IEEE Electrification - September 2020 - 21

Penetration of Wind and Solar (%)

Take wind and solar generation as an example to
examine the impact of power electronics. With the wind
and solar generation capacity factor (20-35%) limited by
the weather dependency and the diurnal and seasonal
nature of wind and solar, instantaneous power penetration
can reach 80% or higher during certain periods if wind and
solar are to supply a large share of the annual energy to
the power system in future. Figure 4 shows that a high
instantaneous power penetration of wind and solar generation already existed in some power systems as of 2019.
During these high-penetration periods, inverter-based resources will
120
dominate the dynamics of the
power system. The new fast dy---
Ta'u Island
namics caused by ubiquitous power
100
electronics will present two funda100%
mental challenges not present in
80
the conventional power system: low
system inertia and active participa60
tion of many small devices. For
small-size systems such as those
40
on Ta'u Island, American Samoa
and King Island, Australia, the plan20
ning and operational solutions
could be relatively easy. However,
0
those solutions are not scalable eco0.0001
0.001
nomically and reliably for largersize systems.

adequate frequency response. Frequency response measures the initial frequency deviation when a disturbance
causes a sudden change in generation or load. The slowing down of frequency changes allows an important time
window for the rest of the system to respond, which is
also slow because of the heavy inertia. However, recent
studies performed by the North American Electricity
Reliability Corporation (NERC) clearly indicate that the
frequency response capability is declining in the U.S.
Eastern Interconnection power system (Figure 5).

Maui

Ireland Australia CAISO
ERCOT
84%
80%
70%

King Island

70%

58%
50%

65%

37%

21% 20% 20%
0.01

0.1

100

1,000

Highest % Instantaneous Power

Figure 4. Instantaneous wind and solar power and annual energy penetration reported in 2019
(data sources: HECO, EirGrid, AEMO, CAISO, ERCOT, and NREL).

4,000
*1999 Data Interpolated
3,500
3,000
MW/0.1 Hz

2,500
2,000
1,500
1,000
500
1994
1995
1996
1997
1998
1999*
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020

Inertia is a dynamic system's inherent property that resists a change.
Power system inertia has been
dominated by the mechanical inertia associated with the large mass
of the shaft, which is an essential
part of the conventional turbinegenerator system. This mechanical
inertia in the power system is
diminishing as conventional power
generation such as fossil-fuel generation is being retired and displaced by more inverter-connected
generations.
Although it is not a desired feature by design, mechanical inertia
has historically been utilized to
help power system operation
because it slows down power system dynamics in response to disturbances, i.e., the frequency of the
power system does not change
quickly because of the large inertia
in the system. Power system operation has been relying on such

System Size (GW)

% Annual Energy

The Impact on Power
System Inertia

36%

Year
Annual Mean Primary Frequency Response
Short-Term Trend
Long-Term Trend
Figure 5. Frequency response declines in the U.S. Eastern Interconnection. (Source: NERC; used
with permission.)

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

IEEE Electrification - September 2020

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