IEEE Electrification - September 2020 - 24

Power Output (MW)

conventional hydro generator, a Type 3 wind generator,
and a PV generator, all providing electricity to the power
system. They represent three inertia scenarios-conventional mechanical inertia from a synchronous turbine generator, partial mechanical inertia from the
hybrid ac/dc connection of the wind generator, and zero
mechanical inertia from PV panels. They are modeled
using the typical approved models in the U.S. Western
Interconnection model database.
Assuming available hydro, wind, and solar resources,
these three generation scenarios are simulated in
response to a short circuit fault at the generation terminal bus. Their power outputs during this fault situation

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are shown in Figure 8. In terms of the speed of responses,
it is clear that the PV generator has the fastest recovery
to its rated 100-MW output, the hydro generator has the
slowest, and wind generator recovery falls in the middle
but is still much faster than the hydro generator. The
oscillatory dynamics are typical for a hydro generator.
The speed of the rotating shaft mass in the hydro turbine
generator cannot change quickly because of its heavy
inertia. It has to cycle several times to reach the control
objective-100-MW output. The wind turbine generator
has a smaller inertia and exhibits some minor cycling.
The solar generator does not show any of this cycling
behavior. This simple example clearly illustrates the
advantage of fast control for a low-inertia system. Similar effects were also reported in a recent NERC report
about fast frequency response.

Multiple Control Functions of Inverters

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Besides power control, inverters can achieve many more
control functions. There are two major categories of
inverters-current source inverters and voltage source
inverters. A current source inverter provides current injection from the system perspective, while voltage source
inverters inject a voltage source into the system and
attempt to maintain it at the target value. Both inverters
are capable of multiple control functions, as indicated in
Time (s)
the following paragraphs using voltage source inverters
for illustration purposes.
Wind Type 3
PV
Hydro
The voltage source from a voltage source inverter can
be in parallel or in series with the system, depending on
Figure 8. The power output of three different generators in response
how the inverter is connected to the power system. For
to a hypothetical fault situation.
example, inverters of PV generation are connected in parallel with
the system, a static synchronous
series compensator in the FACTS
Vsyst
V1
Vinv
Vsyst
family is in series, and a unified
Vinv
System
System
power flow controller has both
parallel and serial connections
with the system. Either in parallel
or in series or both, the injected
±
voltage source could have four
±
quadrant control capabilities if
Vinv
there is enough energy support
V1
behind the inverter. These four
quadrant voltage sources are
θ
Vinv
illustrated in the phasor diagrams
θ
Vsyst
in Figure 9.
Vsyst
In either the parallel or serial
connections, the relative voltage
angle i determines whether the
inverter is injecting or absorbing real
power. This capability can be imple(a)
(b)
mented as real power control or
frequency control. The relative
Figure 9. Four-quadrant voltage sources for the multiple control functions of inverters: (a) a circuit
amplitude of the system voltage,
and phasor diagram of an inverter connected in series, and (b) a circuit and phasor diagram of an
inverter connected in parallel.
Vsyst , with respect to voltage V1 , for

I E E E E l e c t r i f i cati o n M agaz ine / SEPTEMBER 2020

IEEE Electrification - September 2020

Table of Contents for the Digital Edition of IEEE Electrification - September 2020