  As renewable energy penetration increases and system inertia levels drop,
primary frequency control is becoming a critical concern in relatively small
interconnections such as the Electric Reliability Council of Texas (ERCOT). To
address this problem ERCOT is implementing a number of market rule changes
including the introduction of a new Fast Frequency Response (FFR) reserve type
to the electricity market. This FFR reserve type aims to help the traditional
Primary Frequency Response (PFR) reserve type in arresting frequency decline in
the event of a large generator outage. This paper derives reserve requirements
to ensure sufficient reserve to arrest frequency decline before reaching the
critical frequency threshold while coupling PFR reserve, FFR reserve, and
system inertia. The general reserve requirement places limits on the amount of
PFR reserve that can be provided by each unit based on its ramping
capabilities. Two such limits are derived from first principles and another is
proposed that is capable of accommodating the equivalency ratio introduced in
previous work. These PFR reserve limits also provide first principles insight
into equivalency ratios, which have only been studied empirically in the past.
High-level insights are provided on a large Texas test case.
