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Power S stems En ineerin Research Center
PSERC Background Paper
Distributed GenerationRobert H. Lasseter
Professor Emeritus, Department of Electrical and Computer
EngineeringUniversity of Wisconsin-Madison
September 4, 2003
The electrical power system consists of three major components:
generation, ahigh voltage transmission grid, and a distribution
system. The high voltagetransmission system links the generators to
substations, which supply power tothe user through the distribution
system. Interruptions in these connecting linkscan disrupt the flow
of power from generators to the users.
Over the last ten years the reserve capacity in transmission has
been falling dueto increases in both electricity demand and
generation capacity. This hasresulted in an over utilized
transmission system with an increasing likelihood ofloss of power
during a disturbance.
One obvious solution is to build more transmission lines and
improve reliabilityprotocols. A not so obvious alternative is to
integrate generation within thedistribution system reducing the
dependence of local loads on the transmissiongrid. This latter
approach is referred to as distributed generation.
It is generally agreed that any power production that is
integrated within thedistribution system is called distributed
generation. Distributed generationencompasses a wide range of
generator technologies, such as internalcombustion engines, gas
turbines, microturbines, fuel cells, photovoltaics andwind power.
The larger units are normally gas turbines, large internal
combustion
engines, wind turbines and some fuel cells. Their ratings are in
millions of wattsof power at thousands of volts. The applications
include direct power support atsubstations and deferral of the need
to build more transmission.
While placing distributed generation at substations will reduce
the reliance on thehigh voltage transmission grid, it does
introduce problems of control and dispatchof each unit. At a
minimum, additional central real time monitoring and controlsystems
need to be designed and implemented. The net result is an increase
in
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the power system’s complexity. An example that neglects this
complexity is IEEE1547 Standard for Interconnecting Distributed
Resources with Electric PowerSystems. This standard requires that
distributed generators disconnect from thegrid when there is a
disturbance. In the case of a grid disturbance due tooverload,
disconnecting is exactly the wrong action since the system needs
loads
not generation to disconnect.
It is obvious that indiscriminant application of distributed
generation with centralcontrol can cause as many problems as it may
solve. A better way to realize theemerging potential of distributed
generation is to take a system approach whichviews generation and
associated loads as a subsystem or a “microgrid”. Thisapproach
allows for control of distributed generation through load
followingtechniques, reducing or eliminating the need for central
dispatch. Duringdisturbances, the generation and corresponding
loads can separate from thedistribution system to isolate the
microgrid’s load from the disturbance (andthereby maintaining
service) without harming the transmission grid’s integrity.
This ability to island generation and loads together has a
potential to provide ahigher local reliability than that provided
by the power system as a whole.
Smaller units, having power ratings in thousands rather than
millions of watts,can provide even higher reliability and fuel
efficiency. These units are alsoclustered with loads creating
microgrid services to customer sites such as officebuildings,
industrial parks and homes. Since the smaller units are modular,
sitemanagement could decide to have more units than required by the
customerload, providing local, online backup if one of the
operating units failed.
Most existing power plants, central or distributed, deliver
electricity to user sites atan overall fuel-to-electricity
efficiency in the range of 20-40%. This represents a lossof around
70% of the primary energy provided to the generator. To reduce
thisenergy loss, it is necessary to either increase the
fuel-to-electricity efficiency of thegeneration plant and/or use
the waste heat. For site-based microgrids, it is mucheasier to
place generators near the heat loads therebyallowing more effective
useof waste heat than substation-based systems. Of course, there
are specialsituations where the waste heat from large generators
can also be utilized. Theseapplications produce both electricity
and the byproduct ofonsite thermal energy,converting 80 percent or
more of the input fuel into useable energy. It also has
thepotential to dramatically reduce industrial sector carbon and
air pollutant emissions.
Economic, technology and environmental incentives, along with
end userdemands, are changing the face of electricity generation
and transmission.Centralized generating facilities are giving way
to smaller, more distributed units.Either type of microgrid could
help shoulder the nation’s demand for more power,less complexity
and higher local reliability without further overburdening
thetransmission grid.
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Contact Information
Robert H. Lasseter
Site DirectorProfessor EmeritusElectrical and Computer
EngineeringUniversity of Wisconsin-Madison1415 Engineering
DriveMadison, WI 53706Phone: 608-262-0186Fax: 608-262-1267e-mail:
[email protected]
Power Systems Engineering Research Center 428
Phillips HallCornell UniversityIthaca, NY 14853-5401Phone:
607-255-5601
© 2003 University of Wisconsin Board of Regents
mailto:[email protected]:[email protected]://pserc.org/http://pserc.org/http://pserc.org/mailto:[email protected]
