Frequency response approaches for power system reliability
Frequency and voltage control provide the foundations for reliable, stable electric power system operation. Today, every utility and operating entity must abide by certain frequency and voltage control guidelines.
Frequency response is of paramount interest to the industry, especially due to the potential grid impact from integrating increasing levels of variable energy resources (VERs), such as wind. Utilities and stakeholders are exploring how energy storage can help support VER integration and frequency regulation applications.
NERC is considering comments on the Resource and Demand Balancing standards recently proposed for frequency response (see: BAL-003-1 and Project 2007-12). As defined under the NERC project, frequency response is a measure of an interconnection’s ability to stabilize frequency immediately following the sudden loss of generation or load. This is a critical aspect of managing reliable bulk power system operations, particularly during disturbances and restoration.
The decline in frequency response
During the past 10 years, there has been a continuous decline in frequency response. NERC is planning to collect data to analyze this trend. When the electric energy industry has identified the issues and potential solutions, it is expected that NERC will refine the proposed Resource and Demand Balancing standards and issue final standards to bring the industry into compliance with frequency response. Non-compliance ultimately will result in financial penalties.
There are a number of theories about what might be causing the decline in frequency response, including the ratio of regulating generators (such as steam units) to peaking units and renewable resources. Another theory is the growing lack of new regulating units located in the same area as large load pockets—it is increasingly likely that large regulating units will be located away from populated areas. An additional consideration is the continuous increase in renewable VERs, which will be a major contributing factor to frequency response decline.
Options to address frequency response
There are a number of options to address frequency regulation, including demand-side management, frequency response obligation, supplemental service, and overlap regulation service. These options can help a utility be prepared in advance for NERC frequency regulation standards on the horizon.
Demand-side management (DSM) – A growing number of utilities and independent system operators (ISOs) are employing DSM solutions to decrease load peaks by altering consumer load profiles. DSM is accomplished when the consumer voluntarily allows the utility to turn off specific appliances, such as hot water heaters, pool filters, and air conditioners, for a limited period of time.
Frequency response obligation – Today, the primary source of frequency response to meet a balancing authority’s (BA) obligation resides with generation companies. A second source comes from programs, such as ERCOT’s Load Acting as a Resource, or from emergency load-shedding schemes. These solutions tend to work when the disturbance is within a utility’s area. However, they may be less effective when the disturbance is in a neighboring area.
Augmenting generation frequency response – Utilities can also obtain frequency response through supplemental service (SS) or overlap regulation service (ORS). ORS uses a reserve sharing group, which is a viable option as long as the sharing area is not experiencing the disturbance. For SS, utilities might look at DSM as an option. When experiencing significant frequency deviations, the duration of the impact might be lessened by augmenting the generator’s response with DSM, which can be calibrated and directed by the utility, controlling the impact.
By using DSM to shorten the period and stabilize the frequency, the BA lessens the impact on the frequency response measure (FRM) and decreases the potential of incurring NERC’s BAL-003-1 standard Violation Severity Level.
Designing a frequency response process
Designing and implementing an effective utility frequency response process is a key component of NERC BAL-003-1 compliance. A process to maintain the FRM within NERC’s assigned limits requires the ability to detect an external disturbance by monitoring tie line flows and frequencies, then activating DSM. The process might also require accumulating residual FRM from event-to-event to apply to subsequent events. This helps keep the utility’s annual frequency response within NERC’s guidelines and avoiding potential penalties.
In some cases, this process can be implemented as an application in the utility’s energy management system (EMS), if it can get a one-second periodicity. However, since EMS systems are typically not designed for a one-second resolution, this process might be better implemented external to the EMS by reading one-second frequency data and triggering DMS. Another consideration is if the one-second process isn’t reading frequency directly, the frequency data must be time stamped to prevent unknowingly reading the same frequency, creating false positives. There are other process design considerations to ensure customers are minimally inconvenienced and the annual FRM calculation is correct, which will arise as the technology evolves.
For additional insight into the use of DSM relative to frequency response solutions, contact KEMA’s expert: Raymond Gilby, principle consultant.