Fast Frequency Response Concepts and Bulk Power System Reliability Needs

The whitepaper, Fast Frequency Response Concepts and Bulk Power System Reliability Needs, discusses factors determining rate of change of frequency, fast frequency response concepts, and critical inertia.

Topics Covered

The whitepaper covers topics such as:

• Fast frequency response
• Frequency control
• Frequency deviation
• Bulk power system reliability
• Inverter-based resources
• Rate of change of frequency
• Essential reliability service.

Key Takeaways

The authors of this whitepaper begin by noting that bulk power system synchronous inertia will decrease as we transition toward higher levels of inverter-based resources. The lower level of inertia means that interconnections will experience a higher initial rate of change of frequency (ROCOF) in an unplanned event. This increases the risk that if there are large unplanned losses of generation, the frequency will quickly go low enough to reach the settings for underfrequency load shedding before appropriate recovery methods are applied. One of these methods is faster active power injection, which can dampen the increasing ROCOF. This quick-acting frequency response is called fast frequency response, which NERC considers an essential reliability service for the reliability of the bulk power system.

Fundamentals of Frequency Response

The paper then discusses fundamentals of frequency response and presents an illustration to describe a frequency excursion and the types of frequency response needed to control the frequency both during the excursion and while the frequency is brought back to stability.

Chapter 1: Factors Determining Rate of Change of Frequency

Chapter one provides a definition of the rate of change of frequency and discusses a common way of calculating it. The whitepaper discusses the main factors that affect the initial ROCOF and recommends that for planning purposes, the calculation for the most severe single contingency per NERC Reliability Standard BAL-003 should be reevaluated for interconnections with high levels of distributed energy resources, for which it may need to be updated.  

Chapter 2: Fast Frequency Response Concepts

Chapter two gives a definition for fast frequency response and explains various methods and controls used to manipulate active power injection or reduction in load to achieve the desired effect. The whitepaper provides simulation results to illustrate the concepts presented. 

Chapter 3: Concept of Critical Inertia

Chapter three defines critical inertia and uses simulation results from studies to describe critical inertia and to illustrate the relationship between frequency response, critical inertia, and ROCOF.

Appendices

Appendix A lists NERC, industry and U.S. Department of Energy national laboratory studies and relevant literature.

Appendix B gives fast frequency response capabilities for specific technologies including synchronous machines, wind turbines, various types of photovoltaic inverters, battery energy storage systems, static synchronous compensators, and controllable load resources.

Appendix C provides example calculations of ROCOF in the Texas interconnection and the  Western Electricity Coordinating Council interconnection.