Managing an evolving grid

The California ISO is a leader in the global movement to decarbonize the electricity sector, and is already managing growing amounts of renewable energy into its resource mix in support of state climate action goals. While the transition to cleaner energy is essential to California’s economy and environment, the variability of renewable energy is driving changes in how grids operate, and the laws and policies supporting the grid of the future.

With the rapid shift to clean energy sources, the ISO is identifying emerging operational trends and exploring new policies to reliably run the grid while best leveraging renewable resource investments.

Increased ramping needs
Reduced renewable energy production during multi-day weather events
Capacity shortfall in 2020 and beyond
Oversupply and curtailment

Increased ramping needs

Currently, quick increases in energy demand, called “ramps,” are being met primarily by natural gas resources and energy imported from outside the ISO balancing area.

(chart – see mock up)

Relying on natural gas, however, is counter to state legislation calling for low-carbon power grids. And imports are uncertain, especially as other states adopt renewable energy portfolio requirements and feel the effects of extreme weather events.

During the middle of the day, California’s robust renewable resources sometimes generate more electricity than is needed. The ISO’s market automatically reduces, or curtails, renewable generation to match supply with demand. Solar curtailment is already occurring in the ISO market, at increased frequency and higher rates in spring and fall when demand falls, to smooth the ramp and take strain off the system. While curtailment is an acceptable operational tool, as increasing amounts of renewable energy come online, oversupply conditions will happen more often and in greater amounts.

By 2030, solar is expected to contribute to increasing ramping needs

(chart – see mock up; note for SG: need to identify lines as net load)

The ISO is supporting actions to address ramp and curtailment issues:

Expanding western markets for solar energy to be distributed across a larger area
Promoting inverter-based technology that promises to make renewable power plants more commercially useful and competitive
Increasing visibility and control of “behind-the-meter” commercial and residential solar panels
Dynamic pricing to shift load to times of high solar production
Exploring a diverse mix of renewable resources to match output with system needs, including offshore wind development
Advocating for regional collaboration to enhance flexibility and diversity of geography and resources

The ISO also supports longer-term efforts aimed at shaving the ramp, including:

Advancing long and short-duration storage technology
Adoption of time-of-use rates
Electric vehicle integration

Reduced renewable energy generation during multi-day weather events

Consecutive days of cloud cover or low winds cause sustained reductions in solar and wind generation. During these weather events, storage resources with shorter durations will not be able to recharge.

In those cases, the ISO grid will need to rely heavily on natural gas and imports to close the resource gap. To reduce reliance and maximize renewable energy use, the ISO is exploring the following strategies:

Diversify resource mix both technologically and geographically
Use resources smartly to support the grid during consecutive-day weather events
Consider multiple-day low production events in resource planning studies

(chart – see mock up)

Capacity shortfall in 2020 and beyond

As the electricity system transitions to more renewable resources, studies by the California Public Utilities Commission (CPUC) and the California ISO show that power supplies could fall short of needs in the early evening, when solar production ends and demand remains high. The gap could cause reliability issues for the electrical grid, and natural gas are currently used to satisfy the steep evening ramp.

The ISO’s analysis projects the following energy shortages:

2,300 MW in 2020
4,400 MW in 2021
4,700 MW in 2022

However, California’s import energy supply is not under contract, and scheduled retirement of natural gas plants could leave the state without adequate electricity in peak hours.

The ISO recommends the following actions:

Increase resource adequacy contracting from operational and mothballed resources

Secure available import capacity

Extend once-through cooling compliance date on critical units until CPUC identifies alternatives

Natural gas and imports support high demand after sun sets

On July 25, 2018 the peak demand of 46,424 MW occurred just before 5:30 p.m. but by 8:00 p.m. solar resources had faded and the system still needed to serve 44,000 MW of demand. While there are slight increases in wind, the bulk of the evening peak demand is met with imported energy and energy from natural gas generation.

Resource shortage starting in 2020

Solar production increases from new resources coming online while natural gas reduces due to retirements of once-through cooling (OTC) power plants. In response, the grid must rely more and more on imported energy, much of which is not under a resource adequacy contract.

Curtailment

Sometimes, during the middle of the day, California's renewable resources can generate more electricity than is needed, and the ISO's market automatically curtails the production of energy from renewable resources to maintain the balance between supply and demand.

As increasing amounts of renewable resources are added to the grid, oversupply conditions are expected to occur more often. The ISO is seeking solutions to avoid curtailment of renewable power and maximize the use of clean energy sources.

(chart – wind and solar curtailment totals by month; pull from managing oversupply page)