Page 141 - Decision Making Applications in Modern Power Systems

106 Decision Making Applications in Modern Power Systems

loads, and to calculate the harmonic elements, a harmonic extractor block is
needed in this structure. To compensate the voltage harmonics of the PCC,
since the PCC load current is not accessible for MFDG, PCC voltage com-
pensation would be indirectly done by measuring the PCC voltage. The basic
idea is to provide all the load currents inside the microgrid by MFDG so that
the grid current would be almost harmonic free (4.4); as a result the PCC
voltage will be harmonic free also. It is worth mentioning that, when com-
pensating the PCC voltage, some other disturbances may appear inside the
microgrid. Since the main objective is to compensate the PCC voltage har-
monics, these disturbances could be neglected. This effect is called the
whack-a-mole effect; it means that in some cases, compensating harmonics
in a point will result in the appearance of harmonics in other points of micro-
grid; then when a compensating effort is being applied to a microgrid, all
aspects of power quality issues should be considered and a trade-off between
different objectives should be done.
I ref 5 I ref f 1 I ref h
ð4:3Þ
5 I ref f 1 H D ðsÞUI local
where H D ðsÞ is the harmonic detector to extract the harmonic elements of the
local load current.
I ref 5 I ref f 1 I ref h
V PCC ð4:4Þ
f 2 H D ðsÞU
5 I ref
R V
where R V is the equivalent MFDG resistor at harmonic frequencies. It is
worth mentioning that the only difference between PCC voltage harmonic
and local load harmonic current compensation lies in calculating the
harmonic current reference in Eqs. (4.3) and (4.4). As is shown in Fig. 4.7,
the presented CCM is based on a stationary reference frame with PR (propor-
tional and parallel resonant) controllers at harmonic and fundamental
frequencies.

Voltage-controlled method Although CCM is used for most of the grid-
connected MFDGs, VCM is increasingly being used in stand-alone applica-
tions of MFDGs to simulate the behavior of a synchronous generator; on the
other hand for autonomous control of microgrids, for voltage and frequency
control, VCM should be applied to interfacing converters. Another advantage
of using VCM is the control of several MFDG units to share the power in a
decentralized way by means of a droop controller without any need to com-
municate between MFDGs. Since there are no closed-loop line current regu-
lators in the controller, it could be hardly used to regulate the MFDG output
current; as a result, VCM is rarely used to address any harmonic compensat-
ing issues. However, PQI by means of VCM could be possible with virtual