In a power system context, indirect control is a way of
managing a DER by issuing signals which might or might
not affect its operation. Heussen et. al. in [6] define two
characteristics of the indirect control: ”indirectness of the
relationship between control objective and observables”, and
”local and independent decision making of the DER makes
its behavior non-deterministic”. The DER accepts the control
signals but it is not obliged to either react to the signal or
send any feedback. Lack of the feedback implies that indirect
control requires only one-way communication to transfer the
control signal from external controller to a DER. Indirect
control can uses broadcast to propagate messages, any DER
can simply choose to listen to commands at any time.
Scalability, as defined by Tanenbaum [9], is a characteristic
of a system that remains effective when there is a significant
increase in number of resources or users. Broadcast signals
propagated in the indirect control scheme can be received
by any number of units, in contrast to single addressing in
unicast communication in the direct control scheme. Scalability
of the indirect control makes it suitable for systems with
high uncertainty in DER number, type and external influence
factors, for example power systems with changing topology.
The indirect control advantages are simplified protocols,
exchanged data and interfaces. DERs can follow thair private
and dynamic objectives and prioritize local goals. In comparison
to direct control, predictability of the DER response
is lower, due to its non-deterministic behavior. To reduce
mismatch between control objective and the actual response,
DER models including relationship between signal and the
response are needed [6]. If the field measurements of the power