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Distribution Transformers
Distribution Transformers convert high-voltage electricity to lower voltage levels acceptable for use in homes and business.
Electrical energy is passed through distribution
transformers to reduce high-distribution voltage levels down to end-use
levels. Nearly all energy used in the United States passes through at
least one distribution transformer before being consumed by an end-use
appliance, motor, or other piece of equipment. Transformers are found in
all sectors of the economy: residential, commercial, and industrial. General Purpose Distribution Transformers They are generally used for supply appliance, lighting, motorized machine and power loads from electrical distribution systems. They are either ventilated or totally enclosed, and are available with either aluminum or copper windings in standard ratings from 50VA up to 750 kVA. Non-Linear (K-Factor) Distribution Transformers The use of electronic equipment has continued to grow in both offices and industrial plants. Offices now include computers, fax machines, copier, printers, cash registers, UPS's and solid-state ballasts. They all contribute to the distortion of the current waveform and the generation of Harmonics. The K-Factor transformer are designed to tolerate the harmonic distortion associated with these loads. Energy Efficient Distribution Transformers These distribution transformers are energy efficient, dry type, general purpose transformers to meet NEMA TP-1 specifications. In the past several years, there has been an accelerated rate of change to introduce energy efficiency standards for transformers. Many products have preceded the initiatives for transformers in an overall effort to optimize manufactured goods for environmental concerns. Governments are encouraging users to consider energy efficient products like dry type transformers, in an effort to help reduce carbon dioxide emissions. In addition to the benefits to the environment, energy efficient transformers also can realize substantial savings in operating costs thereby have a direct impact on the initial investment evaluated over a period of time. Distribution Transformer Basics Copper or aluminum conductors are wound around a magnetic core to transform current from one voltage to another. Liquid insulation material or air (dry-type) surrounds the transformer core and conductors to cool and electrically insulate the transformer. Since small distribution transformers do not generate much heat, a higher proportion of theses tend to be dry-type. Dry-types are less flammable, and are therefore often selected for use when they must be located in confined spaces on a customer's premises. Distribution transformers are used in electric power systems. The final part of the distribution system at medium voltage are the distribution transformers. Due to the Iow impedance voltage, this type of power distribution transformer will not substantially limit the short circuit current in the case of a fault on its secondary side. It is therefore common practice that power distribution transformers have to be type tested to their ultimate short-circuit conditions. Power distribution transformers may be oil filled or dry-filled. Manufacturer offers a complete range of power distribution transformers, which can be certified in one test access very efficiently. You use a power distribution transformer when you have no interest in what happens below the LV terminal of the power distribution transformer. You use a power distribution transformer element if you are interested in modeling loads or lines on the LV side of the power distribution transformers. If you have a node with a power distribution transformer, any loads on that node are assumed to be applied at the LV side of the power distribution transformer. Modeling the power distribution transformer give you the benefit of computer values for secondary voltages, and transformer losses due to series (or copper) losses and due to no load (or core energization losses. Transformer Design Characteristics:
Transformer Loss Basics Core Losses are constant. Winding Losses increase exponentially with the square of the load. A Transformer uses the core's magnetic properties and current in the primary winding (connected to the source of electricity) to induce a current in the secondary winding (connected to the output or load). Alternating current in the primary winding induces a magnetic flux in the core, which in turn induces a voltage in the secondary winding. A voltage step-down results from the exchange of voltage to current, and its magnitude is determined by the ratio of turns in the primary and secondary windings. A transformer with 50 primary turns and five secondary turns would step the voltage down by a factor of 10, for example from 13,500 volts to 1,350 volts. A given transformer's energy output is lower than the level specified by the nameplate rating due to inefficiencies in both the core and the windings. In general, transformer losses are less than two percent of the total transformer load. The magnitude of the losses is dependent upon the loading of the transformer. Core losses remain constant while winding losses increase with the square of the load. Thus, for a transformer with an average load of 25 percent, the core losses may represent approximately 75 percent of total energy losses. Conversely at 100 percent of rated load, the winding losses may represent more than 80 percent of total energy losses. Thus, core losses make-up a greater share of total losses at lower transformer loads, while the winding losses make-up a greater share of total losses at higher transformer loads. Many different distribution transformer designs are available, depending on the loading patterns and needs of the end-user. Transformer engineers modify transformer design and vary material depending upon circumstances. Transformer design includes variation of :
The following describe these factor in more detail. Core Material Transformer cores are usually made of either grain-oriented silicon steel or amorphous metal. Silicon steel comes in a variety of grades, each with its own conductive and efficiency characteristics. Amorphous metal, a more costly but highly efficient material, can significantly reduce core losses. Constructing the core of laminated sheets, insulated from each other, also reduce losses, but adds to the cost, weight and volume of the transformers. The type of core material preferred by a utility is usually dependent on the cost of its core losses and the expected transformer loading levels. Since the marginal cost of energy for electricity utilities is usually rather low, the financial incentive for moving to high efficiency transformer materials may be limited. Winding Material Generally, copper and aluminum are used for transformer windings. As with silicon steel, these materials are available in a variety of grades and thicknesses, each with their own efficiency characteristics. The types of windings chosen by the transformer designer are also dependent on the cost of a specific utility's losses and on assumed transformer loading levels. Insulating Material The majority of utility distribution transformers are liquid filled. The non-conducting liquid (mineral oil is most commonly used) serves to electrically insulate and cool the transformer. As the core temperature of the transformer rises, the efficiency decreases, so an efficient cooling method improves performance. Typically, transformers perform best at temperatures below 55 C above the ambient temperature. Liquid-filled transformers transfer heat more efficiently than dry-type transformers and are generally preferred for larger applications. Most liquids used in transformers now are non-flammable. Phase Transformers may be designed to step down a single alternating current from one voltage to another, call single-phase transformers, or contain three primary and three secondary windings and therefore provide the output in three-phases. Three-phase transformers induce a more constant magnetic flux and output voltage necessary for motors, heating, ventilating, air-conditioning and other large equipment. Technically, the three-phase transformer is equally efficient to the single-phase transformer. In the US, over 95% of average annual transformer sales comprise single-phase distribution units, mostly in the 15kVA to 25kVa range. Since the larger transformers tend to be three-phase, these comprise 38% of the total MVA sold each year. Mounting Distribution transformers are either mounted on an overhead pole or on a concrete pad at ground level. There is some evidence to suggest that pole mounted transformers dissipate heat more easily than pad mounted units and may therefore be more fully loaded. Correct Sizing Properly sizing a transformer for a given application has a significant impact on the overall transformer efficiency and energy loss level. Oversized transformers are lightly loaded, and consequently lose more energy from excess core losses than optimally sized transformers. Conversely, undersized transformers operate at higher load levels and experience high load losses. Energy Losses from New Distribution Transformers A new 50kVA single-phase utility transformer will produce approximately 70 MWh in energy losses. An average 1,500kVA liquid-filled three-phase transformer will produce approximately 1.4GWh in losses over its life time. Studies in the US have shown that new, efficient transformer designs could reduce energy losses and associated air emissions by 10 percent to more than 40 percent, depending on materials used and the loading pattern of the transformer. Advance in transformer design have produced substantial transformer efficiency improvements over the past 20 years. The most significant improvements have been made in core technologies with the use of high-efficiency silicon-steel and amorphous metal. Due to the large numbers of transformers in service and the constant nature of energy losses, only small increases in efficiency are need to produce significant economic and environmental gains. Sources of Transformer Efficiency Transformer energy losses can be reduced by improving the efficiency of the core or windings. The relative importance of core and winding losses depends on the loading on the transformer and the cost of each type of loss to the utility. Core Loss Reductions Since the majority of transformer losses at low load levels are due to core inefficiencies, much of the research on reducing transformer losses has concentrated on building more efficient cores. Core losses result from cyclic changes in the magnetic state of iron, and "eddy-current" losses caused by the flow of small currents in the iron. Core losses can be reduced by improving the magnetic permeability of the core material or by using a core material that offers less magnetic resistance. Considerable progress in reducing core losses has been made over the past twenty years, primarily through material improvements. In the early 1970's manufactures introduced more efficient silicon-steels. The four main grades of silicon-steel used in transformers are M2, M3, M4 and M6(decreasing in efficiency). Differences are due mainly to the chemical composition and the rolling techniques used in manufacture of the core. The increased domestic availability of higher grades of silicon-steel (M2 and M3) and new manufacturing processes has led to the improved efficiency of silicon-steel distribution transformers. Amorhous metal, a highly efficient material used in transformer cores, possesses good magnetic properties, low inherent magnetic resistance losses, and high resistively. Due to its ability to be constructed into very thin sheets, "eddy-current" losses are significantly reduced. Amorphous metals have been found to reduce core losses by as much as 70 percent. However, the cost of transformers with more efficient cores increases due to the following factors:
Winding Loss Reductions Winding losses, or load-losses, arise from the conducting material's inherent resistance to the flow of electrical current. Winding losses increase with the square of the transformer load. Efficiency gains can be achieved by using materials with lower resistively or greater diameters. For example, distribution transformer coils made with low resistively conductors, such as copper, can have considerable lower load losses than those made with other materials. However, low resistively conductors often cost more than other conducting material. Importance of Sizing Overall transformer efficiency depends critically on the percent of time that the transformer is heavily or lightly loaded. the load factor. Transformers need to be sized to cope with expected peak loads, rather that average loads, and therefore where there is a large disparity between these two, the load factor will be small. For example, distribution transformers serving primarily residential loads regularly carry average loads that are only 15 percent to 20 percent of the transformer's rated capacity but also must be designed to support peak morning and evening loads. Because of the wide gap between peak and non-peak loads, and the relatively limited amount of time that the transformer is peak-loaded, average transformer loading tends to be fairly low. In this case, total losses may be mainly attributed to core losses. Larger distribution transformers, used more often in transforming power for commercial or industrial customers, tend to be loaded at higher average levels over the course of the year. Transformers that serve businesses operating from 9:00 am to 5:00 pm, for example, typically experience a consistent and relatively higher load throughout the day. In this circumstance, it is likely that load losses will make the major contribution to total losses. Correctly sizing a transformer is therefore critical to the quantity and source of losses, and optimizing transformer design for efficiency remains a complex task. To complicate matters, the marginal cost of energy varies dramatically throughout the day, altering the cost of energy losses and the cost-benefit of installing more efficient transformers from the utility perspective. In addition, transformer loading patterns tend to change over time. Homeowners may accumulate more appliances and equipment (or new houses built in the area), or businesses may expand and consequently increase the load on the transformer. Generally, utilities estimate load growth when sizing and purchasing transformers. In the Us it has been calculated that, on average, utilities size single-phase transformers so that transformer peak load at installation is approximately 88 percent of its capacity, and 157 percent of capacity at the end of its service life.
In an effort to improve transformer
sizing practices, the U.S. Environmental
Protection Agency as part of the Energy Star Transformer
Program, has developed technical tools to enable
utilities to enter utility-specific loading information
in order to optimize transformer size and total owning
cost which providing reliability and energy-efficiency.
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