IEEE Power & Energy Magazine - March/April 2021 - 30

This technology needs to provide an equally flexible upward
capacity to match the ramping capability of variable
resources. Figure 3 presents how the 3-h evening ramp has
increased year after year. This ramp exceeded 15,000 MW
in 2019. A large portion of this ramp is currently absorbed
by gas-fired generation. When there is a limitation on gas
availability, the risk of not being able to support this ramp
increases and poses a reliability concern.
Every year, CAISO conducts its summer loads and
resources assessment to study the upcoming summer supply-and-demand outlook for the CAISO balancing authority
area and identify issues that could lead to potentially challenging operating conditions. This assessment uses a variety
of inputs from external and internal sources as well as stochastic production-simulation models and more than 2,000
scenarios of forecasted load and renewable generation with
different weather conditions.
In recent years, this assessment has included the potential impact of operating restrictions on the Aliso Canyon gas
storage facility. As explained in greater detail in subsequent
sections, a leak in this gas facility had profound implications
on the supply availability of gasfired resources in the southern part
of the CAISO system.
For the load, this study considers a
Market Operator
distribution
of load patterns that include
Puget Sound
CAISO
Powerex
Energy
the
range
of
one-in-two, one-in-five, and
EIM Entity
one-in-10-year
peak-demand forecasts.
Seattle
Active Participant
City Light
The forecast accounts for load reducPlanned EIM Entry 2021
Tacoma
tions from behind-the-meter solar proAvista
Power
duction and energy efficiency programs.
Planned EIM Entry 2022
NorthWestern
Portland
For generation, it considers the expected
Energy
General
Bonneville
generation from hydro resources based
Electric
Power
on expected hydro conditions and
Administration Idaho
the reliance on imported resources.
PacifiCorp
Power
The availability of imported energy is
uncertain but a function of contracted
NV
PacifiCorp
resource-adequacy capacity, the load
BANC
Energy
in external areas demand, transmisTurlock
sion and resource outages, and other
Irrigation
fuel supply conditions. It also considDistrict
ers the variation in the contribution
Xcel Energy-Colorado
California
ISO
of renewable resources, retirements or
mothballing of existing units, and new
Arizona Public
generation reaching commercial operPublic Service
Service
Los Angeles
Company
of
ation. This effort assesses CAISO's
Department of
New Mexico
Water and
resource adequacy for capacity, ancilPower
lary services and flexible capacity, and
Salt River
the potential risk for load interruption.
Project Tucson
In preparation for the summer, CAISO
Electric
Power
releases its assessments to start coordinating with other agencies involved
in the sector, including the Western
Electricity Coordinating Council,
figure 1. The CAISO and EIM footprint. BANC: Balancing Authority of Northern
transmission operators, neighboring
California.

environmental regulations and air permits, but they generally
had reliable and predictable fuel sources that allowed them
to produce energy where and when the system needed. With
these characteristics, the resource-adequacy program's focus
was on the overall system-capacity need, which required jurisdictional load-serving entities to secure the capacity to meet
up to 115% of their expected peak load needs.
Over the years and with the transition to achieving environmental goals, the generation fleet that CAISO uses to
operate the grid has become more diverse and variable. This
has required CAISO to refine certain aspects of its resourceadequacy tariff provisions to ensure it has the flexible capacity needed to operate the grid reliably. The flexible resourceadequacy requirement secures capacity with the needed
flexibility to sustain the ramping requirements arising during
the evening peak when solar production is decreasing.
Ramping needs have been growing rapidly in the last
years as more solar and variable generation come online
in the CAISO system. When aggregated variable-source
generations decrease production, other types of resource
technologies will have to ramp up to offset the reduction.

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IEEE Power & Energy Magazine - March/April 2021

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