IEEE Electrification - June 2021 - 55

All controllers are developed using an abc-dq axis
transformation by which all sinusoidally rotating signals
are transformed into equivalent dc signals, which simplifies
control design. A phase-locked loop (PLL) is used to
extract the phase angle of point of common coupling
(PCC) voltage for transforming the voltage and current signals
from the abc-frame to the dq-frame and vice versa.
The PLL control is further utilized to regulate Vpcc,q (q-axis
component of PCC voltage) to zero. Hence, the Vpcc,d
(d-component of PCC voltage) becomes equal to the
amplitude of PCC voltage.
In the d-q reference frame, due to decoupled control,
the d-axis inverter current loop controls active power and
the q-axis inverter current loop controls reactive power
output of the solar PV inverter. The reference values for
both inner current control loops are obtained from the
outer loop PV-STATCOM controllers. Both inner current
control loops use PI controllers to regulate the d and q
components of inverter current to the reference values
idref and iqref, generated by active
and reactive control loops, respectively.
The inner current control
loops generate modulation indices
that are used for pulse width modulation
in conjunction with a carrier
switching frequency to produce
switching pulses for the insulatedgate
bipolar transistor (IGBT)
switches of the PV inverter. The
closed loop bandwidth of the inner
current controllers is chosen to be
at least 10 times lower than the
switching frequency of the inverter.
The PWM-based switching of VSC
produces several high frequency
components that need to be filtered
out to allow mainly the fundamental
voltage to appear at the VSC
output. A low-pass LC filter is utilized
for this purpose. The filter
should be so designed that the
unfiltered harmonics do not get
amplified by existing network resonances
and create unacceptable
harmonic distortions in the grid.
The control system of PV-STATCOM
is depicted in Figure 11. This
includes the PV-STATCOM controls
for generating the reactive current
reference iqref and active current
reference idref, respectively. However,
the conventional PV inverter
controls are not shown.
The PV-STATCOM controller
consists of: 1) a nighttime charging
circuit, 2) a dc voltage controller, 3)
S7
Load
M1
5 hp (3.7 kW)
Filter
10-kVA PV-STATCOM
Figure 9. The PV-STATCOM installation in the network of Bluewater Power Distribution Corp., Sarnia.
∆
Bluewater
PCC
a reactive power controller, 4) a power factor controller,
and 5) an operation mode selector, as shown in Figure 11.
During a system disturbance, the PV-STATCOM controller
generates the reference signals for real current idref
reactive current iqref
and
for the inner current controller to prevent
stalling of the critical motor. The functioning of different
PV-STATCOM controls is described here.
Nighttime Charging Circuit
Because at night the dc input power to the PV-STATCOM
inverter is zero, the dc link capacitor of the PV inverter is
charged from the grid using a " nighttime charging circuit "
utilizing the diodes across the IGBT switches of the inverter.
The charging circuit consists of a resistor that is used to
limit the inrush current during charging. Once the dc link
capacitor is charged, the charging circuit is disabled to prevent
losses in the charging circuit. Thereafter, the dc voltage
reference is switched to the constant value Vdcrated
STATCOM operation.
for
Bluewater Power
600 V/4.16 kV, 3 Ph, Pole #325
S1
10 kVA
600/208 V
L1
S4 L2
S3
Y
10 kVA
208/140 V
S2
S5
S9
M2
3 hp (2.2 kW)
S6
10-kW Solar Panels
10-kW PV Inverter
Y
∆
S8
Distribution Line
208 V
PVSTATCOM
(10
kVA)
Solar
Panel
Figure 10. An equivalent study system.
IEEE Electrification Magazine / JUNE 2021
55
Vdc
Large
Load
(3 kW,
10 kvar)
IM
5 hp

IEEE Electrification - June 2021

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