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Major Topic Sections

Fossil-fueled Power
Non-Fossil Generation
End-Use Efficiency
Electricity T&D
Carbon Sequestration
Non-CO2 Reductions
Other GHG Reductions

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Elec. Transmission
Elec. Distribution
Smart Grid Technology

 

 Electricity Distribution

 
 Background


The distribution system represents the link between the transmission grid and the customer.  The distribution system in many instances involves voltages up to 35kV.

All options that reduce system energy losses will have a direct impact on reducing emissions of greenhouse gases. There are many different ways to improve the efficiency and reduce losses in the distribution system. In addition to options for improving distribution line performance, improvements can be made in the equipment used on the system, such as transformers, which are a major source of energy loss. 

The text below discusses several specific areas for distribution improvements:

  Compensation to Reduce Reactive Power Losses
  Distribution System Automation
  System Voltage Optimization and Phase Current Balancing
  Lower Loss Transformers
  Dispersed Energy Storage
  Point of Use Automated Billing/Metering


Compensation to Reduce Reactive Power Losses

Energy losses in transmission lines and transformers are of two kinds: resistive and reactive. Losses caused by the resistive component of the load cannot be avoided, while losses coming from reactive component of the load can be. Reactive losses come from circuit capacitance (negative), and circuit inductance (positive).

The use of electric motors requires that the distribution system deliver a form of power known as “reactive power.”  In residential areas, this is generally not a problem because of the small size and limited number of motors.  For commercial and industrial customers, the situation may be quite different.  In rural areas with high concentrations of oil and/or water wells, reactive power is a significant potion of the load.  Supplying large amounts of reactive power through the distribution system increases current and energy losses.   

When capacitors of appropriate size are added to the grid at appropriate locations, the energy losses can be minimized by reducing the reactive power component, thereby reducing the observed power demand. Connecting capacitors to the distribution system compensates for the reactive power and reduces current energy losses back through the power system. 

Compensating for reactive power (correcting the power factor by adding capacitors) improves the efficiency of the electric system by reducing the amount of current flowing in a line, thereby reducing the line losses and reducing greenhouse gas emissions. Power factor correction capacitors for application on primary distribution feeders are commercially available for use on the full range of voltage levels and in practical kVAr sizes. Power factor correction capacitors can be installed with switches and relays that sense low and high voltage.


Distribution System Automation

Distribution Automation (DA) refers to a system that enables an electric utility to remotely monitor, coordinate, and operate distribution components in a real-time mode.  In a DA system, there are feeder automation options that include: demand side management (DSM), remote switch control, integrated volt-var control, service restoration, feeder configuration, trouble call, fault location/isolation; load check, and safety checks.  There are also customer automation options that include: remote metering, load control, load shedding and shaping for emergencies; economic operation; cold load pickup, remote connect/disconnect, trouble call, and tamper detection. 

Some of these services may have the net effect of increasing the overall level of efficiency of the electric system, in addition to improving overall electric service.  The efficiency increases are gained by optimizing power flows on lines, which reduces line losses.  Increased efficiency not only means reduced costs; it also means reduced greenhouse gas emissions as less generation is used to provide the same level of service.

The reliable and cost-effective operation of the distribution power system will become increasingly reliant on an overlaying, complex information infrastructure of field equipment, communications links, integrated sets of applications, and data from other systems. For the power system to operate reliably, this information infrastructure also must be reliable and well-managed for DA functions to be efficiently and effectively implemented and maintained over the long term. Only with a reliable information infrastructure can DA functions be trusted and, therefore, fully used in distribution operations by distribution dispatchers.

Together, these services help optimize line power flow and increase system efficiency (and reduce cost), which reduces generation demand and the emission of greenhouse gases, while providing the same level of service. Work on superconducting technology (and other products) is underway that is expected to further increase efficiency of distribution controls.


System Voltage Optimization & Phase Current Balancing
 

Electric energy is supplied to customers at a utilization voltage that is maintained within prescribed limits to insure proper operation of customer equipment. Maintaining the voltage as close to the standard as is practical controls electrical losses and contributes to improved system efficiency. Improved reliability through reduction in forced outages will have a secondary benefit in the form of fewer hours of lost production and a higher quality of life for customers.

Careful engineering of distribution system components and the use of voltage-regulating equipment are required. Connecting single-phase load in a careful way eliminates losses associated with residual current flow. These reductions in distribution system losses in turn reduce generation demand and GHG emissions.

Voltage regulation is accomplished by adjusting the turns ratio of transformers and by the control of reactive power. Automatic control of transformer taps and shunt capacitors and shunt reactors can be accomplished with Supervisory Control and Data Acquisition (SCADA) and automation systems.


Lower Loss Transformers

Distribution transformers are one of the most widely used elements in the U.S. electric distribution system. Transformers are the devices that change the voltage of an AC electric circuit, converting electricity from the high voltage levels in utility transmission systems to voltages that can safely be used in businesses and homes. Although used throughout the electric system, they are most commonly used to reduce the voltage from the distribution level of 4 - 69 kV to the level required by the customer.

Over 40 million distribution transformers are currently in service on electric utility distribution systems, and utilities nationwide purchase more than one million new units annually. Transformers are a crucial link in the utility industry’s efforts to bring American consumers safe, reliable, and cost-effective electricity. One of the most striking features of transformers is their dependability and long service lives. On average, transformers remain in service for over 30 years, during which time they perform their vital function reliably and with little degradation in service quality.

When a transformer is energized, an electrical loss in the transformer known as “core loss” occurs. The losses are small, and many transformers operate at efficiency levels that often exceed 98 percent.  Despite those high average efficiencies, transformers have a significant overall energy impact. Of the 9% total losses attributable to transmission and distribution from the point of generation to the point of use, 2-3% can be assigned to losses in feeder conductors and transformers. 

Core loss reduction has the greatest potential because these losses are present anytime the transformer is energized, regardless of the load. In addition, a decrease in winding loss, which is a function of transformer load, is also achievable, especially when compared to the loss in some of the older units on a system.

Transformer energy losses can be reduced cost-effectively by 10 to 40 percent using a variety of available transformer technologies, not withstanding the already high average efficiency of new transformers. These small efficiency improvements can significantly reduce energy losses and emission levels associated with distribution transformers.  Because of the large number of transformers installed throughout the country, there is a significant potential for reducing greenhouse gas emissions.

Another way to reduce core losses in a transformer is to change the metal used in the core to one which offers less magnetic resistance. In recent years, transformer cores utilizing amorphous steel have been developed. Unlike most metals that take on a regularly patterned crystalline structure as they cool, amorphous metals retain a more random internal structure that gives them unusual physical and magnetic properties.

Efficiency of distribution transformers can be improved by various means such as 1) using copper instead of aluminum wire, 2) increasing the size of the core, 3) utilizing advanced high efficiency core materials, and 4) configuration of the core and coal arrangement within the tank, all with a higher initial cost.

In 1995, ENERGY STAR qualified utility distribution transformers were introduced to encourage manufacturers and utilities to produce and purchase high-efficiency distribution transformers, increasing profitability and reducing lost or wasted electricity. 


Dispersed Energy Storage

Distributed resources are small generation (1kW to 50MW) and/or energy storage devices typically sited near customer loads or distribution and sub-transmission substations. Distributed Resources provide grid, system, or customer benefits, such as standby generation, peak shaving, combined heat-and-power (CHP), prime power, premium power, or renewable power. DR options may operate in grid-connected or grid-independent modes, or in transition between these states.

The distributed utility concept has been set forth as one view of the electric utility of the future. In this vision, customer demand, energy needs, and service requirements are met by a combination of conventional bulk power sources and small, modular generation and/or storage systems strategically located throughout the electric distribution system.  Utilities may wish to take advantage of new technologies to supply energy from dispersed sources, delay facility upgrades, enhance the use of their distribution network, mitigate the risks of new construction, and improve reliability.

Dispersed sources, which may not all be under the ownership of the utility, will pose challenges in the control, protection, operation, and maintenance of a distribution system. The integration of dispersed generation, particularly renewable energy technologies that are intermittent generators (i.e., solar and wind power) is facilitated by dispersed energy storage systems. Energy storage systems use electricity during non-peak hours or from intermittent sources to convert water to ice or chilled water (for cooling), or to store energy in batteries. During peak periods, this stored energy can be converted back to electricity for use.

The use of these systems, in addition to filling the traditional role of meeting peak demand needs, increases overall system efficiency and reduces total system losses. The resulting reduction in generation demand also reduces GHG emissions. The utilization of dispersed energy storage systems also reduces GHG emissions by allowing greater use of local low- or non-carbon fueled generation systems at the local level.

Use of dispersed storage systems will enable utilities to lower costs through deferrals of upgrades and new construction, supply new generation to customers, and improve reliability. The use of dispersed energy storage systems throughout the distribution system will improve dynamic operating capabilities and asset utilization, allowing existing generation to function more efficiently and improve the overall efficiency of the system.

Uninterruptible Power Supplies and other forms of disbursed storage and generation range from a few kilowatts to several hundred kilowatts. Emerging energy storage systems, which can be dispersed throughout the distribution system, include batteries, flywheels and superconducting magnetic energy storage (SMES). 


Point of Use Automated Billing/Metering

Improved customer service can be described in terms of improved reliability. Service reliability is measured in terms of frequency and duration of outages. Other measures of service include prompt and accurate billing and payment procedures, as well as providing information to users. In the case of commercial and industrial customers, service assistance may include energy audits and surveys. Some service programs include assistance in conversion to energy efficient lighting.

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 References, Sources, and Other Useful Data


BTE Corporation, “Capacitor Control Concepts”
http://www.btecentral.com/NTMC/capacitor_control.htm

BTE provides highly customized, quality control systems and consulting services. Capacitor Control is usually done to achieve as a reduction in losses due to reactive load current, a reduction in kVA demand, decreased customer energy consumption, an improved voltage profile, and/or increased revenue. This paper discusses some of the concepts in capacitor control.

Congressional Budget Office, “Prospects for Distributed Electricity Generation” (Sept. 2003)
http://www.cbo.gov/ftpdocs/45xx/doc4552/09-16-Electricity.pdf

This CBO paper examines the current state of and prospects for distributed generation, its benefits and risks, and barriers to the wider adoption of small-scale, customer-owned technologies. The paper also discusses what types of policy changes could help reduce barriers while limiting the downside risks of greater reliance on distributed generation.

EC&M magazine, “Improving Power Factor with Variable Speed AC Drives” (July 1, 2003)
http://www.ecmweb.com/powerquality/electric_improving_power_factor/ 

EC&M magazine is the technical authority for 140,000+ electrical professionals. This article discusses some of the benefits, and some of the not-so-predictable side effects, of installing variable-speed AC drives.  By installing variable speed AC drives, you can improve process controls, increase energy savings, and reduce wear on the machinery. Such drives also have the added benefit of improving PF (the ratio of real power to reactive power).

Electric Power Research Institute, Wade Malcolm and Mark McGranaghan, “EPRI's 2006 Portfolio in Distribution Systems and Automation (May 2006)
http://www.electricdistribution.ctc.com/pdfs/ED06/Malcolm.pdf

This 24-slide presentation describes EPRI's base program and application project opportunities for Power Delivery and Markets.

Electric Power Research Institute, “Primer on Distributed Energy Resources for Distribution Planning” (Oct. 2002)
http://my.epri.com/portal/server.pt?Abstract_id=000000000001004644

Many factors -- including restructuring of the electric utility industry and an increased demand for electricity -- are driving the adoption of distributed energy technologies. This primer outlines the potential impacts that distributed generation and energy storage technologies (collectively called distributed energy resources) may have on utility distribution company planning. The primer focuses on distributed generation technologies with a capacity of 500 kW to 5 MW as well as energy storage systems with capacities up to 15 MW and ride-through times as high as several hours.

Electric Power Research Institute, “Distribution Knowledge-Based Services” (May 2006)
http://my.epri.com/portal/server.pt?Abstract_id=000000000001013650

Distribution companies are facing a wide variety of pressures and technical challenges. Despite continually shrinking budgets, utility planning, engineering, and operations personnel must keep up-to-date with the latest technologies, software tools, standards, and procedures for optimizing distribution system performance. EPRI's Distribution Knowledge-Based Services cost-effectively supports utility distribution engineering managers and staff with exclusive technical resources, training, and standards information. This service provides subscribers with access to the best distribution engineering expertise in the industry to help deal with specific challenges in a timely manner and stay informed on key technical developments.

Electric Power Research Institute, "Engineering Handbook for Dispersed Energy Systems on Utility Distribution Systems," November 1995, Publication TR-105589
http://my.epri.com/portal/server.pt?Abstract_id=TR_105589

The deployment of small, modular generation and storage systems within the electric distribution system presents a number of new technical and economic challenges for electric utilities. This handbook provides a digest of material on planning and implementing distributed storage and generation (DSG) systems. The handbook is especially applicable for electric utility distribution engineers, distribution planners, consultants, and end users.

Electric Power Research Institute, “Technical and System Requirements for Advanced Distribution Automation",  Report #1010915 (June 2004)
http://mydocs.epri.com/docs/public/000000000001010915.pdf 

Traditional distribution systems were designed to perform one function: distribute electrical
energy to end-users. Advanced Distribution Automation (ADA) is a concept for a fully
controllable and flexible distribution system that will facilitate the exchange of both electrical
energy and information between participants and system components. This report presents
background information on distribution automation technologies and develops a roadmap to
achieve the ADA systems required for future power delivery systems.

Office of Gas and Electricity Markets (UK), “Electricity Distribution Losses Workshop” (14-Apr-2003)
http://www.ofgem.gov.uk/Networks/ElecDist/Policy/DistChrgs/Documents1/3353-TimetableApril-2003workshop.pdf

OFGEM is the regulator for Britain's gas and electricity industries. Its role is to promote choice and value for all customers. The purpose of this workshop was to develop further thinking on the subject of electricity distribution losses.

Tepel, R.C. et al., 1987, "Customer System Efficiency Improvement Assessment: Supply Curves for Transmission and Distribution Conservation Options", Battelle Pacific Northwest Laboratory, PNL-6076.
http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=5562193&query_id=0

This report was part of the Customer System Efficiency Improvement (CSEI) Assessment Project. A principal objective of this project was to assess the potential for energy conservation in the T&D systems of electric utilities in the BPA service area. The scope of this assessment covers BPA customers in the Pacific Northwest region and all non-federal T&D systems, including those that currently place no load on the BPA system. Supply curves were developed to describe the conservation resource potentially available from T&D-system efficiency improvements. These supply curves relate the levelized cost of upgrading existing equipment to the estimated amount of energy saved. Stated in this form, the resource represented by T&D loss reductions can be compared with other conservation options and regional electrical generation resources to determine the most cost-effective method of supplying power to the Pacific Northwest.

U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability
http://www.oe.energy.gov/

The Office of Electricity Delivery and Energy Reliability (OE) is leading Federal efforts related to several sections of the bill and is responsible for completing a number of activities and studies. The mission of this office is to lead a national effort to modernize and expand America's electric delivery system.

U.S. Energy Association and U.S. AID, “Handbook of Climate Change Mitigation Options for Developing Country Utilities and Regulatory Agencies”, Chapter 6, "Distribution System Actions"
http://www.usea.org/Archive/Climatechange/PDF/chapter6.pdf

The U.S. Agency for International Development and the U.S. Energy Association authorized the compilation of this handbook to increase awareness of the climate mitigation benefits from each practice among utility personnel and regulators in developing countries. Chapter 6 discusses Distribution System actions.

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