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