IEEE Electrification - September 2022 - 49

further in the 50 and 75% scenarios as battery energy storage
effectively reduces peak loads.
Even though annual generation and displacement values
are important for public policy and long-term system
planning, they provide little information on day-to-day,
hourly, or subhourly operations. Because system load
changes from hour to hour and solar resources are variable,
understanding chronological generation by unit and
resource type is critical. A production cost analysis performs
a chronological commitment and dispatch of the
power grid to minimize system cost in a fashion similar to
that of the grid operator. The commitment determines
which units should be online while dispatch determines
the megawatt output from each generator.
The dispatch diagrams in Figure 5 show a relatively
" normal " day of operation for each respective case. The
dashed line shows the load level for each given hour. Battery
storage is depicted as two shades; when the battery
storage (dark pink) is above the dashed line it is charging,
and when it is directly below the dashed line (light pink)
the battery storage is discharging.
As DER penetration increases along with the buildout
of battery storage, battery storage fulfills a larger portion
of load during the morning and evening hours. As mentioned
previously, by the 75% DER case, most of the generation
formerly provided by ST units is also replaced by
solar PV.
With this increase in DERs, solar and storage becomes
the largest resource on the system in most of the hours of
the day, with CC-and to a lesser extent, ST-fossil units
dispatching in the morning and evening hours when solar
generation is reduced. The peak solar hours of the day are
not only the prime hours for charging battery storage
resources but also the hours where most fossil fuel-based
generation is either reduced to lower loading levels or
turned off entirely. This is most noticeable in the 75% DER
case where all generation, save for a small portion of CC
generation, is displaced during the middle of the day by
solar. It is important to note that this is happening even
with a large amount of solar generation being directly
charged by battery storage for use at a later time.
The decision to turn down a generator or entirely turn
it off is based on several variables, including the resource's
start-up and shutdown costs, minimum loading level,
spinning reserve requirements, and the expected amount
of time that the generator can be turned off.
The study closely evaluated periods with high instantaneous
solar and battery output. This is because both solar
and batteries (as well as wind) resources are IBRs. IBRs utilize
a power electronic inverter rather than a spinning
generator to inject controlled levels of electric power into
the grid, which is critical for maintaining grid stability
moment to moment throughout time. It is important to
note that because solar and wind resources are variable,
they may at times reach very high levels of penetration (as
a percentage of the grid's total resource mix), even if their
annual generation levels are relatively modest.
Figure 6 shows that as more IBRs are added with each
scenario, all hours have a greater total generation and
percentage of IBRs providing generation. Of particular
note is that in the 50% DER and 75% DER cases, there are
hours with 100% of the generation coming from IBRs,
even after using storage to shift much of the surplus generation.
At the time of the study, the inverter control technologies
needed to manage these conditions are still
under development, so reliability challenges are
addressed through operational changes as well as consideration
for synchronous condensers in higher-penetration
cases. In the 50% DER case, only 4 h across the entire year
have all their energy coming from IBRs, suggesting that
this challenge could be mitigated with relatively brief
operational changes. However, in the 75% DER case, nearly
1,250 h have 100% of the generation coming from IBRs,
which is slightly more than 14% of all hours of the year.
One could expect inverter technology advancing in the
upcoming years to mitigate these situations, but if not,
the introduction of synchronous condensers could provide
needed stability.
Current
25% DERs
4,000
3,500
3,000
2,500
2,000
1,500
1,000
500
One Day
Storage
ST
Figure 5. The dispatch diagrams for a " normal " day.
IEEE Electrification Magazine / SEPTEMBER 2022
49
One Day
Solar
CC
Wind
Coal
One Day
Hydro
Load
GT
One Day
50% DERs
75% DERs
(MW)
IEEE Electrification - September 2022

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