IEEE Electrification - September 2021 - 58

Inverter
PCC
DGU
+
-
PWM
Voltage
Controller
uabc
abc
dq
i*dq
Current
Controller
Grid-Forming Control
udq
Figure 4. A grid-forming inverter.
blocks, and a PWM block constitute the grid-forming control
of a grid-connected inverter. The functions of the " abcdq "
blocks and the PWM block are the same as those of
their counterparts in grid-following control. The angle i of
the " abc-dq " block is set by the power controller, which
comprises a power calculator and a droop controller. The
power calculator computes the fundamental components
of real and reactive power P and Q according to the
instantaneous measurements of three-phase terminal
voltage vabc
and terminal current iabc
tunes i and the setpoint vdq
)
. The droop controller
by comparing , P Q with their
setpoints d~ , dE , which are specified by the secondary
control of the inverter's host microgrid. The voltage controller
takes vdq
setpoint
i .dq
)
According to idq
)
) and vdq as input, and it outputs a current
and ,idq
the current controller
computes ,udq based on which the modulation signal
m is obtained.
System-Level Control
The system-level control includes secondary control for a
microgrid and tertiary control for a system consisting of
several networked microgrids. Secondary control aims to
Tertiary
Control
Eref
ωref
Secondary
Control
µMS
δE1, δω1
δE2, δω2
δE
δω
δEn, δωn
The Cyber Vulnerability of Energy
Transactions in Distribution Systems
Both inverter-level and system-level control schemes
entail feedback structures that drive the physical infrastructure
according to real-time measurements. As
examples, the grid-forming/grid-following controller
tunes the modulation index m of the inverter by measuring
terminal voltage vabc
and current iabc
in Figures 3
and 4. The secondary controller measures bus voltage
magnitudes and frequencies at the microgrid network
and dispatches control commands to each controlled
grid-forming inverter. Because there are such tight
interactions between the physical infrastructure and
the cyberlayers, adversaries may compromise the physical
infrastructure by maliciously manipulating the realtime
measurements.
For example, if an attacker manipulates one of the voltPhysical
Configuration of a
Grid-Connected Microgrid
Grid-Forming
Inverter 1
Grid-Forming
Inverter 2
Grid-Forming
Inverter n
Microgrid
Network
E, ω
PCC
age measurements E that is fed to the secondary controller,
the control commands computed by the secondary
controller will be wrong, leading to nonoptimal operation
of the microgrid. Similar scenarios can occur in the other
controllers described in the last section. Table 1 summarizes
vulnerable measurements in the control schemes of
a future distribution system. Compared
with signals encapsulated in
the controllers, measurements are
more vulnerable to cyberattack as
the sensing devices of the measurements
may be exposed outside
the controllers. For example, the
sensors of voltage magnitudes and
frequencies are distributed over the
microgrid and may be far away
from the MSn
P, Q
Distribution
System
. In such a case, even
Figure 5. A microgrid with system-level control.
58
IEEE Electrification Magazine / SEPTEMBER 2021
though the MSn is well secured,
adversaries can assess the distributed
measurements, thereby compromising
the microgrid.
v*dq
idq
dc
ac
m
abc
dq
Output LC
Filter
iabc
vabc
abc
dq
vdq
Power
Calculator
Droop
P, Q
δω, δE
Calculator
θ
Power
Controller
regulate frequency and voltage at some critical buses to
their nominal values. This can be achieved in either a
centralized or distributed manner. Figure 5 presents the
block diagram of a centralized secondary controller. In
Figure 5, the secondary controller is located at the MSn of
a microgrid that contains several distribution generations
with grid-forming inverters. The secondary controller
measures the frequencies ~ and voltage magnitudes E
from the microgrid network and tunes the setpoints of the
grid-forming inverters (i.e.,
secondary controller has setpoints Eref
d E and d~ ) accordingly. The
re ,f
and ~ which are
set by tertiary control. The goal of the tertiary controller is
to regulate real and reactive power exchange between the
microgrid and its host distribution system. According to
the measurements of real and reactive power P and
Q ,
the tertiary controller tunes the setpoints of the secondary
controller Eref
and ~
re .f
⋅⋅⋅
IEEE Electrification - September 2021

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