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With Java chosen as the programming language, no further
adjustment of the byte code is necessary after the deployment
onto the target system.
Network forming and security issues as authenticity and integrity
of messages are taken care of by the Zigbee stack
on the communication modems, which are attached to
the embedded controller through a standard serial interface
(see fi gure below) and are thus easily exchangeable by
other, potentially longer range protocols, e.g. WiMax (IEEE
802.16), extending the segment size from typically 100m to
several kilometres.
A useful feature of the underlying IEEE 802.15.4 protocol, is
its ability to establish hierarchical (i.e. tree-like) or mesh (i.e.
peer to peer) networks.
While the former are a necessary prerequisite for the correct
functioning of the optimisation algorithms, the latter
increase the robustness of the communication, as dead
links can be circumvented.
In this case the tree-structure must be implemented on top
of the mesh, i.e. by the application.
Until today two different algorithms have been tested in the
environment described:
_A Distributed Constraint Optimisation Protocol (DCOP)
_And a Distributed Langrangian Optimisation Protocol (DLOP)
The difference between the two is largely given by the message
exchange on the logical tree, as visualized by the arrows
in the fi gure above.
In the DCOP scenario a parent node broadcasts so-called
VALUE messages to its children towards the leafs of the
tree, specifying requirements and the child nodes report their
utility and cost back to the parent with UTIL messages.
In DLOP the relationship between two nodes is described
by the exchange of a price parameter for a certain utility,
which is derived from a local optimisation procedure, e.g.
Quadratic Programming, aiming at a bilateral equilibrium.
The cluster head then selects its preferred power fl ow on
cost criteria, where the cost function is assembled by contributions
from the difference of supply and demand matching
and battery use.
This means that a cost arises from over– or undersupply, as
well as from charging or discharging a battery pack, as this
leads to power loss (effectivity is typically around 70%) and
the number of charge cycles of a battery is limited.
While the price negotiation approach in DLOP can be proved
to converge against a fi xed value, the number of messages
needed to achieve this leads to a higher risk of collisions
and consequently lower number of nodes in a communication
cell as in the DCOP scenario.
Both algorithms support the asynchronous nature of the
network, i.e. the loss of a node for communication or power
availability reasons does not block the grid as a whole.
At present the focus of the development lies on the integration
of a cluster into mainstream protocols as IP to enable the link of
power grids and SCADA systems through the internet.
A PhD project at the University of Luxembourg supported
by the Fonds National de la Recherche.
Dipl. phys Ralf Hoben
CAHIER SCIENTIFIQUE | REVUE TECHNIQUE LUXEMBOURGEOISE 2 | 2010
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_The network structures foreseen
within Zigbee , where each node
is a stamp-sized wireless communication
modem, and on the left
a Zigbee modem attached to an
embedded battery controller