Distributed Renewable Energy Operating Impacts and Valuation Study

Section 5
However, discussions with APS reveal that their system operators – APS staff that manage and
administer the moment-to-moment operation of the electric system – monitor and predict the
impact and timing of storm fronts on the APS system quite well. So long as the system operators
have accounted for the quantity of solar DE on the APS system, APS believes that they should be
able to reasonably predict the impact of a storm front on solar DE operation and, subsequently,
on the electric system. They believe they will be able to commit and dispatch resources
sufficient to manage potential adverse effects that weather events may have on solar DE
operation and the electric system.
Another possible event that would cause a significant loss of solar DE output was discussed in
Section 4 where an under- or over-frequency condition could cause a significant number of
inverters for solar PV installations to trip off-line. However, such conditions would be rare since
they effectively represent a double contingency event. As a result, PV inverter trips are typically
not treated as a condition for which APS is required to maintain reserves.
While there may exist other events that could cause a significant number of solar DE resources to
simultaneously stop producing, the two events described are thought to represent the
preponderance of potential causes. As such, solar DE installations are not anticipated to result in
a significant increase in spinning and operating reserve requirements for APS. However,
because the quantity of solar DE electricity production is projected to be greater than the current
levels of spinning reserves maintained by APS in some hours, the utility should carefully weigh
its ability to predict and manage such events when determining whether to increase its spinning
and operating reserve requirements as solar DE levels increase on the APS system.
5.4.3 Impact of Solar DE on Regulation Reserves
Regulation reserves represent the quantity of capacity that APS must have committed and
available to meet instantaneous, moment-to-moment variations in load. These variations are
generally caused by customer appliances and equipment turning off and on in unpredictable
patterns. Much of the variability observed for any individual customer will be mitigated across
the electric system since all customer loads do not turn on or off simultaneously. However, net
variability in total load can still be large, varying by as much as 2 to 3 percent of the average
load within an hour under normal operations. Variability is usually worse in the morning and
evening hours when load is growing or declining significantly.
With regard to solar DE installations, the concern is that fluctuations in the output of individual
or small groups of solar DE facilities could increase the variability already observed in system
loads. Any increase could require APS to carry more regulation reserves, which would increase
APS’s operating costs. Variability in solar DE facility output could be caused by forced outage
events (random electronic or mechanical failures of the solar DE facility) or by weather events,
such as a random passing cloud. However, it is also anticipated that the distributed nature of the
solar DE installations and the natural non-coincidence of outages that would occur across a large
number of solar DE facilities could effectively mitigate the variability that might be observed for
any one solar DE resource.
To appropriately evaluate the impact that solar DE facilities have on APS regulation reserves, it
would be necessary to track a statistically significant sample of solar DE facilities randomly
distributed over the entire APS service area. Data from these facilities would need to be
monitored and recorded continuously, down to perhaps one-minute intervals for at least one year.
5-24 R. W. Beck, Inc. Arizona Public Service