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The California Independent System Operator is curtailing growing amounts of renewable generation, Mark Rothleder, the grid operator's vice president for market quality and integrating renewables, said.

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Speaking Wednesday during a meeting of the Western energy imbalance market board, Rothleder said the ISO curtailed about 60,000 MWh in February and about 80,000 MWh in March, up from about 21,000 MWh and 47,000 MWh a year earlier.

Matching the increased curtailment volumes, the ISO experienced curtailments in 31.1% of its market intervals so far this year, up from 21.1% a year ago, 15.9% in 2015 and 9.6% in 2014, he added.

California's net load -- actual load minus wind and solar production -- and its late afternoon ramps are at levels the ISO expected to see in 2020, according to Rothleder.



When the ISO first put out its "duck curve" chart several years ago, the grid operator underestimated the amount of behind-the-meter solar that would be added in the state, he said, noting that there is about 5,000 MW of rooftop solar, with more being added every day.

On April 9, the ISO had a net load of 10,386 MW, roughly 1,700 MW less than the grid operator forecast in its duck curve chart for a typical spring day in 2020.

The ISO has roughly 10,000 MW of nuclear, hydroelectric and qualifying facilities on its system that "you can't move," Rothleder said, which leads to renewable curtailments in low load periods.

Most of the curtailments come via economic bids, but some are manual, he said.

The Western energy imbalance market has provided an opportunity for California to sell energy from renewables that would be otherwise have been curtailed, according to Rothleder. In some periods of oversupply, California exports up to 2,000 MW, he said.

The ISO has also been able to import energy via the EIM to address its ramping periods when solar production falls off in the late afternoon, Rothleder said.

However, so far this year, the amount of avoided curtailments has dropped compared with a year ago. It is unclear why this is happening, Rothleder said.

In an effort to better address oversupply conditions, the ISO about three weeks ago started issuing oversupply forecasts so that market participants can prepare ahead of time for the oversupply conditions, Rothleder said.

Meanwhile, Portland General Electric is on track to begin participating in the EIM on October 1, according to Petar Ristanovic, ISO vice president for technology and EIM executive sponsor.

PGE's participation will add about 450 MW of bi-directional transfer capacity between the utility's balancing authority area and the western part of PacifiCorp's footprint, Ristanovic said. It will also add 450 MW of transfer capacity to the north and 300 MW to the south, he said.

PGE expects joining the EIM will save it up to $4 million a year while it is expected to bring up to $3.7 million in benefits to the rest of the market.

PGE has 3,843 MW of generating resources and gets about 15% of its electricity from non-hydro renewables, according to Larry Bekkedahl, PGE vice president for transmission and distribution. Oregon has a renewable portfolio standard that ramps up to 50% by 2040.

PGE is working with smaller balancing authority areas to help them join the EIM, Bekkedahl said. "Our hope is to make it as easy as possible" for them to join, he said.

Also, the ISO is preparing to start a stakeholder process by July that aims to develop rules for non-EIM entities to donate transmission capacity for EIM transfers in exchange for congestion rents, according to Brad Cooper, ISO manager for market policy.

A proposal is expected to go to the EIM governing body and the ISO board by the end of the year, he said.

The ISO is also preparing to start a process to establish an EIM wheeling charge to address a drop in transmission revenue caused by EIM transfers across participating balancing authority areas, Cooper said. Before the process starts late this year, a working group will determine exactly how much transmission revenues have dropped, he said.

--Ethan Howland, newsdesk@spglobal.com

--Edited by Keiron Greenhalgh, keiron.greenhalgh@spglobal.com