IEEE Power & Energy Magazine - November/December 2015 - 86

Additional Sources of Flexibility
Although wind and solar generators can provide fast and
accurate control, integrating additional wind and solar

60.10

operator's suite of flexible resources. These sources can
allow decommitting conventional generation and increasing the amount of renewable energy that the power system is able to reliably and economically integrate. Existing
resources often can be operated in a flexible manner if there
are incentives and if the institutional structure does not prevent it. Two groups of relatively new emerging sources are
demand response and other sources of flexibility.
Demand Response
Demand response can allow conventional generation that

Bus Frequency (Hz)

generation is often aided by broadening the power system

60.00
59.90
59.80
59.70
59.60
59.50
59.40
0.00

5.00

10.00 15.00
Time (s)

20.00

25.00

1 MW FFR ≈ 2.35 MW PFR

would otherwise be kept online to be decommitted to provide fast reserves. Historically, a utility would ensure that
adequate unloaded conventional generation was online and
had active governors to ensure that power system frequency
could recover from the worst credible contingency without
triggering any underfrequency load shedding. For example,

Load = 25 GW, Wind = 7.2 GW
Disconnect Two Nuclear Units (2,750 MW Total)
4,700 MW of PFR from
Conventional Synchronous Generators
1,400 MW of FFR from Load Replaces
3,300 MW of PFR from Generation

it could be challenging for ERCOT to keep the frequency
nadir above 59.4 Hz immediately after a major contingency
if light load and abundant wind generation had displaced
much of the conventional generation. Rather than relying
exclusively on generator response, ERCOT has found that
FFR from load (FFR that responds in fewer than 30 cycles
once system frequency reaches 59.7 Hz) provides at least

figure S2. An example of FFR to the loss of two
nuclear units (2,750 MW total) from load (black) compared to PFR from conventional generators (blue) in
ERCOT. FFR responds in fewer than 30 cycles. Primary
frequency response is the governor response, typically
within 10 s.

the same reliability benefit as conventional generation providing primary frequency response (PFR). For example, at

Other Sources of Flexibility

low-load, high-wind conditions, as shown in Figure S2, it

Improvements in conventional generation flexibility also

was found that 1,400 MW of FFR provides the same reliabil-

help with effective renewable integration. Fast-start recip-

ity benefit as 3,300 MW of conventional generation provid-

rocating engines and combustion turbines that have lower

ing PFR. This shows that FFR from load can be 2.35 times as

minimum loads and higher efficiencies and can be started

effective as the frequency response from conventional gen-

within minutes without incurring start-up costs can help

eration. In addition, decommitting conventional generation

provide ramping and nonspinning reserves. Shortening the

operating at low load simply to provide PFR also reduces

start-up time (and lowering the start-up cost) lets the sys-

wind and solar curtailment, which reduces emissions and

tem operator delay response until wind and solar conditions

saves money. Carrying the same reserves with wind or solar

are more certain, which improves forecast accuracy and

would require spilling zero marginal cost energy to create

reduces the requirement for online reserves. Retrofitting

the response headroom. Although renewables could pro-

existing conventional generators to reduce start-up times,

vide the reliability response, obtaining the response from

minimum loads, and cycling costs provides similar benefits.

the load allows the renewables to supply more energy, and

Storage provides similar technical benefits and should be

ERCOT has found this to be more cost-effective.

used whenever it is the least-cost resource.

explored in more detail in the accompanying article in this
issue by Ahlstrom et al.
It is also important to note the rapid spread of distributed generation. This will have a larger impact on transmission and distribution systems in which the current planning
86

ieee power & energy magazine

is still largely centralized. The challenge of the electricity
industry is twofold: first, there must be a vision of a market
structure that accommodates these changes, and second, a
transition is needed that can be done with minimal disruption, both physically and financially.
november/december 2015



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