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Power quality issues of smart microgrids Chapter | 4 107

E f K p
E f
Q P V DG
if
V ref_f 2K ω b s
G (s) =
2
Q–E Droop Ctrl P–f Droop Ctrl R s + 2 b s + ω 2
ω
( )
f
V PCC V ref
H D (S) τ V ref_h 2K ω
V PCC Comp (s) = Σ ih b s
V ref_h G H
2
ω
Harmonic extraction 3,5,7,9,11 s + 2 b s + (hω ) 2
h= f
I DG Comp I DG
H D (S) R v
I Local G * inner s – *
I Local-Load H D (S) –R v () K inner V Out
Comp
I ind
FIGURE 4.8 Overall view of VCM with harmonic current reference calculation. VCM,
Voltage-controlled method.
harmonic impedance control. An overall view of the VCM applied to micro-
grids to fulfill the three objectives is shown in Fig. 4.8. It is worth mention-
ing that to control the reactive power in zero steady state, an integral control
term should also be added to voltage magnitude reference. As is shown in
Fig. 4.8, similar to CCM, a double-loop PR controller is used to control the
voltage.
To compensate the PCC voltage harmonics, for voltage reference a feed-
forward term could be used as
V ref 5 V ref f 1 V ref h
ð4:5Þ
5 V ref f 2 H D ðsÞUV PCC
where H D ðsÞ is the harmonic detector block and τ is the feed-forward gain.
This way, MFDG will play the role of small impedance in selected harmonic
frequencies with equivalent harmonic impedance of that harmonic so that it
could absorb selected harmonic currents.
Z DG
Z DG;eq 5 ð4:6Þ
1 1 τ
To compensate the local load harmonic using VCM is not easy since, by
default, it is not capable of compensating local load harmonic. It could be
possible by adding a feed-forward high-bandwidth controller such as hystere-
sis band controller, model predictive controller, or multiple harmonic reso-
nant controllers to the inner control loop that should replace G Inner in
Fig. 4.8. Although it would increase the complexity of the controller, it
would make local load harmonic compensation by means of HCM. Using
the same strategy in CCM, the harmonic elements of the local load current
should be added to the current reference.
To have a harmonic-free output current of MFDG, it could not be con-
trolled directly, instead current regulation could be done considering
τ 52 1, so that virtually it would create a large closed-loop harmonic
impedance at the terminal of MFDG, which would reject any harmonic