IEEE Power Electronics Magazine - June 2023 - 24

Some commercial inverters need to pause to change modes.
The functional changes in inverter operation include the
following:
■ From managing and optimizing power and energy on the
grid to managing voltage and frequency when off-grid.
■ Island detection function used to transition to off-grid
mode, replaced by a " grid returns " sensing function.
Fault Currents in Inverters
Fault current operation of inverters is characterized by the
following:
a) Short-Circuit Capability-Typically, the short-circuit
current of an inverter is in the range of 1.5 pu
based on the inverter rating; this current is limited by
the inverter switching device's current ratings and is a
short-time rating. Therefore, either
the protection
system needs to be changed to become more sensitive
and activate at lower currents, or other mechanisms
need to be employed to achieve higher fault currents
than the inverters can naturally supply.
b) Nature of the Fault Currents-The inverter can produce
only positive sequence voltage and, when feeding a
balanced grid, positive sequence current. Under unbalanced
fault conditions, the inverter cannot supply negative
sequence current as an SG can. It can be equipped
with a current control loop that can provide some negative
sequence current if required.
Grid Code Requirements for Inverters
Ride-Through Requirements
The codes and standards that relate to these ride-through
issues have evolved over time, and there have been major
equipment recalls and recertifications of wind and PV
inverters in leading renewable grids such as Germany,
China, and Australia over the past 15 years. Forensics
have revealed that many of the issues relate to problems
with following phase (and frequency) during transient
events and often the setting of parameters related to voltage
and frequency ranges, which incorrectly result in the
tripping of substantial resources. The Blue Cut, Canyon,
and Odessa grid events are all hailed by many in the GFM
community as fundamental GFM events. In truth they
were all ride-through trip events related to frequency and/
or phase deviations.
Ride-through standards began as no-trip standards
from FERC Order 661 for wind power in the bulk power
system. Ride-through, or no-trip, requirements typically
defined in grid codes include the following as shown in IEEE
2800-2022 (Figures 4 and 5) [12]:
Grid Support Functions
Many functions can be programmed in the inverter control
system to take advantage of the independent control of the
active and reactive power control of the inverter. The more
useful functions include the following:
a) Reactive Power Control-Volt-Var Function-This
function implements voltage regulation and support by
injecting inductive or capacitive power as the voltage
rises above, or drops below, set values.
b) Active Power Control-Volt-Watt Function-This function
implements active power reductions, as the voltage
varies outside the allowable range. In particular, it
allows active power curtailment as the voltage at the
inverter terminal increases. If this function is active, the
MPPT function must be disabled.
c) Active Power Control-Frequency-Watt
Function-
This
function implements
active power increase, as the
frequency drops below a set
value, in support of the frequency,
or reductions, as the
frequency increase above a
set value.
Another function requested
FIG 4 Voltage ride-through requirements for IBRs (voltages below 500 kV) [12].
24 IEEE POWER ELECTRONICS MAGAZINE z June 2023
in recent grid codes is the capability
for inverters to inject
negative sequence current components
in proportion to the
amount of unbalance in grid
voltages. These are typically
the result of unbalanced short
circuits in the grid, mostly single-phase
to ground faults. Negative
sequence current injection
is implemented to allow the use
of conventional sequence component
relays to detect faults.
IEEE Power Electronics Magazine - June 2023

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