impulse-ed 2-060417

IMPULSE 2017
DEE, RSET
ELECTRIC SPRING (ES) - A PROMISING SMART GRID
TECHNOLOGY
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. 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. 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.
Smart inverters are poised to fill a big need in the fast-evolving electric-utility 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 for
helping utilities transition to the grid of the future, in which vast amounts of wind- and solargenerated
electricity will be the norm. A smart inverter can “ride through” voltage or frequency
dips and other short-term grid disturbances and if these inverters have communications
capabilities, they can let grid operators monitor and control them in response to changing
conditions.
The 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 responding instantaneously 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 will naturally align and picture several weights hanging from
springs that are connected to a rigid and fixed board. As the weights are given a pull to set the
springs bouncing, the springs will tend to bounce in an uncoordinated fashion. However, if the
board itself is suspended from the ceiling on springs, it becomes a feedback mechanism that
responds inversely to the pushes and pulls from the bouncing weights. Within a short period of
time, the weights will all start bouncing at the same frequency, all in lockstep.