Ceyhun Sahin of ABB explains how a new range of dry-type distribution transformers can play a vital role in improving energy efficiency in power distribution networks
Transformers in operation incur two types of losses: no-load loss that occurs in the transformer cores due to hysteresis and eddy current losses - which is always present and is constant during normal operation, and load loss that occurs in the transformer’s electrical circuit, including windings and components, due to resistive loss and is a function of loading conditions. Although distribution transformers are very efficient, there is still a large total loss of energy due to the vast installed bases of distribution transformers. Globally, these losses are estimated to account for around 2-3% of all electric energy production – some 25 GW. According to a 2008 study by SEEDT (Strategies for development and diffusion of Energy- Efficient Distribution Transformers) in the EU alone, there are some 4.5 million distribution transformers, causing 38 TWh of losses each year – more than the entire amount of electricity consumed by Denmark – and 30 million tons of CO2.
Load profile – a key consideration
The growing recognition that transformer losses constitute a significant economic cost is driving programmes to implement energy efficiency standards for distribution transformers. This includes the US, with its DOE (Department of Energy) minimum energy efficiency requirements for liquid-filled and dry-type distribution transformers. The required efficiency is typically quoted for a reference load value – in the case of the DOE this is at 50% of the transformers rating. However the overall efficiency of a transformer in service depends very much on the load profile. Depending on the loading, the effective efficiency can vary significantly from the reference value. This variation requires the selection of transformers to consider the load profile
Distribution transformers are typically custom designed to meet customer specifications and requirements which are normally in compliance with the technical requirements of national or international standards. In addition, the design reflects an optimisation between the cost of materials and labour used in the production of the transformer and the cost of losses.
Some customers look for the lowest possible purchase price, ignoring the cost of losses over the transformer lifetime. These customers are usually not responsible for the ownership or operational costs of the transformers. Customers with operational responsibility seek to reduce losses and specify loss capitalisation, or loss evaluation, values. These capitalised losses reflect the cost of energy consumed by the transformer during an expected economical lifetime. Therefore, when comparing transformer designs it is best to consider the TCO (Total Cost of Ownership), which is the summation of the purchase price and the loss capitalisation. Typically, the design with the lowest TCO results in the most desirable unit for each individual customer based on its specific loss capitalisation factors.
An important option for customers seeking to optimise their TCO by specifying ultra high efficiency distribution transformers is amorphous metal. The use of AMDT (amorphous metal distribution transformers) core technology, combined with optimised coil designs, can provide significant reduction in no-load losses, resulting in higher energy efficiency. Amorphous core technology is at the heart of ABB’s new generation EcoDry ultra high efficiency dry-type transformers.
Amorphous metal transformer cores
Historically, there was an initial interest in amorphous core transformers which stemmed from the first oil shock in the mid-1970s when improved energy efficiency in power distribution systems was increasingly desirable. This interest fell away in the mid-1990s when energy costs decreased. Furthermore, the initial costs of an amorphous core transformer are higher than of a crystalline silicon steel core transformer: first, the amorphous material itself is more expensive than crystalline silicon steel and second, the saturation magnetic flux density of amorphous steel is lower than that of silicon steel. This means larger sizes of amorphous core transformers are required, which results in a higher cost per unit. However, the higher initial costs can be compensated by lower operating costs over the lifetime of the transformers due to their increased energy efficiency.
Nowadays, amorphous metal core transformers have become commercially available and are cost-competitive with conventional core transformers. There has also been significant technical progress in increasing the saturation magnetic flux density of iron-based amorphous alloys, resulting in smaller transformers and reduced material costs.
The amorphous metal used in transformer cores is a unique alloy of Fe–Si–B (iron, silicon and boron) that is produced by extremely rapid solidification from the alloy melt. This causes the metal atoms to form a random pattern, as opposed to conventional Cold-Rolled Grain-Oriented (CRGO) silicon steel (a Fe–Si alloy), which has an organized crystalline structure. The amorphous structure, usually associated with non-metallic systems looks like glass - which has prompted the name ‘glassy metal’ widely used for such materials.
The absence of a crystalline structure in amorphous metal allows easy magnetization of the material, leading to lower hysteresis losses. The eddy current losses are also lower in amorphous metal due to a combination of its low thickness and a high electrical resistivity of 130 μΩ-cm compared to the 51 μΩ-cm in CRGO silicon steels. Thus, amorphous metal has a much lower total loss than even the best grades of CRGO steel, by up to 70 percent.
Amorphous metal cores have a proven track-record of over 20 years in liquid-filled transformers and this technology is now being applied to dry-type transformers.
Advantages of dry-type transformers
Dry-type distribution transformers offer significant practical advantages, including: no fire risk; no risk of escape of pollutants or fire-hazardous substances; long lifetime; high mechanical strength; ability to cope with load changes, overloads, short-circuits and over-voltages; and reduced installation footprint. This means they can be installed near their place of use – saving on cabling and reducing losses in cables and terminals on the low-voltage side.
ABB’s EcoDry range includes transformers that reduce no-load losses by up to 70%, and by more than 30% at full load, when compared with international reference standards such as the European CENELEC HD538 standard and the US Department of Energy energy conservation standard for distribution transformers introduced in 2010.
For each GW saved, there is the potential for an annual reduction of five million tonnes of CO2 emissions – a single 1,000 kVA unit can save 7 tonnes of CO2 a year. Lower losses also generate less heat, so there is a reduced aging effect on the transformer insulation.
EcoDry transformers achieve higher efficiency levels through the use of state-of-the-art materials and components, including amorphous metal as the core material, as well as the latest simulation methods for loss-optimized design. They are available in ratings from 100 to 3,150 kVA, with operating voltage up to 36 kV.
The EcoDry range includes three models, each designed to meet the different needs of applications where losses are either predominantly ‘no-load’ losses (caused by fluctuating magnetization of, and eddy currents in, the transformer core), or ‘load’ losses (which occur in the conductors due to ohmic loss and eddy currents, and increase quadratically with the load), or a combination of the two.
EcoDryBasic – low-load efficiency for power utilities
Distribution transformers at power utilities often see only a low mean load in actual operation. With low load profiles, it is the no-load losses that account for the major proportion of total losses and they are three to five times higher than the load losses. This means a significant reduction in no-load losses is one of the paramount considerations for the EcoDryBasic transformer, a high-tech product, based on 30 years of experience, and developed using the very latest simulation methods for a loss-optimised design.
The EcoDryBasic transformer is specifically designed to meet the needs of power utilities by providing low-load efficiency that enables losses and CO2 emissions to be reduced by more than 50%.
EcoDry99plus – full-load efficiency for industrial applications
In an ideal world, industrial plant is operating at or near full capacity, and mean loading of the distribution transformer of 60% or more is not uncommon. The costs of load losses, and their reduction, can be significant.
In a typical industrial application, an EcoDry99Plus transformer rated at 1,000 kVA, with 10,000 V primary voltage, would reduce annual power losses by more than 30,000 kWh, and cut CO2 emissions by some 18 tonnes per year. At full load, the transformer operates at over 99% efficiency.
EcoDryUltra – efficiency across the load range
EcoDryUltra transformers combine the advantages of the EcoDryBasic and EcoDry99Plus to minimise no-load and load losses simultaneously. This transformer type is ideal for variable loads – such as renewable energy applications – and in applications where the supply is fed through two transformers at the same time (for redundancy) and so each is continuously operated at medium load – such as in pumping or ventilation systems.
Distribution transformers have a vital role to play in helping power utilities and general industry meet targets for reducing carbon emissions, as well as boosting efficiency and cutting running costs. The next generation of low-loss dry amorphous distribution transformers can effectively reduce overall losses, contributing to energy savings, lower operating costs and reduced environmental impact.
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