IEEE Electrification - June 2021 - 56

DC Voltage Controller
DC voltage controller generates the active current reference
to control active power fed into the grid. In a conventional
solar PV system, the dc link reference voltage V ,edc rf
is provided by the MPPT controller. During a large system
disturbance, the PV-STATCOM curtails active power generation
to zero and uses the entire inverter capacity to
exchange (inject/absorb) reactive power. This objective is
achieved by setting V c,
dref to open circuit voltage Voc
of the
solar panels, thereby ceasing any power generation from
the solar panels. Once the system returns to stable operation,
the PV-STATCOM controller ramps up active power to
its predisturbance level. The power is ramped up by varying
V ,dc ref
from the open circuit voltage Voc
mum power point voltage V .MPP
During the daytime, the dc voltage controller uses solar
power to compensate the inverter switching losses, and
thereby keep the dc link capacitor charged to the desired
voltage reference level. However, during the nighttime, the
dc voltage reference is switched to a constant value
which is chosen to be twice the magnitude of Vpcc
V ,dcrated
. The
required active power for compensating switching losses
(1-3%) and for maintaining dc voltage at the reference
value Vdcrated
is provided from the grid, as in STATCOMs.
The closed loop bandwidth of the dc link voltage controller
should be lower than at least one-third of the bandwidth
of inner current controllers.
to the maxiReactive
Power Controller
The reactive power controller block generates the reactive
current reference iqref
for different operating modes of PVSTATCOM.
It consists of PCC voltage controller and a reactive
current limiter. The closed loop voltage controller is a
proportional integral controller. Its closed loop bandwidth
should also be lower than at least one-third of the bandwidth
of the inner reactive current controller. The voltage
controller is designed to respond within one to two cycles,
as in STATCOM.
The measurement of PCC voltage involves a low-pass
filter. The time constant of this filter should be kept
small to avoid controller instability. Studies have shown
that for a given filter time constant, systems with lower
X/R ratios (more resistive networks) are more likely to
cause instability. A slope or droop Kdroop
is often introduced
in the PV-STATCOM voltage controller similar to
the slope in the V-I characteristic of SVCs and STATCOMs.
The reactive current limit is set to the available
inverter capacity in Partial-STATCOM mode, while it is
set to the rated inverter current Imax
COM mode.
during Full-STATPower
Factor Controller
The power factor controller allows the PV-STATCOM to
dynamically provide constant (nonunity) power factor
control. It generates reactive current reference to operate
the solar system at the specified power factor.
Vdc
Vdc
Idc
Idc
M2
Md
Idref
Vcdq
M1
M2
Idref
pf
M0
VL1
Vcdq Vcdq
PPCC
Vcdq
VH1
VL2
VH2
Vcdq
Vref
√I2
Voltage
Controller
max-I2
dref
Reactive Power Controller
Idref
tan(cos-1(pf))
M0
Power Factor Controller
+-
+-
+-
+-
Mode Selector
PPCC
Pt
M1
M0
M2
Md
Mode 1: Partial-STATCOM Mode
(Represented by Flag M1)
This mode is activated during small voltage variations
(sags or swells). The mode selector compares the PCC voltage
Vcdq
^h with the threshold settings VL1
and VH1 . If the
measured voltage if greater than threshold for voltage
swell VH1
or lower than threshold for voltage sag VL1
mode is initiated. The dc voltage controller regulates Vdc
Figure 11. The PV-STATCOM controller.
56
IEEE Electrification Magazine / JUNE 2021
, this
at
reference provided by MPPT. The PV-STATCOM exchanges
reactive power using remaining inverter capacity, with
active power priority.
Ivc
Min
M2
M1
∑
∑
iqref
dc Voltage Controller
MPPT
Voc M21
1
Vdc
dc Voltage
Controller
Vdcrated 0 Vdcref
Md
idref
Mode Selector
The PV-STATCOM controller functions in three operating
modes determined by the operation mode selector (OMS)
block based on the grid voltage
V .cdq The OMS compares
Ppcc with a small value Pt (selected as 0.05 pu) to check if
PV-STATCOM operation is being performed during the day
(Ppcc
> 0.05 pu) or night (Ppcc
or 0, respectively.
Mode 0: Conventional PV Power Generation
Mode (Represented by Flag M0)
In this default mode, the PV-STATCOM generates maximum
available power. The dc voltage controller regulates
Vdc
at the MPPT reference. The power factor control is
enabled by the power factor controller.
< 0.05 pu), setting flag Md
to 1
IEEE Electrification - June 2021

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