IEEE Electrification - September 2020 - 25

the serial inverter or Vinv for the parallel inverter, determines whether the inverter is injecting or absorbing reactive power. This capability can be implemented as reactive
power control or voltage control. Furthermore, if these controls are modulated according to system oscillations,
inverters can provide damping control. An inverter can
also provide controlled harmonic voltage components in
its output. These harmonic components can be controlled
such that the resulting harmonic current cancels the system harmonic current. In this way, the inverter functions
as an active harmonic filter.
The key point is that in a power-electronics-heavy
power system, inverters that have the multiple control
capabilities described previously can and must provide
all the control functions that are provided by conventional generators today. Because of the high-speed control capabilities of inverters, all these functions can be
faster in handling the low-inertia dynamics of the
power system. When implementing these functions in
a conventional power system, the inverter relies on
well-established system frequency and voltage as references so its output can be synchronized and coordinated with the rest of the system. These frequency and
voltage references are regulated and maintained primarily by the conventional synchronous generators. In
a future power-electronics-heavy system, such references may not be established or maintained, and the
inverters will need to regulate and maintain the system
frequency and voltage by using the collective interactions of the inverters themselves. These regulation
functions are much like the droop controls of real-power-frequency and reactive-power-voltage relationships
for conventional synchronous generators. Inverters are
capable of providing these droop control functions.

Currently, extensive research efforts are developing the
droop controls for inverters, and multiple solutions
have been proposed.
It is worth pointing out that many of these functions
can be provided in a simultaneous manner as indicated
in the phasor diagrams shown in Figure 9. It is also
worth noting that the limitation of the energy support
behind the inverter will limit the quadrants of these control functions. For example, PV generation alone cannot
absorb power, so it can only be controlled in the first and
second quadrants, but PV generation combined with
energy storage can operate in all four quadrants. These
phasor diagrams still govern the construction of inverter
control functions.

Hierarchical Control of a Large Number of Inverters
Multiple control functions that an inverter could offer are
illustrated in the previous section. These functions
include frequency control, voltage control, active power
control, reactive power control, damping control, active
and reactive power droop controls, power factor correction, and harmonic filtering. Beyond these fast control
functions offered by individual inverters, the large number of inverters collectively offer a further control capability for adaptive and scalable reconfigurations of the
inverters to achieve system-level performance, such as
flexibility and resilience.
An example is a hierarchical control structure with
coordination across the regional control layers (Figure 10).
The inverters can communicate with and among each
other and communicate upward across the layers, so
they can maintain awareness of the system-level conditions and exchange control signals for their set
points. The increase in wind and solar generation in

System Operator

Regional Operator

Conventional Generators

Regional Operator

Self-Organizing Microgrid

Figure 10. The hierarchical coordinated control of many inverters.

IEEE Elec trific ation Magazine / S EP T EM BE R 2 0 2 0

IEEE Electrification - September 2020

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