IEEE Power & Energy Magazine - July/August 2015 - 45

Wind and solar are the primary
renewable resources available for future development
in California.

allowing additional conventional generation to stay online
and reducing the required ramp to the available capability.
As a result, firm load curtailment is avoided.
Note that the renewable curtailment must be done prospectively. In this example, the renewables must be curtailed from 13:00 to 17:00 to avoid loss of load at 20:00.
This type of curtailment is different in character and in
implementation from curtailment driven simply by oversupply conditions.
The presence of a dispatch flexibility constraint on this
day means that the system operator must choose between
curtailing firm load in the evening or curtailing renewable
generation during the daytime. In such a situation, the system operator must consider the consequences of load and
renewable curtailment.
✔✔ Load curtailment: Studies have placed the economic value of lost load at between US$5,000 and
US$50,000/MWh.
✔✔ Renewable curtailment: Curtailed output must be
replaced with additional renewable energy genera-
tion to ensure compliance with the target. The value
of the curtailed renewable output might range from
US$50 to US$250/MWh, depending on the cost of replacement resources.
Given this vast disparity in value, renewable curtailment can be thought of as a "default" renewable integration
strategy in the presence of dispatch flexibility constraints.
Renewable integration "solutions" can then be assessed for
their effectiveness at reducing curtailment by increasing dispatch flexibility. This is the approach taken in this study.

Results
REFLEX model runs were conducted for four scenarios:
1) the 33% scenario, 2) the 40% scenario, 3) the 50% large
solar scenario, and 4) the 50% diverse scenario, as well as
for variations of the 50% large solar scenario that include the
implementation of several potential renewable integration
solutions. Due to time and resource constraints, the study did
not attempt to find an optimal generation mix or set of renewable integration solutions under the 50% renewable scenarios. Rather, it explored the operational challenges of a 50%
renewable grid, providing directional information about the
potential benefits and cost savings of integration solutions.
The largest integration challenge is overgeneration.
Overgeneration occurs when must-run generation-nondispatchable renewables, combined heat and power, nuclear
july/august 2015

generation, run-of-river hydro, and thermal generation
that is needed for grid stability-is greater than loads plus
exports. The study finds that overgeneration is pervasive at
penetration levels above 33%, particularly when the renewable portfolio is dominated by solar resources. This occurs
even after thermal generation is reduced to the minimum
levels necessary to maintain reliable operations.
Figure 4 shows an April day in 2030 under the 33, 40, and
50% large solar scenarios on which the system experiences
both low load conditions and high solar output. A very small
amount of overgeneration is observed at 33% penetration.
The 40% scenario experiences over 5,000 MW of overgeneration, while the 50% large solar scenario experiences over
20,000 MW of overgeneration.
Table 1 shows overgeneration statistics for the 33, 40,
and 50% large solar scenarios. In the 33% scenario, overgeneration occurs during 1.6% of all hours, amounting to
0.2% of available renewable energy. In the 50% large solar
case, overgeneration must be mitigated in 23% of hours,
amounting to 9% of available renewable energy, and reaches
25,000 MW in the highest hour. Potential solutions must
therefore be available during large portions of the year and
comprise a large total capacity.
To ensure reliable operations, REFLEX utilizes "prospective" curtailment, in which the system operator looks
ahead one or more hours, subject to uncertainty and
table 1. 2030 overgeneration statistics for
the 33, 40, and 50% large solar scenarios.
33%
RPS

40%
RPS

50% RPS
Large
Solar

GWh/year

190

2,000

12,000

Percentage of available
RPS energy

0.2%

1.8%

8.9%

Hours per year

140

750

2,000

Percent of hours

1.6%

8.6%

23%

99th percentile (MW)

610

5,600

15,000

Maximum observed (MW)

6,300

14,000

25,000

Overgeneration Statistics
Total overgeneration

Overgeneration frequency

Extreme overgeneration
events

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