IEEE Electrification - December 2020 - 78

substation-voltage source. This can
be seen in Figure 7, where there is a
PG1
PG 3
PG 2
loss of the substation voltage and
PG 4
PG 5 and PG 6
60
automatic operation to island the
PG 7
three microgrids. Figure 7(a) shows
59.5
1,000
the frequency transient, and Figure 7(b) shows the active power
59
output of each of the generation
units shown in Table 1.
58.5
From Figure 7, it can be seen that
500
58
Microgrids 2 and 3 experience sigMG 1
nificant frequency transients during
MG 2
57.5
the unintentional islanding event-
MG 3
transitioning from 1(a) to 1(c) in Fig0
57
0
10
20
0
10
20
ure 1. The transients result from the
Time (s)
Time (s)
instantaneous mismatch in the gen(a)
(b)
eration and load, which results in
the rotational energy of the generaFigure 7. The microgrid frequency transients (a) and generator loading (b) during an unintentional
tors being dissipated through rotor
islanding event at t = 5.0 s.
deceleration. The control on the
generators admits additional fuel to
increase the mechanical torque and
1,500
60.5
restore the frequency, but the res--
PG1
PG 3
PG 2
PG 4
ponse is typically slow due to the
PG 5 and PG 6
PG 7
time delays in the governors and
engines. In contrast, Microgrid 1
1,000
experiences a minimal frequen--
cy transient because of the fast
60
res--ponse of the grid-forming, in--
verter-based DER. This is a sig500
nificant advantage of inverters
MG 1
using power electronic-based,
MG 2
grid-forming controls.
MG 3
It also can be seen in Figure 7
0
59.5
that the primary frequency res--
10
20
30
10
20
30
ponse has left the microgrids operTime (s)
Time (s)
ating at off-nominal values. This
(a)
(b)
is because of the implementation
of the previously-mentioned droop
Figure 8. (a) and (b) The restoration of the frequency to nominal value (secondary
frequency -control).
controls. Once the transients of
the islanding operation have setfor the loads in gray. The other end-use loads experience
tled sufficiently to ensure stable operation, the
a loss of service. At this stage, it is assumed that there is
microgrid controllers would use the OpenFMB architecstill an operating communications infrastructure, and the
ture, over wireless infrastructure in this example, to
individual microgrids can communicate via an OpenFMB
restore the frequency to nominal. This can be seen in
harness. Additionally, it is assumed that the switchgear
Figure 8, where the generator set points are adjusted to
between the microgrids, between nodes 151 and 300 and
restore frequency.
between nodes 97 and 197, have autosynchronizing capaFrom Figure 8, it can be seen that the frequency is
bilities and are also connected to the OpenFMB bus. If
adjusted with minimal adjustment to the generator
these conditions are not met, then the microgrids would
active-power outputs. This is achieved by the microgrid
continue to operate independently as long as their
controller adjusting the active-power set points of the
resources would permit. However, the desire to interconcontrollers to bias frequency toward the nominal value of
nect micro--grids for resiliency and efficiency pushes
60 Hz. Additional control signals could be dispatched by
toward network--ed operations.
the microgrid controllers for tertiary frequency control to
During the initial loss of substation power, there can
adjust the generators' dispatches, but this example will
be significant transients due to the loss of the stiff
assume that does not immediately occur. Instead, the
1,500

Frequency (Hz)

Active Power (kW)

Frequency (Hz)

Active Power (kW)

60.5

I E E E E l e c t r i f i cati o n M agaz ine / DECEMBER 2020

IEEE Electrification - December 2020

Table of Contents for the Digital Edition of IEEE Electrification - December 2020