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Average temperature against current density for copper and aluminium under a short-circuit condition with duration of 4 s
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Copper and aluminium are the two conductors most commonly used in transformer windings. This study presents a comprehensive comparison of distribution transformers built either with copper or with aluminium windings. The comparison is based on winding material conductivity, density, cost, connectivity, oxidation, machinability and behaviour under s...
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Context 1
... is strongly dependent on physical and mechanical properties of the workpiece: hard, brittle metals being generally more difficult to machine than soft, ductile ones. Machinability is also strongly dependent on the type and geometry of tool used, the cutting operation, the machine tool, metallurgical structure of the tool and workpiece, the cutting / cooling fluid and the machinist’s skill and experience. The main cause of machinability problems are always excessive heat. This may arise by frictional heating, and / or by insufficient cooling. All tools should be kept sharp and in good condition at all times (carbide tools retain sharp edges over a longer period between regrinds than carbon or high-speed steel tools). Tools geometry must be maintained within the established requirements for aluminium alloys. Cutter must have the optimum number of flutes and the optimum spiral configurations for each application. The cutting speed should be as high as is practical in order to save time and to minimise temperature rise in the part. As cutting speed is increased above 30– 60 m / min, the probability of forming a built-up edge on the edge cutter is reduced, chips breaks more readily and finish is improved. In particular, a slow feed rate (dwelling) and high spindle speeds are especially troublesome. An adequate and continuous flow of cutting fluid directly at the cutting edges is essential. The flow of cutting fluid should begin before cutting and must continue until the cutter has been removed from the part. During a short-circuit, the current increases sharply in the transformer windings. High currents generate an increase in temperature of the conductor. When the current is very high, the temperature rises rapidly. The melting point of aluminium is 660.2 8 C, while that of copper is 1084.88 8 C. However, it is more important to consider the speed at which the temperature of the conductor increases during the short-circuit than during the melting point. Fig. 4 presents a curve of temperature rise against current density for a short-circuit of 4 s. The curve for copper was obtained from [30, ...
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... u 0 is the initial temperature ( 8 C), J is the short-circuit current density (A / mm 2 ), t is the duration of short-circuit (s), u 1 is the highest average temperature attained by the windings after a short-circuit ( 8 C), E 2 1⁄4 ( J r / J ) × 100%, where E 2 is the impedance voltage (%) and J r is the rated current density (A / mm 2 ). Equations (10a) and (10b) were obtained assuming that the entire heat developed during the short-circuit is retained in the winding itself raising its temperature (adiabatic conditions) because of the short duration of the short-circuit ( , 10 s). The short-circuit current depends on percentage impedance between the transformer and fault point. If the faults occur at the transformer terminals, only the % impedance of the transformer should be taken into calculation of the fault current. In Fig. 4 a curve of temperature rise against current density for copper and aluminium, for a short-circuit duration of 4 s is observed. Considering a short-circuit of 4 s with a current density of 30 A / mm 2 , the temperature in an aluminium winding is 81.8 8 C, whereas in copper it is 54.95 8 C. The maximum allowed temperature for oil-immersed transformers with the insulation system temperature of 105 8 C (thermal class A) is 250 8 C for copper conductor, whereas the same is 200 8 C for an aluminium conductor without any detriment to mechanical properties. A maximum temperature of 250 8 C is allowed for aluminium alloys that have resistance to annealing properties at 250 8 C equivalent to electrical conductor (EC) aluminium at 200 8 C, or for applications of EC aluminium where the characteristics of the fully annealed material satisfy the mechanical requirements [32]. This limiting temperature is specified mainly to limit the ageing of paper insulation in contact with the conductor. It has been shown that high-conductivity copper would not be appreciably softened in the lifetime of a transformer by occasional excursions up to 250 8 C even if the copper is deliberately cold-worked to increase its strength. The same applies to the usual 99.5% aluminium alloys used as conductors [33]. The transformer insulation is burned long before the aluminium or copper is melted. The thermal expansion of conductors can break the insulation and then the failure is originated in the transformer. Copper and aluminium are the primary materials used as conductors in transformer windings. While aluminium is lighter and generally less expensive than copper, a larger cross-section of aluminium conductor must be used to carry a current with similar performance as copper. Copper has higher mechanical strength and it is used almost exclusively in large power transformers, where extreme forces are encountered, and materials such as silver-bearing copper can be used for even greater strength. The windings have to be strong enough to withstand the mechanical forces of short-circuit. Electrical engineers working in the design departments of transformer manufacturers use transformer TOC as an objective function when optimising transformer design [34 – 36]. The usefulness of TOC objective function is also very important when new transformer materials are being introduced [37]. The TOC takes into account not only the transformer bid price (BP) but also the transformer losses throughout the transformer lifetime. The TOC is computed as ...
Citations
... Percentages of overall costs of materials used to manufacture a transformer[35]. ...
Offshore wind power has attracted significant attention due to its high potential, capability for large-scale farms, and high capacity factor. However, it faces high investment costs and issues with subsea power transmission. Conventional high-voltage AC (HVAC) methods are limited by charging current, while high-voltage DC (HVDC) methods suffer from the high cost of power conversion stations. The low-frequency AC (LFAC) method mitigates the charging current through low-frequency operation and can reduce power conversion station costs. This paper aims to identify the economically optimal frequency by comparing the investment costs of LFAC systems at various frequencies. The components of LFAC, including transformers, offshore platforms, and cables, exhibit frequency-dependent characteristics. Lower frequencies result in an increased size and volume of transformers, leading to higher investment costs for offshore platforms. In contrast, cable charging currents and losses are proportional to frequency, causing the total cost to reach a minimum at a specific frequency. To determine the optimal frequency, simulations of investment costs for varying capacities and distances were conducted.
... In a quest to innovate and develop a better replacement for copper conductors, Al alloy came into play. Al's acceptability over Cu was because it was more affordable, its density was 232% lower than Cu, it passivates more than Cu, and it had an apt electrical conductivity that was 200% above Cu by weight [11,12]. Even though monolithic Al conductor enjoyed all these superior characteristics over Cu in the transmission of electricity, it still face with some functional deficiencies as follows: its strength can hardly withstand ice loads and wind vibrations; it sags at moderate loads which can obstruct Rights-of-Ways (ROW); its maximum operating temperature is below 100 °C; and it has limited ampacity due to its low temperature of operation and a high coefficient of thermal expansion (CTE). ...
Transmission conductor forms the essential pathway where electric power traverses from the generating centre station to the distribution sub-station. Some glitches in power delivery have been attributed to that occasioned by defective transmission conductors. Challenges accruing from transmission conductors can be handled proactively by designing and developing robust conductors. This review was aimed at studying the challenges witnessed in power transmission, ways of ameliorating them, and prospective conductors for future power transmission. In the study, it was observed that lightning, bush fire, short-circuiting, and grid overload are some of the challenges in the transmission grid. It was also observed that aluminium conductor composite core (ACCC) and aluminium conductor composite reinforced (ACCR) are the two best transmission conductors existing presently based on ampacity and efficiency. It was concluded that Al-based composites of CNTs, graphene, BN, Si3N4, and TiC could perform more favourably than the existing transmission conductors. It was recommended that these new materials should be studied further to verify their applicability in transmitting electric power.
... When compared at the same power factor parameter, a 2% lower efficiency was observed when using aluminium winding instead of copper winding on distribution transformers. However, for the same current density, aluminium winding is lighter than copper winding [5][6][7][8][9]. At frequencies above 500 Hz, the ratio of AC/DC resistance for aluminium winding approaches the resistance ratio of copper winding [10]. ...
Currently, both limited fossil fuel resources and environmental factors have increased the use of renewable energy sources. Renewable energy resources, such as wind energy systems, are gaining popularity, resulting in increased competition among manufacturers. This study aims to achieve a cost-efficient wind energy conversion system by designing and analysing the performance of a 250 kVA aluminium wound double-fed induction generator (DFIG). The advantages and disadvantages of aluminium windings are compared to those of copper windings, and three DFIG models are created: Model-1 with a copper winding set, Model-2 with the same geometry as Model-1 but designed with an aluminium winding set, and Model-3 with an aluminium winding set and slightly different stator and rotor diameters. The three DFIG models were analysed using finite element analysis (FEA) in ANSYS Maxwell, and the simulation results were obtained. According to the FEA results, Model-1 with copper windings had a higher efficiency compared to Model-2 with aluminium windings, but Model-2 had better cost and weight performance compared to Model-1. Model-3 and Model-1 had similar efficiencies, but Model-3 had slightly greater torque ripple compared to Model-1 due to a slightly different stator and rotor diameter. Although the total machine weight of aluminium-wound DFIGs was slightly increased, the total manufacturing costs were less than copper-wound DFIGs at the same efficiency levels.
... Many researchers in the literature worked on the design, analysis, and development of transformers [1][2][3][4][5][6][7], while transformer performance characteristics [8]. Sarac and Cogelja [8] studied the variation of the output voltage and efficiency of a small-rating, three-phase distribution transformer at various loads. ...
Electrical power passes through several transformers after generation through consumption. The final stage of any power system consists of many small-rating distribution transformers. Since the loads connected to these transformers vary with time, their effective design and operation with variable loading conditions are crucial for the system to operate efficiently and reliably. In most previous works, loads connected to these transformers are treated as constant power and stray losses of the transformers are neglected in the calculations. Neglecting the load variation and stray losses reduces the transformer life span noticeably and impairs grid quality and reliability. In this work, a small-rating distribution transformer is designed and analyzed analytically and numerically. Detailed procedures on the conventional design are presented with the general design issues. The effect of variable loading on the transformer’s power losses, efficiency, temperature rise, voltage regulation, and output voltage variation is analytically and numerically investigated. Results show that the rating and power factor of the load significantly influence the performance characteristics of the transformer.
... Cu metal is the best non-precious metal conductor of electricity as well as being the safest, thus it is used widely in commercial and residential building wiring [4]. In addition, Cu alloys are an essential component of energy-efficient generators [5], motors [6,7], transformers [8], and renewable energy production systems [9]. Renewable energy sources such as solar, wind, geothermal, fuel cells, and other technologies, are all heavily reliant on Cu alloys due to their excellent conductivity [10,11]. ...
A Cu–15Fe alloy was fabricated using a powder metallurgy (PM) route, with the addition of different solid lubricants (i.e., paraffin wax (PW) and stearic acid (SA) as well as their composites (PW+SA)). Green compacts were produced via cold compaction at different pressure levels of 50 MPa, 200 MPa, and 350 MPa, then sintered for 60 min under vacuum at 1050 °C. The systematic evolution of the densification, porosity, and pore-size behavior were studied. Vickers Hardness Tests were used to measure hardness. The analysis of the morphological alterations was performed using scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. Moreover, under dry sliding conditions, pin-on-disk wear tests were conducted in order to determine tribological properties such as the coefficient of friction (µ), specific wear rate (K), and friction temperature gain. Results revealed that the lubrication process and compaction pressure play a crucial role in defining the characteristics of the final compact. Higher sintered densities and hardnesses were achieved at 50 MPa when PW was used as a solid lubricant, and became reduced as the compaction pressure increased. In contrast, in the case of SA, higher sintered densities and hardnesses were obtained at a compaction pressure of 350 MPa, and increased with increasing pressure. Moreover, PW samples exhibited lower coefficients of friction and wear properties. The addition of SA improves the wear loss of friction materials as well as their coefficients of friction. Compared to blank and PW samples, SA samples show a nearly 50% reduction in wear rate.
... has a tensile strength of about 450-500 MPa [6]. At the same time, for pure technical aluminium, it is 70 MPa [7]. ...
In this study, the Al-15 wt % Fe powders were prepared using two methods: (1) milling of the initial alloy in a
ball or a planetary mill; (2) centrifugal atomization. The effect of milling time and distribution of temperature
field during atomization on the morphology and microstructure of the Al-15 wt % Fe alloy powders were
investigated. After milling, the powders have an agglomerate structure with a size of Al13Fe4 about 4.1–5.4 μm.
Using centrifugal atomization at a blades rotation speed of 2800 rpm resulted in a microstructure consisting of
α-Al-matrix, stable Al13Fe4 (10–15 μm) and random oriented short fibres of metastable Al6Fe (2.3 ± 2.2 μm).
Randomly oriented fibres are formed due to deformation processes that act on the particles during cooling and
crystallization. The formation of the metastable Al6Fe is associated with a high cooling rate of nearly 106
–108 ◦C/
min. The influence of the particle shape on the temperature field was simulated. The stable Al13Fe4 occurs in the
centre of the powders according to the low solidification velocity. The crystallization of a uniform structure is
possible for particles of regular shape. The matrix of the atomized powder has a cellular microstructure, which
introduces an additional reinforcing effect to the Al-15 wt % Fe alloy.
... Copper and aluminum are the common LFT winding materials. The transformer short circuit current has an impact on melting points, which plays a critical role in the selection of winding material [179]. In addition, due to the skin and proximity effects, the circularshaped conductors are not a good choice for the HFT. ...
The emergence of DC fast chargers for electric vehicle batteries (EVBs) has prompted the design of ad-hoc microgrids (MGs), in which the use of a solid-state transformer (SST) instead of a low-frequency service transformer can increase the efficiency and reduce the volume and weight of the MG electrical architecture. Mimicking a conventional gasoline station in terms of service duration and service simultaneity to several customers has led to the notion of ultra-fast chargers, in which the charging time is less than 10 min and the MG power is higher than 350 kW. This survey reviews the state-of-the-art of DC ultra-fast charging stations, SST transformers, and DC ultra-fast charging stations based on SST. Ultra-fast charging definition and its requirements are analyzed, and SST characteristics and applications together with the configuration of power electronic converters in SST-based ultra-fast charging stations are described. A new classification of topologies for DC SST-based ultra-fast charging stations is proposed considering input power, delta/wye connections, number of output ports, and power electronic converters. More than 250 published papers from the recent literature have been reviewed to identify the common understandings, practical implementation challenges, and research opportunities in the application of DC ultra-fast charging in EVs. In particular, the works published over the last three years about SST-based DC ultra-fast charging have been reviewed.
... Copper wires have been used for long years for stator windings which have good processing properties and high electrical conductivity. Copper and aluminium are the two conductors most commonly used in transformer windings [1,2]. But ores of copper is decreasing while raw material prices are continuously increasing. ...
... With advantages on both sides, copper and aluminum have become interchangeable in many applications and are given preference in accordance with their price ratio. Olivares-Galvan et al. (2010) describe this interchangeability in the production of windings for distribution transformers. ...
The paper focuses on minor metals and coupled elements and aspires to understand individual incidents of imbalance on the mineral markets during the last 100 years and gain insight into the acting dynamics—those dynamics are commodity-specific but remain largely unchanged in their nature to date—and to identify the factors in play. The conclusions allow for a critical analysis of the widespread security-of-supply narrative of industrialized countries. They point at a market that is mostly a buyers’ market, in which prices and their volatility are largely dictated by shifting demand patterns and much less by supply constraints. Neither high country concentration nor poor governance seem to have a substantial or lasting impact on market balance. Short-term market imbalances are generally neutralized by a dynamic reaction on the demand side via substitution, efficiency gains or technological change. The paper also assesses the impact of those quickly shifting demand patterns and the related price volatilities on producing countries. It shows how mineral price volatilities can expose developing countries’ economies to significant economic risk, if their economy is heavily dependent on mineral production. Two cases that illustrate country exposure are explored in detail—the saltpeter crisis in Chile and the tin crisis in Bolivia. Both led to state bankruptcy. The paper concludes with an attempt to quantify economic exposure of producing countries to price volatilities of specific metals and suggests policies that adapt to the characteristic challenges of highly volatile demand.
... Typical choice of a winding material is copper or aluminum due to low electrical resistance. However, winding material selection is influenced by transformer short circuit current due to their melting points and an extensive discussion on selection of winding material is found in [142]. In addition, losses such as skin and proximity effects are dominant at higher frequencies. ...
Increase in global energy demand and constraints from fossil fuels have encouraged a growing share of renewable energy resources in the utility grid. Accordingly, an increased penetration of direct current (DC) power sources and loads (e.g., solar photovoltaics and electric vehicles) as well as the necessity for active power flow control has been witnessed in the power distribution networks. Passive transformers are susceptible to DC offset and possess no controllability when employed in smart grids. Solid state transformers (SSTs) are identified as a potential solution to modernize and harmonize alternating current (AC) and DC electrical networks and as suitable solutions in applications such as traction, electric ships, and aerospace industry. This paper provides a complete overview on SST: concepts, topologies, classification, power converters, material selection, and key aspects for design criteria and control schemes proposed in the literature. It also proposes a simple terminology to identify and homogenize the large number of definitions and structures currently reported in the literature.
