IEEE Spectrum November, 2016 - 44

W
When Steve Johnson had solar panels

installed on the roof of his Boulder, Colo., home several years
ago, he considered it his personal contribution to making the
world a little cleaner. And if his grid-connected photovoltaic
(PV) system would occasionally cause his electricity meter
to spin backward during the day and trim his utility bills, so
much the better.
It was only later, during a tour of the National Renewable
Energy Laboratory (NREL), in nearby Golden, that he learned
about solar's potential downside: Most PV systems are set up
to disconnect from the grid whenever they detect a significant fault. If a single home's PV system trips off-line, it's only
a headache for the owner. But if hundreds or thousands of
them do so simultaneously, it could upset the network's delicate balance, turning an otherwise small disturbance into
an outage blacking out an entire city or county.
Throughout much of the developed world, electric utilities are facing an unprecedented challenge. Growing numbers of customers are installing solar PV systems on their
homes or businesses. In the United States alone, the installation of PV systems has seen a compound annual growth
rate of nearly 60 percent since 2010, resulting in an installed
capacity of 32 gigawatts. The power they're injecting into
distribution lines is causing voltage- and frequency-control

problems that threaten to
destabilize the grid. While
this is not yet a major problem, it could become one as
distributed solar systems
proliferate.
The cause of the problem is the inverter, an electronic system that converts
the direct current (DC) supplied by the PV panels into
the alternating current (AC)
that flows on the power grid.
The vast majority of inverters sold to homes and businesses today, including the
one on Johnson's house in
Boulder, are "dumb" inverters. Although they supply
AC at the right voltage and
frequency to sync with the
distribution grid, they are
otherwise passive. They
can't sense what is happening on the grid and adjust
themselves accordingly.
But newer "smart" inverters can prevent a PV system
from going off-line when it
doesn't have to. By doing so,
they can actually make the
grid more stable, by preventing the sudden deterioration
of voltage and frequency that
would otherwise occur when
hundreds or thousands of PV
panels are suddenly taken
off-line, according to recent

research by NREL and its
partners. My colleagues at
NREL have also been developing an innovative method
for controlling inverters that
will keep the grid stable even
when all of the power is coming from solar, wind, and
other forms of generation
that connect to the grid via
an inverter.
Smart inverters are poised
to fill a big need in the fastevolv ing elec t r ic-ut ilit y
industry. As more and more
homeowners put PV panels
on their roofs, the power
they are supplying is reducing the need for big, centralized generating plants.
The upshot is that increasing numbers of these traditional power plants are
getting retired, and grid
operators are scrambling for
ways to keep their networks
running with the same high
level of reliability that their
customers have long taken
for granted. The combination of smart inverters and
new control methods will
be essential to helping utilities transition to the grid
of the future, in which vast
amounts of wind- and solargenerated electricity will be
the norm.

Previous Pages: solarCity

StEADY StAtE: A new technique known as virtual oscillator control allows smart solar inverters to sense
and adjust to grid disturbances, such as a sudden change in frequency or voltage. As distributed solar
grows and big power plants are retired, VOC will help keep the grid stable.

Nov 2016

North AmericAN

SPectrUm.ieee.orG

ILLuStRAtIOn:

Mark Montgomery

http://SPectrUm.ieee.orG

Table of Contents for the Digital Edition of IEEE Spectrum November, 2016