IEEE Electrification - March 2021 - 81

case, the GFM BESS, PV plant, and wind turbine are operating with a frequency droop for load sharing. The system restoration is a gradual process-based on priorities as defined
in the restoration plans-until the totality of the customers
are reenergized. Restoration strategies depend on the types
of systems being restored and the location of the black-start
resource in the grid. PV-wind-BESS plants with GFM capability can participate in various restoration schemes and will
become a critical component of resilient island grids.

Real-Time Monitoring of System Inertia and PFR
Operation of island power systems that have very low
inertia is perhaps the biggest stability challenge as high
levels of IBRs are integrated. The system frequency can be
stabilized for low-inertia systems by designing FFR in IBRs.
However, implementation of FFR functionality in IBRs
requires the continuous maintenance of some active
power reserve, which may necessitate curtailment and
other generation dispatch compromises. Real-time monitoring of the system inertia and PFR can help optimize the
active power reserve in IBRs.
The inertia and PFR of an island power system can
be estimated by injecting small-signal, noninvasive
active power perturbations into the network via IBRs
and measuring the system frequency. The frequency of

the active power perturbations is varied from a fraction of a hertz to a few tens of hertz, and the recorded
system response is used to obtain a transfer function
from the injected active power perturbations to the
system frequency. The response of this so-called frequency response transfer function can be used for
estimating the system inertia and PFR. Figure 7 demonstrates this approach for the real-time inertia and
PFR monitoring of an IEEE nine-bus system simulated
in PSCAD and a real 2-MW diesel generator at NREL's
Flatirons campus.

Selected Case Studies
Puerto Rico: Demonstration of Frequency
and Voltage Services From PVs
Puerto Rico has a modern electric power system that
serves the territory's entire main island. Currently, the
total installed generation capacity is close to 6 GW, which
includes almost 200 MW of wind and solar PV generation
(Figure 8). The bulk of the electric generation is still largely based on petroleum and coal, with increasing participation by natural gas generators. Puerto Rico's
transmission system consists of 230-kV and 115-kV lines
as well as 38-kV subtransmission lines, with a total of

Line-Neutral Bus Voltages (kV)

9
8
7
6
5
4
3
2
1
0
-50

Voltage 1
0

Net PV Power (kW)

450
400
350
GFM BESS Inverter
300
Energizes System
250
at t = 0 s
200
150
100
50
0
-50 -50
0
0
-100

Time (s)

100

Voltage 2
150

Voltage 3
200

Initialization Time for
PV Inverter 1 = 60 s
Nominal Power Operation
R
h d After
Aft 130 s
Reached
Initialization Time for
PV Inverter 2 = 80 s
50

Time (s)

100

150

200

Figure 4. The soft black start of a PV system using the GFM BESS at the NREL Flatirons test facility.

IEEE Electrific ation Magazine / MARCH 2 0 2 1

IEEE Electrification - March 2021

Table of Contents for the Digital Edition of IEEE Electrification - March 2021