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 new way to safely, simply and effectively connect
electricity-pro- ducing photovoltaic (PV) solar systems to utility
company power grids has been developed by the Department of Energy's
Sandia National Laboratories. Several manufacturers have already
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. 
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