IEEE Electrification - June 2021 - 65

output voltage and frequency of an
IBR. Here, the electrical active
power and reactive power output of
the IBR are measured at its terminals
and used as input terms to the
droop equation. The resultant value
of frequency and voltage magnitude
subsequently governs the nature of
the sinusoidal voltage generated by
the IBR. Finally, time domain nonlinear
control methods have also been
proposed in research literature. An
example is the use of virtual oscillator
control wherein electrical oscillator
circuits are constructed based
upon the principals of the Van der
Pol equation or Andronov-Hopf bifurcation to generate
limit cycles of a certain specified frequency and voltage
magnitude. These limit cycles then serve as the " timing "
mechanism of the IBR control structure governing the
generated ac voltage. As a side note, although all these
concepts are still being researched and developed, and
there may yet be even more options investigated in
the near future, there are already some such grid-forming
converters that have been developed, built and
installed by vendors in both microgrids and larger grids
around the world. Most of these are of the virtual synchronous
machine type.
It is appropriate for a system planner/operator to
define expected performance from a future IBR rather
Around the world, the
present state of the
art is the use of
positive sequence
phasor-domain
simulation platforms
for bulk power system
planning studies.
than focus on the explicit type of
control structure used within an individual
inverter and plant. However,
from the perspective of a transmission
system planner/operator, to
increase the percentage of inverterbased
ge neration, rather than asking
the question from the perspective of
how the control structure of a future
IBR should be implemented, it might
be more beneficial to define interconnection
requirements from the
perspective of the services that are
expected to be provided by inverters
with advanced controls while respecting
individual equipment limits. An
example listing of these services can be as follows.
1) The ability to create an open circuit voltage at the
point of interconnection of the IBR plant. Note that
this characteristic is defined at the plant point of
interest and not at the terminal of an individual
inverter within the plant. This implicitly assumes that
the IBR plant is capable of
x serving its own auxiliary load, and
x operating in the absence of a synchronous machine.
2) The ability to synchronize and operate in conjunction
with other sources of energy in the grid. These other
sources can include conventional rotating machines
as well as other forms of IBR control methods. This
also includes different types of loads.
Rate limits on reactive current for recovery after fault.
Upward limit is active when Qgeno > 0.
Downward limit is active Qgeno
< 0.
Qgeno
Iqrmax
Iqcmd
-1
1 + s Tg
s0
Iqrmin
Vt
-1
1 + s Tfltr
s2
RateFlag
1
0 1
Ipcmd
-1
1 + s Tg
s1
Figure 2. A block diagram of an REGC_B model illustrating the voltage source interface of the model with the network while assuming near ideal
behavior of inner current control loops and PLL.
IEEE Electrification Magazine / JUNE 2021
65
rrpwr
(Rate Limit)
Numerator
1
1
Ip
Id ( = Ip)
s4
0.01
Ed
1
1 + s Te
Eq
1
1 + s Te
s3
+
j
+
V
~
Iq
Iq
re Xe
Eq = Vtqo + iqre + idXe
Ed = Vtdo + idre - iqXe

IEEE Electrification - June 2021

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