Balance of Power

Deck: 

Large grids can integrate more wind—without major burdens.

Fortnightly Magazine - May 2011

The desire of policymakers to add significant amounts of renewable energy to the generation mix is well understood. It’s also widely known that because the output of renewables such as wind power can be highly variable, power system operators have concerns about the ability to economically and reliably integrate these resources with system operations. Fortunately, a large body of knowledge has been developed in the last few years about integrating wind.

Despite their variable nature, renewable resources like wind and solar can be managed so they won’t impair the reliability of a utility system.

Given the diversity of resources and likelihood of improved forecasting of the output of these resources, it might be possible to reduce the need for quick-start and spinning operating reserves because the availability of the variable resources may actually offset unplanned variations in retail power demand. For this reason, it’s important that wind developers focus on working with existing balancing authorities rather than developing wind-only balancing authorities, which tend to have challenging economics.

Balancing authorities are the responsible entities that integrate resource plans ahead of time within specific service areas; they need to be attentive to their ability to meet control performance requirements (CPR) in the 10-minute timeframe as renewable capacity comes on line. This can be done with considerably less new investment than many market participants might think.

A key concern involves the basic need to maintain a balance of generation and load. Both can be subject to unexpected changes that aren’t necessarily exclusive to renewables, such as when a base-load steam plant suddenly trips off-line and 800 MW disappears, or when cold weather moves through an area 12 hours ahead of the forecast and causes an unexpected and rapid increase in power demand. While wind typically ramps down or up in unanticipated ways, it isn’t as problematic as dealing with a large generator that suddenly goes offline, and the variability of wind isn’t viewed as creating the type of frequency fluctuation problem that such an outage would cause. (See sidebar “Balancing Authorities and Frequency Control.”) A recent report funded by the Federal Energy Regulatory Commission (FERC) acknowledges this when it “clarifies that the events the interconnection is expected to withstand and the set points for under-frequency load shedding will not be affected by integrating variable renewable generation.”1

FERC, Reliability and Renewables

FERC plays a major role in the integration of renewables. The agency must approve reliability criteria, including requirements that affect their integration, and must approve tariff provisions related to open access of transmission.

FERC received many responses to its January 2010 Notice of Inquiry seeking comments on barriers to the integration of renewables to the grid. On Nov. 18, 2010, FERC issued a notice of proposed rulemaking in Docket RM10-11, proposing, in part, to require public utility transmission providers to offer all customers the option to schedule transmission service at 15-minute intervals instead of the current hourly scheduling procedure. This proposal, if adopted, would greatly enhance the ability of any balancing authority with limited internal flexible resources to integrate wind. The proposed rule would allow a balancing authority to look outside of its own boundaries for resources to offset unanticipated changes in wind output within a given hour. Every 15 minutes (within the hour) the balancing authority could change its intertie schedule by arranging for power supplies outside of its area to offset wind changes within its boundaries.

Early attempts to study the ability of a balancing authority to integrate wind required the modeling of possible future events. Now that several balancing authorities are actually integrating significant amounts of wind, the modeling can be calibrated with real-life experience. Modeling techniques vary from simple spreadsheets of 10-minute loads and resources to more elaborate dispatch models that attempt to replicate the commitment of generating units and dispatch decisions in time frames of less than one hour.

No matter what tool is used, the sub-hourly components are critical: forecast load, actual load, forecast wind and solar, actual wind and solar, and the resources whose output can be changed in slices of time of less than an hour’s duration. Black & Veatch analyzed the wind-integration capabilities and operating reserve requirements at several entities and found important differences between large, small, and wind-only balancing authorities.

Large Balancing Authority: This example considers a large balancing authority (peak load greater than 6,000 MW) with a minor amount of existing wind. The authority is looking to add approximately 2,500 MW of equal amounts of wind and solar generation. The study was conducted with different assumptions regarding the ability to forecast load and scheduled hourly megawatts from wind and solar.

The study also looked at targeting the 90 percent of allowable control performance standard (CPS) in time periods of 10 minutes (CPS2 value) and targeting a more conservative 95 percent CPS2 value.2 It was understood at the outset of the study that if all of wind or solar to be added was assumed to have the same hourly pattern, then there wouldn’t be any diversity associated with the added renewables. To avoid this unrealistic result, the study considered the likely locations of wind and solar facilities and the hourly patterns of generation.

For purposes of forecasting load and renewables, it was assumed that forecasts would be persistence-based. A persistence-based forecast derives its forecast value for an upcoming scheduled hour from actual values in a recent historical time period. For example, a two-hour persistence load forecast would estimate the load in the next scheduling hour based on the actual load two hours before the scheduling hour commences.

EES North America

Cases were run with two-hour and one-hour persistence-based load forecasts and two-hour persistence-based forecasts of wind and solar generation. The study allowed for the possibility that certain percentage improvements could be made in the persistence-based forecasts. For example, a two-hour forecast was assumed without any improvement. Then sensitivity analyses were performed to determine whether this two-hour persistence forecast for load, wind and solar could be improved by 10 percent and 20 percent.

The study first determined how much operating reserve (quick-start and spinning) would be needed to meet CPS2 requirements if the 2,500 MW of wind and solar wasn’t added. Then the study determined how much additional operating reserve (quick-start or spinning) would be needed if the 2,500 MW of renewable wind and solar was added in the balancing authority (see Figure 1).

The tables show the incremental operating reserves needed to integrate the additional 2,500 MW of wind and solar as opposed to the amount of operating reserves needed if no additional wind and solar power was added to the balancing authority. The capacity is needed in both the upward (increasing MW) and downward (decreasing MW) direction. The results are lower than they would have been if there was no diversity in the technology or the location of the wind and solar generation. The study was done with both 90 percent and 95 percent CPS2 targets. FERC’s first level violation doesn’t occur until the CPS2 value falls below 90 percent. However, many utilities have expressed a desire to target at least 95 percent for additional safety purposes. The table shows the additional amount of operating reserves in megawatts needed when attempting to achieve the 95 percent level of performance as compared to what would be needed if shooting for the FERC-indicated level of 90 percent.

The negative values in Figure 1 indicate that if improvements in load forecasts are accomplished at the same time as improvements in the ability to forecast renewables, then the renewables can be added even with reductions in operating reserves and still meet CPS goals.

The conclusion of this study is that the additional operating reserves needed are very much influenced by the ability to forecast the renewable output as well as the CPS2 target that is desired. This study also demonstrates that the diversity of the load and the added renewables can possibly cause a reduction in the need for operating reserves in comparison to a future without any new renewables, as long as a reasonable forecast can be made of the renewable generation.

Wind-Only Balancing Authority: Wind developers often run into difficulty in acquiring needed balancing services. Balancing authorities either claim they can’t perform the balancing services or state that if requested, they will file a tariff with FERC that has a very high charge for providing these services. As a result, wind developers occasionally consider forming their own balancing services and building gas-fired generation to accomplish the balancing.

Analyses performed for two such developers concluded that a wind developer can’t provide the balancing services it needs nearly as cost effectively as an existing balancing authority. First, a wind project doesn’t have retail load to include in its proposed wind-only balancing authority. Load and wind together provide a diversity that’s more effective than wind alone.

Second, balancing authorities generally have non-wind supplies already in place—such as resources that help meet planning reserve requirements. These assets can provide some of the operating reserves needed to perform any additional balancing services.

Finally, most balancing authorities are already integrating other wind and solar. As a result, the diversity of those resources coupled with the diversity of the developers’ proposed new wind plant reduces the need for new operating reserve capacity.

• Small Balancing Authority: A small balancing authority has few resources that can be used to provide balancing needs. When asked to integrate new wind, this balancing authority asserts that it will need all new regulated resources in large amounts to integrate the wind.

Analysis of a small balancing authority, performed for a wind developer, concluded that the amount of operating reserve needed to integrate existing wind by the balancing authority was considerably less than what had been put in place.

Several insights were gained by examining the differences in the studies performed by Black & Veatch and the study performed by the balancing authority. First, it wasn’t clear how the balancing authority established its final hourly interchange schedules with other balancing authorities. The balancing authority didn’t keep records of the load forecast it used when it finalized its interchange schedule. And although the balancing authority engaged a wind forecasting consultant to prepare forecasts for the scheduled hour, it didn’t use those forecasts. It wasn’t clear what forecasts the balancing authority used because it didn’t keep those records.

Because current FERC rules make it difficult to change intertie schedules within the operating hour, forecasting and scheduling activities that are done “before the hour” are crucial for meeting CPS2 requirements. Changing the output of resources located within a balancing authority “within the hour” in order to maintain intertie schedules is clearly allowed; regional transmission organizations and independent system operators with centralized dispatch are designed to do exactly that. The small balancing authority that was the subject of the study, however, didn’t change any of its in-area resources within the hour, thus causing it to need more regulating reserves to meet its CPS2. It isn’t clear why these changes weren’t being made within the hour consistent with its ability to do so.

The Black & Veatch analysis of the need for regulating reserves for this small balancing authority doesn’t correspond with the balancing authority’s own study of its needs. However, both studies agree that if future additional renewables are provided from smaller plants in diversified locations, less new operating reserve will be needed than if added renewable generation comes from larger plants in more concentrated locations.

Findings and Recommendations

The analyses yield two recommendations to further assist and enable integration of large amounts of renewable generation into the grid.

First, wind developers should take a collaborative stance and work with existing balancing authorities rather than develop a wind-only balancing authority.

Second, balancing authorities should consider keeping records of load and wind forecasts used in actual intertie scheduling activities. These records will help the utility demonstrate that it is prudently scheduling its purchases and sales outside of its balancing authority.

Improvements in near-term forecasting of renewable generation output and power demand—coupled with geographic, generation and load diversity—will enable providers to integrate large amounts of wind and solar generation capacity into the grid. And providers will be able to do so in a manner that doesn’t place a large burden on the balancing authority’s ability to plan bulk power operations in its area—particularly if the FERC-proposed rule on transmission scheduling is fully adopted.

 

Endnotes:

1. Use of Frequency Response Metrics to Assess the Planning and Operating Requirements for Reliable Integration of Variable Renewable Generation, December 2010, Lawrence Berkeley National Laboratory, LBNL-4142E, pg. 25.

2. Control Performance Standard (CPS) is defined as the reliability standard that sets the limits of a Balancing Authority’s control error over a specified time period. The CPS1 has a time period of one minute and CPS2 has a time period of 10 minutes. For wind projects, the CPS2 standard is more difficult to meet. For more detail on this standard, see “Standard BAL-001-0.1a—Real Power Balancing Control Performance.”