IEEE Electrification - June 2021 - 52

Active Power Priority Mode
The inverter capacity remaining after active power production
is utilized for operation as STATCOM. Active
power is not curtailed. This mode is referred to as " PartialSTATCOM
with active power priority. " IEEE Standard 15472018
requires that DER inverters shall supply at least 44%
reactive power even while generating rated active power.
If this is achieved by oversizing the inverter, the PV inverter
can provide reactive power modulation in Partial-STATCOM
mode with this additional reactive power capacity,
even while producing its rated active power output.
Reactive Power Priority Mode
Active power is curtailed to make additional inverter capacity
available for STATCOM operation. Different variants of
PV-STATCOM control in this mode are presented here.
Reactive Power Modulation After
Full Active Power Curtailment
During any large system disturbance, if significant reactive
power support is needed to support the grid, the PV
A
100
B
B
inverter rapidly reduces its active power from its predisturbance
level to zero, making the entire inverter capacity
available for STATCOM operation. This is known as " Daytime
Full-STATCOM mode " and is illustrated in an
expanded manner in Figure 2. Once the system need is
fulfilled, the PV inverter rapidly restores active power to its
predisturbance level.
However, if the need for reactive power is moderate,
the PV active power output is curtailed to an intermediate
level, making additional (albeit less than rated)
inverter capacity available for STATCOM operation. This
intermediate level of curtailment may be determined
from prior offline studies for the potential contingencies
in the system.
50
Night
Active Power
Reactive Power
12 a.m.
6 a.m.
12 p.m.
Time of Day
A: Full-STATCOM
6 p.m.
B: Partial-STATCOM
Figure 1. PV-STATCOM capability of a PV inverter in active power priority mode.
Simultaneous Active and Reactive Power Modulation
After Partial Active Power Curtailment
Dynamic modulation of both active and reactive
power (as STATCOM) is performed after curtailing
active power from its predisturbance level to an intermediate
level. This operation is performed during
large disturbances requiring critical
grid support. For instance,
the maximum available active
power Pmax based on the available
solar irradiance is curtailed to an
intermediate value (for example,
0.5 Pmax). Active power is then
dynamically modulated between
Pmax and 0.5 Pmax. Simultaneously,
reactive power is also dynamically
modulated between zero and the
inverter capacity remaining after
active power modulation. This
mode is depicted in an expanded
manner in Figure 3. Once the system
need is fulfilled, the PV inverter
rapidly restores its active power
to the predisturbance level.
In the reactive power priority
A
Night
12 a.m.
A
100
B
A
B
A
50
Night
Night
Active Power
Reactive Power
12 a.m.
6 a.m.
12 p.m.
Time of Day
A: Full-STATCOM
B: Partial-STATCOM
Figure 2. PV-STATCOM operation in reactive power priority mode.
52
IEEE Electrification Magazine / JUNE 2021
6 p.m.
12 a.m.
mode, the entire operation of
active power curtailment and
restoration is typically performed
in less than a minute. It is, however,
ensured that no adverse
impact is created on the overall
system frequency by the active
power curtailment.
Simultaneous Active and
Reactive Power Modulation
With Preexisting Active
Power Curtailment
Active power is kept curtailed in
this mode for a substantial duration.
Large system disturbances
PV Inverter Capacity (%)
PV Inverter Capacity (%)
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