ERCOT runs its Security-Constrained Economic Dispatch (SCED) every five minutes and sends resulting targeted output levels for the next five-minute dispatch interval to each generator. This set of output levels results in the most economic generation being used to serve the expected generation requirement, taking into account the operating characteristics of the generation units, the current wind and solar outputs, and relevant transmission system constraints.
However, neither the load nor the generation output on the system is constant within these five-minute intervals. Load varies with consumers’ constantly changing demands. Aggregate generation input varies due to generating units ramping and variation in wind and solar inputs. Because ERCOT comprises an entire electrical interconnection, any imbalance between generation and load results in an increase or decrease in system frequency. If frequency deviates too far, the system’s Primary Frequency
Response (PFR) will be deployed, potentially leaving insufficient PFR available to respond to generating unit trips.
Regulation Service is an Ancillary Service that provides a mechanism to adjust the system generation output, at the margin, to better match system load and maintain frequency within a dispatch interval. With Regulation Service, ERCOT sends a signal to generators every few seconds to increase or decrease their output. This service is automatically deployed in Real-Time Operations by ERCOT’s Energy Management System every four seconds as needed to maintain system frequency within a desired range around 60 Hz.
The quantification of the required amount of Regulation Service has evolved over time. Currently, ERCOT determines the amount of Regulation Service needed based on historic net load variability, by month and hour of the day. For example, to determine the quantity of Reg-Up for hour ending 0800 in May, ERCOT calculates the decrease in net load (load minus wind minus solar) for each historic five-minute interval between 7:00 a.m. and 8:00 a.m. in May of the last two years and then calculates the 95th percentile of these values. A factor is then added to account for the increased variability due to additional installed wind generation (this factor is less than 1 MW per 100 MW of new wind in most hours). This calculation is performed in December for each hour of each month, in increasing (Reg-Up) and decreasing (Reg-Down) direction, to determine the quantities that will be procured for the upcoming year. The actual procurement of Regulation Service occurs through the Day-Ahead Market for each hour of the next day.
The average hourly quantity of Regulation Service required has gradually decreased from ~580 MW in late 2010, when the five-minute dispatch market was first implemented, to ~300 MW in 2017. This reduction has primarily been due to been due to the continuing refinement in determining the required amount of Regulation Service. This decrease in Regulation Service requirements has occurred in spite of the fact that the installed capacity of wind generation on the ERCOT grid has risen from 9,400 MW in January 2011 to over 21,000 MW today because the refinement in determining the required quantities has offset the aforementioned increase that is added to each hour to account for the variability of increasing installed wind generation.
The mechanism for assessing whether ERCOT is procuring sufficient Regulation Service to maintain good frequency control is the Control Performance Standard 1 (CPS1) score, calculated for NERC Standard BAL-001-01 R1. The CPS1 score is assessed monthly on a scale of 0 to 200 and must be above 100 to achieve compliance. ERCOT’s 12-month rolling average CPS1 score has generally increased from 150 in January 2011 to nearly 180 in May 2018, indicating very good frequency control.
In addition to improvements in determining the required quantity, ERCOT is in the process of making two improvements that should increase the efficiency of Regulation Service deployment.
ERCOT has added a short-term wind forecast (five-minute interval forecast for the next two hours). This forecast will soon be included in the determination of generation to be dispatched for the next five minutes, rather than the current practice that assumes that the wind and solar outputs persist at the current level over the next five minutes in the economic dispatch. ERCOT’s analysis has shown significant accuracy improvement in determining the generation to be dispatched for the next five minutes during hours when wind is ramping by using this forecast instead of the persistence assumption. Economic dispatch will now cover the wind energy ramps during each five-minute period, similar to how it currently covers load ramps.
The second improvement, which is still under consideration, is to automate the process of running the SCED more frequently when a net load ramp results in significant Regulation Service deployment.
Both of these improvements should reduce the energy currently deployed through Regulation Service and shift it to being deployed on the more efficient resources through the economic dispatch.
Dan Woodfin, Senior Director, System Operations