IEEE Power & Energy Magazine - March/April 2021 - 26

ERCOT identified the need for a reactive power control tool in
operations to improve the control and coordination of various
reactive devices.
managed through generic transmission constraints
enforced in real-time security-constrained economic dispatch.
✔ Real-time transient security assessment (currently
under implementation): Most dynamic stability-related generic transmission constraints are based on
offline studies that generally include conservative assumptions to account for various potential real-time
system conditions. ERCOT is planning to implement
the transient security assessment in real-time operation to determine the limits based on the most recent
operating conditions.

Situational Awareness Tools
✔ Inertia monitoring tool: Since IBRs do not naturally

contribute to system inertia as the number of these resources continues to increase and displace synchronous generators in a power system's generation mix,
the synchronous inertia will inevitably decline. ERCOT identified a critical inertia level that needs to be
maintained at all times to minimize the risk of underfrequency load shedding after a large generation trip.
In 2015, ERCOT implemented the real-time inertia
monitoring tool. It continuously calculates the current
total inertia contribution of all online synchronous
generators based on the inertia parameters of individual units in the network model and on the online
status of the units in the energy management system.
The tool also calculates future inertia conditions for
the next 168 h on a rolling basis. This calculation is
based on the generation unit commitment plans that
every generator submits to ERCOT every hour (spanning the next 168 h). The tool then identifies any periods where the expected system inertia is less than the
critical level, in which case ERCOT system operators
will follow procedures to deploy nonspinning reserve
or start other synchronous generators to increase synchronous inertia online.
As of now, ERCOT's all-time-minimum inertia
level is still about 30% higher than the determined
critical inertia level. This means that ERCOT has
not yet had to deploy any of the above measures to
increase inertia.
✔ Reserve adequacy tool: Another situational awareness tool developed at ERCOT is the reserve adequacy tool. ERCOT publishes RRS amounts before
the start of each year for every hour of the upcoming
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year. These requirements are based on historic inertia
conditions from the past two years and vary by month
and time of day. In real time, however, the system inertia may be different from what was expected based
on historic data. The reserve adequacy tool allows
operators to verify whether the available frequencyresponsive capacity, including procured reserves, is
sufficient based on actual inertia and calculated future
inertia conditions. The tool uses information from the
inertia monitoring tool and, using empirical equations
obtained from the series of dynamic studies, calculates the amount of reserves needed to protect against
the largest generation contingency. The tool then compares all available frequency-responsive capacity with
the amount of reserves needed.
In this comparison, the tool also takes into account
the prevailing equivalency ratio between faster
responding resources and traditional PFR, based on
inertia conditions. If the available frequency-responsive capacity, including the RRS procured in the dayahead market, is insufficient, ERCOT may open a
supplemental ancillary service market and procure
additional reserves as needed.
✔ Physical responsive capability monitoring tool: This
tool is a monitoring tool used in operations as an indicator of overall system " health " in terms of available
frequency-responsive capability. The tool summarizes
all of the available frequency-responsive capacity
from online resources. If the available capability is
below a certain threshold and cannot be restored to
a higher level over a given time, ERCOT enters into
an energy emergency alert, and system operators follow procedures and steps to recover the system back to
normal conditions with sufficient physical responsive
capability available.
✔ Capacity adequacy tool: This tool was developed
at ERCOT to assess whether the generation capacity is sufficient to serve the forecasted load demand
for the next 1-24 h. It tracks and monitors online
generation capacity and reserves as well as offline
generation capacity that can potentially be started
in the future hours, up to 24 h ahead. To account
for the variability and uncertainty of intermittent
resources, the tool also allows users to evaluate the
impacts of different levels of load and wind forecast uncertainties on system capacity adequacy.
Thus, operators have a better awareness of the
march/april 2021

IEEE Power & Energy Magazine - March/April 2021

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