10. Reduce Losses in the Transmission and Distribution System10. Reduce Losses in the Transmission andDistribution System1. ProfileElectricity losses occur at each stage of the powerdistribution process,1 beginning with the stepup transformers2 that connect power plants tothe transmission system, and ending with thecustomer wiring beyond the retail meter. The systemconsists of several key components: step-up transformers,transmission lines, substations, primary voltage distributionlines, line or step-down transformers, and secondarylines that connect to individual homes and businesses.Figure 10-1 shows a diagram of these system components.These electricity losses are often referred to genericallyas “line losses,” even though the losses associated withthe conductor lines themselves represent only one typeof electricity loss that occurs during the process oftransmitting and distributing electricity. System averageline losses are in the range of six to ten percent on mostUS utility grids, but they increase exponentially as powerlines become heavily loaded. Avoiding a small amount ofelectricity demand in the highest peak hours can reduceline losses by as much as 20 percent. At such levels oflosses, disproportionately more generation resources needto be operated to deliver the same amount of electricity toend-users.Each of the stages identified in Figure 10-1 is subject tolosses, and therefore provides opportunity for efficiencyimprovements. The cumulative benefits can be very significant. This is because a one-kilowatt (kW) load reductionat the customer’s end translates into more than a one-kWload reduction – sometimes very much more – moving“upstream” to the distribution, transmission, and generation levels because of losses compounding along the way.Each component of the distribution system can beoptimized to reduce line losses. This chapter discusses eachcomponent, and how equipment choices can affect efficiencyFigure 10-1Simple Diagram of an Electric Transmission and Distribution System3Power Plant1Step UpTransformerTransmissionLinesStep DownTransformer“Distribution” is, regrettably, an ambiguous term whendiscussing electric power. As used in this sentence, it reflectsthe overall process of delivering electricity from power plants(where it is generated) to end-users (where it is consumedby homes, businesses, and institutions). Distribution is alsoa technical term of art, however, which refers to the lowervoltage, later stages in the electricity delivery process, asillustrated in Figure 10-1. The reader should remain cognizantof the potential for the confusion this ambiguity creates.DistributionLinesStep DownTransformerResidence2Transformers are used to increase voltage for more effectivetransportation of electricity and to decrease voltage back tolevels suitable for industrial, commercial, and residential use.3Adapted from: Cowlitz County (WA) Public UtilityDistrict. (Undated). Electricity-Transmission (How ElectricityMoves). Available at: lectricity%20-%20Transmission.pdf.10-1
Implementing EPA’s Clean Power Plan: A Menu of Optionsand, by extension, greenhouse gas (GHG) emissions.In addition, line losses can be significantly affected byend-use energy efficiency policies (detailed in Chapters 11through 15) and demand response programs (Chapter 23).Engineering FundamentalsLosses occur in both transmission and distribution linesand in transformers, the fundamental components of theelectricity distribution system or “the grid.” Some losses,called “core” or “no-load” losses, are incurred to energizetransformers in substations and on the distribution system.A larger share is labeled “resistive” or “copper” losses; theselosses reflect the resistance of the materials themselves tothe flow of electricity.Core losses are typically 25 to 30 percent of total distribution losses, and do not increase (or decrease) withchanges in load. They are largely influenced by the characteristics of the steel laminations used to manufacture thecore of transformers.Resistive losses are analogous to friction losses in the linesand transformers. As loads increase, the wires (includingthose in the transformers) get hotter, the material becomesmore resistive, and line losses increase. For this reason, resistive losses increase exponentially with the current on a line.4At low-load periods, system losses are almost entirelycore losses, and may be as low as three percent.5 Duringpeak electrical demand periods, however, resistive lossesbecome dominant. At the highest load hours, averageline losses increase into the 10- to 15-percent range,but marginal line losses (those that are avoided if loadis reduced) may increase to 20 percent or more. Thisconcept is analogous to a freeway at rush hour – even asmall reduction in traffic volumes can produce very largereductions in “friction” and improve traffic flow. At peakextremes, it can take five power plants operating to providethe end-use electricity normally provided by four.Therefore, line loss reduction is partly a function ofsystem design and construction, but is also heavily affectedby operation of the underlying electrical loads and by howwell peak loads are managed. Chapters 11 through 15 and23 address energy efficiency and peak load management,both of which are very important in reducing line losses.Key Units for Measuring ElectricityThis chapter necessarily involves technologies andterminology that may be foreign to air quality regulators,but are quite well understood by the utilities that theyregulate. Several terms reflecting common units ofelectrical measurement – and their abbreviations – aredefined below. Amperes (A): A measure of the current flowthrough lines and transformers. It is analogous tothe flow of water through a pipe. Kilovolts (kV): Thousand volts, the unit ofmeasure for generation, transmission, anddistribution lines. Kilowatt-hour (kWh): A measure of energy orpower consumed in one hour. Volts (V): Voltage is what drives current throughlines and transformers to end-use appliances inhomes and businesses. It is analogous to pressure4This is reflected mathematically as I2R, meaning the lossesincrease with the square of the current (“I” or amperage)multiplied by the resistance (R) of the transformer windingor line conductor.in a water pipe. Voltage must be delivered within anarrow range of between 110 and 124 volts at alltimes for residential appliances and equipment tooperate properly. Watts (W): A measure of the quantity of poweror work (horsepower) that electricity can do atany moment. Watts is the product of amperesmultiplied by volts. For example, 220 volts at 20amps equals 4400 watts, about the amount that atypical residential electric water heater uses. A onehorsepower (1 hp) swimming pool pump motoruses 746 watts.A later section will discuss additional terms, includingpower factor and reactive power, which slightly modifythese units of measurements to reflect the character ofelectricity usage. Reactive power is measured by voltamperes (VA) and by volt-ampere reactive (VAR).510-2Because the current is low, the square of the current is alsosmall.
10. Reduce Losses in the Transmission and Distribution SystemComponents of the System ThatContribute to LossesEach component of the utility transmission anddistribution system contributes to losses, so a loss avoidedat the customer’s end-use or meter compounds, movingback up the system to the generation level. Table 10-1below illustrates typical line losses at each stage below thetransmission receipt point. Transmission system line lossesgenerally involve two (or more) additional transformationstages and one (or more) additional set of lines. Dependingon voltage and distance, transmission line losses range fromtwo to five percent.Table 10-1Losses at Each Stage of Electricity Distribution6Estimated Loss as aPercentage of Energy SoldComponentTypical UrbanTypical RuralSubtransmission Lines0.10.7Power Transformers0.10.7Distribution Lines0.92.5Distribution Transformers No Load1.21.7Distribution Transformers Load0.80.8Secondary Lines0.50.9Total3.67.3The following section describes each segment of thetransmission and distribution system, with an indication ofhow losses occur and how they can be mitigated.Step-Up Transformers. These are the transformerslocated at generating facilities, which convert the powerproduced at generating plants to voltages suitable fortransmission lines. Typical large generators produce powerat 6600 volts, 13,800 volts, 18,000 volts, or even 22,000volts, whereas typical transmission voltages in the UnitedStates are 115 kV, 138 kV, 230 kV, 345 kV, 500 kV, and 765kV. Step-up transformers are typically sized to the generating units, with modest losses at normal operating levels.If, however, they carry more power than their originalintended capacity, losses increase sharply. This can be aproblem when generating units have been “uprated” toprovide higher output without similar uprating of the stepup transformers. Also, if the generators are operating at anon-optimal power factor (explained below), the resultingincreased reactive power output (also explained below) canincrease system losses at every level.Transmission System Conductors. Long-distancetransmission lines bring power from generators to theservice territory of the distribution utility. In the westernUnited States, these distances can exceed 1000 miles (forexample, power from the Canadian border to Los Angeles).Although the conductors themselves have low resistance,the length of the lines and the sizing of the conductorsaffect losses. Losses along the line may be greatly reducedin direct current (DC) long-distance transmission systems,making DC transmission desirable for very long-distancetransmission lines. However, additional losses of up to 1.5percent occur in the converter stations at each end of a DCtransmission line.Distributing Stations. Many utilities have anintermediate step on their systems, with power taken from“distributing stations,” which receive power at high voltage(230 kV and higher) and deliver that power to multipledistribution substations at 69 kV or 115 kV. Transformerlosses that occur in substations are incurred twice – firstin transforming power from high-voltage transmissionto an intermediate voltage, then again at the substationstransforming it down to primary voltage. The principallosses in distributing stations are transformer losses. Thereason utilities use separate voltage levels is to isolatebulk power transfers from power that is serving load. Thisapproach increases system reliability.Substation Transformers. These take power from thetransmission system, typically at 115 kV or higher, andconvert it to the distribution voltage levels of 4 kV to 34 kV.Sized specifically for their maximum expected loads, theyvery seldom carry power near that limit in order to allowfor load transfer between circuits, but there are two issuesof concern. The first is core losses that may be too highwhen they are lightly loaded. The second is resistive lossesthat may be too high when they are heavily loaded.Voltage Regulators. These are transformers withmultiple taps installed along distribution circuits toenable increasing or decreasing voltage at variouspoints. Historically these were installed along long ruraldistribution lines to enable a step-up of voltage at distantpoints, offsetting reduced voltage caused by resistance610-3Hydro One. Distribution Line Loss Study. Ontario EnergyBoard Docket. No. RP-2005-0020. Available at: 6-rates/hydroone 0-%20Schedule%202.pdf.
Implementing EPA’s Clean Power Plan: A Menu of Optionsof the lines. Today there are additional functions forthese devices. They enable acceptance of higher levelsof distributed resources, such as residential solar, onto acircuit, by allowing the grid operator to ensure that “hotspots” do not result from the injection of power at midcircuit. In addition, they enable more rigorous conservationvoltage regulation along a distribution line, which canreduce total power consumption (see Chapter 5). Becausethey are transformers, they involve both core losses andresistive losses, and attention to both the materials and thesizing of these affects the level of line losses.Primary Distribution Lines. Primary lines connectsubstations to circuits that bring power into businessdistricts and neighborhoods. These typically run at 4 kV to34 kV. The higher the voltage, the lower the current, andthus the lower the resistive losses on these lines. However,higher voltages require taller poles (or more expensiveundergrounding technology), so there is a cost/efficiencytradeoff.Line Transformers. These are the garbage-can-sizedcylinders you see mounted on neighborhood power polesor in metal boxes mounted on concrete pads. They convertprimary voltage distribution power to the voltages we usein our homes and businesses, typically 120 V, 208 V, 240 V,277 V, and 480 V.Secondary Distribution Lines. These connect linetransformers to individual homes and businesses. They aretypically very short, in part because at these lower voltages,the amperage needed to move power is significant, whichrequires larger (and thus more expensive) conductors.Losses can be quite high owing to the high current. This isespecially true if the secondary load has grown beyond orfaster than original projections.Reducing Transformer LossesRecall that transformer losses are caused in two differentways, core (no-load) losses and resistive (copper) losses.Core losses are the losses incurred to energize thetransformer. These vary with the size of the transformerand the materials used to construct the transformer.It is essential to “right-size” transformers to minimizecore losses. In a situation in which, for example, a largeindustrial customer with heavy machinery and high powerdemand moves out of a l