IEEE Spectrum November, 2016 - 46

46

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Nov 2016

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North AmericAN

% of Daytime
minimum LoaD
on oaHu
Less than 50%
50 to 100%
100 to 250%
250% and greater
This map shows the
relative density of
grid-connected PV
systems installed on
the Hawaiian island
of Oahu.

addition, the voltage spikes
lasted only microseconds
to milliseconds, an acceptably short duration. The
tests demonstrated that
when smart inverters were
installed, voltage spikes
posed less of a problem than
had been feared.
Persuaded by these tests,
H ECO l i f ted it s morato rium on solar systems on
Oahu in early 2015, prov ided t h at new c u stomers i nst a l led one of t he
advanced inverters vetted
by NREL. The utility is now
working with inverter makers to certify other models
of inverters, using the NREL
test methods. Separately,
HECO worked with Enphase
Energy, of Petaluma, Calif.,
to re mote ly repro g r a m
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800,000 microinverters
already installed on the
Oahu grid, to allow them to
ride through frequency and
voltage anomalies and help
improve grid stability during such disturbances. And
HECO has more than doubled its limit on PV capacity on each distribution line,
to 250 percent of the minimum daytime load. The
utility's moves may eventually have a national or even
global impact, as the IEEE
and UL, which set international technical standards
for inverters, are now preparing to include overvoltage protec t ions in t heir
smart-inverter standards.

W

hile the current generation of smart
inverters
can solve some of the problems confronting utilities
like HECO, an even larger
issue looms. Today's power
grids still rely on large, centralized power plants with a
massive generator rotating
at just the right speed to produce power at a certain frequenc y-50 c yc les per
second in most of the world,
60 cycles per second in the
United States and parts of
Asia. These generators collectively g ive the entire

power grid mechanical inertia: They are very large and
rigidly synchronized with
each other, so only major disturbances can affect the
grid's frequency.
But distributed energ y
sources, including solar,
wind, fuel cells, and batteries, don't have synchronous
generators; they supply variable and uncertain amounts
of electricity, and the inverters they use to connect to the
grid are designed to simply lock onto the grid's frequency and follow it. And as
renewable-energy generation spreads, many large central power plants-especially
coal-are being retired. Eventually, the grid will lack the
inertia it has today to maintain a stable voltage and frequency in the event of a large
disturbance.
Fo r t h i s r e a s o n , t h e
power grids of the future
may need advanced inverters that don't simply follow
what the grid is doing but
actually help form the grid,
by respond i ng i nst a nt aneously to disturbances and
working in concert to help
keep the grid stable. This
approach, known as virtual
oscillator control (VOC), was
developed by an NREL team
led by Brian Johnson, working with Sairaj Dhople of the
University of Minnesota;
Francesco Bullo at the University of California, Santa
Barbara; and Florian Dörfler
at ETH Zurich.
The basic idea behind
VOC is to leverage the properties of oscillators-that is,
anything that moves in a
periodic fashion, such as a
mechanical spring, a metronome, or a motor. When
two oscillators are paired,
or coupled, their motions

HawaiiaN eleCtriC ComPaNy

Energy's SunShot Initiative,
NREL researchers Sudipta
Chakrabort y and A nderson Hoke worked with engineers at HECO and SolarCity,
the country's largest solarpower provider, to study
ways to allow more solar on
the Oahu grid. They started
by running inverter tests at
NREL's Energy Systems Integration Facility to check different scenarios in which an
overvoltage occurred. The
goal was to see if smar t
inverters could respond to
such overvoltages by quickly
reducing the amount of
power they were releasing
onto the grid.
The researchers ran the
tests using five commercially available advanced
inverters ranging in capacity from 3 to 12 kilowatts.
Among the types of inverters tested were string inverters, which draw DC power
from one or more strings
of solar panels, and microinverters, which draw from
a single panel. They connected each inverter to a
grid simulator and a load
that simulated the home
to which the panels would
be supplying power. They
then monitored the inverter's response when the grid
simulator's voltage spiked.
They repeated the tests for
a range of ratios of solar
generation to load, from
an even match on up to 10
times as much power generation as load.
The resu lts were reassuring: The inverters all
responded immediately by
cutting their output, and so
the maximum overvoltage
never exceeded twice the
normal voltage in any test,
with the typical overvoltage significantly lower. In
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Table of Contents for the Digital Edition of IEEE Spectrum November, 2016
