IEEE Power & Energy Magazine - March/April 2021 - 22

Various types of reserves defined and procured as ancillary
services are deployed when needed to maintain frequency
close to the nominal frequency (60 Hz).
the upcoming day are co-optimized. After the day-ahead
market is cleared, day-ahead reliability unit commitment is
carried out to ensure that sufficient generation is committed to serving the next day's load forecast while satisfying
reliability criteria. Day-ahead reliability unit commitment is
reevaluated every hour based on updated system conditions,
load, and renewable resource power production forecasts.
This is called hourly reliability unit commitment.
Security-constrained economic dispatch is executed every
5 min in the operating hour, and dispatch instructions (dispatch basepoint), effective immediately, are sent to all of
the resources. The resources adjust their output to attain the
basepoint within 5 min. Being able to adjust the system every
5 min in real-time operation significantly reduces the need
for system reserves to balance the uncertainty and variability
of renewable generation and load.

Ancillary Services
Various types of reserves defined and procured as ancillary services are deployed when needed to maintain frequency close
to the nominal frequency (60 Hz). Annually, ERCOT reviews
and revises the methodology for determining ancillary service
requirements. In recent years, ERCOT observed declining system inertia due to the decreasing number of online synchronous

Day 0

generators. Declining system inertia leads to an increasing rate
of change of frequency during events and an increasing need
for faster frequency response and additional ramping to maintain adequate frequency performance. To address this issue,
two new ancillary services were introduced: the Fast Frequency
Response (FFR) Service and ERCOT Contingency Reserve
Service (ECRS), implemented in March 2020 and June 2024,
respectively. A summary of reserves expected to be procured
and maintained through the ERCOT ancillary services, including two new ancillary service products, is provided next.
✔ Responsive reserve service (RRS): This is the frequency containment reserve deployed during low-frequency
events. The needed RRS amount is determined based on
the largest generation contingency (simultaneous loss of
two largest generation units, ~2,805 MW) and anticipated
system inertia conditions. RRS can be provided by generation resources through traditional governor response, also
called primary frequency response (PFR), and by load resources with under-frequency relays. The recently implemented FFR product is also procured as a part of the RRS.
Studies found that, during low-inertia conditions, faster
responding resources, such as load and FFR resources,
are more effective than the PFR from the generators. An
equivalency ratio was introduced, showing how many

Day 1

The hourly reliability unit commitment runs
every hour of the day.

DayDay-Ahead
Ahead
Reliability
Market
Unit
for
Commitment
Day 1
for Day 1
The day-ahead
reliability unit
commitment
solves for the
period from
00:00 to
23:00 for the
next day.

The hourly reliability unit commitment runs
every hour of the day.

DayAhead
Market
for the
Next
Day

Day-Ahead
Reliability
Unit
Commitment
for the
Next Day

figure 5. The ERCOT market processes.
22

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march/april 2021

IEEE Power & Energy Magazine - March/April 2021

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