IEEE Power & Energy Magazine - March/April 2021 - 19

ERCOT's generation mix has dramatically changed in
the past 20 years-from being heavily dominated by coal
and gas-steam generation in the late 1990s to now having
a substantial share of renewable generation and simple and
combined cycle generation, as presented in Figure 2. Furthermore, nearly 6,700 MW of coal and gas-steam units
were retired in the past four years.

Operational Challenges with High
Inverter-Based Resource Penetration

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To date, the highest percentage of load served by wind generation in ERCOT is 59.3% at 2:10 a.m. on 2 May 2020,
with a total of 19,426 MW served by wind generation.
Most wind and solar generation projects in ERCOT are
interconnected at the transmission level (i.e., 69 kV, 138 kV,
or 345 kV). The best wind resource potential within the
ERCOT region is in the Panhandle, which consists of the

northernmost 26 counties, and West Texas as well as the
area along the Gulf Coast (Figure 3). These areas have
an average wind generation capacity factor of about 43%
(with consideration of curtailments due to transmission constraints).
However, while the Gulf Coast area is generally more attractive due to higher energy prices and favorable wind generation
patterns better correlated with ERCOT load peak (Figure 4),
more wind generation has been built in West Texas and the Panhandle area due to land availability and a higher annual energy
yield. Similar considerations apply to solar projects as well. It
should be noted that the renewable resources in the ERCOT
region are generally far from the load centers where most
conventional synchronous generation is also concentrated.
This results in high power transfers over a long transmission
path to deliver renewable power to the load centers (Figure 3).
In this environment, to ensure the reliable operation of the
ERCOT grid, adequate voltage
support must be maintained
and associated voltage stability
must be assessed.
Wind and solar resources are
connected to the grid through
inverters. These renewable inverter-based resources (IBRs)
have a near-zero marginal cost
and, thus, displace more expensive conventional synchronous
generation in unit commitment
and dispatch. As a consequence,
the decline in system inertia and
the corresponding increase in
the rate of change of frequency
during events must also be assessed. Sufficient system reserves and flexibility are needed
to manage variability and uncertainty of renewable generation
during real-time grid operations.
To address these challenges and
efficiently and reliably adapt to
the ever-changing generation
mix, ERCOT relies on its energy market, ancillary services,
wind and solar forecasting, security assessment studies, and
tools in real-time operation.

march/april 2021

Overview of ERCOT
Market Processes
ERCOT is the sole energy-only market in the United States,
i.e., there is no capacity market. Energy is co-optimized
with ancillary services in the
ieee power & energy magazine

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IEEE Power & Energy Magazine - March/April 2021

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2021