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Renewable
energies such as wind, sun and biogas are set to become increasingly
important in generating electricity. If increasing numbers of wind
turbines and photovoltaic systems feed electrical energy into the grid,
it becomes denser—and more distributed. Therefore, instead of a small
number of large power plants, it links a larger number of small,
decentralized power plants with the washing machines, computers and
industrial machinery of consumers. Such a dense power grid, however may
not be as vulnerable to power outages as some experts fear. One might
assume that it is much harder to synchronize the many generators and
machines of consumers, that is, to align them into one shared grid
frequency, just as a conductor guides the musicians of an orchestra into
synchronous harmony.
In
contrast, scientists at the Max Planck Institute for Dynamics and
Self-Organization in Göttingen have now discovered in model simulations
that consumers and decentralized generators may rather easily
self-synchronise. Their results also indicate that a failure of an
individual supply line in the decentralized grid less likely implies an
outage in the network as a whole, and that care must be taken when
adding new links: paradoxically, additional links can reduce the
transmission capacity of the network as a whole.
Synchronization,
the coordinated dynamics of many units to the same timing is found
throughout the natural world. Neurons in the brain often fire
simultaneously, fireflies synchronize their blinking lights, and
crickets chirp in shared rhythm. A similar form of harmony is also
necessary in electricity networks, in that all generators and all
machines that consume electricity must be tuned to the grid frequency of
50 Hz. The generators of large power plants are regulated in such a way
that they stay in rhythm with the power grid. The grid, in turn,
imposes its frequency on the washing machines, vacuum cleaners and
fridges at the other end of the line, so that all elements remain in
synchrony, avoiding short circuits and emergency shutdowns.
In
the course of the energy turnaround, however, the structure of the
power grid will change. Today’s large power plants that supply energy to
the surrounding areas will be largely replaced by multiple photovoltaic
panels on roofs, biogas systems on fields, and wind turbines on hills
and offshore. Power lines will no longer form star-like networks and
only transmit energy from large power plants to nearby consumers, but
will look more like dense fishing nets linking many generators with the
consumers. Experts believe it will be very difficult to bring this
multiplicity of small generators into synchronous harmony. In effect, it
would be like conducting a huge orchestra with thousands of musicians,
instead of a chamber orchestra. However, as the Network Dynamics Group,
headed by Marc Timme at the Max Planck Institute for Dynamics and
Self-Organisation in Göttingen has now discovered, synchronization in a
decentralized power grid may actually be easier than previously thought,
as a grid with many generators finds its own shared rhythm of
alternating current.
In a decentralized grid, power plants and consumers synchronize themselves
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The
Göttingen-based scientists have simulated a dense network of small
generators and consumers. Their computer model calculates the grid for
an entire country (for practical reasons, they chose Great Britain) and
takes into account the oscillations of all generators and electric
motors that are connected to the grid. Combining this level of detail
with this grid size is a new departure. Previously, the dynamics of the
oscillating 50 Hz AC current was basically only simulated for small
networks. Simulations for larger grids did exist, but they were
generally used only to make predictions regarding the static properties
of the network, such as how much electricity would be transmitted from A
to B. They completely ignored the oscillations of the generators and
electric motors.
“Our
model is sufficiently complex and extensive to simulate collective
effects in complex networks and, just as importantly, it is simple
enough that we can understand these effects too,” says Dirk Witthaut,
project leader within the research group.
The
scientists simulated a very large number of networks, each with a
different structure. The networks consisted of different mixes of large
and small generators with lines of varying capacities, a little like
country lanes and motorways for electrical current. This enabled them to
identify differences between centralized and decentralized power grids.
A dense grid can compensate more easily for a line outage
The
scientists in Göttingen examined additional aspects that are discussed
in relation to the transition from a centralized grid to a decentralized
one. What happens, for example, if a single transmission line is
damaged or malfunctions? In existing grids, this can have a kind of
domino effect, as seen in the 2006 power outage around Europe, caused by
the shutting down of a single line in Northern Germany. The simulations
of the Göttingen-based team indicate that decentralized grids are much
more robust when single lines are cut. This is because a dense grid
always more often has neighbouring lines that can take on the extra load
of a downed line. Unlike the case of large-meshed networks, they have
few indispensable main links with the potential to cripple the whole
grid.
Nonetheless,
the expansion of renewable energy also holds challenges for the
stability of the supply network. Another simulation showed the
scientists that a highly decentralized grid is more vulnerable to strong
fluctuations in consumption, as occurs, for example, when millions of
people turn on their washing machines at the same time. Large power
plants can buffer these fluctuations in demand more easily than small
ones, as their rotating generators store more kinetic energy. The grid
can tap into these spinning reserves at short notice to cover supply
gaps – an option which is not available in the case of solar cells.
Adding new lines can hinder power transmission
In
a second study using the same mathematical model, Marc Timme and Dirk
Witthaut revealed another effect that is known from road traffic and is
counterintuitive. Building a new road and thereby increasing the
capacity of the network does not necessarily improve traffic flow; on
the contrary, even more congestion may occur with the same volume of
traffic. This is the case when the new stretch of road provides a
shortcut for many drivers, but has been poorly chosen in that it links
bottlenecks which were previously avoided by most.
Witthaut
and Timme have shown that this situation, known as Braess’s Paradox,
can also be observed in power grids, specifically in decentralized
networks. If such a dense network self-synchronizes, it might be assumed
that synchronization would become easier with each new link; however,
this is not always the case: the addition of a new line may actually
disrupt self-synchronization.
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In
order to understand this paradox, it is helpful to consider two
machines in a dense network. Additional machines are located along the
line that connects them. The two machines are in synchrony as long as
the phases of their oscillations—where phase describes the state of
oscillation at a given time and place—remain in a fixed, mathematically
defined relationship to one another. This may be visualized as two
pendulums. The phase of the pendulum describes its degree of deflection
at a given point in time. If the pendulums swing at the same frequency,
their oscillations will be in a fixed phase relationship to one another.
This does not mean that the two pendulums are swinging exactly parallel
to each other, i.e. that they always have the same degree of
deflection; in fact, they may even shift out of phase with each other.
However, the distance between the swinging pendulums is fixed for every
point in time, and the points at which they have the same degrees of
deflection recur at regular intervals.
How does the grid react to supply fluctuations?
If
two machines in a power grid are to be synchronized, that is, if their
fixed phase relationship is to be fulfilled, they must always reach
minimum and maximum voltage at the same time. This means that they must
not be out of phase, or only by a full wave train. Every line in the
network now yields a fixed phase relationship, either directly or
indirectly. If a new line is now built to link the two machines
directly, their oscillations must conform to a new phase relationship;
however, this may not be compatible with the old one. Because the latter
is consistent with the other machines on the old line, there is a
conflict between the shortcut and the old line, which has the potential
to desynchronize the entire network.
“Care
must therefore be taken when adding lines to a decentralized grid,”
cautions Witthaut. Careful consideration should be given to which nodes
can be linked without risk. However, Witthaut sees the results of the
simulations as encouraging for the construction of decentralized
networks.
“Until
now, concerns rather centred on the possible collective impact that a
large number of small generators could have in a dense grid,” says the
physicist. The fear was more frequent power outages. “But our work shows
that the opposite is the case and that collective effects can be very
useful.”
The
Network Dynamics Group based in Göttingen currently starts
collaborating with engineers and network operators to ensure that their
findings can be put to practical use. Initial contacts have already been
made, and, in the meantime, the scientists are improving the model.
Their current focus is to integrate weather-related fluctuations in
renewable energy sources into their simulations.
Source: Max Planck Institute
