TODAY’S STUDY: WHY THOSE WIND TURBINES AREN’T TURNING
Wind and Solar Energy Curtailment: Experience and Practices in the United States
Lori Bird, Jaquelin Cochran, and Xi Wang, March 2014 (National Renewable Energy Laboratory)
Curtailment is a reduction in the output of a generator from what it could otherwise produce given available resources, typically on an involuntary basis. Curtailment of generation has been a normal occurrence since the beginning of the electric power industry. However, owners of wind and solar generation, which have no fuel costs, are concerned about the impacts of curtailment on project economics. Operator-induced curtailment typically occurs because of transmission congestion or lack of transmission access, but it can occur for a variety of other reasons, such as excess generation during low load periods, voltage, or interconnection issues. Market-based protocols that dispatch generation based on economics can also result in wind and solar energy plants generating less than what they could potentially produce.
This report examines U.S. curtailment practices regarding wind and solar generation, with a particular emphasis on utilities in the western states. The information presented here is based on a series of interviews conducted with utilities, system operators, wind energy developers, and other stakeholders. The report provides case studies of curtailment experience and examines the reasons for curtailment, procedures, compensation, and practices that can minimize curtailment.
Key findings include:
• In the largest markets for wind power, the amount of curtailment appears to be declining even as the amount of wind power on the system increases. Curtailment levels have generally been 4% or less of wind generation in regions where curtailment has occurred. Many utilities in the western states report negligible levels of curtailment. The most common reasons for curtailment are insufficient transmission and local congestion and excessive supply during low load periods.
• Definitions of curtailment and data availability vary. Understanding curtailment levels can be complicated by relatively new market-based protocols or programs that dispatch wind down or limit wind generation to schedules and the lack of uniformity in data collection.
• Compensation and contract terms are changing as curtailment becomes of greater concern to solar and wind plant owners. Increasingly there are negotiated contract provisions addressing use of curtailment hours and there is greater explicit sharing of risk between the generator and off-taker.
• Automation can reduce curtailment levels. Manual curtailment processes can extend curtailment periods because of the time needed for implementation and hesitancy to release units from curtailment orders.
• Market solutions that base dispatch levels on economics offer the advantages of creating transparency and automation in curtailment procedures, which apply equally to all generators.
• Curtailed wind and solar resources may provide ancillary services to aid in system operations.
• A variety of solutions is being used to reduce curtailments: transmission expansion and interconnection upgrades; operational changes such as forecasting and increased automation of signaling; and better management of reserves and generation.
Curtailment of variable renewable generation, particularly wind and solar energy, is becoming more widespread as wind and solar energy development expands across the country and penetrations increase. Curtailment can affect the revenue of wind and solar energy projects.
These impacts are specific to each balancing area due to differences in grid characteristics, operating practices, and other factors such as weather.
In this paper, we define curtailment as a reduction in the output of a generator from what it could otherwise produce given available resources (e.g., wind or sunlight), typically on an involuntary basis. Curtailments can result when operators or utilities command wind and solar generators to reduce output to minimize transmission congestion or otherwise manage the system or achieve the optimal mix of resources. Curtailment of wind and solar resources typically occurs because of transmission congestion or lack of transmission access, but it can also occur for reasons such as excess generation during low load periods that could cause baseload generators to reach minimum generation thresholds, because of voltage or interconnection issues, or to maintain frequency requirements, particularly for small, isolated grids. Curtailment is one among many tools to maintain system energy balance, which can also include grid capacity, hydropower and thermal generation, demand response, storage, and institutional changes. Deciding which method to use is primarily a matter of economics and operational practice.
“Curtailment” today does not necessarily mean what it did in the early 2000s. Two sea changes in the electric sector have shaped curtailment practices since that time: the utility-scale deployment of wind power, which has no fuel cost, and the evolution of wholesale power markets. These simultaneous changes have led to new operational challenges but have also expanded the array of market-based tools for addressing them.
Practices vary significantly by region and market design. In places with centrally-organized wholesale power markets and experience with wind power, manual wind energy curtailment processes are increasingly being replaced by transparent offer-based market mechanisms that base dispatch on economics. Market protocols that dispatch generation based on economics can also result in renewable energy plants generating less than what they could potentially produce with available wind or sunlight. This is often referred to by grid operators by other terms, such as “downward dispatch.” In places served primarily by vertically integrated utilities, power purchase agreements (PPAs) between the utility and the wind developer increasingly contain financial provisions for curtailment contingencies.
This report delineates several types of practices under the broad rubric of curtailment done for wind or solar generation. Some reductions in output are determined by how a wind operator values dispatch versus non-dispatch. Other curtailments of wind are determined by the grid operator in response to potential reliability events. Still other curtailments result from overdevelopment of wind power in transmission-constrained areas. Responses to all types of curtailment largely reflect the operating context, including whether the wind power is part of an centrally-organized wholesale market, or whether it is in a balancing authority area operated by a vertically integrated utility.
Dispatch below maximum output (curtailment) can be more of an issue for wind and solar generators than it is for fossil generation units because of differences in their cost structures. The economics of wind and solar generation depend on the ability to generate electricity whenever there is sufficient sunlight or wind to power their facilities. Because wind and solar generators have substantial capital costs but no fuel costs (i.e., minimal variable costs), maximizing output improves their ability to recover capital costs. In contrast, fossil generators have higher variable costs, such as fuel costs. Avoiding these costs can, depending on the economics of a specific generator, to some degree reduce the financial impact of curtailment, especially if the generator's capital costs are included in a utility's rate base.
Ascertaining the level of curtailment of wind and solar generation and its impacts is challenging. Often system operators or utilities do not track it or make data publicly available, and there are differences in terminology as well. Manual curtailment processes for wind have been replaced by economic dispatch protocols in a number of regions, and under the new protocols, dispatch below maximum output is typically not referred to as curtailment. In addition, energy lost due to line outages, and limits placed on deviations from schedule can all reduce wind generator’s production; some operators call these actions curtailment while others do not.
This report examines curtailment practices for wind and solar energy in the United States, with a particular emphasis on utilities in the western states. Much of the experience documented in this report pertains to curtailment of wind power, which has reached higher penetrations of bulk system power, although solar curtailment is included where information is available.
This report builds on earlier reviews of domestic curtailment experience by Rogers et al. (2010) and Fink et al. (2009) and a recent review of international practices by Lew et al. (2013). The information presented here is based on a series of interviews conducted with utilities, system operators, wind energy developers and owners, and non-governmental organizations as well as other available data sources. This review was conducted to better understand the diversity of practices in place and the magnitude of curtailment that has been occurring. The report provides case studies of curtailment experience and examines the reasons for curtailment, curtailment procedures, compensation, and practices that can minimize curtailment of wind and solar.
Overview: Levels of Curtailment in the United States
Curtailment levels, where curtailment has occurred, are often in the range of 1% to 4% of wind generation, but higher levels have been reported by the Electric Reliability Council of Texas (ERCOT) in past years, as can be seen in Figure 1.
However, the levels of wind curtailment experienced to date in the United States differ substantially by region and utility service territory, as discussed in Section 3. In many regions, curtailment is very low and not even tracked. Table 1 provides a summary of curtailment levels and causes for all of the utilities and grid operators interviewed for this study. Further discussion of the reason for curtailments is included in Section 3. Table A-1 in Appendix A summarizes experiences with wind and solar energy curtailment from all of the utilities and grid operators interviewed, including utilities that reported relatively low levels of curtailment…
While a greater number of regions are experiencing some form of curtailment of wind and solar resources, the relative magnitude of curtailment appears to be declining in the largest markets for wind power even as the amount of wind power on the system increases. New transmission capacity and better operating practices, such as greater automation and the use of forecasting and other operational practices, are now resolving challenges for grid operators, often circumventing the need for curtailment. As penetrations of wind and solar energy increase, curtailment practices and the use of strategies to mitigate the potential for curtailment may become increasingly important and may impact wind and solar energy project economics. Nevertheless, as wind and solar energy penetrations increase, there may come a time when changes in operating protocols would not lead to reduced curtailments, and rather that curtailment volumes could rise as a fraction of total wind and solar generation.
Curtailment levels have generally been 4% or less of wind energy generation in regions where curtailment has occurred. A notable exception is ERCOT, where curtailment levels reached 17% in one year, primarily because wind generation came online ahead of transmission capacity.
These levels have since receded to less than 2%. Many utilities in the western states report negligible levels of curtailment. The most common reasons for curtailment are insufficient transmission and local congestion, and excessive supply during low load periods. One challenge to determining curtailment levels is that data are not uniformly collected.
Definitions of curtailment vary. Understanding curtailment levels can be complicated by relatively newly implemented market-based protocols or programs that limit wind generation to schedules. Now that economic dispatch is being used in several areas, wind generators can be dispatched down based on market prices, but this reduction of output is not characterized as curtailment. In some cases, wind generators are not able to exceed scheduled levels—a process that is referred to in the BPA balancing area as limiting output rather than curtailment.
Curtailment order varies and is often based on plant economics or ability to alleviate local congestion. For curtailments that are needed to address balancing or system operations, the most expensive generators are often curtailed first. For wind projects, one consideration is whether the project utilizes the federal PTC or ITC. Generators reliant on the PTC, which is provided based on project output, face greater financial impacts from curtailment (the value of the PTC as well as the energy) than wind generators that received the upfront ITC. To address local congestion issues, curtailment is often applied equitably across generators that are most able to alleviate congestion. Hawaii provides preference to projects based on the order that they were installed (i.e., curtailing the most recently installed resources first), which limits the financial impacts and risks of curtailment for existing renewable energy facilities.
Compensation and contract terms are changing for curtailment. Contracts between generators and off-takers have in some cases included provisions whereby the off-taker will compensate for curtailment for reasons such as congestion, scheduled maintenance, and operator errors, but typically not for curtailment ordered by other entities. However, contracts are increasingly reflecting a negotiated number of annual hours in which curtailments are not compensated, despite the cause, and there is greater sharing of risk between the generator and off-taker. In some cases, when new curtailment procedures are adopted, the need to renegotiate contract language has posed implementation challenges. In wholesale power markets, the grid operator sometimes compensates if it calls for units to deviate from initial dispatch orders.
Automation can reduce curtailment levels. Manual curtailment processes for wind have been found to extend curtailment periods because of the time required to implement curtailment and hesitancy to release units from curtailment orders. Automatic communication procedures can speed the implementation of curtailment orders and reduce overall curtailment time.
Market solutions that base dispatch levels on economics offer the advantages of creating transparency and efficiency in curtailment procedures, which apply equally to all generation sources. Programs like the MISO DIR utilize economic dispatch to determine which units will generate at a given time. During periods of oversupply, the use of negative pricing to determine dispatch order can eliminate the need for manual curtailments. Some wind developers have expressed a preference of the market-based dispatch framework because it reduces market distortions and allows wind generators to participate alongside conventional generators.
The use of market-based approaches with their associated automation can also minimize curtailment by improving operational efficiency and reducing the burden on grid operators of implementing manual processes.
Curtailed wind resources can provide ancillary services to aide in system operations. PSCO uses curtailed wind resources to provide both up and down regulation reserves for the balancing area. Wind turbines can provide quick response to signals, which can be valuable for the system. MISO assessed this and found it was economically viable only 2% of the time, however. ERCOT requires all wind turbines that can be retrofitted with governor response to do so in order to provide primary frequency response if they are curtailed.
Transmission expansion and interconnection upgrades can be one of the most direct ways to reduce curtailments. ERCOT’s expansion of transmission in recent years through CREZ has alleviated wind generator curtailments. A key challenge is that renewable energy projects can be built much more rapidly than transmission lines. The CREZ program identified the need for transmission to particular regions to facilitate wind energy expansion. SPP is building new transmission capacity that is expected to alleviate current curtailment levels, while MISO is pursuing multi-value transmission projects to move wind to load centers and more robust parts of the grid.
Forecasting can decrease uncertainty associated with wind and solar resources, reducing the need for curtailments due to unexpected changes. Improved forecasting can enable utilities or grid operators to turn down conventional resources when sufficient wind generation is predicted and to reduce curtailment from oversupply. Improved forecasting can also reduce curtailments related to ramping. Wind forecasting can provide generators better information and enable them to participate more fully in the day-ahead market. Improved data on wind profiles may help grid operators provide more precise not-to-exceed instructions, enabling increased wind generation output. Grid operators could also improve visibility of distributed solar, which appears to grid operators as reduced load, to help understand system changes that can influence unit commitment and balancing to minimize curtailments.
Firming resources can decrease curtailments and increase financial certainty for generators, but they are not necessarily the most cost-effective system-wide solution. Iberdrola in BPA has found it more cost-effective to balance its own resources than it is to pay integration charges and be exposed to uncompensated curtailments under DSO 216. Nevertheless, developer-specific balancing, through options such as storage and natural gas, may likely be less cost-effective at a system-wide level than it would be for the utility or grid operator to adopt operating practices that minimize the need to curtail, such as dynamic reserves, negative pricing, and improved forecasting.