The California ISO on May 22 unveiled a new initiative that would expand the pool of power plants it can call on in the real-time market to meet the state's peak demand in the evening, when power use is ramping up but solar output is decreasing.
And after forecasting tight supply conditions for the coming summer, the grid operator is hoping to implement the changes as soon as possible. "To better prepare for the upcoming summer and winter months, CAISO's market and operators will need the ability to view daily load peaks and economically optimize resources to meet system needs," according to a proposal discussed on a web conference May 22.
In particular, the plan would extend the timeframe for the real-time market's short-term unit commitment, or STUC, process from 4.5 hours to 18 hours, so the grid operator can weigh both the morning and evening peaks and bring more long-start generators online if needed.
Proposal gives plants more notice, optimizes resources
The proposal would also give power plants more notice that they will run and better match resources to the time of day they are needed. "The purpose of the STUC modifications is to provide earlier notification to resources that are needed to meet the evening peak, which increases the probability these resources will be available, and better optimize the use of resources with limited starts over the entire day," the proposal said.
CAISO's STUC process currently looks ahead 4.5 hours to determine whether the grid operator needs to call on short-start generators, which can start up in less than two hours, or medium-start generators, which can start up in less than five hours.
But there are some problems with using such a short time horizon because the STUC might commit a resource for the morning peak when it should have been saved for the evening peak, the proposal said. Or the STUC might decommit a unit in the middle of the day for economic reasons when it should have kept it online for the evening peak.
"Both of these scenarios may result in the real-time market committing more expensive units with shorter start-up times and more flexible ramp rates," the proposal said. "These considerations are not considered under the existing 4.5-hour STUC horizon because STUC cannot see far enough in the future to make informed decisions."
Since the STUC process can only consider short- and medium-start units, there is a limited pool of power plants to use in the real-time market, the proposal said. This means that if the day-ahead market has not procured enough energy to meet the evening peak and there is not enough capacity from short- and medium-start resources, the only option the grid operator has is to commit a long-start resource through an exceptional dispatch, the proposal said.
The proposal will allow the grid operator to look at the entire so-called duck curve when making short-term unit commitments, Megan Poage, a CAISO market design policy official, said on the web conference. "But again, the neck of the duck is the driver," she said.
Poage was referring to a duck-shaped chart of net load in California over the course of the day. The belly of the duck represents the middle of the day when solar supply is highest and net load is lowest. The neck of the duck represents the evening ramp when solar output is dropping and consumers are returning home and increasing energy use.
Plan allows CAISO to call on more long-start plants
Under the proposal, CAISO would extend the STUC horizon to 18 hours so the real-time market can commit long-start power plants. The proposal would preserve the day-ahead commitments for long-start units but also allow the grid operator to add incremental real-time commitments for these plants if needed, it said.
For example, a gas plant with a day-ahead award can go ahead and procure gas knowing its commitment will not change, Poage said. And if a gas plant did not get a day-ahead award, she added, it can bid into the real-time market if it can get gas supplies.
The proposal will help the grid operator prevent or reduce uneconomic cycling, respond faster and more efficiently to demand changes, and reduce reliance on short-start peakers whose daily number of startups might be exhausted before the evening peak, it said.
As part of the plan, CAISO is also planning to modify certain bid cost recovery rules that allocate start-up costs to either the day-ahead or real-time market. Also, the grid operator would require energy imbalance market participants to submit base schedules for 20 hours, up from the current six-hour time frame.
Comments on the straw proposal are due May 29. CAISO is hoping to bring the plan before its board of governors in July. The grid operator would also need to submit the proposal to the Federal Energy Regulatory Commission for approval.
"By expediting this stakeholder process and implementing changes with the fall 2018 release we will be better prepared for summer 2019," CAISO spokeswoman Anne Gonzalez said. "If we did not expedite this stakeholder process, changes would not be implemented until fall of 2019."
Kate Winston is a reporter for S&P Global Platts which, like S&P Global Market Intelligence, is owned by S&P Global Inc.
