Synchronous inertia characterizes the ability of a power system to oppose changes in electric frequency after large and sudden changes in active power production or consumption. The level of inertia present in a system at any time is dependent on the amount of kinetic energy stored in rotating masses of synchronously-interconnected machines: both generators and synchronously operating motor loads. Synchronous inertia determines the initial rate of frequency decline after a sudden loss of generation, and therefore, time available for frequency response mechanisms to deploy and arrest frequency decay above the system’s involuntary under-frequency load shed (UFLS) set points.
As penetration levels of inverter-based generation resources (e.g., wind, solar, batteries) that do not naturally contribute inertia to the system continue to increase and displace synchronous generators in a power system’s generation mix, the synchronous inertia will inevitably decline, especially during operating conditions when system load is low and wind and solar power production are high. These conditions vary depending on the season and the time of day.
In recent years, ERCOT carried out a number of dynamic studies and found that the amount of frequency containment reserve (called Responsive Reserve Service or RRS) needed to arrest the frequency above the UFLS trigger after the largest generation loss depends on system inertia conditions.
Additionally, it was determined that there is a critical inertia level below which existing frequency response mechanisms are not fast enough to arrest the frequency before it reaches UFLS after the largest generation loss, per NERC Standard BAL-003. If system inertia is expected to fall below this value, ERCOT system operators will follow procedures to start more synchronous generators in order to increase synchronous inertia online. Since these generators are not needed for energy, starting them is likely to result in power output reduction from lower cost resources, including curtailment of wind and solar generation. Currently, ERCOT’s all-time minimum inertia level is still about 30% higher than the determined critical inertia level.
With these considerations in mind, it becomes essential for ERCOT operators to monitor system inertia in real time. Additionally, it is important to forecast system inertia for the upcoming hours since it may take several hours to start additional synchronous generation if inertia levels are expected to fall below the critical level. ERCOT has developed a number of tools in the control room to monitor and forecast synchronous inertia and frequency containment reserve sufficiency.
ERCOT’s Inertia Monitoring Tool 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 the online status of the units in the Energy Management System. The tool also is capable of calculating future inertia conditions for the next 168 hours on a rolling basis. This calculation is based on the unit commitment plans (for the next 168 hours) that every generator submits to ERCOT every hour. The tool then identifies any time periods where the expected system inertia is less than the critical level.
Another situational awareness tool developed at ERCOT is the Reserve Sufficiency Tool. ERCOT publishes frequency containment reserve amounts before the start of each year for every hour of the upcoming 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, system inertia may be different from what was expected based on historic data. As discussed previously, there is a direct correlation between system inertia and the frequency containment reserve. ERCOT’s Reserve Sufficiency Tool allows operators to verify whether available frequency responsive capacity, including procured reserves, is sufficient based on actual inertia and projected conditions. The Reserve Sufficiency Tool uses inertia information from the Inertia Monitoring Tool and calculates the amount of reserves needed to protect against ERCOT’s NERC BAL-003 defined generation loss event, using empirical equations obtained from the aforementioned series of dynamic studies. The tool then compares available frequency responsive capacity with what is needed based on projected inertia conditions at the time. If available frequency responsive capacity, including reserves procured in the Day-Ahead Market is insufficient, ERCOT may open a Supplemental Ancillary Services Market and procure additional reserves as needed.
It is important to note that the tools and mitigation measures in place today are based on current frequency control mechanisms and operating practices that ERCOT system operators have at their disposal. As system conditions change in the future, so may ERCOT’s inertia management practices. For example, if a portion of frequency containment reserve is provided by faster resources responding to a higher frequency trigger compared to the present setting, then the critical inertia level can potentially be lowered. A more fundamental future change is the possibility of grid forming inverter technology to allow for very low or even zero-inertia operation. In any case, as the grid continues to incorporate more low or zero-inertia generator technologies, it is necessary to develop a clear transition path from current operating practices towards new practices that will allow ERCOT to continue to maintain reliability in the most efficient and economical manner. Additional thoughts from me and my colleagues here at ERCOT will be provided on this topic in next month’s blog.
Lead Planning Engineer