IEEE Power & Energy Magazine - March/April 2017 - 66

Grid-following controllers represent the most
prevalent type of control strategy for grid-connected
PV and wind inverters.
loads and VRE when the magnitude of the frequency deviation
is inversely proportional to the net inertia on the system. Consequently, a system with low inertia is vulnerable to larger and
undesirable frequency deviations.
Another important factor determining the dynamic behavior of existing power systems is the synchronizing torque produced by synchronous generators. The synchronizing torque
along with inertia has a crucial role in determining the initial
rotor speed behavior of conventional generators following a
contingency event in the grid. The active power injected by
synchronous machines maintains synchronism and damps
mechanical oscillations through their synchronizing and
the damping torque components of the total electric torque.
The abundance of inertia and synchronous torque from synchronous machines along with their controls allows for the
mitigation of the large active and reactive power imbalances
in the grid. This fundamentally important characteristic of
power systems would change dramatically with growing penetrations of inverter-based generation.
In contrast, VRE technologies utilize a fundamentally
different set of technologies for energy conversion and
interfacing to the grid. VRE sources typically connect to
the grid through a power electronics interface called an
inverter. The inverter converts dc electricity to ac power
and manages the flow of energy by controlling switching
semiconductor devices at a fast timescale. In contrast to
a generator, an inverter is strictly electronic and does not
contain any mechanical components or rotating masses.
Accordingly, it does not exhibit the physical properties of
the machines described previously. Inverter technologies are
especially important because they are used across a wide
variety of applications.
Wind turbines that use power electronics interfaces
include what are called Type III or doubly fed induction generators (DFIGs), in which the rotor windings are connected
to the grid via slip rings and an inverter. Type IV wind turbines convert all power delivered by the wind turbine generator to dc and then back to grid-compatible ac power through
an inverter. PV systems always require an inverter because
PVs natively generate dc electricity and the inverter must
deliver this power to the ac grid. Although battery storage is
not a VRE source in itself, it will play a key role in managing
energy balance in systems with high penetrations of VRE,
and it is also interfaced to the grid through inverters.
Regardless of the VRE type that interfaces to an inverter,
a closed-loop controller is required to regulate the energy
flow from the dc input, through the power electronics, and
66

ieee power & energy magazine

ultimately to the ac grid. These controllers are typically executed on digital controllers where real-time measurements
are processed and user-defined controls are programmed and
executed. Of particular importance, the characteristics of
the chosen control strategy, not the inverter's physical properties, dictate the electrical dynamics of the inverter during
disturbances and how it interacts with the grid on its ac side.
In other words, its physical response is dictated by how its
digital control is programmed. This is in contrast to synchronous machines, where the physical properties of the machine
itself, such as the amount of mechanical inertia and electrical
parameters, play the largest role in determining its transient
behavior. To highlight this difference between inverters and
electrical machinery, inverters are often described as having
zero inertia because their response depends almost entirely
on the particular control strategy they utilize and they have
no moving parts.
Broadly speaking, there are two classes of inverter controllers: grid following and grid forming. Grid-following
controllers represent the most prevalent type of control
strategy for grid-connected PV and wind inverters. At the
core of its operation, a grid-following controller utilizes a
phase-locked loop to estimate the instantaneous angle of the
sinusoidal voltage at the inverter terminals. Subsequently,
the power electronics are manipulated to inject a controlled
current into the grid that tracks the sinusoidal terminal voltage. In essence, a grid-following inverter acts like a sinusoidal current source that "follows" the voltage at its terminals;
hence, it is called a grid-following unit.
As one limitation, grid-following inverters work under
the presumption that a "stiff" ac voltage with minimal
amplitude and frequency deviations is maintained at its terminals such that it can simply follow its local voltage and
inject a controlled current. In practice, this translates to the
assumption that the collective behavior of the synchronous
machines, the generator and system controllers, and voltage
regulating equipment on the system provides a sufficiently
stiff frequency and voltage at any point on the grid. Historically, this assumption has held up relatively well because the
cumulative amount of VRE with grid-following inverters has
been relatively small compared to conventional synchronous
generators that regulate the system frequency and voltages
(see the left side of Figure 4). However, what will happen as
we transition to a system that may be dominated by or built
entirely on inverter-interfaced VRE sources? If one tried to
build a zero-inertia system entirely with grid-following inverters, it is unclear which grid assets would regulate the
march/april 2017



Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2017

IEEE Power & Energy Magazine - March/April 2017 - Cover1
IEEE Power & Energy Magazine - March/April 2017 - Cover2
IEEE Power & Energy Magazine - March/April 2017 - 1
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IEEE Power & Energy Magazine - March/April 2017 - Cover3
IEEE Power & Energy Magazine - March/April 2017 - Cover4
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