2Al6(OH)12(SO4)4) in medicine as an astringent sulfate, The metal originally obtained its name from the Latin word for alum, alumen. Scientists suspected than an unknown metal existed in alum but they did not have a way to extract it until 1825. Hans Christian Oersted, a Danish chemist, was the first to produce tiny amounts of aluminium. it would take years upon years of research to find an efficient method to extract the metal from its ore and even more years to create a production process that would allow the metal to be commercially practical. History of Aluminium Unknown about known Mid 19th century, Deville, French chemist , develop process for commercial production of aluminium. As a result, the price of aluminium dropped pp from around $1200 p per kilogram g in 1852 to around $40 per kilogram in 1859. With an easy way to extract aluminium from aluminium oxide and an easy way to extract large amounts of aluminium oxide from b bauxite, it th the era off aluminium l i i h had db begun. In 1888, the Pittsburgh Reduction Company, which is now known as Alcoa produced about 25 kilograms of aluminium a day. By 1909 company was producing about 41 1909, 41,000 000 kilograms of aluminium a day. As a result of this huge increase of supply, the price of aluminium fell rapidly to about $0.60 per kilogram. Today, y aluminium and aluminium alloys y are used in a wide varietyy of products: cars, cans, foils and kitchen utensils, airplanes, rockets and other items that require a strong, light material but also in electrotechnic industry: transmission lines, cables and p products. Conductor Materials in Transformers Informed Choice Considering technical and economical characteristics and availability, the conductor materials used in transformers are the following: Copper (Cu) Aluminium (Al) The informed choice of Al and Cu conductors shall be based on technical, commercial/economical and environmental reasons such as: Conductivity Density Skin effect Connectivity Manufacturability Thermal & mechanical properties Cost Environmental effects prescribe for copper pp windings g only, y, for different reasons like Some customers strictlyy p conservatism, established custom and practice as well as perception of better quality. © ABB Group October 12, 2012 | Slide 5 Conductor Materials in Transformers Historical price fluctuation of Cu Cu, Al Al, Tin & Zinc LME STOCK LME STOCK LME PRICE LME STOCK © ABB Group October 12, 2012 | Slide 6 LME PRICE LME PRICE LME STOCK LME PRICE Conductor Materials in Transformers Copper historical price fluctuation In less than 10 years, between 2002 to 2012, 2012 copper price have increased b 500% by © ABB Group October 12, 2012 | Slide 7 The price of copper has historically been unstable. It jumped from 60-year low of 1.32USD/kg in June 1999 to 8.27USD/kg in May 2006. It dropped to 5.29USD/kg in Feb. 2007, then rebounded to 7.71USD/kg in April 2007. In Feb. 2009, weakening global demand and a steep fall in commodity prices since th previous the i year's ' highs hi h lleft ft copper prices i att 3.32USD/kg. 3 32USD/k Conductor Materials in Transformers Copper vs Aluminium price fluctuation © ABB Group October 12, 2012 | Slide 8 Cu iss currently cu e t y much uc more oee pe s e tthan a Al in te so / g expensive terms of US USD/kg. The price of Cu has recently moved ahead much faster than the price of Al. Conductor Materials in Transformers Cu Al reserves and production Cu-Al COPPER ALUMINIUM © ABB Group October 12, 2012 | Slide 9 World copper pp reserves are estimated to be 480x106 metric tons Considering an average consumption of 15x106 tons gives 32 years to reach full depletion Recycling is a major source of copper in the modern world and the availability of copper can be longer. World bauxite (Al) reserves are estimated to be 25000x106 which is more than 5 times of world copper reserves Aluminium world production is almost 2 times as that of copper world production Conductor Materials in Transformers Surge and Overload Current Mass of aluminium transformers is 0.488 0 488 times the copper ones for a similar electrical performance Heat (calorific) capacity = M x c Heat _ capacity _ Al m Al c Al 904 = × = 0.488 × ( ) = 1.15 H t _ capacity Heat it _ Cu C mCu cCu 385 © ABB Group October 12, 2012 | Slide 10 Aluminium transformers have superior p thermal storage g capacity p y compared to Copper wound units (15% higher for Al) Conductor Materials in Transformers Copper – Aluminium properties PHYSICAL PROPERTIES PHYSICAL PROPERTIES COPPER ALUMINIUM 0.01754 0.0282 Density 8.9 2.7 Thermal Conductivity (W/m K) 398 203 Specific Heat (J/kg K) 385 904 11000 6500 154 102 Resistivity (micro ohm‐m) Young’s modulus of elasticity ((dN/mm2) / ) S.C (1 sec) thermal current density (A/mm2) © ABB Group October 12, 2012 | Slide 11 Conductivity of Aluminium is approximately 62% of Copper conductivity This means for the same resistance, cross section of Al conductors must be 62% larger than the cross section of Cu conductor Larger cross section of aluminium is needed for having the same resistance. This increases quantity of other materials used in power transformers such as the core, tank and oil But Aluminium density is less than Copper (2.7 vs 8.9 kg/dm3). We need a mass of aluminium half of the mass of copper for the same resistance and the same winding length ( the same losses). Conductor Materials in Transformers Copper – Aluminium properties Yield stress ( N / mm2) – typical value Copper Aluminium Soft strand conductor after annealing 40 25 Soft strand conductor after mechanical work – hardening 80 65 Controlled hardned conductors by heat treatment 120 – 180 110 - 130 COPPER vs ALUMINIUM CONDUCTOR FOR SAME LOAD LOSSES © ABB Group October 12, 2012 | Slide 12 Copper Aluminium C Cross-section ti 100 160 Weight 100 48.5 S.C. Thermal Current 100 106 Aluminium has a lower tensile strength than Copper however the increased cross section of aluminium, compared with copper for reaching the same resistivity, works to its advantage, since the increased section helps to offset the lower ultimate strength of Aluminium Aluminium wound transformers have better thermal performances and in the most of cases also mechanical during S.C w.r.t copper wound one, one for the same load losses Harmonics, Transformers and K-Factors Eddy loss due to high harmonics: Al vs Cu According to classical theory, theory the AC losses can be computed with the following formula: The penetration depth of Cu at 50 Hz is 9.4 mm, while for Al is 12 3 mm. 12.3 mm The conductivity of Cu at 75 75°C C is 0.4703 0 4703 · 108 [Ohm m]-1, while for Al is 0.28935 · 108 [Ohm m]-1 Conductivity Penetration Depth Eddy losses © ABB Group October 12, 2012 | Slide 13 Cu 100% 100% 100% Al 62% 127% 62% Eddy loss in aluminium winding is 38 % less w.r.t eddy losses in pp with same volume of conductor copper Harmonics, Transformers and K-Factors Eddy loss due to high harmonics Once the KFactor of the load has been determined, it is a simple matter to specify a transformer with a higher K-rating from the standard range of 4 4, 9 9, 13, 20, 30,40,50 K-Factor In US practice, the K-factor is the ratio of eddy current losses when driving non-linear and linear loads : Harmonics, Transformers and K-Factors Eddy loss due to high harmonics Harmonic currents are generated whenever a non non-linear linear load is connected to the mains supply. The problems caused by harmonic currents include overheating of cables, cables especially overheating and vibration in induction motors and increased losses in transformers. What is effect of harmonics on the transformers and how designers can use to account for them and ensure reliability. Losses in transformers are due to magnetic losses in the core, and d eddy dd currentt and d resistive i ti llosses iin th the windings. i di Eddy current losses are of most concern when harmonics are present because they increase with the square of the frequency. The harmonic spectrum of the load current must be known to determine eddy losses due to harmonics Harmonics, Transformers and K-Factors Eddy loss due to high harmonics: Al vs Cu Copper Windings Al mini m Windings Aluminium K-Factor © ABB Group October 12, 2012 | Slide 16 Pdc Peddy Ptotal Pdc Peddy Ptotal K1 4997 111 5108 4997 70 5067 K4 4997 444 5441 4997 280 5277 K9 4997 999 5996 4997 630 5627 K13 4997 1443 6440 4997 910 5907 K20 4997 2220 7217 4997 1400 6397 Ratio Eddy (Al/Cu) 63% Below B l iis an example, l ffor 400KVA VSD ttransformer f with ith equall N No Load and Load losses designed with Copper and Aluminium conductors Eddy losses are reduced by approx. 37% in case of aluminium As we can see from table, a K20 rated aluminium wound transformer have approximately Eddy losses as K13 copper wound transformer Aluminium Conductor in Transformers Economic Considerations © ABB Group October 12, 2012 | Slide 17 Copper represents one of the most expensive materials used in transformers Besides its cost, price of copper is highly unstable. The price of aluminium is lower than the price of copper and it fluctuates, but not as much as copper. World copper reserves are limited and a big debate on, when will be the full depletion. It is reasonable to expect future copper prices to be both high and volatile. The amount of Al now produced annually exceeds Cu production by two. Economies of scale, supply and demand, and other factors such as Al being g the most p plentiful metal in the earth have the potential to make the Al prices more stable and lower than the Cu prices in the future. Aluminium Conductor in Transformers Environmental Considerations © ABB Group October 12, 2012 | Slide 18 For evaluating the environmental impact of Cu and Al, Al European Union “Eco-Indicator” is used as an indication. The values given in “Eco-Indicator 99 Manual for Designers 2000” for primary Cu and Al are 1400 mpt/kg and 780 mpt/kg 2000 respectively. As with cost, comparisons based on volume are more useful than comparisons on mass: per volume, volume the values can be expressed as 12.5 mpt/cm3 for Cu and 2.1 mpt/cm3 for Al. Even considering the 62% additional Al needed for equal resistivity, i ti it Al has h much h less l environmental i t l impact i t than th C Cu needed for the same winding by this measure. Eco-indicator method quantifies environmental impact in terms of “ i t ” where “points”, h 1000 points i t roughly hl corresponds d to t the th annuall impact of an average European. Millipoints (mpt) are often the appropriate units for evaluating practical decisions in design work work. Practical issues in using Al in Transformers Termination & Connection © ABB Group October 12, 2012 | Slide 19 To avoid interconnection problems, ABB uses proper connectors for internal connections Using g cables,, rings g and special weldings for proper connections it is possible to provide necessary elasticity in joints without compressing aluminium too much and having the same efficiency as copper terminals. Aluminium Conductor in Transformers Reliability Considerations © ABB Group October 12, 2012 | Slide 20 Transformer’s life is defined by the life of its insulation system. In transformers, limiting temperatures are specified mainly to limit the ageing of paper insulation in contact with the conductor. Hot spot temperature is mainly located where eddy losses concentrated For every 6 - 8 ˚C C rise in temperature, temperature the life of paper insulation is halved The higher resistivity of Aluminium gives inherently lower eddy l losses iin th the windings. i di Thi This mitigates iti t the th risk i k ffor hot h t spots. t The transformers are designed to comply with the temperature limits, whichever the conductor type is. The maximum allowed temperature during S.C for oil-immersed transformers is 250oC for copper conductor, whereas the same is 200oC for an aluminium conductor without any detriment to mechanical properties. ABB Aluminium Wound Transformers Experience © ABB Group October 12, 2012 | Slide 21 • More than 40 years ears of experience with aluminium wound transformers • Experience of several million aluminium wound di t ib ti ttransformers distribution f • Experience of several thousand aluminium wound power transformers above 10 MVA Well proven and established ABB technology up to 170 kV class and 63 MVA ABB Aluminium Wound Transformers Some customers using aluminium transformers © ABB Group October 12, 2012 | Slide 22 Aluminium Conductor in Transformers Summary Transformers with Aluminium windings having the same losses as that of Copper windings are technically feasible and economically advantageous. Copper is extensively used in power transformer windings. The price of copper fluctuates heavily during the last years and the price level is expected to be at high levels, due to limited copper reserves. As the price of Cu has recently moved ahead much faster than the price Al, usage of Al in winding conductors has become more viable option. Descriptions Aluminium price is more stable because of its market availability (twice copper pp worldwide p production); ); Aluminium windings are lighter than copper windings; True X X Copper winding transformers better bear short-circuit efforts; X Al i i Aluminium t transformers f have h bigger bi llosses; X Aluminium winding transformers are not compatible with copper connectors; X Copper transformers are more compact than aluminium ones; © ABB Group October 12, 2012 | Slide 23 False X Interesting facts about Al Every minute of every day, an average of more than 123,000 aluminium cans are recycled. Since 1972, an estimated 660-plus billion beverage cans have been recycled—placed end-to-end, they could stretch to the moon nearly 300 times times. If each person recycles one aluminium can each month, energy savings equal 1,750 to 3,500 gallons of gas. American consumers and industry throw away enough aluminium to rebuild the entire U.S. commercial air fleet every three months. One pound of aluminium can replace twice that weight in steel in most applications. Rubies, emeralds and sapphires consist mainly of crystalline aluminium oxide. Aluminium is light, strong, corrosion-resistant, nonmagnetic, nontoxic and naturally good looking looking. © ABB Group October 12, 2012 | Slide 25
