Power and distribution transformers are key to providing efficiencies for the utilities, as well as for some other electrotechnical disciplines. Recent technological breakthroughs have made lower transformer costs of ownership possible. James Hunt looks a
The markers determining the recent evolution of power and distribution transformers are essentially those of efficiencies, both in manufacture and operation. It's no good being able to design a transformer with a high-efficiency core if it cannot be made easily and cheaply enough. Moreover, careful manufacture is key to success in achieving optimum performance. Indeed, one of the senior development engineers in charge of distribution transformers at ABB, said that "while ABB is proceeding with developments in stacked core designs and is always looking at the latest materials, it is essential always to ensure that every design is optimised for production".
For example, ABB's UK transformer production plant at Dundee has implemented the company's TrafoStar common platform concept for the design and manufacture of power transformers up to 300MVA and 400kV. This uses standardised, service-proved components and modules to create flexible and reliable transformer designs.
The drive for energy efficiency:
The Environmental Protection Agency (EPA) in the USA has determined that 61 billion kWh of electricity is wasted each year in transformer losses. As a result, energy-efficient distribution transformers are beginning to gain momentum in the market. Until recently, energy-efficient transformers were hard to find and prices were high. More recently, the number of manufacturers producing such transformers has increased significantly.
In the USA, only three manufacturers produced high efficiency distribution transformers as recently as four years ago. Up to that time, high-efficiency transformers were mostly custom-designed. More recently, after the US transformer industry defined and published efficiency standards through NEMA standard TP-1, the number of such manufacturers has increased roughly four-fold. Prices moreover, have fallen significantly. The update itself described how CEE, NEMA, and ENERGY STAR* teamed up with federal and state governments, utilities and transformer manufacturers to move this market forward.
Distribution transformers:
Distribution transformers are energised every hour of every year. Because of this, even small efficiency improvements can yield big energy savings. Moreover, the decision to purchase an energy-efficient transformer locks in long-term savings over the life of the equipment (typically 30-35 years).
Studies on LV transformer loads in commercial, industrial, and public buildings have shown that dry-type LV transformers are relatively lightly loaded with an average load factor of 16%. Such findings are significant because at low loads, core losses dominate. Transformers having low-loss cores are especially advantageous at such low operating levels.
Transformers meeting the NEMA standard can reduce losses by about 50%, yielding paybacks in as little one to three years. Another recent study has shown that around 350 million kWh would be saved each year if all transformers purchased in the US met NEMA TP-1. If low-loss dry-type amorphous core transformers (using amorphous metals developed by Allied Signal and now part of Honeywell) were brought to market, annual US national savings could approach 620 million kWh. Similar savings could translate to Europe.
Ultra-low-loss transformers go one step further than many TP-1 rated transformers and this type has achieved 98.5% efficiency at 35% load.
Amorphous core transformers:
Amorphous metal distribution transformers (or AMDTs) are thought to be new but have been around for 20 years or so. They use a metallic glass alloy in their cores. Amorphous metals can be defined as bulk, structural, metallic materials whose microstructure in the solid state is, unlike that of conventional metals, non-crystalline, amorphous or 'glassy'. As a result of this novel microstructure, amorphous alloys exhibit unique combinations of properties such as magnetic performance, hardness, strength, damage tolerance and corrosion resistance.
Transformers, when used to change the voltage between different segments of the transmission and distribution system, provide a major source of system losses. There are winding impedance losses but there are also losses due to hysteresis and eddy currents in the steel core. Ultra-efficient transformer cores made - for example - with Honeywell's METGLAS amorphous metal alloy, make lower core losses possible. However, amorphous core transformers are still only available for lower ratings.
AMDTs were once said to be key to improving utility economics and enhancing energy conservation efforts worldwide. As a result of TP-1, the US has been at the forefront of recent applications in amorphous core transformers. Indeed, most applications for these are now found in the US.
Amorphous core advantages:
- They are less expensive to operate
- They save energy and therefore reduce greenhouse gases and other pollutants
- In the US, the local utility offers a rebate for the installation of TP-1 rated transformers
- It is possible to improve power quality and mitigate harmonics with the devices
- Replacing older inefficient transformers improves reliability
Amorphous core disadvantages:
- It affects almost every transformer purchased by a non-utility
- The cost of TP-1 rated equipment can be twice as great as a standard efficiency transformer
- Such transformers are even now often not readily available
- There are still limited numbers of manufacturers high efficiency transformers
- Amorphous metal cores are up to 30% bigger than conventional cores
Evaluating AMDT cost of ownership:
Transformers should always be optimally loaded to obtain maximum efficiency. The maximum efficiency in a conventional transformer occurs at such a load where the iron losses are equal to copper losses. For this reason, proper load management of the transformers will lead to substantial savings in electricity bills. A similar principle applies to the use of AMDTs.
One method of illustrating how AMDTs provide economic savings is by evaluating on a Total Owning Cost (TOC) basis. TOC covers both the initial cost of the transformer and the future cost of the energy losses over the life of the equipment. A user who saves on the initial purchase price of the transformer may be losing money by not properly considering the value of the energy losses over the transformer's life.
Customers who pay for electricity on a per unit basis can easily calculate their operating costs and determine how long it takes to recover the price premium associated with the purchase of higher-efficiency transformers. A simple payback calculation demonstrates that, for a wide range of price premiums, the energy savings result in a relatively short payback period for AMDTs.
Despite the fact that difficulties have been experienced in amorphous core transformer manufacture, personnel with experience in lacing transformers have found it easy to work with amorphous cores, since the expanded lapped sets are not rigid. In addition, the inter-leafing process has minimal stress impact. Honeywell states that 'lowest losses and exciting power are achieved when the distribution transformer core is in its lowest stress conditions'. The company makes recommendations for achieving this.
Other types of high-efficiency core:
There are other ways of making high-efficiency transformers. For example, major advances have been made using steels with improved magnetic permeability. Core losses have also been reduced in the crystalline, grain-oriented silicon steel currently used in utility transformers through metallurgical advances in the basic alloys. Such advances have led to better materials and manufacture and include magnetic domain refinement, reduced core steel sheet thickness, and increased lamination stacking factors through increased perfection in steel surface quality, thickness uniformity, and interlaminar insulations.
Some transformers:
ABB Power Technology Products' Dundee Transformer Division is the focal point for all ABB transformer projects in the UK. Products made include various ranges of both TrafoStar power transformers for transmission networks and distribution transformers for local supply networks.
The company has delivered a 66/33 kV, 25 MVA transformer to a distribution substation in Sunderland and a further six of these 65 tonne transformers will be delivered to other substations in the area.
Typical of high efficiency transformers now on the market are those of Square D. The company offers a complete line of transformers that can significantly reduce energy losses. The Type EE energy efficient transformers are optimised for maximum efficiency at the most common loading levels.
The company's Watchdog 115°C or 80°C rise transformers offer significant energy savings in less common applications where the average daily loading exceeds 50% of nameplate rating. They give the following advantages:
- Designed for low energy losses at loads greater than 50% of nameplate rating.
- Extra long life expectancy using 220°C insulation system designed for full load operation at a maximum temperature rise of 115°C or 80°C instead of 115°C.
- Continuous emergency overload capability of 15% on 115°C rise and 30% on 80°C rise.
Another from Square D is its Type EE NEMA TP-1 compliant ENERGY STAR transformers. These comply with TP-1 for optimum energy efficiency at 35% load, are more efficient than standard 150°C rise transformers at all load levels, have identical dimensions to standard 150°C rise general purpose transformers and have been cost optimised to allow an energy savings payback in 3 - 5 years.
Eaton (Cutler-Hammer) can supply a wide range of power and distribution transformers. Included are low temperature rise energy efficient single- and three-phase energy efficient types EP, DS-3, EPT and DT-3 60 Hz. Types EP, EPT have a sand- and resin-encapsulated design, and are available in single-phase ratings through 37.5 kVA and 4160 V primary (EP) and in three-phase ratings through 75 kVA and 4160 V primary (EPT). Types DS-3 and DT-3 use a ventilated, NEMA 2 enclosure as standard are available in single-phase ratings 15 - 167 kVA and up to 4160 V primary (DS-3) and in three-phase ratings 15 - 1500 kVA and up to 4160 V primary (DT-3). All are NEMA TP-1-1996 compliant.
Square D offers a full range of MV transformers. The company's scope includes liquid-filled, conventional dry type as well as two premium cast coil designs. All are manufactured in ISO certified and are built and tested to applicable ANSI/IEEE, CSA and NEMA standards. Offered are high efficiency standard designs as well as optional low loss TP-1 designs. The full range covers liquid-filled, dry and cast types, plus package sub-stations.
Hermetically sealed transformers giving reduced maintenance have been added to the Merlin Gerin Fast Trans range. Once installed, these transformers will operate without further attention required to the fluid, making them ideal for petrochemical complexes, water plants and sewage treatment works, as well as many industrial applications.
Future trends:
The transformer industry is notorious for 'price being king' when it comes to purchasing the product. This is changing to some extent with the professional buyer looking more at high quality and good design and not just how much they must pay for the product. There is also growing trend towards the use of higher frequency wound components within their equipment. This has a number of benefits but poses new design requirements.
The main advantage of high frequency components is that they result in generally much smaller products. This, however, does not always result in a lower price as specialist materials need to be used in the design. Examples include winding wires, multi-stranded conductors (Litz) and new core materials. The latter, instead of being standard 3% silicon steels (widely available) are now more expensive alloys such as 6.5% silicon steel, amorphous cores, sinter metals, (powder cores) and so on. These materials are relatively new and not widely manufactured and are substantially more expensive than standard electrical steels.
Amorphous metal and other low-loss cores will also be increasingly used for applications where high efficiency is a pre-requisite. Their take up into the mainstream is low however and now seems less likely than was at first thought, as they are still expensive. Overall cost of ownership will become more appreciated, rather than first cost.
Other factors likely to affect this sector include a global pricing structure and acquisitions by the major companies.
*ENERGY STAR for commercial and industrial transformers was launched a few years ago by the U.S. Environmental Protection Agency (EPA).
ABB Power Technology Products
Web: www.abb.com
Eaton (Cutler-Hammer)
Web: www.ch.cutler-hammer.com
Square D
Web: www.squared.com
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