IEEE Electrification - March 2022 - 27

A major objective for UniCon is to be applicable across a
wide range of inverter topologies and system/network
applications-ranging from large VSCs connected to transmission
all of the way to microgrids and microgrid clusters.
The objective is to be agnostic to the vendor and to the
implementation of the inner voltage/current and protection
loops. Most of the UniCon control is slower than typical
inverter inner loops and aims at achieving universality.
The main goal is to create interoperability for all types, ratings,
and brands of inverters that are connected to the grid
and to have a unified control strategy.
The relevance and applicability of the UniCon strategy
in real-grid applications is validated using some of the most
challenging events and disturbances that IBR-rich grids
face today. These include response to major load transients
and faults, operation with varying levels of generator/IBR
mix, abrupt connect/disconnect of grid and microgrids,
massive phase and frequency jump as a system is reconfigured
following a fault, sustained operation of clustered bottom-up
microgrids, P/Q sharing under steady-state and
dynamic conditions, and the ability to mitigate interactions
with other grid-forming inverters and grid elements. While
most grid-forming systems achieve similar power-frequency
droop characteristics in the steady-state, it is the behavior
under these major transients and disturbances that will
determine their suitability in real systems and their ability
to achieve scale.
To evaluate the operation of UniCon grid-forming
inverters in such situations, a modified version of a CiGRE
medium-voltage feeder (Figure 5) is utilized to run comprehensive
scenarios that include multiple major events as
discussed earlier (Moghe et al. 2017, Miranbeigi et al. 2021,
and Rudion et al. 2006). These scenarios are selected as a
representative subset of use cases that are expected in systems
with high IBR penetration, and the results and analysis
presented later can be extended to larger systems at
different voltage/power levels. With this in mind, each IBR
in the system is equipped with the UniCon scheme, as
shown in Figure 4, and has no communication with its
counterparts present in the system or has visibility to system
state or grid status. Every UniCon controller actively
and simultaneously manages many functions that often
interact with each other, as the only final control parameter
for the inverter is the instantaneous point on wave voltage.
The real-time implementation of the UniCon
algorithm has been done in separate TI-F28379D-based
inverter controllers. The feeder is modeled with an OPALRT
controller-hardware-in-the-loop setup. The IBRs are voltage
source inverters equipped with LC filters, as depicted in
Figure 4(a), with a 10-kHz switching frequency. Figure 6
demonstrates the operation and resilience of the system
as it is subject to a continuum of disturbances and events.
These events showcase the capability of UniCon and are
discussed later in more detail.
Initially, IBR1 and IBR2 form Microgrid 1, while IBR3
feeds loads at buses 9 and 10 as Microgrid 2. At t1, the two
microgrids are interconnected with no prior knowledge of
the state of the individual IBR units. This requires all three
inverters to dynamically and rapidly get from their individual
frequency/phase operating points to a common
point with minimal transients and within the capabilities
of the inverters. This involves simultaneous operation of
current limit functions in multiple inverters, phase jumps
to a new point on the instantaneous waveforms, rapid
convergence of inverter frequencies, and settling without
oscillations to appropriate levels of P/Q sharing as dictated
by the outside transactive layer. These functionalities are
harmoniously and autonomously performed by the UniCon
scheme. More specifically, the current limit and phase
jump are managed by the phase-jump algorithm depicted
in Figure 4(c), and its efficacy is demonstrated in Figure 6(c)
[detailed in Figure 6(a)]. This algorithm utilizes the inverter's
local current/voltage measurements and input filter
inductor value to correct the voltage phase between the
converter and its point-of-connection, thus controlling the
inrush current during the interconnection. In addition,
notice that during the synchronization transient, both the
power and instantaneous frequency of each IBR exhibits
their own trajectory until they converge in a smooth manner,
much like if the converters had provided inertia and
damping. In fact, due to the variable inertia capability of
UniCon [see Figure 4(c)] every inverter in the system is
able to dynamically change its inertia and damping based
on the power error and measured power to achieve the
new operating point stably. This variable inertia not only
helps keep the system stable under steady-state and
small-signal disturbances-it also adapts continuously to
230 kV
13.8 kV
1
2
Grid Connection
Breaker
3
Microgrids
Interconnection
Breaker
4
11
10
5
9
IBR 3
IBR 2
Fault
Microgrid 2
Location
IBR 1
Load
Step
Figure 5. The inverter-based system under study (Sources: Miranbeigi
et al. 2021, Rudion et al. 2006).
IEEE Electrification Magazine / MARCH 2022
27
6
Microgrid 1
8
7
IEEE Electrification - March 2022

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