In order to reliably and securely operate the grid, grid operators must constantly maintain the balance between power generation and consumption in real time, but also must maintain voltage stability, frequency stability, and be prepared to recover from major grid disruptions28. System operator must procure special services and functions beyond energy to achieve this balance, which are referred as ancillary services (AS). While definitions and terminology vary drastically across regions, we provide general definitions which we will use throughout this discussion29:
Non-event Reserves: corrects short timescale differences between supply and demand to maintain system frequency
• Regulation: automatic response to cover a limited range of output change. Typically required of generators.
• Following: manually dispatched resources to cover larger unscheduled changes in supply/demand. Participants are typically paid to have their capacity available, and paid for the service provided.
28 https://www.logicenergy.com/what-are-ancillary-services-and-why-do-power-grids-need-them/
29 Utilizes the framework from NREL’s paper on operating reserves: https://www.nrel.gov/docs/fy11osti/51978.
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5940 Figure 4-1 Event and non-event operating reserve
Event Reserves: resources that can be quickly dispatched in the case of an unexpected loss of supply o Primary Reserves: (also known as frequency response reserves) Immediate response that
helps stabilize the grid after a loss of supply for long enough for additional reserves to replace the lost supply
o Secondary Reserves: (usually further divided into spinning reserves and non-spinning reserves depending on response time) capable covering lost supply within 10 minutes of the event.
Typically provided by generators already operating or able to start up quickly. Demand-side resources can also provide spinning reserves if they are able to reduce their load within the required time frame.
o Tertiary Reserves (also known as supplemental reserves): more economic resources, but with slower response times (30- 60 mins), coming online to fill the capacity lost from the event
Figure 4-2 Illustration of event reserve response50
Figure 14: Event and non-event operating reserve
Event Reserves: resources that can be quickly dispatched in the case of an unexpected loss of supply
• Primary Reserves (also known as frequency response reserves): Immediate response that helps stabilize the grid after a loss of supply for long enough for additional
reserves to replace the lost supply
• Secondary Reserves (usually further divided into spinning reserves and non-spinning reserves depending on response time): capable covering lost supply within 10 minutes of the event. Typically provided by generators already operating or able to start up quickly. Demand-side resources can also provide spinning reserves if they are able to reduce their load within the required time frame.
• Tertiary Reserves (also known as supplemental reserves): more economic resources, but with slower response times (30- 60 mins), coming online to fill the capacity lost from the event
40 Figure 4-1 Event and non-event operating reserve
Event Reserves: resources that can be quickly dispatched in the case of an unexpected loss of supply o Primary Reserves: (also known as frequency response reserves) Immediate response that
helps stabilize the grid after a loss of supply for long enough for additional reserves to replace the lost supply
o Secondary Reserves: (usually further divided into spinning reserves and non-spinning reserves depending on response time) capable covering lost supply within 10 minutes of the event.
Typically provided by generators already operating or able to start up quickly. Demand-side resources can also provide spinning reserves if they are able to reduce their load within the required time frame.
o Tertiary Reserves (also known as supplemental reserves): more economic resources, but with slower response times (30- 60 mins), coming online to fill the capacity lost from the event
Figure 4-2 Illustration of event reserve response50
Figure 15: Illustration of event reserve response30
Increasingly, a new reserve classification is emerging in these markets, referred to generally as ramping reserve that can load-follow or ramp for sustained periods with substantial ramp mileage to cover for sudden losses in VRE.
These products are generally procured at volumes according to reliability standards established by NERC (typically 15%). These are required to be procured by the balancing authorities, who may procure them by several methods:
30 https://eta-internal-publications.lbl.gov/sites/default/files/spinning-reserves.pdf
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• Require each generator to provide as a part of their normal dispatch (typical of a vertically integrated utility and some services also considered complimentary with DA & RT
dispatch)
• Administratively set prices for the service and pay those generators contributing
• Creating a market for competitive selection of AS providers
We will predominately focus on the market-based approach, since the most aggressive renewable integration is typically happening in market territories and by defining AS market products, the role of these service definitions in maximizing flexibility is better exposed.
Table 4: Overview of the ancillary services offered by CAISO and ERCOT31
Increasing renewable penetration creates more fluctuations in grid supply, and therefore a greater need for ancillary services to maintain reliable operation32,33. The most typical operations discussed in this case are:
• Frequency Regulation: generators increase or decrease output (known as ”regulation up” and ”regulation down”) to maintain system frequency. As renewable output varies moment by moment, the need for frequency regulation increases, specifically the ramp mileage required to cover changes in renewable output. This has largely been addressed by shortening the timescales by which real-time markets clear resources, and by
optimizing energy market and ancillary service scheduling (discussed in section 3.3). For sustained ramping, this is addressed by introducing new ramping products, discussed in section 3.4.
• Over-frequency events: this typically happens slowly and grid operators can respond by reducing output from some generators. Large, unpredicted surpluses of solar energy
31 Definitions per each service vary between different ISO/RTOs and although those nuanced differences do result in material differences in system operation, for the purposes of this discussion, the broad classifications of spinning, non-spinning, and regulation should be considered parallel with secondary contingency reserves, tertiary contingency reserves, and a combination of regulating and ramping reserves, respectively.
32 http://www.caiso.com/Documents/2019FinalFlexibleCapacityNeedsAssessment.pdf 33 https://www.caiso.com/Documents/FinalFlexibleCapacityNeedsAssessmentFor2017.pdf