Page 139 - Decision Making Applications in Modern Power Systems

104 Decision Making Applications in Modern Power Systems

have a variety of applications in decision-making processes in smart grids
that would be discussed in detail in Section 4.3.4.4.3. Another classification
of control methods divides the methods into three groups of voltage-
controlled method (VCM), current-controlled method (CCM), and hybrid
control method (HCM), for which a detailed discussion will be done defining
control objective. Based on the influence area of MFDGs, different control
objectives should be pursued, that is, for local systems the harmonic-free out-
put current of an MFDG is satisfactory, and CCM could be applied, while
for a regional control of MFDGs, other objectives such as point of common
coupling (PCC) voltage THD and harmonic-free PCC current should be con-
sidered as objectives; therefore different control methods could be used for
each case [7,34 38].

4.3.4.4.1 The Proportional 1 Resonant control method
To overcome the harmonic reference tracking problems of a PI controller,
the PR controller was developed; the main difference between PI and PR
controllers is the parallel resonant loops that have the duty of tracking the
harmonic references. It has been widely used to control MFDGs and hierar-
chical microgrid control [7,39,40]. In some references the PR control method
has been used as a hierarchical control method; to have a higher supervision
over the system, the included control levels could be energy management,
voltage and current control at the first control level, power quality and power
flow issues at the secondary control level, and economic dispatch, DSM, and
microgrid supervision are done at the tertiary control level [36,41,42]. Since
the third level is not the focus area, in this chapter, first and second levels of
control methods will be discussed; in this regard, to have a better understand-
ing of PR control method, it would be discussed in three parts: CCM, VCM,
and HCM. In each case a solution for each control objective would be pro-
posed; to make the following operation of these control methods easier, an
overall view of harmonic compensation in microgrids is shown in Fig. 4.6.
In this configuration, MFDGs are considered to play a role in PQI-related
tasks, such as harmonic compensation, in addition to participating in load
sharing.

Current-controlled method The CCM is the most used control method in
grid-connected DGs; the main idea in CCM is to create an appropriate cur-
rent reference due to the control objective and then track the current refer-
ence. Fig. 4.7 explains an overall view of the CCM applied to interfacing
converters; as it could be seen in (4.1), the reference current is made up of
two elements: the fundamental current and harmonic current. The fundamen-
tal reference current calculation is done based on P and Q exchange between
microgrid and the main grid. The harmonic current calculation is as shown
in Fig. 4.7; for three different control objectives, there are three harmonic