IEEE Electrification - March 2022 - 26

245
240
235
230
225
220
5 10 15 20
Nodes
(a)
Time of Day
245
240
235
230
225
220
5
10
15
Nodes
20
Time of Day
(b)
Figure 3. Southern Company distribution feeder 8-h voltage profile;
(a) centralized top-down control, and (b) grid-edge real-time control
(Source: Moghe et al. 2017).
have to address include: operation in grid-connected and
grid-independent modes; operation in normal, abnormal,
and fault modes; operation based on local information
with poor topology knowledge and slow communications;
P and Q sharing under steady-state, transient and fault
conditions; no undesired interactions with other inverters,
generators and grid elements; and the ability to form resilient
microgrids that can automatically coalesce or separate
as needed. Additional factors that are important but
are not covered here include mechanisms for dynamic
balancing under energy surplus and scarcity conditions,
grid services, transactive control, interoperability, vendor
agnostic controls, and issues of cybersecurity.
To address these requirements, a universal controller
(UniCon) strategy has been developed for grid-connected
inverters, as shown in Figure 4 (Miranbeigi et al. 2021). UniCon
manages the grid-side behavior of the inverter in
steady-state as well as normal, abnormal, transient, and
fault modes based on specified rules. It does not really
operate in different modes; instead modulating its
response based on what it sees as current needs, prioritizing
or deemphasizing control actions as appropriate. This
is akin to how we behave in real-life situations and is the
hallmark of an intelligent agent. There is basic information
available, such as inverter ratings, switching frequency, and
limitations of its sensors, and so on. The grid voltage and
current are the key instantaneous variables against which
the inverter acts. Frequency is allowed to vary rapidly and
is not tightly controlled but moves toward an average value
that would balance system power flow in the steady-state.
Current Loop and
Virtual Impedance
Rg, Lg
vg
i2
Cf
p = vCi1
X
Low-Pass
Filter
i1
vC
Pfb
+
−
Rf, Lf
i1
SU SV
SPWM
Vi
UniCon
Scheme
P/ω Droop
+
Σ
VN Imax ω∗
(a)
+
−
Power Pfb
Feedback
Virtual Inertia
1/s
+
ω∗
Rated Frequency
(c)
Figure 4. An IBR circuit diagram and UniCon functionalities block diagram: (a) single-phase voltage source inverter schematic diagram,
(b) current-loop and virtual impedance, and (c) power-loop.
26
IEEE Electrification Magazine / MARCH 2022
Σ
ω
1/s
+
Power Loop
Vdc
θ
sin(x)
VN
+
−
Rated Voltage
Voltage
Feedback
(b)
Variable Inertia
Damping
Kd (x)
Vc
I1
Phase Jump
Algorithm
∆θ
+
Σ
θ
Vc
Virtual
Impedance
I1
E∗
Σ
Hvir (s)
+
−
Current
Controller
Current
Feedback
Imax
Adaptive Virtual
Impedance
Rvir
Lvir
I∗
Vi
Σ
Gc (s)
I1
Vc
16:00
17:00
18:00
15:00
14:00
12:00
13:00
11:00
10:50
11:50
12:50
13:50
14:50
15:50
16:50
17:50
18:50
(V)
(V)
IEEE Electrification - March 2022

Table of Contents for the Digital Edition of IEEE Electrification - March 2022
