IEEE Power & Energy Magazine - March/April 2021 - 34

clearing process, resources are required to increase or
decrease generation. This produces the right market signals through prices that incentivize resources to follow
CAISO instructions. Resources that exacerbate congestion on a constraint are exposed to a negative marginal
congestion component that reduces the overall locational
marginal price of the resource.
This price is consistent with the need for the resource
to decrease production. The congestion-management protocol applies regardless of the type of generation technology or fuel type and is based on both economical bids and
resources' effectiveness in relieving congestion. This, in
turn, allows the reliable operation of the system while meeting supply with demand and is possible because the market mechanism has the information to enforce transmission
limits. This minimizes the risks of violating or overloading
transmission constraints.
Even when conditions change, such as when there are
transmission outages or a reduction of capacity (which
result in the need for a different operation of resources),
the market will redispatch resources to comply with the
new and reduced operating limit. When certain operational conditions are not visible to the market mechanism,
there can be a disconnect between the market instructions
and the actual operating condition of the system, which,
if not addressed, can lead to or exacerbate a reliability
condition. For this reason, system operators rely on a
variety of tools and visualizations to know actual system
conditions and adjust as needed. When a condition in the
system cannot be addressed through the market, operators
have tools to adjust for increases or decreases in generation throughout the system. It is less efficient, but it is a
mechanism of last resort.
When Aliso Canyon restrictions took effect, they effectively limited the amount of gas supply available to electric generators. On one hand, if the electric system has
no visibility on these supply restrictions, there are risks
of issuing operating instructions that cannot be followed,
which could lead to an electric reliability condition. On the
other hand, if these restrictions are not followed, it could
jeopardize the gas restriction and potentially lead to a gas
reliability condition. In the absence of an explicit constraint in the electric market to limit electric generation to
support limited gas supply, CAISO grid operators would
need to resort to manual dispatches on potentially all generators that receive service from SoCalGas to reduce their
production and be within the gas limit without regards
to the impact on the electric operation or transmission
constraints. This imposed additional costs on the system
and was not as efficient as an explicit market mechanism.
Furthermore, CAISO seeks to utilize a market constraint
to manage the gas constraint instead of using manual
dispatches to alleviate an untenable situation for electric operators. Without an explicit constraint modeled
in the market that could drive for an optimal dispatch of
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resources, operators will need to manually manage the
constraint on a 5-min basis, requiring the adjustment of
potentially dozens of generators.
Being able to enforce a gas constraint provides CAISO
with an effective tool to accomplish the necessary redispatch
to limit the gas usage on the electric system when it needs
to avoid further stressing the already-constrained gas system
while continuing to reliably and efficiently serve the load.
In 2016, CAISO started using a maximum gas constraint
in both day-ahead and real-time electric markets. This is
a maximum gas-burn constraint that coordinates the gas
limitation with the electric dispatches for generators that
rely on such a limited gas supply. This gas constraint is
analogous to a transmission constraint. It has an upper limit
that defines the maximum gas-burn enforcement. The constraint has a predefined set of resources subject to the gas
limitation. This nomogram limits the dispatch of generators
in the affected areas to comply with maximum gas usage.
The constraints also limit the CAISO market dispatch of the
affected generators in the real-time market to a maximum
gas usage if there is a limitation that relates to differences
between the gas scheduled with the gas company and the
gas consumed during the operating day due to gas system
imbalance limitations.
The gas constraint definition accounts for the different
generators' efficiencies through the heat rates. This allows
the electric system operator to attain more efficient dispatches by first reducing the least-efficient generators in terms of gas usage. This is information that the
market already uses in the determination of generators'
energy costs.
Analogous to a transmission constraint, the enforcement
of the gas limitation is equivalent to congestion management.
The constraint is priced in the electric market and results
in price signals to generators that are part of the restriction
definition. These price signals are consistent and incentivize
generators' decremental dispatches.
When the market enforces a gas constraint concurrent
with the enforcement of all transmission constraints, a natural interaction between electric and gas restrictions can
occur. Resources that are part of the gas limitation can also
be effective for transmission congestion management. In
some instances, the management of transmission constraints
will require certain resources to increase production while,
at the same time, they may be part of the gas constraint
that requires limiting or decreasing production. Under such
competing generation movements, the market will efficiently determine the most optimal dispatch that complies
with both constraint limitations. This also has a secondary
effect that, to comply with the gas limitation, congestion
on the electric transmission system can sometimes increase.
In some cases, this creates power flows from Northern to
Southern California, which can congest certain transmission
elements in between and result in congestion that otherwise
would not occur.
march/april 2021

IEEE Power & Energy Magazine - March/April 2021

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