Page 243 - Decision Making Applications in Modern Power Systems

208 Decision Making Applications in Modern Power Systems

FIGURE 8.2 Primary droop control.
8.4.1 Primary control

A general practice is the application of droop control to perform primary reg-
ulation [16]. This approach is very efficient due to the naturally distributed
characteristic and no requirements for communication. However, these con-
trol actions may sometimes be not sufficient to maintain the network opera-
tion within satisfactory levels, especially in scenarios where the microgrid is
subject to significant demand variations.
A demonstration of primary droop control response is described in
Fig. 8.2. A perturbation in the system caused by an increase in demand
requires the adjustment of the generating units’ contribution to attend this
additional supplying requirement. The primary control adjusts the generators
to meet this new operating condition. However, this adjustment is accom-
plished with an inversely proportional decrease in frequency related to the
machines’ droop coefficients [17,18].

8.4.2 Secondary control
Secondary control strategies are the ones responsible for adjusting the system
operation back to the reference level after the primary control response to sys-
tems perturbation. These controls are typically associated with a small group
of predefined generating units and usually require some sort of communica-
tion between the generating units [22]. Nonetheless, recent works have pro-
posed new methodologies without communication requirements [20,21].
As described in Fig. 8.3, after the system is subjected to a disturbance, the
primary control responses adjust the generators to supply the new demand
requirements. However, it comes with a frequency offset. This frequency
deviation is corrected by secondary control strategies, returning the system to
the operation as close as possible to the reference level.