IEEE Power & Energy Magazine - March/April 2017 - 67
voltage because grid-following inverters merely act as voltage-following current sources. It would
Synchronous
Synchronous
Generator
Generator
be challenging, if not infeasible,
to obtain an inverter-based system
with grid-following control.
To overcome this shortcoming
of grid-following inverters, it is
necessary to develop next-generation grid-forming inverters that
Induction
enable the transition to an inverterInduction
Motor/
Grid ac
Motor/
based infrastructure and are capaGenerator
Waveform
Generator
ble of regulating system voltages
50 or 60 Hz
and frequency through local decentralized control. Before considering the necessary characteristics
that these grid-forming inverter
controllers must have, it is worth
Smart Wind
dc/
looking at a few key aspects of the
dc/
ac
Smart PV
ac
challenges that lie ahead. First,
it must be recognized that nextgeneration inverter-based systems
will be realized gradually over figure 6. The representation of an electric power system showing tight coupling of
synchronous generators and smart VRE systems and loose coupling of induction moseveral years or decades as syntors/generators.
chronous machines are gradually
replaced with renewable sources.
Further, given that power electronics inverters are typically programming a linear relationship between real and reactive
several orders of magnitude smaller in power rating com- power compared to frequency and voltage; however, because
pared to synchronous machines, this implies that the system the computations of real and reactive power are carried out
load in an inverter-based infrastructure must be satisfied with on a relatively slow timescale, droop-controlled inverters are
a much larger population of inverters. For large-scale electric susceptible to a sluggish response during transients. Alternagrids, this will likely translate to the installation of millions tively, researchers have explored methods of emulating variof inverter-interfaced VRE units across large geographical re- ous physical phenomena with inverters to create so-called
gions. Considering these points, these new controllers must virtual synchronous machines or virtual inertia. In this type
have the following features:
of application, the governing equations of a machine or its
✔✔ Grid-forming inverter controllers must be compatible inertial responses are programmed on the inverter controller.
with existing systems and provide a seamless path be- Last, a class of grid-forming methods based on the dynamics
tween the architectures shown in Figure 4 as the sys- of nonlinear oscillators has received recent attention. Drawing inspiration from the emergence of synchronization in
tem evolves over time.
✔✔ Very large collections of geographically dispersed networks of coupled oscillators and leveraging the algorithunits imply the need for decentralized approaches mic flexibility of digital control, these new virtual oscillator
that do not require communications for fast-timescale controllers yield rapid response times and have been shown
to be capable of creating zero-inertia, inverter-based systems.
control.
✔✔ To pave the path toward a resilient and reliable infrastructure that lasts into the foreseeable future, the Power System Stability
grid-forming units must be able to operate in the com- AC power systems rely on the basic physics of synchronous
plete absence of synchronous machines, if needed.
generators to provide grid stability. For all synchronous
✔✔ In addition to active and reactive power controls, generator rotors, the rotating mass electromechanically
controllers of grid-forming inverters must employ ad- couples to each and every one of them through the electric
vanced control methods to maintain adequate power grid so that they rotate in synchrony during stable operating conditions. In synchronized conditions, this is equiquality characteristics of the energy supply to loads
To achieve these objectives, a variety of grid-forming con- valently represented as one large generator shaft running
trol strategies have been proposed. The most established at the nominal ac frequency (typically 50 or 60 Hz). Figapproach, droop control, is motivated from traditional control ure 6 shows a representation of the tight coupling among
methods for synchronous machines and is implemented by synchronous generators. This equivalent rotating mass is
march/april 2017
ieee power & energy magazine
67
Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2017
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IEEE Power & Energy Magazine - March/April 2017 - Cover3
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