ERCOT readies for renewable market integration.
Julia Matevosyan is a senior planning engineer for the Electric Reliability Council of Texas (ERCOT). She is an expert in transmission and distribution system planning, power production planning, and network interconnection of new generation and load. She earned a Ph.D., MSc. and Technical Licentiate in electrical engineering from the Royal Institute of Technology, Stockholm, and is a senior member of IEEE.
The Electric Reliability Council of Texas, or ERCOT, is the independent transmission system and electricity market operator of Texas serving 85 percent of the state's load, with installed generation capacity of almost 85,000 MW. ERCOT conducts a long-range planning study every two years to review generation and transmission needs for the region in accordance with Texas state law. As well, it follows NERC requirements to conduct 10-year assessments of system reliability.
Historically, the scope of long-term transmission plans has always complied with legal and regulatory requirements. With the relative uncertainty created by forecasted renewable energy sources (RES) penetration on the regional transmission grid, ERCOT decided to expand its biannual study to determine necessary operating reserve requirements for accommodating RES. A successful application for DOE funding through the American Recovery and Reinvestment Act of 2009 enabled ERCOT to begin a much more ambitious long-range analysis.
Wind Today; Wind Tomorrow
Current wind generation capacity in Texas is over 12 percent of total capacity, with most of it located in the western part of the state. As Figure 1 shows, there can be wide mismatch between wind power production and actual load in terms of seasonal and daily patterns. This mismatch can lead to operational challenges when trying to integrate large amounts of wind energy into the system.
As a Balancing Authority (BA), ERCOT has the responsibility to continuously balance demand and supply to maintain system frequency close to 60 Hz. Currently, ERCOT system is not synchronously interconnected with any of its neighboring systems. Thus, the required balancing power has to be provided from available resources within the ERCOT system. ERCOT procures the required amount of ancillary services in the day-ahead market to ensure sufficient reserves are available in real time. These ancillary services include products with different characteristics (Responsive Reserve, Regulation, and Non-Spin) to provide required flexibility for real time balancing.
Presently there are adequate reserves to maintain system frequency, but the need for more ancillary services in our study was almost a foregone conclusion, as expected higher penetration levels of wind and solar generation come online in the coming decades. One of the goals for the expanded study was to learn how much reserve capacity was necessary to support an expanded RES portfolio.
Until it began its reserve adequacy study in 2012, ERCOT's long term transmission planning processes relied on production cost simulation tools (PROMOD, UPLAN) and tools for AC analysis (PSS/E, PowerWorld). While powerful for their intended applications, these tools do not capture intra-hourly variability of the renewables and load and system balancing challenges.
Determining the impact of increasing proportions of wind, solar, storage, or demand-response resources on ERCOT's ability to comply with current balancing metrics requires second-by-second time series simulation of the future. Accordingly, ERCOT procured a tool with the following capabilities:
• Intra-hour time series simulation with net-load (load minus wind) changes with one-second resolution.
• Adequate operational representation of emerging or expanding non-traditional resources, including, but not limited to, wind, solar, and storage technologies.
• Adequate representation of the existing ERCOT system, including existing resources, system load, and historic and forecasted variable generation availability patterns.
ERCOT acquired a software application known as the KEMA Renewable Market Integration Tool (KERMIT) for dynamic simulation of second-by-second grid operation on a 24-hour horizon (Figure 2).
The non-linear power plant modules included in the software tool feature different models of thermal, wind and solar generation, demand response, and storage resources, capturing user-defined energy schedules and plant characteristics. They are used to assess the ability of the combined generation fleet to meet reliability metrics, including NERC's Control Performance Standard 1 (CPS1) and disturbance control standards (DCS). For this exercise, KERMIT was used to simulate how much balancing energy is needed to manage both normal net-load intermittency and the largest simulated net-load change.
In this reserve adequacy study, the software tool was calibrated to historical ERCOT system operation data from 2011 that ranged from normal situations to extreme events. Three days from various months were set up in the simulation to represent normal operation and generation trips with actual system load, SCED (Security Constrained Economic Dispatch) instructions, wind power production, and generation outages for each of the three days.
ERCOT performed simulations of two long-term transmission study scenarios with differing proportions of renewables (20 GW and 50 GW). In each scenario 22 representative days covering different times of the year were selected for the study. In two cases modeled (Figures 3A and 3B), ERCOT calculated a need for more resources capable of responding to net-load changes very quickly.
The modeled output from the 20 GW of wind case behaved similarly to current operations, with high intermittent resource contribution during early hours of spring and fall days. However, in the 50-GW study, even peak hours in the summer became troublesome. This result was due to increased share of wind generation being projected in the south and coastal parts of Texas, where wind and load generation patterns are well correlated. That lead to an increased share of load served by intermittent generation and elevated net-load variability during summer peak hours.
Need for Speed
The results were both informative and expected. As net-load ramps increase with higher proportions of interconnected wind, more resources capable of providing power on a zero-to-five minute basis are needed. The most challenging hours are those with intermittent resources serving a high percentage of load. These hours will have less conventional generation online capable to support system frequency. This scenario is largely the case today in current operations with high intermittent contribution during early hours of spring and fall days.
Figure 4 illustrates the ancillary services requirements for the studied scenarios. The ancillary service requirements from 2012 were included in the table for reference. For 20 GW of wind generation, an additional 800 MW of capacity capable of deployment in the zero-to-five minute range is needed to achieve comparable balancing to current frequency maintenance performance standards.
This same need for more zero-to-five minute product increased to 1,300 MW for the 50 GW of wind scenario. Without these additional reserves, CPS1 over a year is below ERCOT's NERC requirement, while the addition of the reserves allows ERCOT to bring CPS1 score up to 165 percent, similar to current CPS1 performance.
The need for a 30-minute (potentially off-line resource) remained unchanged for both scenarios when compared to the initial KERMIT run. Total ancillary service capacity increased from 3,650 MW to 4,450 MW and 4,950 MW in the 20 GW and 50 GW studies, respectively. While the installed intermittent resource base increased 400 percent in the 50-GW study scenario, the amount of additional ancillary services increased only 50 percent. However, the need for a new, more flexible ancillary service product was evident.
Toward Higher Flexibility
The new five-minute product that is projected to be needed in these scenarios does not correspond to any particular technology or currently existing market product. For example, our software solution assumed that any technology that can start up and deploy within five minutes' notice (e.g., batteries, thermal/traditional resources, demand response resources, etc.) may provide this service in the real-time market SCED deployment. While there is no existing market product for resources that can dispatch to full load within five minutes, ERCOT has assumed that should the need arise in the next 20 years, that emerging or existing market technologies would provide a market-based solution. Alternatively, this need could be addressed by increasing capacity procured for existing Regulation Service.
One of the advantages of the software model is its ability to evaluate combinations of capabilities and operating characteristics to satisfy ancillary service needs. This flexibility permits the study of possible resource characteristics different than those associated with existing technologies.
The assessment of ancillary service adequacy, thus far, solely identified viable ancillary service portfolios to accompany varying levels of wind integration and did not forecast how the price and availability of ancillary services may change with increased demand and deployment of ancillary resources. For example, ERCOT routinely procures interruptible loads, quick start and slower resources to support current levels of intermittent resource integration. However, it is not clear if some of these resources, e.g., interruptible loads and quick starts, will be still be willing to provide the ancillary services should the frequency of the deployment increase.
Reserve Decisions: Looking Ahead
Depending on the additional reserve requirement, the economics for the future resource expansion scenarios may change and the generation mix should then be revised - which in turn may affect the entire long-term transmission study. For the studied scenarios, the amount of additional reserves needed was not large enough to affect the generation build.
ERCOT's reserve adequacy assessment is still a work-in-progress. Future work will include an impact analysis of rare events such as large net-load ramps/forecast errors and wind generation drop out due to storms. ERCOT also wants to identify any additional reserves based on currently available technologies (conventional quick-start peaker units, demand response and storage) and what they might cost to remain in compliance with the CPS1 score.