New
Sandia control method opens doors for more photovoltaic
systems being safely linked to electric grid
Sep 26, 2000 - Sandia
ALBUQUERQUE, N.M. A new way to safely, simply
and effectively connect electricity-producing photovoltaic
(PV) solar systems to utility company power grids
has been developed by the Department of Energy’s
Sandia National Laboratories. Already several manufacturers
have adapted it into their systems.
Called the “non-islanding inverter,” it
is receiving a “thumbs up” from standards
organizations and PV system manufacturers around the
country. They see it as a door opener for more freestanding
PV systems to be hooked up to the nation’s grid.
Basically, it is a safety-centered, Underwriters
Laboratories-certified control method that automatically
diverts or turns off electricity flow from grid-connected
PV systems when an electric distribution line shuts
down.
“Electric utility companies are reluctant to
connect photovoltaic, or solar cell, units to the
grid because of the potential for safety hazards and
equipment damage,” says Sandia researcher John
Stevens. “This new method eliminates that possibility.”
The problem has been that if a utility needs to
shut down power in a distribution line for repair
or some other reason, the line must be completely
de-energized. This is for the safety of a line worker
or a passerby coming in contact with the line.
Before the Sandia development, photovoltaic systems
could not determine that a line had been de-energized
under all conditions and it could continue sending
electricity through the line. Currently, in order
for PV systems to be connected to the grid, they must
meet specialized hardware requirements for different
utility jurisdictions.
The new “non-islanding inverter” permits
photovoltaic systems to sense that a line has been
de-energized and to automatically shut off power production
or divert the electricity to the house or business
to which it is connected.
The method comes in the form of computer code built
into the inverter portion of the photovoltaic system.
The inverter takes DC current from the photovoltaic
array and turns it into AC current.
Stevens says the idea emerged a couple of years
ago when he was chairing an Institute of Electrical
and Electronics Engineers (IEEE) working group that
was developing grid-connection PV standards. Among
members of the group were representatives of utility
companies concerned about hooking up photovoltaic
units to the grid.
“We worked on this two years, going down several
blind alleys before coming to a good workable approach,”
he says.
Working on the project were several people from
Sandia’s Photovoltaic System Applications Department
and a researcher from Ascension Technology in Boulder,
Colo. In addition, an advisory group from the photovoltaics
inverter industry provided input.
Stevens says the goal is for inverter manufacturers
to adopt the technology as a way to make solar power
a more viable option for electricity production. Grid
connection means the PV system owner can sell excess
electricity. This is good for both the owners who
can make a profit and for the utilities that have
available additional power resources at peak usage
times.
The non-islanding inverter concept also can be used
with other types of alternate energy such as fuel
cells and microturbines.
In addition, Sandia’s non-islanding inverter
has been added to IEEE standards, becoming the first
standard of its kind allowing utility interconnection
of non-utility-owned distributed generation equipment.
Stevens says older inverters cannot be retrofitted
with the new technology because the fit involves adding
a computer code. However, he anticipates it will be
in nearly every new photovoltaic system built this
decade.
Two companies currently have the system in their
commercial inverters. They are Ascension Technologies
and Omnion Power Electronics Corp. Several other inverter
manufacturers are in the process of adapting the technique
to their products.
Sandia is a multiprogram laboratory
operated by Sandia Corporation, a Lockheed Martin Company,
for the United States Department of Energy under contract
DE-AC04-94AL85000. With main facilities in Albuquerque,
N.M., and Livermore, Calif., Sandia has major research
and development responsibilities in national security,
energy and environmental technologies, and economic
competitiveness.
Media contact:
Chris
Burroughs, coburro@sandia.gov, (505) 844-0948
Technical contacts:
John Stevens,
jwsteve@sandia.gov, (505) 844-7717
Paul Klimas, pcklima@sandia.gov,
(505) 844-8159
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