IEEE Power & Energy Magazine - November/December 2021 - 22

synchronous generators is often determined by first-swing
transient stability to ensure that
the machines remain in
synchronism. As IBRs replace synchronous generators and
reduce system strength, the transient stability concern may
be replaced by a need to maintain adequate voltage and
angular stability across the grid.
New voltage stability concerns are created due to the
following:
✔ the varying number of online voltage regulators with
high gains
✔ differences in IBR responses
✔ potential interaction between IBRs and other dynamic
devices.
For example, if the system strength at a point of interconnection
is low, the critical voltage at the nose of the power-voltage
curve may reside within the normal operating voltage
range. This may mask a voltage condition that exceeds the
reliable operating point. Figure 2 provides an example of
this phenomenon based on the Electric Reliability Council
of Texas's " Panhandle Renewable Energy Zone Study. " Historically,
similar issues have been seen in systems with high
levels of shunt reactive compensation. Such a convergence of
voltage stability limits on the normal operation range contributes
to the apparent " brittleness " of the system, tending
to accelerate the speed at which the network fails.
Contingencies without faults, such as line trips, can lead
to more stressed system conditions. Under fault conditions,
IBRs' fault ride-through mode is triggered, resulting in the
rapid increase of reactive power and the reduction of active
power to assist voltage recovery. In the case of no-fault
contingency conditions, the real power stays at higher levels,
which results in increased reactive losses due to the
1.02
1.04
1.06
0.92
0.94
0.96
0.98
1
Voltage Collapse at
Normal Operating
Voltage Range: 0.95~1.05 p.u.
postcontingency state of the system. This can lead to a fast
voltage collapse. On the other hand, an improper response
from IBRs during the fault ride-through after a fault (for
example, too-aggressive active power recovery) could lead to
a cascading high- or low-voltage collapse in BPSs where IBRs
are concentrated in remote pockets far from load centers.
The high control bandwidth and different dynamic characteristics
of IBR voltage controls may interact in new, and not
fully understood, ways with other reactive devices. Limitations
on the bandwidth of controls for synchronous generators (for
example, 5 Hz in Great Britain) can also apply to IBRs. As
IBR penetration approaches 100%, coordinated control tuning
between nearby plants may no longer be practical. The
concept of angular stability also changes with increased IBR
penetration. Large-signal rotor angle stability has long been
the proxy for synchronous machines maintaining synchronism
during grid faults, a concept driven by the angular displacement
of rotor and stator magnetic fields and accelerating or
decelerating torque. The accelerating energy accumulated by
synchronous machines due to the mismatch between mechanical
power input and reduced electrical power output dominates
fault clearing dynamics. There is a duality between voltage stability
limits, represented by power-voltage curves, and angle
limits, represented by equal area plots (Figure 3).
Figure 3 illustrates the behavior of an exporting, radially
Wind Generation in Panhandle Area (GW)
Bus 1
Bus 4
Bus 7
Bus 2
Bus 5
Bus 3
Bus 6
connected synchronous generator compared to an IBR of the
same rating and initial dispatch. For a typical fault-and-clear
event, the figure includes two time traces and the accompanying
power-angle curves, power-voltage nose curves,
and phasor diagrams. The numbered, colored dots represent
points in the time sequence for the two types of generation.
For simplicity in this example, the receiving system does
not move. The swing behavior of the synchronous machine
is familiar, with accelerating energy balancing decelerating
energy (the orange shading). The IBR behavior (in black) is
radically different, with the terminal voltage phasor moving
only a little. The IBR recovery is relatively crisp, with no
urgent need to dissipate mechanical energy that accumulated
during the fault. The fault severity, in terms of the voltage
retained at the terminals of the resource or fault clearing time,
has little relevance for IBRs, especially solar and battery. The
control of the power electronics determines the phase angle
based on the injection of active current and voltage control.
The phase angle can be controlled to facilitate whatever fault
power is most advantageous, subject to inverter limits. The
challenge of maintaining voltage and angle stability becomes
completely dominated by IBR control and postdisturbance
grid characteristics.
As the time traces and the phasor diagram show, the IBR
figure 2. Power-voltage curves at 345-kV buses, illustrating
a postcontingency voltage collapse in a normal voltage
range (from the 2014 Electric Reliability Council of Texas
" Panhandle Renewable Energy Zone Study Report " ). p.u.:
per unit.
22
ieee power & energy magazine
controls tend to be more stable in this regard, having a much
lower angular swing. This means the initial angle can be
greater, enabling more power to be exported in the predisturbance
condition. This pushes the initial voltage angles farther
apart across the network and closer to the nose of the power-
voltage curve. Care must be taken when the grid is under
november/december 2021
Bus Voltage (p.u.)
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3
3.2
3.4
3.6
IEEE Power & Energy Magazine - November/December 2021

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