IEEE Power Electronics Magazine - December 2016 - 23
and the midpoint of the split capacitors. The main objectives of the neutral leg are to balance the voltages of
the capacitors and to provide a return
path for the dc input current. At the
same time, the inverter leg is used to
generate an ac output voltage with
high power quality and to control the
reactive and active power exchange
between the PV and the grid.
To make the PV inverter grid
friendly, the inverter leg is controlled
to be a synchronverter. There are two
operating modes for the operation
of the single-phase synchronverter
(SPSV): the islanded mode and the
grid-connected mode. In the islanded
mode, the SPSV generates the nominal voltage at the nominal frequency.
The real power and reactive power
generated depend on the local load
connected (when radiation is high
enough). In the grid-connected mode,
the SPSV could be controlled to generate the required amount of both
real and reactive power to take part
in the grid frequency and voltage regulation or to generate the power set
by the reference values.
Figure 10 shows the real-time
simulation results of such a system,
where the grid frequency increased
to 50.02 Hz at t = 4 seconds and
the grid voltage decreased by 2%
at t = 5 seconds. The real power
and reactive power automatically
changed according to the change of
the frequency and the voltage. Note
that the MPPT was not included in
the simulation for brevity.
Removal of PLLs
As discussed in the "Smart Grid Integration" section, it is crucial for DGs to
synchronize with the grid before connection and after connection. This is
often achieved by adding a dedicated
synchronization unit, for example,
PLLs, to the controller. However, PLLs
suffer from nonlinear structure, timeconsuming tuning, and slow performance, all of which bring many problems to grid-connected inverters, for
example, instability [14]. Because of
the inherent synchronization mechanism of synchronous machines
Three-Phase Rectifier
PMSG
vra
ira
vrb
vrc
irb
irc
Vdc
ωn
.
θr
Dmp
Vdc
Tmm
-
Vref
PI
1
- Reset
.
θm
STA
θm
1
s
Jms
vr
Tme
Calculation
Qm
em
vr
ir
-
Qmref
1
-
Kms
PWM
Generation
Mmfimf
FIG 6 A controller for the rotor-side converter in Figure 5 [12].
Three-Phase Rectifier
LCL Filter
vga
vb
ib
Vdc
Grid
va
ia
vgb
vc
ic
vgc
ωn
.
θg
Dp
MPPT
- Reset
Pref
1
.
θn
P
Tm
1
Js
-
-
1
Ks
1
s
vg
θ
θc
Te
Calculation
Q
Qref
.
θ
STA
e
PWM
Generation
i
Mf if
Dq
- Vm
Amplitude
Detection
v
Vgm
FIG 7 A controller for the grid-side converter in Figure 5 [12].
December 2016
z IEEE PowEr ElEctronIcs MagazInE
23
Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - December 2016
IEEE Power Electronics Magazine - December 2016 - Cover1
IEEE Power Electronics Magazine - December 2016 - Cover2
IEEE Power Electronics Magazine - December 2016 - 1
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