IEEE Power & Energy Magazine - July/August 2015 - 44

The study considers four alternative
scenarios that achieve 50% renewables and one
that achieves 40% renewables.

relatively flat and predictable during daylight hours, by 2020
the net load drops by several thousand megawatts during
peak solar production hours. The combination of increased
electric load in the evening hours (the chart represents a cool
spring day with significant residential heating load) and the
reduction in solar output around sundown creates a very significant upward ramp between 17:00 and 20:00. CAISO has
used this chart to argue that California utilities should be
required to procure sufficient upward ramping capability to
meet the projected need. CAISO and the California Public
Utilities Commission jointly developed the flexible resource
adequacy capacity-must-offer obligation (FRAC-MOO)

proposal that a 3-h upward ramping obligation be added to
California's existing resource adequacy standard.

Analytical Approach

Megawatts

Megawatts

The study utilized a novel approach to simulating power system dispatch. E3's Renewable Energy Flexibility (REFLEX)
Model, implemented on ECCO's ProMaxLT platform, has a
number of unique features designed to investigate renewable integration issues under high penetration. These include
economic parameters that reflect the cost of dispatch inflexibility and random draws of stochastic variables such as load,
wind, solar, and hydro conditions taken from a very large
sample to ensure that the economic
results represent a true expected
27,000
Unserved
value. The model uses mixed-inteEnergy
25,000
ger programming techniques to capLimited
ture unit commitment requirements
23,000
Ramping
as well as the capability of each
Capability
21,000
resource to provide reserve products.
2013
The CAISO duck chart illustrates
19,000
Increased
the
potential for a lack of upward
Ramp
17,000
ramping
capability to prevent the
2015
Significant Change
system
from
being able to serve the
15,000
Potential
Starting in 2015
Overgeneration
high loads that occur after sundown.
13,000
However, it does not take into con2020
sideration the potential for the cur11,000
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
tailment of renewable energy output
(a)
to be used as a flexibility resource.
27,000
Renewable curtailment could allow
dispatchable resources to continue
25,000
to operate at their minimum generat23,000
ing levels rather than shutting down,
better positioning the system to meet
21,000
upward ramps.
2013
19,000
This point is demonstrated in
Increased
Ramp
Limited
Figure
3, which uses the duck chart
17,000
Ramping
2015
to
illustrate
the effects of limited
Capability
Significant Change
Potential
15,000
ramping
capability.
In (a), the renewStarting in 2015
Overgeneration
able
generation
is
treated
as "must
13,000
run."
Because
a
limited
quantity
of
2020
11,000
upward
ramping
capability
is
avail0 1 2 3 4 5 6
17 18 19 20 21 22 23
Renewable
able, the system operator is unable
Curtailment
(b)
to meet the load during the evening
hours, resulting in unserved energy.
figure 3. Prospective curtailment of renewable energy output can be used to avoid
In (b), the renewable generation
firm load curtailment when upward ramping capability is limited. (a) The strategy to
is curtailed during the afternoon,
minimize downward violations and (b) the strategy to minimize upward violations.
44	

ieee power & energy magazine	

july/august 2015
Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - July/August 2015
