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capabilities, they can let
grid operators monitor
and control them in
response to changing
conditions.
paired, or coupled, their
motions will naturally
align and picture several
weights hanging from
springs that are connected
to a rigid and fixed board.
g r i d f r e q u e n c y w i l l
s i m i l a r l y c a u s e t h e
inverter to adjust its
p o w e r o u t p u t t o
compensate. The use of
“Electric Springs” is a
voltage instability and
frequency deviations. In
recent years, Electric
Springs (ESs) have been
developed as a new
means of addressing these
issues. Besides regulating
a c v o l t a g e s i n t h e
distribution network, ESs
can also regulate the load
power flow to response
the variable renewable
sources. Therefore, it
brings a new control
paradigm, i.e., the load
d e m a n d f o l l o w i n g
v a r i a b l e p o w e r
generation.
The power grids of the
future may need advanced
inverters that don’t simply
follow what the grid is
doing but actually help
f o r m t h e g r i d , b y
r e s p o n d i n g
i n s t a n t a n e o u s l y t o
disturbances and working
in concert to help keep the
g r i d s t a b l e . T h i s
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,
S a n t a B a r b a r a ; a n d
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
f a s h i o n , s u c h a s a
mechanical spring, a
metronome, or a motor.
When two oscillators are
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
a n d p u l l s f r o m t h e
bouncing weights. Within
a short period of time, the
weights will all start
bouncing at the same
frequency, all in lockstep.
For virtual oscillator
control, the trick is to
m a k e e a c h i n v e r t e r
respond to the grid much
like a spring: If the grid
voltage drops, the inverter
adjusts its output such
that it “pushes” against
the voltage change.
Likewise, a surge in grid
voltage will induce an
inverter to “pull” the
voltage back to the
n o m i n a l r a n g e . A n
increase or decrease in
novel way of distributed
voltage control while
simultaneously achieving
effective Demand-Side
M a n a g e m e n t ( D S M )
through modulation of
noncritical loads in
r e s p o n s e t o t h e
f l u c t u a t i o n s i n
intermittent renewable
energy sources like Wind,
solar etc. The role of
demand side management
is going to be critical once
t h e p e n e t r a t i o n o f
renewable energy sources
becomes significant. This
would bring about a
paradigm shift from the
traditional centralised
control of hundreds of
power plants to match the
demand to decentralised
control of millions of
loads to balance the
supply (generation). In a
distribution network,
uncertainty and variability
associated with renewable
energy sources, such as
solar power and wind
power, can easily cause
p o w e r f l u c t u a t i o n s ,
An ES connected in series
with the noncritical load
(e.g., a water heater) that
has high voltage variation
tolerance forms the socalled
smart load. The
critical load that requires
a well-regulated mains
voltage is parallel with
the smart load branch. By
operating under inductive
or capacitive mode, the
E S c a n i n j e c t a
controllable voltage and
accordingly change the
uncritical load voltage so
as to maintain the voltage
across the smart load at a
c o n s t a n t l e v e l . I n
addition, it also enables
uncritical load to vary its
power consumption and
thus contributes to the
d e m a n d r e s p o n s e .
Overall, the smart load
c a n b e i n t e n d e d t o
provide reactive power
compensation to the
system by adjusting the
output voltage of ES and
ensure a tightly regulated
voltage across the critical
load.