IEEE Power & Energy Magazine - March/April 2021 - 35

Effectively, resources inside the region of the gas constraint will decrease production, which is the intended
outcome to impose a gas limitation. Another effect of the
gas constraint is that it will naturally limit the systemsupply availability and cause higher energy prices in the
overall system. This has the effect of requiring generation
from the rest of the system to increase production. All
of these effects are a result of an efficient dispatch that
properly prices the additional limitations imposed in the
electric market. The maximum gas constraint produces
efficient prices for affected generators because if the
maximum gas constraint is binding, the price decreases
for these resources. This pricing logic is the same for how
a generator behind a transmission constraint is priced
lower to reflect the congestion cost associated with dispatching that supply.
Under certain conditions, it may not be feasible to comply with the enforced limitation on a given constraint in
CAISO's market. Generally, infeasibility occurs when there
is an insufficient supply or short-term ramping capability
to be utilized in the management of the constraint. For
such cases, CAISO provisions allow for constraints to be
relaxed up to the point needed. This infeasibility is short
term in nature and set at administrative prices defined above
any economical bid value.
With the participation of virtual bids in the day-ahead
market, the current rules utilize prices with the gas-constraint effect to settle only physical gas-fired generators that
are within the gas constraint. Virtual bids or congestion revenue rights do not settle on those prices to eliminate the possibility of any adverse impact. For instance, a virtual trader
could take a position with a location exposed to the gas-constraint price and an offset position at another location unaffected by the gas constraint to profit from the gas-constraint
binding between the day-ahead and real-time markets.

Use of Gas Constraints

consistent with the limitations observed on the gas system.
The general process to apply a gas constraint in the electric
market includes the following steps:
1) In coordination with the affected gas company, CAISO estimates and sets hourly limits to the gas burn by generators in the impacted region. In some cases, the limitation is set as a fixed hourly limit. In others, it can be set
as a daily quota. In this latter instance, CAISO creates
an hourly profile that meets the daily limit with a shape
based on gross demand patterns, enforcing lower limits
in the middle of the day, when the load is low, and imposing the highest limits for peak hours, when the load
is highest. CAISO is currently expanding the options
to shape the hourly limitations based on a historical gas
burn or using net load patterns. Figure 5 illustrates the
projected gas burn in May 2020 based on CAISO's energy schedules from the day-ahead process.
2) CAISO verifies with the impacted gas company
whether these hourly limits address the gas limitation
and do not further worsen the gas system constraint.
3) Based on the specifics of the condition, CAISO may
enforce the gas constraints in both the day-ahead and
the real-time markets as needed to ensure that the dispatches and commitments from the CAISO market
do not exceed the maximum gas burn in the specified
region. Since conditions can change rapidly, if CAISO
becomes aware of the need for a gas constraint after
the day-ahead market run, it may enforce the constraint in only the real-time market run since it can
still help manage and coordinate generation with gas
conditions in the impacted area. Depending on the
commitments derived from the day-ahead market,
when gas constraints are enforced only in real time,
the market may have reduced flexibility to fully comply with the gas limitation, such as the case where all
units in the impacted region are already at their minimum operating points, and temporal constraint may
prevent turning them offline.

Gas Burn (MMSCFH)

CAISO's markets have the functionality to enforce gas constraints. CAISO enforces gas constraints in the day-ahead market,
the real-time market, or both to
14
limit the gas burn in specific arCoastal
East of Moreno Valley
Inland
eas previously defined by CAISO
12
Los Angeles Basin
San Diego Area
as deemed necessa r y. T hese
10
constraints come in the form of
potential risks of curtailments or
8
as requests for conservation to
6
noncore customers from the gas
company. CAISO enforces gas
4
constraints only after a full as2
sessment of gas and electric conditions and in coordination with
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
the impacted gas company. This
ensures that limitations imposed
by the constraint in the market are figure 5. The gas usage estimated with the day-ahead solution.
march/april 2021

ieee power & energy magazine

IEEE Power & Energy Magazine - March/April 2021

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2021