FAST FACTS
What the duck curve tells us about
managing a green grid
The electric grid and the requirements to manage it are changing.
Renewable resources increasingly satisfy the state’s electricity
demand. Existing and emerging technology enables consumer control of electricity consumption.
These factors lead to different operating conditions that require flexible resource capabilities toensure green grid reliability. The ISO created future scenarios of net load curves to illustrate thesechanging conditions. Net load is the difference between forecasted load and expected electricityproduction from variable generation resources. In certain times of the year, these curves produce
a “belly” appearance in the mid-afternoon that quickly ramps up to produce an “arch” similar to
the neck of a duck—hence the industry moniker of “The Duck Chart”.
Energy and environmental goals drive change
In California, energy and environmental policy initiatives are driving electric grid changes. Key initiatives
include the following:• 50 percent of retail electricity from renewable power by 2030;• greenhouse gas emissions reduction goal to 1990 levels;• regulations in the next 4-9 years requiring power plants that use coastal water for cooling to either		 repower, retrofit or retire;• policies to increase distributed generation; and• an executive order for 1.5 million zero emission vehicles by 2025.

New operating conditions emerge
The ISO performed detailed analysis for every day of the year from 2012 to 2020 to understandchanging grid conditions. The analysis shows how real-time electricity net demand changes as policy
initiatives are realized. In particular, several conditions emerge that will require specific resource
operational capabilities. The conditions include the following:• short, steep ramps – when the ISO must bring on or shut down generation resources to meet an		 increasing or decreasing electricity demand quickly, over a short period of time;• oversupply risk – when more electricity is supplied than is needed to satisfy real-time electricity		 requirements; and• decreased frequency response – when less resources are operating and available to		 automatically adjust electricity production to maintain grid reliability.
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Green grid reliability requires flexible resource capabilities
To reliably operate in these conditions, the ISO requires flexible resources defined by their operating
capabilities. These characteristics include the ability to perform the following functions:• sustain upward or downward ramp;• respond for a defined period of time;• change ramp directions quickly;• store energy or modify use;
• react quickly and meet expected operating levels;• start with short notice from a zero or low-electricity operating level;• start and stop multiple times per day; and• accurately forecast operating capability.

Reliability requires balancing supply and demand
The net load curves represent the variable portion that ISO must meet in real time. To maintain reliability
the ISO must continuously match the demand for electricity with supply on a second-by-second basis.
Historically, the ISO directed conventional, controllable power plant units to move up or down with
the instantaneous or variable demand. With the growing penetration of renewables on the grid, there
are higher levels of non-controllable, variable generation resources. Because of that, the ISO must
direct controllable resources to match both variable demand and variable supply. The net load curves
best illustrate this variability. The net load is calculated by taking the forecasted load and subtracting
the forecasted electricity production from variable generation resources, wind and solar. These curves
capture the forecast variability. The daily net load curves capture one aspect of forecasted variability.
There will also be variability intra-hour and day-to-day that must be managed. The ISO created curves
for every day of the year from 2012 to 2020 to illustrate how the net load following need varies
with changing grid conditions.
Figure 1

Ramping flexibility

stop

32,000stop

30,000

Megawatts

The ISO needs a resource mix
that can react quickly to adjustelectricity production to meetthe sharp changes in electricity
net demand. Figure 1 shows a
net load curve for the January
11 study day for years 2012
through 2020. This curveshows the megawatt (MW)amounts the ISO must follow
on the y axis over the different
hours of the day shown on thex axis. Four distinct rampperiods emerge.

Net load - January 11

34,00028,0002012(actual)26,000

2013

start

24,000

2015
2016
2017
2018
2019

22,000

2014

2020

start

20,00018,000
0

12am

3am

46am

79am

1012pm

13pm

6pm

9pm

Hour

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The first ramp of 8,000 MW in the upward direction (duck’s tail) occurs in the morning starting around4:00 a.m. as people get up and go about their daily routine. The second, in the downward direction,occurs after the sun comes up around 7:00 a.m. when on-line conventional generation is replaced bysupply from solar generation resources (producing the belly of the duck). As the sun sets starting around4:00 p.m., and solar generation ends, the ISO must dispatch resources that can meet the third and mostsignificant daily ramp (the arch of the duck’s neck). Immediately following this steep 11,000 MW ramp
up, as demand on the system deceases into the evening hours, the ISO must reduce or shut down thatgeneration to meet the final downward ramp.
Flexible resources needed
To ensure reliability under changing grid conditions, the ISO needs resources with ramping flexibility
and the ability to start and stop multiple times per day. To ensure supply and demand match at all times,
controllable resources will need the flexibility to change output levels and start and stop as dictated byreal-time grid conditions. Grid ramping conditions will vary through the year. The net load curve or duck
chart in Figure 2 illustrates the steepening ramps expected during the spring. The duck chart shows thesystem requirement to supply an additional 13,000 MW, all within approximately three hours, to replace
the electricity lost by solar power as the sun sets.
Oversupply mitigation

Figure 2: The duck curve shows steep ramping needs and overgeneration risk

Oversupply is when all anticipatedgeneration, including renewables,exceeds the real-time demand.
The potential for this increases
as more renewable energy isadded to the grid but demandfor electricity does not increase.
This is a concern because if themarket cannot automaticallymanage oversupply it can leadto overgeneration, which requiresmanual intervention of the market
to maintain reliability. During
oversupply times, wholesale prices
can be very low and even go
negative in which generators haveto pay utilities to take the energy. But
the market often remedies the oversupply situation and automatically works to restore the balance
between supply and demand. In almost all cases, oversupply is a manageable condition but it is nota sustainable condition over time — and this drives the need for proactive policies and actions toavoid the situation. The duck curve in Figure 2 shows that oversupply is expected to occur during
the middle of the day as well.
Because the ISO must continuously balance supply and demand, steps must be taken to mitigate

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3

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oversupply risk. These actions can help avoid oversupply conditions from occurring: 1) increasing
demand by expanding the ISO control area beyond California to other states so that low cost surplus
energy can serve consumers over a large geographical area; 2) increase participation in the western
Energy Imbalance Market in which real-time energy is made available in western states; 3) transition ourcars and trucks to electricity; 4) offer consumers time-of-use rates that promote using electricity during theday when there is plentiful solar energy and the potential for oversupply is higher; 5) increase energystorage; and 6) increase the flexibility of power plants to more quickly follow ISO instructions to change
its generation output levels.
Reliable grids have automated frequency response
System frequency measures the extent to which supply and demand are in balance. To ensure reliability,
system frequency must be managed in a very tight band around 60 hertz. When an unexpected event
occurs that disrupts the supply-demand balance, such as a loss of a generator or transmission line,
frequency is impacted. These events do not allow time for manual response and balance is maintained
through automated equipment. Conventional generation resources include frequency-sensing equipment,or governors, that automatically adjust electricity output within seconds in response to frequency tocorrect out-of-balance conditions.
Part of the renewable integration analysis conducted by the ISO uncovered concerns about frequency
response capabilities due to the displacement of conventional generators on the system. The 2020
33% studies show that in times of low load and high renewable generation, as much as 60% of theenergy production would come from renewable generators that displace conventional generationand frequency response capability. Under these operating conditions, the grid may not be able toprevent frequency decline following the loss of a large conventional generator or transmission asset.
This situation arises because renewable generators are not currently required to include automatedfrequency response capability and are operated at full output (they can not increase power). Without
this automated capability, the system becomes increasingly exposed to blackouts when generation ortransmission outages occur.
Policy needed for flexible resources
To reliably manage the green grid, the ISO needs flexible resources with the right operational
characteristics in the right location. The ISO is actively engaged in policy efforts to build awarenessof the new grid needs. Working with the industry and policymakers, the ISO is collaborating on rules
and new market mechanisms that support and encourage the development of flexible resources toensure a reliable future grid.

California Independent System Operator
www.caiso.com | 250 Outcropping Way, Folsom, CA 95630 | 916.351.4400

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